NEUROENDOCRINOLOGY OF MELATONIN IN REPRODUCTION - RECENT DEVELOPMENTS

The circadian melatonin rhythm with high levels in the dark period is important for the synchronization of reproductive response to appropri ate environmental conditions in animals. The target sites of melatonin action on reproductive functions remain to be clarified. Using autora diography (ARG) and radioreceptor binding assays with 2[I-125]iodomela tonin, a melatonin agonist, as the radioligand, studies on the sites o f melatonin action have increased significantly in the last ten years. The recent cloning of melatonin receptor subtypes also allowed the ch aracterization of receptor(s) to the molecular level. Earlier reports have documented that the hypothalamic-pituitary axis plays a vital rol e in the regulation of reproduction by melatonin. This is supported in part by the demonstration of melatonin receptors in the suprachiasmat ic nuclei (SCN) in the brain and pars tuberalis (PT) in the pituitary. However, the nature of SCN and PT involvement in the reproductive act ion of melatonin remains unknown. In addition to the hypothalamus and pituitary, the two classical sites of melatonin action, other targets have been identified. The recent demonstration of 2[I-125]iodomelatoni n binding sites or melatonin receptors in the testis, epididymis, vas deferens, prostate, ovary and mammary gland suggest the concept of mul tiple sites of melatonin action on the reproductive system. The presen ce of melatonin receptors in the said tissues is consistent with earli er reports of direct melatonin actions on different levels of the repr oductive system. This multiple levels of melatonin action, from the hy pothalamus, pituitary, gonads to other reproductive tissues form a rob ust system of photoperiodic control in animal reproduction. This would guarantee successful gestation and delivery of the offspring at a tim e with optimum food availability and ultimately favourable for the sur vival of species. Molecular and cellular studies of melatonin signalin g system(s), its regulation and effects on downstream functional event s in the future may provide new insights and directions for the study of the physiology and pharmacology of fertility and contraception in a nimals and humans. (C) 1998 Elsevier Science B.V. All rights reserved.