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Rat Tickling: Forma de enriqueciemiento social para promover su bienestar

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Rat Tickling: Forma de enriqueciemiento social para promover su bienestar

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NC3R´s

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The importance of positive human-animal interactions is widely recognised for large laboratory animal species, but often ignored for the smaller rodents, despite growing evidence of benefits to their welfare. Here we highlight tickling as a social enrichment to improve rat welfare.
2017 saw Dr Jaak Panksepp, neuroscientist, pioneer of research into animal emotions and creator of the rat tickling technique, sadly pass away this spring. It's been almost 20 years since he first developed the method, so taking a look at this habituation technique seems especially timely.

Designed to mimic the play activity of juvenile rats (see video below), tickling has been shown to be an effective means of generating a positive affect (a positive mental state) in these animals. Tickled rats produce vocalisations at 50 kHz, a range repeatedly shown to be associated with positive welfare ¹.

A systematic review, written by Megan LaFollette (PhD student, Purdue University), Dr Brianna Gaskill (Assistant Professor of Animal Science, Purdue University) and colleagues and published in , examines the current evidence and evaluates the benefits, applications and challenges of the technique ².

The review notes that 22 separate experiments have evaluated the benefits of tickling and have reported tickling as an effective means of improving welfare and reducing anxiety in rats, both towards handlers and new environments (see figure below). Tickling rats before a stressful procedure, such as dosing or blood sampling has been found to reduce the impact of the procedure upon the animal ³, making it both a refinement in the 3Rs sense and a potential means of limiting the experimental variability that such stress responses can cause.

While most of the data on tickling focuses on behavioural changes, some studies have also demonstrated biochemical alterations in rats caused by tickling, from increased circulating levels of the reward hormone dopamine ⁴ to upregulation of several genes associated with feeding and metabolism. Corresponding effects have been seen in humans following laughter, leading the authors to speculate whether tickling could be used as a means of identifying biomarkers for positive affective states ⁵.

Outcomes from tickling. The impact of tickling on various outcomes as assessed by experiments with a control specific to tickling (N = 22). Green = positive results from tickling. Grey = no difference from tickling. Red = negative result from tickling. (Source: LaFollette et al. 2017)

As with similar techniques of this type, the authors highlight that tickling currently needs the establishment of best practices and the best ways to employ this method across different rat strains, sexes and cage set ups. Currently a video journal article is in press at to demonstrate how to properly tickle rats. Additionally, they also begin to identify instances where this technique is most beneficial; for instance, the technique results in the most positive outcomes when performed on singly-housed rats compared to those which had been group-housed.

It also seems that tickling rats for the first time when they are adults may not be as rewarding as tickling rats as juveniles, indicating care must be taken when deciding when and where to implement this method.

For more information on rat tickling the Gaskill Lab has several dedicated pages on the technique and an excellent step-by-step guide on how to tickle a rat:

First conceived in 1997, tickling rats may be a novel technique for improving welfare but it certainly isn't a new one. This raises the question of why the uptake of this refinement, most especially in the field of behavioural biology, hasn't been higher? With the validation that Megan LaFollette and Dr Gaskill's paper provides, perhaps 2018 will see a rise in the reporting of experiments utilising this method.

This month brings the NC3Rs Year of Laboratory Rodent Welfare to a close. You can review the range of activities featured on our Year of Laboratory Rodent Welfare timeline.
References

1. Burke CJ, Kisko TM, Swiftwolfe H, Pellis SM, Euston DR (2017). Specific 50-kHz vocalizations are tightly linked to particular types of behavior in juvenile rats anticipating play. PloS One, 2017; 12(5): e0175841.

2. LaFollette MR, O'Haire ME, Cloutier S, Blankenberger WB, & Gaskill BN (2017). Rat tickling: A systematic review of applications, outcomes, and moderators. PloS One, 12(4), e0175320. doi: 10.1371/journal.pone.0175320

3. Cloutier S, Wahl K, Baker C, & Newberry RC (2014). The social buffering effect of playful handling on responses to repeated intraperitoneal injections in laboratory rats. Journal of the American Association for Laboratory Animal Science, 53(2), 168-173.

4. Hori M, Shimoju R, Tokunaga R, Ohkubo M, Miyabe S, Ohnishi J, Murakami K and Kurosawa M, 2013. Tickling increases dopamine release in the nucleus accumbens and 50 kHz ultrasonic vocalizations in adolescent rats. Neuroreport, 24(5), 241-245.

5. Yamamuro T, Hori M, Nakagawa Y, Hayashi T, Sakamoto S, Ohnishi J, Takeuchi S, Mihara Y, Shiga T, Murakami K,Urayama O (2013). Tickling stimulation causes the up-regulation of the kallikrein family in the submandibular gland of the rat. Behavioural brain research, 236, 236-243.

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Tickling rats: a social enrichment to improve rodent welfare

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The importance of positive human-animal interactions is widely recognised for large laboratory animal species, but often ignored for the smaller rodents, despite growing evidence of benefits to their welfare. Here we highlight tickling as a social enrichment to improve rat welfare.

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Purdue University

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Summary

This article demonstrates the standardized application of playful handling, a tickling technique designed to mimic rat rough-and-tumble play. This technique is effective at reducing fearful reactions to humans and generating positive affect when rats are handled for common husbandry activities and medical and research procedures such as injection.

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Cloutier, S., LaFollette, M. R., Gaskill, B. N., Panksepp, J., Newberry, R. C. Tickling, a Technique for Inducing Positive Affect When Handling Rats. J. Vis. Exp. (135), e57190, doi:10.3791/57190 (2018).

Abstract

Handling small animals such as rats can lead to several adverse effects. These include the fear of humans, resistance to handling, increased injury risk for both the animals and the hands of their handlers, decreased animal welfare, and less valid research data. To minimize negative effects on experimental results and human-animal relationships, research animals are often habituated to being handled. However, the methods of habituation are highly variable and often of limited effectiveness. More potently, it is possible for humans to mimic aspects of the animals' playful rough-and-tumble behavior during handling. When applied to laboratory rats in a systematic manner, this playful handling, referred to as tickling, consistently gives rise to positive behavioral responses. This article provides a detailed description of a standardized rat tickling technique. This method can contribute to future investigations into positive affective states in animals, make it easier to handle rats for common husbandry activities such as cage changing or medical/research procedures such as injection, and be implemented as a source of social enrichment. It is concluded that this method can be used to efficiently and practicably reduce rats' fearfulness of humans and improve their welfare, as well as reliably model positive affective states.

Introduction

Handling of small animals, such as domestic rats ( Rattus norvegicus) kept in laboratories and as pets, can cause significant increases in behavioral and physiological indicators of stress ¹, as well as aversive responses suggestive of emotional distress ². Handling rats can trigger fear-based responses towards humans ³,⁴, which may make further handling more difficult, increase the risk of injury to both animal and handler, and limit the validity of experimental data ⁵.

To avoid negative effects of handling on experimental results and human-animal relationships, it is common practice to allow some time for newly acquired young animals to habituate to human interaction prior to assigning them to experimental treatments. During this acclimation period, caretakers may expose rats to a series of events that can be highly variable in both form and duration. These may include passive exposure to people, touching, lifting, petting, holding, talking, and offering food treats. Although combinations of such procedures have been reported to reduce fear of humans ⁶, it is unclear which specific practices are most effective for use by busy animal caretakers. There are reports that rats can have either adverse responses, or at least no favorable response, to some of these individual techniques ²,⁷,⁸,⁹.

In contrast, abundant evidence supports the conclusion that tickling, a handling technique that mimics aspects of the playful rough-and-tumble behavior of rats, is a robustly beneficial form of social contact, especially for juvenile rats ¹⁰,¹¹,¹²,¹³. Rats are highly social mammals, which is clear from observing the time they spend playing together as juveniles ¹⁴ and their strong motivation to re-establish and maintain social contact with conspecifics ¹⁵,¹⁶,¹⁷. Rat tickling draws upon the playful social nature of rats to convert handling into a positive experience. This concept was originally developed as a model for investigating the neurobiological basis of positive affective states evoked by social play behavior ⁷. Since then, rat tickling has been used in over 56 different experiments, 22 of which compared tickling to other handling techniques ¹³.

The rat's rough-and-tumble play is characterized by two key behaviors: "dorsal contact" and "pin" ( Figure 1) ¹⁸,¹⁹. During rough-and-tumble play, a rat initiates play by making dorsal contact with the partner's nape. The recipient responds by rolling supine and is pinned down by the initiator standing above ¹⁹. The play continues with the rats taking turns being in each position, on top and below ¹⁸. Human handlers are able to mimic the two key components of rough-and-tumble play by alternating between contact with a rat's nape (dorsal contact) and ventral surface (pin) using vigorous, quick movements of the fingers similar to those used when tickling a child. Of course, tickling is not an exact copy of rat rough-and-tumble play because rats cannot pin the human hand during tickling. Indeed, the level of engagement of individuals in rough-and-tumble play and tickling can differ ²⁰,²¹. Despite this, it appears that tickling invokes the same reward mechanisms as rough-and-tumble play among conspecifics. Tickling is actively solicited by the animals and can serve as a reward for operant learning, contributing evidence that being tickled is substantially rewarding for young rats ⁷,¹¹,¹²,¹³,²². Even though rats have also been reported to learn an operant task in exchange for gentle stroking as the reward, they were faster to approach the hand of a person offering tickling compared to stroking ⁷.

The pins used in tickling may seem similar to common restraint procedures but there is evidence that rats perceive these types of handling differently. When rats are repeatedly restrained, such as for an injection procedure, they secrete excessive porphyrins resulting in chromodacryorrhea ("red tears") ⁸. Chromodacryorrhea occurs in response to acute stressors and causes the skin and fur around the eyes and nose to become stained a reddish color ²³. When repeatedly held down and restrained on the back as in a pin, rats showed less attraction to humans than when tickled ¹². These outcomes imply that the repeated brief restraints and releases during tickling are perceived differently than non-playful restraint. Thus, the precise technique used in tickling has important implications for generating a positive affective state as opposed to contributing to handling stress.

Monitoring the effect of tickling on rat affective state can be easily accomplished by attending to the rats' vocalizations. Rats typically produce two categories of ultrasonic vocalizations (USVs), at around 22-kHz and 50-kHz, in addition to audible vocalizations. The 22-kHz USVs typically occur when anticipating aversive events and are accompanied by elevated brain cholinergic activity ²⁴. Therefore, 22-kHz USVs are deemed to indicate a negative affective state of anxiety. The 50-kHz USVs, especially of the frequency-modulated "trill" type ( Figure 2), are generally associated with rewarding situations and stimuli and are accompanied by increased brain dopaminergic activity ²⁴,²⁵. Accordingly, expression of abundant 50-kHz USVs is considered to reflect a positive affective state ²⁴,²⁶,²⁷,²⁸. Finally, audible vocalizations are usually associated with, and therefore used as markers of, physical pain and discomfort. The differential production of these three forms of vocalizations thus offers a reliable record of changes in the affective state of rats ²⁶. Rats produce USVs at around 22-kHz during exposure to predators, exposure to painful stimuli such as foot shock, and inter-male fighting ²⁸. Stroking has also been reported to elicit multiple 22-kHz USVs in naïve rats, inferred as indicating negative affect ². In contrast, when tickled by humans, as when engaged in rough-and-tumble play with other rats, rats produce numerous 50-kHz USVs. It follows that the rate of 50-kHz USVs provides a graded measure of the positive affective response generated by tickling ²⁹,³⁰.

Existing descriptions of "how to tickle a rat" are rather brief and can lead to variable results. Further, novice "rat ticklers" may be overly tentative in their attempts to mimic rough-and-tumble play. The words "tickle vigorously" are easier to understand when demonstrated visually. Therefore, the goal of this article is to provide a detailed description and demonstration of how to mimic rats' positive playful interactions in an effective and consistent manner that reliably elicits positive responses from most rats when handled. This protocol describes a standardized tickling technique, and explains how tickling can be combined with procedures such as restraint and injection to mitigate potentially unpleasant affective experiences.

Protocol

All experiments were carried out in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals ³¹,³² at facilities accredited by AAALAC International and were approved by the Washington State University Institutional Animal Care and Use Committee. The following protocol can expose personnel to laboratory animal allergens. Personal protective equipment should be worn as recommended by the institutional occupational health and safety program and health care providers. The protocol depicts procedures under typical laboratory housing conditions but could be adapted for use in higher-biosecurity facilities (e.g. by using a biosafety cabinet, etc.).

1. Preparation of Equipment and Setup

1. Handling box

1. Prepare an animal's home cage or other enclosure for tickling, hereafter called the handling box.

2. Make sure the handling box is large enough to provide space for the tickler's hand and for the rats to move around while preventing them from jumping out.

3. Make sure the handling box is free of obstacles. Thus, if conducting tickling in the home cage, remove any objects such as huts, gnawing sticks, and crinkled paper.

4. Line the handling box floor with bedding³³. Other types of soft floor cover can be used but are not as effective at eliciting 50-kHz vocalizations¹³.

5. Place the handling box on a table or countertop at a height that allows the handler's hand to comfortably reach the entire floor area of the handling box.

6. Minimize the effect of novelty by habituating the rats to the handling box prior to tickling, if needed.

2. Timer: To indicate the start and end of 15 s bouts, place a timing device near the handling box where it can be easily read. Alternatively, an auditory time signal may be transmitted to the tickler through earphones.

3. Vocalization recording

1. If vocalizations are being recorded to quantify different vocalization types for research purposes, use a microphone capable of capturing 10-120-kHz broadband sound, connected to a device capable of recording high frequency sounds.

2. Suspend or attach the microphone at the top or side of the box, low enough to clearly detect the calls and avoid confusion with calls from rats in nearby cages, but out of physical reach of the rats.

3. If vocalizations only need to be monitored to confirm proper tickling technique, then use a bat detector and set it up on the side of the box. If using a simple, heterodyne detector, set the device at 50-kHz.

4. Laboratory gloves: Use a bare or thinly gloved hand when tickling. The tickling technique was developed using a bare hand. However, a thinly gloved hand is also effective at inducing playful behavior and 50-kHz USV production while allowing the handler to sense the amount of pressure being applied to the rat's body and feel the rat's responses through the glove.

2. Tickling Procedure

1. Put on the laboratory gloves, if required. Bring the home cage to a table or workbench. Gently transfer the rat into the handling box if tickling is not conducted in the home cage. Start the timer, and the audio recording if needed.

2. For consistent application, alternate between 15 s of hand contact and 15 s release from contact for a total of 2 min. This duration of interaction has been found sufficient for increasing attraction of rats to humans¹¹.

1. To begin the tickling session with a 15 s release, place the hand motionless inside one side of the handling box. The presence of the hand allows rats to solicit interaction by directing licks and light nips (nibbles) towards the hand, indicating the rat's interest to interact²². When this occurs, move the hand away from the rat, which can solicit playful chasing of the hand by the rat. Alternatively, leave the hand out of the box during the 15 s release.

2. Follow the release with 15 s of tickling, initiate the interaction with a dorsal contact (2-4 s). Using one hand, vigorously touch the rat's nape (dorsal surface of the neck) with the fingertips using rapid finger movements as commonly performed in human tickling. Avoid contact with the rump as this area of the body is targeted during aggressive encounters³⁴.

3. Follow the dorsal contact with a pin. Place the thumb and middle finger under the rat's forelegs; lift the rat, just high enough to allow quickly turning the rat on its back while ensuring the tail does not get kinked.
NOTE: It may help to lightly push on the back of the rat with the little finger when turning the rat on its back.

4. Hold the rat loosely, but firmly; be assertive but not forceful. Once the rat is on its back on the floor, hold the body down loosely while moving the fingers quickly and vigorously but gently on the belly, as is commonly used in human tickling.

5. Even though the stimulation is rapid and assertive, keep the pinning brisk (2-4 s/pin) to minimize negative responses. Allow the rat to right itself and rapidly follow with a dorsal contact.

6. Continue to alternate between dorsal contacts and pins. On average, there should be 4 to 5 pins/15 s³⁵.

7. Do not pick up the rat with both hands. During the 15 s of tickling, always keep the fingers moving and in contact with the rat's body, even in transitions between pins and dorsal contacts.

8. Perform three additional 15-s tickles and 15-s releases for a total of 2 min.

3. If using a handling box, return the rat to its home cage. If using the home cage, place enrichment objects back in the cage. Clean the handling box with paper towels to remove feces and urine not absorbed by the bedding.

4. Repeat with the next cage of rats, as needed.

3. Tickling Multiple Rats Simultaneously

1. Rats can be tickled as a group³⁶. If the handling duration is not relevant, alternate tickling each rat for 15 s. The duration of the tickling session can be extended beyond 2 min as needed so that each rat receives four bouts of tickling hand contact interspersed by release.

2. If the handling duration is relevant for research results, then use the following procedures:

1. For pairs, tickle one rat for 15 s followed immediately by tickling the second rat for the next 15 s (i.e., during the release of the first rat), and so on. The tickler does not have a rest period.

2. For triplets, use the procedure described in 3.2.1 to tickle two of the rats and tickle the third one at the same time as one of the others using the other hand.

3. For groups of 4 or more rats, tickle each rat or pair of rats for 15 s. The duration of the tickling session can be extended beyond 2 min as needed.

Representative Results

Factors affecting, or potentially affecting, responses to tickling are summarized in Table 1. Positive outcomes from rat tickling have been indicated by multiple measures of rat welfare and positive affect. Common findings include increased production of 50-kHz USVs, increased approach behavior, and increased ease of handling. Reported effects of rat tickling at the genetic and cellular level include altered gene expression in the hypothalamus ³⁷, and increased neurogenesis in the dentate gyrus of the hippocampus ³⁸,³⁹. Outcomes indicative of reduced generalized anxiety and fear of humans are also reported such as reduced latency to approach in 9 different experiments ¹³, less freezing after fear conditioning ⁴⁰,⁴¹, and increased entries into the center of an open field arena by individually housed rats ³⁶. For further information see LaFollette et al., who present a systematic review of the tickling literature that identifies 22 reports investigating the outcomes of rat tickling in comparison to other techniques ¹³.

Next, specific examples of results that can be obtained when the tickling technique is paired with restraint and intra-peritoneal (IP) injection procedures are outlined.

Sprague-Dawley rats (males; 85-94 days of age; n = 48) tickled for 2 min immediately before and after IP saline injection daily for 10 days, especially when combined with the juvenile tickling experience (2 min/day from 25-45 days of age), produced more 50-kHz USV just before (GLM ANOVA, F 3, 44 = 9.59, p < 0.0001; Figure 3A) and just after (F 3, 44 = 8.74, p = 0.0001; Figure 3B) injection (Tukey-Kramer, p < 0.05) compared to rats exposed to a passive hand ¹⁰. During the actual brief delivery of the injection, production of 50-kHz USVs was curbed (mean ± SE, 15 ± 1.4 USVs/min) and handling treatment had no effect on their rate ¹⁰. It was concluded that tickling, when applied both before and after an injection procedure, induced a positive affective state that moderated the aversiveness of the procedure, especially for those that had been exposed to tickling as juveniles.

To find out if tickling would be effective if reduced to before or after an injection rather than both and, if so, which time would be better, rat responses to tickling before, after, and before-after injection were compared. Rates of 50-kHz USVs and audible calls were measured as indicators of positive and negative affect, respectively ¹². Sprague-Dawley rats (males; 32-41 days of age; n = 96) were either restrained and given a daily IP saline injection (INJ) or restrained without receiving an injection (control, CON), and exposed to one of the following treatments: no tickling, the rats were not touched before and after the restraint procedure (TN); tickled for 2 min right before (TB), after (TA), or before and after (TBA) restraint ¹². In the 2 min before restraint, rats from treatments that received tickling before the restraint (TB, TBA) produced the most 50-kHz USVs (GLMM ANOVA, F 3,88 = 49.19, p < 0.0001; Figure 3C). TA and TN rats did not differ in the rate of USVs produced before the restraint procedure. During the procedure, any rat that received tickling (TA, TB, TBA) produced more 50-kHz USV than those that were not tickled (TN; F 3,88 = 50.67, p < 0.0001; Figure 3D). Among tickled rats, those receiving tickling prior to the procedure (TB, TBA) produced more 50-kHz USVs during the restraint compared to the ones tickled afterwards (TA; p < 0.05). In the 2-min period after the procedure, rats in all tickled groups (TA, TB, TBA) produced more 50-kHz USVs than those that were not tickled (TN; F 3,88 = 53.64, p < 0.0001; Figure 3E). Furthermore, during the procedure, rats tickled before restraint (TB, TBA) produced fewer audible calls compared to rats that did not received tickling (TN), whereas rats tickled afterwards (TA) did not differ from those in the other groups (F 3,88 = 3.67, p= 0.02; Figure 3F). The results demonstrate that tickling rats before restraining them for an IP injection produced signs of greater positive affect across the periods before, during, and after the procedure than tickling only after restraint ¹².

Caretakers applied one of the following handling treatments to rats (Sprague-Dawley males; n = 32) from 57 to 74 days of age: (1) Minimal handling once a week at cage change; (2) Exposure to a passive hand for 2 min daily; (3) 2 min of tickling each day, to mimic social play; (4) 2 min daily of restraint on the rat's back, simulating unreciprocated pinning by a dominant ¹¹. In the minute following an IP injection, rats from the Tickled and Passive treatments spent more time rearing in the front half of the cage, close to where the researcher was standing, compared to rats from the Minimal and Restraint treatments (GLM ANOVA, F 3, 27 = 5.70, p = 0.004; Figure 3G). Additionally, Tickled rats uttered more 50-kHz USVs than rats in the Passive treatment when exposed to a familiar or unfamiliar hand for 3 min in a Human approach test (Log-transformed mean ± SE calls/min, Minimal: 0.7 ± 0.26, Passive: 0.6 ± 0.18, Tickled: 1.8 ± 0.30, Restraint: 1.4 ± 0.28; F 3,28 = 4.04, p = 0.02). Thus, the simple presence of a passive hand aided habituation of rats to humans relative to minimal handling, but tickling was the most effective method for producing a positive response towards humans ¹¹.

Figure 1: Rat rough-and-tumble play behavior. ( A) Dorsal contact; ( B) Pin ¹⁸. Please click here to view a larger version of this figure.

Figure 2: 50-kHz ultrasonic vocalizations of the frequency modulated "trill" type. Five trill vocalizations were produced in 1 s by a rat during tickling. The horizontal lines at 17-, 25-, and 70-kHz were produced by electronic equipment in the room ( e.g., computer, microphone, camera).

Figure 3: Responses of rats exposed to tickling prior to, or in combination with, experimental procedures such as intra-peritoneal (IP) injections. Rate (mean ± SE calls/min) of 50-kHz USVs produced ( A) before and ( B) after an injection procedure by adult male rats exposed to minimal handling (M) or tickling (T) as juveniles and a passive hand (P) or tickling (T) immediately before and after injection (yielding four treatment combination: M-P, T-P, M-T, T-T); from Cloutier et al. ¹⁰. Rate of 50-kHz USVs (mean ± SE calls/min) produced ( C) before, ( D) during, and ( E) after restraint (with or without an IP injection) once daily for 10 consecutive days by juvenile male rats that received no tickling (TN) or tickling before (TB), after (TA), or both before and after (TBA) the procedure; and ( F) the rate of audible calls produced during restraint; from Cloutier et al. ¹². ( G) Duration (mean ± SE s) of rearing nearby the researcher during the 1 min following an IP injection by male rats exposed to minimal, passive, tickle, or restraint handling by caretakers; from Cloutier et al. ¹¹. Bars with different letters (a, b, c) indicate differences in pairwise comparisons (Tukey-Kramer, p < 0.05). Please click here to view a larger version of this figure.

Factor Effect References

Intrinsic

Extrinsic

Table 1: List of factors affecting, or potentially affecting, the responses of rats to tickling, a playful handling technique intended to mimic rat rough-and-tumble play.

Discussion

This protocol describes a technique for rat tickling, a playful handling technique mimicking rat rough-and-tumble play behavior that is effective in generating vocalizations indicative of positive affect and increasing approach behavior when rats are handled for common husbandry or medical and research procedures such as IP injection.

Social play, a typically positive form of social contact, has been proposed to assist in the calibration of emotional reactions to frightening situations ⁴²,⁴³, and generate resilience to anxiety and depression ⁴⁴. By mimicking rat social play, tickling appears to have similarly beneficial effects ⁷. Thus, for the application of tickling to be successful at inducing positive affective states, it is important to keep in mind that the hand is intended to act as "the play partner" during the tickling. The hand is attractive because it simulates the behaviors that rats expect and enjoy from another rat during rough-and-tumble play. Hence, lifting and holding the rat with both hands is not mimicking rat play, and may not have the same effect. In fact, laboratory rats will learn to avoid non-playful restraints in which a handler holds them with one hand and strokes them with the other hand ⁴⁵. During rat tickling, it is also important to move the hand confidently and assertively, while being lively and playful. Keeping the pinning brisk (2-4 s/pin) should minimize negative responses because in rough-and-tumble play between rats, repeated pinning leads to uttering of 22-kHz USVs ¹⁸.

Even though juvenile rats are more playful and show stronger responses to tickling than adult rats ¹⁸, tickling in adulthood also has benefits ¹⁰. Tickling rats when young has also been found to have benefits later in life ³⁶, suggesting that rats experience a prolonged positive effect on their emotional reaction to human presence. Sex differences in response to tickling are inconsistent, with reports of no sex differences in juvenile rats ⁴⁶,⁴⁷, juvenile males producing more 50-kHz vocalizations and fewer 22-kHz vocalizations than juvenile females ²⁹,⁴⁸,³⁹, and older females producing more 50-kHz vocalizations than same-aged males ²⁹, similar to findings for social play ⁴⁹.

The effectiveness of the tickling technique can be assessed using rat movement patterns and vocalization. Rats will nibble and lick the handler's fingers during the 15-s periods of release following tickling ¹¹. Nibbles are light, non-injurious bites that have been described as play-solicitation behavior in young rats ²². When the handler moves the hand around the handling box, the rat will follow and chase it. Rats, especially young ones, will also run, scamper around the box between tickling bouts, and show a distinct increase in activity after a tickling bout, similar to the running activity that characterizes rough-and-tumble play. If the rat remains immobile during releases, adjustment of the tickling technique, such as reducing pin duration or giving fewer pins, is probably needed. It is best to give pins of short duration for the first tickling session. Duration and number of pins can be increased as the rats get used to tickling. Rats utter 50-kHz USVs, including trills, during tickling and between tickling bouts. A gradual increase in the number of trills produced during the 15-s releases can be seen over the first two weeks of daily tickling before stabilizing ⁴⁷. Individual differences allow tickled rats to be divided into high and low callers that respond differentially to stress, anxiety, and anhedonia ⁴⁶,⁴⁷,⁵⁰.

Rat tickling has been used successfully to induce positive affect as assessed based on production of 50-kHz USVs during tickling ¹³. It has recently been implemented as a technique to reduce fear of humans in the rats and mitigate their aversiveness of medical and research procedures such as injection. Evidence suggests that, when provided for only 2 min daily per animal, tickling is an effective and practical method for reducing avoidance of humans ¹¹. Considering that juvenile rats will play together for about 60 min daily ⁵¹,⁵²,⁵³, tickling for 2 min daily is practical but presumably too brief to replicate the full effects of social play among rats. Pet rat owners, who generally have more time for handling their rats than laboratory caretakers, may provide rats with additional tickling as long as the rats continue to respond positively. Based on Burgdorf et al. ⁴⁴,⁴⁵, the first 15 min of rough-and-tumble play seems to be the most positive for rats. In turn, it may be more effective to tickle rats for periods of up to 15 min, especially if for a justified reason they cannot be kept in a social group. However, based on play as "training for the unexpected" ³⁸, a more varied technique than the method described here might be needed to maintain the playfulness of longer tickling sessions. Additional tickling sessions each day may also be beneficial as long as other biologically important activities such as sleep and feeding are not impaired. As well as contributing to improving the well-being of laboratory and pet rats, the described standardized tickling technique can be used in continued research aimed at understanding the effects of social play and its components on behavioral, social, and brain development.

When tickling was performed surrounding injection, particularly when combined with previous tickling experience (ending 40-50 days earlier), it increased 50-kHz USV rates before and after injection, and reduced audible call rates during injection ¹⁰. Thus tickling seems to have mitigated the adverse effects of IP injections, especially in rats accustomed to tickling as juveniles ¹⁰. Even though tickling did not prevent discomfort associated with the injection as assessed based on production of audible calls, it increased 50-kHz USVs compared to the control condition, before, during and after the restraint procedure ¹². According to 50-kHz USV data, the most effective time point for applying tickling was just before rather than after the restraint procedure ¹². Tickling rats before the injection had a carry-over effect that elevated positive affect during the procedure ¹². Tickling can therefore minimize potential negative effects of routine medical procedures on rat welfare and behavior. More research is necessary to determine how the technique could be paired effectively with other injection techniques and other medical/experimental procedures.

Overall, we have demonstrated that rat tickling results in positive outcomes such as increased production of 50-kHz USV and approach behavior, as well as decreased avoidance of humans and resistance to handling. We recommend rat tickling as a viable method for improving rat welfare and modeling positive affective states.

Disclosures

The authors declare that they have nothing to disclose.

Acknowledgments

Sylvie Cloutier, Megan LaFollette, Brianna Gaskill, and Ruth Newberry wish to dedicate this article to the memory of Jaak Panksepp, our beloved co-author, who passed away during the final stages of completing this manuscript. We remember Jaak fondly as the "father of rat tickling" - his insights into animal emotions form the foundational inspiration for this work. This material is partially based upon work supported by a Washington State University Veterinary Summer Research Fellowship, an Animal Welfare Enhancement Award from the Animal Welfare Institute, and a "Grant for Laboratory Animal Science" from the American Association for Laboratory Animal Science. Publication of this article was sponsored by Petco Animal Supplies, Inc. We would like to thank Carol Dowell and Purdue University's Laboratory Animal Program for allowing us to tickle their rats for the shoot.

Materials

Name Company Catalog Number Comments

Table or workbench

No specific brand is recommended.

Handling box (rat home cage or another enclosure)

No specific brand is recommended.

Bedding or or other type of soft cover

No specific brand is recommended.

Gloves (thin laboratory gloves)

Optional.

DOWNLOAD MATERIALS LIST

References

1. Lawlor, M. M. Comfortable quarters for rats in research institutions. Comf Quart Lab Anim. 26-32 (2002).

2. Panksepp, J. Affective neuroscience: The foundations of human and animal emotions. Oxford University Press. New York, NY. (1998).

3. National Research Council. Guide for the care and use of laboratory animals. National Academies Press. Washington, D.C. (1996).

4. National Research Council. Guide for the care and use of laboratory animals. National Academies Press. Washington, D.C. (2011).

5. Yamamuro, T., et al. Neurogenesis in the dentate gyrus of the rat hippocampus enhanced by tickling stimulation with positive emotion. Neurosci Res. 68, (4), 285-289 (2010).

6. Hori, M., et al. Effects of repeated tickling on conditioned fear and hormonal responses in socially isolated rats. Neurosci Lett. 536, (1), 85-89 (2013).

7. Hori, M., et al. Tickling during adolescence alters fear-related and cognitive behaviors in rats after prolonged isolation. Physiol Behav. 131, 62-67 (2014).

8. Pellis, S., Pellis, V. The playful brain: venturing to the limits of neuroscience. Oneworld Publications. Oxford. (2009).

9. Burgdorf, J., et al. The effects of selective breeding for differential rates of 50-kHz ultrasonic vocalizations on emotional behavior in rats. Dev Psychobiol. 51, (1), 34-46 (2009).

10. Mällo, T., et al. Tickling-induced 50-kHz ultrasonic vocalization is individually stable and predicts behaviour in tests of anxiety and depression in rats. Behav Brain Res. 184, (1), 57-71 (2007).

11. Panksepp, J. Rough and tumble play: A fundamental brain process. Parent-Child Play. 147-184 (1993).

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Tickling, a Technique for Inducing Positive Affect When Handling Rats | Protocol

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This article demonstrates the standardized application of playful handling, a tickling technique designed to mimic rat rough-and-tumble play. This technique is effective at reducing fearful reactions to humans and generating positive affect when rats are handled for common husbandry activities and medical and research procedures such as injection.

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Video y algunas bases neurofisiológicas del efecto positivo

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Video y algunas bases neurofisiológicas del efecto positivo

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See What Happens When You Tickle a Rat | National Geographic

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Revisión Sistemática de la Aplicación del Rat Tickling

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Revisión Sistemática de la Aplicación del Rat Tickling

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Introduction

Importance

The study of positive affect has previously been difficult to model and study in laboratory animals. Understanding positive affect has wide-reaching importance for humans and animal welfare. Positive affective states in humans confer resilience to depression and anxiety as well as lead to an overall increase in general health [ 1]. Therefore, modeling positive affect in animals could further human health research. Measuring and eliciting positive affect/experiences are also important for assessing animal welfare [ 2]. The technique of tickling, and measuring positive vocalizations in rats, is a promising solution to modeling and quantifying positive affective states in laboratory animals [ 3]. Additionally, by increasing positive affect and decreasing rat fear of humans, tickling may be a promising technique to improve rat welfare [ 4].

Current knowledge

The technique of tickling rats was developed by Dr. Jaak Panksepp and Dr. Jeffrey Burgdorf in 2000. The aim of their initial publication was to determine if a "tickling-type somatosensory stimulation by a human hand" that mimicked juvenile rat rough-and-tumble play was rewarding to rats. In their original article involving eight experiments, juvenile Long Evans rats were tickled for two minutes across four to ten total sessions. The tickling technique involves two main components: a "dorsal contact" on the rat's neck and a "pin" on the rat's stomach. Developing this technique was additionally used to investigate the possibility that 50-kHz ultrasonic vocalizations, produced by rats during play and other rewarding situations, could be used to measure reward value and emotion in rats. Subsequently, vocalizations in the 50-kHz range have repeatedly been shown to be good indicators of positive affect and welfare of rats [ 5, 6]. Some indications that 50-kHz vocalizations are positive are that they are produced in a range of contexts such as during play, mating, exploratory activity, amphetamine administration, and in anticipation of food rewards [ 7]. These vocalizations can be recorded by using specialized microphones and later quantified by trained observers. In contrast to 50-kHz calls, vocalizations in the 22-kHz range have been demonstrated to reflect negative affect and are produced during exposure to predators, pain, and inter-male fighting [ 5].

Rationale

Since the initial publication of the tickling technique, a number of laboratories have used tickling to investigate and measure positive affect [ 8] as well as decrease rat stress, and habituate rats to human contact [ 4]. However, tickling experiments use a range of protocols that find a wide variety of outcomes. Neither tickling protocols nor specific outcomes from tickling that may improve rat welfare have been summarized. Additionally, a number of experiments have also specifically investigated factors that may moderate the outcomes of tickling. However, without a singular article describing these factors, it is unclear how tickling is being used and what factors pertaining to rat characteristics (strain, sex, age) and the tickling protocol (session duration, total number of sessions) influence the efficacy of tickling. As a result, researchers are left with insufficient guidance for using tickling either as a tool for modeling and measuring affect or to improve rat welfare in their experimental colonies. This lack of guidance may also prevent the widespread adoption of this technique for general enrichment since researchers and managers of animal care programs are unsure what time investment this technique requires or its potential effects on their research.

Objectives

Previously, meta-analyses and systematic reviews of animal studies have proven useful for optimizing study design and reporting [ 9]. Although there have been reviews of using tickling and ultrasonic vocalizations to model laughter in humans [ 10, 11], there has not yet been a systematic review of how tickling is utilized in research, its outcomes related to affect and welfare, and methodological considerations for its use. Therefore, the purpose of this review was to provide a comprehensive overview of the empirical research using the tickling technique in rats. Our objective was to systematically identify, summarize, and evaluate the research on tickling in rats to provide direction for future investigation. Our specific aims were to (a) describe current methods used in tickling articles, (b) describe outcomes from tickling, and (c) summarize factors that have the potential to alter tickling outcomes. By summarizing this information in one location, it is possible for researchers, laboratory personnel, those interacting with pet rats, and other interested individuals to make educated decisions about the use and application of tickling.

Methods

Protocol

Before beginning our review, we consulted the preferred reporting guidelines for systematic reviews and meta-analysis (PRISMA) guidelines [ 12, 13], the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) E-learning course and guides, and a veterinary science librarian. We defined our procedures a priori in an unpublished study protocol that specified the search strategy, inclusion and exclusion criteria, and data extraction items.

Article identification

We identified articles by searching complete electronic databases from their first index date through August 2016 and scanning reference lists of identified articles. Databases included Scopus (1966-Present), PubMed (1946-Present), Web of Science (1900-Present), and PsycINFO (1840-Present). Titles, abstracts, and keywords were used for the search. Search terms for all databases included at least one identifier for tickling and rat or rats. Identifiers for tickling included playful handling OR tickle OR tickling OR heterospecific play. No language restrictions were applied. See S1 Text for the full search strategy for each database.

Article selection

After removing duplicate articles, we screened all references for inclusion based on their title and abstract. If we could not determine inclusion by title and abstract, we reviewed the full-text manuscript. We used the following inclusion criteria, in order, to select relevant articles for review: (1) publication in a peer-reviewed journal, (2) collection of original, empirical data, (3) on rats (4) using the handling method of tickling. Collection of original, empirical data is defined as the systematic collection and reporting of original observational or experimental scientific data. All articles were independently assessed by two reviewers (MRL and LMF). Disagreements were resolved via discussion.

Data extraction

One researcher (MRL) extracted information from each included article to achieve the three aims of this review. If the article reported data from several experiments, information was extracted individually for each experiment. In this review, "article" refers to the entire paper while "experiment" refers to a singular experiment within an article. To achieve the first aim-describe the current use of tickling-we extracted data items from all experiments including terminology, aim, design, rat age, sex, strain, and housing; and tickling duration, frequency, total time, and location factors. To achieve the second aim-describe outcomes from tickling-experiments were identified that used a control group specific to the tickling intervention, such as stroking. To achieve the third aim-summarize the factors that have the potential to alter tickling outcomes-experiments were identified that described using variants of a tickling technique or the same tickling technique in different situations or animals. Data items for the second and third aims included type of control or comparison condition, outcomes measured, and whether each outcome was increased or decreased. An outcome was coded as increased or decreased based on the conclusion of the author in the original manuscript. Outcomes were organized by type from most- to least-commonly reported. Effect sizes and confidence intervals were not commonly reported within studies and therefore were not extracted as data items. We also extracted additional data about the publications including first author, publication year, journal name, country, and title.

Reporting quality and risk of bias assessment

Two independent reviewers (MRL and WBB) assessed reporting quality and risk of bias of each experiment included within each article. Reporting quality was assessed by answering one question: did the experiment report any sort of randomization of rats to treatment groups in the methods section. Risk of bias was assessed using a modified version of the Systematic Review Center for Laboratory animal Experimentation (SYRCLE) Risk of Bias Tool for animal intervention studies [ 14]. The SYRCLE Risk of Bias Tool is adapted from the Cochrane Collaboration Risk of Bias Tool [ 15] to address bias specific to animal experiments. In brief, the tool asks reviewers to assess 5 domains of bias including selection bias, performance bias, detection bias, attrition bias, and reporting bias. When assessing selection bias, we considered groups similar at baseline if their number of ultrasonic vocalizations were not statistically different. When discrepancies between reviewers occurred, we reached consensus by discussion.

Three main modifications were made to this tool. First, we split item 5 Blinding Caregivers/Investigators into two parts to assess the blinding of caregivers and investigators separately. This was done because although no studies indicated blinding caregivers, a few studies did blind investigators. Second, we added two potential answers to the bias questions: not applicable or partial. These answers were added to accommodate the use of this tool on a wide variety of experimental designs beyond the traditional randomized control trials. The answer "not applicable" was used when studies applied the same treatment to all animals in the study, therefore making certain techniques to address bias not relevant. The answer "partial" was used for randomization reporting and item 7a to indicate a medium risk of bias when only part of the treatment group was reported as randomized or if not all outcomes were blinded during assessment. Third, we added question 7b to indicate which outcomes were blinded (if any) as a follow-up question on medium risk of bias answer.

Results

Article identification and selection

The literature search resulted in 156 citations. A flowchart of the article selection process is presented in Fig 1 . The final sample included 32 articles (20.5% of the total initial pool) published between 2000 and 2016 that met the inclusion criteria of empirically evaluating outcomes from tickling rats. There was an international representation of researchers including corresponding authors from the North America (n = 14), Europe (n = 12), and Asia (n = 6). The articles were published in a variety of disciplines, primarily related to Neuroscience (n = 14), Behavior, Physiology, or Psychology (n = 13), or Pharmacology (n = 5). We focused this review on descriptive and qualitative synthesis, rather than meta-analysis, because of the large variety of study designs, animals, intervention techniques, reported outcomes, and possibility of bias.

Characteristics of tickling

To achieve our first aim-to describe the characteristics of tickling-we extracted and summarized the key features and applications across the 56 experiments of the 32 articles. All results are presented by experiment in S1 Table.

Terminology

All 32 articles used the word "tickling" at least once although some specified it as "manual tickling," "tickling stimulation," or "tickling-like stimulation". Preferred and additional terms varied across articles. These additional terms most commonly included words such as play or playful (n = 11), heterospecific (n = 9 terms), somatosensory (n = 3), tactile (n = 5), stimulation (n = 4), or wrestling (n = 1). The most common additional term used was heterospecific play (n = 6) followed "playful handling" (n = 4). Nine articles only used one term while the remaining 23 used multiple terms.

Tickling methodology

A variety of methods for tickling were used within each article () of which a summary of tickling dosage is presented in . The most common (46%, n = 26) dosage and technique of tickling used was to replicate the "Panksepp Method" of 15 sec baseline rest followed by 15 sec of dorsal contacts and pins for a total of 2 minutes for at least 4 days [ 16]. The second most common (39%, n = 22) tickling dosage was to use a "Panksepp Variation" which involves the same distinctive dorsal contact and pin components with 15 sec intervals, but with a different session length or number of days. A few experiments (9%, n = 5) also used the "Schwarting Method" which involves using a variety of specific techniques (neck tickle, hand chase, grab and tickle, etc.) with random breaks for at least 3.5 minutes for at least 3 days [ 17].

Tickling occurred in a variety of locations, times of day, light phase, and lighting condition. A majority of experiments reported the flooring of the cage the rats were tickled in (87%, n = 46), including bedding (n = 21), uncovered cage bottom (n = 18), cloth (n = 5), or both bedding and empty flooring (n = 2). Rats were usually tickled individually in a test cage or box (77%, n = 43) although a few experiments tickled rats in their home cage (11%, n = 6) or did not report where rats were tickled (7%, n = 4). Time of day of tickling was also reported for less than a half of experiments (41%, n = 17); this included during the light phase (n = 18) or during the dark phase (n = 3). Less than one third of experiments reported lighting conditions during tickling (21%, n = 12); these included red lighting (n = 6), a light intensity of 150 lux (n = 4), dim (n = 1) or "normal" (n = 1).

Article characteristics

Key article characteristics are summarized in . Tickling was applied to the four of the commonly used rat strains including Long-Evans (39%, n = 22), Sprague-Dawley (29%, n = 16), Wistar (23%, n = 13), and Fisher (5%, n = 3; 2 experiments did not report strain). More than half of experiments investigated tickling only in male rats (57%, n = 32), followed by using both sexes (32%, n = 18) and very few used only females (7%, n = 4; 2 experiments did not report which sex was used). Most experiments tickled juvenile rats less than 6 weeks (68%, n = 38), although some experiments tickled adult rats older than 6 weeks (25%, n = 14; 4 experiments did not report rat age).

More than half of the experiments (55%, n = 31) housed their rats individually though some (20%, n = 11) housed their rats in groups for the entire experiment and others (25%, n = 14) either housed at least some rats socially for part of the experiment. Most experiments (71%, n = 40) housed their rats in "standard" plastic cages. However, the original tickling experiments (23%, n = 13) housed rats in wire mesh cages with wire mesh flooring [ 16, 18]. Three experiments did not report the type of caging their rats were housed on.

Total sample sizes ranged from 8 to 240 with an average of 35 rats. Rats in experiments that reported acclimation days (59%, n = 33) received anywhere from 0 to 29 days with an average of 5.5 days. In general, articles did not report what activities were done during acclimation, however one article noted that all rats were "handled" for 5 minutes per day for 3 days during acclimation [ 19].

Outcomes of tickling

To achieve our second aim-to describe the outcomes of tickling-we extracted and synthesized outcomes from all experiments that included a control comparison to tickling (22 experiments from 17 articles). Although the specific designs and assessments of these experiments varied, we identified key types of outcomes and categorized them according to the number of experiments in which they were reported (Fig 2 ; Table 4 ). Types of control/comparison treatments in each experiment included minimal handling (n = 11), light touch or stroking (n = 7), transfer to test box without tickling (n = 3), restrained on back (n = 3), exposure to a passive hand (n = 2), or food treat (n = 1). Seventeen experiments only used one control/comparison group while four experiments used multiple control/comparison groups.

Table 4

Vocalizations

The most commonly assessed outcome from tickling was production of 50-kHz vocalizations, evaluated in 72% (n = 16) of tickling controlled experiments. Nearly all experiments (94%, n = 15) found that tickled rats produced more 50-kHz vocalizations than control rats. The one experiment that found no effect tickled rats directly after injection [ 20].

Six experiments assessed 22-kHz vocalizations and found mixed results: 33% (n = 2) found fewer 22-kHz vocalizations, 50% (n = 3) found no effect, and 17% (n = 1) found more 22-kHz vocalizations. The two experiments that found an effect of handling treatment used a light touch comparison condition [ 21, 22]. The three experiments that found no effect of tickling treatment used a minimal handling, restraint, or passive hand comparison condition [ 4, 23, 24]. The experiment that found that tickled rats produced more 22-kHz vocalizations specifically found that only the rats that had been tickled as adults, not juveniles, produced more 22-kHz calls and that all rats were tickled after an injection [ 25].

Three experiments assessed audible vocalizations and found mixed results. One experiment found no effect on audible vocalizations during injection [ 20]. Two experiments found tickled rats made fewer audible vocalizations during an injection [ 24, 25]. However, one of these experiments also found that during the actual tickling procedure, tickled rats made more audible vocalizations [ 24], while the other experiment found no effect of tickling treatment on audible vocalizations during the tickling procedure [ 25].

Approach behavior

Ten experiments (48%) assessed outcomes of tickling on approach behavior. Among these, 90% (n = 9) found positive results such as decreased approach latency. Two different methods for evaluating approach were used among the ten experiments: 70% (n = 7) evaluated rats directly after tickling and 30% (n = 3) conducted a human approach test delayed from tickling by three days. Testing rats directly after tickling always led to positive results. Testing rats 3 days post tickling has mixed results: 1 found no effect of tickling [ 20] and the other 2 found tickled rats reared, nibbled, and made more contacts with the hand than controls [ 4].

Other anxiety and fear behaviors

Seven experiments (33%) assessed the outcomes of tickling on anxiety and fear behaviors during standardized tests. Among these, 71% (n = 5) found fewer anxiety and fear behaviors in tickled rats. Three distinct positive results were found: decreased freezing after fear conditioning [ 22, 26], increased locomotor activity and entries into the center of an open field arena [ 23], and increased entries and time in the open arms of an elevated plus maze by high-callers only on day 2 of testing [ 27]. However, elevated plus maze behavior was unaffected by tickling in two other studies [ 4, 20]. Additionally, no behavioral changes due to tickling treatments were observed in a Forced Swim Test [ 27], Cat Odor Test [ 4], Flinch-Jump Test [ 26], or Morris water maze [ 26].

Handling effects

Six experiments (29%) assessed outcomes of tickling on the stress associated with handling. Among these, 83% (n = 5), found reduced stress in tickled rats. Five experiments investigated the effects of tickling on the injection experience. Four of these found that tickling increased 50-kHz vocalizations while decreasing approach latency, audible calls during injection, and injection procedure duration [ 4, 24, 25]. The experiment that found no effect, tickled rats after injection only [ 20]. Later research showed that tickling rats only before or before and after injection, particularly combined with juvenile tickling experience, is more rewarding than tickling rats after injection [ 24]. Additionally, tickling increases 50-kHz calls in anticipation of handling [ 23].

Hormonal effects

Four experiments (19%) assessed effect of tickling on rat hormones. Among these, 50% (n = 2) found at least some positive effects and 50% (n = 2) found no effects. Tickling increased dopamine release in the nucleus accumbuens [ 28] and decreased adrenaline and noadrenaline after fear conditioning [ 22]. However, tickling was never found to effect corticosterone in the feces [ 23, 25] or blood [ 22].

Genetic and cellular level effects

Four experiments used tickling of rats to elicit a positive emotional state for studying the effects of positive affect on cell proliferation, neurogenesis, or gene expression. Tickling increased adult hippocampus cell proliferation, but only in high-callers [ 19]. Adult hippocampus cell proliferation is a factor associated with depression. Tickling altered the expression of genes in the hypothalamus related to feeding regulation [ 21]. This experiment was an attempt to model and investigate the effect seen in humans that blood glucose levels improve after laughter. [ 21]. Tickling increased neurogenesis in the dentate gyrus of the hippocampus [ 29]. The dentate gyrus is an area thought to mediate learning and memory formation. Tickling up-regulated production of kallikrein family mRNAs [ 30], which is one of the serine proteases and related to the production of many bioactive peptides in the submandibular gland of the rat. These authors believe it could be a candidate for a biochemical marker of positive emotional state [ 30].

Other outcomes

Two experiments reported outcomes that have not been replicated. Tickling high-calling rats from normal lines induced a positive cognitive bias compared to no tickled or tickled low-calling rats [ 8]. Tickling adult rats was unsuccessful in inducing a conditioned place preference or sexual partner preference based on the smell of the hand that tickled them [ 31].

Tickling outcome moderation

To achieve our third aim-to summarize factors that could moderate tickling outcomes-we extracted and summarized the key outcomes from experiments that investigated factors that could moderate the results of tickling (40 experiments from 24 articles).

Inter-individual differences in randomly bred rats

Inter-individual differences in randomly bred rats as a moderator of tickling was evaluated in 30% (n = 12) of moderator experiments. All experiments reported at least some significant effects on outcomes which are shown in Table 5

Varied methods were used to group rats according to inter-individual differences, although all experiments used 50-kHz vocalizations. Five experiments (42%) used a median split of their average response on the 12-14 th day of tickling [ 27, 32- 34]. Four experiments (33%) used calling rate as a continuous covariate [ 19, 35, 36]. One experiment used total 50-kHz vocalizations after just 1 session of tickling [ 8]. One experiment split rats into upper and lower quartiles [ 37]. One experiment did not detail the methods of separation [ 38]. Also, one experiment investigated vocalization subtypes and found that only frequency modulated 50-kHz vocalizations differed between groups, but that flat 50-kHz vocalizations and 22-kHz vocalizations were not different [ 37].

Pharmacology

The impact of pharmacological substances on the production of 50-kHz vocalizations during tickling was evaluated in 20% (n = 8) of experiments investigating tickling moderation. Results on 50-kHz are presented in Table 6 .

Housing

The influence of housing type (solitary versus group) on tickling outcomes was evaluated in 20% (n = 8) of experiments investigating tickling moderation. Among these, 88% (n = 7) reported more positive outcomes in solitary rats compared to group-housed rats. Specifically, solitary rats produced more 50-kHz vocalizations during tickling [ 16, 18, 37], had shorter approach latencies [ 16, 18], and had more play bites [ 16]. Conversely, socially housed rats learned to avoid tickling during an instrumental learning task [ 16]. However, 22-kHz vocalizations were not found to differ between housing types [ 37]. No significant differences were found between solitary, pair, or triplet housed rats in 50-kHz or 22-kHz vocalizations in one experiment [ 23]. A key methodological difference with this experiment was that group housed rats were tickled with their cage mates in their home cage, rather than tickled singly in a test cage [ 23].

Genetics

The ability to select rats for their response to tickling, and the subsequent effects of this selection, was evaluated in 18% (n = 7) of experiments investigating tickling moderation. Three experiments successfully bi-directionally selected rats to exhibit high or low levels of 50-kHz vocalizations in response to tickling based on their call rate on the 4 th day of tickling [ 16, 38, 39]. All experiments found that high-line rats produced more 50-kHz vocalizations as compared to random-line or low-line rats [ 16, 38- 40]. Overall high-line rats seem to have a stress-resilient phenotype while low-line rats seem to have a stress prone phenotype. Specific results are presented in .

Other moderations

Ten experiments investigated additional factors that can moderate tickling outcomes including bedding, age, brain lesioning, and tickling timing and technique. Using bedding in the tickling cage increased the reward value of tickling and number of positive vocalizations as compared to no bedding [ 41]. The impact of age is nuanced. Younger juvenile animals produce more positive vocalizations and show higher reward value in one study [ 31], but another experiment shows no difference in 50-kHz or 22-kHz vocalizations [ 38]. However, adult rats tickled as both juveniles and adults show the most positive results as compared to rats only tickled a juveniles or adults [ 25]. Rats tickled as both juveniles and adults produced more 50-kHz vocalizations before and after an injection as an adult, fewer audible calls during injection, and the injection procedure was shorter as compared to tickling rats only as juveniles or adults. Tickling rats 1 hour after restraint stress reduces 50-kHz vocalizations [ 42, 43], while tickling rats 23 hours after restraint stress has no effect on 50-kHz vocalizations [ 42]. Tickling rats after lesioning the parvafox nucleus also decreases 50-kHz vocalizations and approach behavior [ 44].

Tickling outcomes are also moderated by tickling timing [ 24] and technique [ 17]. Tickling before injection is more effective than tickling after injection [ 24]. Different techniques investigated included chasing the rat with a hand at its rear, holding the rat in one hand while tickling its stomach with the other, tickling the belly without flipping the rat over, pressing with one finger between the shoulder blades, and allowing the rat to chase the experimenter's hand. This tickling method had a session duration of 10 minutes per day for 3 days. Per visual inspection of the results, techniques of neck tickle, hand chase, grab & tickle, and push & drill (pressing with one finger between the shoulder blades) evoked the most 50-kHz calls while neck tickle and full back (tickling the entire back) evoked the most 22-kHz calls. However, the "Schwarting Method" of tickling evokes significantly higher 22-kHz calls (227 ± 38.4 (0-420) 22-kHz vocalizations) in comparison to the "Panksepp Method" which typically evokes 10 or fewer 22-kHz vocalizations per minute of tickling

Reporting quality and risk of bias assessment

Relatively few experiments adequately used and reported using techniques to reduce bias (). Many experiments did not report randomizing animals to treatment groups (43%, n = 24). No experiments reported using techniques of sequence generation, allocation concealment, or blinded caregivers. Risk of bias was most commonly minimized by were blinding during outcome assessment (50%, n = 28), having complete outcome data or addressing missing outcome data (41%, n = 23), ensuring similar baseline characteristics between groups (16%, n = 9), and randomly assessing outcomes (14%, n = 8). The least used methods to reduce risk of bias were randomly housing rats throughout the room (9%, n = 5) and blinding investigators to treatments (7%, n = 4). Details of which outcome measures were blinded as well as specific reporting quality and risk of bias information for each experiment are included in .

Discussion

We conducted a systematic review to synthesize the empirical literature involving tickling rats. Our search procedure identified 32 articles with 56 experiments. An international and interdisciplinary field of researchers use tickling to accomplish a variety of objectives. Each article was reviewed to achieve three aims: (a) describe the characteristics of rat tickling, (b) summarize outcomes of rat tickling, and (c) evaluate factors moderating rat tickling outcomes. There are a variety of specific tickling methods used in various rat strains, ages, and housing. The most common tickling method was the original "Panksepp method". Overall, tickling outcomes support positive welfare benefits of tickling for rats including increased production of positive vocalizations, improved approach behavior, decreased behavioral measures of anxiety and fear ins standardized tests, and improved handling. However, these outcomes of tickling can be modified by inter-individual differences, pharmacology, housing, genetics, age, timing, specific technique, and the presence of bedding. Overall tickling appears to be a promising welfare enhancing technique that also measures affect, but specific investigation of the best methods is needed.

Characteristics of tickling

To achieve the first aim of our review, several elements of rat tickling were examined in each article, including terminology, rat sex, strain, and housing, tickling duration and frequency, and study design. Although fifteen different terms were used to describe the technique used, the word "tickling" was mentioned at least once in every article. Terminology is an important tool for communicating research to both others in academia and the public. Often, the original term for a procedure is used. However, the large variety of additional terms in this review seems to indicate a disagreement of the best term. Anecdotally, the term "tickling" has been critiqued based on the perception that it lacks scientific rigor by researchers outside the field, is unprofessional, and inadequately describes the procedure. Although the motion of the fingers used in tickling is similar to the motion used when tickling humans, it may or may not mimic human to human tickling. Rather, tickling rats mimics rat rough-and-tumble play by making contact with body areas that are stimulated during this type of play. Based on our review of the literature, we recommend using the term tickling at least once in each article, preferably in the abstract, for consistency across the field. If researchers wish to use an additional term we recommend either "heterospecific play" which was the second most common description and perhaps more accurately describes the interaction or "playful handling" which also describes the technique and is more easily understood by the general public. Consistent use of terminology may help advance knowledge more quickly and efficiently by allowing the technique to be more easily identified and recognized by both researchers and the public.

The tickling procedure varied across studies, although the most common method was to replicate the procedure developed by Panksepp and Burgdorf, with each tickling session lasting for 2 minutes total, with 1 minute of active tickling by alternating between 15 seconds of rest and 15 seconds of active tickling. Most frequently, tickling was conducted over 5 days. However, there was great diversity in session length, amount of time active tickling, and number of sessions. Reporting of other characteristics of the tickling procedure such as presence of bedding, light intensity, and the time of day tickling took place was poor. We recommend that future studies provide specific details about the tickling procedure since bedding increases positive vocalizations [ 17], and high light intensity decreases social play in juveniles [ 45]. We also recommend an experimental investigation of the ideal tickling duration and frequency. Until then, we recommend that researchers tickle for 2 minutes with 1 minute of active tickling for at least 5 sessions since this is the most common method.

Tickling procedures were applied to the four most common rat strains, both sexes, and across ages with typically short to no acclimation periods. Although the effect of rat strain on tickling outcome has not been evaluated, it is likely that rat strain impacts outcomes considering that different types of rats have different play behaviors [ 46]. There are also sex differences in rat play. Juvenile males engage in rough-and-tumble play fighting attacks more frequently than females [ 47]. Male rats also transition to a rougher form of playful defense post puberty [ 47]. Age is also an important factor, considering that rat play peaks between 30-40 days and declines as rats approach sexual maturity [ 48]. We recommend that, unless sufficient rationale can be given for an alternative, future research include both sexes, focus on juvenile rats, and if possible test multiple strains of rats with an adequate acclimation period.

Outcomes of tickling

To achieve the second aim of our review-summarize outcomes of rat tickling-we reviewed studies that included a control for tickling. Overall, almost all studies that investigated tickling in rats reported positive welfare outcomes and that using tickling was beneficial in studying positive affective states. The most common finding was increased production of 50-kHz vocalizations, which are indicative of positive affect, and in turn improved welfare. Given that the tickling technique was specifically developed to study these vocalizations this result is unsurprising [ 16, 18]. The second most common finding of increased approach behavior is thought to relate to the positive reward value of the interaction which causes rats to want to move back to the hand for more tickling. Outcomes related to reduced generalized anxiety and fear of humans, as well as improved handling, also indicate the promising benefits of tickling to rat welfare. Finally, tickling was successfully used to study the genetic and cellular effects associated with positive affect.

Among the experiments that provided a control for tickling, five explicitly used tickling to induce or measure positive affective states. For example, since previously low hippocampal cell proliferation has been associated with depression, increasing hippocampal cell proliferation is essential for the therapeutic efficacy of anti-depressants [ 49]. However, it has been controversial whether hippocampal cell proliferation is a trait or state effect of this psychopathology. To investigate this trait vs state controversy, one experiment used tickling and individual differences in calling rate to show that hippocampal cell proliferation depends on an interaction between a pre-disposing trait and stimulation-depending variations in the subject's affective state [ 19]. Additionally, a series of studies used tickling to model laughter in humans and study its effects on gene expression and neurogenesis in the brain and salivary glands [ 21, 29, 30]. Tickling is useful for studying positive affect because it can be used quickly, consistently and reliably in contrast to other tests such as cognitive bias or sucrose consumption. For example, counting 50-kHz vocalizations has been repeatedly shown to have higher inter- and intra- rater reliability and is easy to train.

Despite the described benefits from tickling, researchers should remain cautious when implementing this procedure widely across rat sexes, ages, and housing types. Single housing of rats less than 50 days old were used in 81% of experiments, and male rats were used in more than half of experiments. Therefore, some findings may be most relevant to singly housed juvenile male rats. Additionally, as several different control conditions were used and techniques to address bias were not sufficiently reported, different results may be seen with different comparison conditions. Finally, as shown below many variables have the potential to moderate tickling outcomes.

One area to consider in future research is the selection of appropriate control conditions. Eleven experiments used an "active" control that involved at least exposing control rats to a passive hand or light touch. The remaining experiments either gave control rats no additional manipulation or simply transferred them to the test box. Choice of control should be determined by experimenters explicitly depending on their experimental question. Stroking or light touch is aversive and therefore can serve as an aversive control [ 18]. Restraint on the back has been found to have mixed results compared to tickling and should be used with caution as a control. Transferring rats to the test box can be used to control for the novelty and reward of a new environment. However, if tickling is being investigated as a habituation method or method to elicit a positive affective state, minimal handling may be the best option.

A final area of concern related to rat tickling is that no experiments reported the impact of tickling rats on their human caretakers. Animal care personnel, particularly those involved in euthanasia, report mild to moderate traumatic general stress, higher level of work stress, and may have higher employee turnover [ 50]. To combat these effects, several universities have implemented programs to mitigate stress and compassion fatigue such as Washington State University's Animal Caregiver Training. Tickling may help form and strengthen the human-animal bond by increasing direct care for the animals and helping caretakers recognize individual animals [ 51]. Establishing a human-animal bond, which is bolstered by human-animal interactions, can be beneficial for humans working with laboratory animals [ 51]. Therefore, it is important to consider caretaker outcomes, in addition to animal outcomes, when investigating human-animal interactions such as tickling.

Tickling outcome moderation

To achieve the third aim of this review-evaluate factors moderating rat tickling outcomes-we reviewed experiments that specifically investigated factors that could moderate the outcomes of tickling. These experiments fell into five main categories: inter-individual differences in random bred rats, pharmacology, housing, genetics, and other. Overall our results indicate that care must be taken while applying tickling in the laboratory.

Inter-individual differences are one of the most relevant moderators of outcomes of tickling for facility personnel. Rats repeatedly show a large range of 50-kHz vocalizations in response to tickling that impact experimental outcomes. In one study, only high-calling rats showed a positive cognitive bias after tickling [ 8]; thus low-calling rats may experience tickling differently than high-calling rats. Further investigation is needed to determine if low-calling rats indeed find the tickling intervention positive, neutral, or negative. Since you can also bi-directionally select rats for either a high- or low-calling rate and find similar differences in line bred rats, it seems to indicate these are trait differences. In future studies, we strongly recommend that investigators include calling rate as a continuous covariate in all statistical models and determine the calling rate of control rats after the termination of the experiment. Tickling can be used to determine behavioral traits and subsequently how a trait response to tickling affects response to stress [ 52]. Selectively bred high-calling rats could also be useful for understanding the importance of playful joy [ 39].

Tickling rats to investigate the effects of pharmacological substances on positive affect has been used repeatedly. All experiments found differences in 50-kHz vocalizations after application of some substances. One article attempted to use tickling combined with the application of the psychotomimetic drug phencyclidine to model the negative symptoms of schizophrenia, but concluded that it was unclear if this model would be valid or not [ 53]. Another article successfully concluded that 50-kHz calls in response to tickling were mediated by dopamine release as evidenced by a decrease in calls after application of dopamine agonists [ 28]. This article was using tickling to model the importance of play behavior during adolescence. Finally, an additional article has used tickling to evaluate calling rates and drug administration to further investigate the effects of trait differences on chronic variable stress as a model of depressions [ 34]. Overall, tickling is a promising method to evaluate pharmacological compounds designed to improve psychological measures of affect while also considering the individual differences in calling rates.

One aspect of tickling moderation-housing-was investigated in the initial work by Panksepp and Burgdorf in 2000 and 2001. Their results showed that rats housed individually had more numerous positive vocalizations and reduced approach latencies. Socially housed animals actually learned to avoid tickling. However, the one study that investigated the impact of housing more recently did not find any significant differences, which was partially attributed to tickling group-housed animals with their cage mates in their home cage [ 23]. All experiments investigating the impact of housing differences were conducted in the same laboratory. An increasing amount of evidence indicates that environmental standardization within laboratories contributes to poor reproducibility of animal experimental outcomes [ 54]. Therefore, it would be beneficial for experimenters external to this laboratory to investigate housing factors, therefore increasing environmental heterogenization for reproducible results. Until then, we recommended researchers tickle group-housed rats in their home cage with cage mates.

There are a few additional factors that experimenters should keep in mind while tickling rats. Rats should be tickled before aversive procedures, rather than after, especially since stress decreases positive vocalizations during tickling. Bedding should be used in the tickling location whenever possible in order to increase reward value and number of 50-kHz vocalizations [ 41]. Bedding may increase the rats comfort while being turned over on their back. Additionally, it seems that tickling rats for the first time when they are adults may not be as rewarding as tickling rats as juveniles only or as both juveniles and adults. Investigators should tickle rats before stressful procedures, rather than after, since stress decreases 50-kHz vocalizations during tickling. Finally, considering the high rates of 22-kHz calls that are elicited while conducting the Schwarting tickling technique, we recommend that investigators rely on the standard Panksepp method of tickling.

Reporting quality, risk of bias, limitations, and future directions

Poor reporting and use of methods to reduce risk of bias are a serious concern in the reviewed literature on tickling. Our reporting quality and risk of bias assessment demonstrate that crucial pieces of information are missing from many studies, such as randomization to treatment groups, random housing, and blinding during outcome assessment. Following guidance from the SYRCLE Risk of Bias tool, we coded studies as having an unclear risk of bias when insufficient details were reported to properly assess the risk of bias [ 14]. However, it is perhaps more likely that experiments did not report details because they simply did not mitigate these areas of potential bias. In this case, there is a risk that the effects of tickling and impact of tickling moderators have been overestimated and may be unable to be replicated [ 55]. Undoubtedly, more clear reporting is needed throughout experiments from sequence generation, random housing, and even clearly indicating that all animals were used in the analysis. Overall, we encourage authors to check their own articles using the SYRCLE tool before publishing, as well as follow the recommendations outlined by Macleod (2009) and the Animal Research: Reporting In Vivo Experiments (ARRIVE) Guidelines Checklist [ 56].

One limitation of this systematic review is that we did not include theses in our review. Therefore, our conclusions could be limited due to a publication bias or the file drawer problem, which occurs when negative or non-significant positive findings are placed in a file drawer rather than published. However, considering that some of the studies did include non-significant findings we consider this limitation to be a more minor concern. Overall, there is a need for future tickling studies to reduce risk of bias, report these efforts, and publish both significant and non-significant findings.

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Rat tickling: A systematic review of applications, outcomes, and moderators

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The study of positive affect has previously been difficult to model and study in laboratory animals. Understanding positive affect has wide-reaching importance for humans and animal welfare. Positive affective states in humans confer resilience to depression and anxiety as well as lead to an overall increase in general health [ 1].

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Megan R. LaFollette,

Marguerite E. O'Haire,

Sylvie Cloutier,

Whitney B. Blankenberger,

Brianna N. Gaskill

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Citation: LaFollette MR, O'Haire ME, Cloutier S, Blankenberger WB, Gaskill BN (2017) Rat tickling: A systematic review of applications, outcomes, and moderators. PLoS ONE 12(4): e0175320. https://doi.org/10.1371/journal.pone.0175320

Editor: Sergio Pellis, University of Lethbridge, CANADA

Received: January 6, 2017; Accepted: March 23, 2017; Published: April 6, 2017

Copyright: © 2017 LaFollette et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: The authors received no any funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Importance

The study of positive affect has previously been difficult to model and study in laboratory animals. Understanding positive affect has wide-reaching importance for humans and animal welfare. Positive affective states in humans confer resilience to depression and anxiety as well as lead to an overall increase in general health [ 1]. Therefore, modeling positive affect in animals could further human health research. Measuring and eliciting positive affect/experiences are also important for assessing animal welfare [ 2]. The technique of tickling, and measuring positive vocalizations in rats, is a promising solution to modeling and quantifying positive affective states in laboratory animals [ 3]. Additionally, by increasing positive affect and decreasing rat fear of humans, tickling may be a promising technique to improve rat welfare [ 4].

Current knowledge

The technique of tickling rats was developed by Dr. Jaak Panksepp and Dr. Jeffrey Burgdorf in 2000. The aim of their initial publication was to determine if a "tickling-type somatosensory stimulation by a human hand" that mimicked juvenile rat rough-and-tumble play was rewarding to rats. In their original article involving eight experiments, juvenile Long Evans rats were tickled for two minutes across four to ten total sessions. The tickling technique involves two main components: a "dorsal contact" on the rat's neck and a "pin" on the rat's stomach. Developing this technique was additionally used to investigate the possibility that 50-kHz ultrasonic vocalizations, produced by rats during play and other rewarding situations, could be used to measure reward value and emotion in rats. Subsequently, vocalizations in the 50-kHz range have repeatedly been shown to be good indicators of positive affect and welfare of rats [ 5, 6]. Some indications that 50-kHz vocalizations are positive are that they are produced in a range of contexts such as during play, mating, exploratory activity, amphetamine administration, and in anticipation of food rewards [ 7]. These vocalizations can be recorded by using specialized microphones and later quantified by trained observers. In contrast to 50-kHz calls, vocalizations in the 22-kHz range have been demonstrated to reflect negative affect and are produced during exposure to predators, pain, and inter-male fighting [ 5].

Rationale

Since the initial publication of the tickling technique, a number of laboratories have used tickling to investigate and measure positive affect [ 8] as well as decrease rat stress, and habituate rats to human contact [ 4]. However, tickling experiments use a range of protocols that find a wide variety of outcomes. Neither tickling protocols nor specific outcomes from tickling that may improve rat welfare have been summarized. Additionally, a number of experiments have also specifically investigated factors that may moderate the outcomes of tickling. However, without a singular article describing these factors, it is unclear how tickling is being used and what factors pertaining to rat characteristics (strain, sex, age) and the tickling protocol (session duration, total number of sessions) influence the efficacy of tickling. As a result, researchers are left with insufficient guidance for using tickling either as a tool for modeling and measuring affect or to improve rat welfare in their experimental colonies. This lack of guidance may also prevent the widespread adoption of this technique for general enrichment since researchers and managers of animal care programs are unsure what time investment this technique requires or its potential effects on their research.

Objectives

Previously, meta-analyses and systematic reviews of animal studies have proven useful for optimizing study design and reporting [ 9]. Although there have been reviews of using tickling and ultrasonic vocalizations to model laughter in humans [ 10, 11], there has not yet been a systematic review of how tickling is utilized in research, its outcomes related to affect and welfare, and methodological considerations for its use. Therefore, the purpose of this review was to provide a comprehensive overview of the empirical research using the tickling technique in rats. Our objective was to systematically identify, summarize, and evaluate the research on tickling in rats to provide direction for future investigation. Our specific aims were to (a) describe current methods used in tickling articles, (b) describe outcomes from tickling, and (c) summarize factors that have the potential to alter tickling outcomes. By summarizing this information in one location, it is possible for researchers, laboratory personnel, those interacting with pet rats, and other interested individuals to make educated decisions about the use and application of tickling.

Methods

Protocol

Before beginning our review, we consulted the preferred reporting guidelines for systematic reviews and meta-analysis (PRISMA) guidelines [ 12, 13], the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) E-learning course and guides, and a veterinary science librarian. We defined our procedures a priori in an unpublished study protocol that specified the search strategy, inclusion and exclusion criteria, and data extraction items.

Article identification

We identified articles by searching complete electronic databases from their first index date through August 2016 and scanning reference lists of identified articles. Databases included Scopus (1966-Present), PubMed (1946-Present), Web of Science (1900-Present), and PsycINFO (1840-Present). Titles, abstracts, and keywords were used for the search. Search terms for all databases included at least one identifier for tickling and rat or rats. Identifiers for tickling included playful handling OR tickle OR tickling OR heterospecific play. No language restrictions were applied. See S1 Text for the full search strategy for each database.

Reporting quality and risk of bias assessment

Two independent reviewers (MRL and WBB) assessed reporting quality and risk of bias of each experiment included within each article. Reporting quality was assessed by answering one question: did the experiment report any sort of randomization of rats to treatment groups in the methods section. Risk of bias was assessed using a modified version of the Systematic Review Center for Laboratory animal Experimentation (SYRCLE) Risk of Bias Tool for animal intervention studies [ 14]. The SYRCLE Risk of Bias Tool is adapted from the Cochrane Collaboration Risk of Bias Tool [ 15] to address bias specific to animal experiments. In brief, the tool asks reviewers to assess 5 domains of bias including selection bias, performance bias, detection bias, attrition bias, and reporting bias. When assessing selection bias, we considered groups similar at baseline if their number of ultrasonic vocalizations were not statistically different. When discrepancies between reviewers occurred, we reached consensus by discussion.

Three main modifications were made to this tool. First, we split item 5 Blinding Caregivers/Investigators into two parts to assess the blinding of caregivers and investigators separately. This was done because although no studies indicated blinding caregivers, a few studies did blind investigators. Second, we added two potential answers to the bias questions: not applicable or partial. These answers were added to accommodate the use of this tool on a wide variety of experimental designs beyond the traditional randomized control trials. The answer "not applicable" was used when studies applied the same treatment to all animals in the study, therefore making certain techniques to address bias not relevant. The answer "partial" was used for randomization reporting and item 7a to indicate a medium risk of bias when only part of the treatment group was reported as randomized or if not all outcomes were blinded during assessment. Third, we added question 7b to indicate which outcomes were blinded (if any) as a follow-up question on medium risk of bias answer.

Results

Article identification and selection

The literature search resulted in 156 citations. A flowchart of the article selection process is presented in Fig 1. The final sample included 32 articles (20.5% of the total initial pool) published between 2000 and 2016 that met the inclusion criteria of empirically evaluating outcomes from tickling rats. There was an international representation of researchers including corresponding authors from the North America (n = 14), Europe (n = 12), and Asia (n = 6). The articles were published in a variety of disciplines, primarily related to Neuroscience (n = 14), Behavior, Physiology, or Psychology (n = 13), or Pharmacology (n = 5). We focused this review on descriptive and qualitative synthesis, rather than meta-analysis, because of the large variety of study designs, animals, intervention techniques, reported outcomes, and possibility of bias.

A flow chart of the selection process of articles using tickling in rats from an original search of databases.

Characteristics of tickling

To achieve our first aim-to describe the characteristics of tickling-we extracted and summarized the key features and applications across the 56 experiments of the 32 articles. All results are presented by experiment in S1 Table.

Terminology.

All 32 articles used the word "tickling" at least once although some specified it as "manual tickling," "tickling stimulation," or "tickling-like stimulation". Preferred and additional terms varied across articles. These additional terms most commonly included words such as play or playful (n = 11), heterospecific (n = 9 terms), somatosensory (n = 3), tactile (n = 5), stimulation (n = 4), or wrestling (n = 1). The most common additional term used was heterospecific play (n = 6) followed "playful handling" (n = 4). Nine articles only used one term while the remaining 23 used multiple terms.

Tickling methodology.

A variety of methods for tickling were used within each article ( Table 1) of which a summary of tickling dosage is presented in Table 2. The most common (46%, n = 26) dosage and technique of tickling used was to replicate the "Panksepp Method" of 15 sec baseline rest followed by 15 sec of dorsal contacts and pins for a total of 2 minutes for at least 4 days [ 16]. The second most common (39%, n = 22) tickling dosage was to use a "Panksepp Variation" which involves the same distinctive dorsal contact and pin components with 15 sec intervals, but with a different session length or number of days. A few experiments (9%, n = 5) also used the "Schwarting Method" which involves using a variety of specific techniques (neck tickle, hand chase, grab and tickle, etc.) with random breaks for at least 3.5 minutes for at least 3 days [ 17].

Tickling occurred in a variety of locations, times of day, light phase, and lighting condition. A majority of experiments reported the flooring of the cage the rats were tickled in (87%, n = 46), including bedding (n = 21), uncovered cage bottom (n = 18), cloth (n = 5), or both bedding and empty flooring (n = 2). Rats were usually tickled individually in a test cage or box (77%, n = 43) although a few experiments tickled rats in their home cage (11%, n = 6) or did not report where rats were tickled (7%, n = 4). Time of day of tickling was also reported for less than a half of experiments (41%, n = 17); this included during the light phase (n = 18) or during the dark phase (n = 3). Less than one third of experiments reported lighting conditions during tickling (21%, n = 12); these included red lighting (n = 6), a light intensity of 150 lux (n = 4), dim (n = 1) or "normal" (n = 1).

Article characteristics.

Key article characteristics are summarized in Table 3. Tickling was applied to the four of the commonly used rat strains including Long-Evans (39%, n = 22), Sprague-Dawley (29%, n = 16), Wistar (23%, n = 13), and Fisher (5%, n = 3; 2 experiments did not report strain). More than half of experiments investigated tickling only in male rats (57%, n = 32), followed by using both sexes (32%, n = 18) and very few used only females (7%, n = 4; 2 experiments did not report which sex was used). Most experiments tickled juvenile rats less than 6 weeks (68%, n = 38), although some experiments tickled adult rats older than 6 weeks (25%, n = 14; 4 experiments did not report rat age).

More than half of the experiments (55%, n = 31) housed their rats individually though some (20%, n = 11) housed their rats in groups for the entire experiment and others (25%, n = 14) either housed at least some rats socially for part of the experiment. Most experiments (71%, n = 40) housed their rats in "standard" plastic cages. However, the original tickling experiments (23%, n = 13) housed rats in wire mesh cages with wire mesh flooring [ 16, 18]. Three experiments did not report the type of caging their rats were housed on.

Total sample sizes ranged from 8 to 240 with an average of 35 rats. Rats in experiments that reported acclimation days (59%, n = 33) received anywhere from 0 to 29 days with an average of 5.5 days. In general, articles did not report what activities were done during acclimation, however one article noted that all rats were "handled" for 5 minutes per day for 3 days during acclimation [ 19].

Outcomes of tickling

To achieve our second aim-to describe the outcomes of tickling-we extracted and synthesized outcomes from all experiments that included a control comparison to tickling (22 experiments from 17 articles). Although the specific designs and assessments of these experiments varied, we identified key types of outcomes and categorized them according to the number of experiments in which they were reported (Fig 2; Table 4). Types of control/comparison treatments in each experiment included minimal handling (n = 11), light touch or stroking (n = 7), transfer to test box without tickling (n = 3), restrained on back (n = 3), exposure to a passive hand (n = 2), or food treat (n = 1). Seventeen experiments only used one control/comparison group while four experiments used multiple control/comparison groups.

The impact of tickling on various outcomes assessed by experiments with a control specific to tickling (N = 22). Green = positive results from tickling. Grey = no difference from tickling. Red = negative result from tickling.

Vocalizations.

The most commonly assessed outcome from tickling was production of 50-kHz vocalizations, evaluated in 72% (n = 16) of tickling controlled experiments. Nearly all experiments (94%, n = 15) found that tickled rats produced more 50-kHz vocalizations than control rats. The one experiment that found no effect tickled rats directly after injection [ 20].

Six experiments assessed 22-kHz vocalizations and found mixed results: 33% (n = 2) found fewer 22-kHz vocalizations, 50% (n = 3) found no effect, and 17% (n = 1) found more 22-kHz vocalizations. The two experiments that found an effect of handling treatment used a light touch comparison condition [ 21, 22]. The three experiments that found no effect of tickling treatment used a minimal handling, restraint, or passive hand comparison condition [ 4, 23, 24]. The experiment that found that tickled rats produced more 22-kHz vocalizations specifically found that only the rats that had been tickled as adults, not juveniles, produced more 22-kHz calls and that all rats were tickled after an injection [ 25].

Three experiments assessed audible vocalizations and found mixed results. One experiment found no effect on audible vocalizations during injection [ 20]. Two experiments found tickled rats made fewer audible vocalizations during an injection [ 24, 25]. However, one of these experiments also found that during the actual tickling procedure, tickled rats made more audible vocalizations [ 24], while the other experiment found no effect of tickling treatment on audible vocalizations during the tickling procedure [ 25].

Approach behavior.

Ten experiments (48%) assessed outcomes of tickling on approach behavior. Among these, 90% (n = 9) found positive results such as decreased approach latency. Two different methods for evaluating approach were used among the ten experiments: 70% (n = 7) evaluated rats directly after tickling and 30% (n = 3) conducted a human approach test delayed from tickling by three days. Testing rats directly after tickling always led to positive results. Testing rats 3 days post tickling has mixed results: 1 found no effect of tickling [ 20] and the other 2 found tickled rats reared, nibbled, and made more contacts with the hand than controls [ 4].

Other anxiety and fear behaviors.

Seven experiments (33%) assessed the outcomes of tickling on anxiety and fear behaviors during standardized tests. Among these, 71% (n = 5) found fewer anxiety and fear behaviors in tickled rats. Three distinct positive results were found: decreased freezing after fear conditioning [ 22, 26], increased locomotor activity and entries into the center of an open field arena [ 23], and increased entries and time in the open arms of an elevated plus maze by high-callers only on day 2 of testing [ 27]. However, elevated plus maze behavior was unaffected by tickling in two other studies [ 4, 20]. Additionally, no behavioral changes due to tickling treatments were observed in a Forced Swim Test [ 27], Cat Odor Test [ 4], Flinch-Jump Test [ 26], or Morris water maze [ 26].

Handling effects.

Six experiments (29%) assessed outcomes of tickling on the stress associated with handling. Among these, 83% (n = 5), found reduced stress in tickled rats. Five experiments investigated the effects of tickling on the injection experience. Four of these found that tickling increased 50-kHz vocalizations while decreasing approach latency, audible calls during injection, and injection procedure duration [ 4, 24, 25]. The experiment that found no effect, tickled rats after injection only [ 20]. Later research showed that tickling rats only before or before and after injection, particularly combined with juvenile tickling experience, is more rewarding than tickling rats after injection [ 24]. Additionally, tickling increases 50-kHz calls in anticipation of handling [ 23].

Hormonal effects.

Four experiments (19%) assessed effect of tickling on rat hormones. Among these, 50% (n = 2) found at least some positive effects and 50% (n = 2) found no effects. Tickling increased dopamine release in the nucleus accumbuens [ 28] and decreased adrenaline and noadrenaline after fear conditioning [ 22]. However, tickling was never found to effect corticosterone in the feces [ 23, 25] or blood [ 22].

Genetic and cellular level effects.

Four experiments used tickling of rats to elicit a positive emotional state for studying the effects of positive affect on cell proliferation, neurogenesis, or gene expression. Tickling increased adult hippocampus cell proliferation, but only in high-callers [ 19]. Adult hippocampus cell proliferation is a factor associated with depression. Tickling altered the expression of genes in the hypothalamus related to feeding regulation [ 21]. This experiment was an attempt to model and investigate the effect seen in humans that blood glucose levels improve after laughter. [ 21]. Tickling increased neurogenesis in the dentate gyrus of the hippocampus [ 29]. The dentate gyrus is an area thought to mediate learning and memory formation. Tickling up-regulated production of kallikrein family mRNAs [ 30], which is one of the serine proteases and related to the production of many bioactive peptides in the submandibular gland of the rat. These authors believe it could be a candidate for a biochemical marker of positive emotional state [ 30].

Other outcomes.

Two experiments reported outcomes that have not been replicated. Tickling high-calling rats from normal lines induced a positive cognitive bias compared to no tickled or tickled low-calling rats [ 8]. Tickling adult rats was unsuccessful in inducing a conditioned place preference or sexual partner preference based on the smell of the hand that tickled them [ 31].

Tickling outcome moderation

To achieve our third aim-to summarize factors that could moderate tickling outcomes-we extracted and summarized the key outcomes from experiments that investigated factors that could moderate the results of tickling (40 experiments from 24 articles).

Inter-individual differences in randomly bred rats.

Inter-individual differences in randomly bred rats as a moderator of tickling was evaluated in 30% (n = 12) of moderator experiments. All experiments reported at least some significant effects on outcomes which are shown in Table 5

Varied methods were used to group rats according to inter-individual differences, although all experiments used 50-kHz vocalizations. Five experiments (42%) used a median split of their average response on the 12-14 th day of tickling [ 27, 32- 34]. Four experiments (33%) used calling rate as a continuous covariate [ 19, 35, 36]. One experiment used total 50-kHz vocalizations after just 1 session of tickling [ 8]. One experiment split rats into upper and lower quartiles [ 37]. One experiment did not detail the methods of separation [ 38]. Also, one experiment investigated vocalization subtypes and found that only frequency modulated 50-kHz vocalizations differed between groups, but that flat 50-kHz vocalizations and 22-kHz vocalizations were not different [ 37].

Pharmacology.

The impact of pharmacological substances on the production of 50-kHz vocalizations during tickling was evaluated in 20% (n = 8) of experiments investigating tickling moderation. Results on 50-kHz are presented in Table 6.

Housing.

The influence of housing type (solitary versus group) on tickling outcomes was evaluated in 20% (n = 8) of experiments investigating tickling moderation. Among these, 88% (n = 7) reported more positive outcomes in solitary rats compared to group-housed rats. Specifically, solitary rats produced more 50-kHz vocalizations during tickling [ 16, 18, 37], had shorter approach latencies [ 16, 18], and had more play bites [ 16]. Conversely, socially housed rats learned to avoid tickling during an instrumental learning task [ 16]. However, 22-kHz vocalizations were not found to differ between housing types [ 37]. No significant differences were found between solitary, pair, or triplet housed rats in 50-kHz or 22-kHz vocalizations in one experiment [ 23]. A key methodological difference with this experiment was that group housed rats were tickled with their cage mates in their home cage, rather than tickled singly in a test cage [ 23].

Genetics.

The ability to select rats for their response to tickling, and the subsequent effects of this selection, was evaluated in 18% (n = 7) of experiments investigating tickling moderation. Three experiments successfully bi-directionally selected rats to exhibit high or low levels of 50-kHz vocalizations in response to tickling based on their call rate on the 4 th day of tickling [ 16, 38, 39]. All experiments found that high-line rats produced more 50-kHz vocalizations as compared to random-line or low-line rats [ 16, 38- 40]. Overall high-line rats seem to have a stress-resilient phenotype while low-line rats seem to have a stress prone phenotype. Specific results are presented in Table 7.

Other moderations.

Ten experiments investigated additional factors that can moderate tickling outcomes including bedding, age, brain lesioning, and tickling timing and technique. Using bedding in the tickling cage increased the reward value of tickling and number of positive vocalizations as compared to no bedding [ 41]. The impact of age is nuanced. Younger juvenile animals produce more positive vocalizations and show higher reward value in one study [ 31], but another experiment shows no difference in 50-kHz or 22-kHz vocalizations [ 38]. However, adult rats tickled as both juveniles and adults show the most positive results as compared to rats only tickled a juveniles or adults [ 25]. Rats tickled as both juveniles and adults produced more 50-kHz vocalizations before and after an injection as an adult, fewer audible calls during injection, and the injection procedure was shorter as compared to tickling rats only as juveniles or adults. Tickling rats 1 hour after restraint stress reduces 50-kHz vocalizations [ 42, 43], while tickling rats 23 hours after restraint stress has no effect on 50-kHz vocalizations [ 42]. Tickling rats after lesioning the parvafox nucleus also decreases 50-kHz vocalizations and approach behavior [ 44].

Tickling outcomes are also moderated by tickling timing [ 24] and technique [ 17]. Tickling before injection is more effective than tickling after injection [ 24]. Different techniques investigated included chasing the rat with a hand at its rear, holding the rat in one hand while tickling its stomach with the other, tickling the belly without flipping the rat over, pressing with one finger between the shoulder blades, and allowing the rat to chase the experimenter's hand. This tickling method had a session duration of 10 minutes per day for 3 days. Per visual inspection of the results, techniques of neck tickle, hand chase, grab & tickle, and push & drill (pressing with one finger between the shoulder blades) evoked the most 50-kHz calls while neck tickle and full back (tickling the entire back) evoked the most 22-kHz calls. However, the "Schwarting Method" of tickling evokes significantly higher 22-kHz calls (227 ± 38.4 (0-420) 22-kHz vocalizations) in comparison to the "Panksepp Method" which typically evokes 10 or fewer 22-kHz vocalizations per minute of tickling

Reporting quality and risk of bias assessment.

Relatively few experiments adequately used and reported using techniques to reduce bias (Fig 3). Many experiments did not report randomizing animals to treatment groups (43%, n = 24). No experiments reported using techniques of sequence generation, allocation concealment, or blinded caregivers. Risk of bias was most commonly minimized by were blinding during outcome assessment (50%, n = 28), having complete outcome data or addressing missing outcome data (41%, n = 23), ensuring similar baseline characteristics between groups (16%, n = 9), and randomly assessing outcomes (14%, n = 8). The least used methods to reduce risk of bias were randomly housing rats throughout the room (9%, n = 5) and blinding investigators to treatments (7%, n = 4). Details of which outcome measures were blinded as well as specific reporting quality and risk of bias information for each experiment are included in S2 Table.

Reporting Quality was assessed by answered the question if any sort of randomization was reported in the methods section. Risk of Bias was assessed using a modified version of SYRCLE Risk of Bias Tool in 55 experiments that used rat tickling. Unclear risk indicates that studies did not report whether bias was mitigated or not. Low risk indicates there was proper bias mitigation. Medium risk indicates that only part of the randomization was reported or outcome assessment was blinded. High risk of bias indicates there was a clear indication of potential bias. Not applicable bias indicates that the source of bias was not relevant. For example, random allocation of animals when all animals received the same treatment at the same time.

Discussion

We conducted a systematic review to synthesize the empirical literature involving tickling rats. Our search procedure identified 32 articles with 56 experiments. An international and interdisciplinary field of researchers use tickling to accomplish a variety of objectives. Each article was reviewed to achieve three aims: (a) describe the characteristics of rat tickling, (b) summarize outcomes of rat tickling, and (c) evaluate factors moderating rat tickling outcomes. There are a variety of specific tickling methods used in various rat strains, ages, and housing. The most common tickling method was the original "Panksepp method". Overall, tickling outcomes support positive welfare benefits of tickling for rats including increased production of positive vocalizations, improved approach behavior, decreased behavioral measures of anxiety and fear ins standardized tests, and improved handling. However, these outcomes of tickling can be modified by inter-individual differences, pharmacology, housing, genetics, age, timing, specific technique, and the presence of bedding. Overall tickling appears to be a promising welfare enhancing technique that also measures affect, but specific investigation of the best methods is needed.

Characteristics of tickling

To achieve the first aim of our review, several elements of rat tickling were examined in each article, including terminology, rat sex, strain, and housing, tickling duration and frequency, and study design. Although fifteen different terms were used to describe the technique used, the word "tickling" was mentioned at least once in every article. Terminology is an important tool for communicating research to both others in academia and the public. Often, the original term for a procedure is used. However, the large variety of additional terms in this review seems to indicate a disagreement of the best term. Anecdotally, the term "tickling" has been critiqued based on the perception that it lacks scientific rigor by researchers outside the field, is unprofessional, and inadequately describes the procedure. Although the motion of the fingers used in tickling is similar to the motion used when tickling humans, it may or may not mimic human to human tickling. Rather, tickling rats mimics rat rough-and-tumble play by making contact with body areas that are stimulated during this type of play. Based on our review of the literature, we recommend using the term tickling at least once in each article, preferably in the abstract, for consistency across the field. If researchers wish to use an additional term we recommend either "heterospecific play" which was the second most common description and perhaps more accurately describes the interaction or "playful handling" which also describes the technique and is more easily understood by the general public. Consistent use of terminology may help advance knowledge more quickly and efficiently by allowing the technique to be more easily identified and recognized by both researchers and the public.

The tickling procedure varied across studies, although the most common method was to replicate the procedure developed by Panksepp and Burgdorf, with each tickling session lasting for 2 minutes total, with 1 minute of active tickling by alternating between 15 seconds of rest and 15 seconds of active tickling. Most frequently, tickling was conducted over 5 days. However, there was great diversity in session length, amount of time active tickling, and number of sessions. Reporting of other characteristics of the tickling procedure such as presence of bedding, light intensity, and the time of day tickling took place was poor. We recommend that future studies provide specific details about the tickling procedure since bedding increases positive vocalizations [ 17], and high light intensity decreases social play in juveniles [ 45]. We also recommend an experimental investigation of the ideal tickling duration and frequency. Until then, we recommend that researchers tickle for 2 minutes with 1 minute of active tickling for at least 5 sessions since this is the most common method.

Tickling procedures were applied to the four most common rat strains, both sexes, and across ages with typically short to no acclimation periods. Although the effect of rat strain on tickling outcome has not been evaluated, it is likely that rat strain impacts outcomes considering that different types of rats have different play behaviors [ 46]. There are also sex differences in rat play. Juvenile males engage in rough-and-tumble play fighting attacks more frequently than females [ 47]. Male rats also transition to a rougher form of playful defense post puberty [ 47]. Age is also an important factor, considering that rat play peaks between 30-40 days and declines as rats approach sexual maturity [ 48]. We recommend that, unless sufficient rationale can be given for an alternative, future research include both sexes, focus on juvenile rats, and if possible test multiple strains of rats with an adequate acclimation period.

Outcomes of tickling

To achieve the second aim of our review-summarize outcomes of rat tickling-we reviewed studies that included a control for tickling. Overall, almost all studies that investigated tickling in rats reported positive welfare outcomes and that using tickling was beneficial in studying positive affective states. The most common finding was increased production of 50-kHz vocalizations, which are indicative of positive affect, and in turn improved welfare. Given that the tickling technique was specifically developed to study these vocalizations this result is unsurprising [ 16, 18]. The second most common finding of increased approach behavior is thought to relate to the positive reward value of the interaction which causes rats to want to move back to the hand for more tickling. Outcomes related to reduced generalized anxiety and fear of humans, as well as improved handling, also indicate the promising benefits of tickling to rat welfare. Finally, tickling was successfully used to study the genetic and cellular effects associated with positive affect.

Among the experiments that provided a control for tickling, five explicitly used tickling to induce or measure positive affective states. For example, since previously low hippocampal cell proliferation has been associated with depression, increasing hippocampal cell proliferation is essential for the therapeutic efficacy of anti-depressants [ 49]. However, it has been controversial whether hippocampal cell proliferation is a trait or state effect of this psychopathology. To investigate this trait vs state controversy, one experiment used tickling and individual differences in calling rate to show that hippocampal cell proliferation depends on an interaction between a pre-disposing trait and stimulation-depending variations in the subject's affective state [ 19]. Additionally, a series of studies used tickling to model laughter in humans and study its effects on gene expression and neurogenesis in the brain and salivary glands [ 21, 29, 30]. Tickling is useful for studying positive affect because it can be used quickly, consistently and reliably in contrast to other tests such as cognitive bias or sucrose consumption. For example, counting 50-kHz vocalizations has been repeatedly shown to have higher inter- and intra- rater reliability and is easy to train.

Despite the described benefits from tickling, researchers should remain cautious when implementing this procedure widely across rat sexes, ages, and housing types. Single housing of rats less than 50 days old were used in 81% of experiments, and male rats were used in more than half of experiments. Therefore, some findings may be most relevant to singly housed juvenile male rats. Additionally, as several different control conditions were used and techniques to address bias were not sufficiently reported, different results may be seen with different comparison conditions. Finally, as shown below many variables have the potential to moderate tickling outcomes.

One area to consider in future research is the selection of appropriate control conditions. Eleven experiments used an "active" control that involved at least exposing control rats to a passive hand or light touch. The remaining experiments either gave control rats no additional manipulation or simply transferred them to the test box. Choice of control should be determined by experimenters explicitly depending on their experimental question. Stroking or light touch is aversive and therefore can serve as an aversive control [ 18]. Restraint on the back has been found to have mixed results compared to tickling and should be used with caution as a control. Transferring rats to the test box can be used to control for the novelty and reward of a new environment. However, if tickling is being investigated as a habituation method or method to elicit a positive affective state, minimal handling may be the best option.

A final area of concern related to rat tickling is that no experiments reported the impact of tickling rats on their human caretakers. Animal care personnel, particularly those involved in euthanasia, report mild to moderate traumatic general stress, higher level of work stress, and may have higher employee turnover [ 50]. To combat these effects, several universities have implemented programs to mitigate stress and compassion fatigue such as Washington State University's Animal Caregiver Training. Tickling may help form and strengthen the human-animal bond by increasing direct care for the animals and helping caretakers recognize individual animals [ 51]. Establishing a human-animal bond, which is bolstered by human-animal interactions, can be beneficial for humans working with laboratory animals [ 51]. Therefore, it is important to consider caretaker outcomes, in addition to animal outcomes, when investigating human-animal interactions such as tickling.

Tickling outcome moderation

To achieve the third aim of this review-evaluate factors moderating rat tickling outcomes-we reviewed experiments that specifically investigated factors that could moderate the outcomes of tickling. These experiments fell into five main categories: inter-individual differences in random bred rats, pharmacology, housing, genetics, and other. Overall our results indicate that care must be taken while applying tickling in the laboratory.

Inter-individual differences are one of the most relevant moderators of outcomes of tickling for facility personnel. Rats repeatedly show a large range of 50-kHz vocalizations in response to tickling that impact experimental outcomes. In one study, only high-calling rats showed a positive cognitive bias after tickling [ 8]; thus low-calling rats may experience tickling differently than high-calling rats. Further investigation is needed to determine if low-calling rats indeed find the tickling intervention positive, neutral, or negative. Since you can also bi-directionally select rats for either a high- or low-calling rate and find similar differences in line bred rats, it seems to indicate these are trait differences. In future studies, we strongly recommend that investigators include calling rate as a continuous covariate in all statistical models and determine the calling rate of control rats after the termination of the experiment. Tickling can be used to determine behavioral traits and subsequently how a trait response to tickling affects response to stress [ 52]. Selectively bred high-calling rats could also be useful for understanding the importance of playful joy [ 39].

Tickling rats to investigate the effects of pharmacological substances on positive affect has been used repeatedly. All experiments found differences in 50-kHz vocalizations after application of some substances. One article attempted to use tickling combined with the application of the psychotomimetic drug phencyclidine to model the negative symptoms of schizophrenia, but concluded that it was unclear if this model would be valid or not [ 53]. Another article successfully concluded that 50-kHz calls in response to tickling were mediated by dopamine release as evidenced by a decrease in calls after application of dopamine agonists [ 28]. This article was using tickling to model the importance of play behavior during adolescence. Finally, an additional article has used tickling to evaluate calling rates and drug administration to further investigate the effects of trait differences on chronic variable stress as a model of depressions [ 34]. Overall, tickling is a promising method to evaluate pharmacological compounds designed to improve psychological measures of affect while also considering the individual differences in calling rates.

One aspect of tickling moderation-housing-was investigated in the initial work by Panksepp and Burgdorf in 2000 and 2001. Their results showed that rats housed individually had more numerous positive vocalizations and reduced approach latencies. Socially housed animals actually learned to avoid tickling. However, the one study that investigated the impact of housing more recently did not find any significant differences, which was partially attributed to tickling group-housed animals with their cage mates in their home cage [ 23]. All experiments investigating the impact of housing differences were conducted in the same laboratory. An increasing amount of evidence indicates that environmental standardization within laboratories contributes to poor reproducibility of animal experimental outcomes [ 54]. Therefore, it would be beneficial for experimenters external to this laboratory to investigate housing factors, therefore increasing environmental heterogenization for reproducible results. Until then, we recommended researchers tickle group-housed rats in their home cage with cage mates.

There are a few additional factors that experimenters should keep in mind while tickling rats. Rats should be tickled before aversive procedures, rather than after, especially since stress decreases positive vocalizations during tickling. Bedding should be used in the tickling location whenever possible in order to increase reward value and number of 50-kHz vocalizations [ 41]. Bedding may increase the rats comfort while being turned over on their back. Additionally, it seems that tickling rats for the first time when they are adults may not be as rewarding as tickling rats as juveniles only or as both juveniles and adults. Investigators should tickle rats before stressful procedures, rather than after, since stress decreases 50-kHz vocalizations during tickling. Finally, considering the high rates of 22-kHz calls that are elicited while conducting the Schwarting tickling technique, we recommend that investigators rely on the standard Panksepp method of tickling.

Reporting quality, risk of bias, limitations, and future directions

Poor reporting and use of methods to reduce risk of bias are a serious concern in the reviewed literature on tickling. Our reporting quality and risk of bias assessment demonstrate that crucial pieces of information are missing from many studies, such as randomization to treatment groups, random housing, and blinding during outcome assessment. Following guidance from the SYRCLE Risk of Bias tool, we coded studies as having an unclear risk of bias when insufficient details were reported to properly assess the risk of bias [ 14]. However, it is perhaps more likely that experiments did not report details because they simply did not mitigate these areas of potential bias. In this case, there is a risk that the effects of tickling and impact of tickling moderators have been overestimated and may be unable to be replicated [ 55]. Undoubtedly, more clear reporting is needed throughout experiments from sequence generation, random housing, and even clearly indicating that all animals were used in the analysis. Overall, we encourage authors to check their own articles using the SYRCLE tool before publishing, as well as follow the recommendations outlined by Macleod (2009) and the Animal Research: Reporting In Vivo Experiments (ARRIVE) Guidelines Checklist [ 56].

One limitation of this systematic review is that we did not include theses in our review. Therefore, our conclusions could be limited due to a publication bias or the file drawer problem, which occurs when negative or non-significant positive findings are placed in a file drawer rather than published. However, considering that some of the studies did include non-significant findings we consider this limitation to be a more minor concern. Overall, there is a need for future tickling studies to reduce risk of bias, report these efforts, and publish both significant and non-significant findings.

Conclusion

Overall, this systematic review demonstrates many positive effects of tickling rats, including increased 50-kHz vocalizations, increased positive human approach behaviors, decreased fear and anxiety after stress, and decreased stress from handling. Some of these effects are stronger in high-calling tickled animals, indicating inter-individual differences in outcomes from tickling. A review of the characteristics of tickling indicate that a wide variety of techniques are used, with the most common being Panksepp's original 2000 method. Future research is needed to determine the ideal dosage of tickling and the effect of calling rate on outcomes. Overall, we recommend tickling as a viable method for studying positive affective states in rats and improving rat welfare.

Acknowledgments

We would like to Lindsay Fisher for assistance with study selection.

Author Contributions

1. Conceptualization: MRL SC BNG MEO.

2. Data curation: MRL.

3. Formal analysis: MRL.

4. Investigation: MRL WBB.

5. Methodology: MRL SC BNG MEO.

6. Project administration: MRL BNG MEO.

7. Resources: MRL BNG MEO.

8. Supervision: MRL BNG MEO SC.

9. Validation: WBB.

10. Visualization: MRL.

11. Writing - original draft: MRL.

12. Writing - review & editing: MRL SC BNG MEO WBB.

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Rat tickling: A systematic review of applications, outcomes, and moderators

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Introduction Rats initially fear humans which can increase stress and impact study results. Additionally, studying positive affective states in rats has proved challenging. Rat tickling is a promising habituation technique that can also be used to model and measure positive affect. However, current studies use a variety of methods to achieve differential results.