Sensitized corticosterone responses do not mediate the enhanced fear memories in chronically stressed rats

Following a stressful event, the hypothalamus-pituitary-adrenal axis mediates the release of the stress hormone cortisol (corticosterone in rodents; CORT). Elevated CORT binds to glucocorticoid receptors to mediate physiological responses including facilitating memory formation. Previous work from our laboratory demonstrated that male rats exposed to chronic stress demonstrate enhanced contextual fear memories and sensitized CORT responses to subsequent stress exposure; however, this is unknown in female rats. The experiments here tested whether chronic stress enhances fear memory formation in female rats and whether the sensitized CORT response in chronic stress rats contributes to their enhanced fear memory. Studies first examined CORT responses to contextual fear conditioning in male and female rats and examined whether chronic stress enhanced the formation of contextual fear memories 24 h later. Studies then used metyrapone, a CORT synthesis inhibitor, to investigate whether blockade of plasma CORT would eliminate the chronic stress-induced enhancement in contextual fear memory. Results show that female rats have greater CORT responses than males, and chronic stress sensitizes the CORT response to fear conditioning in both sexes. However, female rats do not show enhanced contextual fear memory following chronic stress. Chronically stressed male rats show greater memory acquisition and show greater contextual fear memory 24 h later following fear conditioning. Metyrapone dampens contextual fear memory in all rats but does not eliminate the enhancement in freezing behavior in chronic stress rats. Collectively, these studies indicate sensitized CORT responses in chronically stressed rats is likely not the mechanism by which chronic stress facilitates memory formation.