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.
The ontogenesis of melatonin receptors in the anterior pituitary and pars tuberalis of the Golden hamster was studied using [125l]iodomelatonin as a ligand. The affinity of the binding site to the ligand (K<j) was in the range 21 to 54 pM and it did not change significantly during development. The concentration of the [125lliodomelatonin binding sites in the anterior pituitary was highest in one-day-old hamsters (Bmax=14 fmol/mg protein) and thereafter gradually decreased. In adults it reached to about 6 % of the neonatal values. In contrast, the concentration of the binding sites in pars tuberalis did not change significantly during ontogenesis and it was in the range of 3 to 5 fmol/mg protein.