Melatonin inhibits pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP accumulation and [Ca2+]i in cultured cells of neonatal rat pituitary

Abstract The effects of melatonin on pituitary adenylyl cyclase-activating polypeptide-induced increase of cyclic AMP and [Ca 2+ ] i were studied in neonatal rat pituitary cells. The polypeptide increased cyclic AMP accumulation. In the presence of melatonin the increase of cyclic AMP was inhibited in a dose-dependent manner, the maximal inhibition was achieved with 1–10 nM melatonin. Pituitary adenylyl cyclase-activating polypeptide also increased [Ca 2+ ] i in 30% of the pituitary cells and melatonin inhibited the effect. Most of the cells sensitive to adenylyl cyclase-activating polypeptide (77%) were also sensitive to GnRH, suggesting they are gonadotrophs. The remaining cells were not identified. The polypeptide-induced [Ca 2+ ] i increase was inhibited in Ca 2+ -free medium in 2/3 of the cells indicating that Ca 2+ influx was involved. To examine causal relationship between cyclic AMP and [Ca 2+ ] i increase, we have studied the effect of adenylyl cyclase activation by forskolin on intracellular Ca 2+ concentration. Forskolin had similar effects as adenylyl cyclase-activating polypeptide: it increased [Ca 2+ ] i in the pituitary cells and the increase was dependent on presence of Ca 2+ in the medium. Melatonin inhibited the forskolin induced [Ca 2+ ] i increase. Our observations indicate that increase of cyclic AMP stimulates Ca 2+ influx in the pituitary cells of neonatal rat and that this mechanism is involved in [Ca 2+ ] i increase induced by the pituitary adenylyl cyclase-activating polypeptide. Because melatonin inhibits increase of cyclic AMP induced by pituitary adenylyl cyclase-activating polypeptide or forskolin, the inhibitory effect of melatonin on Ca 2+ -influx may be mediated by the decrease of cyclic AMP concentration. This mechanism of melatonin action has not been described previously. Because melatonin inhibits the polypeptide- or forskolin-induced [Ca 2+ ] i also in the cells not sensitive to GnRH, melatonin receptors seem to be present on both gonadotrophs and non-gonadotrophic pituitary cells.