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

Maternal Aggression Driven by the Transient Mobilisation of a Dormant Hormone-Sensitive Circuit

Aggression, a sexually dimorphic behaviour, is prevalent in males and typically absent in virgin females. Following parturition, however, the transient expression of aggression in adult female mice protects pups from predators and infanticide by male conspecifics. While maternal hormones are known to elicit nursing, their potential role in maternal aggression remains elusive. Here, we show in mice that a molecularly defined subset of ventral premammillary (PMvDAT) neurons, instrumental for intermale aggression, switch from quiescence to a hyperexcitable state during lactation. We identify that the maternal hormones prolactin and oxytocin excite these cells through actions that include T-type Ca2+ channels. Optogenetic manipulation or genetic ablation of PMvDAT neurons profoundly affects maternal aggression, while activation of these neurons impairs the expression of non-aggression-related maternal behaviours. This work identifies a monomorphic neural substrate that can incorporate hormonal cues to enable the transient expression of a dormant behavioural program in lactating females.

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.

Dominance status modulates activity in medial amygdala cells with projections to the bed nucleus of the stria terminalis

The medial amygdala (MeA) controls several types of social behavior via its projections to other limbic regions. Cells in the posterior dorsal and posterior ventral medial amygdala (MePD and MePV, respectively) project to the bed nucleus of the stria terminalis (BNST) and these pathways respond to chemosensory cues and regulate aggressive and defensive behavior. Because the BNST is also essential for the display of stress-induced anxiety, a MePD/MePV-BNST pathway may modulate both aggression and responses to stress. In this study we tested the hypothesis that dominant animals would show greater neural activity than subordinates in BNST-projecting MePD and MePV cells after winning a dominance encounter as well as after losing a social defeat encounter. We created dominance relationships in male and female Syrian hamsters (Mesocricetus auratus), used cholera toxin b (CTB) as a retrograde tracer to label BNST-projecting cells, and collected brains for c-Fos staining in the MePD and MePV. We found that c-Fos immunoreactivity in the MePD and MePV was positively associated with aggression in males, but not in females. Also, dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells compared to their same-sex subordinate counterparts. Another set of animals received social defeat stress after acquiring a dominant or subordinate social status and we stained for stress-induced c-Fos expression in the MePD and MePV. We found that dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells in the MePD after social defeat stress compared to subordinates. Also, dominants showed a longer latency to submit during social defeat than subordinates. Further, in males, latency to submit was positively associated with the proportion of c-Fos+ /CTB+ double-labeled cells in the MePD and MePV. These findings indicate that social dominance increases neural activity in BNST-projecting MePD and MePV cells and activity in this pathway is also associated with proactive responses during social defeat stress. In sum, activity in a MePD/MePV-BNST pathway contributes to status-dependent differences in stress coping responses and may underlie experience-dependent changes in stress resilience.

Chronic social defeat alters behaviors and neuronal activation in the brain of female Mongolian gerbils

Chronic social defeat has been found to be stressful and to affect many aspects of the brain and behaviors in males. However, relatively little is known about its effects on females. In the present study, we examined the effects of repeated social defeat on social approach and anxiety-like behaviors as well as the neuronal activation in the brain of sexually naïve female Mongolian gerbils (Meriones unguiculatus). Our data indicate that repeated social defeats for 20 days reduced social approach and social investigation, but increased risk assessment or vigilance to an unfamiliar conspecific. Such social defeat experience also increased anxiety-like behavior and reduced locomotor activity. Using ΔFosB-immunoreactive (ΔFosB-ir) staining as a marker of neuronal activation in the brain, we found significant elevations by social defeat experience in the density of ΔFosB-ir stained neurons in several brain regions, including the prelimbic (PL) and infralimbic (IL) subnuclei of the prefrontal cortex (PFC), CA1 subfields (CA1) of the hippocampus, central subnuclei of the amygdala (CeA), the paraventricular nucleus (PVN), dorsomedial nucleus (DMH), and ventrolateral subdivision of the ventromedial nucleus (VMHvl) of the hypothalamus. As these brain regions have been implicated in social behaviors and stress responses, our data suggest that the specific patterns of neuronal activation in the brain may relate to the altered social and anxiety-like behaviors following chronic social defeat in female Mongolian gerbils.

The impact of social defeat on basomedial amygdala neuronal activity in adult male rats

Social stressors negatively impact social function, and this is mediated by the amygdala across species. Social defeat stress is an ethologically relevant social stressor in adult male rats that increases social avoidance, anhedonia, and anxiety-like behaviors. While amygdala manipulations can mitigate the negative effects of social stressors, the impact of social defeat on the basomedial subregion of the amygdala is relatively unclear. Understanding the role of the basomedial amygdala may be especially important, as prior work has demonstrated that it drives physiological responses to stress, including heart-rate related responses to social novelty. In the present study, we quantified the impact of social defeat on social behavior and basomedial amygdala neuronal responses using anesthetized in vivo extracellular electrophysiology. Socially defeated rats displayed increased social avoidance behavior towards novel Sprague Dawley conspecifics and reduced time initiating social interactions relative to controls. This effect was most pronounced in rats that displayed defensive, boxing behavior during social defeat sessions. We next found that socially defeated rats showed lower overall basomedial amygdala firing and altered the distribution of neuronal responses relative to the control condition. We separated neurons into low and high Hz firing groups, and neuronal firing was reduced in both low and high Hz groups but in a slightly different manner. This work demonstrates that basomedial amygdala activity is sensitive to social stress, displaying a distinct pattern of social stress-driven activity relative to other amygdala subregions.

The lifetime impact of stress on fear regulation and cortical function

A variety of stressful experiences can influence the ability to form and subsequently inhibit fear memory. While nonsocial stress can impact fear learning and memory throughout the lifespan, psychosocial stressors that involve negative social experiences or changes to the social environment have a disproportionately high impact during adolescence. Here, we review converging lines of evidence that suggest that development of prefrontal cortical circuitry necessary for both social experiences and fear learning is altered by stress exposure in a way that impacts both social and fear behaviors throughout the lifespan. Further, we suggest that psychosocial stress, through its impact on the prefrontal cortex, may be especially detrimental during early developmental periods characterized by higher sociability. This article is part of the Special Issue on 'Fear, Anxiety and PTSD'.

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.

Neural systems that facilitate the representation of social rank

Across species, animals organize into social dominance hierarchies that serve to decrease aggression and facilitate survival of the group. Neuroscientists have adopted several model organisms to study dominance hierarchies in the laboratory setting, including fish, reptiles, rodents and primates. We review recent literature across species that sheds light onto how the brain represents social rank to guide socially appropriate behaviour within a dominance hierarchy. First, we discuss how the brain responds to social status signals. Then, we discuss social approach and avoidance learning mechanisms that we propose could drive rank-appropriate behaviour. Lastly, we discuss how the brain represents memories of individuals (social memory) and how this may support the maintenance of unique individual relationships within a social group. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Corticotropin releasing factor and norepinephrine related circuitry changes in the bed nucleus of the stria terminalis in stress and alcohol and substance use disorders

Alcohol Use Disorder (AUD) affects around 14.5 million individuals in the United States, with Substance Use Disorder (SUD) affecting an additional 8.3 million individuals. Relapse is a major barrier to effective long-term treatment of this illness with stress often described as a key trigger for a person with AUD or SUD to relapse during a period of abstinence. Two signaling molecules, norepinephrine (NE) and corticotropin releasing factor (CRF), are released during the stress response, and also play important roles in reward behaviors and the addiction process. Within the addiction literature, one brain region in which there has been increasing research focus in recent years is the bed nucleus of the stria terminalis (BNST). The BNST is a limbic structure with numerous cytoarchitecturally and functionally different subregions that has been implicated in drug-seeking behaviors and stress responses. This review focuses on drug and stress-related neurocircuitry changes in the BNST, particularly within the CRF and NE systems, with an emphasis on differences and similarities between the major dorsal and ventral BNST subregions.

The challenge hypothesis revisited: Focus on reproductive experience and neural mechanisms

Our review focuses on findings from mammals as part of a Special Issue "30th Anniversary of the Challenge Hypothesis". Here we put forth an integration of the mechanisms through which testosterone controls territorial behavior and consider how reproductive experience may alter these mechanisms. The emphasis is placed on the function of socially induced increases in testosterone (T) pulses, which occur in response to social interactions, as elegantly developed by Wingfield and colleagues. We focus on findings from the monogamous California mouse, as data from this species shows that reproductive status is a key factor influencing social interactions, site fidelity, and vigilance for offspring defense. Specifically, we examine differences in T pulses in sexually naïve versus sexually experienced pair bonded males. Testosterone pulses influence processes such as social decision making, the winner-challenge effect, and location preferences through rewarding effects of T. We also consider how social and predatory vigilance contribute to T pulses and how these interactions contribute to a territory centered around maximizing reproduction. Possible underlying mechanisms for these effects include the nucleus accumbens (rewarding effects of testosterone), hippocampus (spatial memories for territories), and the bed nucleus of the stria terminalis (social vigilance). The development of the challenge effect has provided an ideal framework for understanding the complex network of behavioral, environmental, physiological and neural mechanisms that ultimately relates to competition and territoriality across taxa. The opportunity to merge research on the challenge effect using both laboratory and field research to understand social behavior is unparalleled.

Increased BNST reactivity to affective images is associated with greater α-amylase response to social stress

While rodent research suggests that the bed nucleus of the stria terminalis (BNST) and centromedial amygdala (CM) coordinate the hormonal stress response, little is known about the BNST’s role in the human stress response. The human BNST responds to negatively valenced stimuli, which likely subserves its role in responding to threat. Thus, variation in BNST reactivity to negatively valenced stimuli may relate to differences in the stress response. We measured participants’ blood oxygenated level-dependent response to affective images and salivary cortisol and α-amylase (AA) levels in response to a subsequent Trier social stress test (TSST). Greater BNST activation to emotionally evocative images was associated with a larger TSST-evoked AA, but not cortisol response. This association remained after controlling for CM activation, which was not related to the cortisol or AA response. These results suggest that the BNST response to negatively valenced images subserves its role in coordinating the stress response, a BNST role in the stress response independent from the CM, and highlight the need for investigation of the conditions under which BNST activation predicts the cortisol response. Our findings are critical for the future study of mood and anxiety disorders, as dysregulation of the stress system plays a key role in their pathogenesis.

Extending the neurocircuitry of behavioural inhibition: a role for the bed nucleus of the stria terminalis in risk for anxiety disorders

Behavioural inhibition is a biologically based risk factor for anxiety disorders. Children with behavioural inhibition are shy, cautious and avoidant of new situations. Much research on behavioural inhibition has focused on the amygdala as an underlying neural substrate and has identified differences in amygdala function and volume; however, amygdala findings have yet to lead to meaningful interventions for prevention or treatment of anxiety disorders. The bed nucleus of the stria terminalis (BNST) is a prime candidate to be a neural substrate of behavioural inhibition, given current evidence of BNST function and development in human research and animal models. Children with behavioural inhibition have an increased startle response to safety cues and an increased cortisol response to social evaluative situations, both of which are mediated by the BNST. In rodents, activation of the BNST underlies contextual fear responses and responses to uncertain and sustained threat. Non-human primates with anxious temperament (the macaque equivalent of behavioural inhibition) have increased BNST activity to ambiguous social situations, and activity of the BNST in anxious temperament is significantly heritable. Importantly, the BNST is sexually dimorphic and continues to develop into adulthood, paralleling the development of anxiety disorders in humans. Together, these findings suggest that further investigation of the BNST in behavioural inhibition is necessary and may lead to new avenues for the prevention and treatment of anxiety disorders.

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.

Social stress-potentiated methamphetamine seeking

Human studies of substance use disorder show that psychological stress and drug availability interact following rehabilitation, contributing to the high relapse potential. Social stressors trigger particularly strong motivation for drug, but how this affects neuronal function to increase relapse is unknown. Animal models, which allow for the dissection of neural mechanisms, primarily utilize physical stressors to trigger relapse. To recapitulate psychosocial post-rehabilitation challenges in animals, we developed a model of social stress-potentiated methamphetamine (METH) seeking. Rats receive a single social defeat (SD) session after completion of self-administration and extinction of lever pressing. While a reminder of the SD was insufficient to reinstate METH seeking on its own, rats that received a reminder of SD followed by a METH-priming injection displayed potentiated reinstatement over METH-priming alone. Examination of neuronal activation patterns of the METH-primed reinstatement session identified c-Fos-immunoreactivity in the basolateral amygdala (BLA) as correlated with SD score, a measure of defeat latency. Rapidly defeated rats showed potentiated METH-primed reinstatement and elevated BLA c-Fos compared with controls. Conversely, rats that were undefeated during the social stress did not show potentiated METH-primed reinstatement or elevated BLA c-Fos. Interestingly, inactivation of the BLA with baclofen/muscimol prior to the stress reminder and METH-priming generated a potentiation of METH seeking in the undefeated rats, suggesting the BLA may mediate resilience to the stressor. This model provides a tool for the further dissection of neural mechanisms mediating social stress-potentiated relapse and for the development of relapse-reducing therapeutics.

When mothers neglect their offspring: an activated CRF system in the BNST is detrimental for maternal behavior

Becoming a mother is an intense experience that not only changes a woman's life but is also paralleled by multiple central adaptations. These changes evolve before parturition and continue to persist into lactation, thereby ensuring the full commitment of the mother to care for the newborns. Most of our knowledge on these adaptations that drive the peripartum brain come from rodent animal models. On one side, it is known that maternal behavior is initiated and maternal mood is stabilized by an upregulation of the pro-maternal neuropeptide systems' activity of oxytocin and arginine-vasopressin. On the other side, signaling of the rather anti-maternal corticotropin-releasing factor system triggers maternal neglect and increases maternal anxiety. Here, we discuss how the corticotropin-releasing factor system based in the limbic bed nucleus of the stria terminalis negatively affects maternal behavior and maternal mood. Moreover, we apply microdialysis and acute pharmacological interventions to demonstrate how the corticotropin-releasing factor system potentially interacts with the pro-maternal oxytocin system in the posterior bed nucleus of the stria terminalis to trigger certain aspects of maternal behavior.

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.

Role of the Bed Nucleus of the Stria Terminalis in PTSD: Insights From Preclinical Models

Post-traumatic stress disorder (PTSD) afflicts approximately 8% of the United States population and represents a significant public health burden, but the underlying neural mechanisms of this and other anxiety- and stressor-related disorders are largely unknown. Within the last few decades, several preclinical models of PSTD have been developed to help elucidate the mechanisms underlying dysregulated fear states. One brain area that has emerged as a critical mediator of stress-related behavioral processing in both clinical and laboratory settings is the bed nucleus of the stria terminalis (BNST). The BNST is interconnected with essential emotional processing regions, including prefrontal cortex, hippocampus and amygdala. It is activated by stressor exposure and undergoes neurochemical and morphological alterations as a result of stressor exposure. Stress-related neuro-peptides including corticotropin-releasing factor (CRF) and pituitary adenylate cyclase activating peptide (PACAP) are also abundant in the BNST, further implicating an involvement of BNST in stress responses. Behaviorally, the BNST is critical for acquisition and expression of fear and is well positioned to regulate fear relapse after periods of extinction. Here, we consider the role of the BNST in stress and memory processes in the context of preclinical models of PTSD.

Mom doesn't care: When increased brain CRF system activity leads to maternal neglect in rodents

Mothers are the primary caregivers in mammals, ensuring their offspring's survival. This strongly depends on the adequate expression of maternal behavior, which is the result of a concerted action of "pro-maternal" versus "anti-maternal" neuromodulators such as the oxytocin and corticotropin-releasing factor (CRF) systems, respectively. When essential peripartum adaptations fail, the CRF system has negative physiological, emotional and behavioral consequences for both mother and offspring often resulting in maternal neglect. Here, we provide an elaborate and unprecedented review on the implications of the CRF system in the maternal brain. Studies in rodents have advanced our understanding of the specific roles of brain regions such as the limbic bed nucleus of the stria terminalis, medial preoptic area and lateral septum even in a CRF receptor subtype-specific manner. Furthermore, we discuss potential interactions of the CRF system with other neurotransmitters like oxytocin and noradrenaline, and present valuable translational aspects of the recent research.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Dominance relationships in Syrian hamsters modulate neuroendocrine and behavioral responses to social stress

Stress is a well-known risk factor for psychopathology and rodent models of social defeat have strong face, etiological, construct and predictive validity for these conditions. Syrian hamsters are highly aggressive and territorial, but after an acute social defeat experience they become submissive and no longer defend their home territory, even from a smaller, non-aggressive intruder. This defeat-induced change in social behavior is called conditioned defeat (CD). We have shown that dominant hamsters show increased neural activity in the ventromedial prefrontal cortex (vmPFC) following social defeat stress and exhibit a reduced CD response at social interaction testing compared to subordinates. Although the vmPFC can inhibit the neuroendocrine stress response, it is unknown whether dominants and subordinates differ in stress-induced activity of the extended hypothalamic-pituitary-adrenal (HPA) axis. Here, we show that, following acute social defeat, dominants exhibit decreased submissive and defensive behavior compared to subordinates but do not differ from subordinates or social status controls (SSCs) in defeat-induced cortisol concentrations. Furthermore, both dominants and SSCs show greater corticotropin-releasing hormone (CRH) mRNA expression in the basolateral/central amygdala compared to subordinates, while there was no effect of social status on CRH mRNA expression in the paraventricular nucleus of the hypothalamus or bed nucleus of the stria terminalis. Overall, status-dependent differences in the CD response do not appear linked to changes in stress-induced cortisol concentrations or CRH gene expression, which is consistent with the view that stress resilience is not a lack of a physiological stress response but the addition of stress coping mechanisms. Lay summary Dominant hamsters show resistance to the behavioral effects of acute social defeat compared to subordinates, but it is unclear whether social status modulates the neuroendocrine stress response in Syrian hamsters. This study indicates that dominant social status does not alter stress-induced activity of the extended hypothalamic-pituitary-adrenal (HPA) axis, which suggests that the ability of dominants to cope with social defeat stress is not associated with changes in their neuroendocrine stress response.

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.

The effects of CRF and urocortins on the sociability of mice

The aim of our study was to determine the role of corticotropin-releasing factor (CRF), the urocortins (Ucn1, Ucn2 and Ucn3) and their receptors (CRF₁ and CRF₂) in the sociability of mice. Male CFLP mice were administered intracerebroventricularly (icv) with CRF and urocortins alone or in combination with antalarmin (specific CRF₁ antagonist) and astressin_2B (specific CRF₂ antagonist) and then investigated in a Crawley social interaction test arena, that consists of three chambers. An unknown male in a cage was put in the first chamber and an empty cage was put in the opposite chamber. The tested male was habituated with the middle chamber for 5min and then allowed to explore the remaining chambers for 5min, during which the number of entries and the time of interaction were measured. Intracerebroventricular administration of CRF decreased significantly the number of entries and the time of interaction with the unknown male and these effects were blocked by antalarmin, but not astressin_2B. In contrast, central administration of Ucn1 increased significantly the number of entries into the chamber of the unknown male, without changing the time of interaction and this effect was blocked by astressin_2B, but not antalarmin. Central administration of Ucn2 and Ucn3 didn't influence remarkably the number of entries, but it reduced the time of interaction between the male mice. Our study suggests that CRF and Ucn1 may play important, but different roles in sociability, and that Ucn2 and Ucn3, playing similar roles, must be also involved in social interactions.

Distinct neuronal activation patterns are associated with PCP-induced social withdrawal and its reversal by the endocannabinoid-enhancing drug URB597

The fatty acid amide hydrolase inhibitor, URB597, an endocannabinoid enhancing drug, reverses social withdrawal in the sub-chronic PCP rat model of schizophrenia, but reduces social interaction (SI) in controls. To identify the anatomical substrates associated with PCP-induced social withdrawal and the contrasting effects of URB597 on SI in PCP- versus saline-treated rats, we analyzed SI-induced c-Fos expression in 28 brain areas relevant to schizophrenia and/or social behavior following vehicle or URB597 administration. In saline-treated rats, SI was accompanied by changes in c-Fos expression in the infralimbic and orbitofrontal cortices, dorsomedial caudate putamen, ventrolateral nucleus of the septum, dorsolateral periaqueductal gray (dlPAG) and central amygdala. Except for the dlPAG, these changes were not observed in PCP-treated rats or in saline-treated rats receiving URB597. In the dorsomedial part of the bed nucleus of the stria terminalis (dmBNST), SI-induced c-Fos expression was observed only in PCP-treated rats. Interestingly, URB597 in PCP-treated rats restored a similar c-Fos expression pattern as observed in saline-treated rats: activation of the orbitofrontal cortex, inhibition of the central amygdala and suppression of activation of the dmBNST. These data suggest that orbitofrontal cortex, central amygdala and dmBNST play a critical role in the reversal of PCP-induced social withdrawal by URB597.

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

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

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.

Stress Modulation of Opposing Circuits in the Bed Nucleus of the Stria Terminalis

The anterior bed nucleus of the stria terminalis (BNST) has been recognized as a critical structure in regulating trait anxiety, contextual fear memory, and appetitive behavior, and is known to be sensitive to stress manipulations. As one of the most complex structures in the central nervous system, the intrinsic circuitry of the BNST is largely unknown; however, recent technological developments have allowed researchers to begin to untangle the internal connections of the nucleus. This research has revealed the possibility of two opposing circuits, one anxiolytic and one anxiogenic, within the BNST, the relative strength of which determines the behavioral outcome. The balance of these pathways is critical in maintaining a normal physiological and behavioral state; however, stress and drugs of abuse can differentially affect the opposing circuitry within the nucleus to shift the balance to a pathological state. In this review, we will examine how stress interacts with the neuromodulators, corticotropin-releasing factor, norepinephrine, dopamine, and serotonin to affect the circuitry of the BNST as well as how synaptic plasticity in the BNST is modulated by stress, resulting in long-lasting changes in the circuit and behavioral state.

Grin1 deletion in CRF neurons sex-dependently enhances fear, sociability, and social stress responsivity

The corticotropin releasing factor (CRF) system plays a critical role in responses to stressful stimuli, and is expressed in many areas of the brain involved in processing fear, anxiety, and social behaviors. To better understand the mechanisms by which the CRF system modulates responses to stressful events and social stimuli, we employed a mouse model that selectively disrupts NMDA receptor function via NMDA receptor subunit NR1 (Grin1) knockout specifically in Cre-expressing CRF neurons. These animals (Cre+/(fGrin1+)) were compared with littermates lacking Cre expression (Cre-/(fGrin1+)). Following cue discrimination fear conditioning, male Cre+/(fGrin1+) mice showed increased fear expression to the tone paired with a foot shock (CS+) while still discriminating the CS+ from a tone never paired with a foot shock (CS-). In contrast to males, female mice learned and discriminated fear cues equivalently across the genotypes. Similarly, no genotype differences in sociability or social novelty were observed in female mice, but Cre+/(fGrin1+) males displayed greater naive sociability and preference for social novelty than Cre-/(fGrin1+) littermates. Furthermore, the level of social withdrawal exhibited by male Cre+/(fGrin1+) mice susceptible to social defeat stress relative to same genotype controls was significantly more pronounced than that displayed by susceptible Cre-/(fGrin1+) mice compared to control Cre-/(fGrin1+) mice. Together, these results demonstrate increased fear, social, and stress responsiveness specifically in male Cre+/(fGrin1+) mice. Our findings indicate that NMDA-mediated glutamatergic regulation of CRF neurons is important for appropriately regulating fear and social responses, likely functioning to promote survival under aversive circumstances.

Drug withdrawal conceptualized as a stressor

Drug withdrawal is often conceptualized as an aversive state that motivates drug-seeking and drug-taking behaviors in humans. Stress is more difficult to define, but is also frequently associated with aversive states. Here we describe evidence for the simple theory that drug withdrawal is a stress-like state, on the basis of common effects on behavioral, neurochemical, and molecular endpoints. We also describe data suggesting a more complex relationship between drug withdrawal and stress. As one example, we will highlight evidence that, depending on drug class, components of withdrawal can produce effects that have characteristics consistent with mood elevation. In addition, some stressors can act as positive reinforcers, defined as having the ability to increase the probability of a behavior that produces it. As such, accumulating evidence supports the general principles of opponent process theory, whereby processes that have an affective valence are followed in time by an opponent process that has the opposite valence. Throughout, we identify gaps in knowledge and propose future directions for research. A better understanding of the similarities, differences, and overlaps between drug withdrawal and stress will lead to the development of improved treatments for addiction, as well as for a vast array of neuropsychiatric conditions that are triggered or exacerbated by stress.

Polymorphism in the corticotropin-releasing factor receptor 1 (CRF1-R) gene plays a role in shaping the high anxious phenotype of Marchigian Sardinian alcohol-preferring (msP) rats

INTRODUCTION

Marchigian Sardinian alcohol-preferring (msP) rats exhibit innate preference for alcohol along with anxious phenotype. In these animals, two single-nucleotide polymorphisms in position -1,836 and -2,097 from the first start codon of the CRF1-R transcript have been found.

MATERIALS AND METHODS

Here, we examined whether these point mutations account for the heightened anxiety-like behavior and stress responsiveness of msP rats. We rederived the msP rats to obtain two distinct lines carrying the wild-type (GG) and point mutations (AA), respectively.

RESULTS

CRF1-R gene expression analysis revealed significant dysregulation of the system in the extended amygdala of AA rats. At the behavioral level, using the elevated plus maze, we found that both AA and GG lines had higher basal anxiety compared to Wistar rats. In the defensive burying test, AA rats showed decreased burying behavior compared to the GG and the unselected Wistar lines. Freezing/immobility did not differ among AA and GG but was higher than that of Wistars. The selective CRF1-R antagonist antalarmin (0, 10, and 20 mg/kg) reduced burying behavior in Wistar animals. However, antalarmin (10 mg/kg) tended to increase rather than reducing this behavior when tested in the msP lines, an effect that appeared more marked in the GG as compared to the AA line.

CONCLUSION

The present data suggest that rats with msP genetic background are more anxious and show different sensitivity to stress and CRF1-R blockade than Wistars. The point mutations occurring in the CRF1-R gene do not seem to influence basal anxiety while they appear to affect active responses to stress.

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.

Neuroanatomical and functional characterization of CRF neurons of the amygdala using a novel transgenic mouse model

The corticotropin-releasing factor (CRF)-producing neurons of the amygdala have been implicated in behavioral and physiological responses associated with fear, anxiety, stress, food intake and reward. To overcome the difficulties in identifying CRF neurons within the amygdala, a novel transgenic mouse line, in which the humanized recombinant Renilla reniformis green fluorescent protein (hrGFP) is under the control of the CRF promoter (CRF-hrGFP mice), was developed. First, the CRF-hrGFP mouse model was validated and the localization of CRF neurons within the amygdala was systematically mapped. Amygdalar hrGFP-expressing neurons were located primarily in the interstitial nucleus of the posterior limb of the anterior commissure, but also present in the central amygdala. Secondly, the marker of neuronal activation c-Fos was used to explore the response of amygdalar CRF neurons in CRF-hrGFP mice under different experimental paradigms. C-Fos induction was observed in CRF neurons of CRF-hrGFP mice exposed to an acute social defeat stress event, a fasting/refeeding paradigm or lipopolysaccharide (LPS) administration. In contrast, no c-Fos induction was detected in CRF neurons of CRF-hrGFP mice exposed to restraint stress, forced swimming test, 48-h fasting, acute high-fat diet (HFD) consumption, intermittent HFD consumption, ad libitum HFD consumption, HFD withdrawal, conditioned HFD aversion, ghrelin administration or melanocortin 4 receptor agonist administration. Thus, this study fully characterizes the distribution of amygdala CRF neurons in mice and suggests that they are involved in some, but not all, stress or food intake-related behaviors recruiting the amygdala.

Social stress and CRF-dopamine interactions in the VTA: role in long-term escalation of cocaine self-administration

The nature of neuroadaptations in the genesis of escalated cocaine taking remains a topic of considerable interest. Intermittent social defeat stress induces both locomotor and dopaminergic cross-sensitization to cocaine, as well as escalated cocaine self-administration. The current study examines the role of corticotropin releasing factor receptor subtypes 1 and 2 (CRFR1, CRFR2) within the ventral tegmental area (VTA) during social defeat stress. This study investigated whether injecting either a CRFR1 or CRFR2 antagonist directly into the VTA before each social defeat would prevent the development of later (1) locomotor sensitization, (2) dopaminergic sensitization, and (3) escalated cocaine self-administration in rats. CRFR1 antagonist CP376395 (50 or 500 ng/side), CRFR2 antagonist Astressin2-B (100 or 1000 ng/side), or vehicle (aCSF) was microinjected into the VTA 20 min before social defeat stress (or handling) on days 1, 4, 7, and 10. Ten days later, rats were injected with cocaine (10 mg/kg, i.p.) and assessed for either locomotor sensitization, measured by walking activity, or dopaminergic sensitization, measured by extracellular dopamine (DA) in the nucleus accumbens shell (NAcSh) through in vivo microdialysis. Locomotor sensitization testing was followed by intravenous cocaine self-administration. Intra-VTA antagonism of CRFR1, but not CRFR2, inhibited the induction of locomotor cross-sensitization to cocaine, whereas both prevented dopaminergic cross-sensitization and escalated cocaine self-administration during a 24 h "binge." This may suggest dissociation between locomotor sensitization and cocaine taking. These data also suggest that interactions between CRF and VTA DA neurons projecting to the NAcSh are essential for the development of dopaminergic cross-sensitization to cocaine.

Optogenetic dissection of amygdala functioning

Studies of amygdala functioning have occupied a significant place in the history of understanding how the brain controls behavior and cognition. Early work on the amygdala placed this small structure as a key component in the regulation of emotion and affective behavior. Over time, our understanding of its role in brain processes has expanded, as we have uncovered amygdala influences on memory, reward behavior, and overall functioning in many other brain regions. Studies have indicated that the amygdala has widespread connections with a variety of brain structures, from the prefrontal cortex to regions of the brainstem, that explain its powerful influence on other parts of the brain and behaviors mediated by those regions. Thus, many optogenetic studies have focused on harnessing the powers of this technique to elucidate the functioning of the amygdala in relation to motivation, fear, and memory as well as to determine how the amygdala regulates activity in other structures. For example, studies using optogenetics have examined how specific circuits within amygdala nuclei regulate anxiety. Other work has provided insight into how the basolateral and central amygdala nuclei regulate memory processing underlying aversive learning. Many experiments have taken advantage of optogenetics’ ability to target either genetically distinct subpopulations of neurons or the specific projections from the amygdala to other brain regions. Findings from such studies have provided evidence that particular patterns of activity in basolateral amygdala (BLA) glutamatergic neurons are related to memory consolidation processes, while other work has indicated the critical nature of amygdala inputs to the prefrontal cortex and nucleus accumbens (NA) in regulating behavior dependent on those downstream structures. This review will examine the recent discoveries on amygdala functioning made through experiments using optogenetics, placing these findings in the context of the major questions in the field.

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.

Paradoxical enhancement of fear expression and extinction deficits in mice resilient to social defeat

The exposure to stress has been associated with increased depressive and anxiety symptoms, yet not all individuals respond negatively to the experience of stress. Recent rodent social defeat models demonstrate similar individual differences in response to social stress. In particular, mice subjected to chronic social defeat have been characterized as being either "susceptible" or "resilient" by the level of social interaction following social defeat. Susceptibility is associated with lasting social avoidance as well as increased anxiety-like behavior, and depressive-like symptoms. Resilient animals, however, do not show social avoidance or increased depressive-like symptoms, but retain increased anxiety-like behavior. Thus, it is unclear what "resilience" as measured by social interaction represents in terms of an overall behavioral and physiological phenotype. Here, we use an acute social defeat procedure, which produces distinct behavioral phenotypes in social interaction with no apparent changes in anxiety-like behavior. Susceptible mice display lasting social avoidance, whereas resilient mice display normal social interaction. Susceptible mice also displayed deficits in fear extinction retention but had normal within-session extinction. Paradoxically, resilience was associated with enhanced fear expression, and severe deficits in fear extinction and extinction retention beyond that observed in susceptible mice. These effects in resilient mice were only apparent after the experience of social stress and were not due to impaired behavioral flexibility. These data suggest that mechanisms controlling resilience to acute social defeat as characterized by social interaction leave animals vulnerable to maladaptive fear behavior.

The CRF system and social behavior: a review

The corticotropin-releasing factor (CRF) system plays a key role in a diversity of behaviors accompanying stress, anxiety and depression. There is also substantial research on relationships between social behaviors and the CRF system in a variety of taxa including fish, birds, rodents, and primates. Some of these relationships are due to the broad role of CRF and urocortins in stress and anxiety, but these peptides also modulate social behavior specifically. For example, the social interaction (SI) test is often used to measure anxiety-like behavior. Many components of the CRF system including CRF, urocortin1, and the R1 receptor have been implicated in SI, via general effects on anxiety as well as specific effects depending on the brain region. The CRF system is also highly responsive to chronic social stressors such as social defeat and isolation. Animals exposed to these stressors display a number of anxiety- and stress-related behaviors, accompanied by changes in specific components the CRF system. Although the primary focus of CRF research on social behavior has been on the deleterious effects of social stress, there are also insights on a role for CRF and urocortins in prosocial and affiliative behaviors. The CRF system has been implicated in parental care, maternal defense, sexual behavior, and pair bonding. Species differences in the ligands and CRF receptors have been observed in vole and bird species differing in social behavior. Exogenous administration of CRF facilitates partner preference formation in monogamous male prairie voles, and these effects are dependent on both the CRF R1 and R2 receptors. These findings are particularly interesting as studies have also implicated the CRF and urocortins in social memory. With the rapid progress of social neuroscience and in understanding the complex structure of the CRF system, the next challenge is in parsing the exact contribution of individual components of this system to specific social behaviors.

Sex differences in stress-related receptors: ″micro″ differences with ″macro″ implications for mood and anxiety disorders

Stress-related psychiatric disorders, such as unipolar depression and post-traumatic stress disorder (PTSD), occur more frequently in women than in men. Emerging research suggests that sex differences in receptors for the stress hormones, corticotropin releasing factor (CRF) and glucocorticoids, contribute to this disparity. For example, sex differences in CRF receptor binding in the amygdala of rats may predispose females to greater anxiety following stressful events. Additionally, sex differences in CRF receptor signaling and trafficking in the locus coeruleus arousal center combine to make females more sensitive to low levels of CRF, and less adaptable to high levels. These receptor differences in females could lead to hyperarousal, a dysregulated state associated with symptoms of depression and PTSD. Similar to the sex differences observed in CRF receptors, sex differences in glucocorticoid receptor (GR) function also appear to make females more susceptible to dysregulation after a stressful event. Following hypothalamic pituitary adrenal axis activation, GRs are critical to the negative feedback process that inhibits additional glucocorticoid release. Compared to males, female rats have fewer GRs and impaired GR translocation following chronic adolescent stress, effects linked to slower glucocorticoid negative feedback. Thus, under conditions of chronic stress, attenuated negative feedback in females would result in hypercortisolemia, an endocrine state thought to cause depression. Together, these studies suggest that sex differences in stress-related receptors shift females more easily into a dysregulated state of stress reactivity, linked to the development of mood and anxiety disorders. The implications of these receptor sex differences for the development of novel pharmacotherapies are also discussed.

Corticotropin-releasing factor in the mouse central nucleus of the amygdala: ultrastructural distribution in NMDA-NR1 receptor subunit expressing neurons as well as projection neurons to the bed nucleus of the stria terminalis

Corticotropin-releasing factor (CRF) and glutamate are critical signaling molecules in the central nucleus of the amygdala (CeA). Central amygdala CRF, acting via the CRF type 1 receptor (CRF-R1), plays an integral role in stress responses and emotional learning, processes that are generally known to involve functional NMDA-type glutamate receptors. There is also evidence that CRF expressing CeA projection neurons to the bed nucleus of the stria terminalis (BNST) play an important role in stress related behaviors. Despite the potentially significant interactions between CRF and NMDA receptors in the CeA, the synaptic organization of these systems is largely unknown. Using dual labeling high resolution immunocytochemical electron microscopy, it was found that individual somata and dendrites displayed immunoreactivity for CRF and the NMDA-NR1 (NR1) subunit in the mouse CeA. In addition, CRF-containing axon terminals contacted postsynaptic targets in the CeA, some of which also expressed NR1. Neuronal profiles expressing the CRF type 1 receptor (CRF-R1), identified by the expression of green fluorescent protein (GFP) in bacterial artificial chromosome (BAC) transgenic mice, also contained NR1, and GFP immunoreactive terminals formed synapses with NR1 containing dendrites. Although CRF and GFP were only occasionally co-expressed in individual somata and dendritic profiles, contacts between labeled axon terminals and dendrites were frequently observed. A combination of tract tracing and immunocytochemistry revealed that a population of CeA CRF neurons projected to the BNST. It was also found that CRF, or GFP expressing terminals directly contacted CeA-BNST projection neurons. These results indicate that the NMDA receptor is positioned for the postsynaptic regulation of CRF expressing CeA neurons and the modulation of signals conveyed by CRF inputs. Interactions between CRF and NMDA receptor mediated signaling in CeA neurons, including those projecting to the BNST, may provide the synaptic basis for integrating the experience of stress and relevant environmental stimuli with behaviors that may be of particular relevance to stress-related learning and the emergence of psychiatric disorders, including drug addiction.

Yohimbine anxiogenesis in the elevated plus maze is disrupted by bilaterally disconnecting the bed nucleus of the stria terminalis from the central nucleus of the amygdala

The α2 adrenergic receptor antagonist yohimbine (YO) is a sympathomimetic drug that crosses the blood-brain barrier after systemic administration. YO promotes increased transmitter release from noradrenergic (NA) axon terminals in the central nucleus of the amygdala (CEA), bed nucleus of the stria terminalis (BST), hypothalamus, and other brain regions implicated in physiological and behavioral responses to stressful and threatening stimuli. YO is potently anxiogenic in humans and experimental animals, including rats. To determine whether direct connections between the CEA and anterolateral group of BST nuclei (algBST) are necessary for YO anxiogenesis in rats, neurotoxic ibotenate lesions of the CEA in one hemisphere and the ipsi- or contralateral algBST were conducted to disrupt CEA-algBST communication uni- or bilaterally. Sham-lesioned controls received microinjections of vehicle into the CEA and algBST. Two weeks later, behavior was assessed in the elevated plus maze (EPMZ) in rats after i.p. saline or YO (1.0mg/kg). Central ibotenate lesion placement and extent was assessed post-mortem in NeuN-immunolabeled tissue sections. The ability of YO to increase anxiety-like behavior in the EPMZ was similarly robust in rats with sham lesions or ipsilateral CEA-algBST lesions. Conversely, YO anxiogenesis in the EPMZ was disrupted in rats with asymmetric lesions designed to bilaterally disconnect the CEA and algBST, whereas neither unilateral nor bilateral disconnecting lesions altered EPMZ behavior in rats after i.p. saline. We conclude that the anxiogenic effects of increased NA signaling in rats after YO require direct CEA-algBST interactions that do not shape EPMZ behavior under baseline conditions.

Remembering another aspect of forgetting

Although forgetting is most often thought of in terms of declines in performance (response loss or impairment), another class of memory phenomena, the forgetting of stimulus attributes, has begun to attract experimental attention. In non-human animals, the loss of memory for stimulus features is reflected in the flattening of stimulus generalization gradients as well as in the attenuation of the disrupting effect of a shift in context at testing. In both cases, a delay between the learning episode and testing results in increased responding in the presence of previously ineffective stimuli. Thus, previously discriminable cues become more functionally interchangeable. The implications of the forgetting of attributes for some theoretical issues of memory loss and for methodological strategies have been noted earlier. However, relatively little is known about the neurobiological mechanisms underlying stimulus attribute forgetting, and why some memories are maintained while others are not. In this paper we review the evidence for the forgetting of stimulus attributes, discuss recent findings identifying neurobiological underpinnings of forgetting and generalization of fear responses, and discuss relevant clinical implications of fear generalization.

Presynaptic inhibition of gamma-aminobutyric acid release in the bed nucleus of the stria terminalis by kappa opioid receptor signaling

BACKGROUND

The kappa opioid receptor (KOR) and its endogenous agonist, the neuropeptide dynorphin, are a critical component of the central stress system. Both dynorphin and KOR are expressed in the bed nucleus of the stria terminalis (BNST), a brain region associated with anxiety and stress. This suggests that KOR activation in this region may play a role in the regulation of emotional behaviors. To date, however, there has been no investigation of the ability of KOR to modulate synaptic transmission in the BNST.

METHODS

We used whole-cell patch-clamp recordings from acutely prepared mouse brain slices to examine the actions of KOR on inhibitory transmission in the BNST. Additionally, we used neurochemical and pathway-specific optogenetic manipulations to selectively stimulate gamma-aminobutyric acid (GABA)ergic fibers from the central nucleus of the amygdala (CeA) to the BNST.

RESULTS

We found that activation of KOR reduced GABAergic transmission through a presynaptic mechanism. Furthermore, we examined the signal transduction pathways that mediate this inhibition and provide the first functional information implicating extracellular signal-regulated kinase in KOR-mediated presynaptic modulation. Moreover, we found that at KOR signaling robustly reduced inhibitory synaptic transmission in the CeA to BNST pathway.

CONCLUSIONS

Together, these results demonstrate that KOR provides important inhibitory control over presynaptic GABAergic signaling within the BNST and provides the first direct functional demonstration of KOR-sensitive long-range GABAergic connections between the CeA and the BNST.

Common and distinct neural inputs to the medial central nucleus of the amygdala and anterior ventrolateral bed nucleus of stria terminalis in rats

The central nucleus of the amygdala (CEA) and lateral bed nucleus of stria terminalis (BST) are highly interconnected limbic forebrain regions that share similar connectivity with other brain regions that coordinate behavioral and physiological responses to internal and environmental stressors. Their similar connectivity is frequently referred to when describing the CEA and lateral BST together as a unified "central extended amygdala". However, the CEA and BST reportedly play distinct roles in behavioral and physiological responses associated with fear, anxiety, and social defeat, presumably due to differences in connectivity. To identify common and unique sources of input to the CEA and lateral BST, we performed dual retrograde tracing. Fluorogold and cholera toxin β were iontophoresed into the medial CEA (CEAm) and the anterior ventrolateral BST (BSTvl) of adult male rats. The anatomical distribution of tracer-labeled neurons was mapped throughout the brain. Regions with overlapping populations of CEAm- and BSTvl-projecting neurons were further examined for the presence of double-labeled neurons. Although most regions with input to the mCEA also projected to the BSTvl, and vice versa, cortical and sensory system-related regions projected more robustly to the CEAm, while motor system-related regions primarily innervated the BSTvl. The incidence of double-labeled neurons with collateralized axonal inputs to the CEAm and BSTvl was relatively small (~2 to 13%) and varied across regions, suggesting regional differences in the degree of coordinated CEAm and BSTvl input. The demonstrated similarities and differences in inputs to CEAm and BSTvl provide new anatomical insights into the functional organization of these limbic forebrain regions.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

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'.

Sex differences in social interaction behavior following social defeat stress in the monogamous California mouse (Peromyscus californicus)

Stressful life experiences are known to be a precipitating factor for many mental disorders. The social defeat model induces behavioral responses in rodents (e.g. reduced social interaction) that are similar to behavioral patterns associated with mood disorders. The model has contributed to the discovery of novel mechanisms regulating behavioral responses to stress, but its utility has been largely limited to males. This is disadvantageous because most mood disorders have a higher incidence in women versus men. Male and female California mice (Peromyscus californicus) aggressively defend territories, which allowed us to observe the effects of social defeat in both sexes. In two experiments, mice were exposed to three social defeat or control episodes. Mice were then behaviorally phenotyped, and indirect markers of brain activity and corticosterone responses to a novel social stimulus were assessed. Sex differences in behavioral responses to social stress were long lasting (4 wks). Social defeat reduced social interaction responses in females but not males. In females, social defeat induced an increase in the number of phosphorylated CREB positive cells in the nucleus accumbens shell after exposure to a novel social stimulus. This effect of defeat was not observed in males. The effects of defeat in females were limited to social contexts, as there were no differences in exploratory behavior in the open field or light-dark box test. These data suggest that California mice could be a useful model for studying sex differences in behavioral responses to stress, particularly in neurobiological mechanisms that are involved with the regulation of social 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.

Short- and long-term effects of intermittent social defeat stress on brain-derived neurotrophic factor expression in mesocorticolimbic brain regions

Social defeat stress is an ethologically salient stressor which activates dopaminergic areas and, when experienced repeatedly, has long-term effects on dopaminergic function and related behavior. The mechanism for these long-lasting consequences remains unclear. A potential candidate for mediating these effects is brain-derived neurotrophic factor (BDNF), a neurotrophin involved in synaptic plasticity and displaying alterations in dopaminergic regions in response to various types of stress. In this study, we sought to determine whether repeated social defeat stress altered BDNF mRNA and protein expression in dopaminergic brain regions either immediately after the last stress exposure or 4 weeks later. Male Sprague-Dawley rats were subjected to social defeat stress consisting of brief confrontation with an aggressive male rat every third day for 10 days; control rats were handled according to the same schedule. Animals were euthanized either 2 h or 28 days after the last stress or handling episode. Our results show that 2 h after stress, BDNF protein and mRNA expression increased in the medial prefrontal cortex. At this time-point, BDNF mRNA increased in the amygdala and protein expression increased in the substantia nigra. Twenty-eight days after stress, BDNF protein and mRNA expression were elevated in the medial amygdala and ventral tegmental area. Given the role of BDNF in neural plasticity, BDNF alterations that are long-lasting may be significant for neural adaptations to social stress. The dynamic nature of BDNF expression in dopaminergic brain regions in response to repeated social stress may therefore have implications for lasting neurochemical and behavioral changes related to dopaminergic function.

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.