Evidence for a cytoplasmic melatonin receptor

MELATONIN, a secretion of the pineal gland, has profound inhibitory effects on the development and maturation of gonadal organs in mammals 1 . Pinealectomy of the rat or hamster results in premature enlargement of the uterus and ovary 2,3 in females and gonadal organs in males 4,5 , while melatonin administration reverses the effects of pinealectomy and suppresses gonadal growth and maturation in these species 1 . The physiological site and mechanism of action are uncertain. The hormone seems to act both in the central nervous system and at peripheral sites; previous studies have demonstrated inhibition of luteinising hormone (LH) and follicle stimulating hormone (FSH) secretion by the pituitary 5,6 as well as inhibition of oestrogen and progesterone synthesis by ovarian tissue slices 7,8 . Since other hormones are known to exert their effects through binding to receptor proteins in target organs, we have examined melatonin binding in hamster and rat tissues and human ovaries, and here report evidence for the presence of a specific melatonin receptor in the supernatant fraction of several organs.