This project includes studies on the structural and signaling properties of receptors for gonadotropin-releasing hormone (GnRH), and their role in neuropeptide release and gonadotropin secretion. GnRH receptors were found to be expressed not only in pituitary gonadotrophs but also in the hypothalamus and in immortalized GnRH neurons (GT1 cells). Cultured hypothalamic cells and GT1 cells exhibit pulsatile GnRH release that is modulated by neurotransmitters and neuropeptides including GnRH itself. In structure-function studies on the GnRH receptor, a conserved leucine residue in the second intracellular loop (leu147) was found to be essential for the coupling of the receptor to G protein-mediated signaling responses and for agonist-induced internalization of the ligand-receptor complex. In addition, the aromatic moiety of the conserved tyrosine residue in the atypical NPXXY (DLPIY) sequence of the GnRH receptor was found to be essential for normal coupling to G proteins, and is a critical element in agonist-induced receptor activation and signal transduction. Studies in GT1 cells revealed the presence of large-conductance potassium channels that assist in membrane repolarization but do not participate in the control of neuropeptide release. In both GT1 cells and hypothalamic neurons, nanomolar concentrations of hCG were found to rapidly inhibit the pulsatile release of GnRH in a reversible manner. This process was not mediated by the stimulatory action of hCG on cAMP production, but probably results from a Gi-mediated inhibitory action on plasma-membrane ion channel activity. This effect of hCG or pulsatile GnRH release could be responsible for the suppression of pituitary gonadotropin secretion during pregnancy. Studies of the control of ovarian function by members of the VIP/GHRH/PACAP family of neuropeptides showed that both PACAP and the PACAP-I receptor and expressed in the rat ovary. The presence of PACAP and three splice variants of its type I receptor suggest that PACAP can exert autocrine or paracrine actions on granulosa cell function. In human ovarian cancer cells, endothelin I (ET-1) was found to be produced and to act on ETA receptors to stimulate calcium signaling and thymidine incorporation. The effect of ET-1 on cell proliferation, and the basal growth rate of the tumor cells, were inhibited by the ETA receptor antagonist, BQ123. Thus, ET-1 produced in ovarian cancer cells can act through ETA receptors as an autocrine growth factor, and may contribute to cell growth and proliferation in certain ovarian tumors.