The role of photoperiod and melatonin in the control of seasonal reproduction in mammals

Abstract

<b>Melatonin secreted by pineal cells is a hormone whose biosynthesis is coordinated by neurons of the master clock located in the hypothalamic suprachiasmatic nuclei (SCN), characterized by the generation of a 24-hour rhythm. In many species of mammals, fluctuations in melatonin secretion affect reproductive functions, e.g. by regulating the frequency and amount of pulsatile secretion of hypothalamic and gonadotropic hormones. Seasonal breeding is a common adaptive strategy among mammals, allowing them to reproduce during the periods of the year that are most favourable for the later survival and growth of the offspring. This type of reproduction is characteristic of sheep, with winter reproductive activity, and hamsters, with summer reproductive activity. In these animals, melatonin synthesis is largely regulated by the photoperiod, which indirectly influences the period of reproductive activity or passivity. The aim of this study was to gather available knowledge on melatonin as a key element controlling seasonal reproduction. The paper presents the general shape of the circadian rhythm and the neuroendocrine mechanism regulating animal reproduction depending on the variable photoperiod. The collected results suggest that melatonin, kisspeptins, gonadotropin-releasing hormone (GnRH), sex hormones and thyroid hormones participate in the regulation of seasonal reproduction in mammals. </b>