GnRH-I and GnRH-II-induced calcium signaling and hormone secretion in neonatal rat gonadotrophs.

Two forms of gonadotropin-releasing hormone (GnRH), GnRH-I and GnRH-II, are commonly present in mammals. The main hormone controlling reproduction is GnRH-I acting through its receptor (GnRHR-I), whereas the function of GnRH-II is unknown. In primates, it has been suggested that GnRH-II is a specific agonist for the structurally distinct GnRHR-II. Here we compared effects of GnRH-I and GnRH-II on intracellular calcium and gonadotropin hormone release in neonatal rat gonadotrophs in vitro and the dependence of agonist actions on cyclic nucleotide levels. Both agonists elevated intracellular calcium and stimulated gonadotropin secretion in a concentration-dependent manner, with comparable peak amplitudes, but GnRH-I was three times more potent than GnRH-II. Antide, a specific GnRHR-I antagonist, completely blocked the action of both agonists on gonadotropin release. Inhibition of adenylyl cyclase activity by melatonin and MDL significantly attenuated GnRH-I- and GnRH-II-induced calcium signaling and gonadotropin release, whereas inhibition of soluble guanylyl cyclase activity was ineffective. GnRH-II also generated calcium oscillations in a fraction of gonadotrophs not expressing melatonin receptors. These results indicate that GnRH-I and GnRH-II act on the same GnRHR to stimulate gonadotropin release through intracellular calcium and cyclic nucleotide signaling, and that GnRH-II is less potent agonist for this receptor in neonatal rat gonadotrophs.