This project addresses the molecular and cellar processes involved in the secretion and actions of the hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), in pituitary gonadotrophs and hypothalamic neurons. The GnRH receptor is a unique tail-less member of the G protein-coupled receptor (GPCR) family, and acts primarily through Gq/11 and phosphoinositide/calcium signaling from its second and third intracellular loops. It also activates Gs and the adenylyl cyclase system through its first intracellular loop. G protein coupling of agonist-activated heptahelical receptors is often dependent on the amino-terminal region of their third intracellular loop. Many GPCRs, including the GnRH receptor, contain an apolar amino acid in this region at a constant distance below highly conserved Pro and Tyr/Asn residues in the fifth transmembrane domain. A mutational analysis of this region revealed that the conserved apolar residue (Leu237) in the third intracellular loop is an important determinant of GnRH receptor expression, signaling, and internalization. These findings suggest that the various apolar residues located at this position in many GPCRs are of major importance in receptor coupling to their cognate G proteins.The episodic mode of gonadotropin release from pituitary gonadotrophs is essential for normal reproductive function, and reflects the pulsatile secretory activity of GnRH-producing neurons in the hypothalamus. Pulsatile release of GnRH also occurs in vitro from cultured fetal hypothalamic cells and immortalized GnRH neurons. Both cell types also express GnRH receptors, and their secretory activity is regulated by GnRH agonist and antagonist ligands. Interestingly, unstimulated rat pituitary cultures also release GnRH, as well as gonadotropins, and some gonadotrophs exhibits high-amplitude Ca2+ oscillations. These cells release GnRH that contributes to the basal rate of LH secretion, which is reduced by treatment with a GnRH receptor antagonist or with specific GnRH antiserum. Such tonic GnRH stimulation could serve to maintain optimal responsiveness of the pituitary gonadotrophs to the episodic pulses of GnRH arising in the hypothalamus. The production and actions of GnRH in the anterior pituitary gland, the major site of expression of GnRH receptors, suggest that local regulatory effects of the neuropeptide could supplement the primary hypothalamic mechanism for the control of episodic gonadotropin secretion.
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