The role of the posterior medial bed nucleus of the stria terminalis in modulating hypothalamic-pituitary-adrenocortical axis responsiveness to acute and chronic stress.

The bed nucleus of the stria terminalis (BST) plays a prominent role in brain integration of acute responses to stressful stimuli. This study tests the hypothesis that the BST plays a complementary role in regulation of physiological changes associated with chronic stress exposure. Male Sprague-Dawley rats received bilateral ibotenate lesions or sham lesions of the posterior medial region of the BST (BSTpm), an area known to be involved in inhibition of HPA axis responses to acute stress. Chronic stress was induced by 14-day exposure to twice daily stressors in an unpredictable sequence (chronic variable stress, CVS). In the morning after the end of CVS, stressed and non-stressed controls were exposed to a novel restraint stress challenge. As previously documented, CVS caused adrenal hypertrophy, thymic involution, and attenuated body weight gain. None of these endpoints were affected by BSTpm lesions. Chronic stress exposure facilitated plasma corticosterone responses to the novel restraint stress and elevated CRH mRNA. Lesions of the BSTpm increased novel stressor-induced plasma ACTH and corticosterone secretion and enhanced c-fos mRNA induction in the paraventricular nucleus of the hypothalamus (PVN). In addition, lesion of the BSTpm resulted in an additive increase in CVS-induced facilitation of corticosterone responses and PVN CRH expression. Collectively these data confirm that the BSTpm markedly inhibits HPA responses to acute stress, but do not strongly support an additional role for this region in limiting HPA axis responses to chronic drive. The data further suggest that acute versus chronic stress integration are subserved by different brain circuitry.