MELATONIN INHIBITS INCREASE OF INTRACELLULAR CALCIUM AND CYCLIC-AMP IN NEONATAL RAT PITUITARY VIA INDEPENDENT PATHWAYS

In neonatal rat pituitary, melatonin inhibits GnRH-induced increase of cAMP and [Ca2+](i). Both effects are transduced by specific high-affi nity melatonin receptors coupled with pertussis toxin-sensitive G-prot ein. We have attempted to determine whether melatonin acts via indepen dent pathways on both messengers or whether the indole directly inhibi ts only one of the messengers and the second is decreased as a seconda ry consequence. Melatonin inhibition,of cAMP accumulation was not prev ented by agents known to block melatonin effect on [Ca2+](i) such as N a+- or Ca2+-free medium, Bay K, nifedipine, KCl or gramicidin. Melaton in effect on [Ca2+](i) was not prevented by forskolin or 8-bromo-cAMP. We therefore conclude that melatonin inhibits cAMP accumulation and [ Ca2+](i) increase independently by separate pathways.