Social play experience in juvenile rats is indispensable for appropriate socio-sexual behavior in adulthood in males but not females

Maternal exposure to buprenorphine, but not methadone, during pregnancy reduces social play behavior across two generations of offspring

RATIONALE

The prevalence of newborns exposed to medications for opioid use disorder (MOUD), such as methadone or buprenorphine, during pregnancy is increasing. The opioid system plays a crucial role in regulating and shaping social behavior, and children prenatally exposed to opioids face an increased risk of developing behavioral problems. However, the impact of prenatal exposure to MOUD on offspring's social behavior during adolescence and adulthood, as well as potential intergenerational effects, remains largely unexplored.

OBJECTIVES

Our study employed a translationally relevant animal model to investigate how maternal (F0) exposure to MOUD during pregnancy affects social behavior in young and adult rats across the first (F1) and second (F2) generation of offspring.

METHODS

Female Sprague-Dawley rats were implanted with an osmotic minipump delivering methadone (10 mg/kg/day), buprenorphine (1 mg/kg/day), or sterile water, prior to mating with drug-naïve males. Adult F1 females were mated with treatment-matched F1 males to generate F2 offspring. We assessed social play behavior in juvenile offspring, and social interaction behavior in a three-chamber social interaction test in young adults of the F1 and F2 generations.

RESULTS

Maternal exposure to buprenorphine, but not methadone, during pregnancy reduced social play behavior in both F1 and F2 offspring, expressed by a reduced number of pounces and pins, which are the two most characteristic parameters of social play in rats. Adult social interactions were unaffected by prenatal MOUD exposure across both generations.

CONCLUSIONS

Maternal exposure to buprenorphine during pregnancy may have adverse effects on social play behavior across two generations of offspring.

To Play or Not to Play? Effects of Playmate Familiarity and Social Isolation on Social Play Engagement in Three Laboratory Rat Strains

Social play is a motivating and rewarding behavior displayed by juveniles of many mammalian species, including humans and rats. Social play is vital to the development of social skills. Autistic children show less social play engagement which may contribute to their impairments in social skills. There is limited knowledge about what external conditions may positively or negatively influence social play engagement in humans or other animals. Therefore, we determined how two common external conditions, playmate familiarity and social isolation, modulate social play levels and social play defense tactics in juveniles of three common laboratory rat strains: Long-Evans, Sprague-Dawley, and Wistar. Males and females were socially isolated for either 2h or 48h prior to social play testing and were then exposed to either a familiar (cage mate) or novel playmate, creating four testing conditions: 2h-Familiar, 48h-Familiar, 2h-Novel, and 48h-Novel. Both playmate familiarity and social isolation length influenced social play behavior levels and tactics in juvenile rats, but did so differently for each of the three rat strains. Long-Evans played most with a familiar playmate, irrespective of time isolated, Sprague-Dawley played most in the 48h-Familiar condition, and Wistar played the least in the 2h-Familiar condition, but Wistar played more with a novel playmate than Long-Evans and Sprague-Dawley. Analysis of social play tactics by the playmates in response to nape attacks by the experimental rats revealed strain differences with novel playmates. Here, Sprague-Dawley and Wistar defended more nape attacks than Long-Evans. Sprague-Dawley evaded these attacks, thereby shortening body contact. In contrast, Wistar turned to face their playmate attacker and showed more complete rotations, thereby extending body contact and wrestling longer. Role reversals, which increase social play reciprocity and reflect the quality of social play, were higher in Long-Evans and Sprague-Dawley with familiar playmates. Role reversals decreased for Sprague-Dawley but increased for Wistar after 48h isolation. The effects of playmate familiarity or social isolation length on social play levels and tactics were similar across sex for all three strains. In conclusion, we showed that two common external factors (playmate familiarity and social isolation length) that largely vary across social play studies have a major impact on the level and quality of social play in the three rat strains. Strain differences indicate higher level and quality of social play with familiar playmates in Long-Evans, with familiar playmates after short isolation in Sprague-Dawley, and with novel playmates after longer isolation for Wistar. Future research could determine whether strain differences in neuronal mechanisms underlie these condition-induced variations in social play engagement. Our findings are also informative in suggesting that external conditions like playmate familiarity and social isolation length could influence social play levels and social play quality in typical and atypical children.

The transcriptome of playfulness is sex-biased in the juvenile rat medial amygdala: a role for inhibitory neurons

Social play is a dynamic behavior known to be sexually differentiated; in most species, males play more than females, a sex difference driven in large part by the medial amygdala (MeA). Despite the well-conserved nature of this sex difference and the importance of social play for appropriate maturation of brain and behavior, the full mechanism establishing the sex bias in play is unknown. Here, we explore "the transcriptome of playfulness" in the juvenile rat MeA, assessing differences in gene expression between high- and low-playing animals of both sexes via bulk RNA-sequencing. Using weighted gene co-expression network analysis (WGCNA) to identify gene modules combined with analysis of differentially expressed genes (DEGs), we demonstrate that the transcriptomic profile in the juvenile rat MeA associated with playfulness is largely distinct in males compared to females. Of the 13 play-associated WGCNA networks identified, only two were associated with play in both sexes, and very few DEGs associated with playfulness were shared between males and females. Data from our parallel single-cell RNA-sequencing experiments using amygdala samples from newborn male and female rats suggests that inhibitory neurons drive this sex difference, as the majority of sex-biased DEGs in the neonatal amygdala are enriched within this population. Supporting this notion, we demonstrate that inhibitory neurons comprise the majority of play-active cells in the juvenile MeA, with males having a greater number of play-active cells than females, of which a larger proportion are GABAergic. Through integrative bioinformatic analyses, we further explore the expression, function, and cell-type specificity of key play-associated modules and the regulator "hub genes" predicted to drive them, providing valuable insight into the sex-biased mechanisms underlying this fundamental social behavior.

Behavioral neuroscience's inevitable SABV growing pains

The field of rodent behavioral neuroscience is undergoing two major sea changes: an ever-growing technological revolution, and worldwide calls to consider sex as a biological variable (SABV) in experimental design. Both have enormous potential to improve the precision and rigor with which the brain can be studied, but the convergence of these shifts in scientific practice has exposed critical limitations in classic and widely used behavioral paradigms. While our tools have advanced, our behavioral metrics - mostly developed in males and often allowing for only binary outcomes - have not. This opinion article explores how this disconnect has presented challenges for the accurate depiction and interpretation of sex differences in brain function, arguing for the expansion of current behavioral constructs to better account for behavioral diversity.

Feminization of social play behavior depends on microglia

Many sex differences in brain and behavior are established developmentally by the opposing processes of feminization and masculinization, which manifest following differential steroid hormone exposure in early life. The cellular mechanisms underlying masculinization are well-documented, a result of the fact that it is steroid-mediated and can be easily induced in newborn female rodents via exogenous steroid treatment. However, the study of feminization of particular brain regions has largely been relegated to being "not masculinization" given the absence of an identified initiating trigger. As a result, the mechanisms of this key developmental process remain elusive. Here we describe a novel role for microglia, the brain's innate immune cell, in the feminization of the medial amygdala and a complex social behavior, juvenile play. In the developing amygdala, microglia promote proliferation of astrocytes equally in both sexes, with no apparent effect on rates of cell division, but support cell survival selectively in females through the trophic actions of Tumor Necrosis Factor α (TNFα). We demonstrate that disrupting TNFα signaling, either by depleting microglia or inhibiting the associated signaling pathways, prevents the feminization of astrocyte density and increases juvenile play levels to that seen in males. This data, combined with our previous finding that male-like patterns of astrocyte density are sculpted by developmental microglial phagocytosis, reveals that sexual differentiation of the medial amygdala involves opposing tensions between active masculinization and active feminization, both of which require microglia but are achieved via distinct processes.

RNA-sequencing reveals a shared neurotranscriptomic profile in the medial preoptic area of highly social songbirds and rats

Rough-and-tumble play in juvenile rats and song in flocks of adult songbirds outside a breeding context (gregarious song) are two distinct forms of non-sexual social behavior. Both are believed to play roles in the development of sociomotor skills needed for later life-history events, including reproduction, providing opportunities for low-stakes practice. Additionally, both behaviors are thought to be intrinsically rewarded and are associated with a positive affective state. Given the functional similarities of these behaviors, this study used RNA-sequencing to identify commonalities in their underlying neurochemical systems within the medial preoptic area. This brain region is implicated in multiple social behaviors, including song and play, and is highly conserved across vertebrates. DESeq2 and rank-rank hypergeometric overlap analyses identified a shared neurotranscriptomic profile in adult European starlings singing high rates of gregarious song and juvenile rats playing at high rates. Transcript levels for several glutamatergic receptor genes, such as GRIN1, GRIN2A, and GRIA1, were consistently upregulated in highly gregarious (i.e., playful/high singing) animals. This study is the first to directly investigate shared neuromodulators of positive, non-sexual social behaviors across songbirds and mammals. It provides insight into a conserved brain region that may regulate similar behaviors across vertebrates.

Play partner preferences among groups of unfamiliar juvenile male rats

Like many mammals, as juveniles, rats engage in play fighting, which in the laboratory is typically studied in dyads, and consequently, it is the researcher who determines a rat's play partner. In real-life conditions, a rat would have many partners with whom to play. In a previous study, we found that rats do prefer to play with some individuals more than others, and surprisingly, when given the choice, unfamiliar partners are preferred to familiar ones. In this study, we assessed partner choice when all the available partners are strangers. Eight groups of six unfamiliar juvenile male rats were observed for 10 min play trials. One of the six in each group was selected as the 'focal' rat and his play towards, and received by, the others were scored. Social networks revealed that five of the eight groups formed preferences, with preferred partners also engaging in more play with the focal rat. The mechanism by which these preferences were formed remains to be determined, but it seems that there are individual differences, potentially in the amount and style of play, that allow an individual to select the most suitable partner from a group of strangers.

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.

Conspecific interactions predict social transmission of fear in female rats

Social transmission of fear occurs in a subset of individuals, where an Observer displays a fear response to a previously neutral stimulus after witnessing or interacting with a conspecific Demonstrator during memory retrieval. The conditions under which fear can be acquired socially in rats have received attention in recent years, and suggest that social factors modulate social transmission of information. We previously found that one such factor, social rank, impacts fear conditioning by proxy in male rats. Here, we aimed to investigate whether social roles as determined by nape contacts in females, might also have an influence on social transmission of fear. In-line with previous findings in males, we found that social interactions in the home cage can provide insight into the social relationship between female rats and that these relationships predict the degree of fear acquired by-proxy. These results suggest that play behavior affects the social transfer/transmission of information in female 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.

Adolescent social isolation increases binge-like alcohol drinking in male but not female high-alcohol-preferring mice

AIMS

This study examined how adolescent social isolation affects adult binge-like alcohol drinking and stress-axis function, via basal levels of circulating corticosterone (CORT), in male and female mice with a genetic predisposition toward high alcohol preference (HAP).

METHODS

Male and female HAP2 mice were randomly assigned to a group-housed or social isolation (ISO) group. Social isolation began at postnatal Days 40-42 and lasted for 21 days prior to assessment of binge-like alcohol drinking using a 4-day drinking-in-the-dark (DID) procedure. Blood samples to assess basal CORT were taken 6 days after social isolation ended and 24 h before DID started, and again 60 h after DID ended, during the light portion of the light cycle.

RESULTS

Adolescent social isolation increased adult binge-like alcohol drinking in male but not female mice. All groups showed significantly lower CORT after DID compared to before DID. Pearson bivariate correlation coefficients between the first 2 h of grams-per-kilogram alcohol intake on Day 4 and CORT levels indicated a significant positive correlation in ISO males only after DID and negative correlations in ISO females before and after DID.

CONCLUSIONS

These findings demonstrate that adolescent social isolation increased binge-like alcohol drinking in male but not female adult HAP2 mice. Stress-axis adaptations in male HAP2 mice may be associated with the social-isolation-induced increase in binge-like alcohol drinking.

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.

Neural Control of Sexually Dimorphic Social Behavior: Connecting Development to Adulthood

Rapid advances in the neural control of social behavior highlight the role of interconnected nodes engaged in differential information processing to generate behavior. Many innate social behaviors are essential to reproductive fitness and therefore fundamentally different in males and females. Programming these differences occurs early in development in mammals, following gonadal differentiation and copious androgen production by the fetal testis during a critical period. Early-life programming of social behavior and its adult manifestation are separate but yoked processes, yet how they are linked is unknown. This review seeks to highlight that gap by identifying four core mechanisms (epigenetics, cell death, circuit formation, and adult hormonal modulation) that could connect developmental changes to the adult behaviors of mating and aggression. We further propose that a unique social behavior, adolescent play, bridges the preweaning to the postpubertal brain by engaging the same neural networks underpinning adult reproductive and aggressive behaviors.

Maturation of a cortical-amygdala circuit limits sociability in male rats

Prefrontal cortical maturation coincides with adolescent transitions in social engagement, suggesting that it influences social development. The anterior cingulate cortex (ACC) is important for social interaction, including ACC outputs to the basolateral amygdala (BLA). However, little is known about ACC-BLA sensitivity to the social environment and if this changes during maturation. Here, we used brief (2-hour) isolation to test the immediate impact of changing the social environment on the ACC-BLA circuit and subsequent shifts in social behavior of adolescent and adult rats. We found that optogenetic inhibition of the ACC during brief isolation reduced isolation-driven facilitation of social interaction across ages. Isolation increased activity of ACC-BLA neurons across ages, but altered the influence of ACC on BLA activity in an age-dependent manner. Isolation reduced the inhibitory impact of ACC stimulation on BLA neurons in a frequency-dependent manner in adults, but uniformly suppressed ACC-driven BLA activity in adolescents. This work identifies isolation-driven alterations in an ACC-BLA circuit, and the ACC itself as an essential region sensitive to social environment and regulates its impact on social behavior in both adults and adolescents.

Postweaning cafeteria diet induces a short-term metabolic disfunction and a differential vulnerability to develop anxiety-like and depressive-like behaviors in male but not female rats

Children and adolescents are high consumers of Western diets (rich in fat and sugars), which is a risk factor for overweight and obesity. Moreover, the presence of anxiety and depression among this population has increased significantly. This study explores in young postweaning rats the association between Western diet consumption and the development of metabolic and behavioral disturbances. At postnatal day (PN) 24, Wistar rats of both sexes were weaned and assigned to a control or cafeteria diet (CAF) group. After short-term exposure, a group of rats was euthanized at PN31 to obtain abdominal fat pads and blood samples. Another group of rats was tested in the open-field test, splash test, anhedonia test, and social play across 11 days (PN32-42). The CAF groups exhibited a significantly high level of body fat, serum glucose, triglycerides, leptin, and HOMA index when compared to the control groups. Only CAF males exhibited anxiety-like and depression-like behavior. Present results indicate that postweaning short-term exposure to a CAF diet has immediate detrimental effects on metabolism in both sexes. However, only CAF males showed mood disturbances. This study provides evidence that a CAF diet exerts immediate effects on behavior and metabolism in the postweaning period and that sexes present differential vulnerability.