Characterizing the behavioral and neuroendocrine features of susceptibility and resilience to social stress

Dynamic alterations of depressive-like behaviors, gut microbiome, and fecal metabolome in social defeat stress mice

Gut microbiome is implicated in the onset and progression of major depressive disorder (MDD), but the dynamic alterations of depressive symptoms, gut microbiome, and fecal metabolome across different stages of stress exposure remain unclear. Here, we modified the chronic social defeat stress (CSDS) model to evaluate mice subjected to social defeat stress for 1, 4, 7, and 10 days. Behavioral tests, 16S rRNA, metagenomics, and fecal metabolomics were conducted to investigate the impact of stress exposure on behaviors, gut microbiota and fecal metabolites. We observed that depressive-like behaviors, such as anhedonia and social avoidance, worsened significantly as stress exposure increased. The microbial composition, function, and fecal metabolites exhibited distinct separations across the different social defeat stress groups. Mediation analysis identified key bacteria, such as Lachnospiraceae_UCG-001 and Bacteroidetes, and fecal metabolites like valeric acid and N-acetylaspartate. In our clinical depression cohort, we confirmed that fecal valeric acid levels, were significantly lower in depressive-like mice and MDD patients, correlating closely with stress exposure and anhedonia in mice. Further analysis of serum and brain metabolites in mice revealed sustained changes of N-acetylaspartate abundance in fecal, serum, and cortical samples following increasing stress exposure. Together, this study elucidated the characteristics of depressive-like behaviors, gut microbiome, and fecal metabolome across various social defeat stress exposure, and identified key bacteria and fecal metabolites potentially involved in modulating social defeat stress response and depressive-like behaviors, providing new insights into the pathogenesis and intervention of depression.

Effects of repeated social stress on risk assessment behaviors and response to diazepam in the elevated plus maze in adult male rats

Anxiety is highly common, and stress is a major trigger for anxiety. Anxiety includes heightened threat assessment and avoidance, but we do not fully understand which components are sensitive to stress. Rodents show a balance of exploration and avoidance that incorporates threat assessment prior to making the relatively risky decision to explore an open area. The purpose of this study was to determine if stress impacts risk assessment and if this is tied to the effects of stress on exploration. The present study used elevated plus maze (EPM) to test the effects of repeated social defeat stress (RSDS) on risk assessment behaviors in adult male rats. We then tested the effects of diazepam, an anxiolytic that reduces the impact of stress on EPM exploration, to further clarify the relationship between risk assessment and risky behavior in the EPM. We found that RSDS decreased time in the open arm, similar to prior studies. We also found that RSDS increased the likelihood of the primary risk assessment behavior, stretch and attend posture (SAP), increased SAP prior to entering an open arm, and decreased the likelihood that a rat would enter an open arm after SAP. Diazepam ameliorated the effects of RSDS on both SAP and exploratory behavior, further linking risk assessment and subsequent exploratory behaviors. These results suggest that increased risk assessment and reduced risky choices after risk assessment are tied to effects of stress on exploration and provide novel insight into how stress may increase avoidance by effects on risk assessment. (PsycInfo Database Record (c) 2025 APA, all rights reserved).

Corticosterone effects induced by stress and immunity and inflammation: mechanisms of communication

The body instinctively responds to external stimuli by increasing energy metabolism and initiating immune responses upon receiving stress signals. Corticosterone (CORT), a glucocorticoid (GC) that regulates secretion along the hypothalamic-pituitary-adrenal (HPA) axis, mediates neurotransmission and humoral regulation. Due to the widespread expression of glucocorticoid receptors (GR), the effects of CORT are almost ubiquitous in various tissue cells. Therefore, on the one hand, CORT is a molecular signal that activates the body's immune system during stress and on the other hand, due to the chemical properties of GCs, the anti-inflammatory properties of CORT act as stabilizers to control the body's response to stress. Inflammation is a manifestation of immune activation. CORT plays dual roles in this process by both promoting inflammation and exerting anti-inflammatory effects in immune regulation. As a stress hormone, CORT levels fluctuate with the degree and duration of stress, determining its effects and the immune changes it induces. The immune system is essential for the body to resist diseases and maintain homeostasis, with immune imbalance being a key factor in the development of various diseases. Therefore, understanding the role of CORT and its mechanisms of action on immunity is crucial. This review addresses this important issue and summarizes the interactions between CORT and the immune system.

Illuminating the impact of stress: In vivo approaches to track stress-related neural adaptations

Stressful experiences can affect both daily life and long-term health outcomes in a variety of ways. Acute challenges may be adaptive, promoting arousal and enhancing memory and cognitive function. Importantly, however, chronic stress dysregulates the body's physiological regulatory mechanisms consisting of complex hormone interactions throughout the peripheral and central nervous systems. This disrupted signaling consequently alters the balance of synapse formation, maturation and pruning, processes which regulate neural communication, plasticity, learning, cognitive flexibility and adaptive behaviors - hallmarks of a healthy, functional brain. The chronically stressed brain state, therefore, is one which may be uniquely vulnerable. To understand the development of this state, how it is sustained and how behavior and neural function are transiently or indelibly impacted by it, we can turn to a number of advanced approaches in animal models which offer unprecedented insights. This has been the aim of my recent work within the field and the goal of my new independent research program. To achieve this, I have employed methods to uncover how key brain circuits integrate information to support motivated behaviors, how stress impacts their ability to perform this process and how best to operationalize behavioral readouts. Here I present an overview of research contributions that I find most meaningful for advancing our understanding of the impact of stress and propose new avenues which will guide my own framework to address the salient outstanding questions within the field.

Repeated social stress increases posterior medial amygdala neuronal activity in stress-susceptible adult male rats

The medial amygdala (MeA) is activated by social stimuli and manipulations of the MeA disrupt a wide range of social behaviors. Social stress can shift social behaviors and may accomplish this partly via effects on the MeA. However, very little is known about the effects of social stress on the electrophysiological activity of MeA neurons. The posterior division of the MeA (MeAp) has been implicated in driving social engagement. We hypothesized that repeated social stress would cause parallel changes in in vivo activity of MeAp neurons and social behavior. The resident-intruder paradigm was used to produce repeated social stress in adult male rats. After repeated social stress, MeAp neurons were recorded with in vivo single-unit electrophysiology in anesthetized rats. MeAp neurons, specifically those in the posterodorsal subnucleus (MeApd), fired faster in stressed rats than in controls, and this effect was directly associated with stressor intensity. The MeAp sends dense projections to the posterior bed nucleus of stria terminalis (pBNST) and ventromedial hypothalamus (VMH), and both regions are essential for social engagement and are sensitive to social stressors. MeAp projections to pBNST had higher activity after stress, whereas projections to the VMH were not affected. These effects were significant only in rats that displayed susceptibility to this social stressor, as demonstrated by lower weight gain. Furthermore, the effect of stress on MeApd and MeAp-pBNST neuronal firing was correlated with lower social interaction. These results indicate that heightened MeApd and MeA-pBNST activity may contribute to alterations in social behaviors following social stress.NEW & NOTEWORTHY Social stress contributes to psychiatric disorders and impacts multiple brain regions. However, effects on a crucial area for social function, the medial amygdala (MeA), are unclear. We found that social stress increased firing of posterior MeA neurons, and particularly neurons that project to bed nucleus of the stria terminalis, a region implicated in anxiety. Effects of stress on this circuit were associated with diminished social interaction and help clarify how stress can impact social functions.

Chronic social defeat stress gives rise to social avoidance through fear learning

Chronic social defeat stress (CSDS), a widely used rodent model of stress, reliably leads to decreased social interaction in stress susceptible animals. Here, we investigate a role for fear learning in this response using male 129 Sv/Ev mice, a strain that is more vulnerable to CSDS than the commonly used C57BL/6 strain. We first demonstrate that defeated 129 Sv/Ev mice avoid a CD-1 mouse, but not a conspecific, indicating that motivation to socialize is intact in this strain. CD-1 avoidance is characterized by approach behavior that results in running in the opposite direction, activity that is consistent with a threat response. We next test whether CD-1 avoidance is subject to the same behavioral changes found in traditional models of Pavlovian fear conditioning. We find that associative learning occurs across 10 days CSDS, with defeated mice learning to associate the color of the CD-1 coat with threat. This leads to the gradual acquisition of avoidance behavior, a conditioned response that can be extinguished with 7 days of repeated social interaction testing (5 tests/day). Pairing a CD-1 with a tone leads to second-order conditioning, resulting in avoidance of an enclosure without a social target. Finally, we show that social interaction with a conspecific is a highly variable response in defeated mice that may reflect individual differences in generalization of fear to other social targets. Our data indicate that fear conditioning to a social target is a key component of CSDS, implicating the involvement of fear circuits in social avoidance.

Heightened lateral habenula activity during stress produces brainwide and behavioral substrates of susceptibility

Some individuals are susceptible to chronic stress, and others are more resilient. While many brain regions implicated in learning are dysregulated after stress, little is known about whether and how neural teaching signals during stress differ between susceptible and resilient individuals. Here, we seek to determine if activity in the lateral habenula (LHb), which encodes a negative teaching signal, differs between susceptible and resilient mice during stress to produce different outcomes. After (but not before) chronic social defeat stress, the LHb is active when susceptible mice are in proximity of the aggressor strain. During stress, activity is higher in susceptible mice during aggressor interactions, and activation biases mice toward susceptibility. This manipulation generates a persistent and widespread increase in the balance of subcortical vs. cortical activity in susceptible mice. Taken together, our results indicate that heightened activity in the LHb during stress produces lasting brainwide and behavioral substrates of susceptibility.

Behavioral coping with chronic defeat stress in mice: A systematic review of current protocols

Social stress is the most significant source of chronic stress in humans and is commonly associated with health impairment. Individual differences in the behavioral coping responses to stress have been proposed to mediate the negative effects of stress on physical, behavioral and mental health. Animal models, particularly mice, offer valuable insights into the physiological and neurobiological correlates of behavioral coping strategies in response to chronic social stress. Here we aim to identify differences and similarities among stress protocols in mice, with particular attention to how neuroendocrine and/or behavioral responses vary according to different coping strategies, while highlighting the need for standardized approaches in future research. A systematic review was undertaken following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA statement). A total of 213 references were identified by electronic search, and after the screening, 18 articles were found to meet all the established criteria. We analyzed differences in the stress protocol, the characterization and classification of coping strategies and the physiological and behavioral differences according to coping. The results show that differences in behavioural expression under chronic social stress (coping) may also be associated with physiological differences and differential susceptibility to disease. However, this review also underlines the importance of a cautious interpretation of the results obtained. The lack of consistency in the nomenclature and procedures associated with the study of coping strategies for social stress, as well as the absence of a uniform classification, highlight the importance of using a common language when approaching the study of coping strategies. Thereby, this review encourages the development of a more defined method and criteria for assessing coping strategies, based on both behavioral and biological indicators.

Comparative Evaluation of Antidepressant and Anxiolytic Effects of Escitalopram, Crocin, and their Combination in Rats

Background

Chronic stress can lead to anxiety and depression. Escitalopram is a selective serotonin reuptake inhibitor (SSRI), and crocin is a natural compound derived from saffron. Both of them are used to treat these disorders in clinical and traditional medicine, respectively. This study compared the antidepressant and anxiolytic effects of escitalopram, crocin, and their combination in rats.

Materials and Methods

Rats were divided into nine groups: control, sham, rest-depression, depression-rest, depression-crocin, depression-escitalopram10, depression-escitalopram20, depression-escitalopram10-crocin, and depression-escitalopram20-crocin. Forced swimming and open field tests (FST and OFT, respectively) were used to evaluate depression, anxiety, and locomotor activity.

Results

In the FST, the immobility time on day 28 significantly decreased in all depressed groups that received escitalopram, crocin, and their combination compared to the rest-depression group. Whereas, conversely, the time spent at the center in the OFT was significantly higher in similar comparisons. The total distance traveled by the OFT was significantly lower in all depressed groups, except for the depression-escitalopram10 and depression-escitalopram20 groups. The total distance traveled was significantly higher in the depression-escitalopram20 compared to the rest-depression group.

Conclusion

Crocin, both doses of escitalopram and their combination, reduced depression. A high dose of escitalopram, with and without crocin, was partially more effective than a low dose of escitalopram in reversing depression. There was anxiety-like behavior observed after inducing depression with and without a recovery period. Whereas, crocin alone and both doses of escitalopram, with and without crocin, decreased anxiety-like behaviors in subjects with depression. This effect may be attributed to a modulation of brain neurotransmitter ratios.

Heightened lateral habenula activity during stress produces brainwide and behavioral substrates of susceptibility

Some individuals are susceptible to the experience of chronic stress and others are more resilient. While many brain regions implicated in learning are dysregulated after stress, little is known about whether and how neural teaching signals during stress differ between susceptible and resilient individuals. Here, we seek to determine if activity in the lateral habenula (LHb), which encodes a negative teaching signal, differs between susceptible and resilient mice during stress to produce different outcomes. After, but not before, chronic social defeat stress (CSDS), the LHb is active when susceptible mice are in the proximity of the aggressor strain. During stress itself, LHb activity is higher in susceptible mice during aggressor proximity, and activation of the LHb during stress biases mice towards susceptibility. This manipulation generates a persistent and widespread increase in the balance of subcortical versus cortical activity in susceptible mice. Taken together, our results indicate that heightened activity in the LHb during stress produces lasting brainwide and behavioral substrates of susceptibility.

Neurobiology and systems biology of stress resilience

Stress resilience is the phenomenon that some people maintain their mental health despite exposure to adversity or show only temporary impairments followed by quick recovery. Resilience research attempts to unravel the factors and mechanisms that make resilience possible and to harness its insights for the development of preventative interventions in individuals at risk for acquiring stress-related dysfunctions. Biological resilience research has been lagging behind the psychological and social sciences but has seen a massive surge in recent years. At the same time, progress in this field has been hampered by methodological challenges related to finding suitable operationalizations and study designs, replicating findings, and modeling resilience in animals. We embed a review of behavioral, neuroimaging, neurobiological, and systems biological findings in adults in a critical methods discussion. We find preliminary evidence that hippocampus-based pattern separation and prefrontal-based cognitive control functions protect against the development of pathological fears in the aftermath of singular, event-type stressors [as found in fear-related disorders, including simpler forms of posttraumatic stress disorder (PTSD)] by facilitating the perception of safety. Reward system-based pursuit and savoring of positive reinforcers appear to protect against the development of more generalized dysfunctions of the anxious-depressive spectrum resulting from more severe or longer-lasting stressors (as in depression, generalized or comorbid anxiety, or severe PTSD). Links between preserved functioning of these neural systems under stress and neuroplasticity, immunoregulation, gut microbiome composition, and integrity of the gut barrier and the blood-brain barrier are beginning to emerge. On this basis, avenues for biological interventions are pointed out.

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Repeated social defeat stress differently affects arthritis-associated hypersensitivity in male and female mice

Chronic stress enhances the risk of neuropsychiatric disorders and contributes to the aggravation and chronicity of pain. The development of stress-associated diseases, including pain, is affected by individual vulnerability or resilience to stress, although the mechanisms remain elusive. We used the repeated social defeat stress model promoting susceptible and resilient phenotypes in male and female mice and induced knee mono-arthritis to investigate the impact of stress vulnerability on pain and immune system regulation. We analyzed different pain-related behaviors, measured blood cytokine and immune cell levels, and performed histological analyses at the knee joints and pain/stress-related brain areas. Stress susceptible male and female mice showed prolonged arthritis-associated hypersensitivity. Interestingly, hypersensitivity was exacerbated in male but not female mice. In males, stress promoted transiently increased neutrophils and Ly6Chigh monocytes, lasting longer in susceptible than resilient mice. While resilient male mice displayed persistently increased levels of the anti-inflammatory interleukin (IL)-10, susceptible mice showed increased levels of the pro-inflammatory IL-6 at the early- and IL-12 at the late arthritis stage. Although joint inflammation levels were comparable among groups, macrophage and neutrophil infiltration was higher in the synovium of susceptible mice. Notably, only susceptible male mice, but not females, presented microgliosis and monocyte infiltration in the prefrontal cortex at the late arthritis stage. Blood Ly6Chigh monocyte depletion during the early inflammatory phase abrogated late-stage hypersensitivity and the associated histological alterations in susceptible male mice. Thus, recruitment of blood Ly6Chigh monocytes during the early arthritis phase might be a key factor mediating the persistence of arthritis pain in susceptible male mice. Alternative neuro-immune pathways that remain to be explored might be involved in females.

Chronic Social Defeat Stress Gives Rise to Social Avoidance Through Fear Learning

Chronic social defeat stress (CSDS), a widely used rodent model of stress, reliably leads to decreased social interaction in stress susceptible animals. Here, we investigate a role for fear learning in this response using 129Sv/Ev mice, a strain that is more vulnerable to CSDS than the commonly used C57BL/6 strain. We first demonstrate that defeated 129Sv/Ev mice avoid a CD-1 mouse, but not a conspecific, indicating that motivation to socialize is intact in this strain. CD-1 avoidance is characterized by approach behavior that results in running in the opposite direction, activity that is consistent with a threat response. We next test whether CD-1 avoidance is subject to the same behavioral changes found in traditional models of Pavlovian fear conditioning. We find that associative learning occurs across 10 days CSDS, with defeated mice learning to associate the color of the CD-1 coat with threat. This leads to the gradual acquisition of avoidance behavior, a conditioned response that can be extinguished with 7 days of repeated social interaction testing (5 tests/day). Pairing a CD-1 with a tone leads to second-order conditioning, resulting in avoidance of an enclosure without a social target. Finally, we show that social interaction with a conspecific is a highly variable response in defeated mice that may reflect individual differences in generalization of fear to other social targets. Our data indicate that fear conditioning to a social target is a key component of CSDS, implicating the involvement of fear circuits in social avoidance.

Chronic defeat stress induces monoamine level dysregulation in the prefrontal cortex but not in the hippocampus of OF1 male mice

Chronic social stress can increase susceptibility to chronic diseases such as depression. One of the most used models to study the physiological mechanisms and behavioral outcomes of this type of stress is chronic defeat stress (CDS) in male mice. OF1 male mice were subjected to a stress period lasting 18 days. During that time, non-stressed animals were housed in groups. The cluster analysis of the behavioral profile displayed during the first social interaction divided subjects into two groups: active/aggressive (AA) and passive/reactive (PR). The day after the end of the stress period, the following behavioral analyses were performed: the sucrose preference test (SPT) on day 19, the open field test (OFT) on day 20, and the forced swim test (FST) on day 21. Immediately after completing the last test, animals were weighed, and blood samples were obtained. Then, they were sacrificed, and their prefrontal cortices and hippocampi were removed and stored to analyze monoamine levels. Stressed animals displayed anhedonia, and solely the PR mice continued to show higher levels of immobility in the OFT and FST. All stressed animals, regardless of the coping strategy, presented higher plasma corticosterone levels. In addition, stressed mice showed lower levels of tyrosine, dopamine, DOPAC, MHPG, kynurenine, kynurenic acid, and 5-HIAA levels but higher serotonin levels in the prefrontal cortex, not in the hippocampus. In conclusion, our results show that CSD induces differences in monoamine levels between brain areas, and these differences did not respond to the coping strategy adopted.

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Hidden variables in stress neurobiology research

Among the central goals of stress neurobiology research is to understand the mechanisms by which stressors change neural circuit function to precipitate or exacerbate psychiatric symptoms. Yet despite decades of effort, psychiatric medications that target the biological substrates of the stress response are largely lacking. We propose that the clinical advancement of stress response-based therapeutics for psychiatric disorders may be hindered by 'hidden variables' in stress research, including considerations of behavioral study design (stressors and outcome measures), individual variability, sex differences, and the interaction of the body's stress hormone system with endogenous circadian and ultradian rhythms. We highlight key issues and suggest ways forward in stress neurobiology research that may improve the ability to assess stress mechanisms and translate preclinical findings.

The cortisol switch between vulnerability and resilience

In concert with neuropeptides and transmitters, the end products of the hypothalamus-pituitary-adrenal (HPA) axis, the glucocorticoid hormones cortisol and corticosterone (CORT), promote resilience: i.e., the ability to cope with threats, adversity, and trauma. To exert this protective action, CORT activates mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) that operate in a complementary manner -as an on/off switch- to coordinate circadian events, stress-coping, and adaptation. The evolutionary older limbic MR facilitates contextual memory retrieval and supports an on-switch in the selection of stress-coping styles at a low cost. The rise in circulating CORT concentration after stress subsequently activates a GR-mediated off-switch underlying recovery of homeostasis by providing the energy for restraining the primary stress reactions and promoting cognitive control over emotional reactivity. GR activation facilitates contextual memory storage of the experience to enable future stress-coping. Such complementary MR-GR-mediated actions involve rapid non-genomic and slower gene-mediated mechanisms; they are time-dependent, conditional, and sexually dimorphic, and depend on genetic background and prior experience. If coping fails, GR activation impairs cognitive control and promotes emotional arousal which eventually may compromise resilience. Such breakdown of resilience involves a transition to a chronic stress construct, where information processing is crashed; it leads to an imbalanced MR-GR switch and hence increased vulnerability. Novel MR-GR modulators are becoming available that may reset a dysregulated stress response system to reinstate the cognitive flexibility required for resilience.

Stress, associative learning, and decision-making

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

Advancing social behavioral neuroscience by integrating ethology and comparative psychology methods through machine learning

Social behavior is naturally occurring in vertebrate species, which holds a strong evolutionary component and is crucial for the normal development and survival of individuals throughout life. Behavioral neuroscience has seen different influential methods for social behavioral phenotyping. The ethological research approach has extensively investigated social behavior in natural habitats, while the comparative psychology approach was developed utilizing standardized and univariate social behavioral tests. The development of advanced and precise tracking tools, together with post-tracking analysis packages, has recently enabled a novel behavioral phenotyping method, that includes the strengths of both approaches. The implementation of such methods will be beneficial for fundamental social behavioral research but will also enable an increased understanding of the influences of many different factors that can influence social behavior, such as stress exposure. Furthermore, future research will increase the number of data modalities, such as sensory, physiological, and neuronal activity data, and will thereby significantly enhance our understanding of the biological basis of social behavior and guide intervention strategies for behavioral abnormalities in psychiatric disorders.

From helplessness to controllability: toward a neuroscience of resilience

"Learned helplessness" refers to debilitating outcomes, such as passivity and increased fear, that follow an uncontrollable adverse event, but do not when that event is controllable. The original explanation argued that when events are uncontrollable the animal learns that outcomes are independent of its behavior, and that this is the active ingredient in producing the effects. Controllable adverse events, in contrast, fail to produce these outcomes because they lack the active uncontrollability element. Recent work on the neural basis of helplessness, however, takes the opposite view. Prolonged exposure to aversive stimulation per se produces the debilitation by potent activation of serotonergic neurons in the brainstem dorsal raphe nucleus. Debilitation is prevented with an instrumental controlling response, which activates prefrontal circuitry detecting control and subsequently blunting the dorsal raphe nucleus response. Furthermore, learning control alters the prefrontal response to future adverse events, thereby preventing debilitation and producing long-term resiliency. The general implications of these neuroscience findings may apply to psychological therapy and prevention, in particular by suggesting the importance of cognitions and control, rather than habits of control.

Behavioural and dopaminergic signatures of resilience

Chronic stress can have lasting adverse consequences in some individuals, yet others are resilient to the same stressor1,2. Susceptible and resilient individuals exhibit differences in the intrinsic properties of mesolimbic dopamine (DA) neurons after the stressful experience is over3-8. However, the causal links between DA, behaviour during stress and individual differences in resilience are unknown. Here we recorded behaviour in mice simultaneously with DA neuron activity in projections to the nucleus accumbens (NAc) (which signals reward9-12) and the tail striatum (TS) (which signals threat13-16) during social defeat. Supervised and unsupervised behavioural quantification revealed that during stress, resilient and susceptible mice use different behavioural strategies and have distinct activity patterns in DA terminals in the NAc (but not the TS). Neurally, resilient mice have greater activity near the aggressor, including at the onset of fighting back. Conversely, susceptible mice have greater activity at the offset of attacks and onset of fleeing. We also performed optogenetic stimulation of NAc-projecting DA neurons in open loop (randomly timed) during defeat or timed to specific behaviours using real-time behavioural classification. Both open-loop and fighting-back-timed activation promoted resilience and reorganized behaviour during defeat towards resilience-associated patterns. Together, these data provide a link between DA neural activity, resilience and resilience-associated behaviour during the experience of stress.

Distinct forms of regret linked to resilience versus susceptibility to stress are regulated by region-specific CREB function in mice

Regret describes recognizing alternative actions could have led to better outcomes. It remains unclear whether regret derives from generalized mistake appraisal or instead comprises dissociable, action-specific processes. Using a neuroeconomic task, we found that mice were sensitive to fundamentally distinct types of regret following exposure to chronic social defeat stress or manipulations of CREB, a transcription factor implicated in stress action. Bias to make compensatory decisions after rejecting high-value offers (regret type I) was unique to stress-susceptible mice. Bias following the converse operation, accepting low-value offers (regret type II), was enhanced in stress-resilient mice and absent in stress-susceptible mice. CREB function in either the prefrontal cortex or nucleus accumbens was required to suppress regret type I but bidirectionally regulated regret type II. We provide insight into how maladaptive stress response traits relate to distinct forms of counterfactual thinking, which could steer therapy for mood disorders, such as depression, toward circuit-specific computations through a careful description of decision narrative.

Identification of hyper-ramified microglia in the CA1 region of the mouse hippocampus potentially associated with stress resilience

Recent studies have indicated that some individuals are less affected by stress, and such individuals are called resilient. This study aimed to determine whether the specific phenotype of microglia might be involved in resilience using the social defeat stress paradigm. Male C57BL/6J (B6) mice were attacked by aggressive male ICR mice for five consecutive days. After stress exposure, the social behaviour was reduced in about half of the B6 mice (vulnerable), whereas no such change was observed in the remaining half of the B6 mice (resilient). Anxiety-like behaviour was increased in vulnerable mice compared with resilient mice and non-stressed controls. However, depression-related behaviour was comparable between the three groups. The morphological characteristics of microglia in the CA1 region of the dorsal hippocampus in non-stressed controls and resilient mice differed from those in vulnerable mice. Interestingly, the voxel densities of GABAergic and glutamatergic synaptic puncta colocalized with microglia were higher in resilient mice than in non-stressed controls and vulnerable mice. Microglia were then objectively classified into three morphological types by hierarchical cluster analysis. The appearance of type I microglia resembled the so-called resting ramified microglia and represented the major population of microglia in non-stressed controls. Type II microglia exhibited a de-ramified morphology and accounted for 60% of the microglia in vulnerable mice. Type III microglia showed a hyper-ramified morphology and represented more than half of the microglia in resilient mice. These results suggest that hyper-ramified microglia in the hippocampus may be associated with stress resilience via the modulation of synaptic transmission.

Dietary Long-Chain n-3 Polyunsaturated Fatty Acid Supplementation Alters Electrophysiological Properties in the Nucleus Accumbens and Emotional Behavior in Naïve and Chronically Stressed Mice

Long-chain (LC) n-3 polyunsaturated fatty acids (PUFAs) have drawn attention in the field of neuropsychiatric disorders, in particular depression. However, whether dietary supplementation with LC n-3 PUFA protects from the development of mood disorders is still a matter of debate. In the present study, we studied the effect of a two-month exposure to isocaloric diets containing n-3 PUFAs in the form of relatively short-chain (SC) (6% of rapeseed oil, enriched in α-linolenic acid (ALA)) or LC (6% of tuna oil, enriched in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) PUFAs on behavior and synaptic plasticity of mice submitted or not to a chronic social defeat stress (CSDS), previously reported to alter emotional and social behavior, as well as synaptic plasticity in the nucleus accumbens (NAc). First, fatty acid content and lipid metabolism gene expression were measured in the NAc of mice fed a SC (control) or LC n-3 (supplemented) PUFA diet. Our results indicate that LC n-3 supplementation significantly increased some n-3 PUFAs, while decreasing some n-6 PUFAs. Then, in another cohort, control and n-3 PUFA-supplemented mice were subjected to CSDS, and social and emotional behaviors were assessed, together with long-term depression plasticity in accumbal medium spiny neurons. Overall, mice fed with n-3 PUFA supplementation displayed an emotional behavior profile and electrophysiological properties of medium spiny neurons which was distinct from the ones displayed by mice fed with the control diet, and this, independently of CSDS. Using the social interaction index to discriminate resilient and susceptible mice in the CSDS groups, n-3 supplementation promoted resiliency. Altogether, our results pinpoint that exposure to a diet rich in LC n-3 PUFA, as compared to a diet rich in SC n-3 PUFA, influences the NAc fatty acid profile. In addition, electrophysiological properties and emotional behavior were altered in LC n-3 PUFA mice, independently of CSDS. Our results bring new insights about the effect of LC n-3 PUFA on emotional behavior and synaptic plasticity.

Long-term changes of parvalbumin- and somatostatin-positive interneurons of the primary motor cortex after chronic social defeat stress depend on individual stress-vulnerability

Chronic stress is a major risk factor for developing mental illnesses and cognitive deficiencies although stress-susceptibility varies individually. In a recent study, we established the connection between chronic social defeat stress (CSDS) and impaired motor learning abilities accompanied by chronically disturbed structural neuroplasticity in the primary motor cortex (M1) of mice. In this study, we further investigated the long-term effects of CSDS exposure on M1, focusing on the interneuronal cell population. We used repeated CSDS to elicit effects across behavioral, endocrinological, and metabolic parameters in mice. Susceptible and resilient phenotypes were discriminated by symptom load and motor learning abilities were assessed on the rotarod. Structural changes in interneuronal circuits of M1 were studied by immunohistochemistry using parvalbumin (PV+) and somatostatin (SST+) markers. Stress-susceptible mice had a blunted stress hormone response and impaired motor learning skills. These mice presented reduced numbers of both interneuron populations in M1 with layer-dependent distribution, while alterations in cell size and immunoreactivity were found in both susceptible and resilient individuals. These results, together with our previous data, suggest that stress-induced cell loss and degeneration of the GABAergic interneuronal network of M1 could underlay impaired motor learning, due to their role in controlling the excitatory output and spine dynamics of principal neurons required for this task. Our study further highlights the importance of long-term outcomes of chronically stressed individuals which are translationally important due to the long timecourses of stress-induced neuropsychiatric disorders.