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

Preclinical Models of Chronic Stress: Adaptation or Pathology?

The experience of prolonged stress changes how individuals interact with their environment and process interoceptive cues, with the end goal of optimizing survival and well-being in the face of a now-hostile world. The chronic stress response includes numerous changes consistent with limiting further damage to the organism, including development of passive or active behavioral strategies and metabolic adjustments to alter energy mobilization. These changes are consistent with symptoms of pathology in humans, and as a result, chronic stress has been used as a translational model for diseases such as depression. While it is of heuristic value to understand symptoms of pathology, we argue that the chronic stress response represents a defense mechanism that is, at its core, adaptive in nature. Transition to pathology occurs only after the adaptive capacity of an organism is exhausted. We offer this perspective as a means of framing interpretations of chronic stress studies in animal models and how these data relate to adaptation as opposed to pathology.

Depressive symptoms, anxiety and social stress are associated with diminished cardiovascular reactivity in a psychological treatment-naive population

BACKGROUND

There is now an increasing appreciation of how psychological health can contribute to cardiovascular disease, called the mind-heart connection. A blunted cardiovascular reactivity to depression and anxiety may be responsible for the potential mechanism, however, with inconsistent results. Anti-psychological drugs have an effect on the cardiovascular system and, thus, may disturb their relationship. However, in treatment-naive individuals with psychological symptoms, no research has specifically evaluated the relationship between psychological state and cardiovascular reactivity.

METHODS

We included 883 treatment-naive individuals who came from a longitudinal cohort study of Midlife in the United States. Symptoms of depression, anxiety, and stress were assessed by the Center for Epidemiologic Studies Depression Scale (CES-D), Spielberger Trait Anxiety Inventory (STAI), the Liebowitz Social Anxiety scale (LSAS) and the Perceived Stress Scale (PSS), respectively. Cardiovascular reactivity was measured using standardized, laboratory-based stressful tasks.

RESULTS

Treatment-naive individuals with depressive symptoms (CES-D ≥ 16), anxiety symptoms (STAI ≥ 54), and higher stress levels (PSS ≥ 27) had lower cardiovascular reactivity as assessed by systolic blood pressure (SBP) reactivity, diastolic blood pressure (DBP) reactivity and heart rate (HR) reactivity (P < 0.05). Pearson analyses showed that psychological symptoms were correlated with lower SBP reactivity, DBP reactivity, and heart rate reactivity (P < 0.05). Multivariate linear regression showed that depression and anxiety were negatively related to lower cardiovascular reactivity (SBP, DBP and HR reactivity) after full adjustments (P < 0.05). Stress was associated with reduced SBP and DBP reactivity but with a nonsignificant association with HR reactivity (P = 0.056).

CONCLUSION

Depression, anxiety, and stress symptoms are associated with blunted cardiovascular reactivity in treatment-naive adult Americans. These findings suggest that blunted cardiovascular reactivity is an underlying mechanism linking psychological health and cardiovascular diseases.

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'.

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

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

Encore: Behavioural animal models of stress, depression and mood disorders

Animal studies over the past two decades have led to extensive advances in our understanding of pathogenesis of depressive and mood disorders. Among these, rodent behavioural models proved to be of highest informative value. Here, we present a comprehensive overview of the most popular behavioural models with respect to physiological, circuit, and molecular biological correlates. Behavioural stress paradigms and behavioural tests are assessed in terms of outcomes, strengths, weaknesses, and translational value, especially in the domain of pharmacological studies.

IL-6 and IL-8 are likely associated with psychological status in treatment naïve general population

BACKGROUND

Levels of inflammatory markers are elevated in patients with psychological disorders. However, anti-psychological drugs have an effect on proinflammatory cytokine production and disturb their relationship. Limited evidence focuses on the inflammatory marker profile of psychological status before treatment. This study aimed to investigate the inflammatory biomarker profiles of psychological treatment-naive individuals.

METHODS

We included 790 psychological treatment-naive individuals from a longitudinal cohort study of Midlife in the United States (MIDUS). Symptoms of depression, anxiety, and stress were assessed by the Center for Epidemiologic Studies Depression Scale (CES-D) subscales, the Social Anxiety Scale (STAI), and Liebowitz Social Anxiety Scale (LSAS), the Perceived Stress Scale (PSS), respectively.

RESULTS

Spearman correlation analysis showed that a higher CESD total score was correlated with higher CRP (p=0.009), IL-6 (p=0.007), fibrinogen (p=0.036), E-selectin (p=0.018), ICAM-1 (p=0.013), and IL-8 (p=0.05) levels. Multivariate linear regression analysis showed that the CESD total score was positively associated with the levels of IL-6 (p=0.024) after adjustments. Moreover, the perceived stress score (PSS) was negatively associated with the levels of IL-8 (p=0.025). However, these associations were not significant after multiple testing (p=0.088, 0.091, respectively).

LIMITATIONS

The casual relationship cannot be drawn due to the cross-sectional design CONCLUSION: Overall, our results suggested IL-6 and IL-8 might play a important role in the pathogenesis of psychological disorder. Larger and longitude studies are needed to confirm our results.

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.

Individual baseline behavioral traits predict the resilience phenotype after chronic social defeat

Chronic social defeat (CSD) has been widely used as a psychosocial stress model in mice, with the magnitude of CSD-induced social avoidance as the major behavioral hallmark of the resilient and susceptible groups. Despite significant progress in the study of the neurobiology of resilient and susceptible mice, the nature and ethological relevance of CSD-induced social avoidance and social approach, particularly measured using a CD1 mouse, needs conceptual clarification. Based on the findings of a recent study revealing substantial individuality in genetically homogeneous inbred mice, we investigated whether certain baseline individual characteristics of male C57BL/6J mice predict the resilient outcome after CSD. We focused on two well-studied individual traits that seem to have heritable underpinnings—approach to novelty and avoidance of harm, which are essential for the expression of the exploratory drive. Our results showed that the exploration levels and the approach to novelty and harm were different before and after CSD in resilient and susceptible mice. Before the stress, resilient mice had higher horizontal activity in a novel environment, shorter approach latencies, and higher exploration times for social and non-social targets than susceptible mice. However, susceptible mice performed better in the passive avoidance task than resilient mice as they were more successful in learning to avoid potential adversity by suppressing the spontaneous exploratory drive. Our findings challenge the validity of the current selection criteria for the susceptible and resilient groups and encourage comprehensive assessment of both baseline and stress-induced individual behavioral signatures of mice.

Emotional Impairments and Neuroinflammation are Induced in Male Mice Invulnerable to Repeated Social Defeat Stress

Prolonged stress triggers neuroinflammation, which plays a significant role in the development of depression; however, stressed people do not always suffer from depression because of individual differences in stress vulnerability. Negative cognitive bias (NCB) toward pessimistic judgment often underlies depressive episodes. However, a relationship between stress vulnerability, neuroinflammation, and NCB remains elusive. In addition, an animal model with all the traits would be a powerful tool for studying the etiology of depression and its therapeutic approaches. Accordingly, this study evaluated the effect of stress vulnerability on neuroinflammation and depression-related behaviors, including NCB in males, using a modified version of repeated social defeat stress (mRSDS) paradigm, a validated animal model of psychosocial stress. Exposure to mRSDS, consisting of 5 min of social defeat by unfamiliar CD-1 aggressor mice for five consecutive days, caused NCB, which co-occurred with depressive- and anxiety-like behaviors, and neuroinflammation in male BALB/c mice. Treatment with minocycline, an antibiotic with anti-inflammatory property, blocked mRSDS-induced depressive-like behaviors and neuroinflammation, but not NCB, indicating the limited effect of an anti-inflammatory intervention. In addition, marked differences were found in neuroinflammatory profiles and hippocampal gene expression patterns between resilient and unstressed mice, as well as between susceptible and resilient mice. Therefore, mice resilient to mRSDS are indeed not intact. Our findings provide insights into the unique features of the mRSDS model in male BALB/c mice, which could be used to investigate the etiological mechanisms underlying depression as well as bridge the gap in the relationship between stress vulnerability, neuroinflammation, and NCB in males.

Cellular mechanisms and molecular signaling pathways in stress-induced anxiety, depression, and blood-brain barrier inflammation and leakage

Depression and anxiety are comorbid conditions in many neurological or psychopathological disorders. Stress is an underlying event that triggers development of anxiety and depressive-like behaviors. Recent experimental data indicate that anxiety and depressive-like behaviors occurring as a result of stressful situations can cause blood-brain barrier (BBB) dysfunction, which is characterized by inflammation and leakage. However, the underlying mechanisms are not completely understood. This paper sought to review recent experimental preclinical and clinical data that suggest possible molecular mechanisms involved in development of stress-induced anxiety and depression with associated BBB inflammation and leakage. Critical therapeutic targets and potential pharmacological candidates for treatment of stress-induced anxiety and depression with associated BBB dysfunctions are also discussed.

Stress-induced blood brain barrier disruption: Molecular mechanisms and signaling pathways

Stress is a nonspecific response to a threat or noxious stimuli with resultant damaging consequences. Stress is believed to be an underlying process that can trigger central nervous system disorders such as depression, anxiety, and post-traumatic stress disorder. Though the pathophysiological basis is not completely understood, data have consistently shown a pivotal role of inflammatory mediators and hypothalamo-pituitary-adrenal (HPA) axis activation in stress induced disorders. Indeed emerging experimental evidences indicate a concurrent activation of inflammatory signaling pathways and not only the HPA axis, but also, peripheral and central renin-angiotensin system (RAS). Furthermore, recent experimental data indicate that the HPA and RAS are coupled to the signaling of a range of central neuro-transmitter, -mediator and -peptide molecules that are also regulated, at least in part, by inflammatory signaling cascades and vice versa. More recently, experimental evidences suggest a critical role of stress in disruption of the blood brain barrier (BBB), a neurovascular unit that regulates the movement of substances and blood-borne immune cells into the brain parenchyma, and prevents peripheral injury to the brain substance. However, the mechanisms underlying stress-induced BBB disruption are not exactly known. In this review, we summarize studies conducted on the effects of stress on the BBB and integrate recent data that suggest possible molecular mechanisms and signaling pathways underlying stress-induced BBB disruption. Key molecular targets and pharmacological candidates for treatment of stress and related illnesses are also summarized.

Chronic social defeat-induced social avoidance as a proxy of stress resilience in mice involves conditioned learning

Chronic social defeat (CSD)-induced social avoidance is considered to model a feature of stress-related mental dysfunction, while its absence has been used as a proxy of resilience in rodents. However, knowledge on the mechanisms shaping CSD-induced individual outcomes remains fragmentary. Fear conditioning has been described as a suitable model in humans for better understanding the pathophysiology of stress related mental disorders. We sought to explore the extent to which conditioned learning is involved in CSD-induced social avoidance. In experiment 1 (social avoidance specificity), C57BL/6 J male mice underwent CSD followed by a modified social interaction test offering the simultaneous choice between an unknown mouse from the aggressor's strain or a mouse from a different strain and phenotypic characteristics. In experiment 2 (social avoidance extinction), CSD-extinction sessions involving only the sensory phase of CSD were conducted on one group of defeated mice whereas a second group only received handling, followed by social interaction test with a novel mouse from the aggressor's strain. Our results provide evidence that CSD-induced social avoidance does not generalize to other phenotypic characteristics than those of the aggressors and can be successfully reversed during extinction training. Taken together, our findings strongly point to the involvement of conditioned learning in shaping CSD-induced social avoidance, a finding that is of interest to future studies into the neurobiology of resilience.

Predator stress-induced depression is associated with inhibition of hippocampal neurogenesis in adult male mice

Stress has been suggested to disturb the 5-hydroxytryptamine system and decrease neurogenesis, which contribute to the development of depression. Few studies have investigated the effect of predator stress, a type of psychological stress, on depression and hippocampal neurogenesis in adult mice; we therefore investigated this in the present study. A total of 35 adult male Kunming mice were allocated to a cat stress group, cat odor stress group, cat stress + fluoxetine group, cat odor stress + fluoxetine group, or a control group (no stress/treatment). After 12 days of cat stress or cat odor stress, behavioral correlates of depression were measured using the open field test, elevated plus maze test, and dark-avoidance test. The concentrations of hippocampal 5-hydroxytryptamine and 5-hydroxyindoleacetic acid were measured using high-performance liquid chromatography-electrochemical detection. Neurogenesis was also analyzed using a bromodeoxyuridine and doublecortin double-immunostaining method. Cat stress and cat odor stress induced depression-like behaviors; this effect was stronger in the cat stress model. Furthermore, compared with the control group, cat stress mice exhibited lower 5-hydroxytryptamine concentrations, higher 5-hydroxyindoleacetic acid concentrations, and significantly fewer bromodeoxyuridine⁺/doublecortin⁺-labeled cells in the dentate gyrus, which was indicative of less neurogenesis. The changes observed in the cat stress group were not seen in the cat stress + fluoxetine group, which suggests that the effects of predator stress on depression and neurogenesis were reversed by fluoxetine. Taken together, our results indicate that depression-like behaviors induced by predator stress are associated with the inhibition of hippocampal neurogenesis.

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

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

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

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

Metabolomics reveals distinct neurochemical profiles associated with stress resilience

Acute social defeat represents a naturalistic form of conditioned fear and is an excellent model in which to investigate the biological basis of stress resilience. While there is growing interest in identifying biomarkers of stress resilience, until recently, it has not been feasible to associate levels of large numbers of neurochemicals and metabolites to stress-related phenotypes. The objective of the present study was to use an untargeted metabolomics approach to identify known and unknown neurochemicals in select brain regions that distinguish susceptible and resistant individuals in two rodent models of acute social defeat. In the first experiment, male mice were first phenotyped as resistant or susceptible. Then, mice were subjected to acute social defeat, and tissues were immediately collected from the ventromedial prefrontal cortex (vmPFC), basolateral/central amygdala (BLA/CeA), nucleus accumbens (NAc), and dorsal hippocampus (dHPC). Ultra-high performance liquid chromatography coupled with high resolution mass spectrometry (UPLC-HRMS) was used for the detection of water-soluble neurochemicals. In the second experiment, male Syrian hamsters were paired in daily agonistic encounters for 2 weeks, during which they formed stable dominant-subordinate relationships. Then, 24 h after the last dominance encounter, animals were exposed to acute social defeat stress. Immediately after social defeat, tissue was collected from the vmPFC, BLA/CeA, NAc, and dHPC for analysis using UPLC-HRMS. Although no single biomarker characterized stress-related phenotypes in both species, commonalities were found. For instance, in both model systems, animals resistant to social defeat stress also show increased concentration of molecules to protect against oxidative stress in the NAc and vmPFC. Additionally, in both mice and hamsters, unidentified spectral features were preliminarily annotated as potential targets for future experiments. Overall, these findings suggest that a metabolomics approach can identify functional groups of neurochemicals that may serve as novel targets for the diagnosis, treatment, or prevention of stress-related mental illness.

Indomethacin counteracts the effects of chronic social defeat stress on emotional but not recognition memory in mice

We have previously observed the impairing effects of chronic social defeat stress (CSDS) on emotional memory in mice. Given the relation between stress and inflammatory processes, we sought to study the effectiveness of the anti-inflammatory indomethacin in reversing the detrimental effects of CSDS on emotional memory in mice. The effects of CSDS and indomethacin on recognition memory were also evaluated. Male CD1 mice were randomly divided into four groups: non-stressed + saline (NS+SAL); non-stressed + indomethacin (NS+IND); stressed + saline (S+SAL); and stressed + indomethacin (S+IND). Stressed animals were exposed to a daily 10 min agonistic confrontation (CSDS) for 20 days. All subjects were treated daily with saline or indomethacin (10 mg/kg, i.p.). 24 h after the CSDS period, all the mice were evaluated in a social interaction test to distinguish between those that were resilient or susceptible to social stress. All subjects (n = 10–12 per group) were then evaluated in inhibitory avoidance (IA), novel object recognition (NOR), elevated plus maze and hot plate tests. As in control animals (NS+SAL group), IA learning was observed in the resilient groups, as well as in the susceptible mice treated with indomethacin (S+IND group). Recognition memory was observed in the non-stressed and the resilient mice, but not in the susceptible animals. Also, stressed mice exhibited higher anxiety levels. No significant differences were observed in locomotor activity or analgesia. In conclusion, CSDS induces anxiety in post-pubertal mice and impairs emotional and recognition memory in the susceptible subjects. The effects of CSDS on emotional memory, but not on recognition memory and anxiety, are reversed by indomethacin. Moreover, memory impairment is not secondary to the effects of CSDS on locomotor activity, emotionality or pain sensitivity.

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.

Proteolytic cleavage of proBDNF into mature BDNF in the basolateral amygdala is necessary for defeat-induced social avoidance

Brain-derived neurotrophic factor (BDNF) is essential for memory processes. The present study tested whether proteolytic cleavage of proBDNF into mature BDNF (mBDNF) within the basolateral amygdala (BLA) regulates the consolidation of defeat-related memories. We found that acute social defeat increases the expression of mBDNF, but not proBDNF, in the BLA/central amygdala. We also showed that blocking plasmin in the BLA with microinjection of α2-antiplasmin immediately following social defeat decreases social avoidance 24 h later. These data suggest the proteolytic cleavage of BDNF in the BLA is necessary for defeat-induced social avoidance.

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.

Phenotypic responses to social defeat are associated with differences in cued and contextual fear discrimination

Conflict among individuals is one of the most common forms of stressors experienced across a variety of species, including humans. Social defeat models in mice produce two phenotypic behavioral responses characterized by prolonged social avoidance (susceptibility) or continued social interaction (resistance). The resistant phenotype has been proposed as a model of resilience to chronic stress-induced depression in humans. Previously, we have found that mice that are resistant to social defeat stress display significant impairments in extinction learning and retention, suggesting that continued social interaction following the experience of social defeat may be associated with maladaptive fear responses. Here, we examined how individual differences in response to social defeat may be related to differences in cued and context fear discrimination. Following defeat, resistant mice showed increased fear to a neutral cued stimulus (CS-) compared to control and susceptible mice, but were still able to significantly discriminate between the CS+ and CS-. Likewise, both phenotypes were generally able to discriminate between the training context and neutral context at all retention intervals tested (1, 5, 14 days). However, susceptible mice displayed significantly better discrimination compared to resistant and non-defeated control mice when assessing the discrimination ratio. Thus, at a time when most animals begin exhibiting generalization to contextual cues, susceptible mice retain the ability to discriminate between fearful and neutral contexts. These data suggest that the differences observed in context and cued discrimination between susceptible and resistant mice may be related to differences in their coping strategies in response to social defeat. In particular, resistance or resilience to social defeat as traditionally characterized may be associated with altered inhibitory learning. Understanding why individual differences arise in response to stress, including social confrontation is important in understanding the development and treatment of stress related pathologies such as PTSD.

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 PTSD resilience and susceptibility: Challenges for animal models of fear learning

PTSD occurs in only a small fraction of trauma-exposed individuals, but risk is twice as high in women as in men. The neurobiological basis for this discrepancy is not known, but the identification of biological determinants of resilience and susceptibility in each sex could lead to more targeted preventions and treatments. Animal models are a useful tool for dissecting the circuits and mechanisms that underlie the brain's response to stress, but the vast majority of this work has been developed and conducted in males. The limited work that does incorporate female animals is often inconsistent across labs and does not broadly reflect human populations in terms of female susceptibility to PTSD-like behaviors. In this review, we suggest that interpreting male vs. female comparisons in these models be approached carefully, since common behavioral outcome measures may in fact reflect distinct neural processes. Moreover, since the factors that determine resilience and susceptibility are likely at least in part distinct in men and women, models that take a within-sex approach to response variability may be more useful in identifying critical mechanisms for manipulation.

Two weeks of predatory stress induces anxiety-like behavior with co-morbid depressive-like behavior in adult male mice

Psychological stress can have devastating and lasting effects on a variety of behaviors, especially those associated with mental illnesses such as anxiety and depression. Animal models of chronic stress are frequently used to elucidate the mechanisms underlying the relationship between stress and mental health disorders and to develop improved treatment options. The current study expands upon a novel chronic stress paradigm for mice: predatory stress. The predatory stress model incorporates the natural predator-prey relationship that exists among rats and mice and allows for greater interaction between the animals, in turn increasing the extent of the stressful experience. In this study, we evaluated the behavioral effects of exposure to 15 days of predatory stress on an array of behavioral indices. Up to 2 weeks after the end of stress, adult male mice showed an increase of anxiety-like behaviors as measured by the open field and social interaction tests. Animals also expressed an increase in depressive-like behavior in the sucrose preference test. Notably, performance on the novel object recognition task, a memory test, improved after predatory stress. Taken as a whole, our results indicate that 15 exposures to this innovative predatory stress paradigm are sufficient to elicit robust anxiety-like behaviors with evidence of co-morbid depressive-like behavior, as well as changes in cognitive behavior in male mice.