MECHANISMS OF NEUROENDOCRINE REGULATION

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Investigation has focused on the properties and regulation of the corticotropin releasing factor receptors and the mechanism of interaction between CRF and other regulators of ACTH secretion. A. CRF receptor properties: Gel electrophoresis analysis of detergent-solubilized CRF receptors crosslinked with 1251-Tyr-oCRF indicated that the receptor is a single protein with a molecular weight of 67 kDa. The characteristics of the CRF receptor are similar in the different target tissues, including anterior and intermediate lobes of the pituitary and the cortex, amygdala and olfactory bulb of the brain. B. CRF receptor regulation: Previous studies have shown that pituitary CRF receptor down regulation that accompanies the increase in plasma ACTH following adrenalectomy is dependent on hypothalamic factors, such as CRF and VP. Studies during stress showed transient increases in plasma ACTH following chronic immobilization. The subsequent decrease in plasma ACTH in the continuous presence of stress is accompanied by CRF receptor down- regulation and desensitization of the pituitary. However, pituitary responsiveness in vivo as well as the potentiating effect of VP on CRF action in vitro are maintained emphasizing the importance of the interaction between regulators during physiological control of ACTH secretion. CRF receptors in the intermediate pituitary and brain are unchanged during chronic immobilization stress. C. Mechanism of action of ACTH regulators: Previous studies demonstrated that the synergistic effect of VP on CRF action involves potentiation of CRF-stimulated cAMP production suggesting that protein kinase C activation is part of the mechanism of action of VP. Studies in isolated pituitary cells showed that inhibition of endogenous protein kinase C abolishes the effects of VP in the corticotroph. In addition VP was shown to stimulate inositol phosphate formation and to induce translocation of protein kinase C from cytosol to the membrane compartment.