The Formation, Metabolism, and Physiologic Effects of Melatonin

Abstract

Publisher Summary Melatonin is the prototype of a family of compounds, the methoxyindoles, which are produced in mammals only in the pineal gland. The pineal gland is capable of synthesizing melatonin because it, alone among mammalian tissues, contains the enzyme hydroxyindole-0-methyltransferase (HIOMT). In lower vertebrates, this enzyme is also present in the brain and the eye. Melatonin is released into the circulation and produces a variety of physiological effects, mostly inhibitory, on endocrine organs, the brain, smooth muscle, and epithelial structures. It depresses ovarian growth and the incidence of vaginal smears showing estrous phases in rats; it blocks the rise in pituitary LH levels, which follows castration; it interferes with the uptake of 131I by the thyroid, and the secretion of thyroid hormones. Its administration leads to a rapid decline in the levels of melanocyte stimulating hormone in the rat pituitary. Melatonin relaxes several types of smooth muscle, and antagonizes the contractile effects of 5-HT. The synthesis of melatonin in the pineal varies diurnally, with a rhythm that is generated by environmental lighting. Exposure to continuous illumination depresses the rate of melatonin formation; darkness has an opposite effect.