Developmental changes and daily rhythm in melatonin-induced inhibition of 3',5'-cyclic AMP accumulation in the rat pituitary.

Melatonin's transduction mechanisms were investigated using in vitro cultured anterior hemipituitaries. Melatonin inhibited cAMP and 3',5'-cyclic GMP accumulation in neonatal rat anterior pituitary stimulated with LHRH. Maximal inhibitory effect was reached within 25 min and persisted for at least 20 min. Inhibition of cAMP accumulation is specific for melatonin because its analogs N-acetylserotonin and 5-methoxytryptamine are 1000 times less potent. Melatonin effect is age- and time-dependent. Marked inhibition was observed in 5-, 10-, and 14-day-old rats but not in 29-day-old ones. Melatonin was significantly more potent when applied at the end of the light period as compared with the first half of the day. Melatonin's effects on cAMP correlate with its effect on reproductive functions and on LH release. Cyclic nucleotides may thus serve as second messengers transducing the effect of melatonin on cellular level.