Melatonin inhibits gonadotropin-releasing hormone-induced elevation of intracellular Ca2+ in neonatal rat pituitary cells.

GnRH stimulates LH release by increasing intracellular Ca2+ (Ca2+]i). Melatonin is known to inhibit GnRH-stimulated LH release from neonatal rat pituitary cells. In the present report, the issue of whether melatonin acts through Ca2+]i was addressed. Ca2+]i was studied in cells in suspension, using Fluo-3 as a fluorescent indicator. In neonatal rat pituitary cells, melatonin inhibited the GnRH-induced Ca2+]i increase in a dose-dependent manner; the GnRH-induced increase in Ca2+]i was inhibited 40% by 100 nM melatonin. The relative potencies of several indoles as inhibitors of the GnRH stimulation of Ca2+]i in neonatal pituitary cells (2-iodo-melatonin greater than melatonin greater than 6-hydroxymelatonin) correlate with their known potencies to inhibit LH release and with their binding affinity to high affinity melatonin receptors, which indicates that these receptors probably mediate the effects of melatonin. Further support for this interpretation comes from the observation that melatonin does not inhibit the GnRH effect on Ca2+]i in cells obtained from adolescent rat pituitary glands, which lack melatonin receptors and are insensitive to melatonin as an inhibitor of GnRH-stimulated LH release. The possible involvement of an inhibitory G-protein was also investigated by studying the effects of pertussis toxin. Pretreatment with pertussis toxin antagonized the effects of melatonin on Ca2+]i and LH release. This indicates that melatonin may inhibit the GnRH-induced increase in Ca2+]i through a mechanism involving a pertussis toxin-sensitive G-protein. To examine the role of extracellular Ca2+ in this effect, the effects of melatonin were examined in a low Ca2+ medium. Under these conditions, the effect of melatonin was markedly reduced, which indicates that melatonin may act by inhibiting Ca2+ influx. These observations indicate that melatonin inhibits GnRH stimulation of Ca2+]i in neonatal rat gonadotrophs, and this probably explains the inhibitory action of melatonin on GnRH stimulation of LH release.