Abstract
Night-time pineal levels of tryptophan, 5-hydroxytryptophan, serotonin, N-acetylserotonin, melatonin, 5-hydroxyindoleacetic acid and the activities of the two enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase involved in the cyclic production of melatonin were determined in male albino rats and Syrian hamsters that were implanted with thyroxine or thyroidectomized two weeks earlier. Both treatments depressed nocturnal pineal melatonin content in rats and hamsters. The cause of this depression is not known, although minor alterations in the substrates and the enzymes involved in melatonin production were observed. The data suggest that alterations in thyroid hormone levels may increase the release of nocturnal melatonin from the pineal, thereby allowing less to accumulate in the gland.