Inhibitory Action of Exogenous Melatonin, 5-Methoxytryptamine, and 6-Hydroxymelatonin on Sexual Maturation of Male Rats: Activity of 5-Methoxytryptamine Might be due to its Conversion to Melatonin1

The effect on sexual maturation of 6 different pineal indoles, including melatonin, and of the metabolite 6-hydroxymelatonin was studied in the male rat after daily injections from 20 to 40 days of age. Only 5-methoxytryptamine (5MT) and 6-hydroxymelatonin (6M), in addition to melatonin, inhibited the neuroendocrine-reproductive axis during sexual maturation. Their potencies when injected in the afternoon were in the range of one-twentieth to one-fifth that of melatonin. Like melatonin these two indoles had no effect when injected in the morning. N-acetylserotonin, serotonin, 5-hydroxytryptophol and 5-methoxytryptophol did not influence sexual maturation either when injected in the morning or in the afternoon. Chromatographic separation was performed on plasma extracts from rats injected daily with the biologically active indoles and killed 10-120 min after the last injection. This procedure confirmed that 6M injections did not increase plasma melatonin levels. In contrast, plasma melatonin levels in 5MT-treated rats were increased 1 h after the 5MT injection. These results suggest that 5MT or part of it might be acetylated to melatonin; thus inhibition of sexual maturation might be mainly due to melatonin. These results indirectly support the contention that melatonin is the principal pineal indoleamine playing a role during sexual maturation.