Acute Corticosteriod Actions in the Hypothalamus

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DESCRIPTION (provided by applicant): Stress is associated with activation of the hypothalamic-pituitary-adrenal (HPA) axis and increased levels of circulating adrenal corticosteroids. Glucocorticoids feed back onto the hypothalamus to inhibit not only corticotropin releasing hormone (CRH) secretion and HPA activation, but also many other hypothalamic neuroendocrine systems. The negative feedback regulation by glucocorticoids occurs in two stages: an acute inhibition of the release of CRH, and a slower down-regulation of CRH and vasopressin synthesis in neurons of the hypothalamic paraventricular nucleus (PVN). To date, it is not known where in the brain glucocorticoids exert their negative feedback regulation of hypothalamic neurosecretion, and what the cellular mechanisms of this feedback are. These questions have profound significance for the treatment of widespread affective disorders, including stress, depression, and eating disorders, that impact large numbers of people. The overall purpose of this proposal is to determine whether acute negative glucocorticoid regulation of PVN neuroendocrine systems occurs directly at the hormone-secreting neurons in the PVN, and to characterize the cellular mechanisms of this regulation. We have preliminary evidence for rapid inhibitory glucocorticoid effects in the PVN mediated by membrane receptor activation and the release of a retrograde endocannabinoid messenger that suppresses excitatory and facilitates inhibitory inputs to PVN neurons. Based on these findings, we propose to conduct whole-cell patch-clamp recordings in acute rat hypothalamic slices to test the following specific hypotheses: 1. glucocorticoids inhibit PVN neurons directly by stimulating an endocannabinoid-mediated suppression of glutamate release and facilitation of GABA release; 2. glucocorticoids elicit retrograde endocannabinoid release from PVN neurons by activation of a postsynaptic G protein-coupled receptor and lipid messenger signaling cascade; 3. the endocannabinoid-mediated changes in glutamate and GABA release in the PVN are mediated by presynaptic, G protein- and protein kinase-dependent signaling mechanisms. These studies will reveal for the first time the site and physiological mechanisms of fast glucocorticoid actions in the hypothalamus. Fast glucocorticoid feedback in the hypothalamus plays a critical role in the organism's holistic response to stress, and understanding the mechanisms of glucocorticoid actions in the hypothalamus will provide important cellular targets for the treatment of HPA-related pathologies. Furthermore, the glucocorticoid-endocannabinoid link opens interesting possibilities for interactions between hypothalamic function and the cannabinoids.