MECHANISMS OF NEUROENDOCRINE REGULATION

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Investigation has focused on the mechanism of regulation of the hypothalamic and pituitary function during stress. These studies have defined distinct types of responses of the hypothalamic-pituitary-adrenal axis during adaptation in different chronic stress paradigms: a) desensitization of the ACTH to the primary stress, but hyperresponsiveness to a novel stimulus, b) sustained ACTH response to the primary stimulus and hyperresponsivenes to a novel stimulus, and c) minor ACTH responses to the primary stimulus and reduced sensitivity to a novel stress. To determine the mechanism of this differential responses, the hypothalamic expression and secretion of the major regulators of ACTH secretion, corticotropin releasing hormone (CRH) and vasopressin (VP), were studied in the various stress paradigms. A major finding was that activation of the HPA axis during chronic stress is accompanied by activation of the parvicellular vasopressinergic system, which provides VP to the pituitary portal blood. On the other hand, increasing systemic VP levels by osmotic stimulation failed to potentiate the effects of CRH on ACTH secretion and CRH receptor regulation. At the pituitary level, increased ACTH responses were associated to increases in POMC mRNA levels, but no relationship was found between CRH receptor levels and pituitary responsiveness. Previous studies demonstrated that alpha-1 adrenergic receptors are important mediators of stress-induced release from the median eminence. Extension of these studies, using electron microscopy immunocytochemical techniques, showed that alpha1-adrenergic agonists stimulate CRH release from the subpopulation of VP-containing CRH neurons. Additional studies provided two types of evidence indicating that long term, but not acute central alpha1-adrenergic receptor activation increases CRH mRNA in the hypothalamus. First, repeated injection of the alpha1-receptor agonist, methoxamine, every 6 hr for 24 hr, but not 4 hr after a single injection, resulted in significant increases in CRH mRNA in the PVN, and second, central alpha1-adrenergic blockade prevented the increases in CRH mRNA observed after 24 hr stress, but not those after 4 hr stress. Studies are in progress to determine the precise role of the multiple neurotransmitters implicated in hypothalamic regulation.