Melatonin increases cGMP and decreases cAMP levels in rat medial basal hypothalamus in vitro.

Measurement of cyclic nucleotide accumulation in rat medial basal hypothalamus (MBH) incubated in vitro with methoxyindoles indicated that melatonin (10(-8) M or greater) increased significantly cGMP and depressed cAMP levels. Only the effect on MBH cAMP was shared by 5-methoxytryptophol which exhibited a greater activity than melatonin. 6-Fluoromelatonin (10(-7) M) increased cGMP, whereas the effect of 6-hydroxymelatonin was not significant. Both 6-fluoro- and 6-hydroxymelatonin (10(-5) M) depressed MBH cAMP accumulation while only 6-fluoromelatonin affected it at 10(-7) M concentrations. These data suggest that melatonin and 5-methoxytryptophol affect differently cyclic nucleotide content of MBH at physiological concentrations.