The relative playfulness of juvenile Lewis and Fischer-344 rats

Quality not quantity: Deficient juvenile play experiences lead to altered medial prefrontal cortex neurons and sociocognitive skill deficits

Reduced play experience over the juvenile period leads to adults with impoverished social skills and to anatomical and physiological aberrations of the neurons found in the medial prefrontal cortex (mPFC). Even rearing rats from high-playing strains with low-playing strains show these developmental consequences. In the present study, we evaluated whether low-playing rats benefit from being reared with higher playing peers. To test this, we reared male Fischer 344 rats (F344), typically thought to be a low-playing strain, with a Long-Evans (LE) peer, a relatively high-playing strain. As juveniles, F344 rats reared with LE rats experienced less play and lower quality play compared to those reared with another F344. As adults, the F344 rats reared with LE partners exhibited poorer social skills and the pyramidal neurons of their mPFC had larger dendritic arbors than F344 rats reared with same-strain peers. These findings show that being reared with a more playful partner does not improve developmental outcomes of F344 rats, rather the discordance in the play styles of F344 and LE rats leads to poorer outcomes.

Effects of reduced kinematic and social play experience on affective appraisal of human-rat play in rats

BACKGROUND

Play is a common and developmentally important behaviour in young mammals. Specifically in Norway rats (Rattus norvegicus), reduced opportunity to engage in rough-and-tumble (RT) play has been associated with impaired development in social competence. However, RT play is a complex behaviour having both a kinematic aspect (i.e., performing complex 3D manoeuvres during play fights) and a social aspect (interacting with a playful partner). There has been little research so far on disentangling the two aspects in RT play, especially on how these two aspects affect the affective appraisal of the intense physical contact during play.

RESULTS

To examine the developmental effects of kinematic and social play reduction on affective appraisal in rats, we subjected male Long-Evans rats from 21 days old to RT play experience that was reduced either kinematically (through playing in a low ceiling environment) or socially (through playing with a less playful Fischer-344 rat). Starting at 35 days, we measured their production of positively (50-kHz) and negatively (22-kHz) valenced ultrasonic vocalisations (USVs) in a 2-min standardised human-rat play procedure that mimicked the playful sequences of nape contact, pinning, and belly stimulation ('tickling') for ten days. We hypothesised that the rats with kinematically or socially reduced play would perceive the 'tickling' less positively and thus emit positive ultrasonic vocalisations at lower rates compared to control rats with non-reduced play experience. Our results confirmed that each of the treatments reduced play differently: while the kinematic reduction abolished playful pinnings entirely, the social reduction decreased the pinnings and made play highly asymmetric. During the tickling procedure, rats mostly produced 50 kHz USV, indicating that they appraised the procedure as positive. There was a wide inter individual variance and high individual consistency in rats' USV responses to 'tickling'. Crucially, neither the kinematically nor the socially reduced play experience affected either type of USV production when rats were 'tickled'.

CONCLUSIONS

This finding indicates that the ability to appraise play-like interactions as positive remains unaffected even when the kinematic or the social aspect of play experience was substantially curtailed.

When the individual comes into play: The role of self and the partner in the dyadic play fighting of rats

Social play in rats is rewarding and important for the development of brain and social skills. There are differences in the amount of play behavior displayed among individuals, with earlier studies suggesting that, despite variation across trials, individual differences tend to be consistent. In the present study, juvenile Lister-hooded rats were paired with a different, unfamiliar same-sex partner on three days and based on the amount of play each individual initiated, it was characterized as a high, medium or low player. Using this categorization, we explored three features related to individual differences. First, we show that by increasing the number of test days from two, as was done in a previous study (Lesscher et al., 2021), to three, characterization was effectively improved. Secondly, while the earlier study only used males, the present study showed that both sexes exhibit a similar pattern of individual differences in the degree of playfulness. Even though low players consistently initiated less play than medium and high players, all rats varied in how much play they initiated from one trial to the next. Thirdly, we assessed two potential mechanisms by which the playfulness of one rat can modify the level of playfulness of the other rat (i.e., emotional contagion vs homeostasis). Analyses of individuals' contribution to the play of dyads suggest that rats consistently adjust their play behavior depending on the behavior displayed by the partner. Since this adjustment can be positive or negative, our data support a homeostatic mechanism, whereby individuals increase or decrease the amount of play they initiate, which results in the experience of an overall stable pattern of play across trials. Future research will investigate the neural bases for individual differences in play and how rats maintain a preferred level of play.

Transient strain differences in an operant delayed non-match to position task

Working memory refers to the temporary retention of a small amount of information used in the execution of a cognitive task. Working memory impairments are one of the common hallmarks of many neuropsychiatric and neurological disorders including schizophrenia and Alzheimer's disease. Here, we investigated Fischer 344 and Long-Evans rats for strain and sex differences in working memory using the operant-based DNMTP task. Rats were required to press one of two levers presented during a sample phase and followed by a 2-32 second delay, the rats were then required to press the opposite, nonmatch, lever during the choice phase. We found a transient strain difference with Fischer 344 rats performing better than Long-Evans early in training. The Fischer 344 strain showed stable performance across sessions while the performance of Long-Evans increased in the later sessions. Since different background rat strains are used for transgenic rat models, it is critical to be able to compare the behavioral performance across different strains. These findings have implications in behavioral neuroscience research as understanding the typical behavioral endpoints in different background strains will aid our understanding of how different models affect behavioral performance.

The neurobiology of social play behaviour: Past, present and future

Social play behaviour is a highly energetic and rewarding activity that is of great importance for the development of brain and behaviour. Social play is abundant during the juvenile and early adolescent phases of life, and it occurs in most mammalian species, as well as in certain birds and reptiles. To date, the majority of research into the neural mechanisms of social play behaviour has been performed in male rats. In the present review we summarize studies on the neurobiology of social play behaviour in rats, including work on pharmacological and genetic models for autism spectrum disorders, early life manipulations and environmental factors that influence play in rats. We describe several recent developments that expand the field, and highlight outstanding questions that may guide future studies.

Noradrenergic modulation of play in Sprague-Dawley and F344 rats

RATIONALE

For many mammals, engaging in social play behavior as a juvenile is important for cognitive, social, and emotional health as an adult. A playful phenotype reflects a dynamic interplay between genetic framework and experiences that operate on hard-wired brain systems so the relative lack of play in an otherwise playful species may be useful for identifying neural substrates that modulate play behavior. The inbred F344 rat has been identified as a strain that is consistently less playful than other strains commonly used in behavioral research. Norepinephrine (NE) acting on alpha-2 receptors has an inhibitory effect on play and F344 rats differ from a number of other strains in NE functioning. As such, the F344 rat may be particularly useful for gaining insight into NE involvement in play.

OBJECTIVE

The objective of this study was to determine whether the F344 rat is differentially sensitive to compounds that affect NE functioning and that are known to affect play behavior.

METHODS

Using pouncing and pinning to quantify play, the effects of the NE reuptake inhibitor atomoxetine, the NE alpha-2 receptor agonist guanfacine, and the NE alpha-2 receptor antagonist RX821002 on play behavior were assessed in juvenile Sprague-Dawley (SD) and F344 rats.

RESULTS

Atomoxetine and guanfacine reduced play in both SD and F344 rats. RX821002 increased pinning to a comparable extent in both strains but F344 rats were more sensitive to the play-enhancing effects of RX821002 on pounces.

CONCLUSIONS

Strain differences in NE alpha-2 receptor dynamics may contribute to the lower levels of play in F344 rats.

Comparing the ontogeny, neurobiology, and function of social play in hamsters and rats

Syrian hamsters show complex social play behavior and provide a valuable animal model for delineating the neurobiological mechanisms and functions of social play. In this review, we compare social play behavior of hamsters and rats and underlying neurobiological mechanisms. Juvenile rats play by competing for opportunities to pin one another and attack their partner's neck. A broad set of cortical, limbic, and striatal regions regulate the display of social play in rats. In hamsters, social play is characterized by attacks to the head in early puberty, which gradually transitions to the flanks in late puberty. The transition from juvenile social play to adult hamster aggression corresponds with engagement of neural ensembles controlling aggression. Play deprivation in rats and hamsters alters dendritic morphology in mPFC neurons and impairs flexible, context-dependent behavior in adulthood, which suggests these animals may have converged on a similar function for social play. Overall, dissecting the neurobiology of social play in hamsters and rats can provide a valuable comparative approach for evaluating the function of social play.

Atypical play experiences in the juvenile period has an impact on the development of the medial prefrontal cortex in both male and female rats

In rats reared without play, or with limited access to play during the juvenile period, the dendrites of pyramidal neurons of the medial prefrontal cortex (mPFC) exhibit more branching than rats reared with more typical levels of play. This suggests that play is critical for pruning the dendritic arbor of these neurons. However, the rearing paradigms typically used to limit play involve physical separation from a peer or sharing a cage with an adult, causing stress that may disrupt pruning. To limit this potentially confounding source of stress, we used an alternative approach in this study: pairing playful Long Evans rats (LE) with low playing Fischer 344 (F344) rats throughout the juvenile period. We then examined the morphology of medial prefrontal cortex (mPFC) neurons, predicting that pruning should be reduced. LE rats reared with another LE rat had significantly greater pruning of mPFC pyramidal neurons compared to LE rats reared with a F344 partner. Furthermore, in previous studies, only one sex or the other was used, whereas in the present rearing paradigm, both sexes were tested, showing that play influences neuronal pruning in both. The neurons of the play deficient LE rats not only occupied more space, as determined by convex hull analyses, but the dendrites were also longer than in rats with more typical play experiences. Unlike studies using more stressful rearing paradigms, the present effects were limited to the apical dendritic projections, suggesting that the previously reported effects on the basilar dendrites may have resulted from developmental disruptions caused by stress. If correct, the present findings indicate that play experienced over the juvenile period affects how mPFC neurons develop and function.

Play fighting and the development of the social brain: The rat's tale

The benefits gained by young animals engaging in play fighting have been a subject of conjecture for over a hundred years. Progress in understanding the behavioral development of play fighting and the underlying neurobiology of laboratory rats has produced a coherent model that sheds light on this matter. Depriving rats of typical peer-peer play experience during the juvenile period leads to adults with socio-cognitive deficiencies and these are correlated with physiological and anatomical changes to the neurons of the prefrontal cortex, especially the medial prefrontal cortex. Detailed analysis of juvenile peer play has shown that using the abilities needed to ensure that play fighting is reciprocal is critical for attaining these benefits. Therefore, unlike that which was posited by many earlier hypotheses, play fighting does not train specific motor actions, but rather, improves a skill set that can be applied in many different social and non-social contexts. There are still gaps in the rat model that need to be understood, but the model is well-enough developed to provide a framework for broader comparative studies of mammals from diverse lineages that engage in play fighting.

Measuring Play Fighting in Rats: A Multilayered Approach

Rough-and-tumble play or play fighting is an important experience in the juvenile period of many species of mammals, as it facilitates the development of social skills, and for some species, play fighting is retained into adulthood as a tool for assessing and managing social relationships. Laboratory rats have been a model species for studying the neurobiology of play fighting and its key developmental and social functions. However, play fighting interactions are complex, involving competition and cooperation; therefore, no single measure to quantify this behavior is able to capture all its facets. Therefore, in this paper, we present a multilayered framework for scoring all the relevant facets of play that can be affected by experimental manipulations and the logic of how to match what is measured with the question being asked. © 2022 Wiley Periodicals LLC.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Wistar rats and C57BL/6 mice differ in their motivation to seek social interaction versus food in the Social versus Food Preference Test

Here we characterized the Social versus Food Preference Test, a behavioral paradigm designed to investigate the competition between the choice to seek social interaction versus the choice to seek food. We assessed how this competition was modulated by internal cues (social isolation, food deprivation), external cues (stimulus salience), sex (males, females), age (adolescents, adults), and rodent model (Wistar rats, C57BL/6 mice). We found that changes in stimulus preference in response to the internal and external cue manipulations were similar across cohorts. Specifically, social over food preference scores were reduced by food deprivation and social familiarly in Wistar rats and C57BL/6 mice of both sexes. Interestingly, the degree of food deprivation-induced changes in stimulus investigation patterns were greater in adolescents compared to adults in Wistar rats and C57BL/6 mice. Strikingly, baseline stimulus preference and investigation times varied greatly between rodent models: across manipulations, Wistar rats were generally more social-preferring and C57BL/6 mice were generally more food-preferring. Adolescent Wistar rats spent more time investigating the social and food stimuli than adult Wistar rats, while adolescent and adult C57BL/6 mice investigated the stimuli a similar amount. Social isolation did not alter behavior in the Social versus Food Preference Test. Together, our results indicate that the Social versus Food Preference Test is a flexible behavioral paradigm suitable for future interrogations of the peripheral and central systems that can coordinate the expression of stimulus preference related to multiple motivated behaviors.

A naturalistic method to test depression: Anticipation of play

The Wistar-Kyoto (WKY) rat was developed as a control for the spontaneous hypertensive rat but has subsequently also been used as a genetic animal model of depression due to its hyper-responsiveness to stress. We used anticipation of social reward (i.e., a play partner) to assess behavioural and vocal differences between the WKY and normal Wistar (WI) rats in the juvenile period. We found marked differences between groups; the WKY rats, were less active, vocalized less, and used significantly fewer types of 50-kHz calls in comparison to their WI counterparts. The animals were re-tested in adulthood and the same differences existed in overall activity, types of vocalizations and the behavioural vocal profiles used by the two groups of animals. These findings provide a robust baseline for an animal model of depression using a social paradigm. This paradigm may be useful to evaluate the efficacy of pharmaceutical interventions as potential treatments of depression in WKY rats.

How strain differences could help decipher the neurobiology of mammalian playfulness: What the less playful Fischer 344 rat can tell us about play

Play is common among the young of many mammalian species. How that play is exhibited results from a dynamic interplay between genetic framework and experiential influences that, in turn, operate on hard-wired brain systems. One approach towards understanding how genes and environment interact with brain substrates to yield a particular playful phenotype is to take advantage of inbred strains of rats that come with a known genetic identity and assess the effects of varying early social experiences and targeted neurobiological interventions on rats of these strains. This paper primarily summarizes research utilizing the F344 inbred strain, a rat that consistently plays less than most other strains.

Involvement of orexin/hypocretin in the expression of social play behaviour in juvenile rats

Social play is a highly rewarding and motivated behaviour displayed by juveniles of many mammalian species. We hypothesized that the orexin/hypocretin (ORX) system is involved in the expression of juvenile social play behaviour because this system is interconnected with brain regions that comprise the social behaviour and mesocorticolimbic reward networks. We found that exposure to social play increased recruitment of ORX-A neurons in juvenile rats. Furthermore, central administration of ORX-A decreased social play duration, while central blockade of ORX-1 receptors differentially altered social play duration in juvenile rats with low versus high baseline levels of social play (increasing social play in low baseline social play individuals and decreasing social play in high baseline social play individuals). Together, our results provided the first evidence of a role for the ORX system in the modulation of juvenile social play behaviour.

Bridging the species gap in translational research for neurodevelopmental disorders

The prevalence and societal impact of neurodevelopmental disorders (NDDs) continue to increase despite years of research in both patient populations and animal models. There remains an urgent need for translational efforts between clinical and preclinical research to (i) identify and evaluate putative causes of NDD, (ii) determine their underlying neurobiological mechanisms, (iii) develop and test novel therapeutic approaches, and (iv) translate basic research into safe and effective clinical practices. Given the complexity behind potential causes and behaviors affected by NDDs, modeling these uniquely human brain disorders in animals will require that we capitalize on unique advantages of a diverse array of species. While much NDD research has been conducted in more traditional animal models such as the mouse, ultimately, we may benefit from creating animal models with species that have a more sophisticated social behavior repertoire such as the rat (Rattus norvegicus) or species that more closely related to humans, such as the rhesus macaque (Macaca mulatta). Here, we highlight the rat and rhesus macaque models for their role in previous psychological research discoveries, current efforts to understand the neurobiology of NDDs, and focus on the convergence of behavior outcome measures that parallel features of human NDDs.

Basal ganglia involvement in the playfulness of juvenile rats

Play is an important part of normal childhood development and can be readily studied in the laboratory rat in the form of rough-and-tumble play. Given the robust nature of rough-and-tumble play, it has often been assumed that the basal ganglia would have a prominent role in modulating this behavior. Recent work using c-fos expression as a metabolic marker for neural activity combined with temporary inactivation of relevant corticostriatal regions and pharmacological manipulations of opioid, cannabinoid, and dopamine systems has led to a better understanding of how basal ganglia circuitry may be involved in modulating social play in the juvenile rat. Studies using selective play deprivation have also provided insight into the consequences of playful experiences on basal ganglia function. Data reviewed in this paper support a role for the basal ganglia in social play and also suggest that corticostriatal functioning also benefits from playful activities.

Effects of neonatal handling on play and anxiety in F344 and Lewis rats

Play is an important part of normal childhood development and seen in many mammals, including rats. To better understand the interplay between genotype and postnatal experiences, the effects of neonatal handling on play were assessed in Lewis (LEW) and Fischer 344 (F344) rats. Handled litters experienced brief periods of separation during the first two postnatal weeks. F344 rats were less likely to direct nape contacts toward an untreated Sprague-Dawley (SD) partner and less likely to rotate to a supine position in response to a nape contact. When compared to rats from control litters, handled LEW, and F344 rats were more likely to respond to nape contacts with complete rotations, suggesting that handling increased playful responsiveness to a comparable extent in both strains. SD rats paired with handled inbred rats had more nape contacts than those paired with non-handled rats. While handled LEW rats also tended to direct more nape contacts to the SD partner than non-handled LEW rats there was no difference between handled and non-handled F344 rats. These results could not be readily explained by handling-induced changes in either maternal care or anxiety. These data suggest that the behavioral consequences of neonatal handling may not depend to a great extent on the genetic platform that these manipulations are acting on. These data also suggest that the ability to maintain the ebb and flow between playful solicitation and playful responsiveness may be compromised in F344 rats and may contribute to the lower levels of play in this strain.

Pair Housing Alters Delay Discounting in Lewis and Fischer 344 Rats

Impulsive choice underlies several psychological disorders and can be assessed in laboratory rats using delay-discounting tasks, in which choice is for either one food pellet immediately or three food pellets after a delay. Choice for the smaller, immediate reinforcer is considered the impulsive choice. Lewis (LEW) and Fischer 344 (F344) rats differ in the number of impulsive choices made during this task when singly housed, with LEW choosing the impulsive option more often. Due to increasing recommendations to provide environmental enrichment as a component of animal-husbandry practices, a systematic replication of two previous studies was conducted using pair-housed LEW and F344. Delay discounting was assessed with pair-housed LEW and F344 and compared to previous data from singly housed LEW and F344 collected from the same laboratory. Results showed that differences in impulsive choice between the two strains were attenuated with pair housing. The main result driving this change appears to be an increase in impulsive choice in pair-housed F344 relative to singly housed F344.

Effects of cross-fostering on play and anxiety in juvenile Fischer 344 and Lewis rats

A cross-fostering design was used to assess the relative involvement of genetic variability and early postnatal experiences on differential levels of playfulness in juvenile Fischer 344 (F344) and Lewis (LEW) rats and the extent to which strain differences in anxiety may influence play in these two strains. F344 dams were found to lick and groom their pups less than LEW dams and this was not dependent upon the strain of the pups in the litter. As expected, F344 rats were less playful than LEW rats as demonstrated by fewer playful contacts directed to the nape of a Sprague-Dawley (SD) partner and a decreased likelihood of rotating completely to a supine position when their nape was contacted by the SD partner. These differences could not be readily explained by parallel strain differences in anxiety. The pattern of effects on play as a function of cross-fostering depended on both the genetic background of the pup and the motivational state of the pup prior to testing. Whereas in-fostered LEW pups solicited more play as isolation prior to testing increased from 4 to 24h, cross-fostered pups of both strains as well as in-fostered F344 pups were relatively insensitive to the motivational modulation of play solicitation. Responsiveness to play solicitations in pups of both strains reared by F344 dams was insensitive to prior isolation whereas pups reared by LEW dams were less likely to respond with a complete rotation to a supine position when solicited as isolation increased from 4 to 24h prior to testing. These data suggest that the overall level of playfulness in a particular strain of rat is fairly resistant to cross-fostering and may be particularly sensitive to genetic variation whereas how this urge is titrated and modified by motivational factors may be influenced more by epigenetic factors.

The neurobiology of social play and its rewarding value in rats

In the young of many mammalian species, including humans, a vigorous and highly rewarding social activity is abundantly expressed, known as social play behaviour. Social play is thought to be important for the development of social, cognitive and emotional processes and their neural underpinnings, and it is disrupted in pediatric psychiatric disorders. Here, we summarize recent progress in our understanding of the brain mechanisms of social play behaviour, with a focus on its rewarding properties. Opioid, endocannabinoid, dopamine and noradrenaline systems play a prominent role in the modulation of social play. Of these, dopamine is particularly important for the motivational properties of social play. The nucleus accumbens has been identified as a key site for opioid and dopamine modulation of social play. Endocannabinoid influences on social play rely on the basolateral amygdala, whereas noradrenaline modulates social play through the basolateral amygdala, habenula and prefrontal cortex. In sum, social play behaviour is the result of coordinated activity in a network of corticolimbic structures, and its monoamine, opioid and endocannabinoid innervation.

Adolescent social rejection alters pain processing in a CB1 receptor dependent manner

Experiences of social rejection represent a major source of distress and in particular peer rejection during adolescence has been implicated in various psychiatric disorders. Moreover, experimentally induced acute social rejection alters pain perception in humans, implicating overlapping neurocircuits for social and physical pains. We recently demonstrated that rearing of adolescent Wistar rats with inadequate, less playful play partners (Fischer 344) persistently decreases pain sensitivity, although the detailed mechanisms mediating the aversiveness during the social encounter remained unsettled. With the present study we examined the behavioral performance during acute interaction of female adolescent Wistar rats with either age-matched same-strain partners or rats from the Fischer 344 strain. We here identify the low responsiveness upon playful attacks, which appears to be characteristic for social play in the Fischer 344 strain, as one of the main aversive components for adolescent Wistar animals during cross-strain encounters, which subsequently diminishes thermal pain reactivity. A detailed behavioral analysis further revealed increased ultrasonic vocalization at 50kHz and an increased frequency of playful attacks for adolescent Wistar animals paired with a Fischer 344 rat compared to same-strain control pairs. Finally, an acute injection of a subthreshold dose of the cannabinoid type 1 receptor inverse agonist/antagonist SR141716 before the social encounter abolished enhanced play-soliciting behavior in Wistar/Fischer 344 pairs as well as the behavioral consequences of the rejection experience in adolescent Wistar rats, further emphasizing an important modulatory role of the endocannabinoid system in mediating the effects of social behavior and social pain.

Behavioral Phenotyping of Juvenile Long-Evans and Sprague-Dawley Rats: Implications for Preclinical Models of Autism Spectrum Disorders

The laboratory rat is emerging as an attractive preclinical animal model of autism spectrum disorder (ASD), allowing investigators to explore genetic, environmental and pharmacological manipulations in a species exhibiting complex, reciprocal social behavior. The present study was carried out to compare two commonly used strains of laboratory rats, Sprague-Dawley (SD) and Long-Evans (LE), between the ages of postnatal day (PND) 26-56 using high-throughput behavioral phenotyping tools commonly used in mouse models of ASD that we have adapted for use in rats. We detected few differences between young SD and LE strains on standard assays of exploration, sensorimotor gating, anxiety, repetitive behaviors, and learning. Both SD and LE strains also demonstrated sociability in the 3-chamber social approach test as indexed by spending more time in the social chamber with a constrained age/strain/sex matched novel partner than in an identical chamber without a partner. Pronounced differences between the two strains were, however, detected when the rats were allowed to freely interact with a novel partner in the social dyad paradigm. The SD rats in this particular testing paradigm engaged in play more frequently and for longer durations than the LE rats at both juvenile and young adult developmental time points. Results from this study that are particularly relevant for developing preclinical ASD models in rats are threefold: (i) commonly utilized strains exhibit unique patterns of social interactions, including strain-specific play behaviors, (ii) the testing environment may profoundly influence the expression of strain-specific social behavior and (iii) simple, automated measures of sociability may not capture the complexities of rat social interactions.

Play in juvenile mink: litter effects, stability over time, and motivational heterogeneity

Mink are potentially ideal for investigating the functions of play: deleterious effects of early social isolation suggest a crucial developmental role for play; and huge numbers of highly playful juvenile subjects can be studied on farms. We collected descriptive data on 186 pairs from 93 litters, half provided with play-eliciting environmental enrichment objects in their home cages, to test three hypotheses: (1) play frequency is subject to litter effects; (2) relative playfulness is stable over time; (3) play sub-types share a single, common motivational basis. We found weak litter effects that were driven by stronger litter effects on general activity, and weakly stable individual differences in both total and rough-and-tumble play. Experimentally increasing object play did not inhibit rough-and-tumble play, showing these sub-types are not motivational substitutes. Frequencies of these sub-types were also uncorrelated, and changed differently with time of day and age, further supporting this conclusion.

A Brain Motivated to Play: Insights into the Neurobiology of Playfulness

Play is an important part of normal childhood development and is seen in varied forms among many mammals. While not indispensable to normal development, playful social experiences as juveniles may provide an opportunity to develop flexible behavioral strategies when novel and uncertain situations arise as an adult. To understand the neurobiological mechanisms responsible for play and how the functions of play may relate to these neural substrates, the rat has become the model of choice. Play in the rat is easily quantified, tightly regulated, and can be modulated by genetic factors and postnatal experiences. Brain areas most likely to be involved in the modulation of play include regions within the prefrontal cortex, dorsal and ventral striatum, some regions of the amygdala, and habenula. This paper discusses what we currently know about the neurobiological substrates of play and how this can help illuminate functional questions about the putative benefits of play.

Social play behavior, ultrasonic vocalizations and their modulation by morphine and amphetamine in Wistar and Sprague-Dawley rats

RATIONALE

Social play behavior is the most characteristic social behavior in young mammals. It is highly rewarding and crucial for proper neurobehavioral development. Despite the importance of genetic factors in normal and pathological social behaviors, little information is available about strain influences on social play.

OBJECTIVE AND METHODS

The aim of this study was to investigate differences in social play behavior, 50-kHz ultrasonic vocalizations (USVs) and their modulation by acute morphine and amphetamine administration in two rat strains widely used in behavioral pharmacology studies, i.e., Wistar and Sprague-Dawley rats.

RESULTS

Sprague-Dawley rats showed higher levels of social play than Wistar rats. In both strains, no correlation was found between the performance of social behaviors and the emission of 50-kHz USVs. In Wistar and Sprague-Dawley rats, morphine increased and amphetamine decreased social play. The effects of morphine, however, were more pronounced in Wistar than Sprague-Dawley animals. In both strains, morphine did not affect USV emission, while amphetamine increased it during cage exploration. In Sprague-Dawley rats only, amphetamine decreased USVs during social interaction.

CONCLUSIONS

Wistar and Sprague-Dawley rats differ in their absolute levels of social play behavior and 50-kHz USVs, and quantitative differences exist in their response to pharmacological manipulations of social play. The emission of 50-kHz USVs and the behavioral parameters thought to reflect rewarding social interactions in adolescent rats are dissociable.

Adolescent peer-rejection persistently alters pain perception and CB1 receptor expression in female rats

Peer-interactions are particularly important during adolescence and teenagers display enhanced sensitivity toward rejection by peers. Social rejection has been shown to induce alterations in pain perception in humans. However, the neurobiological consequences of adolescent social rejection have yet to be extensively characterized, and no appropriate animal model is available. Here, we propose inadequate playful interactions in adolescent rats as a novel animal model for social peer-rejection and examine potential long-term consequences into adulthood. Acute social pairing of female adolescent Wistar rats with an age-matched rat from the less playful Fischer344 strain was found to alter social play and decrease pain reactivity, indicating Fischer rats as inadequate social partners for Wistar animals. Therefore, in a second experiment, adolescent female Wistar rats were either reared with another Wistar rat (adequate social rearing; control) or with a Fischer rat (inadequate social rearing; play-deprived). Beginning on day 50, all Wistar rats were group housed with same-strain partners and tested for behavioral, neurobiological and endocrine differences in adulthood. Playful peer-interactions were decreased during adolescence in play-deprived animals, without affecting social contact behavior. Consequently, adult play-deprived rats showed decreased pain sensitivity and increased startle reactivity compared to controls, but did not differ in activity, anxiety-related behavior or social interaction. Both groups also differed in their endocrine stress-response, and expression levels of the cannabinoid CB1 receptor were increased in the thalamus, whereas FAAH levels were decreased in the amygdala. The present animal model therefore represents a novel approach to assess the long-term consequences of peer-rejection during adolescence.

Prepubertal Fischer 344 rats display stronger morphine-induced taste avoidance than prepubertal Lewis rats

The present report asked if the previously reported differences in morphine-induced conditioned taste avoidance between adult F344 and LEW rats (F344 > LEW) are also evident in prepubescence (early adolescence). To assess this possibility, adult (Experiment 1) and prepubertal (Experiment 2) F344 and LEW rats were assessed for their ability to acquire morphine-induced taste avoidance (0, 3.2, 10, or 18 mg/kg) in a modified taste avoidance procedure. In each experiment, rats of both strains were given repeated pairings of saccharin and morphine followed by a final two-bottle avoidance test. Adult and prepubertal F344 subjects displayed a more rapid acquisition of the avoidance response as well as stronger suppression of consumption than their LEW counterparts. These data suggest the strains differ in their sensitivity to the aversive effects of morphine and that this differential sensitivity is evident early in development and is developmentally stable. The basis for these strain differences in morphine-induced avoidance was discussed.

The effects of cocaine self-administration on dendritic spine density in the rat hippocampus are dependent on genetic background

Chronic exposure to cocaine induces modifications to neurons in the brain regions involved in addiction. Hence, we evaluated cocaine-induced changes in the hippocampal CA1 field in Fischer 344 (F344) and Lewis (LEW) rats, 2 strains that have been widely used to study genetic predisposition to drug addiction, by combining intracellular Lucifer yellow injection with confocal microscopy reconstruction of labeled neurons. Specifically, we examined the effects of cocaine self-administration on the structure, size, and branching complexity of the apical dendrites of CA1 pyramidal neurons. In addition, we quantified spine density in the collaterals of the apical dendritic arbors of these neurons. We found differences between these strains in several morphological parameters. For example, CA1 apical dendrites were more branched and complex in LEW than in F344 rats, while the spine density in the collateral dendrites of the apical dendritic arbors was greater in F344 rats. Interestingly, cocaine self-administration in LEW rats augmented the spine density, an effect that was not observed in the F344 strain. These results reveal significant structural differences in CA1 pyramidal cells between these strains and indicate that cocaine self-administration has a distinct effect on neuron morphology in the hippocampus of rats with different genetic backgrounds.

Peering into the dynamics of social interactions: measuring play fighting in rats

Play fighting in the rat involves attack and defense of the nape of the neck, which if contacted, is gently nuzzled with the snout. Because the movements of one animal are countered by the actions of its partner, play fighting is a complex, dynamic interaction. This dynamic complexity raises methodological problems about what to score for experimental studies. We present a scoring schema that is sensitive to the correlated nature of the actions performed. The frequency of play fighting can be measured by counting the number of playful nape attacks occurring per unit time. However, playful defense, as it can only occur in response to attack, is necessarily a contingent measure that is best measured as a percentage (#attacks defended/total # attacks X 100%). How a particular attack is defended against can involve one of several tactics, and these are contingent on defense having taken place; consequently, the type of defense is also best expressed contingently as a percentage. Two experiments illustrate how these measurements can be used to detect the effect of brain damage on play fighting even when there is no effect on overall playfulness. That is, the schema presented here is designed to detect and evaluate changes in the content of play following an experimental treatment.

Serotonin, motivation, and playfulness in the juvenile rat

The effects of the selective 5HT(1A) agonist 8-OH-DPAT were assessed on the play behavior of juvenile rats. When both rats of the test pair were comparably motivated to play, the only significant effect of 8-OH-DPAT was for play to be reduced at higher doses. When there was a baseline asymmetry in playful solicitation due to a differential motivation to play and only one rat of the pair was treated, low doses of 8-OH-DPAT resulted in a collapse of asymmetry in playful solicitations. It did not matter whether the rat that was treated initially accounted for more nape contacts or fewer nape contacts, the net effect of 8-OH-DPAT in this model was for low doses of 8-OH-DPAT to decrease a pre-established asymmetry in play solicitation. It is concluded that selective stimulation of 5HT(1A) receptors changes the dynamic of a playful interaction between two participants that are differentially motivated to play. These results are discussed within a broader framework of serotonergic involvement in mammalian playfulness.

Dysfunctional play and dopamine physiology in the Fischer 344 rat

Juvenile Fischer 344 rats are known to be less playful than other inbred strains, although the neurobiological substrate(s) responsible for this phenotype is uncertain. In the present study, Fischer 344 rats were compared to the commonly used outbred Sprague-Dawley strain on several behavioral and physiological parameters in order to ascertain whether the lack of play may be related to compromised activity of brain dopamine (DA) systems. As expected, Fischer 344 rats were far less playful than Sprague-Dawley rats, with Fischer 344 rats less likely to initiate playful contacts with a playful partner and less likely to respond playfully to these contacts. We also found that Fischer 344 rats showed less of a startle response and greater pre-pulse inhibition (PPI), especially at higher pre-pulse intensities. The increase in PPI seen in the Fischer 344 rat could be due to reduced DA modulation of sensorimotor gating and neurochemical measures were consistent with Fischer 344 rats releasing less DA than Sprague-Dawley rats. Fast scan cyclic voltammetry (FSCV) revealed Fischer 344 rats had less evoked DA release in dorsal and ventral striatal brain slices and high-performance liquid chromatography revealed Fischer 344 rats to have less DA turnover in the striatum and prefrontal cortex. We also found DA-dependent forms of cortical plasticity were deficient in the striatum and prefrontal cortex of the Fischer 344 rat. Taken together, these data indicate that deficits in play and enhanced PPI of Fischer 344 rats may be due to reduced DA modulation of corticostriatal and mesolimbic/mesocortical circuits critical to the execution of these behaviors.

Play fighting and corticotropin-releasing hormone in the lateral septum of golden hamsters

This study was focused on determining the possible role of corticotropin-releasing hormone (CRH) on play fighting in juvenile golden hamsters. As no specific neural sites have been proposed, we looked for changes in CRH innervations at the peak of play-fighting activity on postnatal day 35 (P-35) from a week before on P-28. We noted that the increase in play-fighting activity between these two dates was associated with a 100% increase of the density of CRH fibers within the lateral septum. We, then, tested the possible role of CRH receptors on play fighting within the lateral septum through microinjections of alpha-helical CRH, a CRH receptor antagonist (either 0, 30, or 300 ng), directly into the area. The treatments inhibited play-fighting attacks and pins as well as reduced the duration of time that the resident hamsters spent in contact with the intruders, though locomotor activity remained unaffected. The possible source of CRH release in the lateral septum was addressed by quantification of CRH neurons also labeled with a marker of cellular activity, c-Fos, after consummation of play fighting. CRH neurons in the horizontal part of the diagonal band, an area reciprocally connected with the lateral septum, showed a 75% increase in double labeling with c-Fos as compared to controls. Together, these data show that CRH receptors in the lateral septum have a general role on play fighting, not just facilitating its consummation, but also likely enhancing appetitive aspects as well. In addition, this effect is associated with enhanced CRH availability in the area and enhanced neuronal activity within interconnected areas.

Genetic and early environmental contributions to alcohol's aversive and physiological effects

Genetic and early environmental factors interact to influence ethanol's motivational effects. To explore these issues, a reciprocal cross-fostering paradigm was applied to Fischer and Lewis rats. The adult female offspring received vehicle or the kappa opioid antagonist nor-BNI (1 mg/kg) followed by assessments of conditioned taste aversion (CTA), blood alcohol concentrations (BACs) and hypothermia induced by 1.25 g/kg intraperitoneal ethanol. CTA acquisition in the in-fostered Fischer and Lewis animals did not differ; however, the Fischer maternal environment produced stronger acquisition in the cross-fostered Lewis rats versus their in-fostered counterparts. CTAs in the Fischer rats were not affected by cross-fostering. In extinction, the in-fostered Lewis animals displayed stronger aversions than the Fischer groups on two trials (of 12) whereas the cross-fostered Lewis differed from the Fischer groups on nine trials. Despite these CTA effects, Lewis rats exhibited higher BACs and stronger hypothermic responses than Fischer with no cross-fostering effects in either strain. No phenotypes were affected by nor-BNI. These data extend previous findings dissociating the aversive and peripheral physiological effects of ethanol in female Fischer and Lewis rats, and highlight the importance of genetic and early environmental factors in shaping subsequent responses to alcohol's motivational effects in adulthood.

Effects of cross-fostering on cocaine-induced conditioned taste aversions in Fischer and Lewis rats

The systematic comparison between Fischer and Lewis rats is a popular animal model of genetic factors in drug abuse. Although genetic and environmental factors interact to affect drug abuse in humans, analogous effects have not yet been reported within the Fischer-Lewis model. In order to assess the contributions and interaction of genotype and early maternal environment on responses to a drug of abuse, the present study employed a cross-fostering design, where male and female Fischer and Lewis pups were reared by unrelated dams of their own strain (in-fostered) or of the other strain (cross-fostered). As adults, rats from both strains were tested for their ability to acquire a conditioned taste aversion to a novel saccharin solution that had been repeatedly paired with an injection of cocaine (32 mg/kg, subcutaneous). In-fostered Fischer females acquired significantly weaker aversions than in-fostered Lewis females across the multiple saccharin-cocaine pairings. However, cross-fostered Fischer females exhibited aversions that were not only significantly stronger than their in-fostered Fischer counterparts, but identical to all groups of the Lewis genotype. No strain differences or cross-fostering effects were observed in the males. The data with the female subjects cannot be accounted for simply by the genetic strain of the subjects and demonstrate a clear gene-environment interaction effect on responses to the aversive effects of cocaine in Fischer and Lewis rats. Implications for studying maternal behavior as a source of epigenetic modulation of drug abuse vulnerability were discussed.

Selective breeding for infant rat separation-induced ultrasonic vocalizations: developmental precursors of passive and active coping styles

Human depression and anxiety disorders show inherited biases across generations, as do antisocial disorders characterized by aggression. Each condition is preceded in children by behavioral inhibition or aggressive behavior, respectively, and both are characterized by separation anxiety disorders. In affected families, adults and children exhibit different forms of altered autonomic nervous system regulation and hypothalamic-pituitary-adrenal activity in response to stress. Because it is difficult to determine mechanisms accounting for these associations, animal studies are useful for studying the fundamental relationships between biological and behavioral traits. Pharmacologic and behavioral studies suggest that infant rat ultrasonic vocalizations (USV) are a measure of an early anxiety-like state related to separation anxiety. However, it was not known whether or not early ultrasound emissions in infant rats are markers for genetic risk for anxiety states later in life. To address these questions, we selectively bred two lines of rats based on high and low rates of USV to isolation at postnatal (P) 10 days of age. To our knowledge, ours is the only laboratory that has ever selectively bred on the basis of an infantile trait related to anxiety. The High and Low USV lines show two distinct sets of patterns of behavior, physiology and neurochemistry from infancy through adulthood. As adults High line rats demonstrate "anxious"/"depressed" phenotypes in behavior and autonomic nervous system (ANS) regulation to standard laboratory tests. In Lows, on the other hand, behavior and autonomic regulation are consistent with an "aggressive" phenotype. The High and Low USV lines are the first genetic animal models implicating long-term associations of contrasting "coping styles" with early attachment responses. They thus present a potentially powerful model for examining gene-environment interactions in the development of life-long affective regulation.

Genetics of dopamine and its contribution to cocaine addiction

Cocaine addiction is a major health and social problem for which there are presently no effective pharmacotherapies. Many of the most promising medications target dopamine based on the large literature that supports its role in addiction. Recent studies show that genetic factors are also important. Rodent models and gene knock-out technology have helped elucidate the involvement of specific genes in the function of the dopamine reward system and intracellular cascades that lead to neuronal changes in this system. Human epidemiological, linkage, and association studies have identified allelic variants (polymorphisms) that give rise to altered metabolism of dopamine and its functional consequences. Individuals with these polymorphisms respond differently to psychostimulants and possibly to pharmacotherapies. Here we review the literature on genetic variations that affect dopamine neurotransmission, responses to psychostimulants and potential treatments for cocaine addiction. Behavioral responses to psychostimulants in animals with different or modified genetics in dopamine signaling are discussed. We also review polymorphisms in humans that affect dopaminergic neurotransmission and alter the subjective effects of psychostimulants. Pharmacotherapies may have increased efficacy when targeted to individuals possessing specific genetic polymophisms in dopamine's metabolic and intracellular messenger systems.

The effects of selective breeding for infant ultrasonic vocalizations on play behavior in juvenile rats

For over 25 generations, two lines of rats (High and Low USV lines) have been selectively bred for extreme rates of infantile (45 kHz) ultrasonic vocalizations (USV) in response to maternal separation at postnatal day (P)10. High and low line juveniles (P30-P40) were socially isolated and allowed to play in same-sex sibling pairs for 10 min per day over three days. Measures of play were nape contacts and pinning. Other social and nonsocial behaviors were also scored during the three sessions; two of these, 55 kHz USV and walk-overs, were statistically associated with play. Compared to the Random control line, both High and Low line juveniles showed deficits in play behavior. In the High line, play initiatory behavior (nape contacts) was reduced, but pinning, USV and walk-overs were relatively unchanged. In contrast, nape contacts, pinning, USV and walk-overs were all reduced in Low line juveniles compared to Random line controls. The results suggest that selection for extremes of infant USV rates has produced temperamental differences that are expressed in juvenile play in the High and Low USV lines.

Play fighting between kindling-prone (fast) and kindling-resistant (slow) rats

Differences in the play behavior of 2 strains of rats suggest that different components of play fighting can be modified independently. The development of play fighting in cross-strain pairs of familiar and unfamiliar rats was examined to determine whether interacting with a non-congruent pair-mate would alter the pattern of play typical for each strain. In both strains, changes in play fighting were observed throughout development, but partner identity appeared to influence play fighting in different ways depending on age. These data suggest that some components of play may be more impervious to changes in social environment than other components.

Play fighting in androgen-insensitivetfm rats: Evidence that androgen receptors are necessary for the development of adult playful attack and defense

The frequency of playful attack and the style of playful defense, are modifiable by gonadal steroids and change after puberty in male and female rats. The present study examined the play behavior exhibited by testicular feminized mutation (tfm)-affected males, who are insensitive to androgens but can bind estrogens aromatized from androgens, to determine the relative contributions of androgens and estrogens to the age-related changes in play behavior. tfm males did not exhibit a decrease in playful attack with age and were more likely to maintain the use of complete rotations, a juvenile form of playful defense, into adulthood. tfm males did however, show age related changes in the use of partial rotations and upright postures, two other forms of playful defense, that were similar to normal males. These data suggest that the development of play fighting and defense in males is dependent on both androgen- and estrogen-receptor-mediated effects.

Development of play fighting in kindling-prone (FAST) and kindling-resistant (SLOW) rats: How does the retention of phenotypic juvenility affect the complexity of play?

Rats selectively bred for susceptibility to amygdala kindling (FAST) have been shown to retain neural and behavioral features of the juvenile phase into adulthood. In contrast, rats selectively bred for resistance to amygdala kindling (SLOW) are neurobehaviorally more typically adult. The development of play fighting in male and female rats of both selected lines was studied. Given the apparent association of juvenility and play often noted in the literature for mammals in general, it was predicted that the FAST rats should be more playful and be more likely to retain the juvenile tactics of play that lead to more prolonged and complex patterns of social contact. As expected, FAST rats initiated more playful attacks and were more likely to defend against attacks than SLOW rats as both juveniles and adults. Unexpectedly, however, both selected lines exhibited patterns of defense that reduced the likelihood of complex and prolonged social contact. Importantly, the two selected lines did so by very different means. The FAST rats did so by avoiding contact whereas the SLOW rats did so by responding in an adult-typical manner that blocks contact. That is, the FAST rats exaggerated the changes typically occurring at puberty whereas the SLOW rats, at all ages, responded in a more adult manner. These data suggest that the different components of play fighting do not change uniformly with changes in the neurobehavioral underpinnings of juvenility.