Functional Pleiotropy of Melatonin in the Regulation of Reproduction: An Overview

This brief review covers recent important findings to form an informative pool depicting unique role of the pineal hormone melatonin in the regulation of reproductive axis in vertebrates. Since last decades, melatonin research witnessed an enormous progress in understanding the nature and mechanism of actions of this tiny, but versatile, product of tryptophan in the regulation of diverse body functions, especially reproduction in different groups of vertebrates. Demonstration of melatonin receptors on different peripheral organs and parts of brain essentially supported the contention of a hormonal effect of melatonin. Several lines of evidence suggest that melatonin regulates, rather modulates, vertebrate reproduction by two pathways- one through controlling the hypothalamo-hypophyseal- gonadal axis and another by inducing direct effect on the gonads through G-protein coupled MT1 and MT2 receptors. The conventional idea of the regulatory actions of melatonin on gonads via the hypothalamo-hypophyseal- gonadal axis involving gonadotrophic releasing hormone (GnRH) and gonadotrophins, has been greatly enriched by the discovery of other hypothalamic peptides like, gonadotrophic inhibitory hormone (GnIH) and kisspeptin. Additional findings that melatonin, due to its lipophilicity and potential antioxidant property, scavenges and detoxifies various free radicals, opened up a possibility of its significant contribution to the physiology of oocytes during growth and maturation. As an obvious outcome of unequivocal evidence suggesting pleiotropic functions of melatonin in the regulation of vertebrate reproduction, studies on the array of molecular events occurring at different sites of melatonin actions have emerged as a new exciting area of investigation.