Sex-dependent regulation of social avoidance by oxytocin signaling in the ventral tegmental area

It has been hypothesized that oxytocin increases the salience of social stimuli, whether the valence is positive or negative, through its interactions with the ventral tegmental area (VTA). Indeed, oxytocin neurons project to the VTA and activate dopamine neurons that are necessary for social experiences with positive valence. Surprisingly, though, there has not been an investigation of the role of oxytocin in the VTA in mediating social experiences with negative valence (e.g., social stress). Given that there are sex differences in how oxytocin regulates the salience of positively-valenced social interactions, we hypothesized that oxytocin acting in the VTA also alters the salience of social stress in a sex-dependent manner. To test this, female and male Syrian hamsters were site-specifically infused with either saline, oxytocin (9 μM), or oxytocin receptor antagonist (90 μM) into the VTA. Subjects were then exposed to either no defeat or a single, 15 min defeat by one RA. The day following social defeat, subjects underwent a 5 min social avoidance test. There was an interaction between sex and drug treatment, such that the oxytocin antagonist increased social avoidance compared to saline treatment in socially stressed females, while oxytocin decreased social avoidance compared to saline treatment in socially stressed males. Contrary to expectations, these results suggest that oxytocin signaling generally acts to decrease social avoidance, regardless of sex. These sex differences in the efficacy of oxytocin and oxytocin receptor antagonists to alter negatively-valenced social stimuli, however, should be considered when guiding pharmacotherapies for disorders involving social deficits.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. Hippocampal area CA2 is known to play a key role in these behaviors in mice and responds to social stimuli in rats, but CA2 has yet to be characterized in hamsters or voles, which are also used in studies of social behaviors. Here, we used immunofluorescence to determine whether CA2 could be molecularly identified in tissue from voles and hamsters. We found that  staining for many CA2 markers was similar in these three species, with labeling seen in neurons at the distal end of the mossy fibers . In contrast, although perineuronal nets (PNNs) surround CA2 cells in mice, PNN staining differed across species. In voles, both CA2 and CA3 were labeled, whereas in hamsters, labeling was seen primarily in CA3. These results demonstrate that CA2 can be molecularly distinguished from neighboring CA1 and CA3 areas in voles and hamsters with several antibodies commonly used in mice. However, PNN staining is not useful for identifying CA2 in voles or hamsters, suggestive of differing roles for either PNNs or for the hippocampal subregions in social behavior. These findings reveal commonalities across species in the molecular profile of CA2 and should facilitate future studies of CA2 in these species.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and rats (Rattus norvegicus, for example) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. The CA2 region of the hippocampus is known to play a key role in social memory and aggression in mice and responds to social stimuli in rats, likely owing to its high expression of oxytocin and vasopressin 1b receptors. However, CA2 has yet to be identified and characterized in hamsters or voles. In this study, we sought to determine whether CA2 could be identified molecularly in vole and hamster. To do this, we used immunofluorescence with primary antibodies raised against known molecular markers of CA2 in mice and rats to stain hippocampal sections from voles and hamsters in parallel with those from mice. Here, we report that, like in mouse and rat, staining for many CA2 proteins in vole and hamster hippocampus reveals a population of neurons that express regulator of G protein signaling 14 (RGS14), Purkinje cell protein 4 (PCP4) and striatal-enriched protein tyrosine phosphatase (STEP), which together delineate the borders with CA3 and CA1. These cells were located at the distal end of the mossy fiber projections, marked by the presence of Zinc Transporter 3 (ZnT-3) and calbindin in all three species. In addition to staining the mossy fibers, calbindin also labeled a layer of CA1 pyramidal cells in mouse and hamster but not in vole. However, Wolframin ER transmembrane glycoprotein (WFS1) immunofluorescence, which marks all CA1 neurons, was present in all three species and abutted the distal end of CA2, marked by RGS14 immunofluorescence. Staining for two stress hormone receptors-the glucocorticoid (GR) and mineralocorticoid (MR) receptors-was also similar in all three species, with GR staining found primarily in CA1 and MR staining enriched in CA2. Interestingly, although perineuronal nets (PNNs) are known to surround CA2 cells in mouse and rat, we found that staining for PNNs differed across species in that both CA2 and CA3 showed staining in voles and primarily CA3 in hamsters with only some neurons in proximal CA2 showing staining. These results demonstrate that, like in mouse, CA2 in voles and hamsters can be molecularly distinguished from neighboring CA1 and CA3 areas, but PNN staining is less useful for identifying CA2 in the latter two species. These findings reveal commonalities across species in molecular profile of CA2, which will facilitate future studies of CA2 in these species. Yet to be determined is how differences in PNNs might relate to differences in social behavior across species.

Sex differences in the impact of social status on social reward and associated mesolimbic activation

Social stress plays an important role in the etiology of many neuropsychiatric disorders and can lead to a variety of behavioral deficits such as social withdrawal. One way that social stress may contribute to psychiatric disorders is by reducing social motivation and the rewarding properties of social interactions. We investigated the impact of social stress on social reward in the context of winning versus losing agonistic encounters in Syrian hamsters (Mesocricetus auratus). First, we tested the hypothesis that social stress resulting from either stable low, or subordinate, social status or from social defeat reduces the rewarding properties of social interactions. Using an Operant Social Preference (OSP) task to measure social reward/motivation, we found that both subordinate and socially defeated males made significantly fewer entries into chambers containing novel, same-sex conspecifics compared to males who were dominant (i.e., stably won the agonistic encounters). In females, however, there were no differences in social entries between winners and losers. In a second experiment, we found more activation of the mesolimbic dopamine system (MDS) as assessed with cFos immunohistochemistry in the lateral ventral tegmental area (lVTA) and the nucleus accumbens (NAc) shell of male winners compared to losers. In females, however, there were no differences in activation in the lVTA between winners and losers. Surprisingly, however, winning females displayed significantly more activation in the NAc shell as compared to losing females, despite the lack of behavioral differences. Thus, behavioral and histological data suggest that there are sex differences in the impact of social status on social reward and associated mesolimbic activation.

Sex influences the effects of social status on socioemotional behavior and serotonin neurochemistry in rhesus monkeys

BACKGROUND

Despite observed sex differences in the prevalence of stress-related psychiatric conditions, most preclinical and translational studies have only included male subjects. Therefore, it has not been possible to effectively assess how sex interacts with other psychosocial risk factors to impact the etiology and maintenance of stress-related psychopathology. One psychosocial factor that interacts with sex to impact risk for stress-related behavioral and physiological deficits is social dominance. The current study was designed to assess sex differences in the effects of social status on socioemotional behavior and serotonin neurochemistry in socially housed rhesus monkeys. We hypothesized that sex and social status interact to influence socioemotional behaviors as well as serotonin 1A receptor binding potential (5HT1AR-BP) in regions of interest (ROIs) implicated in socioemotional behavior.

METHODS

Behavioral observations were conducted in gonadally intact adult female (n = 14) and male (n = 13) rhesus monkeys. 5HT1AR-BP was assessed via positron emission tomography using 4-(2'-Methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p[18F]fluorobenzamido]ethylpiperazine ([18F]MPPF).

RESULTS

Aggression emitted was greater in dominant compared to subordinate animals, regardless of sex. Submission emitted was significantly greater in subordinate versus dominant animals and greater in females than males. Affiliative behaviors emitted were not impacted by sex, status, or their interaction. Anxiety-like behavior emitted was significantly greater in females than in males regardless of social status. Hypothalamic 5HT1AR-BP was significantly greater in females than in males, regardless of social status. 5HT1AR-BP in the dentate gyrus of the hippocampus was significantly impacted by a sex by status interaction whereby 5HT1AR-BP in the dentate gyrus was greater in dominant compared to subordinate females but was not different between dominant and subordinate males. There were no effects of sex, status, or their interaction on 5HT1AR-BP in the DRN and in the regions of the PFC studied.

CONCLUSIONS

These data have important implications for the treatment of stress-related behavioral health outcomes, as they suggest that sex and social status are important factors to consider in the context of serotonergic drug efficacy.

CRISPR-Cas9 editing of the arginine-vasopressin V1a receptor produces paradoxical changes in social behavior in Syrian hamsters

Studies from a variety of species indicate that arginine–vasopressin (AVP) and its V1a receptor (Avpr1a) play a critical role in the regulation of a range of social behaviors by their actions in the social behavior neural network. To further investigate the role of AVPRs in social behavior, we performed CRISPR-Cas9–mediated editing at the Avpr1a gene via pronuclear microinjections in Syrian hamsters (Mesocricetus auratus), a species used extensively in behavioral neuroendocrinology because they produce a rich suite of social behaviors. Using this germ-line gene-editing approach, we generated a stable line of hamsters with a frame-shift mutation in the Avpr1a gene resulting in the null expression of functional Avpr1as. Avpr1a knockout (KO) hamsters exhibited a complete lack of Avpr1a-specific autoradiographic binding throughout the brain, behavioral insensitivity to centrally administered AVP, and no pressor response to a peripherally injected Avpr1a-specific agonist, thus confirming the absence of functional Avpr1as in the brain and periphery. Contradictory to expectations, Avpr1a KO hamsters exhibited substantially higher levels of conspecific social communication (i.e., odor-stimulated flank marking) than their wild-type (WT) littermates. Furthermore, sex differences in aggression were absent, as both male and female KOs exhibited more aggression toward same-sex conspecifics than did their WT littermates. Taken together, these data emphasize the importance of comparative studies employing gene-editing approaches and suggest the startling possibility that Avpr1a-specific modulation of the social behavior neural network may be more inhibitory than permissive.

Sex and social status modify the effects of fluoxetine on socioemotional behaviors in Syrian hamsters and rhesus macaques

Social subordination increases risk for psychiatric disorders, while dominance increases resilience to these disorders. Fluoxetine, a selective serotonin (5HT) reuptake inhibitor whose actions are mediated in part by the 5HT1A receptor (5HT1AR), has sex- and social status-specific effects on socioemotional behavior and aggressive behavior. However, the impact of social status on these sex-specific effects remains unclear. The current study evaluated the impact of acute fluoxetine treatment and social status on dominance-related behaviors in female and male hamsters, and the impact of chronic fluoxetine treatment on socioemotional behavior and 5HT1AR binding potential (5HT1AR_BP) in female rhesus macaques. We hypothesized that sex differences in the effects of fluoxetine on aggression in hamsters would be diminished in dominant and enhanced in subordinate males and that aggression in female hamsters would be enhanced in dominants and diminished in subordinates. In female rhesus macaques, we hypothesized that chronic fluoxetine would alter socioemotional behaviors and site-specific 5HT1AR_BP in a status-dependent manner. Male (n = 46) and female (n = 56) hamsters were paired with conspecifics for three days to establish social rank. Hamsters received a single dose of 20 mg/kg fluoxetine or vehicle two-hours prior to a test with a non-aggressive intruder. Female rhesus monkeys (n = 14) housed were administered fluoxetine (2.8 mg/kg/day) or vehicle injections chronically for 14-days, separated by a three-week washout period. On Day 15, positron emission tomography neuroimaging for 5HT1AR_BP was conducted. Fluoxetine treatment decreased aggression in subordinate female monkeys and subordinate female hamsters but not in dominant females of either species. Fluoxetine decreased aggression in dominant but not in subordinate male hamsters. Fluoxetine also reduced and increased prefrontal 5HT1AR_BP in dominant and subordinate females, respectively. Taken together, these results provide cross-species evidence that social status and sex impact how increased 5HT modulates agonistic behavior.

Acute administration of fluoxetine increases social avoidance and risk assessment behaviors in a sex- and social stress-dependent manner in Syrian hamsters (Mesocricetus auratus)

Most studies investigating the effects of acute administration of selective serotonin reuptake inhibitors (SSRI) on responses to social stress have been conducted with males. This is despite the fact that SSRIs remain the primary pharmacotherapy for social stress-related disorders for both sexes and that the prevalence of these disorders is twofold higher in women than in men. To determine whether acute treatment with the SSRI, fluoxetine, alters behavioral responses to social defeat stress in a sex- or social stress-dependent manner, male and female Syrian hamsters were subjected to one of three social defeat conditions: no defeat (placed into an empty resident aggressor (RA) cage), a single defeat by one RA for 15 min, or three consecutive defeats using different RAs for 5 min each. The day following social defeat, subjects were infused with either vehicle or fluoxetine (20 mg/kg, I.P.) 2 h prior to a 5 min social avoidance test. Overall, we found that fluoxetine increased social vigilance regardless of sex or defeat condition. We also found that fluoxetine affected social avoidance in a sex by stress intensity interaction, such that fluoxetine increased avoidance in no defeat males and in males defeated once but significantly increased avoidance in females only after three defeats. These data suggest that treatment with an SSRI could initially exacerbate the effects of social stress in both sexes. These data also emphasize the importance of including sex as a biological variable when investigating the efficacy of pharmacotherapy for stress-related disorders.

Ingestion of probiotic (Lactobacillus helveticus and Bifidobacterium longum) alters intestinal microbial structure and behavioral expression following social defeat stress

Social stress exacerbates anxious and depressive behaviors in humans. Similarly, anxiety- and depressive-like behaviors are triggered by social stress in a variety of non-human animals. Here, we tested whether oral administration of the putative anxiolytic probiotic strains Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 reduces the striking increase in anxiety-like behavior and changes in gut microbiota observed following social defeat stress in Syrian hamsters. We administered the probiotic at two different doses for 21 days, and 16S rRNA gene amplicon sequencing revealed a shift in microbial structure following probiotic administration at both doses, independently of stress. Probiotic administration at either dose increased anti-inflammatory cytokines IL-4, IL-5, and IL-10 compared to placebo. Surprisingly, probiotic administration at the low dose, equivalent to the one used in humans, significantly increased social avoidance and decreased social interaction. This behavioral change was associated with a reduction in microbial richness in this group. Together, these results demonstrate that probiotic administration alters gut microbial composition and may promote an anti-inflammatory profile but that these changes may not promote reductions in behavioral responses to social stress.

Sex-dependent effects of social status on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT), and serotonin (5-HT) 1A receptor binding and aggression in Syrian hamsters (Mesocricetus auratus)

Dominance status in hamsters is driven by interactions between arginine-vasopressin V1a, oxytocin (OT), and serotonin 1A (5-HT1A) receptors. Activation of V1a and OT receptors in the anterior hypothalamus (AH) increases aggression in males, while decreasing aggression in females. In contrast, activation of 5-HT1A receptors in the AH decreases aggression in males and increases aggression in females. The mechanism underlying these differences is not known. The purpose of this study was to determine if dominance status and sex interact to regulate V1a, OT, and 5-HT1A receptor binding. Same-sex hamsters (N = 47) were paired 12 times across six days in five min sessions. Brains from paired and unpaired (non-social control) hamsters were collected immediately after the last interaction and processed for receptor binding using autoradiography. Differences in V1a, OT, and 5-HT1A receptor binding densities were observed in several brain regions as a function of social status and sex. For example, in the AH, there was an interaction between sex and social status, such that V1a binding in subordinate males was lower than in subordinate females and V1a receptor density in dominant males was higher than in dominant females. There was also an interaction in 5-HT1A receptor binding, such that social pairing increased 5-HT1A binding in the AH of males but decreased 5-HT1A binding in females compared with unpaired controls. These results indicate that dominance status and sex play important roles in shaping the binding profiles of key receptor subtypes across the neural circuitry that regulates social behavior.

Sex-dependent effects of social isolation on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5HT) 1a receptor binding and aggression

It is widely held that social isolation produces higher rates of mortality and morbidity and has deleterious effects on an individual's sociality. Relatedly, it is widely observed that socially isolated adult rodents display significantly higher levels of aggression when placed in a social situation than do their conspecifics living in social groups. In the following study, we investigated the effects of social isolation on several neurochemical signals that play key roles in the regulation of social behavior in adults. More specifically, we examined the effects of social isolation on vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5-HT)1a receptor binding within the neural circuit controlling social behavior. Male and female Syrian hamsters were housed individually or with two other hamsters for four weeks and were then tested with a same-sex nonaggressive intruder in a neutral arena for 5 min. Social isolation significantly increased aggression in both males and females and altered receptor binding in several brain regions in a sex-dependent manner. For example, V1a receptor binding was greater in socially isolated males in the anterior hypothalamus than it was in any other group. Taken together, these data provide substantial new support for the proposition that the social environment can have a significant impact on the structural and neurochemical mechanisms regulating social behavior and that the amount and type of social interactions can produce differential effects on the circuit regulating social behavior in a sex-dependent manner.

Transcriptomic Analysis Reveals Sex-Dependent Expression Patterns in the Basolateral Amygdala of Dominant and Subordinate Animals After Acute Social Conflict

The basolateral amygdala (BLA) is a critical nucleus mediating behavioral responses after exposure to acute social conflict. Male and female Syrian hamsters both readily establish a stable dominant-subordinate relationship among same-sex conspecifics, and the goal of the current study was to determine potential underlying genetic mechanisms in the BLA facilitating the establishment of social hierarchy. We sequenced the BLA transcriptomes of dominant, subordinate, and socially neutral males and females, and using de novo assembly techniques and gene network analyses, we compared these transcriptomes across social status within each sex. Our results revealed 499 transcripts that were differentially expressed in the BLA across both males and females and 138 distinct gene networks. Surprisingly, we found that there was virtually no overlap in the transcript changes or in gene network patterns in males and females of the same social status. These results suggest that, although males and females reliably engage in similar social behaviors to establish social dominance, the molecular mechanisms in the BLA by which these statuses are obtained and maintained are distinct.

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.

Histone deacetylase and acetyltransferase inhibitors modulate behavioral responses to social stress

Histone acetylation has emerged as a critical factor regulating learning and memory both during and after exposure to stressful stimuli. There are drugs that we now know affect histone acetylation that are already in use in clinical populations. The current study uses these drugs to examine the consequences of acutely increasing or decreasing histone acetylation during exposure to social stress. Using an acute model of social defeat in Syrian hamsters, we systemically and site-specifically administered drugs that alter histone acetylation and measured subsequent behavior and immediate-early gene activity. We found that systemic administration of a histone deacetylase inhibitor enhances social stress-induced behavioral responses in males and females. We also found that systemic administration completely blocks defeat-induced neuronal activation, as measured by Fos-immunoreactivity, in the infralimbic cortex, but not in the amygdala, after a mild social defeat stressor. Lastly, we demonstrated that site-specific administration of histone deacetylase inhibitors in the infralimbic region of the prefrontal cortex, but not in the basolateral amygdala, mimics the systemic effect. Conversely, decreasing acetylation by inhibiting histone acetyltransferases in the infralimbic cortex reduces behavioral responses to defeat. This is the first demonstration that acute pharmacological manipulation of histone acetylation during social defeat alters subsequent behavioral responses in both males and females. These results reveal that even systemic administration of drugs that alter histone acetylation can significantly alter behavioral responses to social stress and highlight the importance of the infralimbic cortex in mediating this effect.

Immediate post-defeat infusions of the noradrenergic receptor antagonist propranolol impair the consolidation of conditioned defeat in male Syrian hamsters

Social defeat occurs when an animal is attacked and subjugated by an aggressive conspecific. Following social defeat, male Syrian hamsters fail to display species-typical territorial aggression and instead exhibit submissive or defensive behaviors even when in the presence of a non-aggressive intruder. We have termed this phenomenon conditioned defeat (CD). The mechanisms underlying CD are not fully understood, but data from our lab suggest that at least some of the mechanisms are similar to those that mediate classical fear conditioning. The goal of the present experiment was to test the hypothesis that noradrenergic signaling promotes the consolidation of CD, as in classical fear conditioning, by determining whether CD is disrupted by post-training blockade of noradrenergic activity. In Experiment 1, we determined whether systemic infusions of the noradrenergic receptor antagonist propranolol (0, 1.0, 10, or 20mg/kg) given immediately after a 15 min defeat by a resident aggressor would impair CD tested 48 h later. Hamsters that were given immediate post-training infusions of propranolol (1.0, but not 10 or 20mg/kg) showed significantly less submissive behavior than did those given vehicle infusions supporting the hypothesis that there is noradrenergic modulation of the consolidation of a social defeat experience. In Experiment 2, we demonstrated that propranolol (1.0mg/kg) given immediately, but not 4 or 24h, after defeat impaired CD tested 48 h after defeat indicating that the window within which the memory for social defeat is susceptible to beta-adrenergic modulation is temporary. In Experiment 3, we examined whether central blockade of noradrenergic receptors could recapitulate the effect of systemic injections by giving an intracerebroventricular infusion of propranolol immediately after defeat and examining the effect on CD 24h later. Centrally administered propranolol (20 μg/3 μl but not 2 μg/3 μl) was also effective in dose-dependently reducing consolidation of CD. Collectively, the present results indicate that noradrenergic activity promotes the consolidation of CD and suggest that CD is a valuable model to study the processes by which emotion and stress modulate memory in an ethologically relevant context. These data also suggest that the popular conception in the clinical literature that the anxiolytic effect of propranolol is primarily due to the drug's peripheral effects may need to be reconsidered.

Dopamine in the nucleus accumbens modulates the memory of social defeat in Syrian hamsters (Mesocricetus auratus)

Conditioned defeat (CD) is a behavioral response that occurs in Syrian hamsters after they experience social defeat. Subsequently, defeated hamsters no longer produce territorial aggression but instead exhibit heightened levels of avoidance and submission, even when confronted with a smaller, non-aggressive intruder. Dopamine in the nucleus accumbens is hypothesized to act as a signal of salience for both rewarding and aversive stimuli to promote memory formation and appropriate behavioral responses to significant events. The purpose of the present study was to test the hypothesis that dopamine in the nucleus accumbens modulates the acquisition and expression of behavioral responses to social defeat. In Experiment 1, bilateral infusion of the non-specific D1/D2 receptor antagonist cis(z)flupenthixol (3.75 μg/150 nl saline) into the nucleus accumbens 5 min prior to defeat training significantly reduced submissive and defensive behavior expressed 24h later in response to a non-aggressive intruder. In Experiment 2, infusion of 3.75 μg cis-(Z)-flupenthixol 5 min before conditioned defeat testing with a non-aggressive intruder significantly increased aggressive behavior in drug-infused subjects. In Experiment 3, we found that the effect of cis-(Z)-flupenthixol on aggression was specific to defeated animals as infusion of drug into the nucleus accumbens of non-defeated animals did not significantly alter their behavior in response to a non-aggressive intruder. These data demonstrate that dopamine in the nucleus accumbens modulates both acquisition and expression of social stress-induced behavioral changes and suggest that the nucleus accumbens plays an important role in the suppression of aggression that is observed after social defeat.

Oxytocin induces social communication by activating arginine-vasopressin V1a receptors and not oxytocin receptors

Arginine-vasopressin (AVP) and oxytocin (OT) and their receptors are very similar in structure. As a result, at least some of the effects of these peptides may be the result of crosstalk between their canonical receptors. The present study investigated this hypothesis by determining whether the induction of flank marking, a form of social communication in Syrian hamsters, by OT is mediated by the OT receptor or the AVP V1a receptor. Intracerebroventricular (ICV) injections of OT or AVP induced flank marking in a dose-dependent manner although the effects of AVP were approximately 100 times greater than those of OT. Injections of highly selective V1a receptor agonists but not OT receptor agonists induced flank marking, and V1a receptor antagonists but not OT receptor antagonists significantly inhibited the ability of OT to induce flank marking. Lastly, injection of alpha-melanocyte-stimulating hormone (α-MSH), a peptide that stimulates OT but not AVP release, significantly increased odor-induced flank marking, and these effects were blocked by a V1a receptor antagonist. These data demonstrate that OT induces flank marking by activating AVP V1a and not OT receptors, suggesting that the V1a receptor should be considered to be an OT receptor as well as an AVP receptor.

Effects of inescapable versus escapable social stress in Syrian hamsters: the importance of stressor duration versus escapability

Social avoidance is a common characteristic of many clinical psychopathologies and is often triggered by social stress. Our lab uses Syrian hamsters to model stress-induced social avoidance, and we have previously established that both inescapable and escapable social defeat result in increased social avoidance when compared with no-defeat controls. Our previous work suggested, however, that social avoidance was significantly greater after inescapable defeat. The goal of this study was to determine if this difference in behavior after the two types of defeat was due to experimental differences in the controllability (i.e., escapability) of the defeat or simply to differences in the overall duration of the defeat. In Experiment 1, we used a yoked design to hold constant the duration of defeat between escapable and inescapable defeat conditions. This design resulted in only a very brief social defeat, yet when comparing defeated animals with no-defeat controls, a significant increase in social avoidance was still observed. In Experiment 2, we also used the yoked design, but the escape task was made more difficult to ensure a longer defeat experience. Again, we observed no effect of controllability. Together, these data suggest that the ability to escape a social stressor does not reduce the impact of the stressful experience. These results emphasize that social stressors need not be prolonged or uncontrollable to produce marked effects on subsequent behavior.

Copulatory and agonistic behavior in Syrian hamsters following social defeat

Syrian hamsters are highly aggressive animals that reliably defend their home territory. After social defeat, however, hamsters no longer defend their home cage but instead display submissive and defensive behavior toward an intruder, a response that we have termed conditioned defeat. Plasma testosterone is significantly reduced in Syrian hamsters following repeated defeat suggesting that social defeat might also impair copulatory behavior. The present study aimed to determine whether copulatory behavior in male Syrian hamsters is suppressed following repeated social defeats and additionally whether exposure to a hormone-primed stimulus female after social defeat reduces the behavioral response to defeat. Hamsters were paired with an aggressive opponent for one or nine defeats using a resident-intruder model, while controls were placed into the empty cage of a resident aggressor. On the day after the last treatment, half of the hamsters were paired with a receptive female for 10 min. There were no significant differences in the copulatory behavior of defeated versus non-defeated hamsters, and the opportunity to copulate had no effect on subsequent conditioned defeat testing, as defeated animals displayed significantly more submissive behavior than did non-defeated animals. The current data suggest that conditioned defeat is not necessarily a maladaptive response to social stress, at least in terms of reproductive behavior, but may instead represent a viable behavioral strategy adopted by losing animals following social defeat. Further, these data indicate that conditioned defeat is relatively persistent and stable, as the opportunity to copulate does not reduce the subsequent display of submissive behavior.

Differential brain-derived neurotrophic factor expression in limbic brain regions following social defeat or territorial aggression

Syrian hamsters readily form dominant-subordinate relationships under laboratory conditions. Winning or losing in agonistic encounters can have striking, long-term effects on social behavior, but the mechanisms underlying this experience-induced behavioral plasticity are unclear. The present study tested the hypothesis that changes in brain-derived neurotrophic factor (BDNF) may at least in part mediate this plasticity. Male hamsters were paired for 15-min using a resident-intruder model, and individuals were identified as winners or losers on the basis of their behavior. BDNF was examined with in situ hybridization 2 hr after treatment during the consolidation period of emotional learning. Losing animals had significantly more BDNF mRNA in the basolateral (BLA) and medial (MeA) nuclei of the amygdala when compared with winning animals as well as novel cage and home cage controls. Interestingly, winning animals had significantly more BDNF mRNA in the dentate gyrus of the dorsal hippocampus than did losing animals, novel, and home cage controls. No conflict-related changes in BDNF mRNA were observed in several other regions including the bed nucleus of the stria terminalis and central amygdala. Next, we demonstrated that K252a, a Trk receptor antagonist, significantly reduced the acquisition of conditioned defeat when administered within the BLA. These data support a model in which BDNF-mediated plasticity within the BLA supports learning of submission or subordinate social status in losing animals, whereas BDNF-mediated plasticity within the hippocampus may instantiate aspects of winning such as control of a territory in dominant animals.

GABAA receptor activation in the lateral septum reduces the expression of conditioned defeat and increases aggression in Syrian hamsters

Exposure to social stressors can cause profound changes in an individual's physiology and behavior. In Syrian hamsters, even a single social defeat results in conditioned defeat, which includes an abolishment of territorial aggression and the emergence of high levels of submissive behavior. The purpose of the current study was to determine whether the lateral septum (LS) is a component of the putative neural circuit underlying conditioned defeat. Experiment 1 explored the possibility that plasticity in the LS is necessary for the induction of conditioned defeat. Infusions of the protein synthesis inhibitor, anisomycin, prior to defeat training, however, failed to alter conditioned defeat during testing on the following day, suggesting that synaptic plasticity in the LS is not critical for defeat-induced suppression of aggression. Experiment 2 tested whether the LS is necessary for the expression of conditioned defeat. Infusions of the GABA(A) agonist muscimol into the LS prior to testing significantly increased aggression and decreased submission in previously defeated animals suggesting that the LS is an important component of the neural circuit mediating the expression of both aggression and submission in conditioned defeat. Experiment 3 examined whether the effects of muscimol on aggression were dependent on prior social defeat. Non-defeated animals receiving muscimol infusions prior to testing with a non-aggressive intruder displayed significantly more aggression than did hamsters receiving control injections. Thus, these data suggest that the activation of GABA(A) receptors in the LS increases aggression regardless of whether or not a hamster has previously experienced social defeat.

The medial prefrontal cortex is both necessary and sufficient for the acquisition of conditioned defeat

We have previously demonstrated that the basolateral amygdala (BLA) is a key component of a neural circuit mediating memory formation for emotionally relevant stimuli in an ethologically-based model of conditioned fear, termed conditioned defeat (CD). In this model, subjects are socially defeated by a larger, more aggressive hamster. Upon subsequent exposure to a smaller, non-aggressive intruder, the defeated animal will show high levels of submissive behaviors and fail to defend its territory. Here we examined whether the medial prefrontal cortex (mPFC), an area with extensive connections with the amygdala, is also a component of this circuit. Temporary inactivation of the mPFC using muscimol, a GABA(A) receptor agonist, significantly enhanced the acquisition but not expression of CD, while blockade of GABA(A) receptors in the mPFC using bicuculline, a GABA(A) antagonist, impaired acquisition of CD. Given these findings, we next sought to test whether plasticity related to the defeat experience occurs in the mPFC. We infused anisomycin, a protein synthesis inhibitor, in the mPFC but this treatment did not alter the acquisition of CD. In our final experiment, we demonstrated that bicuculline failed to alter the acquisition of CD. Together, these results demonstrate for the first time that while the mPFC is both necessary and sufficient for the acquisition of CD, it does not appear to mediate plasticity related to the defeat experience. In contrast, while plasticity underlying CD does appear to occur in the BLA, GABAergic receptor inhibition in the BLA is not sufficient to enhance CD. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

The effect of escapable versus inescapable social defeat on conditioned defeat and social recognition in Syrian hamsters

Male Syrian hamsters are naturally aggressive animals that reliably defend their home territory against intruding conspecifics. Hamsters that lose agonistic encounters subsequently exhibit a striking change in their agonistic behavior, however, expressing no aggression and instead becoming highly submissive, a behavioral change that we have termed conditioned defeat. We have generally employed an inescapable defeat training protocol when studying conditioned defeat. The purpose of the present study was to determine if conditioned defeat is an epiphenomenon of the inescapable defeat experience by comparing the behavior of hamsters exposed to inescapable versus escapable defeat. In the conditioned defeat model, defeated hamsters subsequently generalize their submission and social avoidance to a novel, non-aggressive opponent, suggesting that hamsters subjected to inescapable defeat may not form a specific memory of their aggressive opponent. Thus, a secondary purpose of the present study was to determine whether hamsters subjected to our defeat protocol have the ability to recognize a familiar opponent following defeat. Our results provide evidence that conditioned defeat is not solely a by-product of inescapable defeat because all experimental animals, regardless of the type of defeat, expressed conditioned defeat during testing. We also found that animals experiencing an inescapable defeat avoided a familiar aggressor significantly more than they did an unfamiliar aggressor, demonstrating that these animals have the ability to recognize their previous attacker. Thus, we maintain that a variety of social defeat models, and conditioned defeat in particular, represent generalizable and ethologically valid models with which to study the effects of social stress on physiology and behavior.

NR2B subunit of the NMDA receptor in the basolateral amygdala is necessary for the acquisition of conditioned defeat in Syrian hamsters

Reversible inactivation of the basolateral amygdala (BLA) disrupts the acquisition and expression of conditioned defeat (CD), an ethological model of conditioned fear, suggesting that the BLA may be a critical component of the neural circuit mediating behavioral plasticity associated with the experience of social defeat. We have also shown that this effect is N-methyl-d-aspartic acid (NMDA) receptor-dependent, because infusion of d,l-2-amino-5-phosphovalerate (APV) into the BLA also impairs the acquisition of CD. APV is a non-selective NMDA antagonist, however, thus it disrupts the entire heteromeric receptor complex, making it difficult to distinguish the relative contributions of either the NR2A or NR2B receptor subtypes on the acquisition of CD. There is ample evidence, however, that the NR2B subunit of the NMDA receptor in the amygdala is critical for mediating long-term potentiation and plasticity related to fear learning. The purpose of the present experiment was to determine whether infusion of ifenprodil, a selective antagonist of the NR2B subunit, into the BLA would block the acquisition (but not expression) of CD. In Experiment 1, infusion of ifenprodil immediately before defeat training significantly decreased submissive behaviors and restored territorial aggression when hamsters were later paired with a non-aggressive intruder (NAI). Conversely, infusion of ifenprodil immediately before CD testing failed to inhibit the expression of submissive behaviors in previously defeated hamsters. These results support the hypothesis that the BLA is a critical site for the plasticity underlying social defeat-induced changes in behavior.

Blocking corticotropin-releasing factor-2 receptors, but not corticotropin-releasing factor-1 receptors or glucocorticoid feedback, disrupts the development of conditioned defeat

Several neuroendocrine signals of the hypothalamic-pituitary-adrenal (HPA) axis are released following exposure to stressful events. It has long been proposed that the signals in this cascade each act to modify ongoing and future behavior. In this study we investigated whether blocking glucocorticoid synthesis, corticotropin-releasing factor (CRF)-1 receptors, or CRF-2 receptors during social defeat would alter subsequent behavioral responses. We used a conditioned defeat model in Syrian hamsters in which social defeat results in a dramatic shift from territorial aggression to increased submissive and defensive behavior in future social encounters. We found that intracerebroventricular administration of anti-sauvagine-30, a CRF-2 receptor antagonist, prior to social defeat training reduced the acquisition of conditioned defeat. In contrast, the acquisition of conditioned defeat was not altered by the CRF-1 receptor antagonist CP-154,526 or the glucocorticoid synthesis inhibitor metyrapone. Our results suggest that CRF, and perhaps related neuropeptides such as urocortins, act at CRF-2 receptors to promote the development of defeat-induced changes in social behavior, whereas signaling at CRF-1 and glucocorticoid receptors plays a negligible role in this process.

Role of amygdala and hippocampus in the neural circuit subserving conditioned defeat in Syrian hamsters

We examined the roles of the amygdala and hippocampus in the formation of emotionally relevant memories using an ethological model of conditioned fear termed conditioned defeat (CD). Temporary inactivation of the ventral, but not dorsal hippocampus (VH, DH, respectively) using muscimol disrupted the acquisition of CD, whereas pretraining VH infusions of anisomycin, a protein synthesis inhibitor, failed to block CD. To test for a functional connection between the VH and basolateral amygdala (BLA), we used a classic functional connectivity design wherein injections are made unilaterally in brain areas either on the same or opposite sides of the brain. A functional connection between the BLA and VH necessary for the acquisition of CD could not be found because unilateral inactivation of either BLA alone (but not either VH alone) was sufficient to disrupt CD. This finding suggested instead that there may be a critical functional connection between the left and right BLA. In our final experiment, we infused muscimol unilaterally in the BLA and assessed Fos immunoreactivity on the contralateral side following exposure to social defeat. Inactivation of either BLA significantly reduced defeat-induced Fos immunoreactivity in the contralateral BLA. These experiments demonstrate for the first time that whereas the VH is necessary for the acquisition of CD, it does not appear to mediate the plastic changes underlying CD. There also appears to be a critical interaction between the two BLAs such that bilateral activation of this brain area must occur in order to support fear learning in this model, a finding that is unprecedented to date.

Gonadal hormones modulate the display of conditioned defeat in male Syrian hamsters

It has been widely reported that gonadal hormones influence the display of aggression in Syrian hamsters; conversely, much less is known about whether gonadal hormones modulate submissive/defensive behaviors in these animals. Following social defeat, male hamsters no longer display normal territorial aggression but instead display submissive/defensive behavior in the presence of a smaller opponent, a phenomenon we have termed conditioned defeat (CD). The purpose of the present study was to examine the effect of gonadal hormones on the display of CD in male hamsters. In Experiment 1, males were castrated or sham-operated. The castrated males were significantly more submissive following social defeat relative to their intact counterparts. The increased submissive behavior in the castrated males during CD testing was particularly surprising, given the fact that they were attacked significantly less during CD training. In Experiment 2a, males were castrated and given hormone replacement. Castrated males treated with testosterone or dihydrotestosterone displayed significantly less submissive behavior following social defeat than did those treated with cholesterol or estradiol. Finally, in Experiment 2b, there was no effect of hormone replacement on aggressive behavior in non-defeated hamsters suggesting that the decrease in submissive behavior in males treated with dihydrotestosterone or testosterone is specific to being previously defeated. Taken together the data indicate that the presence of androgens reduces the display of submission in defeated male hamsters. More importantly, these findings suggest that androgens may have a protective effect against the development of depression-like or anxiety-like behaviors following exposure to an ethologically relevant stressor.

Aggressive encounters alter the activation of serotonergic neurons and the expression of 5-HT1A mRNA in the hamster dorsal raphe nucleus

Serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN) have been implicated in stress-induced changes in behavior. Previous research indicates that stressful stimuli activate 5-HT neurons in select subregions of the DRN. Uncontrollable stress is thought to sensitize 5-HT neurons in the DRN and allow for an exaggerated 5-HT response to future stimuli. In the current study, we tested the hypothesis that following aggressive encounters, losing male Syrian hamsters would exhibit increased c-Fos immunoreactivity in 5-HT DRN neurons compared to winners or controls. In addition, we tested the hypothesis that losers would have decreased 5-HT1A mRNA levels in the DRN compared to winners or controls. We found that a single 15-min aggressive encounter increased c-Fos expression in 5-HT and non-5-HT neurons in losers compared to winners and controls. The increased c-Fos expression in losers was restricted to ventral regions of the rostral DRN. We also found that four 5-min aggressive encounters reduced total 5-HT1A mRNA levels in the DRN in losers compared to winners and controls, and that differences in mRNA levels were not restricted to specific DRN subregions. These results suggest that social defeat activates neurons in select subregions of the DRN and reduces message for DRN 5-HT1A autoreceptors. Our results support the hypothesis that social stress can activate 5-HT neurons in the DRN, reduce 5-HT1A autoreceptor-mediated inhibition, and lead to hyperactivity of 5-HT neurons.

Role of the bed nucleus of the stria terminalis in the acquisition and expression of conditioned defeat in Syrian hamsters

When Syrian hamsters (Mesocricetus auratus) are defeated by a larger, more aggressive opponent, they subsequently produce more defensive and submissive behaviors and less chemosensory investigation and aggression, even when they are paired with a smaller, non-aggressive intruder. This persistent change in the behavior of defeated animals has been termed conditioned defeat. In the present study, we tested the hypothesis that the bed nucleus of the stria terminalis (BNST) is important for the acquisition and expression of conditioned defeat. We found that the GABA(A) receptor agonist muscimol infused into the BNST immediately prior to initial defeat training failed to disrupt the acquisition of conditioned defeat, while muscimol infused prior to testing caused a significant reduction in submissive/defensive behaviors and an increase in investigatory behaviors of the non-aggressive intruder. These results indicate that (1) the BNST, unlike the amygdala, does not appear to be critically involved in the consolidation process related to the memory of social defeat and (2) the BNST may be an important site for the execution of fear behaviors associated with social defeat. Considering the high degree of connectivity between the BNST and the amygdala, these findings provide further insight into the neural circuitry governing conditioned defeat and support the view of a functional dissociation between the amygdala and the BNST in the modulation of conditioned fear in an ethologically relevant model.

Activation of 5-HT1A autoreceptors in the dorsal raphe nucleus reduces the behavioral consequences of social defeat

In animal models, serotonin (5-HT) activity contributes to stress-induced changes in behavior. Syrian hamsters (Mesocricetus auratus) exhibit a stress-induced change in behavior in which social defeat results in increased submissive and defensive behavior and a complete loss of normal territorial aggression directed toward a novel, non-aggressive opponent. We refer to this defeat-induced change in agonistic behavior as conditioned defeat. In this study we tested the hypothesis that 5-HT activity in the dorsal raphe nucleus (DRN) contributes to the acquisition and expression of conditioned defeat. We investigated whether injection of the selective 5-HT1A agonist flesinoxan (200 ng, 400 ng, or 800 ng in 200 nl saline) into the DRN would reduce the acquisition and expression of conditioned defeat. Additionally, we investigated whether injection of the selective 5-HT1A antagonist WAY 100635 (400 ng in 200 nl saline) into the DRN would enhance the acquisition and expression of conditioned defeat following a sub-optimal social defeat experience. We found that injection of flesinoxan into the DRN before exposure to a 15-min social defeat reduced the amount of submissive and defensive behavior shown at testing. We also found that injection of flesinoxan into the DRN before testing similarly reduced submissive and defensive behavior. In addition, we found that WAY 100635 enhanced conditioned defeat when injected either before social defeat or before testing. These data support the hypothesis that the activity of 5-HT cells in the DRN, as regulated by 5-HT1A autoreceptors, contributes to the formation and display of conditioned defeat. Further, our results suggest that 5-HT release in DRN projection regions augments defeat-induced changes in social behavior.

Corticotropin-releasing factor receptors in the dorsal raphe nucleus modulate social behavior in Syrian hamsters

RATIONALE

In Syrian hamsters (Mesocricetus auratus), social defeat produces a prolonged change in subsequent agonistic behavior termed conditioned defeat. This stress-induced change in behavior is marked by increased submissive and defensive behavior toward a novel, nonaggressive opponent and a complete loss of normal territorial aggression. Corticotropin-releasing factor (CRF) has been shown to affect serotonergic neurons in the dorsal raphe nucleus (DRN) and to modulate learned helplessness via a CRF type-2 receptor (CRF-R2) mechanism.

OBJECTIVES

In this study, we tested the hypothesis that a nonselective CRF receptor antagonist (experiment 1: 250 or 500 ng D: -Phe CRF in 200 nl saline), or a selective CRF-R2 antagonist (experiment 2: 500 ng anti-Svg-30 in 200 nl saline), injected into the DRN would reduce the acquisition of conditioned defeat in male hamsters. We also tested similar hypotheses for the expression of conditioned defeat (experiments 3 and 4).

RESULTS

Infusion of D: -Phe CRF into the DRN significantly reduced both the acquisition and expression of conditioned defeat compared to vehicle controls, whereas infusion of anti-Svg-30 into the DRN reduced expression but not acquisition. In particular, CRF antagonism in the DRN decreased fleeing from novel opponents but did not reinstate normal territorial aggression after social defeat.

CONCLUSIONS

Our results suggest that the increased flight associated with conditioned defeat is modulated by CRF-R2 activation within the DRN. Overall, social defeat is an ethologically relevant stressor that appears to activate at least some of the same neural substrates that have been implicated in the control of learned helplessness.

Sex and estrous cycle differences in the display of conditioned defeat in Syrian hamsters

We have reported that there is a sex difference in the behavioral response to social defeat in hamsters. While previously defeated male hamsters fail to display normal territorial aggression and instead produce submissive/defensive behavior, a phenomenon that we have termed conditioned defeat (CD), only a small portion of previously defeated females exhibit CD. In Experiment 1, we tested the hypothesis that CD varies over the estrous cycle and found that previously defeated female hamsters tested on diestrus 2 and proestrus were more likely to exhibit CD than were females tested on diestrus 1 and estrus. In Experiment 2, we found that regardless of hormonal status, non-defeated females displayed normal territorial aggression, indicating that the behavioral changes observed in Experiment 1 were not due to a cyclic variation in submissive behavior independent of a previous defeat encounter. In Experiment 3, we found that females tested 4 days after defeat responded similarly to those tested 1 day after defeat suggesting that the hormonal status of females on the day of testing is a more important determinant of the behavioral response to defeat than is the hormonal status on the day of defeat training. Finally, in Experiment 4, we monitored anxiety-like behaviors in diestrous 1 and proestrous females in an open field arena and found that there was no effect of cycle on any of the observed behavioral measures, suggesting that the observed differences in CD are not the result of differences in generalized anxiety-like behaviors across the estrous cycle.

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.

Social defeat and footshock increase body mass and adiposity in male Syrian hamsters

Obesity is a world-wide epidemic, and many factors, including stress, have been linked to this growing trend. After social stress (i.e., defeat), subordinate laboratory rats and most laboratory mice become hypophagic and, subsequently, lose body mass; the opposite is true of subordinate Syrian hamsters. After social defeat, Syrian hamsters become hyperphagic and gain body mass compared with nonstressed controls. It is unknown whether this increase in body mass and food intake is limited to subordinate hamsters. In experiment 1, we asked, do dominant hamsters increase food intake, body mass, and adiposity after an agonistic encounter? Subordinate hamsters increased food intake and body mass compared with nonstressed controls. Although there was no difference in food intake or absolute body mass between dominant and nonstressed control animals, cumulative body mass gain was significantly higher in dominant than in nonstressed control animals. Total carcass lipid and white adipose tissue (WAT) (i.e., retroperitoneal and epididymal WAT) masses were significantly increased in subordinate, but not dominant, hamsters compared with nonstressed controls. In experiment 2, we asked, does footshock stress increase food intake, body mass, and adiposity. Hamsters exposed to defeat, but not footshock stress, increased food intake relative to nonstressed controls. In animals exposed to defeat or footshock stress, body mass, as well as mesenteric WAT mass, increased compared with nonstressed controls. Collectively, these data demonstrate that social and nonsocial stressors increase body and lipid mass in male hamsters, suggesting that this species may prove useful for studying the physiology of stress-induced obesity in some humans.

Social defeat increases food intake, body mass, and adiposity in Syrian hamsters

Overeating and increases in body and fat mass are the most common responses to day-to-day stress in humans, whereas stressed laboratory rats and mice respond oppositely. Group housing of Syrian hamsters increases body mass, adiposity, and food intake, perhaps due to social confrontation-induced stress. In experiment 1 we asked, Does repeated social defeat increase food intake, body mass, and white adipose tissue (WAT) mass in Syrian hamsters? Male hamsters subjected to the resident-intruder social interaction model and defeated intermittently 15 times over 34 days for 7-min sessions significantly increased their food intake, body mass, and most WAT masses compared with nondefeated controls. Defeat significantly increased terminal adrenal norepinephrine, but not epinephrine, content. In experiment 2 we asked, Are 15 intermittent resident-intruder interactions necessary to increase body mass and food intake? Body mass and food intake of subordinate hamsters defeated only once were similar to those of nondefeated controls, but four or eight defeats similarly and significantly increased these responses. In experiment 3 we asked, Do intermittent defeats increase adiposity and food intake more than consecutive defeats? Four intermittent or consecutive defeats similarly and significantly increased food intake and body mass compared with nondefeated controls, but only intermittent defeats significantly increased all WAT masses. Consecutive defeats significantly increased mesenteric and inguinal WAT masses. Plasma leptin, but not insulin, concentrations were similarly and significantly increased compared with nondefeated controls. Collectively, social defeat, a natural stressor, significantly increased food intake, body mass, and adiposity in Syrian hamsters and may prove useful in determining mechanisms underlying human stress-induced obesity.

Role of V1a vasopressin receptors in the control of aggression in Syrian hamsters

The present study investigated the hypothesis that social isolation increases aggression by increasing the number of V1a vasopressin receptors in the anterior hypothalamus (AH). Male hamsters were randomly assigned to a group that was allowed to interact with a small nonaggressive hamster three times each week for 3 weeks (socially experienced) or to a group that did not interact socially with other hamsters (social isolates). On the final day of the experiment, hamsters in both groups were placed in a neutral arena with a small, nonaggressive intruder, and agonistic behavior was scored for 10 min. In social isolates, the duration of aggression and the number of attacks were significantly greater than in socially experienced hamsters. There were no significant between-group differences in the latency to the onset of aggression, the number of flank marks or in the duration of defensive/submissive, social or nonsocial behavior. The amount of V1a receptor binding was significantly greater in the AH, the paraventricular nucleus of the hypothalamus and the lateral hypothalamus in the social isolates than in the socially experienced hamsters. The amount of V1a receptor binding was significantly greater in the central amygdala of socially experienced hamsters than in socially isolated hamsters. Serum concentrations of testosterone were significantly higher in the socially experienced hamsters than in social isolates. These data support the hypothesis that social isolation increases aggression by increasing the number of V1a vasopressin receptors in the AH.

Repeated agonistic encounters in hamsters modulate AVP V1a receptor binding

Arginine vasopressin (AVP) regulates aggression in male Syrian hamsters. In this study, we used radioligand receptor autoradiography to examine whether changes in agonistic behavior following acute and repeated social defeat are accompanied by changes in AVP V1a receptor binding. Social defeat produced high levels of submissive behavior and a loss of territorial aggression when hamsters were subsequently tested with a novel intruder, and repeated agonistic encounters produced similar behavioral changes in subordinates. AVP V1a receptor binding was not reduced by acute social defeat but was affected by repeated agonistic encounters. Dominants had significantly more AVP V1a receptor binding in lateral portions of the ventromedial hypothalamus (VMHL) than did their subordinate opponents, but subordinates were no different from controls. In contrast, receptor binding did not differ in most other brain regions examined. The changes in receptor binding appear to be independent of testosterone levels, as testosterone levels did not differ among dominants, subordinates, and controls. Our results suggest that changes in AVP V1a receptors do not account for the changes in agonistic behavior produced by acute social defeat but AVP V1a binding in the VMHL correlates with, and may modulate, the behavioral changes that occur following repeated experiences of victory.

Memory of social defeat is facilitated by cAMP response element-binding protein overexpression in the amygdala

The cAMP-responsive element binding protein (CREB) is a transcription factor that regulates synaptic plasticity and memory formation. Studies that have used conditioned fear models have established that CREB is important for the acquisition and consolidation of fear learning. The authors demonstrate that overexpression of CREB within the basolateral amygdala (BLA) of animals that are exposed to social defeat enhances subsequent defeat-induced changes in social behavior. This effect is specific to the acquisition of defeat-induced behaviors; overexpression of CREB has no effect on the expression of these behaviors if the overexpression occurs after the initial defeat. These data demonstrate that CREB is important for regulating learning not only to explicit cues but also for mediating behavioral plasticity in ethologically relevant social contexts.

Gonadal hormones modulate the display of submissive behavior in socially defeated female Syrian hamsters

There are striking differences in the behavioral response to social defeat between male and female Syrian hamsters. Whereas males exhibit a prolonged behavioral response to defeat (i.e., conditioned defeat), many females remain aggressive or show only a transient submissive response following defeat. The current study tested the hypothesis that sex steroids underlie this differential behavioral responsivity to social defeat. Female hamsters were ovariectomized and implanted with Silastic capsules containing estradiol (E(2)), testosterone (T), progesterone (P), dihydrotestosterone (DHT), or a blank capsule (no hormone replacement). After a 3-week recovery period, each subject was placed inside the home cage of a larger, more aggressive female for four 5-min defeat trials. The following day, each animal was tested for conditioned defeat by testing it in its own home cage in the presence of a smaller, non-aggressive intruder. Submissive, aggressive, social, and nonsocial behaviors were subsequently scored. Hamsters receiving E(2) or T displayed significantly lower levels of submissive behavior than did animals receiving P, DHT, or no hormone replacement. There were no significant differences in aggressive behavior among groups. These data suggest that gonadal hormones can influence submissive behavior in female hamsters. Collectively, these results suggest that the sex differences observed in conditioned defeat may, in part, be explained by sex differences in circulating gonadal hormones.

Involvement of central amygdalar and bed nucleus of the stria terminalis corticotropin-releasing factor in behavioral responses to social defeat

The authors investigated whether corticotropin-releasing factor (CRF) within the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST) is a critical component of the neural circuitry mediating conditioned defeat. In this model, hamsters that have experienced social defeat subsequently display only submissive-defensive agonistic behavior instead of territorial aggression. Conditioned defeat was significantly reduced following infusion of the CRF receptor antagonist D-Phe CRF((12-41)) into the BNST but not into the CeA. In another experiment, hamsters given unilateral lesions of the CeA and infusions of D-Phe CRF((12-41)) into the contralateral BNST displayed significantly less submissive behavior than did controls. These data suggest that CRF acts within a neural circuit that includes the amygdala and the BNST to modulate agonistic behavior following social defeat.

Corticotropin-releasing factor type II (CRF₂) receptors in the bed nucleus of the stria terminalis modulate conditioned defeat in Syrian hamsters (Mesocricetus auratus)

In Syrian hamsters (Mesocricetus auratus), social defeat produces a subsequent increase in submissive and defensive behavior and a loss of normal territorial aggression, which the authors have called conditioned defeat. In this study, the authors investigated the effect of blocking corticotropin-releasing factor (CRF) Type I and Type II receptors on conditioned defeat. Intracerebroventricular infusion of the CRF-sub-2 receptor antagonist antisauvagine-30 prior to testing significantly reduced conditioned defeat compared with vehicle controls, whereas the CRF-sub-1 receptor antagonist CP-154,526 had no effect. Also, infusion of antisauvagine-30 into the bed nucleus of the stria terminalis (BNST) 15 min, but not immediately, prior to testing reduced conditioned defeat in a dose-dependent manner. The authors' results provide evidence that CRF-sub-2 receptors in the BNST, but not CRF-sub-1 receptors, are an important component in the neural circuitry regulating conditioned defeat.

GABA(B) receptor activation in the suprachiasmatic nucleus of diurnal and nocturnal rodents

Diurnal (day-active) and nocturnal (night-active) animals have very different daily activity patterns. We recently demonstrated that the suprachiasmatic nucleus (SCN) responds to GABAergic stimulation differently in diurnal and nocturnal animals. Specifically, GABAA receptor activation with muscimol during the subjective day causes phase delays in diurnal grass rats while producing phase advances in nocturnal hamsters. The aim of the following experiments was to determine if diurnal and nocturnal animals differ in their response to GABAB receptor activation in the SCN. Baclofen, a GABAB receptor agonist, was microinjected into the SCN region of grass rats or hamsters under free-running conditions and phase alterations were analyzed. Changes in phase were not detected after baclofen treatment during the subjective day in either grass rats or hamsters. During the night, however, GABAB receptor activation significantly decreased the ability of light to induce phase delays in grass rats. Taken together with previous data from our laboratory, these results demonstrate that, in both hamsters and grass rats, GABAB receptor activation in the SCN significantly affects circadian phase during the night, but not during the day.

N-methyl-D-aspartate receptors in the amygdala are necessary for the acquisition and expression of conditioned defeat

Here, we describe a biologically relevant model called conditioned defeat that is used to examine behavioral responses to social defeat in Syrian hamsters. In this model experimental animals that are normally aggressive experience social defeat and consequently display high levels of submissive/defensive behavior even in response to non-threatening conspecifics. N-methyl-D-aspartate (NMDA) receptors within the amygdala play an important role in conditioned fear; therefore, the purpose of this study was to examine whether NMDA receptors within the amygdala are necessary for the acquisition and expression of conditioned defeat. Specifically, the present study examined whether bilateral infusions of the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5; 0.625, 1.25, 2.5, 5.0, 10.0 microg) into the amygdala would block the acquisition of conditioned defeat. Subsequently, we examined whether bilateral infusions of AP5 (0.625, 1.25, 2.5, 5.0 microg) into the amygdala prior to testing would block the expression of conditioned defeat. Infusions of AP5 into the amygdala immediately before the initial social defeat significantly reduced submissive/defensive behavior when hamsters were tested the following day with a non-aggressive intruder. Similarly, infusions of AP5 into the amygdala immediately before exposure to a non-aggressive intruder significantly attenuated the display of submissive/defensive behavior. These data demonstrate that NMDA receptors are necessary for both the acquisition and expression of conditioned defeat. We believe that conditioned defeat is a unique and valuable animal model with which to investigate the neurobiology of fear-related changes in social behavior.

Conditioned defeat in male and female Syrian hamsters

A brief exposure to social defeat in male Syrian hamsters (Mesocricetus auratus) leads to profound changes in the subsequent agonistic behavior exhibited by the defeated animals. Following defeat in the home cage of an aggressive conspecific, male hamsters will subsequently fail to defend their home territory even if the intruder is a smaller, nonaggressive male. This phenomenon has been called conditioned defeat. In Experiment 1, we examined the duration of conditioned defeat by repeatedly testing (every 3-5 days) defeated hamsters with a nonaggressive intruder. We found that conditioned defeat occurs in all defeated male hamsters and persists for a prolonged period of time (at least 33 days) in the majority of male hamsters tested despite the fact that these animals are never attacked by the nonaggressive intruders. In Experiment 2, we examined whether conditioned defeat could be induced in female Syrian hamsters. While conditioned defeat occurred in some females, they displayed only low levels of submissive/defensive behavior and, in contrast to males, the conditioned defeat response did not persist beyond the first test. These results suggest that in male hamsters conditioned defeat is a profound, persistent behavioral change characterized by a total absence of territorial aggression and by the frequent display of submissive and defensive behaviors. Conversely, social defeat in female hamsters does not appear to induce long-term behavioral changes. Finally, in Experiment 3, we determined that plasma adrenocorticotropin-like immunoreactivity increases in females following social defeat in a manner similar to that seen in males, suggesting that the disparate behavioral reactions of males and females are not due to sex differences in the release of, or response to, plasma adrenocorticotropin.

Oxytocin inhibits aggression in female Syrian hamsters

Dominant subordinate relationships are formed as the result of social conflict and are maintained at least in part by communication. At this time, little is known about the neural mechanisms that are responsible for coordinating the social behaviours (e.g. aggression) that occur in association with the formation and maintenance of these relationships. The purpose of the present study was to investigate the role of oxytocin (OXT) within the medial preoptic anterior hypothalamic continuum (MPOA-AH) in the control of aggression in female hamsters. OXT injected into the MPOA-AH immediately before testing significantly reduced the duration of aggression in a dose-dependent manner. Injection of an OXT antagonist 30 min before testing significantly increased the duration of aggression. In contrast, the duration of aggression was not altered when hamsters were tested either 30 min after injection of OXT or immediately following injection of an OXT-antagonist. These data support the hypothesis that OXT release within the MPOA-AH regulates social behaviours important in the formation and maintenance of dominant subordinate relationships in female hamsters.

Short days and exogenous melatonin increase aggression of male Syrian hamsters (Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, "winter-like" compared with long "summer-like" day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters (Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 microg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.