Repeated exposure to social stress alters the development of agonistic behavior in male golden hamsters

Early life stress and altered social behaviors: A perspective across species

Childhood and adolescent affiliations guide how individuals engage in social relationships throughout their lifetime and adverse experiences can promote biological alterations that facilitate behavioral maladaptation. Indeed, childhood victims of abuse are more likely to be diagnosed with conduct or mood disorders which are both characterized by altered social engagement. A key domain particularly deserving of attention is aggressive behavior, a hallmark of many disorders characterized by deficits in reward processing. Animal models have been integral in identifying both the short- and long-term consequences of stress exposure and suggest that whether it is disruption to parental care or social isolation, chronic exposure to early life stress increases corticosterone, changes the expression of neurotransmitters and neuromodulators, and facilitates structural alterations to the hypothalamus, hippocampus, and amygdala, influencing how these brain regions communicate with other reward-related substrates. Herein, we describe how adverse early life experiences influence social behavioral outcomes across a wide range of species and highlight the long-term biological mechanisms that are most relevant to maladaptive aggressive behavior.

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

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

Lifelong enhancement of body mass from adolescent stress in male hamsters

In hamsters, exposure to stress in adulthood causes increased body weight. We addressed how social stress during puberty would impact food intake and body weight. Stressed hamsters started gaining significantly more weight than controls after only two days of stress exposure. Over a two-week period, stressed subjects gained 10% more weight and consumed more food than controls. At the end of the stress period, stressed hamsters collected nearly twice as many palatable sugar pellets from an arena than controls. Stressed subjects presented 15-20% more body fat in mesenteric, inguinal, and retroperitoneal fat pads. In order to assess the duration of these effects, we analyzed data from previous studies keeping hamsters for over two months past the stress period in puberty. Our analysis shows that stressed hamsters stopped gaining more weight after the stress period, but their body weights remained elevated for over two months, consistently weighing 10% more than their non-stressed counterparts. We also analyzed conditioning training data collected after the period of stress in late puberty and early adulthood (P56 to P70) that was part of the original studies. Training consisted of lever pressing for palatable food rewards. At these times, previously stressed hamsters retrieved similar numbers of food pellets from the conditioning chambers, suggesting no difference in appetite after the stress period. These data showing a long-lasting effect of stress on body weight may be relevant to studies on the ontogeny of lifelong obesity.

Agonistic behaviors and neuronal activation in sexually naïve female Mongolian gerbils

Agonistic interaction is important for establishing social hierarchy and determining access to limited resources. Although there are substantial studies investigating the neural mechanisms of aggressive or defensive behavior in male rodents, little attention has been paid to the mechanisms underlying agonistic behaviors in females. In the present study, we depicted patterns of agonistic behaviors in sexually naïve female Mongolian gerbils (Meriones unguiculatus) and examined the neuronal activation in the brain by Fos-immunoreactive (Fos-ir) staining. We found that the winner-loser relationship was established rapidly. Winners displayed higher levels of aggression, environmental exploration, scent marking, and self-grooming, but less defensive behavior, in comparison to losers. Several patterns of Fos-ir expression emerged following agonistic interactions. Winners had the number of Fos-ir cells in the ventrolateral subnucleus of the ventromedial hypothalamus (VMHvl) and dorsal periaqueductal grey (PAGd) more than the controls but less than the losers. Losers also had more Fos-ir cells in the paraventricular nucleus of the hypothalamus (PVN), anterior medial (BSTam) and anteriolateral (BSTal) subnuclei of the bed nucleus of the stria terminalis (BST), and the ventral subnucleus of the lateral septum (LSv), as well as less Fos-ir cells in the dentate gyrus of the hippocampus (DG), compared to the controls. In addition, the number of Fos-ir cells showed similar increases in the principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the BST and amygdala (AMYG) in both the winners and losers, compared to the controls. Together, these data illustrate the patterns of altered neuronal activation in a behavior-, social status-, and brain region-specific manner, implicating potential roles of the brain neural circuit in mediating agonistic interactions in female Mongolian gerbils.

Contemporary Pharmacotherapeutics and the Management of Aggressive Behavior in an Adolescent Animal Model of Maladaptive Aggression

Objective

Antipsychotic and anticonvulsant medications are increasingly being used as pharmacotherapeutic treatments for maladaptive aggression in youth, yet no information is available regarding whether these drugs exhibit aggression- specific suppression in preclinical studies employing adolescent animal models of maladaptive aggression. This study examined whether the commonly used antipsychotics medications haloperidol and risperidone and the anticonvulsant medication valproate exert selective aggression-suppressing effects using a validated adolescent animal model of maladaptive aggression.

Methods

Twenty-seven-day old Syrian hamsters (Mesocricetus auratus) were administered testosterone for 30 consecutive days during the first 4 weeks of adolescent development. The following day (during late adolescence), experimental animals received a single dose of haloperidol (0.00, 0.025, 0.50, 1.0 mg/kg), risperidone (0.00, 0.01, 0.03, 1.0 mg/kg), or valproate (0.00, 1.0, 5.0, 10.0 mg/kg) and tested for offensive aggression using the Resident/Intruder Paradigm. As a baseline, non-aggressive behavioral control, a separate set of pubertal hamsters was treated with sesame oil vehicle during the first 4 weeks of adolescence and then tested for aggression during late adolescence in parallel with the haloperidol, risperidone or valproate-treated experimental animals.

Results

Risperidone and valproate selectively reduced the highly impulsive and intense maladaptive aggressive phenotype in a dose-dependent fashion. While haloperidol marginally reduced aggressive responding, its effects were non-specific as the decrease in aggression was concurrent with reductions in a several ancillary (non-aggressive) behaviors.

Conclusion

These studies provide pre-clinical evidence that the contemporary pharmacotherapeutics risperidone and valproate exert specific aggression-suppressing effects in an adolescent animal model of maladaptive aggression.

Pre-adult aggression and its long-term behavioural consequences in crickets

Social experience, particularly aggression, is considered a major determinant of consistent inter-individual behavioural differences between animals of the same species and sex. We investigated the influence of pre-adult aggressive experience on future behaviour in male, last instar nymphs of the cricket Gryllus bimaculatus. We found that aggressive interactions between male nymphs are far less fierce than for adults in terms of duration and escalation. This appears to reflect immaturity of the sensory apparatus for releasing aggression, rather than the motor system controlling it. First, a comparison of the behavioural responses of nymphs and adults to mechanical antennal stimulation using freshly excised, untreated and hexane-washed antennae taken from nymphs and adults, indicate that nymphs neither respond to nor produce sex-specific cuticular semiochemicals important for releasing aggressive behaviour in adults. Second, treatment with the octopamine agonist chlordimeform could at least partially compensate for this deficit. In further contrast to adults, which become hyper-aggressive after victory, but submissive after defeat, such winner and loser effects are not apparent in nymphs. Aggressive competition between nymphs thus appears to have no consequence for future behaviour in crickets. Male nymphs are often attacked by adult males, but not by adult females. Furthermore, observations of nymphs raised in the presence, or absence of adult males, revealed that social subjugation by adult males leads to reduced aggressiveness and depressed exploratory behaviour when the nymphs become adult. We conclude that social subjugation by adults during pre-adult development of nymphs is a major determinant of consistent inter-individual behavioural differences in adult crickets.

Aggressive behavior and brain neuronal activation in sexually naïve male Mongolian gerbils

Aggressive behavior plays an important role in animal's survival and reproductive success. Although there has been growing interests in studying neural mechanisms underlying aggressive behavior using traditional laboratory animal models, little is known about mechanisms controlling naturally occurring aggression in sexually naïve animals. In the present study, we characterized aggressive behavior displayed by sexually naïve male Mongolian gerbils (Meriones unguiculatus) and examined the subsequent neuronal activation in the brain measured by Fos-immunoreactive (Fos-ir) staining. We found that resident males initiated attacks and showed intense levels of aggression (including chase, bite, offensive sideway, lunge and on-top) towards a conspecific male intruder. Furthermore, attacks from the resident males towards the intruder produced a nonrandom distribution of bites, with the most on the rump, flank, back and tail and few on the limbs, ventrum and head. In contrast, control males that were exposed to a woodblock (control for novelty) never attacked the woodblock and showed higher levels of object/environmental investigation. Male gerbils exposed to an intruder had significantly higher levels of Fos-ir density in the medial (MeA) and anterior cortical (ACo) subnuclei of the amygdala, principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the bed nucleus of the stria terminalis, ventrolateral subdivision of the ventromedial hypothalamus (VMHvl), and paraventricular nucleus of the hypothalamus (PVN), compared to control males. Together, our results indicate that sexually naïve, group housed male gerbils naturally display aggression towards conspecific strangers, and such aggressive behavior is associated with special patterns of neuronal activation in the brain.

Peripubertal stress increases play fighting at adolescence and modulates nucleus accumbens CB1 receptor expression and mitochondrial function in the amygdala

Play fighting is a highly rewarding behavior that helps individuals to develop social skills. Early-life stress has been shown to alter play fighting in rats and hamsters as well as to increase aggressive behaviors at adulthood. However, it is not known whether individual differences in stress-induced play fighting are related to differential developmental trajectories towards adult aggression. To address this question, we used a rat model of peripubertal stress (PPS)-induced psychopathology that involves increased aggression at adulthood. We report that, indeed, PPS leads to enhanced play fighting at adolescence. Using a stratification approach, we identify individuals with heightened levels of play fighting as the ones that show abnormal forms of aggression at adulthood. These animals showed as well a rapid habituation of their corticosterone responsiveness to repeated stressor exposure at peripuberty. They also showed a striking increase in mitochondrial function in the amygdala-but not nucleus accumbens-when tested ex vivo. Conversely, low, but not high players, displayed increased expression of the CB1 cannabinoid receptor in the nucleus accumbens shell. Our results highlight adolescence as a potential critical period in which aberrant play fighting is linked to the emergence of adult aggression. They also point at brain energy metabolism during adolescence as a possible target to prevent adult aggression.

Dissociation of Puberty and Adolescent Social Development in a Seasonally Breeding Species

Alongside the development of sexual characteristics and reproductive competence, adolescents undergo marked cognitive, social, and emotional development [1]. A fundamental question is whether these changes are triggered by activation of the hypothalamic-pituitary-gonadal (HPG) axis at puberty (puberty dependent) or whether they occur independently of HPG activation (puberty independent). Disentangling puberty-dependent from puberty-independent mechanisms is difficult because puberty and adolescence typically proceed concurrently. Here, we test a new approach that leverages natural adaptations of a seasonally breeding species to dissociate pubertal status from chronological age. Siberian hamsters (Phodopus sungorus) reared in a long, summer-like day length (LD) exhibit rapid pubertal development, whereas those reared in a short, winter-like day length (SD) delay puberty by several months to synchronize breeding with the following spring [2, 3]. We tested whether the SD-induced delay in puberty delays the peri-adolescent decline in juvenile social play and the rise in aggression that characterizes adolescent social development in many species [4-6] and compared the results to those obtained after prepubertal gonadectomy. Neither SD rearing nor prepubertal gonadectomy altered the age at which hamsters transitioned from play to aggression; SD-reared hamsters completed this transition prior to puberty. SD rearing and prepubertal gonadectomy, however, increased levels of play in male and female juveniles, implicating a previously unknown role for prepubertal gonadal hormones in juvenile social behavior. Levels of aggression were also impacted (decreased) in SD-reared and gonadectomized males. These data demonstrate that puberty-independent mechanisms regulate the timing of adolescent social development, while prepubertal and adult gonadal hormones modulate levels of age-appropriate social behaviors.

An acute social defeat stressor in early puberty increases susceptibility to social defeat in adulthood

Syrian hamsters readily display territorial aggression. If they lose even a single agonistic encounter, however, hamsters show striking reductions in aggressive behavior and increases in submissive behavior, a distinct behavioral change that we have previously termed conditioned defeat. This acute social defeat stressor is primarily psychological and is effective in both males and females. Therefore, we maintain that this procedure presents an ideal model for studying behavioral and physiological responses to social stress. Here, we demonstrate that social avoidance following social defeat is a particularly useful dependent measure because of its sensitivity and stability between sexes and across the estrous cycle. In addition, we demonstrate that peripubertal hamsters exposed to a single, 15min social defeat exhibit significantly more social avoidance 24h later when compared with no-defeat controls. Later, defeated and non-defeated hamsters display similar agonistic behavior in adulthood indicating that the peripubertal defeat does not alter adult territorial aggression. After experiencing an additional social defeat in adulthood, however, the hamsters that experienced the pubertal defeat respond to the adult defeat with increased social avoidance when compared with hamsters that were defeated only in adulthood and with no-defeat controls. These data are the first to show that a single social defeat in puberty increases susceptibility to later social defeat in both males and females.

Effects of the social environment during adolescence on the development of social behaviour, hormones and morphology in male zebra finches (Taeniopygia guttata)

Background

Individual differences in behaviour are widespread in the animal kingdom and often influenced by the size or composition of the social group during early development. In many vertebrates the effects of social interactions early in life on adult behaviour are mediated by changes in maturation and physiology. Specifically, increases in androgens and glucocorticoids in response to social stimulation seem to play a prominent role in shaping behaviour during development. In addition to the prenatal and early postnatal phase, adolescence has more recently been identified as an important period during which adult behaviour and physiology are shaped by the social environment, which so far has been studied mostly in mammals. We raised zebra finches (Taeniopygia guttata) under three environmental conditions differing in social complexity during adolescence - juvenile pairs, juvenile groups, and mixed-age groups - and studied males’ behavioural, endocrine, and morphological maturation, and later their adult behaviour.

Results

As expected, group-housed males exhibited higher frequencies of social interactions. Group housing also enhanced song during adolescence, plumage development, and the frequency and intensity of adult courtship and aggression. Some traits, however, were affected more in juvenile groups and others in mixed-age groups. Furthermore, a testosterone peak during late adolescence was suppressed in groups with adults. In contrast, corticosterone concentrations did not differ between rearing environments. Unexpectedly, adult courtship in a test situation was lowest in pair-reared males and aggression depended upon the treatment of the opponent with highest rates shown by group-reared males towards pair-reared males. This contrasts with previous findings, possibly due to differences in photoperiod and the acoustic environment.

Conclusion

Our results support the idea that effects of the adolescent social environment on adult behaviour in vertebrates are mediated by changes in social interactions affecting behavioural and morphological maturation. We found no evidence that long-lasting differences in behaviour reflect testosterone or corticosterone levels during adolescence, although differences between juvenile and mixed-age groups suggest that testosterone and song behaviour during late adolescence may be associated.

Corticosterone may interact with peripubertal development to shape adult resistance to social defeat

Studies of social stress in adult mice have revealed two distinct defeat-responsive behavioral phenotypes; "susceptible" and "resistant," characterized by social avoidance and social interaction, respectively. Typically, these phenotypes are observed at least 1day after the last defeat in adults, but may extend up to 30days later. The current study examined the impact of peripubertal social defeat on immediate (1day) and adult (30day) social stress phenotypes and neuroendocrine function in male C57BL/6 mice. Initially, peripubertal (P32) mice were resistant to social defeat. When the same mice were tested for social interaction again as adults (P62), two phenotypes emerged; a group of mice were characterized as susceptible evidenced by significantly lower social interaction, whereas the remaining mice exhibited normal social interaction, characteristic of resistance. A repeated analysis of corticosterone revealed that the adult (P62) resistant mice had elevated corticosterone following the social interaction test as juveniles. This was when all mice, regardless of adult phenotype, displayed equivalent levels of social interaction. Peripubertal corticosterone was positively correlated with adult social interaction levels in defeated mice, suggesting early life stress responsiveness impacts adult social behavior. In addition, adult corticotropin-releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus (PVN) was elevated in all defeated mice, but there were no differences in CRF mRNA expression between the phenotypes. Thus, there is a delayed appearance of social stress-responsive phenotypes suggesting that early life stress exposure, combined with the resultant physiological responses, may interact with pubertal development to influence adult social behavior.

The Unexpected Effects of Beneficial and Adverse Social Experiences during Adolescence on Anxiety and Aggression and Their Modulation by Genotype

Anxiety and aggression are part of the behavioral repertoire of humans and animals. However, in their exaggerated form both can become maladaptive and result in psychiatric disorders. On the one hand, genetic predisposition has been shown to play a crucial modulatory role in anxiety and aggression. On the other hand, social experiences have been implicated in the modulation of these traits. However, so far, mainly experiences in early life phases have been considered crucial for shaping anxiety-like and aggressive behavior, while the phase of adolescence has largely been neglected. Therefore, the aim of the present study was to elucidate how levels of anxiety-like and aggressive behavior are shaped by social experiences during adolescence and serotonin transporter (5-HTT) genotype. For this purpose, male mice of a 5-HTT knockout mouse model including all three genotypes (wildtype, heterozygous and homozygous 5-HTT knockout mice) were either exposed to an adverse social situation or a beneficial social environment during adolescence. This was accomplished in a custom-made cage system where mice experiencing the adverse environment were repeatedly introduced to the territory of a dominant opponent but had the possibility to escape to a refuge cage. Mice encountering beneficial social conditions had free access to a female mating partner. Afterwards, anxiety-like and aggressive behavior was assessed in a battery of tests. Surprisingly, unfavorable conditions during adolescence led to a decrease in anxiety-like behavior and an increase in exploratory locomotion. Additionally, aggressive behavior was augmented in animals that experienced social adversity. Concerning genotype, homozygous 5-HTT knockout mice were more anxious and less aggressive than heterozygous 5-HTT knockout and wildtype mice. In summary, adolescence is clearly an important phase in which anxiety-like and aggressive behavior can be shaped. Furthermore, it seems that having to cope with challenge during adolescence instead of experiencing throughout beneficial social conditions leads to reduced levels of anxiety-like behavior.

Stress and the social brain: behavioural effects and neurobiological mechanisms

Stress often affects our social lives. When undergoing high-level or persistent stress, individuals frequently retract from social interactions and become irritable and hostile. Predisposition to antisocial behaviours - including social detachment and violence - is also modulated by early life adversity; however, the effects of early life stress depend on the timing of exposure and genetic factors. Research in animals and humans has revealed some of the structural, functional and molecular changes in the brain that underlie the effects of stress on social behaviour. Findings in this emerging field will have implications both for the clinic and for society.

The Programming of the Social Brain by Stress During Childhood and Adolescence: From Rodents to Humans

The quality and quantity of social experience is fundamental to an individual's health and well-being. Early life stress is known to be an important factor in the programming of the social brain that exerts detrimental effects on social behaviors. The peri-adolescent period, comprising late childhood and adolescence, represents a critical developmental window with regard to the programming effects of stress on the social brain. Here, we discuss social behavior and the physiological and neurobiological consequences of stress during peri-adolescence in the context of rodent paradigms that model human adversity, including social neglect and isolation, social abuse, and exposure to fearful experiences. Furthermore, we discuss peri-adolescent stress as a potent component that influences the social behaviors of individuals in close contact with stressed individuals and that can also influence future generations. We also discuss the temporal dynamics programmed by stress on the social brain and debate whether social behavior alterations are adaptive or maladaptive. By revising the existing literature and defining open questions, we aim to expand the framework in which interactions among peri-adolescent stress, the social brain, and behavior can be better conceptualized.

Chronic social stress in puberty alters appetitive male sexual behavior and neural metabolic activity

Repeated social subjugation in early puberty lowers testosterone levels. We used hamsters to investigate the effects of social subjugation on male sexual behavior and metabolic activity within neural systems controlling social and motivational behaviors. Subjugated animals were exposed daily to aggressive adult males in early puberty for postnatal days 28 to 42, while control animals were placed in empty clean cages. On postnatal day 45, they were tested for male sexual behavior in the presence of receptive female. Alternatively, they were tested for mate choice after placement at the base of a Y-maze containing a sexually receptive female in one tip of the maze and an ovariectomized one on the other. Social subjugation did not affect the capacity to mate with receptive females. Although control animals were fast to approach females and preferred ovariectomized individuals, subjugated animals stayed away from them and showed no preference. Cytochrome oxidase activity was reduced within the preoptic area and ventral tegmental area in subjugated hamsters. In addition, the correlation of metabolic activity of these areas with the bed nucleus of the stria terminalis and anterior parietal cortex changed significantly from positive in controls to negative in subjugated animals. These data show that at mid-puberty, while male hamsters are capable of mating, their appetitive sexual behavior is not fully mature and this aspect of male sexual behavior is responsive to social subjugation. Furthermore, metabolic activity and coordination of activity in brain areas related to sexual behavior and motivation were altered by social subjugation.

Extended effect of chronic social defeat stress in childhood on behaviors in adulthood

Individuals exposed to social stress in childhood are more predisposed to developing psychoemotional disorders in adulthood. Here we use an animal model to determine the influence of hostile social environment in adolescence on behavior during adult life. One-month-old adolescent male mice were placed for 2 weeks in a common cage with an adult aggressive male. Animals were separated by a transparent perforated partition, but the adolescent male was exposed daily to short attacks from the adult male. After exposure to social stress, some of the adolescent mice were placed for 3 weeks in comfortable conditions. Following this rest period, stressed young males and adult males were studied in a range of behavioral tests to evaluate the levels of anxiety, depressiveness, and communicativeness with an unfamiliar partner. In addition, adult mice exposed to social stress in adolescence were engaged in agonistic interactions. We found that 2 weeks of social stress result in a decrease of communicativeness in the home cage and diminished social interactions on the novel territory. Stressed adolescents demonstrated a high level of anxiety in the elevated plus-maze test and helplessness in the Porsolt test. Furthermore, the number of dividing (BrdU-positive) cells in the subgranular zone of the dentate gyrus was significantly lower in stressed adolescents. After 3 weeks of rest, most behavioral characteristics in different tests, as well as the number of BrdU-positive cells in the hippocampus, did not differ from those of the respective control mice. However, the level of anxiety remained high in adult males exposed to chronic social stress in childhood. Furthermore, these males were more aggressive in the agonistic interactions. Thus, hostile social environment in adolescence disturbs psychoemotional state and social behaviors of animals in adult life.

From the stressed adolescent to the anxious and depressed adult: investigations in rodent models

Anxiety and depression are the most prevalent of the psychiatric disorders. The average age of onset of these disorders is in adolescence, and stressful experiences are recognized as an important pathway to such dysfunction. Until recently, however, most animal models of these disorders involved adult males. We provide a brief overview of anxiety and depression and the extent to which adolescent rodents are a valid model for their investigation, and briefly review the main measures of anxiety-like and depressive behaviour in rodents. The focus of the review is investigations in which adolescent rodents were exposed to chronic stressors, describing our research using social instability stress and that of other researchers using various social and non-social stressors. The evidence to date suggests stress in adolescence alters the trajectory of brain development, and particularly that of the hippocampus, increasing anxiety and depressive behaviour in adulthood.

Development of agonistic encounters in dominance hierarchy formation in juvenile crayfish

We have characterized the behavioural patterns of crayfish during agonistic bouts between groups of crayfish of four different body lengths (9–19, 20–32, 41–48 and 69–75 mm) to characterize changes in the patterns of agonistic encounter during development. The behaviour of both dominant and subordinate animals was analysed by single frame measurement of video recordings. Behavioural acts that occurred during agonistic bouts were categorized as one of seven types: capture, fight, contact, approach, retreat, tailflip and neutral. Dominant–subordinate relationships were formed between juvenile crayfish as early as the third stage of development. Patterns of agonistic bouts to determine social hierarchy became more aggressive during development. The dominant–subordinate relationship was usually determined after contact in crayfish of less than 20 mm and 20–32 mm in length, while several bouts of fights were necessary for crayfish of 41–48 and 69–75 mm in length. Furthermore, social hierarchy was formed more rapidly in small crayfish. In larger animals, the number of approaches by dominant animals that promoted retreat in subordinate animals increased after the establishment of the winner–loser relationship. In smaller crayfish, in contrast, no measurable changes in these behaviour patterns were observed before and after the establishment of the winner–loser relationship. With increasing body size, the probability of tailflips decreased while that of retreats increased as the submissive behavioural act of subordinate animals.

Contrasting hippocampal and amygdalar expression of genes related to neural plasticity during escape from social aggression

Social subjugation has widespread consequences affecting behavior and underlying neural systems. We hypothesized that individual differences in stress responsiveness were associated with differential expression of neurotrophin associated genes within the hippocampus and amygdala. To do this we examined the brains of hamsters placed in resident/intruder interactions, modified by the opportunity to escape from aggression. In the amygdala, aggressive social interaction stimulated increased BDNF receptor TrK(B) mRNA levels regardless of the ability to escape the aggressor. In contrast, the availability of escape limited the elevation of GluR(1) AMPA subunit mRNA. In the hippocampal CA(1), the glucocorticoid stress hormone, cortisol, was negatively correlated with BDNF and TrK(B) gene expression, but showed a positive correlation with BDNF expression in the DG. Latency to escape the aggressor was also negatively correlated with CA(1) BDNF expression. In contrast, the relationship between amygdalar TrK(B) and GluR(1) was positive with respect to escape latency. These results suggest that an interplay of stress and neurotrophic systems influences learned escape behavior. Animals which escape faster seem to have a more robust neurotrophic profile in the hippocampus, with the opposite of this pattern in the amygdala. We propose that changes in the equilibrium of hippocampal and amygdalar learning result in differing behavioral stress coping choices.

The vasopressin 1b receptor and the neural regulation of social behavior

To date, much of the work in rodents implicating vasopressin (Avp) in the regulation of social behavior has focused on its action via the Avp 1a receptor (Avpr1a). However, there is mounting evidence that the Avp 1b receptor (Avpr1b) also plays a significant role in Avp's modulation of social behavior. The Avpr1b is heavily expressed on the anterior pituitary cortiocotrophs where it acts as an important modulator of the endocrine stress response. In the brain, the Avpr1b is prominent in the CA2 region of the hippocampus, but can also be found in areas such as the paraventricular nucleus of the hypothalamus and the olfactory bulb. Studies that have employed genetic knockouts or pharmacological manipulation of the Avpr1b point to the importance of central Avpr1b in the modulation of social behavior. However, there continues to be a knowledge gap in our understanding of where in the brain this is occurring, as well as how and if the central actions of Avp acting via the Avpr1b interact with the stress axis. In this review we focus on the genetic and pharmacological studies that have implicated the Avpr1b in the neural regulation of social behaviors, including social forms of aggressive behavior, social memory, and social motivation. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Juvenile social subjugation induces a sex-specific pattern of anxiety and depression-like behaviors in adult rats

Child abuse is the most significant environmental risk factor for the development of mood disorders, which occur twice as frequently in women as in men. To determine whether juvenile social subjugation (JSS) of rats induces mood disorder-like symptoms, we exposed 28 day-old male and female rats to daily aggressive acts from aggressive male residents. Each rat received pins, kicks, and dominance postures from the resident for 10 min per day for 10 days. When the rats were adults, we tested their anxiety- and depression-like behaviors. In addition, we measured circulating basal and stress-evoked corticosterone (CORT) levels, and weighed the adrenal glands. Although the amount of JSS was indistinguishable between males and females, females were nonetheless more severely affected by the experience. Subjugated females became immobile more quickly during forced swim tests, and made fewer investigatory approaches during the social interaction test than control females. Juvenile social subjugation increased closed arm time in the elevated plus maze of males and females, but the effect of social subjugation was greater in females. Finally, stress-evoked CORT levels were significantly higher, and adrenal gland weights were significantly heavier, in subjugated females relative to their controls and to subjugated males. Our results demonstrate that JSS increases depression- and anxiety-like behaviors and sensitizes the stress response system in a sex-specific manner.

Male Wistar rats are more susceptible to lasting social anxiety than Wild-type Groningen rats following social defeat stress during adolescence

Adolescence is an important period for the development of adult social competences. Social stress during adolescence may contribute not only to an inadequate social development but also to adult vulnerability to social anxiety. There seems to be a clear individual differentiation, however, in the vulnerability to the long-term negative consequences of social stress. The current study further explores this individual vulnerability and is aimed at the influence of social stress during adolescence on adult social anxiety and its context specificity. Rats from different strains (Wistar and Wild-type Groningen rats) were exposed to the resident-intruder paradigm five times during 10 min each in the period between postnatal day 45 and 58. Three and 7 weeks later, the animals were re-exposed to the context in the presence of either a dominant male or an anestrous female behind a wire mesh screen. Wistar rats that were socially defeated spent less time exploring the social stimulus in comparison with socially defeated Wild-type rats and their non-defeated controls. We conclude that the stressed Wistar rat shows signs of generalized social anxiety indicating that the Wistar rat can be considered as a vulnerable phenotype to effects of adolescent social stress.

Post-weaning social isolation induces abnormal forms of aggression in conjunction with increased glucocorticoid and autonomic stress responses

We showed earlier that social isolation from weaning (a paradigm frequently used to model social neglect in children) induces abnormal forms of attack in rats, and assumed that these are associated with hyperarousal. To investigate this hypothesis, we deprived rats of social contacts from weaning and studied their behavior, glucocorticoid and autonomic stress responses in the resident-intruder paradigm at the age of 82 days. Social isolation resulted in abnormal attack patterns characterized by attacks on vulnerable targets, deficient social communication and increased defensive behaviors (defensive upright, flight, freezing). During aggressive encounters, socially deprived rats rapidly switched from one behavior to another, i.e. showed an increased number of behavioral transitions as compared to controls. We tentatively term this behavioral feature "behavioral fragmentation" and considered it a form of behavioral arousal. Basal levels of plasma corticosterone regularly assessed by radioimmunoassay between 27 and 78 days of age were not affected. In contrast, aggression-induced glucocorticoid responses were approximately doubled by socially isolation. Diurnal oscillations in heart rate assessed by in vivo biotelemetry were not affected by social isolation. In contrast, the aggression-induced increase in heart rate was higher in socially isolated than in socially housed rats. Thus, post-weaning social isolation induced abnormal forms of aggression that developed on the background of increased behavioral, endocrine and autonomic arousal. We suggest that this paradigm may be used to model aggression-related psychopathologies associated with hyperarousal, particularly those that are triggered by adverse rearing conditions.

Early adolescence as a critical window during which social stress distinctly alters behavior and brain norepinephrine activity

Many neural programs that shape behavior become established during adolescence. Adverse events at this age can have enduring consequences for both adolescent and adult mental health. Here we show that repeated social stress at different stages of adolescent development differentially affects rat behavior and neuronal activity. Early-adolescent (PND 28, EA), mid-adolescent (PND 42, MA), and adult (PND 63) rats were subjected to resident-intruder social stress (7 days) and behavior was examined 24-72 h later. In EA rats selectively, resident-intruder stress increased proactive responses in the defensive burying and forced swim tests. In adult rats, resident-intruder stress decreased burying behavior regardless of whether the animal was stressed as an adult or during early adolescence. As the locus coeruleus (LC)-norepinephrine system has been implicated in proactive defense behaviors, LC neuronal activity was quantified in separate cohorts. Stressed EA rats had elevated spontaneous LC discharge rates and diminished responses to sensory stimuli compared with controls. Microinjection of a CRF antagonist into the LC selectively inhibited neurons of stressed EA rats, suggesting that EA social stress induces tonic CRF release onto LC neurons, shifting the mode of discharge to an activated state that promotes active defensive behaviors. In all adult groups, resident-intruder stress resulted in an increased phasic response to sensory stimuli with no change in spontaneous rates. MA was a transition period during which social stress did not affect behavior or LC activity. The results suggest that social stress interacts with the brain norepinephrine system to regulate defensive strategies in an age-dependent manner.

Anterior hypothalamic dopamine D2 receptors modulate adolescent anabolic/androgenic steroid-induced offensive aggression in the Syrian hamster

In the Syrian hamster, treatment with anabolic/androgenic steroids (AAS) throughout adolescence increases dopamine and D2 receptor expression in the anterior hypothalamus (AH), a brain region implicated in the control of aggression. D2 receptor antagonists have reduced aggression in various species and animal models. However, these studies used systemic administration of drugs and reported concomitant changes in mobility. These data complicate the question of whether pharmacology targeting D2 receptors is specific to aggression or whether these drugs exert their antiaggressive effects through nonspecific mechanisms. To resolve this discrepancy, the current studies investigate whether administration of the D2 receptor antagonist eticlopride (0.01-10.0 microg in a final volume of 0.5 microl) into the AH modulates AAS-induced aggression. Antagonism of AH D2 receptors effectively suppressed AAS-induced aggression beginning at the 0.1 microg dose, with higher doses producing a floor effect, when compared with AAS-treated animals injected with saline into the AH. Importantly, these reductions in aggressive responding occurred in the absence of changes in locomotor behavior. Our findings identify a neuroanatomical locus where D2 receptor antagonism suppresses adolescent AAS-induced aggression in the absence of alterations to general mobility.

Adaptive modulation of behavioural profiles by social stress during early phases of life and adolescence

The development of individual behavioural profiles can be powerfully influenced by stressful social experiences. Using a comparative approach, we focus on the role of social stressors for the modulation of behavioural profile during early phases of life and adolescence. For gregarious species, the stability of the social environment in which the pregnant and lactating female lives is of major importance for foetal brain development and the behavioural profile of the offspring in later life. Social instability during these critical periods of development generally brings about a behavioural and neuroendocrine masculinisation in daughters and a less pronounced expression of male-typical traits in sons. Moreover, when mothers live in a socially threatening world during this time, anxiety-like behaviour of their offspring often is elevated in adulthood. These effects of the social environment are likely to be mediated by maternal hormones and/or maternal behaviour. In addition, they can be modulated significantly by offspring genotype. We favour the hypothesis that the behavioural effects of social stress during this phase of life are not necessarily "pathological" (nonadaptive) consequences or constraints of adverse social conditions. Rather, mothers could be adjusting the offspring to their environment in an adaptive way. Adolescence is another period in which behavioural development is particularly susceptible to social influences. There is some evidence that stressful social events experienced at this time alter and canalize behaviour in an adaptive fashion, so that earlier influences on behavioural profile development can be complemented and readjusted, if necessary, to meet current environmental conditions. In terms of underlying neuroendocrine mechanism, a central role for the interaction of testosterone and stress hormones is suggested. In summary, the modulation of behavioural profiles by social stress from the prenatal phase through adolescence appears to represent an effective mechanism for repeated and rapid adaptation.

Adolescent exposure to anabolic/androgenic steroids and the neurobiology of offensive aggression: a hypothalamic neural model based on findings in pubertal Syrian hamsters

Considerable public attention has been focused on the issue of youth violence, particularly that associated with drug use. It is documented that anabolic steroid use by teenagers is associated with a higher incidence of aggressive behavior and serious violence, yet little is known about how these drugs produce the aggressive phenotype. Here we discuss work from our laboratory on the relationship between the development and activity of select neurotransmitter systems in the anterior hypothalamus and anabolic steroid-induced offensive aggression using pubertal male Syrian hamsters (Mesocricetus auratus) as an adolescent animal model, with the express goal of synthesizing these data into an cogent neural model of the developmental adaptations that may underlie anabolic steroid-induced aggressive behavior. Notably, alterations in each of the neural systems identified as important components of the anabolic steroid-induced aggressive response occurred in a sub-division of the anterior hypothalamic brain region we identified as the hamster equivalent of the latero-anterior hypothalamus, indicating that this sub-region of the hypothalamus is an important site of convergence for anabolic steroid-induced neural adaptations that precipitate offensive aggression. Based on these findings we present in this review a neural model to explain the neurochemical regulation of anabolic steroid-induced offensive aggression showing the hypothetical interaction between the arginine vasopressin, serotonin, dopamine, gamma-aminobutyric acid, and glutamate neural systems in the anterior hypothalamic brain region.

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

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

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Risk assessment and avoidance in juvenile golden hamsters exposed to repeated stress

Juvenile hamsters are typically less vulnerable to social subjugation than adults, although they will avoid aggressive individuals in some situations. The purpose of this study was to determine the extent to which social subjugation stimulates fear- or anxiety-like behavior in juvenile hamsters in both social and non-social contexts. Social context testing was conducted in a Y-maze while the non-social context apparatus consisted of an open field arena and a lat-maze. In the Y-maze, subjects were exposed to an unfamiliar aggressive adult hamster. Compared with non-subjugated controls, subjugated juveniles spent significantly more time in the area furthest from the aggressive adult stimulus. In addition, socially stressed animals were more likely to avoid the arm of the maze containing the social stimulus. When they did walk in the arm containing the social stimulus, subjugated individuals were more likely to ambulate slowly. Subjugated hamsters also performed fewer olfactory investigations in the proximity of the unfamiliar aggressive individual. Despite these behavioral differences detected between groups during testing in a social context, we observed no differences between groups in the open field and lat-maze. This suggests that the effects of subjugation observed in the Y-maze are specific to exposure to a social context and that social subjugation in juvenile hamsters does not result in a generalized state of fear. Instead, subjugated juveniles learned to avoid adult males and were otherwise behaviorally similar to non-subjugated controls.

Neural circuitry of play fighting in golden hamsters

In hamsters, play fighting matures gradually into adult aggression. As these two behaviors share many similarities in this species, we predicted that a single neural circuitry controls their offensive component. The goal of the present study was to identify neural systems associated with offensive play fighting in male juvenile golden hamsters. The neural circuitry related to this behavior was identified through quantification of c-Fos immunolabeling. We also looked for vasopressin cells possibly associated with play fighting. We found that areas previously associated with offensive aggression in adult hamsters, including the ventrolateral hypothalamus, the medial amygdala, and the bed nucleus of the stria terminalis, also showed enhanced c-Fos expression after play fighting. In addition, vasopressin neurons in the nucleus circularis and the medial division of the supraoptic nucleus expressed enhanced c-Fos immunolabeling in juveniles after play fighting, as previously reported in adult hamsters after aggression. Finally, enhanced c-Fos expression associated with play fighting was also found in areas previously unexplored in adult hamsters, such as the prefrontal cortex. Together, our results support the hypothesis of a single core neural circuitry controlling the offensive components of play fighting and adult aggression throughout puberty in hamsters.

Prepubertal social subjugation and anabolic androgenic steroid-induced aggression in male rats

Abused children are more prone to abuse drugs, such as anabolic androgenic steroids (AAS), as teenagers and display violence as adults. AAS use has been linked with elevated aggression. Thus, exposure to child abuse and AAS may potentiate aggression. A social subjugation paradigm was used as an animal model of childhood abuse to determine whether prior subjugation increases AAS-induced aggression in male rats. Prepubertal gonadally intact male rats were exposed to social subjugation, a novel cage experience, or remained undisturbed in their home cages. Experimental males were socially subjugated by being placed in the home cage of an adult male. At puberty, both subjugated and nonsubjugated rats were injected with either the AAS testosterone or vehicle. AAS treatment continued for 5 weeks. Aggression was measured during the last week of AAS exposure. AAS was then discontinued. Aggression was again tested 12 weeks after AAS withdrawal. Aggression was tested under three conditions: (i) physical provocation of the experimental male; (ii) provocation of the intruder male; and (iii) without provocation. Both AAS-treated males and socially subjugated males displayed significantly more aggression than did controls. Elevated aggression by subjugated males was still present 17 weeks after social subjugation. AAS males also showed increased aggression 12 weeks after AAS withdrawal. However, exposure to both social subjugation and AAS had no long-term effects on aggression. The results of the present study indicate that social subjugation may have lasting consequences on the expression of adaptive social behaviours.

Individual differences in offensive aggression in golden hamsters: a model of reactive and impulsive aggression?

In humans, reactive aggression is associated with impulsivity. The purpose of this study is to relate reactive and impulsive aggression in humans with offensive aggression in animals and identify neurobiological correlates associated with certain forms of the behavior. We predicted that individual differences in offensive aggression are associated with individual differences in impulsivity. Adult male hamsters were repeatedly tested for offensive responses and divided into High-Aggression or Low-Aggression groups. They were then trained and tested under a delay-discounting paradigm to assess impulsivity. High-Aggression animals consistently attacked and bit more frequently and faster, and showed highly repetitive behavior, indicated by repeated attacks per contact bout. In addition, these animals engaged in more fragmented and shorter contact bouts. During impulsivity testing, High-Aggression animals preferred immediate smaller rewards over delayed larger rewards. Furthermore, 5-HT and vasopressin (AVP) innervation was compared between the groups. High-Aggression animals showed decreased 5-HT varicosities in several key brain areas involved in aggressive and/or impulsive behavior and decreased AVP fibers in the anterior hypothalamus. Together, these data show a convergence of behavioral phenotypes through individual differences in offensive aggression and impulsivity. As such, this association provides support for an animal model of reactive and impulsive aggression. Furthermore, this behavioral convergence is supported by a concurrent reduction in 5-HT innervation of brain areas controlling aggression and impulsivity, providing a common neural mechanism for this phenotype.

Stress, aggression, and puberty: neuroendocrine correlates of the development of agonistic behavior in golden hamsters

During puberty, agonistic behaviors undergo significant transitions. In golden hamsters, puberty is marked by a transition from play fighting to adult aggression. During early puberty, male golden hamsters perform play-fighting attacks. This response type is gradually replaced by adult attacks over the course of puberty. Interestingly, this behavioral transition does not appear to be controlled by changes in gonadal steroids. Instead, the shift from play fighting to adult aggression in male golden hamsters is driven by pubertal changes in glucocorticoid levels. Specifically, the transition from play fighting to adult aggression coincides with developmental increases in glucocorticoid levels, and external manipulations such as social stress or treatment with corticosteroid receptor agonists accelerate this behavioral shift. Moreover, the consequences of social stress differ greatly between juvenile and adult male golden hamsters. Although a single defeat during adulthood causes severe and long lasting behavioral and neuroendocrine consequences, socially subjugated juveniles show only transient behavioral effects. As such, it is likely that pubertal changes in the HPA axis are not only linked to the maturation of offensive responses but also determine the consequences of social stress. Combined, these studies in golden hamsters provide a novel mechanism for the development of agonistic behavior and suggest that age related differences in behavioral plasticity are mediated by the development of the HPA axis.

Interactions between the neural regulation of stress and aggression

Socially aggressive interaction is stressful. What is more, social aggression is stressful for both dominant and subordinate animals. Much of the neurocircuitry for stress and aggression overlap. The pattern of neurochemical and hormonal events stimulated by social interaction make it clear that subtle differences in this pattern of response distinguish social rank. The neurotransmitter serotonin (5-HT) responds rapidly to stress, and also appears to play the most important role for inhibitory regulation of aggressive interactions. In addition, the adrenocortical/interrenal steroid hormones corticosterone and cortisol are responsive to stress and influence aggression. However, while 5-HT and glucocorticoids can both be inhibitory to aggression, the relationship between 5-HT and glucocorticoids is not straightforward, and much of the distinctions in function depend upon timing. Neither is inhibitory during the early stressful phase of aggression. This transmitter-hormone combination follows and influences a four-stage functional pattern of effect: (1) predisposed (positively or negatively) toward aggression, (2) motivated toward behavior, (3) responsive to stress (including aggression) and passively allowing aggression, and finally (4) chronically applied 5-HT and glucocorticoids inhibit aggression.

Characterization of offensive responses during the maturation of play-fighting into aggression in male golden hamsters

In hamsters, the maturation of aggression during puberty is associated with a gradual reduction of offensive responses. The purpose of this study was to analyze the changes during this decrease to provide an enhanced description of the behavior. During early puberty, play-fighting is characterized by long and continuous contact duration throughout the encounter and repetitive attacks within bouts of agonistic interaction. By mid-puberty, adult patterns of offensive behavior emerge. Contact time becomes shorter in duration and shifts to the beginning of the test, while attacks become less repetitive per bout. In late puberty, animals show an enhanced efficiency of behavior, as indicated by an increased percentage of attacks followed by bites. This study provides a better understanding of the development of aggression by characterizing the differences between juvenile play-fighting and adult aggression and the process of the maturation of aggression.

Cortisol controls the pubertal development of agonistic behavior in male golden hamsters via type II corticosteroid receptors

In male golden hamsters, agonistic behavior undergoes a pubertal transition from play fighting to adult aggression. Previous studies have shown that this aspect of behavioral development is associated with pubertal increases in glucocorticoids and that daily social stress or injections of a synthetic glucocorticoid accelerate the transition. The goals of this study were to confirm the effects of cortisol on the development of agonistic behavior and to investigate the role of type II corticosteroid receptors in this process. First, animals treated with cortisol during early puberty [from postnatal days 31 (P-31) to P-36] showed an accelerated transition from play fighting to adult aggression. In a second experiment, the behavioral effects of cortisol were blocked by a co-treatment with a type II corticosteroid receptor antagonist. These findings are the first to show a facilitating role for type II corticosteroid receptors in the pubertal development of a social behavior. As such, these findings provide new insights into the neuroendocrine mechanisms controlling behavioral development during puberty.

Social conflict models: can they inform us about human psychopathology?

Social conflict models have been proposed as a powerful way to investigate basic questions of how brain and behavior are altered by social experience. Social defeat, in particular, appears to be a major stressor for most species, and in humans, this stressor is thought to play an important role in the onset of a variety of psychiatric disorders including depression and post-traumatic stress disorder. Aggressive experience, on the other hand, may promote disorders involving inappropriate aggression and violence. Current research using animal models of social conflict involves multiple levels of analysis from genetic and molecular to systems and overt behavior. This review briefly examines a variety of these animal models of social conflict in order to assess whether they are useful for advancing our understanding of how experience can shape brain and behavior and for translating this information so that we have the potential to improve the quality of life of individuals with mental illness and behavioral disorders.

Amygdala size and hypothalamus size predict social play frequency in nonhuman primates: a comparative analysis using independent contrasts

The amygdala and hypothalamus become sexually differentiated by gonadal hormones giving rise to sexually differentiated behaviors, which include play behavior. Phylogenetic comparative analyses test for relationships between social play and brain structure volumes. Relative volume of the amygdala and hypothalamus correlated with social play, but not nonsocial play, even after controlling for the size of other brain structures. The authors propose that behaviors such as social assessment, recognizing and responding to facial expression, and social response appropriateness, which are mediated by the amygdala, are developed through social play. Additionally, the hypothalamus may regulate the motivation to engage in play through positive reinforcement of pleasurable activity. Thus, the instinctive socio-emotive aspects of play in primates appear to be those regulated by the amygdala and hypothalamus.

Intermale aggression in corticotropin-releasing factor receptor 1 deficient mice

The anxiogenic neuropeptide, corticotropin-releasing factor (CRF), has a complex effect on intermale aggression. CRF receptor 1 (CRFR1) is the primary receptor for CRF and in this study, we examined in detail isolation-induced intermale aggression in CRFR1 deficient mice. All mice contained a mixed 50:50 inbred/outbred background to improve aggressive performance. Mice were isolated for 4 weeks prior to 2 consecutive days of aggression testing using the resident-intruder paradigm. Mice were also tested for anxiety on the elevated plus maze. Relative to littermate wild-type (WT) controls, CRFR1-mutant mice exhibited normal levels of intermale aggression over the 2 test days in terms of percentage showing aggression, number of attacks, time aggressive, and latency to first attack. In terms of sites of attacks on intruders, CRFR1-deficient mice attacked the ventral portion of the mid-section (including belly) significantly less frequently than WT males on test day 1, but these differences did not reach significance on test day 2. No other differences in sites of attacks were observed. Tail rattling also did not differ between groups. Importantly, KO males showed decreased anxiety relative to WT mice (consistent with previous reports) as evidenced by spending significantly more time on the open arms and significantly less time on the closed arms of the elevated plus maze. Plus maze performance did not correlate with any measure of levels of aggression, suggesting a dissociation between altered levels of anxiety and aggressive performance. Taken together, the results suggest that the activation CRFR1 is not necessary for the normal production of isolation-induced intermale aggression.

Lead exposure alters the development of agonistic behavior in golden hamsters

We tested the effects of exposure to different doses of lead acetate (either 0, 25, 100, or 400 ppm) on the development of aggressive behavior in male golden hamsters. Pups were tested for offensive responses across puberty, as they were maturing from play fighting to adult aggression. Our data show a dose-specific effect of lead exposure on the development of aggression during puberty at doses resulting in blood levels well below 20 microg/dl. Animals exposed to 25 ppm lead acetate were faster and performed more than twice as many attacks on intruders by late puberty. They were also twice as likely to initiate adult instead of play-fighting attacks around mid-puberty. These observations were independent of any effect on growth. Thus, exposure to low doses of lead enhanced aggression and accelerated its maturation. As such, our data support the association between exposure to low doses of lead and aggressive behavior in boys.

Glucocorticoids and the development of agonistic behaviour during puberty in male golden hamsters

During puberty, the agonistic behaviour of male golden hamsters undergoes a transition from play fighting to adult aggression. Repeated exposure to social stress early in puberty accelerates this transition. The present study investigated the possible role of glucocorticoids on the maturation of agonistic behaviour. First, we compared serum cortisol levels following a 20-min restraint stress during early puberty, mid-puberty or adulthood. Across puberty, animals exhibited a two-fold increase in post-restraint cortisol levels. We also compared corticotrophin-releasing hormone (CRH) immunoreactive fibres projecting to the median eminence between animals in early puberty and adulthood. The CRH fibre density was two-fold greater in adults compared to juveniles. Furthermore, we investigated the effects of stress hormones on the maturation of agonistic behaviour. Male hamsters were injected daily with dexamethasone, a corticosteroid receptor type II agonist (0, 10 or 40 microg/100 g), early in puberty from postnatal day 31 (P-31) to P-36. When paired with a smaller and younger intruder on P-37, attack frequency did not differ between groups. However, dexamethasone-treated animals showed a dose-dependent decrease in the percentage of play-fighting attacks and an increase in the percentage of adult attacks. In summary, puberty can be described as a period of increasing hypothalamic-pituitary-adrenal activity in male golden hamsters. Moreover, increasing glucocorticoid levels influence the maturation of agonistic behaviour. These data shed new light on the neuroendocrine mechanisms that regulate the maturation of social behaviours during puberty.

Glucocorticoid interaction with aggression in non-mammalian vertebrates: reciprocal action

Socially aggressive interaction is stressful, and as such, glucocorticoids are typically secreted during aggressive interaction in a variety of vertebrates, which may both potentiate and inhibit aggression. The behavioral relationship between corticosterone and/or cortisol in non-mammalian (as well as mammalian) vertebrates is dependent on timing, magnitude, context, and coordination of physiological and behavioral responses. Chronically elevated plasma glucocorticoids reliably inhibit aggressive behavior, consistent with an evolutionarily adaptive behavioral strategy among subordinate and submissive individuals. Acute elevation of plasma glucocorticoids may either promote an actively aggressive response via action in specialized local regions of the brain such as the anterior hypothalamus, or is permissive to escalated aggression and/or activity. Although the permissive effect of glucocorticoids on aggression does not suggest an active role for the hormone, the corticosteroids may be necessary for full expression of aggressive behavior, as in the lizard Anolis carolinensis. These effects suggest that short-term stress may generally be best counteracted by an actively aggressive response, at least for socially dominant proactive individuals. An acute and active response may be evolutionarily maladaptive under chronic, uncontrollable and unpredictable circumstances. It appears that subordinate reactive individuals often produce compulsorily chronic responses that inhibit aggression and promote submissive behavior.