The structural and signaling properties of receptors for gonadotropin- releasing hormone (GnRH), and their role in neuropeptide release and gonadotropin secretion, were investigated in hypothalamic and pituitary cells. GnRH receptors were previously cloned and found to be expressed not only in pituitary gonadotrophs but also in the hypothalamus and in immortalized GnRH neurons (GT1 cells). In both perifused GT1 cells and hypothalamic neurons, GnRH agonists influence the normal pulsatile secretion of GnRH by their action on GnRH receptors. GT1 cells also express receptor channels for glutamate (NMDA and AMPA/kainate receptors) and for gamma-amino-butyric acid (GABA/A receptors). The combined effects of glutamate and GABA on cytoplasmic calcium responses were additive and were sensitive to nifedipine. However, their combined effects on GnRH release were more than additive, consistent with non- linear coupling of calcium to exocytosis. In pituitary gonadotrophs the effects of progesterone on GnRH-stimulated LH release were accompanied by significant changes in the calcium signaling responses to agonist stimulation, with stimulatory or inhibitory actions according to the duration of progesterone treatment. Also, GnRH was found to activate both phospholipase C and D in a sequential and synchronized manner via activation of protein kinase C. Studies on the control of exocytosis in permeabilized pituitary gonadotrophs showed that while elevated cytoplasmic calcium per se can promote LH release, concomitant activation of protein kinase C is necessary to support hormone secretion at the physiological calcium levels that prevail in agonist-stimulated intact cells. Thus, sensitization of the calcium-dependent secretory mechanism by protein kinase C may be an important step in the GnRH-induced release of LH from pituitary gonadotrophs. The rat GnRH receptor gene was found to consist of three exons that encode the receptor protein, and to span about 20 kb of DNA. The reading frame is interrupted by two exons of 12 and 2.5 kb in the 4th and between the 5th and 6th transmembrane domains, respectively, and the promoter region of the gene contains a putative TATA box and several potential regulatory elements. In structure- function studies of the GnRH receptor, a conserved hydrophobic residue (Leu147) in the second intracellular loop was found to be essential for receptor signaling and endocytosis. In contrast, restoration of tyrosine in place of the unusual serine residue present in the DRS motif that replaces the conserved DRY sequence present in most G protein-coupled receptors did not affect receptor signaling but increased agonist binding affinity and receptor internalization.
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