NEUROENDOCRINE CONTROL OF THE STRESS RESPONSE

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Studies of this laboratory demonstrated stress-induced expression of corticotropin-releasing hormone (CRH) receptors in the PVN, parallel to increases in CRH peptide expression. To test the hypothesis that CRH has a positive autoregulatory effect in the CRH neuron, experiments were performed in rats subjected to adjuvant-induced arthritis, chronic stress model associated with decreased CRH expression in the PVN. In these studies adjuvant injection failed to increase CRH receptors in the PVN, either after 7 days, the onset of the inflammation or at 14 days when severe inflammation is established. This finding supports the view that positive feedback by CRH is necessary for the enhanced CRH expression during chronic stress. Studies on the regulation of CRH receptor in the pituitary showed that corticosterone injection at doses increasing plasma levels in the stress range, causes transient decreases in CRH receptor mRNA. This suggest that glucocorticoids contribute to the transient decrease in CRH receptor mRNA observed during the early stress response. While glucocorticoids regulate pituitary CRH receptors directly, previous studies have suggested that the transient decreases in CRH receptor mRNA following adrenalectomy are mediated by hypothalamic CRH release. This hypothesis was tested in rats subjected to PVN lesion or median eminence deafferentation to eliminate the source of hypothalamic CRH. While the lesions abolished CRH binding loss and the increases in POMC mRNA after long term adrenalectomy (4 days), only partially prevented the decrease in CRH receptor mRNA and increase in POMC expression after 18 hr adrenalectomy. The data show that increased hypothalamic CRH following glucocorticoid withdrawal is largely responsible for the changes in CRH receptors and POMC expression, and suggest the involvement of additional factors during the early phases after adrenalectomy. Regulation of the HPA axis during chronic stress includes sensitization of the adrenal to ACTH. Studies in isolated adrenal fasciculata cells from repeatedly stressed rats revealed increased cAMP responses to ACTH indicating that increased activity of the ACTH receptor contributes to the enhanced responses. Increased steroidogenic capacity of the adrenal is mediated by increases in side chain cleavage (scc) enzyme activity and P-450scc mRNA levels, and hypertrophy of the zona fasciculata of the adrenal.