The neurobiology of social play and its rewarding value in rats

Opioid Regulation of Social Homeostasis: Connecting Loneliness to Addiction

Loneliness heightens the risk of substance use disorder, and a desire to escape this negative feeling motivates drug use. Opioid drugs in particular are believed to target neurobiological circuits involved in social bonding, increasing vulnerability to opioid addiction when social connectedness is lacking. In this narrative review, we consider how current understanding of μ opioid modulation of reward and threat processing across domains sheds light on the mechanisms that link loneliness and substance use. We discuss evidence for state- and context-dependent μ opioid modulation of social affect and behaviors, which appears to promote prioritization of high-value reward options also in the context of threat. Tying this literature to the model of social homeostasis, we argue for a role of μ opioids in regulating social homeostasis across species. Finally, we explore how disruption of social homeostasis in chronic opioid use contributes to continued drug use. We highlight how increasing patients' psychosocial resources and opportunities for social bonding can improve recovery from drug addiction. Throughout, we consider the translational robustness and generalizability of the nonhuman animal evidence in light of existing human research.

Operant social seeking to a novel peer after social isolation is associated with activation of nucleus accumbens shell in rats

RATIONALE AND OBJECTIVE

Deprivation of social interaction promotes social reward seeking in rodents, assessed primarily by the conditioned place preference procedure. Here, we used an operant social procedure in rats and examined the effect of the housing condition (pair-housing vs. single-housing) during or after social self-administration on social reward seeking.

METHODS

We first trained paired-housed or single-housed rats to gain access to an age- and sex-matched novel peer. On post-training day 1 (PTD1), we tested both groups for social seeking without the presence of the novel peer. Next, we divided each group into pair-housing or single-housing conditions and tested all four groups (pair-pair, pair-single, single-pair, and single-single) for social seeking on post-training day 12 (PTD12). Finally, we analyzed Fos expression in the striatum associated with social seeking on PTD12.

RESULT

Single-housed rats earned more social rewards during social self-administration than pair-housed rats. Social isolation during social self-administration also promoted social seeking on PTD1 and PTD12, regardless of their housing conditions after social self-administration training. Additionally, in pair-housed rats, social isolation during the post-training period led to a time-dependent increase of social seeking on PTD12 compared with PTD1. Finally, the Fos analyses revealed an increase of Fos expression in NAc shell of single-single rats after social seeking test on PTD12 compared with pair-pair rats.

CONCLUSION

Our data suggest that social isolation promotes operant social self-administration and social seeking. In addition, neuronal activation of NAc shell is associated with social seeking after social isolation.

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.

Mapping the landscape of social behavior

Social interaction is integral to animal behavior. However, lacking tools to describe it in quantitative and rigorous ways has limited our understanding of its structure, underlying principles, and the neuropsychiatric disorders, like autism, that perturb it. Here, we present a technique for high-resolution 3D tracking of postural dynamics and social touch in freely interacting animals, solving the challenging subject occlusion and part-assignment problems using 3D geometric reasoning, graph neural networks, and semi-supervised learning. We collected over 110 million 3D pose samples in interacting rats and mice, including seven monogenic autism rat lines. Using a multi-scale embedding approach, we identified a rich landscape of stereotyped actions, interactions, synchrony, and body contacts. This high-resolution phenotyping revealed a spectrum of changes in autism models and in response to amphetamine not resolved by conventional measurements. Our framework and large library of interactions will facilitate studies of social behaviors and their neurobiological underpinnings.

Integrating the COM-B model into behavioral neuroscience: A framework for understanding animal behavior

In light of the intricate nature of animal behavior regulation, a theoretical model is proposed, grounded in the COM-B (Capability, Opportunity, Motivation - Behavior) framework, which has gained considerable traction in the domain of human behavioral intervention. When extending the COM-B model to behavioral neuroscience, we first discuss the utility of the model in animal research, particularly its capacity to integrate environmental and social factors, and enhance cross-species comparisons, including animal-to-human translations and evolutionary considerations. The subsequent discussion then summarizes the advantages of utilizing the COM-B model in neuroscience are summarized, including the facilitation of a systems-level understanding of behavior and the establishment of a link between neural mechanisms and specific behavioral components. The experimental design for the application of the COM-B model in neuroscience is proposed to elucidate the brain regulatory processes that govern behavior. Finally, three specific examples are provided to illustrate the theoretical considerations, namely feeding and social behavior, and the role of neuromodulators in the control of behavior.

The Serotonergic Control of Play Fighting in Male Juvenile Hamsters: Opposite Effects of 5-HT1A and 5-HT3 Receptor Manipulations

In male hamsters, puberty is associated with increased serotonin innervation and unusual responses to fluoxetine, such as enhanced play-fighting activity against intruders but also an acceleration of its maturation from attacks focused on the face (frontal attacks) to the lower belly and rump, suggesting a role for serotonin (5-HT). We tested the role of 5-HT1A and 5-HT3 receptor subtypes on play-fighting behavior observed during resident intruder tests through peripheral treatment with receptor agonists and antagonists. Contrary to observations in adult hamsters, we did not observe any overarching effects of treatment on measures of play-fighting activity, nor its maturation from frontal attacks. However, secondary analyses highlighted variability within the datasets. A subgroup of animals presented inhibited play-fighting activity in response to treatment with DPAT, a 5-HT1A receptor agonist, but these animals also showed enhanced locomotor activity and reduced interest in engaging their opponents. In addition, early juvenile agonistic behavior was predictive of responsiveness to other treatments. The 5-HT1A receptor antagonist, WAY, caused a reduction in play-fighting activity in high attackers and an increase in low attackers. Though high attackers under pretest conditions were equally inhibited by CBG, a 5-HT3 receptor agonist, they performed a higher proportion of frontal attacks. Finally, the density of 5-HT1A and 5-HT3 receptor immunoreactivity was compared among subjects sampled at postnatal Day 35 (early puberty) or postnatal Day 70 (adulthood) within areas mediating the control of social behavior in adults. Adult males showed a higher density of immunolabeling for 5-HT1A receptors in the anterior hypothalamus and medial amygdala, as well as 5-HT3 receptors in the lateral septum. The data suggest that the development of 5-HT receptor expression participates in the control of play-fighting activity and its maturation during puberty in male hamsters.

Timing matters: modeling the effects of gestational cannabis exposure on social behavior and microglia in the developing amygdala

Cannabis is the most frequently used illicit drug during pregnancy, with use steadily increasing in the United States as legalization and decriminalization expand to more states. Many pregnant individuals use cannabis to reduce adverse symptoms of pregnancy, considering it to be less harmful than other pharmaceuticals or alcohol. The primary psychoactive component of cannabis, delta-9-tetrahydrocannabinol (THC), acts on the endocannabinoid (eCB) system, yet whether it perturbs neural development of the fetus is poorly understood. Previously we have shown that androgen mediated eCB tone in the developing amygdala promotes microglial phagocytosis of newborn astrocytes which has enduring consequences on the neural circuits regulating sex differences in social behavior. Microglia are the resident immune cells of the brain and express both receptors of the eCB system, CB1R and CB2R, making them likely targets of modulation by THC. It is also plausible that exposure to THC at differing gestational timepoints can result in distinct outcomes, as is the case with alcohol exposure. To model human cannabis use during either late or early pregnancy, we exposed rodents to THC either directly during the early postnatal period via intraperitoneal (IP) injection or in utero during the prenatal period via dam IP injection respectively. Here we show that postnatal THC exposure results in sex specific changes in microglial phagocytosis during development as well as social behavior during the juvenile period. Interestingly prenatal exposure to THC resulted in inverse changes to phagocytosis and social behavior. These findings highlight the differential effects of THC exposure across gestation.

The role of the dopamine D1 receptor in anticipatory pleasure and social play

Social play is a highly rewarding activity seen across mammalian species that is vital for neurobehavioural development. Dysfunctions in social play are seen across psychiatric and neurodevelopmental disorders positing the importance of understanding the neurobiological mechanisms underlying social play. A multitude of neurotransmitter systems have been implicated in social play, with the present study focused on the role of dopamine, specifically the dopamine D1 receptor. Pharmacological manipulations of dopamine and the D1 receptor reveal mixed findings. Given the limited selectivity of pharmacological tools, we explored the role of the dopamine D1 receptor in social play using dopamine D1 mutant (DAD1-/-) rats which have a genetic reduction in functional D1 receptors. Aligning with the rewarding properties of social play, the present study also examined anticipatory behaviour for the opportunity to engage in social play. Contrary to our predictions, DAD1-/- mutants initiated and engaged in social play similarly to wildtype controls with only subtle differences in specific elements of play behaviour. Subjects did not differ in 50 kHz vocalisations emitted during play, suggesting similar levels of consummatory pleasure. Although subjects initiated and engaged in play similarly, as predicted, DAD1-/- mutants displayed deficits in anticipatory behaviour and pleasure for the opportunity to engage in social play. These findings support a prominent role of the D1 receptor in anticipatory behaviour, with further research needed to elucidate its role in social play.

Playful brains: a possible neurobiological pathway to cognitive health in aging

Healthy cognitive aging emphasizes preserving cognitive functions essential for independence and well-being. Developing interventions that promote cognition and resilience in older individuals is crucial. Social playfulness, characterized by spontaneity and mutual enjoyment, allows individuals to step away from routine roles and engage in novel and surprising exchanges. Emerging evidence suggests that social playfulness is a promising approach for supporting cognitive functions in aging in a joyful and engaging way. In this theory and hypothesis manuscript, we propose a neurobiological pathway mediating the effects of social playfulness on cognition. Playful interactions generate high levels of uncertainty, requiring continuous adaptation and exploration. We suggest that these demands engage the locus coeruleus-noradrenaline (LC-NA) system, which is crucial for navigating uncertainty and sustaining arousal and flexibility needed to adapt to the dynamic and unpredictable nature of playful interactions. Importantly, the collaborative and safe environment of playfulness transforms this uncertainty-driven noradrenergic activation into an engaging and rewarding experience, enhancing focus, positive affect, and flexibility. In older adults, where LC-NA functionality may decline with age, social playfulness could counteract cognitive decline by upregulating this system. We review evidence linking LC-NA integrity to cognitive health and explore how playfulness might mitigate the deterioration of cognitive functioning by training executive functions and promoting novelty and exploration. This framework bridges neuroscience, cognitive psychology, and creative-arts therapies, highlighting social playfulness as a tool for healthy aging. We emphasize the need for further research to validate this hypothesis and explore its implications for designing interventions that leverage social playfulness to enhance cognitive resilience in older populations.

Exploring the role of orexins in the modulation of social reward

RATIONALE

positive social interactions are essential for mental health, by offering emotional support, reducing stress levels, and promoting resilience against drugs of abuse effects. However, not all individuals perceive social interaction as rewarding.

OBJECTIVES

the goal of this study was to investigate whether the modulation of the orexin system can shift passive coping and non-social behavior (vulnerable) to active coping and social behavior (resilient). This knowledge is primordial for stress- and addiction-related disorders, and for other psychiatric disorders involving impairment in social interaction.

METHODS

male C57/BL6N mice categorized into social and non-social groups, received injections of SB334867, a selective orexin 1 receptor (OX1R) antagonist, before the conditioning sessions with a male conspecific of the same weight and age.

RESULTS

our results from the conditioned place preference test (CPP) show that SB334867 has no effect on social preference in non-social mice, but it reduces their stress levels and depression-like behavior. These effects appear to be due to a higher OX1R expression in the basolateral amygdala (BLA), a stress-related brain area, of non-social mice compared to their social counterparts.

CONCLUSIONS

these data suggest that the orexin system may be a target to alleviate stress and depression-like behavior in non-social individuals rather than to promote social reward.

Exploring the effect of play on heart rate variability as a measure of positive emotional states in pigs

Play behaviour has been suggested to be inherently rewarding for animals, inducing positive emotional states. The psychophysiological effect of emotions can be assessed through heart rate variability (HRV), serving as a proxy measure of sympathovagal balance. This study investigated how the performance of play influences heart rate (HR) and HRV parameters (RMSSD, SDNN) in pigs. It was hypothesized that HRV would increase during and immediately after play due to predominant vagal activation compared to baseline, indicating a positive emotional state. Gilts (n = 32, 18 and 19 weeks of age), housed in standard partly-slatted pens, were selected from two pen-level play treatments: Novelty (NOV) and Play Pen (PLP). Play treatment pigs were reared with intermittent play promotion (3x/week) from 10 weeks of age. For HRV recordings, play was promoted for 15-min in pairs of gilts within treatment, with destructible novel objects given either in the home pen (NOV, 1 m2/pig), or in an enclosed 'playpen' area providing extra space (PLP, 2.9 m2/ pig). HRV was measured during a play session in three consecutive periods: (i) baseline (before play bout, no play occurring), (ii) play bout (play expression), and (iii) after-play (immediately after play bout). Twenty-six gilts played at least once. Play bouts lasted between 10 and 30 s (10 s: n = 60, 20 s: n = 18, 30 s: n = 6). In 10-s bouts, compared to baseline, RMSSD was higher during play (p = 0.027) and after-play (p = 0.015), while SDNN increased during play (p ≤ 0.001) and after-play (p = 0.008) only with ambulation (pig moving forward: walking or running). HR did not differ across periods but was higher at ambulation (p = 0.003). Twenty-sec bouts followed the same relationship with only numerical differences, while HRV in 30-s bouts did not differ. Treatments did not influence HRV. Results suggest that engaging in play increases HRV, with this effect persisting into the period immediately after play. This indicates that play contributes to positive emotional states in pigs. Physical activity involved in play influences HRV. More dynamic and energetic play involving ambulation might be more rewarding for pigs. This study provides evidence for assessing positive emotions in pigs and underscores the importance of incorporating rewarding experiences into conventional farming practices.

Recognising depression in non-human primates: a narrative review of reported signs of depression

Major depressive disorder (depression) is a highly heterogenous human mental disorder that may have equivalents in non-human animals. Research into non-human depression teaches us about human depression and can contribute to enhance welfare of non-human animals. Here, we narratively review how signs of depression in non-human primates (NHPs) can be observed based on symptoms of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Furthermore, we propose diagnostic criteria of NHP depression and we review reports on signs of depression in NHPs. We diagnose an NHP with depression when it shows a core sign (depressed mood or anhedonia) alongside at least three other DSM-5-derived signs of depression. Results show that four out of six observable signs of depression are present in NHPs, occasionally lasting for months. However, only a group of six NHPs in one study met our proposed criteria for a diagnosis of depression. We call for more research into the co-occurrence of depressive symptoms in individual NHPs to establish the prevalence of depression in NHPs.

A, Guadarrama Gándara CG, Escobar C, Guerrero-Vargas NN. Maternal exposure to dim light at night induces behavioral alterations in the adolescent and adult offspring Wistar rat

Introduction

Access to electric light has exposed living organisms to varying intensities of light throughout the 24 h day. Dim light at night (DLAN) is an inappropriate signal for the biological clock, which is responsible for the circadian organization of physiology. During the gestational period, physiological adaptations occur to ensure a successful pregnancy and optimal fetal development. Environmental maternal conditions, such as disruptions of maternal circadian rhythms, could negatively affect offspring health. We have previously demonstrated that exposure of female Wistar rats to DLAN results in circadian, metabolic, and behavioral alterations. A relevant behavior during adolescence is social play, primarily regulated by the nucleus accumbens (NAc) which is crucial for the proper performance of important behaviors in adulthood. Throughout development, microglia are responsible for the remodeling of diverse brain regions via synaptic pruning. During adolescence, this process occurs within the NAc, where immune-mediated remodeling directly impacts social play behavior.

Methods

This study investigated the effects of maternal exposure to DLAN or a light-dark cycle (LD) before (5 weeks) and during the gestational period (21-23 days) on the metabolism and behavior of offspring in adolescence and adulthood. Body mass was measured every 5 days from postnatal day 1 (PN1) to PN25 and every 10 days from PN40 to PN90; food consumption was monitored weekly from PN40 to PN90. Social play behavior was evaluated at PN40. The quantification and morphology of microglia in the NAc were measured on PN30. An open field test was conducted at PN60, and anhedonia test was assessed at PN90.

Results and discussion

Male and female offspring from mothers exposed to DLAN showed increased body mass gain at PN25. DLAN male offspring had lower food consumption, while DLAN females exhibited increased food consumption. In social play behavior, no differences were found between DLAN and LD female offspring. In contrast, DLAN male offspring exhibited a significant decrease in social play behavior compared to LD animals, which was associated with higher numbers of microglia in the NAc that had more ramified morphology. Importantly, at PN90, DLAN offspring presented increased anxiety-like behaviors. These results demonstrate that DLAN exposure induces intergenerational behavioral alterations that persist until adulthood.

Possible Worlds Theory: How the Imagination Transcends and Recreates Reality

The imagination is central to human social life but undervalued worldwide and underexplored in psychology. Here, we offer Possible Worlds Theory as a synthetic theory of the imagination. We first define the imagination, mapping the mental states it touches, from dreams and hallucinations to satire and fiction. The conditions that prompt people to imagine range from trauma to physical and social deprivation, and they challenge the sense of reality, stirring a need to create possible worlds. We theorize about four cognitive operations underlying the structure of the mental states of the imagination. We then show how people embody the imagination in social behaviors such as pretense and ritual, which give rise to experiences of a special class of feelings defined by their freedom from reality. We extend Possible Worlds Theory to four domains-play, spirituality, morality, and art-and show how in flights of the imagination people create new social realities shared with others.

Tackling Hominin Tickling: Bonobos Share the Social Features and Developmental Dynamics of Play Tickling With Humans

It is under debate whether intersubjectivity-the capacity to experience a sense of togetherness around an action-is unique to humans. In humans, heavy tickling-a repeated body probing play that causes an automatic response including uncontrollable laughter (gargalesis)-has been linked to the emergence of intersubjectivity as it is aimed at making others laugh (self-generated responses are inhibited), it is often asymmetrical (older to younger subjects), and it elicits agent-dependent responses (pleasant/unpleasant depending on social bond). Intraspecific tickling and the related gargalesis response have been reported in humans, chimpanzees, and anecdotally in other great apes, potentially setting the line between hominids and other anthropoids. Here we investigated this phenomenon in bonobos and predicted that in this species (sharing with humans and chimpanzees the last common ancestor) the presence of tickling would be modulated depending on the players' age, play session initiators, and familiarity. In April-June 2018, we collected videos on play sessions-including tickling-on a bonobo group housed at La Vallée des Singes (France). We showed that tickling received decreased while tickling performed increased with age, with tickling being mostly directed from older to younger individuals. Moreover, tickling was mostly performed by the individuals that started the play interaction and most of it occurred in strongly bonded dyads, particularly mother-infant ones. Bonobo tickling features, especially age profile and social modulation, mirror those of heavy tickling in humans thus suggesting a common evolutionary origin and shared patterns of basic intersubjectivity in hominins.

The Incentive-Sensitization Theory of Addiction 30 Years On

The incentive-sensitization theory (IST) of addiction was first published in 1993, proposing that (a) brain mesolimbic dopamine systems mediate incentive motivation ("wanting") for addictive drugs and other rewards, but not their hedonic impact (liking) when consumed; and (b) some individuals are vulnerable to drug-induced long-lasting sensitization of mesolimbic systems, which selectively amplifies their "wanting" for drugs without increasing their liking of the same drugs. Here we describe the origins of IST and evaluate its status 30 years on. We compare IST to other theories of addiction, including opponent-process theories, habit theories of addiction, and prefrontal cortical dysfunction theories of impaired impulse control. We also address critiques of IST that have been raised over the years, such as whether craving is important in addiction and whether addiction can ever be characterized as compulsive. Finally, we discuss several contemporary phenomena, including the potential role of incentive sensitization in behavioral addictions, the emergence of addiction-like dopamine dysregulation syndrome in medicated Parkinson's patients, the role of attentional capture and approach tendencies, and the role of uncertainty in incentive motivation.

Prenatal Valproic Acid Induces Autistic-Like Behaviors in Rats via Dopaminergic Modulation in Nigrostriatal and Mesocorticolimbic Pathways

Autism spectrum disorder (ASD) is a complex developmental disorder characterized by several behavioral impairments, especially in socialization, communication, and the occurrence of stereotyped behaviors. In rats, prenatal exposure to valproic acid (VPA) induces autistic-like behaviors. Previous studies by our group have suggested that the autistic-like phenotype is possibly related to dopaminergic system modulation because tyrosine hydroxylase (TH) expression was affected. The objective of the present study was to understand the dopaminergic role in autism. Wistar rats on gestational day 12.5 received VPA (400 mg/kg) and behaviors related to rat models of ASD were evaluated in juvenile offspring. Neurochemical and genetic dopaminergic components were studied in different brain areas of both juvenile and adult rats. Prenatal VPA-induced autistic-like behaviors in comparison to a control group: decreased maternal solicitations by ultrasonic vocalizations, cognitive inflexibility and stereotyped behavior in the T-maze test, decreased social interaction and play behavior, as well as motor hyperactivity. Prenatal VPA also decreased dopamine synthesis and activity in the striatum and prefrontal cortex, as well as dopamine transporter, D1 and D2 receptors, and TH expressions. Moreover, prenatal VPA increased TH+ immunoreactive neurons of the ventral tegmental area-substantia nigra complex. In conclusion, the dopaminergic hypoactivity associated with the behavioral impairments exhibited by the rats that received prenatal VPA suggests the important role of this system in the establishment of the characteristic symptoms of ASD in juvenile and adult males. Dopamine was demonstrated to be an important biomarker and a potential pharmacological target for ASD.

The jeong and haan of Vincent van Gogh: neuropeptides of bondedness and loss

We introduce two Korean-named yet transcultural feelings, jeong and haan, to fill gaps in neuroscientific understanding of mammalian bondedness, loss, and aggression. Jeong is a visceral sense of connectedness to a person, place, or thing that may arise after proximity, yet does not require intimacy. The brain opioid theory of social attachment (BOTSA) supports the idea that jeong involves increased activity of enkephalins and beta-endorphins. We propose that withdrawal of jeong-related neuropeptides leads to original haan, a sense of "missingness" that is too subtle to be grossly dysphoric. Through narrative, cognitive appraisals, or moral assignments, however, original haan may transform into the feeling of constructed haan-resentment, bitterness, grievance, sorrow, or suppressed anger. In males, the transformation may be driven by arginine vasopressin, an ancient fight-or-flight neurohormone. Constructed haan may also be driven by vasopressin in females, though data is more sparse, and in both sexes it may depend on situational or societal context. Endogenous opioids inhibit vasopressin, so that when jeong diminishes, vasopressin release may become disinhibited. This relationship implies a companion to the BOTSA, which we articulate as the brain opioid and vasopressin theory of original and constructed haan (BOVTOCH). To illustrate, we reflect on borderline personality disorder, and Vincent van Gogh's self-severing of his ear while living and working with Paul Gauguin, and fearing abandonment by him; yet to understand Van Gogh more completely we also present the brain opioid theory of stable euphoric creativity (BOTSEC), to model the subjective "highs" associated with creative flow states. Together these brain opioid theories may help to explain how feelings related to social bondedness can influence a range of phenomena. For example, opioid drug dependence may be, at least partly, a maladaptive response to feelings of isolation or disconnectedness; the health protective effects of social bonds could be related to tonic exposure to endogenous opioids and their anti-inflammatory properties; endogenous opioid-based social relational enhancement may contribute to placebo responding. Finally we conclude by pointing out the possibility of virtuous cycles of social connectedness and creativity, when feelings of bondedness and euphoric flow reinforce one another through endogenous opioid elevation.

Rat Models in Post-Traumatic Stress Disorder Research: Strengths, Limitations, and Implications for Translational Studies

Post-Traumatic Stress Disorder (PTSD) is a multifaceted psychiatric disorder triggered by traumatic events, leading to prolonged psychological distress and varied symptoms. Rat models have been extensively used to explore the biological, behavioral, and neurochemical underpinnings of PTSD. This review critically examines the strengths and limitations of commonly used rat models, such as single prolonged stress (SPS), stress-re-stress (S-R), and predator-based paradigms, in replicating human PTSD pathology. While these models provide valuable insights into neuroendocrine responses, genetic predispositions, and potential therapeutic targets, they face challenges in capturing the full complexity of PTSD, particularly in terms of ethological relevance and translational validity. We assess the degree to which these models mimic the neurobiological and behavioral aspects of human PTSD, highlighting areas where they succeed and where they fall short. This review also discusses future directions in refining these models to improve their utility for translational research, aiming to bridge the gap between preclinical findings and clinical applications.

Social play in African savannah elephants may inform selection against aggression

In social groups, competition for individual advantage is balanced with cooperation, for the collective benefit. Selection against aggression has favored cooperation and non-aggressive competitive strategies. Because social play is a behavioral system that fluctuates between cooperation and competition, selection against aggression might have especially influenced this behavior. African savannah elephants (Loxodonta africana) are a low aggressive species, therefore suitable to investigate this aspect. We collected all occurrences observational audio-video data on social play, aggression/threats, and affiliation on an African elephant colony housed in a 25-ha open space at Parque de la Naturaleza de Cabarceno (Cantabria, Spain) and composed of 4 family groups (3 immature males, 3 immature females, and 7 adult females) and 2 adult males. Anticipating the influence of reduced aggression, we found that social play decreased with age, persisting in adults, and that it was highest in males. Social play was associated with affiliation (informing cooperation). Indeed, individuals that were central in the social play network were also central in the affiliation network. For immature subjects, we found a correlation between social play and affiliation sociomatrices. However, such correlation was absent in adults and social play mostly occurred between families. Despite the limitations related to dealing with a small captive group, this study largely supports the idea that the features of social play in African savannah elephants may be related to low aggression. This investigation hints toward a non-purely cooperative use of play, possibly as a non-aggressive interaction that accommodates different levels of cooperation and competition.

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.

Compositional pretraining improves computational efficiency and matches animal behavior on complex tasks

1Recurrent neural networks (RNN) are ubiquitously used in neuroscience to capture both neural dynamics and behaviors of living systems. However, when it comes to complex cognitive tasks, training RNNs with traditional methods can prove difficult and fall short of capturing crucial aspects of animal behavior. Here we propose a principled approach for identifying and incorporating compositional tasks as part of RNN training. Taking as target a temporal wagering task previously studied in rats, we design a pretraining curriculum of simpler cognitive tasks that reflect relevant sub-computations. We show that this pretraining substantially improves learning efficacy and is critical for RNNs to adopt similar strategies as rats, including long-timescale inference of latent states, which conventional pretraining approaches fail to capture. Mechanistically, our pretraining supports the development of slow dynamical systems features needed for implementing both inference and value-based decision making. Overall, our approach is an important step for endowing RNNs with relevant inductive biases, which is important when modeling complex behaviors that rely on multiple cognitive computations.

Mapping the landscape of social behavior

Social interaction is integral to animal behavior. However, we lack tools to describe it with quantitative rigor, limiting our understanding of its principles and neuropsychiatric disorders, like autism, that perturb it. Here, we present a technique for high-resolution 3D tracking of postural dynamics and social touch in freely interacting animals, solving the challenging subject occlusion and part assignment problems using 3D geometric reasoning, graph neural networks, and semi-supervised learning. We collected over 140 million 3D postures in interacting rodents, featuring new monogenic autism rat lines lacking reports of social behavioral phenotypes. Using a novel multi-scale embedding approach, we identified a rich landscape of stereotyped actions, interactions, synchrony, and body contact. This enhanced phenotyping revealed a spectrum of changes in autism models and in response to amphetamine that were inaccessible to conventional measurements. Our framework and large library of interactions will greatly facilitate studies of social behaviors and their neurobiological underpinnings.

Nuancing 'Emotional' Social Play: Does Play Behaviour Always Underlie a Positive Emotional State?

This review focuses on social play, a complex behaviour that is often difficult to categorize. Although play has been typically associated with positive emotional states, a thorough examination of the literature indicates that it may relate to different emotional systems, from attachment to conflict. Play oscillates between competition and cooperation, and includes a spectrum in between; thus, quantitatively identifying and demonstrating the emotional nature of play remains challenging. We considered examples from human and non-human animal studies and explored the emotional and neuro-hormonal systems involved in play. We assessed ethological data possibly indicating the emotional states underlying play, and we focused on the cooperative and competitive elements of play. We investigated the relationship between play and affiliative/aggressive behaviours, the communicative meaning of play signals (especially primate play faces), and the motor and possibly emotional contagion function of rapid motor mimicry during play. From all the literature on play, this review selects and combines studies in an innovative way to present the methods (e.g., play indices and social network analysis), tools (e.g., sequential analysis and facial coding software), and evidence indicative of the emotional states underlying play, which is much more complex than previously thought.

A subset of dopamine receptor-expressing neurons in the nucleus accumbens controls feeding and energy homeostasis

Orchestrating complex behaviors, such as approaching and consuming food, is critical for survival. In addition to hypothalamus neuronal circuits, the nucleus accumbens (NAc) also controls appetite and satiety. However, specific neuronal subtypes of the NAc that are involved and how the humoral and neuronal signals coordinate to regulate feeding remain incompletely understood. Here we decipher the spatial diversity of neuron subtypes of the NAc shell (NAcSh) and define a dopamine receptor D1-expressing and Serpinb2-expressing subtype controlling food consumption in male mice. Chemogenetics and optogenetics-mediated regulation of Serpinb2+ neurons bidirectionally regulate food seeking and consumption specifically. Circuitry stimulation reveals that the NAcShSerpinb2→LHLepR projection controls refeeding and can overcome leptin-mediated feeding suppression. Furthermore, NAcSh Serpinb2+ neuron ablation reduces food intake and upregulates energy expenditure, resulting in reduced bodyweight gain. Our study reveals a neural circuit consisting of a molecularly distinct neuronal subtype that bidirectionally regulates energy homeostasis, providing a potential therapeutic target for eating disorders.

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.

Roles of AMPA receptors in social behaviors

As a crucial player in excitatory synaptic transmission, AMPA receptors (AMPARs) contribute to the formation, regulation, and expression of social behaviors. AMPAR modifications have been associated with naturalistic social behaviors, such as aggression, sociability, and social memory, but are also noted in brain diseases featuring impaired social behavior. Understanding the role of AMPARs in social behaviors is timely to reveal therapeutic targets for treating social impairment in disorders, such as autism spectrum disorder and schizophrenia. In this review, we will discuss the contribution of the molecular composition, function, and plasticity of AMPARs to social behaviors. The impact of targeting AMPARs in treating brain disorders will also be discussed.

Social Isolation Induces Changes in the Monoaminergic Signalling in the Rat Medial Prefrontal Cortex

(1) Background: The effects of short-term social isolation during adulthood have not yet been fully established in rats behaviourally, and not at all transcriptomically in the medial prefrontal cortex (mPFC). (2) Methods: We measured the behavioural effects of housing adult male rats in pairs or alone for 10 days. We also used RNA sequencing to measure the accompanying gene expression alterations in the mPFC of male rats. (3) Results: The isolated animals exhibited reduced sociability and social novelty preference, but increased social interaction. There was no change in their aggression, anxiety, or depression-like activity. Transcriptomic analysis revealed a differential expression of 46 genes between the groups. The KEGG pathway analysis showed that differentially expressed genes are involved in neuroactive ligand-receptor interactions, particularly in the dopaminergic and peptidergic systems, and addiction. Subsequent validation confirmed the decreased level of three altered genes: regulator of G protein signalling 9 (Rgs9), serotonin receptor 2c (Htr2c), and Prodynorphin (Pdyn), which are involved in dopaminergic, serotonergic, and peptidergic function, respectively. Antagonizing Htr2c confirmed its role in social novelty discrimination. (4) Conclusions: Social homeostatic regulations include monoaminergic and peptidergic systems of the mPFC.

A non-adaptationist hypothesis of play behaviour

Play is a suite of apparently non-functional, pleasurable behaviours observed in human and non-human animals. Although the phenomenon has been studied extensively, no adaptationist behavioural theory of how play evolved can be supported by the available evidence. However, the advancement of the extended evolutionary synthesis and developments in systems biology offer alternative avenues for non-adaptationist physiological hypotheses. I therefore propose a hypothesis of play, based upon a complex ACh activity that is under agential control of the organism, whereby play initiates ACh-mediated feedforward and feedback processes which act to: (i) regulate metabolic processes; (ii) form new ACh receptors via ACh mRNA activity; (iii) mediate attention, memory consolidation and learning; and (iv) mediate social behaviours, reproduction and embryonic development. However, play occurs across taxa, but does not occur across all taxonomic groups or within all species of a taxonomic group. Thus, to support the validity of the proposed hypothesis, I further propose potential explanations for this anomaly, which include sampling and observer biases, altricial versus precocial juvenile development, and the influence of habitat niche and environmental conditions on behaviour. The proposed hypothesis thus offers new avenues for study in both the biological and social sciences, in addition to having potential applications in applied sciences, such as animal welfare and biomedical research. Crucially, it is hoped that this hypothesis will promote further study of a valid and behaviourally significant, yet currently enigmatic, biological phenomenon.

Neurophysiological analysis of disadvantageous social inequity: Exploring emotional behavior changes and c-Fos expression in a male rat model

Humans and other animals exhibit aversive behavioral and emotional responses to unequal reward distributions compared with their conspecifics. Despite the significance of this phenomenon, experimental animal models designed to investigate social inequity aversion and delve into the underlying neurophysiological mechanisms are limited. In this study, we developed a rat model to determine the effects of socially equal or unequal reward and stress on emotional changes in male rats. During the training session, the rats were trained to escape when a sound cue was presented, and they were assigned to one of the following groups: all escaping rats [advantageous equity (AE)], freely moving rats alongside a restrained rat [advantageous inequity (AI)], all restrained rats [disadvantageous equity (DE)], and a rat restrained in the presence of freely moving companions [disadvantageous inequity (DI)]. During the test session, rats in the advantageous group (AE and AI) escaped after the cue sound (expected reward acquisition), whereas rats in the disadvantageous group (DE and DI) could not escape despite the cue being presented (expected reward deprivation). Emotional alteration induced by exposure to restraint stress under various social interaction circumstances was examined using an open field test. Notably, the DI group displayed reduced exploration of the center zone during the open field tests compared with the other groups, indicating heightened anxiety-like behaviors in response to reward inequity. Immunohistochemical analysis revealed increased c-Fos expression in the medial prefrontal and orbitofrontal cortices, coupled with reduced c-Fos expression in the striatum and nucleus accumbens under DI conditions, in contrast to the other experimental conditions. These findings provide compelling evidence that rats are particularly sensitive to reward inequity, shedding light on the neurophysiological basis for distinct cognitive processes that manifest when individuals are exposed to social equity and inequity situations.

Communal nesting shapes the sex-dependent glutamatergic response to early life stress in the rat prefrontal cortex

Introduction

Early social environment, either positive or negative, shapes the adult brain. Communal nesting (CN), a naturalistic setting in which 2-3 females keep their pups in a single nest sharing care-giving behavior, provides high level of peer interaction for pups. Early social isolation (ESI) from dam and siblings represents, instead, an adverse condition providing no peer interaction.

Methods

We investigated whether CN (enrichment setting) might influence the response to ESI (impoverishment setting) in terms of social behavior and glutamate system in the medial prefrontal cortex (mPFC) of adult and adolescent male and female rats.

Results

Pinning (a rewarding component of social play behavior) was significantly more pronounced in males than in females exposed to the combination of CN and ESI. CN sensitized the glutamate synapse in the mPFC of ESI-exposed male, but not female, rats. Accordingly, we observed (i) a potentiation of the glutamatergic neurotransmission in the mPFC of both adolescent and adult males, as shown by the recruitment of NMDA receptor subunits together with increased expression/activation of PSD95, SynCAM 1, Synapsin I and αCaMKII; (ii) a de-recruiting of NMDA receptors from active synaptic zones of same-age females, together with reduced expression/activation of the above-mentioned proteins, which might reduce the glutamate transmission. Whether similar sex-dependent glutamate homeostasis modulation occurs in other brain areas remains to be elucidated.

Discussion

CN and ESI interact to shape social behavior and mPFC glutamate synapse homeostasis in an age- and sex-dependent fashion, suggesting that early-life social environment may play a crucial role in regulating the risk to develop psychopathology.

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.

Psychostimulants and social behaviors

Mounting evidence from animal models and human studies indicates that psychostimulants can significantly affect social behaviors. This is not surprising considering that the neural circuits underlying the regulation and expression of social behaviors are highly overlapped with those targeted by psychostimulants, which in most cases have strong rewarding and, consequently, addictive properties. In the present work, we provide an overview regarding the effects of illicit and prescription psychostimulants, such as cocaine, amphetamine-type stimulants, methylphenidate or modafinil, upon social behaviors such as social play, maternal behavior, aggression, pair bonding and social cognition and how psychostimulants in both animals and humans alter them. Finally, we discuss why these effects can vary depending on numerous variables such as the type of drug considered, acute versus long-term use, clinical versus recreational consumption, or the presence or absence of concomitant risk factors.

Female mice prenatally exposed to valproic acid exhibit complex and prolonged social behavior deficits

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized mainly by deficits in social communication and stereotyped and restricted behavior and interests with a male to female bias of 4.2/1. Social behavior in ASD animal models is commonly analyzed in males, and seldomly in females, using the widely implemented three-chambers test procedure. Here, we implemented a novel procedure, the Live Mouse Tracker (LMT), combining artificial intelligence, machine learning procedures and behavioral measures. We used it on mice that were prenatally exposed to valproic acid (VPA) (450 mg/kg) at embryonic day 12.5, a widely recognized and potent ASD model that we had previously extensively characterized. We focused on female mice offspring, in which social deficits have been rarely documented when using the 3-CT procedure. We recorded several parameters related to social behavior in these mice, continuously for three days in groups of four female mice. Comparisons were made on groups of 4 female mice with the same treatment (4 saline or 4 VPA) or with different treatments (3 saline and 1 VPA). We report that VPA females show several types of social deficits, which are different in nature and magnitude in relation with time. When VPA mice were placed in the LMT alongside saline mice, their social deficits showed significant improvement as early as 1 h from the start of the experiment, lasting up to 3 days throughout the duration of the experiment. Our findings suggest that ASD may be underdiagnosed in females. They also imply that ASD-related social deficits can be ameliorated by the presence of typical individuals.

Increasing CB2 Receptor Activity after Early Life Stress Prevents Depressive Behavior in Female Rats

Early adversity, the loss of the inhibitory GABAergic interneuron parvalbumin, and elevated neuroinflammation are associated with depression. Individuals with a maltreatment history initiate medicinal cannabis use earlier in life than non-maltreated individuals, suggesting self-medication. Female rats underwent maternal separation (MS) between 2 and 20 days of age to model early adversity or served as colony controls. The prelimbic cortex and behavior were examined to determine whether MS alters the cannabinoid receptor 2 (CB2), which has anti-inflammatory properties. A reduction in the CB2-associated regulatory enzyme MARCH7 leading to increased NLRP3 was observed with Western immunoblots in MS females. Immunohistochemistry with stereology quantified numbers of parvalbumin-immunoreactive cells and CB2 at 25, 40, and 100 days of age, revealing that the CB2 receptor associated with PV neurons initially increases at P25 and subsequently decreases by P40 in MS animals, with no change in controls. Confocal and triple-label microscopy suggest colocalization of these CB2 receptors to microglia wrapped around the parvalbumin neuron. Depressive-like behavior in MS animals was elevated at P40 and reduced with the CB2 agonist HU-308 or a CB2-overexpressing lentivirus microinjected into the prelimbic cortex. These results suggest that increasing CB2 expression by P40 in the prelimbic cortex prevents depressive behavior in MS female rats.

Different Effects of Peer Sex on Operant Responding for Social Interaction and Striatal Dopamine Activity

When rats are given discrete choices between social interactions with a peer and opioid or psychostimulant drugs, they choose social interaction, even after extensive drug self-administration experience. Studies show that like drug and nondrug food reinforcers, social interaction is an operant reinforcer and induces dopamine release. However, these studies were conducted with same-sex peers. We examined if peer sex influences operant social interaction and the role of estrous cycle and striatal dopamine in same- versus opposite-sex social interaction. We trained male and female rats (n = 13 responders/12 peers) to lever-press (fixed-ratio 1 [FR1] schedule) for 15 s access to a same- or opposite-sex peer for 16 d (8 d/sex) while tracking females' estrous cycle. Next, we transfected GRAB-DA2m and implanted optic fibers into nucleus accumbens (NAc) core and dorsomedial striatum (DMS). We then retrained the rats for 15 s social interaction (FR1 schedule) for 16 d (8 d/sex) and recorded striatal dopamine during operant responding for a peer for 8 d (4 d/sex). Finally, we assessed economic demand by manipulating FR requirements for a peer (10 d/sex). In male, but not female rats, operant responding was higher for the opposite-sex peer. Female's estrous cycle fluctuations had no effect on operant social interaction. Striatal dopamine signals for operant social interaction were dependent on the peer's sex and striatal region (NAc core vs DMS). Results indicate that estrous cycle fluctuations did not influence operant social interaction and that NAc core and DMS dopamine activity reflect sex-dependent features of volitional social interaction.

A coming-of-age story: adult neurogenesis or adolescent neurogenesis in rodents?

It is surprising that after more than a century using rodents for scientific research, there are no clear, consensual, or consistent definitions for when a mouse or a rat becomes adult. Specifically, in the field of adult hippocampal neurogenesis, where this concept is central, there is a trend to consider that puberty marks the start of adulthood and is not uncommon to find 30-day-old mice being described as adults. However, as others discussed earlier, this implies an important bias in the perceived importance of this trait because functional studies are normally done at very young ages, when neurogenesis is at its peak, disregarding middle aged and old animals that exhibit very little generation of new neurons. In this feature article we elaborate on those issues and argue that research on the postnatal development of mice and rats in the last 3 decades allows to establish an adolescence period that marks the transition to adulthood, as occurs in other mammals. Adolescence in both rat and mice ends around postnatal day 60 and therefore this age can be considered the onset of adulthood in both species. Nonetheless, to account for inter-individual, inter-strain differences in maturation and for possible delays due to environmental and social conditions, 3 months of age might be a safer option to consider mice and rats bona fide adults, as suggested by The Jackson Labs.

Influence of maternal immune activation on autism-like symptoms and coping strategies in male offspring

Maternal immune activation (MIA) caused by exposure to pathogens or inflammation during critical periods of gestation increased susceptibility to neurodevelopmental disorders, including autism, in the offspring. In the present work, we aimed to provide characterization of the long-term consequences on anxiety-like behavior and cardiovascular stress response of MIA in the offspring. This study aimed to evaluate the effect of MIA by lipopolysaccharide (LPS) in adult male offspring. In our study, the animals were subjected to a range of behavioral and physiological tests, including the elevated plus maze, social interaction, cat odor response, open field behavior, contextual fear conditioning, and cardiovascular responses during restraint stress. In the offspring of MIA, our study unveiled distinct anxious behaviors. This was evident by fewer entries into the open arms of the maze, diminished anti-thigmotaxis in the open field, and a decrease in social interaction time. Moreover, these rats showed heightened sensitivity to cat odor, exhibited prolonged freezing during fear conditioning, and presented elevated 22 Hz ultrasonic vocalizations. Notably, during restraint stress, these animals manifested an augmented blood pressure response, and this was associated with an increase in c-fos expression in the locus coeruleus compared to the control group. These findings collectively underline the extensive behavioral and physiological alterations stemming from MIA. This study deepens our understanding of the significance of maternal health in predisposing offspring to neurobehavioral deficits and psychiatric disorders.

Social Interaction in Adolescent Rats with Neonatal Ethanol Exposure: Impact of Sex and CE-123, a Selective Dopamine Reuptake Inhibitor

Children with fetal alcohol spectrum disorders (FASDs) demonstrate deficits in social functioning that contribute to early withdrawal from school and delinquency, as well as the development of anxiety and depression. Dopamine is involved in reward, motivation, and social behavior. Thus, we evaluated whether neonatal ethanol exposure (in an animal model of FASDs) has an impact on social recognition memory using the three-chamber social novelty discrimination test during early and middle adolescence in male and female rats, and whether the modafinil analog, the novel atypical dopamine reuptake inhibitor CE-123, can modify this effect. Our study shows that male and female rats neonatally exposed to ethanol exhibited sex- and age-dependent deficits in social novelty discrimination in early (male) and middle (female) adolescence. These deficits were specific to the social domain and not simply due to more general deficits in learning and memory because these animals did not exhibit changes in short-term recognition memory in the novel object recognition task. Furthermore, early-adolescent male rats that were neonatally exposed to ethanol did not show changes in the anxiety index but demonstrated an increase in locomotor activity. Chronic treatment with CE-123, however, prevented the appearance of these social deficits. In the hippocampus of adolescent rats, CE-123 increased BDNF and decreased its signal transduction TrkB receptor expression level in ethanol-exposed animals during development, suggesting an increase in neuroplasticity. Thus, selective dopamine reuptake inhibitors, such as CE-123, represent interesting drug candidates for the treatment of deficits in social behavior in adolescent individuals with FASDs.

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.

Dopamine D2 receptors in pyramidal neurons in the medial prefrontal cortex regulate social behavior

Drugs acting on dopamine D2 receptors are widely used for the treatment of several neuropsychiatric disorders, including schizophrenia and depression. Social deficits are a core symptom of these disorders. Pharmacological manipulation of dopamine D2 receptors (Drd2), a Gi-coupled subtype of dopamine receptors, in the medial prefrontal cortex (mPFC) has shown that Drd2 is implicated in social behaviors. However, the type of neurons expressing Drd2 in the mPFC and the underlying circuit mechanism regulating social behaviors remain largely unknown. Here, we show that Drd2 were mainly expressed in pyramidal neurons in the mPFC and that the activation of the Gi-pathway in Drd2+ pyramidal neurons impaired social behavior in male mice. In contrast, the knockdown of D2R in pyramidal neurons in the mPFC enhanced social approach behaviors in male mice and selectively facilitated the activation of mPFC neurons projecting to the nucleus accumbens (NAc) during social interaction. Remarkably, optogenetic activation of mPFC-to-NAc-projecting neurons mimicked the effects of conditional D2R knockdown on social behaviors. Altogether, these results demonstrate a cell type-specific role for Drd2 in the mPFC in regulating social behavior, which may be mediated by the mPFC-to-NAc pathway.

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.

Role of environmental enrichment on social interaction, anxiety, locomotion, and memory in Wistar rats under chronic methylphenidate intake

Introduction

Chronic use of various compounds can have long-lasting effects on animal behavior, and some of these effects can be influenced by the environment. Many environmental enrichment protocols have the potential to induce behavioral changes.

Aim

The aim of the present study was to investigate how environmental enrichment can mitigate the effects of chronic methylphenidate consumption on the behavior of Wistar rats.

Methods

The animals were housed for 20 days under either an environmental enrichment protocol (which included tubes of different shapes) or standard housing conditions. After seven days, half of the rats received 13 days of oral administration of methylphenidate (2 mg/kg). After seven days, the rats underwent behavioral tests, including the elevated plus maze (anxiety), open field (locomotion), object-in-place recognition test (spatial memory), and a test for social interaction (social behavior).

Results

The results showed that the enriched environmental condition reversed the enhanced time in the open arms of the elevated plus maze induced by methylphenidate (F[1,43] = 4.275, p = 0.045). Methylphenidate also enhanced exploratory rearing in the open field (F[1,43] = 4.663, p = 0.036) and the time spent in the open area of the open field (H[3] = 8.786, p = 0.032). The enriched environment mitigated the inhibition of social interaction with peers induced by methylphenidate (H[3] = 16.755, p  < 0.001) as well as the preference for single exploratory behavior (H[3] = 9.041, p = 0.029).

Discussion

These findings suggest that environmental enrichment can counteract some of the effects of methylphenidate. These results are relevant for the clinical treatment of the long-lasting secondary effects associated with methylphenidate pharmacological treatment.

Sex and age differences in social and cognitive function in offspring exposed to late gestational hypoxia

BACKGROUND

Gestational sleep apnea is a hypoxic sleep disorder that affects 8-26% of pregnancies and increases the risk for central nervous system dysfunction in offspring. Specifically, there are sex differences in the sensitivity of the fetal hippocampus to hypoxic insults, and hippocampal impairments are associated with social dysfunction, repetitive behaviors, anxiety, and cognitive impairment. Yet, it is unclear whether gestational sleep apnea impacts these hippocampal-associated functions and if sex and age modify these effects. To examine the relationship between gestational sleep apnea and hippocampal-associated behaviors, we used chronic intermittent hypoxia (CIH) to model late gestational sleep apnea in pregnant rats. We hypothesized that late gestational CIH would produce sex- and age-specific social, anxiety-like, repetitive, and cognitive impairments in offspring.

METHODS

Timed pregnant Long-Evans rats were exposed to CIH or room air normoxia from GD 15-19. Behavioral testing of offspring occurred during either puberty or young adulthood. To examine gestational hypoxia-induced behavioral phenotypes, we quantified hippocampal-associated behaviors (social function, repetitive behaviors, anxiety-like behaviors, and spatial memory and learning), hippocampal neuronal activity (glutamatergic NMDA receptors, dopamine transporter, monoamine oxidase-A, early growth response protein 1, and doublecortin), and circulating hormones in offspring.

RESULTS

Late gestational CIH induced sex- and age-specific differences in social, repetitive, and memory functions in offspring. In female pubertal offspring, CIH impaired social function, increased repetitive behaviors, and elevated circulating corticosterone levels but did not impact memory. In contrast, CIH transiently induced spatial memory dysfunction in pubertal male offspring but did not impact social or repetitive functions. Long-term effects of gestational CIH on social behaviors were only observed in female offspring, wherein CIH induced social disengagement and suppression of circulating corticosterone levels in young adulthood. No effects of gestational CIH were observed in anxiety-like behaviors, hippocampal neuronal activity, or circulating testosterone and estradiol levels, regardless of sex or age of offspring.

CONCLUSIONS

Our results indicate that hypoxia-associated pregnancy complications during late gestation can increase the risk for behavioral and physiological outcomes in offspring, such as social dysfunction, repetitive behaviors, and cognitive impairment, that are dependent on sex and age.

Locomotor play behavior evolves by random genetic drift but not as a correlated response to selective breeding for high voluntary wheel-running behavior

Locomotor play is vigorous and seemingly purposeless behavior, commonly observed in young mammals. It can be costly in terms of energy expenditure, increased injury risk, and predator exposure. The main hypothesized benefit of locomotor play is enhancement of neuromuscular development, with effects persisting into adulthood. We hypothesized that levels of locomotor play would have evolved as a correlated response to artificial selection for increased voluntary exercise behavior. We studied mice from 4 replicate lines bred for voluntary wheel running (High Runner or HR) at 6-8 weeks of age and four non-selected Control (C) lines. Mice were weaned at 21 days of age and play behavior was observed for generations 20 (22-24 days old), 68 (22-23 days old), and 93 (15 days old). We quantified locomotor play as (1) rapid, horizontally directed jerk-run sequences and (2) vertical "bouncing." We used focal sampling to continuously record behavior in cages containing 4-6 individuals during the first 2-3 h of the dark cycle. Observations were significantly repeatable between observers and days. A two-way, mixed-model simultaneously tested effects of linetype (HR vs. C), sex, and their interaction. Contrary to our hypothesis, HR and C lines did not differ in any generation, nor did we find sex differences. However, differences among the replicate HR lines and among the replicate C lines were detected, and may be attributed to the effects of random genetic drift (and possibly founder effects). Thus, play behavior did evolve in this selection experiment, but not as a correlated response to selection for voluntary exercise.

The role of sex and drug use during adolescence in determining the risk for adverse consequences of amphetamines

Use of amphetamines during adolescence, a critical period of brain development and reorganization, may lead to particularly adverse outcomes that are long-lasting. Similarly, female users may be uniquely vulnerable to certain aspects of drug use. A recognition of the role of use during adolescence and sex on outcomes of amphetamine and methamphetamine exposure are of critical importance in understanding and treating substance use disorders. This chapter highlights what human research, which has been largely epidemiological, suggests about sex and age differences in drug use patterns and outcomes. We also discuss work in laboratory animals that has typically utilized rats or mice exposed to drugs in a non-contingent manner (i.e., involuntarily) or through volitional self-administration. Lastly, we draw attention to the fact that advancing our understanding of the effects of amphetamine and methamphetamine use, the development of problematic drug taking, and the mechanisms that contribute to relapse will require an emphasis on inclusion of age and sex as moderating factors in future studies.

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.

The Relevance of Animal Models of Social Isolation and Social Motivation for Understanding Schizophrenia: Review and Future Directions

BACKGROUND AND HYPOTHESES

Social dysfunction in schizophrenia includes symptoms of withdrawal and deficits in social skills, social cognition, and social motivation. Based on the course of illness, with social withdrawal occurring prior to psychosis onset, it is likely that the severity of social withdrawal/isolation contributes to schizophrenia neuropathology.

STUDY DESIGN

We review the current literature on social isolation in rodent models and provide a conceptual framework for its relationship to social withdrawal and neural circuit dysfunction in schizophrenia. We next review preclinical tasks of social behavior used in schizophrenia-relevant models and discuss strengths and limitations of existing approaches. Lastly, we consider new effort-based tasks of social motivation and their potential for translational studies in schizophrenia.

STUDY RESULTS

Social isolation rearing in rats produces profound differences in behavior, pharmacologic sensitivity, and neurochemistry compared to socially reared rats. Rodent models relevant to schizophrenia exhibit deficits in social behavior as measured by social interaction and social preference tests. Newer tasks of effort-based social motivation are being developed in rodents to better model social motivation deficits in neuropsychiatric disorders.

CONCLUSIONS

While experimenter-imposed social isolation provides a viable experimental model for understanding some biological mechanisms linking social dysfunction to clinical and neural pathology in schizophrenia, it bypasses critical antecedents to social isolation in schizophrenia, notably deficits in social reward and social motivation. Recent efforts at modeling social motivation using effort-based tasks in rodents have the potential to quantify these antecedents, identify models (eg, developmental, genetic) that produce deficits, and advance pharmacological treatments for social motivation.