Estrogen‐Mediated Mechanisms of Social Stress Susceptibility within the Central Amygdala

Abstract

Social stress is the most common form of stress humans experience and is a risk factor for anxiety disorders. Women are 2‐3 times more likely to suffer from stress‐related anxiety disorders than men, but this heightened susceptibility is limited to the timeframe between the onset of puberty and menopause, suggesting a role for ovarian hormones. While the underlying neural mechanisms of stress susceptibility are unclear, many anxiety‐like behaviors in rodents including burying, startle responses and avoidance of elevated open spaces are regulated in part by corticotropin releasing factor (CRF) in the stress‐sensitive central nucleus of the amygdala (CeA). Recent evidence also highlights a critical role for estrogen (E) in regulating social stress susceptibility in female rats, and the CeA expresses estrogen receptor‐β (ER‐β). Although a large body of literature suggests that E may signal through ER‐β to reduce anxiety‐like behaviors during non‐stressful conditions, we hypothesize that in the context of social stress, E exacerbates anxiety‐like behaviors through increased expression of CRF in the CeA of female rats. This study examines the role for ER‐β in CeA in regulating stress susceptibility in female rats via region‐specific pharmacological blockade of ER‐β using PHTPP, an ER‐β antagonist. Following recovery from surgical implantation of indwelling bilateral CeA cannulae, adult female Sprague‐Dawley rats were injected with vehicle or PHTPP (0 or 10 µM; 0.5 µL over 1 min). One hour later, rats were subjected to control handling or witnessing an aggressive social defeat encounter between two male rats (WS, 15min/day x 5 days). WS and control rats were video‐recorded on days 1 and 5, and burying, freezing and rearing behaviors were quantified. Following 2 days of undisturbed rest in the absence of drug, all rats underwent a battery of tests to assess anxiety‐like behaviors including elevated plus maze (EPM), acoustic startle (ASR), and marble burying (MB). Each test was separated by 24 hours, and brain and blood were collected 2 days after MB. This study demonstrated that WS‐induced burying behavior was reduced by intra‐CeA PHTPP on days 1 and 5 of WS. There was no effect of WS nor PHTPP on freezing or rearing behavior during WS/control. The anxiogenic effects of WS are persistent as indicated by an enhanced ASR, increased MB and reduced open arm exploration during EPM among vehicle‐treated witnesses. PHTPP administration during WS attenuated these persistent anxiety‐like behaviors after WS exposure. Interestingly, PHTPP had opposing effects in WS and control rats such that PHTPP‐treated controls exhibited increased anxiety‐like behaviors indicated by increased marble burying and EPM closed arm time compared to vehicle‐treated controls. Brain tissue is currently being analyzed to determine the mechanism by which ER‐β regulates stress susceptibility, including measurement of CRF in the CeA and its projection regions. The findings from this study highlight a paradoxical role for E in regulating anxiety‐like behaviors, and further, identify that under conditions of social stress, E, acting through ER‐β, promotes stress susceptibility in female rats. Understanding the specific role for E in heightened stress susceptibility in females will help us to determine the physiological mechanisms underlying exaggerated social stress susceptibility among women.