OPIATE RECEPTOR CONTROL OF POMC PEPTIDE SECRETION

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DESCRIPTION (Adapted from applicant's abstract): The goal of the proposed research is to elucidate the mechanisms subserving opiate receptor control of pro-opiomelanocortin (POMC) peptide release from the pituitary intermediate lobe (IL). Administration of opiate agonists, such as morphine, results in profound alterations in the activity of brain and pituitary POMC systems but the mechanisms underlying these effects are not well understood. Recent experiments with the super-active opiate agonist (D-Met2, Pro5)-enkephalinamide (DMPEA) suggest that opiate receptors in the pituitary neural lobe (NL) may mediate the stimulatory effects of DMPEA on POMC peptide release from the IL in vitro. Experiments outlined in the current proposal will utilize a rodent model to investigate the potential role of NL opiate receptors in controlling IL POMC peptide secretion in vitro. The central hypothesis to be tested is that activation of opiate receptors in the NL promotes the release of a chemical modulator that, in turn, stimulates the secretory activity of IL POMC cells. Two major questions will be addressed. First, which opiate receptor subtype (mu, delta, or kappa) mediates the stimulatory effect of DMPEA on IL POMC peptide release in vitro? Second, does DMPEA stimulate IL cells indirectly by acting upon opiate receptors in the adjacent NL to promote the secretion of a POMC peptide-releasing factor (PPRF)? Selective u, o, and k receptor antagonists will be used to determine the relative contribution of these receptor subtypes to DMPEA stimulation of POMC peptide release using an established, fixed-volume in-vitro incubation procedure. Selective receptor agonists and antagonists will be used to determine the relative contribution of known NL neurochemicals to PPRF activity in conditioned incubation medium from DMPEA-treated NLs. If the PPRF proves to be an unidentified chemical substance, preliminary biochemical characterization will be performed to determine if the putative PPRF is a protein or peptide and to determine its approximate molecular weight. Results from the proposed work will lead to a better understanding of the receptor subtypes mediating opiate agonist effects on POMC cells. Findings generated from this work will establish a basis for future investigations on the interactions between opiate agonists and endogenous neurochemical systems, specifically those in the NL, that modulate the activity of pituitary POMC cells.