Repeated witness social stress causes cardiomyocyte contractile impairment and intracellular Ca2+ derangement in female rats

Individual differences in behavioral responses to predator odor predict subsequent stress reactivity in female rats

Stress-induced neuropsychiatric disorders are among the most prevalent medical conditions and have widespread effects on both patients and society. Females experience over twice the rates of stress-related anxiety and depression when compared to males and often exhibit worse symptomatology and treatment outcomes. However, preclinical experiments exploring the neurobiological mechanisms of stress susceptibility in females have been traditionally understudied. Previous data from our lab has determined that females are selectively vulnerable to the consequences of vicarious witness stress, and these experiments were designed to determine specific behavioral and physiological factors that could predict which groups would be more susceptible to the effects of stress. Adult, female, Sprague-Dawley rats were first exposed to a ferret predator odor to determine baseline individual differences in behavioral responses. Rats were stratified by the duration of freezing behavior exhibited in response to the ferret odor and equally balanced into non-stressed controls and vicarious witness stress exposed groups. These female rats were then assessed on a battery of behavioral tasks including sucrose preference, elevated plus maze, acoustic startle, and the ferret odor and witness stress cue exposures to determine if baseline differences in stress responding can predict the behavioral response to future stress and stress cues. High freezing in response to the ferret odor was associated with behavioral sensitization to witness stress and hypervigilant responses to stress cues that was accompanied by exaggerated neuroimmune responses. These experiments establish a powerful behavioral predictor of stress susceptibility in females and begin to address neurobiological correlates that underlie this response.

The functional role of locus coeruleus microglia in the female stress response

Neuropsychiatric disorders that result from stress exposure are highly associated with central inflammation. Our previous work established that females selectively exhibit heightened proinflammatory cytokine production within the noradrenergic locus coeruleus (LC) along with a hypervigilant behavioral phenotype in response to witnessing social stress. Notably, ablation of microglia using pharmacological techniques prevents this behavioral response. These studies were designed to further investigate the impact of stress-induced neuroimmune signaling on the long-term behavioral and neuronal consequences of social stress exposure in females using chemogenetics. We first characterized the use of an AAV-CD68-Gi-DREADD virus targeted to microglia within the LC and confirmed viral transduction, selectivity, and efficacy. Clozapine-n-oxide (CNO) was used for the suppression of microglial reactivity during acute and chronic exposure to vicarious/witness social defeat in female rats. Chemogenetic-mediated inhibition of microglial reactivity during stress blunted the neuroimmune response to stress and prevented both acute and long-term hypervigilant behavioral responses. Further, a history of microglial suppression during stress prevented the heightened LC activity typically observed in response to stress cues. These studies are among the first to use a chemogenetic approach to inhibit central microglia in vivo and establish LC microglia as a key driver of the behavioral and neuronal responses to social stress in females.

Sex-specific behavioral, cardiac, and neuroendocrine responses to repeated witness social stress in adult rats

In humans, sex disparities exist in the prevalence of social stress-related disorders, yet our understanding of the predisposing factors and underlying mechanisms is still elusive. Also at the preclinical level, the investigation of sex differences in social stress responses is limited. In this study, adult male and female wild-type Groningen rats were repeatedly exposed to witness social defeat stress (WS) to assess sex-specific behavioral, neuroendocrine, and cardiac responses to the same social stress paradigm. Male and female rats bore witness to an aggressive social defeat episode between two males for nine consecutive days or were exposed to a control (CTR) procedure. Stress-related parameters were assessed in correspondence to the first and last WS/CTR exposure and also during subsequent exposure to the stress context alone in the absence of social defeat. During WS, rats of both sexes displayed larger amounts of burying behavior and smaller amounts of rearing and grooming behaviors, but with a greater extent in female witnesses. Cardiac autonomic responses to WS were similar between the sexes, yet only females displayed higher plasma corticosterone levels after the first WS exposure compared to CTRs, and had a larger corticosterone increase than male witnesses upon repeated WS. Exposure to the stress context alone (i.e., without the presence of the aggressive resident rat) elicited greater amount of burying behavior and more pronounced and persistent tachycardic responses in females than males with a history of WS. Our findings suggest sex-disparities in the response of adult rats to WS at multiple behavioral, cardiac, and neuroendocrine levels, highlighting the utility of this social stress paradigm for investigating predisposing factors and pathophysiological mechanisms underlying sex-specific vulnerabilities to stress-related pathologies.

The functional role of locus coeruleus microglia in the female stress response

Neuropsychiatric disorders that result from stress exposure are highly associated with central inflammation. Our previous work established that females selectively exhibit heightened proinflammatory cytokine production within the noradrenergic locus coeruleus (LC) along with a hypervigilant behavioral phenotype in response to witnessing social stress, and ablation of microglia using pharmacological techniques prevents this behavioral response. These studies were designed to further investigate the impact of stress-induced neuroimmune signaling on the long-term behavioral and neuronal consequences of social stress exposure in females using chemogenetics. We first characterized the use of an AAV-CD68-Gi-DREADD virus targeted to microglia within the LC and confirmed viral transduction, selectivity, and efficacy. Clozapine-n-oxide (CNO) was used for the suppression of microglial reactivity during acute and chronic exposure to vicarious/witness social defeat in female rats. Chemogenetic-mediated inhibition of microglial reactivity during stress blunted the neuroimmune response to stress and prevented both acute and long-term hypervigilant behavioral responses. Further, a history of microglial suppression during stress prevented the heightened LC activity typically observed in response to stress cues. These studies are among the first to use a chemogenetic approach to inhibit microglia within the female brain in vivo and establish LC inflammation as a key mechanism underlying the behavioral and neuronal responses to social stress in females.