Neonatal pituitary cells express MT1 and MT2 subtype of melatonin receptors that are coupled to pertussis toxin-sensitive G proteins. Their activation by melatonin leads to a decrease in cAMP production and activity of protein kinase A, and attenuation of gonadotropin-releasing hormone (GnRH)-induced gonadotropin secretion. Single cell calcium and electrophysiological recordings have revealed that a reduction in gonadotropin release results from melatonin-induced inhibition of GnRH-stimulated calcium signaling. Melatonin inhibits both calcium influx through voltage-dependent calcium channels and calcium mobilization from intracellular stores. Inhibition of calcium influx, probably in a cAMP/protein kinase C-dependent manner, and the accompanying calcium-induced calcium release from ryanodine-sensitive intracellular pools by melatonin results in a delay of GnRH-induced calcium signaling. Melatonin-
induced attenuation of GnRH-induced and inositol (1,4,5)-trisphosphate-mediated calcium release from intracellular pools attenuates the amplitude of calcium signal. The potent inhibition of GnRH-induced calcium signaling and gonadotropin secretion by melatonin provides an effective mechanism to protect premature initiation of pubertal changes that are dependent on plasma gonadotropin levels. During the development, such tonic inhibitory effects of melatonin on GnRH action gradually decline due to a decrease in expression of functional melatonin receptors. In adult animals, melatonin does not have obvious direct effects on pituitary functions, whereas the connections between
melatonin release and hypothalamic functions, including GnRH release, are preserved, and are critically important in synchronizing the external photoperiods and reproductive functions through still not well characterized mechanisms.
The role of [Ca2 + ]j and cAMP in transduction of the melatonin inhibitory effect on GnRH-induced LH release from neonatal rat gonadotrophs has been studied, because melatonin inhibits the increase of both intracellular messengers. Treatments increasing Ca2+ influx (S( —) Bay K8644 or KC1) or cAMP concentration (8-bromo-cAMP or 3-isobutyl-l-methylxanthine) potentiated the GnRH-induced LH release and partially diminished the inhibitory effect of melatonin. Combination of the treatments increasing cAMP and calcium concentrations blocked completely the melatonin inhibition of LH release. The combined treatment with 8-bromo-cAMP and S(-) Bay K8644 also blocked the melatonin inhibition of GnRH-induced [Ca2+]j increase in 89% of the gonadotrophs, while any of the treatments alone blocked the melatonin effect in about 25 % of these cells. These observations suggest that a cAMP-dependent pathway is involved in regulation of Ca2+ influx by melatonin and melatonin inhibition of LH release may be mediated by the decrease of both messengers.