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

Play as Occupation: The State of Our Science and a Research Agenda for the Profession

Occupational therapists view play through a unique lens. Considering play as a human occupation, and one to which all people have a right, places occupational therapists among a special group of professionals championing play. This State of the Science article seeks to increase awareness regarding the occupational therapy profession's contributions and to situate those contributions within the larger body of literature on play while also promoting further study of play as an occupation.

The evolution of social play in songbirds, parrots and cockatoos - emotional or highly complex cognitive behaviour or both?

Social play has been described in many animals. However, much of this social behaviour among birds, particularly in adults, is still relatively unexplored in terms of the environmental, psychological, and social dynamics of play. This paper provides an overview of what we know about adult social play in birds and addresses areas in which subtleties and distinctions, such as in play initiation and social organisation and its relationship to expressions of play, are considered in detail. The paper considers emotional, social, innovative, and cognitive aspects of play, then the environmental conditions and affiliative bonds, suggesting a surprisingly complex framework of criteria awaiting further research. Adult social play has so far been studied in only a small number of avian species, exclusively in those with a particularly large brain relative to body size without necessarily addressing brain functions and lateralization. When lateralization of brain function is considered, it can further illuminate a possibly significant relevance of play behaviour to the evolution of cognition, to management of emotions, and the development of sociality.

Disentangling developmental effects of play aspects in rat rough-and-tumble play

Animal play encompasses a variety of aspects, with kinematic and social aspects being particularly prevalent in mammalian play behaviour. While the developmental effects of play have been increasingly documented in recent decades, understanding the specific contributions of different play aspects remains crucial to understand the function and evolutionary benefit of animal play. In our study, developing male rats were exposed to rough-and-tumble play selectively reduced in either the kinematic or the social aspect. We then assessed the developmental effects of reduced play on their appraisal of standardized human-rat play ('tickling') by examining their emission of 50 kHz ultrasonic vocalizations (USVs). Using a deep learning framework, we efficiently classified five subtypes of these USVs across six behavioural states. Our results revealed that rats lacking the kinematic aspect in play emitted fewer USVs during tactile contacts by human and generally produced fewer USVs of positive valence compared with control rats. Rats lacking the social aspect did not differ from the control and the kinematically reduced group. These results indicate aspects of play have different developmental effects, underscoring the need for researchers to further disentangle how each aspect affects animals.

Opportunities for risk-taking during play alters cognitive performance and prefrontal inhibitory signalling in rats of both sexes

Social play behaviour is a rewarding activity that can entail risks, thus allowing young individuals to test the limits of their capacities and to train their cognitive and emotional adaptability to challenges. Here, we tested in rats how opportunities for risk-taking during play affect the development of cognitive and emotional capacities and medial prefrontal cortex (mPFC) function, a brain structure important for risk-based decision making. Male and female rats were housed socially or social play-deprived (SPD) between postnatal day (P)21 and P42. During this period, half of both groups were daily exposed to a high-risk play environment. Around P85, all rats were tested for cognitive performance and emotional behaviour after which inhibitory currents were recorded in layer 5 pyramidal neurons in mPFC slices. We show that playing in a high-risk environment altered cognitive flexibility in both sexes and improved behavioural inhibition in males. High-risk play altered anxiety-like behaviour in the elevated plus maze in males and in the open field in females, respectively. SPD affected cognitive flexibility in both sexes and decreased anxiety-like behaviour in the elevated plus maze in females. We found that synaptic inhibitory currents in the mPFC were increased in male, but not female, rats after high-risk play, while SPD lowered prefrontal cortex (PFC) synaptic inhibition in both sexes. Together, our data show that exposure to risks during play affects the development of cognition, emotional behaviour and inhibition in the mPFC. Furthermore, our study suggests that the opportunity to take risks during play cannot substitute for social play behaviour.

Animal play and evolution: Seven timely research issues about enigmatic phenomena

The nature of play in animals has been long debated, but progress is being made in characterizing play and its variants, documenting its distribution across vertebrate and invertebrate taxa, describing its mechanisms and development, and proposing testable theories about its origins, evolution, and adaptive functions. To achieve a deeper understanding of the functions and evolution of play, integrative and conceptual advances are needed in neuroscience, computer modeling, phylogenetics, experimental techniques, behavior development, and inter- and intra-specific variation. The special issue contains papers documenting many of these advances. Here, we describe seven timely areas where further research is needed to understand this still enigmatic class of phenomena more fully. Growing empirical and theoretical evidence reveals that play has been crucial in the evolution of behavior and psychology but has been underestimated, if not ignored, in both empirical and theoretical areas of evolutionary biology and neuroscience. Play research has important ramifications for understanding the evolution of cognition, emotion, and culture, and research on animals can be both informative and transformative.

Fatty Acid Amide Hydrolase and Cannabinoid Receptor Type 1 Genes Regulation is Modulated by Social Isolation in Rats

Social isolation is a state of lack of social connections, involving the modulation of different molecular signalling cascades and associated with high risk of mental health issues. To investigate if and how gene expression is modulated by social experience at the central level, we analyzed the effects of 5 weeks of social isolation in rats focusing on endocannabinoid system genes transcription in key brain regions involved in emotional control. We observed selective reduction in mRNA levels for fatty acid amide hydrolase (Faah) and cannabinoid receptor type 1 (Cnr1) genes in the amygdala complex and of Cnr1 in the prefrontal cortex of socially isolated rats when compared to controls, and these changes appear to be partially driven by trimethylation of Lysine 27 and acetylation of Lysine 9 at Histone 3. The alterations of Cnr1 transcriptional regulation result also directly correlated with those of oxytocin receptor gene. We here suggest that to counteract the effects of SI, it is of relevance to restore the endocannabinoid system homeostasis via the use of environmental triggers able to revert those epigenetic mechanisms accounting for the alterations observed.

Neurexin1α knockout in rats causes aberrant social behaviour: relevance for autism and schizophrenia

RATIONALE

Genetic and environmental factors cause neuropsychiatric disorders through complex interactions that are far from understood. Loss-of-function mutations in synaptic proteins like neurexin1α have been linked to autism spectrum disorders (ASD) and schizophrenia (SCZ), both characterised by problems in social behaviour. Childhood social play behaviour is thought to facilitate social development, and lack of social play may precipitate or exacerbate ASD and SCZ.

OBJECTIVE

To test the hypothesis that an environmental insult acts on top of genetic vulnerability to precipitate psychiatric-like phenotypes. To that aim, social behaviour in neurexin1α knockout rats was assessed, with or without deprivation of juvenile social play. We also tested drugs prescribed in ASD or SCZ to assess the relevance of this dual-hit model for these disorders.

RESULTS

Neurexin1α knockout rats showed an aberrant social phenotype, with high amounts of social play, increased motivation to play, age-inappropriate sexual mounting, and an increase in general activity. Play deprivation subtly altered later social behaviour, but did not affect the phenotype of neurexin1α knockout rats. Risperidone and methylphenidate decreased play behaviour in both wild-type and knockout rats. Amphetamine-induced hyperactivity was exaggerated in neurexin1α knockout rats.

CONCLUSION

Deletion of the neurexin1α gene in rats causes exaggerated social play, which is not modified by social play deprivation. This phenotype therefore resembles disinhibited behaviour rather than the social withdrawal seen in ASD and SCZ. The neurexin1α knockout rat could be a model for inappropriate or disinhibited social behaviour seen in childhood mental disorders.

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.

Prenatal allergic inflammation in rats confers sex-specific alterations to oxytocin and vasopressin innervation in social brain regions

Prenatal exposure to inflammation via maternal infection, allergy, or autoimmunity increases one's risk for developing neurodevelopmental and psychiatric disorders. Many of these disorders are associated with altered social behavior, yet the mechanisms underlying inflammation-induced social impairment remain unknown. We previously found that a rat model of acute allergic maternal immune activation (MIA) produced deficits like those found in MIA-linked disorders, including impairments in juvenile social play behavior. The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) regulate social behavior, including juvenile social play, across mammalian species. OT and AVP are also implicated in neuropsychiatric disorders characterized by social impairment, making them good candidate regulators of social deficits after MIA. We profiled how acute prenatal exposure to allergic MIA changed OT and AVP innervation in several brain regions important for social behavior in juvenile male and female rat offspring. We also assessed whether MIA altered additional behavioral phenotypes related to sociality and anxiety. We found that allergic MIA increased OT and AVP fiber immunoreactivity in the medial amygdala and had sex-specific effects in the nucleus accumbens, bed nucleus of the stria terminalis, and lateral hypothalamic area. We also found that MIA reduced ultrasonic vocalizations in neonates and increased the stereotypical nature of self-grooming behavior. Overall, these findings suggest that there may be sex-specific mechanisms underlying MIA-induced behavioral impairment and underscore OT and AVP as ideal candidates for future mechanistic studies.

Behavioral responses to natural rewards in developing male and female rats

Reward deficits are a hallmark feature of multiple psychiatric disorders and often recapitulated in rodent models useful for the study of psychiatric disorders, including those employing early life stress. Moreover, rodent studies have shown sex differences during adulthood in response to natural and drug rewards under normative conditions and in stress-based rodent models. Yet, little is known about the development of reward-related responses under normative conditions, including how these may differ in rats of both sexes during early development. Comparing reward-related behavioral responses between developing male and female rats may be useful for understanding how these processes may be affected in rodent models relevant to psychiatric disorders. To this end, we tested behavioral responses to natural rewards in male and female rats using sucrose consumption, sweet palatable food intake and social play tests at two timepoints (peripuberty, adolescence). Our results suggest comparable responses to consummatory and social rewards in male and female rats during peripuberty and adolescence as no sex differences were found for sucrose preference, chocolate candy intake or a subset of play behaviors (dorsal contacts, pins). These findings suggest that sex differences in response to these natural rewards emerge and may be more robust during adulthood.

Social play exclusion model in adolescent rats: Monitoring locomotor and emotional behavior associated with social play and examining c-Fos expression in the brain

The exclusion of social play within an adolescent group interferes with learning and the acquisition of essential social behavior during development and can cause modulations in the social brain areas. However, despite the importance of social play in adolescence, an in-depth explanation of its physiological mechanisms is limited because of the lack of experimental animal models that embody social play exclusion in human society. To determine the mechanism of social play in adolescence, we identified differences in emotional behavior and brain activity in animal models of social play exclusion that mimicked human society. Emotional changes in the social play exclusion and non-exclusion groups were examined by tracking social play-related social interaction behavior, social play-related space preference, social play-related locomotor behavior, and anxiety-like behavior using a behavioral data analysis program. Differences in brain activity among groups were identified using immunohistochemical staining. During the social play exclusion model, the rats preferred the partition zone to the other areas in the test chamber. The exclusion group preferred the partition and the center zone over the non-exclusion group. When comparing before and after the social play exclusion, the exclusion group showed a decrease in mobility and an increase in anxiety-like behavior compared to the non-exclusion group. We found that c-Fos expression in the dentate gyrus (DG) of the exclusion group was lower than that in the non-exclusion group, whereas c-Fos expression in the lateral habenula (LHb) of the exclusion group was higher than that in the non-exclusion group. Taken together, in adolescence, exclusion from social play with peers can increase anxiety-like behavior in the exclusion group and change the neuronal activity of the DG and LHb, suggesting that exclusion from social play is linked to modifications in the DG and LHb, which are regions associated with mood regulation.

Adulthood effects of developmental exercise in rats

Exercise is known to promote efficient function of stress circuitry. The developing brain is malleable and thus exercise during adolescence could potentially exert lasting beneficial effects on the stress response that would be detectable in adulthood. The current study determined whether adolescent wheel running was associated with reduced stress response in adulthood, 6 weeks after cessation of exercise. Male and female adolescent rats voluntarily ran for 6 weeks and then were sedentary for 6 weeks prior to 10 days of chronic restraint stress in adulthood. Fecal corticosterone levels were measured during stress, and escape from the restraint tube was assessed on the final day as a proxy for depressive-like behavior. Anxiety-like behavior was measured 24 h later with the elevated plus maze and locomotor behaviors with the open field. Brain and body measurements were taken immediately following behavioral testing. Developmental exercise and adulthood stress both exerted independent effects on physiological and behavioral outcomes in adulthood. Exercise history increased the odds ratio of escape from restraint stress in males, but did not influence other stress-induced behaviors. In summary, exercise early in life exerted lasting effects, but did not substantially alter the adulthood response to restraint stress.

Shaping the development of complex social behavior

Early life experiences can have an enduring impact on the brain and behavior, with implications for stress reactivity, cognition, and social behavior. In particular, the neural systems that contribute to the expression of social behavior are altered by early life social environments. However, paradigms that have been used to alter the social environment during development have typically focused on exposure to stress, adversity, and deprivation of species-typical social stimulation. Here, we explore whether complex social environments can shape the development of complex social behavior. We describe lab-based paradigms for studying early life social complexity in rodents that are generally focused on enriching the social and sensory experiences of the neonatal and juvenile periods of development. The impact of these experiences on social behavior and neuroplasticity is highlighted. Finally, we discuss the degree to which our current approaches for studying social behavior outcomes give insight into "complex" social behavior and how social complexity can be better integrated into lab-based methodologies.

Play behavior in ectothermic vertebrates

Until a few decades ago, play was considered a behavior unique to birds and mammals. Although play in other vertebrates is still a neglected research subject, data on it has been slowly accumulating, and are reviewed here. Now we know that animals as diverse as stingrays, cichlid fishes, monitor lizards, softshell turtles, and crocodiles can be unexpectedly playful. This knowledge has implications to broader theoretical problems, but much more research attention is needed to utilize its potential.

Social isolation leads to mild social recognition impairment and losses in brain cellularity

Chronic social stress is a significant risk factor for several neuropsychiatric disorders, mainly major depressive disorder (MDD). In this way, patients with clinical depression may display many symptoms, including disrupted social behavior and anxiety. However, like many other psychiatric diseases, MDD has a very complex etiology and pathophysiology. Because social isolation is one of the multiple depression-inducing factors in humans, this study aims to understand better the link between social stress and MDD using an animal model based on social isolation after weaning, which is known to produce social stress in mice. We focused on cellular composition and white matter integrity to establish possible links with the abnormal social behavior that rodents isolated after weaning displayed in the three-chamber social approach and recognition tests. We used the isotropic fractionator method to assess brain cellularity, which allows us to robustly estimate the number of oligodendrocytes and neurons in dissected brain regions. In addition, diffusion tensor imaging (DTI) was employed to analyze white matter microstructure. Results have shown that post-weaning social isolation impairs social recognition and reduces the number of neurons and oligodendrocytes in important brain regions involved in social behavior, such as the anterior neocortex and the olfactory bulb. Despite the limitations of animal models of psychological traits, evidence suggests that behavioral impairments observed in patients might have similar biological underpinnings.

Brief and long maternal separation in C57Bl6J mice: behavioral consequences for the dam and the offspring

Introduction

Animal models, especially rodents, have become instrumental to experimentally investigate the effects of an adverse post-natal environment on the developing brain. For this purpose, maternal separation (MS) paradigms have been widely used in the last decades. Nonetheless, how MS affects maternal behavior and, ultimately, the offspring depend on multiple variables.

Methods

To gain further insights into the consequences of MS, we decided to thoroughly measure and compare the effects of short (15 min, 3 times/day) vs. long (3 h, 1 time/day) separation on multiple maternally-associated behaviors and across the entire post-natal period.

Results

Compared to unhandled control litters, our results confirmed previous studies and indicated that SMS enhanced the time and variety of maternal care whereas LMS resulted in poor caregiving. We also showed that SMS-accrued caregiving persisted during the whole post-natal period. In contrast, LMS effects on maternal behavior were restricted to the early life (P2-P10). Finally, we also analyzed the behavioral consequences of these different rearing social environments on the offspring. We found that MS has profound effects in social tasks. We showed that affiliative touch, a type of prosocial behavior that provides comfort to others, is particularly sensitive to the modification of maternal caregiving.

Discussion

Our results provide further support to the contention that interactions during the early post-natal period critically contribute to emotional processing and brain co-construction.

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.

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.

The practicality of practice: A model of the function of play behaviour

The function of play has been a long-debated topic in animal behaviour. One popular class of accounts is that play offers practice for serious adult behaviour, but little has been done to model the circumstances where this could be true. In this paper, we model an individual who, over the juvenile and subadult ontogenetic periods, has a choice between three behaviours: foraging, playing and rest, where playing improves an individual's ability in some component of a serious adult behaviour. Using stochastic dynamic programming, we show that even when play is more energetically costly and an inferior form of practice than foraging itself, it still may be optimal to play under a variety of circumstances. We offer several instantiations of the play as practice concept to show the possibility of play improving a variety of different adult abilities: antipredatory, foraging and reproductive behaviour. These models show the environmental conditions where play might be expected, as well as the predicted occurrences of play throughout ontogeny. This is a first step in showing the ecological feasibility of the practice hypothesis of play and raises further questions about why playful activity is more beneficial than more deliberate directed practice.

A review of interspecific social play among nonhuman animals

BROOKS, H.J.B. & G.M. Burghardt. A comparative review of interspecific social play among nonhuman animals. NEUROSCI BIOBEHAV REV XX(X) XXX-XXX, XXXX.- Few species play socially with another species, hereafter called interspecific social play (ISP). ISP involves reading and responding appropriately to social cues of other species, often taxonomically remote, and has implications for perception, communication, and cognition. We reviewed information on non-human ISP from both print media and videos from YouTube and Reddit. We found over 200 instances of ISP. The literature predominantly featured wild primates, carnivores, and marine mammals. Carnivores and terrestrial ungulates were common in videos. ISP in avian and reptile species were found in both sources, including instances of playing with mammals. Animals may engage in ISP because it is risky and stimulating, they lack age-appropriate conspecifics, the play motivation is high, or to maintain social bonds in mixed-species groups. Cataloguing ISP uncovers which species are interacting and how. Systematic studies of ISP are difficult and many reports are brief and anecdotal. Minimally, future research should record information about each observation, including the age, sex, and history of participants.

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.