MELATONIN INHIBITS GNRH-INDUCED INCREASE OF CFOS IMMUNOREACTIVITY IN NEONATAL RAT PITUITARY

In neonatal rat gonadotrophs, melatonin inhibits several GnRH-induced effects: stimulation of LH release as well as the increase of several second messengers as cAMP, diacylglycerol and [Ca2+](i). Recently, GnR H has been shown to induce expression of immediate early genes of fos and jun family in adult rat gonadotrophs. The purpose of this study wa s to determine, whether melatonin inhibits the GnRH-induced induction of cFos in neonatal rat pituitary cells. The effects of GnRH and/or me latonin on cFos immunoreactivity was determined in primary cultures of neonatal rat pituitary cells attached to the coverslip. GnRH (3 nM) i nduced a time-dependent increase of cFos immunoreactivity in about 10 to 15% of the cultured cells. Significant increase was observed alread y 30 min after GnRH administration, the maximal increase occurred afte r about 60 min and then gradually decreased. Melatonin (100 nM) marked ly attenuated the GnRH-induced increase, GnRH increased cFos immunorea ctivity in the cells in a dose-dependent manner (EC(50)=36 pM) and mel atonin (100 nM) attenuated the response at all GnRH concentrations tes ted. Melatonin had no effect on basal cFos immunoreactivity, but it in hibited the GnRH-induced (10 nM) increase of cFos in dose-dependent ma nner (EC(50)=12 pM). In conclusion, this is the first report showing t he inhibitory effect of melatonin on gene transcriptional activity in gonadotrophs.