Melatonin in the rat testis: evidence for local synthesis

The vertebrate pineal gland rhythmically produces melatonin, a hormone involved in regulation of several physiological and behavioral processes. Melatonin is synthesized from serotonin essentially by two enzymatic steps involving N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase activities. We have previously demonstrated the presence of melatonin binding sites in the rat testes, and an inhibitory effect of melatonin on testicular gonadotrophin-stimulated androgen production. It is unknown whether these effects are mediated by melatonin synthesized locally or by melatonin from pineal origin. To assess the potential capacity of melatonin production by the testis, we used radiolabeled precursors to measure the activities of N-acetyltransferase and hydroxyindole-O-methyltransferase. The production of N-acetylserotonin was time-dependent during over 10 min of incubation. Melatonin had a linear increase throughout the 30 min incubation period with S-adenosyl-L-[methyl-14C]methionine. Identities of melatonin and N-acetylserotonin were confirmed by thin-layer chromatography. The ability of the testis to synthesize melatonin during sexual maturation was also analyzed. When activity of NAT was expressed per mg of protein, the maximal activity was observed on day 40. In contrast, when activity of NAT is expressed by the testis, the amount of NAT increased to peak on day 40 and remained elevated through day 70. We determined that both activities were predominantly localized in interstitial cells. NAT activity in seminiferous tubules was substantially decreased, representing 6.4% of NAT activity in interstitial cells. We concluded that rat testes are capable of synthesizing melatonin due to the presence of the enzymes necessary for the transformation of serotonin to melatonin.