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

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.

Stress, associative learning, and decision-making

Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.

The hippocampus in stress susceptibility and resilience: Reviewing molecular and functional markers

Understanding the individual variability that comes with the likelihood of developing stress-related psychopathologies is of paramount importance when addressing mechanisms of their neurobiology. This article focuses on the hippocampus as a region that is highly influenced by chronic stress exposure and that has strong ties to the development of related disorders, such as depression and post-traumatic stress disorder. We first outline three commonly used animal models that have been used to separate animals into susceptible and resilient cohorts. Next, we review molecular and functional hippocampal markers of susceptibility and resilience. We propose that the hippocampus plays a crucial role in the differences in the processing and storage of stress-related information in animals with different stress susceptibilities. These hippocampal markers not only help us attain a more comprehensive understanding of the various facets of stress-related pathophysiology, but also could be targeted for the development of new treatments.

Quantifying Sex Differences in Behavior in the Era of "Big" Data

Sex differences are commonly observed in behaviors that are closely linked to adaptive function, but sex differences can also be observed in behavioral "building blocks" such as locomotor activity and reward processing. Modern neuroscientific inquiry, in pursuit of generalizable principles of functioning across sexes, has often ignored these more subtle sex differences in behavioral building blocks that may result from differences in these behavioral building blocks. A frequent assumption is that there is a default (often male) way to perform a behavior. This approach misses fundamental drivers of individual variability within and between sexes. Incomplete behavioral descriptions of both sexes can lead to an overreliance on reduced "single-variable" readouts of complex behaviors, the design of which may be based on male-biased samples. Here, we advocate that the incorporation of new machine-learning tools for collecting and analyzing multimodal "big behavior" data allows for a more holistic and richer approach to the quantification of behavior in both sexes. These new tools make behavioral description more robust and replicable across laboratories and species, and may open up new lines of neuroscientific inquiry by facilitating the discovery of novel behavioral states. Having more accurate measures of behavioral diversity in males and females could serve as a hypothesis generator for where and when we should look in the brain for meaningful neural differences.

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.

Anxious to see you: Neuroendocrine mechanisms of social vigilance and anxiety during adolescence

Social vigilance is a behavioral strategy commonly used in adverse or changing social environments. In animals, a combination of avoidance and vigilance allows an individual to evade potentially dangerous confrontations while monitoring the social environment to identify favorable changes. However, prolonged use of this behavioral strategy in humans is associated with increased risk of anxiety disorders, a major burden for human health. Elucidating the mechanisms of social vigilance in animals could provide important clues for new treatment strategies for social anxiety. Importantly, during adolescence the prevalence of social anxiety increases significantly. We hypothesize that many of the actions typically characterized as anxiety behaviors begin to emerge during this time as strategies for navigating more complex social structures. Here, we consider how the social environment and the pubertal transition shape neural circuits that modulate social vigilance, focusing on the bed nucleus of the stria terminalis and prefrontal cortex. The emergence of gonadal hormone secretion during adolescence has important effects on the function and structure of these circuits, and may play a role in the emergence of a notable sex difference in anxiety rates across adolescence. However, the significance of these changes in the context of anxiety is still uncertain, as not enough studies are sufficiently powered to evaluate sex as a biological variable. We conclude that greater integration between human and animal models will aid the development of more effective strategies for treating social anxiety.

Brain-derived neurotrophic factor signaling mitigates the impact of acute social stress

Brain-derived neurotrophic factor (BDNF) is known to promote fear learning as well as avoidant behavioral responses to chronic social defeat stress, but, conversely, this peptide can also have antidepressant effects and can reduce depressant-like symptoms such as social avoidance. The purpose of this study was to use a variety of approaches to determine whether BDNF acting on tropomyosin receptor kinase B (TrkB) promotes or prevents avoidant phenotypes in hamsters and mice that have experienced acute social defeat stress. We utilized systemic and brain region-dependent manipulation of BDNF signaling before or immediately following social defeat stress in Syrian hamsters, TrkB^F616A knock-in mice, and C57Bl/6J mice and measured the subsequent behavioral response to a novel opponent. Systemic TrkB receptor agonists reduced, and TrkB receptor antagonists enhanced, behavioral responses to social defeat in hamsters and mice. In the neural circuit that we have shown mediates defeat-induced behavioral responses, BDNF in the basolateral amygdala, but not the nucleus accumbens, also reduced social avoidant phenotypes. Conversely, knockdown in the basolateral amygdala of TrkB signaling in TrkB^F616A mice enhanced defeat-induced social avoidance. These data demonstrate that systemic administration of BDNF-TrkB drugs at the time of social defeat alters the behavioral response to the defeat stressor. These drugs appear to act, at least in part, in the basolateral amygdala and not the nucleus accumbens. These findings were generalizable to two rodent species with very different social structures and, within mice, to a variety of strains providing converging evidence that BDNF-TrkB signaling reduces anxiety- and depression-like symptoms following short-term social stress.

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.

Dominance status alters restraint-induced neural activity in brain regions controlling stress vulnerability

Understanding the cellular mechanisms that control resistance and vulnerability to stress is an important step toward identifying novel targets for the prevention and treatment of stress-related mental illness. In Syrian hamsters, dominant and subordinate animals exhibit different behavioral and physiological responses to social defeat stress, with dominants showing stress resistance and subordinates showing stress vulnerability. We previously found that dominant and subordinate hamsters show different levels of defeat-induced neural activity in brain regions that modulate coping with stress, although the extent to which status-dependent differences in stress vulnerability generalize to non-social stressors is unknown. In this study, dominant, subordinate, and control male Syrian hamsters were exposed to acute physical restraint for 30min and restraint-induced c-Fos immunoreactivity was quantified in select brain regions. Subordinate animals showed less restraint-induced c-Fos immunoreactivity in the infralimbic (IL), prelimbic (PL), and ventral medial amygdala (vMeA) compared to dominants, which is consistent with the status-dependent effects of social defeat stress. Subordinate animals did not show increased c-Fos immunoreactivity in the rostroventral dorsal raphe nucleus (rvDRN), which is in contrast to the effects of social defeat stress. These findings indicate that status-dependent changes in neural activity generalize from one stressor to another in a brain region-dependent manner. These findings further suggest that while some neural circuits may support a generalized form of stress resistance, others may provide resistance to specific stressors.

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.

Social housing and social isolation: Impact on stress indices and energy balance in male and female Syrian hamsters (Mesocricetus auratus)

Although Syrian hamsters are thought to be naturally solitary, recent evidence from our laboratory demonstrates that hamsters may actually prefer social contact. Hamsters increase their preference for a location associated with an agonistic encounter regardless of whether they have "won" or "lost". It has also been reported that social housing as well as exposure to intermittent social defeat or to a brief footshock stressor increase food intake and body mass in hamsters. By contrast, it has also been suggested that housing hamsters in social isolation causes anxiety-induced anorexia and reductions in body mass selectively in females. The purpose of this study was to determine the physiological consequences of housing hamsters in social isolation versus in social groups. Male and female hamsters were housed singly or in stable groups of 5 for 4weeks after which they were weighed and trunk blood was collected. In addition, fat pads and thymus and adrenal glands were extracted and weighed. Serum and fecal cortisol were measured using an enzyme-linked immunoassay. Housing condition had no effect on serum or fecal cortisol, but socially housed hamsters displayed modest thymus gland involution. Socially housed females weighed more than did any other group, and socially housed females and males had more fat than did socially isolated hamsters. No wounding or tissue damage occurred in grouped hamsters. Overall, these data suggest that Syrian hamsters tolerate both stable social housing and social isolation in the laboratory although social housing is associated with some alteration in stress-related and bioenergetic measures.

De novo assembly, annotation, and characterization of the whole brain transcriptome of male and female Syrian hamsters

Hamsters are an ideal animal model for a variety of biomedical research areas such as cancer, virology, circadian rhythms, and behavioural neuroscience. The use of hamsters has declined, however, most likely due to the dearth of genetic tools available for these animals. Our laboratory uses hamsters to study acute social stress, and we are beginning to investigate the genetic mechanisms subserving defeat-induced behavioural change. We have been limited, however, by the lack of genetic resources available for hamsters. In this study, we sequenced the brain transcriptome of male and female Syrian hamsters to generate the necessary resources to continue our research. We completed a de novo assembly and after assembly optimization, there were 113,329 transcripts representing 14,530 unique genes. This study is the first to characterize transcript expression in both female and male hamster brains and offers invaluable information to promote understanding of a host of important biomedical research questions for which hamsters are an excellent model.

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.

Activation of 5-HT2a receptors in the basolateral amygdala promotes defeat-induced anxiety and the acquisition of conditioned defeat in Syrian hamsters

Conditioned defeat is a model in Syrian hamsters (Mesocricetus auratus) in which normal territorial aggression is replaced by increased submissive and defensive behavior following acute social defeat. The conditioned defeat response involves both a fear-related memory for a specific opponent as well as anxiety-like behavior indicated by avoidance of novel conspecifics. We have previously shown that systemic injection of a 5-HT2a receptor antagonist reduces the acquisition of conditioned defeat. Because neural activity in the basolateral amygdala (BLA) is critical for the acquisition of conditioned defeat and BLA 5-HT2a receptors can modulate anxiety but have a limited effect on emotional memories, we investigated whether 5-HT2a receptor modulation alters defeat-induced anxiety but not defeat-related memories. We injected the 5-HT2a receptor antagonist MDL 11,939 (0 mM, 1.7 mM or 17 mM) or the 5-HT2a receptor agonist TCB-2 (0 mM, 8 mM or 80 mM) into the BLA prior to social defeat. We found that injection of MDL 11,939 into the BLA impaired acquisition of the conditioned defeat response and blocked defeat-induced anxiety in the open field, but did not significantly impair avoidance of former opponents in the Y-maze. Furthermore, we found that injection of TCB-2 into the BLA increased the acquisition of conditioned defeat and increased anxiety-like behavior in the open field, but did not alter avoidance of former opponents. Our data suggest that 5-HT2a receptor signaling in the BLA is both necessary and sufficient for the development of conditioned defeat, likely via modulation of defeat-induced anxiety.

5-HT1A receptor activation reduces fear-related behavior following social defeat in Syrian hamsters

Social defeat leads to selective avoidance of familiar opponents as well as general avoidance of novel, non-threatening intruders. Avoidance of familiar opponents represents a fear-related memory whereas generalized social avoidance indicates anxiety-like behavior. We have previously shown that serotonin signaling alters responses to social defeat in Syrian hamsters, although it is unclear whether serotonin modulates defeat-induced fear, anxiety, or both. In this study we focus on 5-HT1A receptors, in part, because their activation had been linked to the acquisition of conditioned fear. We hypothesized that pharmacological activation of 5-HT1A receptors prior to social defeat would reduce avoidance of familiar opponents and impair Arc expression in the basolateral amygdala (BLA), but not alter anxiety-like behavior. We administered 8-OH-DPAT, a 5-HT1A receptor agonist, prior to 3, 5-minute social defeats and 24h later exposed hamsters to a social interaction test to measure the conditioned defeat response immediately followed by either a Y-maze test or an open field test. In a separate experiment, we administered 8-OH-DPAT prior to 3, 5-minute social defeats and later removed the brains for Arc immunohistochemistry. Social defeat increased the number of Arc immunopositive cells in the central amygdala (CeA), prelimbic cortex (PL), and BLA, and 8-OH-DPAT treatment reduced Arc immunoreactivity in the PL. These results suggest that 5-HT1A receptor activation impairs the fear memory associated with social defeat, but does not alter defeat-induced anxiety. Overall, 5-HT1A receptor activation may impair Arc expression in select brain regions such as the PL and thereby disrupt the development of a fear memory essential for the conditioned defeat response.

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.

A low-cost automated apparatus for investigating the effects of social defeat in Syrian hamsters

We describe an automated apparatus that can be used to investigate the effects of defeat in hamsters. It consists of a covered alleyway that leads to a box, or arena, where hamsters can be kept separate or allowed to fight. The alleyway is divided into seven equal-sized chambers. Low-power lasers and laser detectors are used to keep track of a hamster's position in the alleyway. A CFL flood lamp placed over the chamber farthest from the arena generates a light gradient in the alleyway that engenders in the subjects a preference for the darker chambers near the arena. A computer automatically records the interruption of the laser beams and yields three measures: average position, the frequency of visits to each chamber, and the frequency of changes in direction of travel in each chamber. The results of a pilot study indicated that when a dominant hamster was placed behind a screened gate in the arena and a subordinate hamster was placed in the alleyway, the subordinate maintained a significantly greater distance from the dominant than did a nondefeated hamster. The subordinate hamster also changed its direction of travel more frequently than did the nondefeated hamster. The results suggest that conditioned fear was elicited in the defeated hamster by proximity to the dominant hamster, an effect that is consistent with published results in which the data were recorded manually or by using commercially available event-tracking software.

Prior fighting experience increases aggression in Syrian hamsters: implications for a role of dopamine in the winner effect

Winning an aggressive encounter enhances the probability of winning future contests. This phenomenon, known as the winner effect, has been well studied across vertebrate species. While numerous animal models have been developed to study the winner effect in the laboratory setting, large variation in experimental design, choice of species, and housing conditions have resulted in conflicting reports on the behavioral outcomes. The Syrian hamster (Mesocricetus auratus) presents as a novel species with face validity to study the effects of repeated fighting on subsequent agonistic encounters. After a 14-day training period, "trained fighter" hamsters displayed elevated fighting behaviors characterized by more intense and severe displays of aggression along with increased displays of dominant postures compared to naïve residents with no prior social experience. To determine whether these phenotypic changes in fighting behavior reflect alterations in neurochemistry, brains of aggressive and naïve hamsters were examined for changes in dopaminergic innervation in key regions known to control social and motivational behavior. Interestingly, changes in tyrosine hydroxylase, the rate limiting enzyme for dopamine production, were observed in brain regions within the social decision-making network. These increases in aggression observed after repeated winning may reflect a learned behavior resulting from increases in neurotransmitter activity which serve to reinforce the behavior. The data implicate the presence of a winner effect in hamsters and provide evidence for a neural mechanism underlying the changes in aggressive behavior after repeated agonistic encounters.

Social stress models in depression research: what do they tell us?

Interest has recently surged in the use of social stress models, especially social defeat. Such interest lies both in the recognition that stressors of social origin play a major role in human psychopathologies and in the acknowledgement that natural and hence ethologically-based stress models have important translational value. The use of the most recent technology has allowed the recognition of the mechanisms through which social defeat might have enduring psychoneuroendocrine effects, especially social avoidance and anhedonia, two behaviours relevant to human depression. In view of the sensitivity of these behavioural outcomes to repeated antidepressant treatments, the social defeat model has been proposed as a possible animal model of depression. The present survey is aimed at examining the limits of such an interpretation and focuses on methodological aspects and on the relevance of social defeat to the study of anxiety-related pathologies.

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.