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

Dynamics in brain activation and behaviour in acute and repeated social defensive behaviour

In nature, confrontations between conspecifics are recurrent and related, in general, due to the lack of resources such as food and territory. Adequate defence against a conspecific aggressor is essential for the individual's survival and the group integrity. However, repeated social defeat is a significant stressor promoting several behavioural changes, including social defence per se. What would be the neural basis of these behavioural changes? To build new hypotheses about this, we here investigate the effects of repeated social stress on the neural circuitry underlying motivated social defence behaviour in male mice. We observed that animals re-exposed to the aggressor three times spent more time in passive defence during the last exposure than in the first one. These animals also show less activation of the amygdalar and hypothalamic nuclei related to the processing of conspecific cues. In turn, we found no changes in the activation of the hypothalamic dorsal pre-mammillary nucleus (PMD) that is essential for passive defence. Therefore, our data suggest that the balance between the activity of circuits related to conspecific processing and the PMD determines the pattern of social defence behaviour. Changes in this balance may be the basis of the adaptations in social defence after repeated social defeat.

Coordination of social behaviors by the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a sexually dimorphic, neuropeptide-rich node of the extended amygdala that has been implicated in responses to stress, drugs of abuse, and natural rewards. Its function is dysregulated in neuropsychiatric disorders that are characterized by stress- or drug-induced alterations in mood, arousal, motivation, and social behavior. However, compared to the BNST's role in mood, arousal, and motivation, its role in social behavior has remained relatively understudied. Moreover, the precise cell types and circuits underlying the BNST's role in social behavior have only recently begun to be explored using modern neuroscience techniques. Here, we systematically review the existing literature investigating the neurobiological substrates within the BNST that contribute to the coordination of various sex-dependent and sex-independent social behavioral repertoires, focusing largely on pharmacological and circuit-based behavioral studies in rodents. We suggest that the BNST coordinates social behavior by promoting appropriate assessment of social contexts to select relevant behavioral outputs and that disruption of socially relevant BNST systems by stress and drugs of abuse may be an important factor in the development of social dysfunction in neuropsychiatric disorders.

Mean Girls: Social Stress Models for Female Rodents

Social stressors are known to have strong negative impacts on mental health. There is a long history of preclinical social defeat stress studies in rodents focusing on males that has produced important insights into the neural mechanisms that modulate depression- and anxiety-related behavior. Despite these impressive results, a historical weakness of rodent social stress models has been an under-representation of studies in females. This is problematic because rates of depression and anxiety are higher in women versus men. Recently there has been a surge of interest in adapting social stress methods for female rodents. Here we review new rodent models that have investigated numerous facets of social stress in females. The different models have different strengths and weaknesses, with some model systems having stronger ethological validity with other models having better access to molecular tools to manipulate neural circuits. Continued use and refinement of these complementary models will be critical for addressing gaps in understanding the function of neural circuits modulating depression- and anxiety-related behavior in females.

Extrahypothalamic oxytocin neurons drive stress-induced social vigilance and avoidance

Oxytocin increases the salience of both positive and negative social contexts and it is thought that these diverse actions on behavior are mediated in part through circuit-specific action. This hypothesis is based primarily on manipulations of oxytocin receptor function, leaving open the question of whether different populations of oxytocin neurons mediate different effects on behavior. Here we inhibited oxytocin synthesis in a stress-sensitive population of oxytocin neurons specifically within the medioventral bed nucleus of the stria terminalis (BNSTmv). Oxytocin knockdown prevented social stress-induced increases in social vigilance and decreases in social approach. Viral tracing of BNSTmv oxytocin neurons revealed fibers in regions controlling defensive behaviors, including lateral hypothalamus, anterior hypothalamus, and anteromedial BNST (BNSTam). Oxytocin infusion into BNSTam in stress naïve mice increased social vigilance and reduced social approach. These results show that a population of extrahypothalamic oxytocin neurons plays a key role in controlling stress-induced social anxiety behaviors.

Extrahypothalamic oxytocin neurons drive stress-induced social vigilance and avoidance

Oxytocin increases the salience of both positive and negative social contexts and it is thought that these diverse actions on behavior are mediated in part through circuit-specific action. This hypothesis is based primarily on manipulations of oxytocin receptor function, leaving open the question of whether different populations of oxytocin neurons mediate different effects on behavior. Here we inhibited oxytocin synthesis in a stress-sensitive population of oxytocin neurons specifically within the medioventral bed nucleus of the stria terminalis (BNSTmv). Oxytocin knockdown prevented social stress-induced increases in social vigilance and decreases in social approach. Viral tracing of BNSTmv oxytocin neurons revealed fibers in regions controlling defensive behaviors, including lateral hypothalamus, anterior hypothalamus, and anteromedial BNST (BNSTam). Oxytocin infusion into BNSTam in stress naïve mice increased social vigilance and reduced social approach. These results show that a population of extrahypothalamic oxytocin neurons plays a key role in controlling stress-induced social anxiety behaviors.

Threat imminence dictates the role of the bed nucleus of the stria terminalis in contextual fear

Recent work indicates that the bed nucleus of the stria terminalis (BNST) is critically involved in the regulation of conditioned fear responses to unpredictable threats. Here we examined whether the involvement of the BNST in contextual fear conditioning in male rats depends on the imminence of shock after placement in the conditioning chamber. Specifically, we hypothesized that the BNST supports contextual freezing after conditioning with delayed, but not imminent, footshock (relative to placement in the context). Rats were implanted with cannulae targeting the BNST and underwent a contextual fear conditioning procedure in which a single footshock unconditioned stimulus (US) was delivered either 1 min or 9 min after the rat was placed in the context; the rats received a total of four identical conditioning sessions over two days, all with equivalent exposure to the context. Contexts associated with either imminent or delayed US onsets produced distinct patterns of freezing and shock-induced activity but freezing in each case was context-dependent. Reversible inactivation of the BNST reduced the expression of contextual freezing in the context paired with delayed (9 min), but not imminent (1 min), footshock onset. Implications of these data are discussed in the light of recent conceptualizations of BNST function, as well as for anxiety behaviors.

Neural circuits for coping with social defeat

When resources, such as food, territory, and potential mates are limited, competition among animals of the same species is inevitable. Over bouts of agonistic interactions, winners and losers are determined. Losing is a traumatic experience, both physically and psychologically. Losers not only need to deploy a set of species-specific defensive behaviors to minimize the physical damage during defeat, but also adjust their behavior towards the winners to avoid future fights in which they are likely disadvantaged. The expression of defensive behaviors and the fast and long-lasting changes in behaviors accompanying defeat must be supported by a complex neural circuit. This review summarizes the brain regions that have been implicated in coping with social defeat, one centered on basolateral amygdala and the other on ventromedial hypothalamus. Gaps in our knowledge and hypotheses that may help guide future experiments are also discussed.

Complementary Neural Circuits for Divergent Effects of Oxytocin: Social Approach Versus Social Anxiety

Oxytocin (OT) is widely known for promoting social interactions, but there is growing appreciation that it can sometimes induce avoidance of social contexts. The social salience hypothesis posed an innovative solution to these apparently opposing actions by proposing that OT enhances the salience of both positive and negative social interactions. The mesolimbic dopamine system was put forth as a likely system to evaluate social salience owing to its well-described role in motivation. Evidence from several sources supports the premise that OT acting within the nucleus accumbens and ventral tegmental area facilitates social reward and approach behavior. However, in aversive social contexts, additional pathways play critical roles in mediating the effects of OT. Recent data indicate that OT acts in the bed nucleus of the stria terminalis to induce avoidance of potentially dangerous social contexts. Here, we review evidence for neural circuits mediating the effects of OT in appetitive and aversive social contexts. Specifically, we propose that distinct but potentially overlapping circuits mediate OT-dependent social approach or social avoidance. We conclude that a broader and more inclusive consideration of neural circuits of social approach and avoidance is needed as the field continues to evaluate the potential of OT-based therapeutics.

Bed nucleus of the stria terminalis regulates fear to unpredictable threat signals

The bed nucleus of the stria terminalis (BNST) has been implicated in conditioned fear and anxiety, but the specific factors that engage the BNST in defensive behaviors are unclear. Here we examined whether the BNST mediates freezing to conditioned stimuli (CSs) that poorly predict the onset of aversive unconditioned stimuli (USs) in rats. Reversible inactivation of the BNST selectively reduced freezing to CSs that poorly signaled US onset (e.g., a backward CS that followed the US), but did not eliminate freezing to forward CSs even when they predicted USs of variable intensity. Additionally, backward (but not forward) CSs selectively increased Fos in the ventral BNST and in BNST-projecting neurons in the infralimbic region of the medial prefrontal cortex (mPFC), but not in the hippocampus or amygdala. These data reveal that BNST circuits regulate fear to unpredictable threats, which may be critical to the etiology and expression of anxiety.

Adolescent environmental enrichment prevents behavioral and physiological sequelae of adolescent chronic stress in female (but not male) rats

The late adolescent period is characterized by marked neurodevelopmental and endocrine fluctuations in the transition to early adulthood. Adolescents are highly responsive to the external environment, which enhances their ability to adapt and recover from challenges when given nurturing influences, but also makes them vulnerable to aberrant development when exposed to prolonged adverse situations. Female rats are particularly sensitive to the effects of chronic stress in adolescence, which manifests as passive coping strategies and blunted hypothalamo-pituitary adrenocortical (HPA) stress responses in adulthood. We sought to intervene by exposing adolescent rats to environmental enrichment (EE) immediately prior to and during chronic stress, hypothesizing that EE would minimize or prevent the long-term effects of stress that emerge in adult females. To test this, we exposed male and female rats to EE on postnatal days (PND) 33-60 and implemented chronic variable stress (CVS) on PND 40-60. CVS consisted of twice-daily unpredictable stressors. Experimental groups included: CVS/unenriched, unstressed/EE, CVS/EE and unstressed/unenriched (n = 10 of each sex/group). In adulthood, we measured behavior in the open field test and forced swim test (FST) and collected blood samples following the FST. We found that environmental enrichment given during the adolescent period prevented the chronic stress-induced transition to passive coping in the FST and reversed decreases in peak adrenocortical responsiveness observed in adult females. Adolescent enrichment had little to no effect on males or unstressed females tested in adulthood, indicating that beneficial effects are specific to females that were exposed to chronic stress.

Stress, sex, and motivated behaviors

Stress is a major risk factor for development of psychiatric disorders such as depression and development of substance use disorder. Although there are important sex differences in the prevalence of these disorders, most preclinical models used to study stress-induced disorders have used males only. Social defeat stress is a commonly used method to induce stress in an ethologically relevant way but has only recently begun to be used in female rodents. Using these new female models, recent studies have examined how social defeat stress affects males and females differently at the behavioral, circuit, and molecular levels. This Mini-Review discusses sex differences in the effects of social defeat stress on social behavior and drug-seeking behavior as well as its impact on the mesolimbic dopamine system and the highly connected region of the bed nucleus of the stria terminalis. © 2016 Wiley Periodicals, Inc.

Evaluating social defeat as a model for psychopathology in adult female rodents

Social conflict is a predominant stressor in humans and is associated with increased risk for developing psychological illnesses including depression and anxiety. Overwhelmingly, more women suffer from these disorders, which may be due to increased stress sensitivity. Like humans, rodents experience a myriad of physiological and behavioral sequelae due to prolonged stress exposure. Although the motivation for social conflict may differ between humans and rodents, female rodents may provide an opportunity to explore the underlying mechanisms by which stress confers risk for psychopathology in women. Because most female rodents do not express spontaneous aggression, the majority of basic research examines the physiological and behavioral outcomes of social conflict in male rodents. However, there are instances where female rodents exhibit territorial (California mice and Syrian hamsters) and maternal aggression (rats, mice, and hamsters) creating a venue to examine sex differences in physiology and behavior in response to stress. While many studies rely upon nonsocial behavioral assays (e.g., elevated plus maze, forced swim test) to assess the impact of stress on emotionality, here we primarily focus on behavioral outcomes in social-based assays in rodents. This is critically important given that disruptions in social relationships can be a cause and consequence of neuropsychiatric diseases. Next, we briefly discuss how sex differences in the recruitment of neural circuitry and/or neurochemistry in response to stress may underlie sex differences in neuroendocrine and behavioral stress responses. Finally, the translational value of females in rodent stress models and considerations regarding behavioral interpretations of these models are discussed. © 2016 Wiley Periodicals, Inc.

Inhibition of vasopressin V1a receptors in the medioventral bed nucleus of the stria terminalis has sex- and context-specific anxiogenic effects

Vasopressin V1a receptors (V1aR) are thought to contribute to the pathophysiology of psychiatric disorders such as anxiety and depression, sparking interest in V1aR as a therapeutic target. Although the global effects of V1aR have been documented, less is known about the specific neural circuits mediating these effects. Moreover, few studies have examined context-specific V1aR function in both males and females. By using the California mouse, we first studied the effects of sex and social defeat stress on V1aR binding in the forebrain. In females but not males, V1aR binding in the bed nucleus of the stria terminalis (BNST) was negatively correlated to social interaction behavior. In females stress also increased V1aR binding in the nucleus accumbens (NAc). Infusions of V1aR antagonist in to the medioventral BNST (BNSTmv) had anxiogenic effects only in animals naïve to defeat. For males, inhibition of V1aR in BNSTmv had anxiogenic effects in social and nonsocial contexts, but for females, anxiogenic effects were limited to social contexts. In stressed females, inhibition of V1aR in the NAc shell had no effect on social interaction behavior, but had an anxiogenic effect in an open field test. These data suggest that V1aR in BNSTmv have anxiolytic and prosocial effects in males, and that in females, prosocial and anxiolytic effects of V1aR appear to be mediated independently by receptors in the BNSTmv and NAc shell, respectively. These findings suggest that males have more overlap in neural circuits modulating anxiety in social and nonsocial contexts than females.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

Chronic social subordination stress modulates glutamic acid decarboxylase (GAD) 67 mRNA expression in central stress circuits

Chronic social subordination is a well-known precipitant of numerous psychiatric and physiological health concerns. In this study, we examine the effects of chronic social stress in the visible burrow system (VBS) on the expression of glutamic acid decarboxylase (GAD) 67 and brain-derived neurotropic factor (BDNF) mRNA in forebrain stress circuitry. Male rats in the VBS system form a dominance hierarchy, whereby subordinate males exhibit neuroendocrine and physiological profiles characteristic of chronic exposure to stress. We found that social subordination decreases GAD67 mRNA in the peri-paraventricular nucleus region of the hypothalamus and the interfascicular nucleus of the bed nucleus of the stria terminalis (BNST), and increases in GAD67 mRNA in the hippocampus, medial prefrontal cortex, and dorsal medial hypothalamus. Expression of BDNF mRNA increased in the dorsal region of the BNST, but remained unchanged in all other regions examined. Results from this study indicate that social subordination is associated with several region-specific alterations in GAD67 mRNA expression in central stress circuits, whereas changes in the expression of BDNF mRNA are limited to the BNST.

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.

Neurobiological mechanisms supporting experience-dependent resistance to social stress

Humans and other animals show a remarkable capacity for resilience following traumatic, stressful events. Resilience is thought to be an active process related to coping with stress, although the cellular and molecular mechanisms that support active coping and stress resistance remain poorly understood. In this review, we focus on the neurobiological mechanisms by which environmental and social experiences promote stress resistance. In male Syrian hamsters, exposure to a brief social defeat stressor leads to increased avoidance of novel opponents, which we call conditioned defeat. Also, hamsters that have achieved dominant social status show reduced conditioned defeat as well as cellular and molecular changes in the neural circuits controlling the conditioned defeat response. We propose that experience-dependent neural plasticity occurs in the prelimbic (PL) cortex, infralimbic (IL) cortex, and ventral medial amygdala (vMeA) during the maintenance of dominance relationships, and that adaptations in these neural circuits support stress resistance in dominant individuals. Overall, behavioral treatments that promote success in competitive interactions may represent valuable interventions for instilling resilience.

Sex differences in stress-induced social withdrawal: role of brain derived neurotrophic factor in the bed nucleus of the stria terminalis

Depression and anxiety disorders are more common in women than men, and little is known about the neurobiological mechanisms that contribute to this disparity. Recent data suggest that stress-induced changes in neurotrophins have opposing effects on behavior by acting in different brain networks. Social defeat has been an important approach for understanding neurotrophin action, but low female aggression levels in rats and mice have limited the application of these methods primarily to males. We examined the effects of social defeat in monogamous California mice (Peromyscus californicus), a species in which both males and females defend territories. We demonstrate that defeat stress increases mature brain-derived neurotrophic factor (BDNF) protein but not mRNA in the bed nucleus of the stria terminalis (BNST) in females but not males. Changes in BDNF protein were limited to anterior subregions of the BNST, and there were no changes in the adjacent nucleus accumbens (NAc). The effects of defeat on social withdrawal behavior and BDNF were reversed by chronic, low doses of the antidepressant sertraline. However, higher doses of sertraline restored social withdrawal and elevated BDNF levels. Acute treatment with a low dose of sertraline failed to reverse the effects of defeat. Infusions of the selective tyrosine-related kinase B receptor (TrkB) antagonist ANA-12 into the anterior BNST specifically increased social interaction in stressed females but had no effect on behavior in females naïve to defeat. These results suggest that stress-induced increases in BDNF in the anterior BNST contribute to the exaggerated social withdrawal phenotype observed in females.

Manganese-enhanced magnetic resonance imaging (MEMRI) reveals brain circuitry involved in responding to an acute novel stress in rats with a history of repeated social stress

Responses to acute stressors are determined in part by stress history. For example, a history of chronic stress results in facilitated responses to a novel stressor and this facilitation is considered to be adaptive. We previously demonstrated that repeated exposure of rats to the resident-intruder model of social stress results in the emergence of two subpopulations that are characterized by different coping responses to stress. The submissive subpopulation failed to show facilitation to a novel stressor and developed a passive strategy in the Porsolt forced swim test. Because a passive stress coping response has been implicated in the propensity to develop certain psychiatric disorders, understanding the unique circuitry engaged by exposure to a novel stressor in these subpopulations would advance our understanding of the etiology of stress-related pathology. An ex vivo functional imaging technique, manganese-enhanced magnetic resonance imaging (MEMRI), was used to identify and distinguish brain regions that are differentially activated by an acute swim stress (15 min) in rats with a history of social stress compared to controls. Specifically, Mn(2+) was administered intracerebroventricularly prior to swim stress and brains were later imaged ex vivo to reveal activated structures. When compared to controls, all rats with a history of social stress showed greater activation in specific striatal, hippocampal, hypothalamic, and midbrain regions. The submissive subpopulation of rats was further distinguished by significantly greater activation in amygdala, bed nucleus of the stria terminalis, and septum, suggesting that these regions may form a circuit mediating responses to novel stress in individuals that adopt passive coping strategies. The finding that different circuits are engaged by a novel stressor in the two subpopulations of rats exposed to social stress implicates a role for these circuits in determining individual strategies for responding to stressors. Finally, these data underscore the utility of ex vivo MEMRI to identify and distinguish circuits engaged in behavioral responses.

Functional mapping of the circuits involved in the expression of contextual fear responses in socially defeated animals

In this study, we have aimed at outlining the neural systems underlying the expression of contextual fear to social defeat. First, we have developed an experimental procedure, where defeated animals could express, without the presence of a dominant aggressive male, robust and reliable conditioned fear responses to the context associated with social defeat. Next, by examining the pattern of Fos expression, we have been able to outline a brain circuit comprising septal and amygdalar sites, as well as downstream hypothalamic paths, putatively involved in the expression of contextual fear to social threat. Of particular relevance, we have found that exposure to a defeat-associated context results in a striking Fos up-regulation in the dorsomedial part of the dorsal premammillary nucleus (PMDdm). To further understand the role of the PMDdm in the circuit organizing conditioned fear to social threats, we have been able to observe that pharmacological blockade of the PMDdm reduced fear responses to a social defeat-associated context. Next, we observed that pharmacological blockade of the dorsomedial part of the periaqueductal gray, one of the main targets of the PMDdm, produced an even higher reduction of conditioned fear in defeated intruders, and appears as an important node for the expression of contextual defensive responses to social threats. The present results help to elucidate the basic organization of the neural circuits underlying contextual conditioned responses to social defeat, and reveal that they share at least part of the same circuit involved in innate responses to social defeat to an aggressive conspecific.

Defeat-induced activation of the ventral medial prefrontal cortex is necessary for resistance to conditioned defeat

The ventral medial prefrontal cortex (vmPFC) controls vulnerability to the negative effects of chronic or uncontrollable stress. Dominance status alters responses to social defeat in the conditioned defeat model, which is a model characterized by loss of territorial aggression and increased submissive and defensive behavior following an acute social defeat. We have previously shown that dominant individuals show a reduced conditioned defeat response and increased defeat-induced neural activation in the vmPFC compared to subordinates. Here, we tested the hypothesis that defeat-induced activation of the vmPFC is necessary to confer resistance to conditioned defeat in dominants. We paired weight-matched male Syrian hamsters (Mesocricetus auratus) in daily 5-min aggressive encounters for 2 weeks and identified dominants and subordinates. Twenty-four hours after the final pairing, animals were bilaterally injected with 200 nl of the GABAA receptor agonist muscimol (1.1 nmol) or 200 nl of saline vehicle 5 min prior to social defeat. Defeat consisted of 3, 5-min encounters with resident aggressor hamsters at 10-min intervals. Twenty-four hours following social defeat, animals received conditioned defeat testing which involved a 5-min social interaction test with a non-aggressive intruder. Muscimol injection prior to social defeat prevented the reduced conditioned defeat response observed in vehicle-treated dominants. Further, there was no effect of muscimol injection on the conditioned defeat response in subordinates or controls. These data support the conclusion that activation of the vmPFC during social defeat is necessary for the protective effects of dominant social status on the acquisition of conditioned defeat.

A double dissociation in the effects of 5-HT2A and 5-HT2C receptors on the acquisition and expression of conditioned defeat in Syrian hamsters

Previous research indicates that serotonin enhances the development of stress-induced changes in behavior, although it is unclear which serotonin receptors mediate this effect. 5-HT2 receptors are potential candidates because activation at these receptors is associated with increased fear and anxiety. In this study, we investigated whether pharmacological treatments targeting 5-HT2 receptors would alter the acquisition and expression of conditioned defeat. Conditioned defeat is a social defeat model in Syrian hamsters in which individuals display increased submissive and defensive behavior and a loss of territorial aggression when tested with a novel intruder 24 hours after an acute social defeat. The nonselective 5-HT2 receptor agonist mCPP (0.0, 0.3, 1.0, or 3.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Also, the 5-HT2A receptor antagonist MDL 11,939 (0.0, 0.5, or 2.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Injection of mCPP prior to testing increased the expression of conditioned defeat, but injection of mCPP prior to training did not alter the acquisition of conditioned defeat. Conversely, injection of MDL 11,939 prior to training reduced the acquisition of conditioned defeat, but injection of MDL 11,939 prior to testing did not alter the expression of conditioned defeat. Our data suggest that mCPP activates 5-HT2C receptors during testing to enhance the display of submissive and defensive behavior, whereas MDL 11,939 blocks 5-HT2A receptors during social defeat to disrupt the development of the conditioned defeat response. In sum, these results suggest that serotonin acts at separate 5-HT2 receptors to facilitate the acquisition and expression of defeat-induced changes in social behavior.

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 role of the nucleus accumbens in the acquisition and expression of conditioned defeat

When Syrian hamsters (Mesocricetus auratus) are defeated by a larger, more aggressive hamster, they subsequently exhibit submissive and defensive behavior, instead of their usual aggressive and social behavior, even toward a smaller, non-aggressive opponent. This change in behavior is termed conditioned defeat, and we have found that the amygdala, bed nucleus of the stria terminalis, and ventral hippocampus, among others, are crucial brain areas for either the acquisition and/or expression of this behavioral response to social stress. In the present study, we tested the hypothesis that the nucleus accumbens is also a necessary component of the circuit mediating the acquisition and expression of conditioned defeat. We found that infusion of the GABA(A) agonist muscimol into the nucleus accumbens prior to defeat training failed to affect acquisition of conditioned defeat, but infusion prior to testing significantly decreased submissive behavior and significantly increased aggressive behavior directed toward the non-aggressive intruder. These data indicate that, unlike the basolateral complex of the amygdala, the nucleus accumbens is not a critical site for the plasticity underlying conditioned defeat acquisition, but it does appear to be an important component of the circuit mediating the expression of the behavioral changes that are produced in response to a previous social defeat. Of note, this is the first component of the putative "conditioned defeat neural circuit" wherein we have found that pharmacological manipulations are effective in restoring the territorial aggressive response in previously defeated hamsters.

Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders

Exposure to traumatic events can increase the risk for major depressive disorder (MDD) as well as posttraumatic stress disorder (PTSD), and pharmacological treatments for these disorders often involve the modulation of serotonergic (5-HT) systems. Several behavioral paradigms in rodents produce changes in behavior that resemble symptoms of MDD and these behavioral changes are sensitive to antidepressant treatments. Here we review two animal models in which MDD-like behavioral changes are elicited by exposure to an acute traumatic event during adulthood, learned helplessness (LH) and conditioned defeat. In LH, exposure of rats to inescapable, but not escapable, tailshock produces a constellation of behavioral changes that include deficits in fight/flight responding and enhanced anxiety-like behavior. In conditioned defeat, exposure of Syrian hamsters to a social defeat by a more aggressive animal leads to a loss of territorial aggression and an increase in submissive and defensive behaviors in subsequent encounters with non-aggressive conspecifics. Investigations into the neural substrates that control LH and conditioned defeat revealed that increased 5-HT activity in the dorsal raphe nucleus (DRN) is critical for both models. Other key brain regions that regulate the acquisition and/or expression of behavior in these two paradigms include the basolateral amygdala (BLA), central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). In this review, we compare and contrast the role of each of these neural structures in mediating LH and conditioned defeat, and discuss the relevance of these data in developing a better understanding of the mechanisms underlying trauma-related depression. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Repeated social defeat increases reactive emotional coping behavior and alters functional responses in serotonergic neurons in the rat dorsal raphe nucleus

Chronic stress is a vulnerability factor for a number of psychiatric disorders, including anxiety and affective disorders. Social defeat in rats has proven to be a useful paradigm to investigate the neural mechanisms underlying physiologic and behavioral adaptation to acute and chronic stress. Previous studies suggest that serotonergic systems may contribute to the physiologic and behavioral adaptation to chronic stress, including social defeat in rodent models. In order to test the hypothesis that repeated social defeat alters the emotional behavior and the excitability of brainstem serotonergic systems implicated in control of emotional behavior, we exposed adult male rats either to home cage control conditions, acute social defeat, or social defeat followed 24h later by a second social defeat encounter. We then assessed behavioral responses during social defeat as well as the excitability of serotonergic neurons within the dorsal raphe nucleus using immunohistochemical staining of tryptophan hydroxylase, a marker of serotonergic neurons, and the protein product of the immediate-early gene, c-fos. Repeated social defeat resulted in a shift away from proactive emotional coping behaviors, such as rearing (explorative escape behavior), and toward reactive emotional coping behaviors such as freezing. Both acute and repeated defeat led to widespread increases in c-Fos expression in serotonergic neurons in the dorsal raphe nucleus. Changes in behavior following a second exposure to social defeat, relative to acute defeat, were associated with decreased c-Fos expression in serotonergic neurons within the dorsal and ventral parts of the mid-rostrocaudal dorsal raphe nucleus, regions that have been implicated in 1) serotonergic modulation of fear- and anxiety-related behavior and 2) defensive behavior in conspecific aggressive encounters, respectively. These data support the hypothesis that serotonergic systems play a role in physiologic and behavioral responses to both acute and repeated social defeat.

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.

Androgenic and oestrogenic influences on tyrosine hydroxylase-immunoreactive cells of the prairie vole medial amygdala and bed nucleus of the stria terminalis

The posterodorsal medial amygdala (MeApd) and principal nucleus of the bed nucleus of the stria terminalis (pBST) are densely interconnected sites integrating steroid hormone and olfactory information necessary for sociosexual behaviours in many rodents. Our laboratory recently reported sexually dimorphic populations of cells containing tyrosine hydroxylase (TH) located in the MeApd and pBST of prairie voles (Microtus ochrogaster), with males having many more TH-immunoreactive (-ir) cells in these sites than do females. Gonadal hormones circulating during adulthood were showm to regulate this sex difference because it was eliminated by castrating adult males or implanting females with testosterone-filled capsules. In the present study, we demonstrate that many (25-65%) TH-ir cells in the MeApd and pBST of adult virgin male and female prairie voles also contain immunoreactivity for either the androgen receptor or oestrogen receptor alpha. Subcutaneous implants of oestradiol benzoate mimicked the effects of testosterone and maintained high numbers of TH-ir cells in these sites in castrated males. However, implants of dihydrotestosterone (DHT) did not, and these males had low numbers of TH-ir cells similar to castrated males given empty capsules. A similar effect was found in females, where testosterone or oestradiol benzoate greatly increased the number of TH-ir cells in these sites compared to intact or ovariectomised controls, but DHT did not. DHT implants did, however, maintain high seminal vesicle weights in males. Thus, many of the TH-ir cells in the prairie vole MeApd and pBST are potentially sensitive to androgens and oestrogens, although maintaining immunocytochemically detectable levels of TH in these cells may depend more on an oestrogen-mediated mechanism in both sexes. These data have implications for understanding how gonadal hormone release across the reproductive cycle modulates these species-specific groups of catecholaminergic cells and socially monogamous behaviours in prairie voles.

Phasic vs sustained fear in rats and humans: role of the extended amygdala in fear vs anxiety

Data will be reviewed using the acoustic startle reflex in rats and humans based on our attempts to operationally define fear vs anxiety. Although the symptoms of fear and anxiety are very similar, they also differ. Fear is a generally adaptive state of apprehension that begins rapidly and dissipates quickly once the threat is removed (phasic fear). Anxiety is elicited by less specific and less predictable threats, or by those that are physically or psychologically more distant. Thus, anxiety is a more long-lasting state of apprehension (sustained fear). Rodent studies suggest that phasic fear is mediated by the amygdala, which sends outputs to the hypothalamus and brainstem to produce symptoms of fear. Sustained fear is also mediated by the amygdala, which releases corticotropin-releasing factor, a stress hormone that acts on receptors in the bed nucleus of the stria terminalis (BNST), a part of the so-called 'extended amygdala.' The amygdala and BNST send outputs to the same hypothalamic and brainstem targets to produce phasic and sustained fear, respectively. In rats, sustained fear is more sensitive to anxiolytic drugs. In humans, symptoms of clinical anxiety are better detected in sustained rather than phasic fear paradigms.

Selective participation of the bed nucleus of the stria terminalis and CRF in sustained anxiety-like versus phasic fear-like responses

The medial division of the central nucleus of the amygdala (CeA(M)) and the lateral division of the bed nucleus of the stria terminalis (BNST(L)) are closely related. Both receive projections from the basolateral amygdala (BLA) and both project to brain areas that mediate fear-influenced behaviors. In contrast to CeA(M) however, initial attempts to implicate the BNST in conditioned fear responses were largely unsuccessful. More recent studies have shown that the BNST does participate in some types of anxiety and stress responses. Here, we review evidence suggesting that the CeA(M) and BNST(L) are functionally complementary, with CeA(M) mediating short- but not long-duration threat responses (i.e., phasic fear) and BNST(L) mediating long- but not short-duration responses (sustained fear or 'anxiety'). We also review findings implicating the stress-related peptide corticotropin-releasing factor (CRF) in sustained but not phasic threat responses, and attempt to integrate these findings into a neural circuit model which accounts for these and related observations.