Historical Perspective and Evaluation of the Mechanisms by which Melatonin Mediates Seasonal Reproduction in Mammals

Ramaswamy Sharma,Russel J Reiter,Dun-Xian Tan

doi:10.32794/mr11250004

Abstract

The discovery of melatonin and its photoperiod-regulated circadian rhythm were important milestones in defining the events associated with the regulation of seasonal reproduction in both short-day and long-day breeding mammals. By means of the seasonal changes in the duration of the nocturnal melatonin rise, which provides both clock and calendar information, animals adjust their reproductive state to the appropriate time of year. Thus, melatonin dictates the proper season for mating which ensures the optimal time for delivery of offspring. The photoperiodic information is transduced into a chemical messenger, melatonin, in the pineal gland. The initial studies related to the importance of day length and the melatonin message in impelling seasonal reproduction involved the use of photoperiod-sensitive rodents, especially several long-day breeding hamster species. Since then, a large group of interested reproductive biologists have extended these findings to numerous other mammals including short-day breeding ungulates. Moreover, critical information related to the molecular processes at the level of the pars tuberalis of the anterior pituitary and the mediobasal hypothalamus has been provided. In this scheme, the pars tuberalis, until recently an almost ignored portion of the adenohypophysis, has been identified as a critical intermediate between the melatonin signal and the functional state of the reproductive organs. It is somewhat ironic that two organs, the pineal gland and the pars tuberalis, which suffered with the image of vestigiality for so long have now been identified as unequivocally essential for this most basic and important function, i.e., regulating seasonal reproduction.

Highlights

Even before melatonin was discovered in the bovine pineal gland by Lerner and colleagues in 1958 [1], there was evidence, albeit not very compelling, that the pineal, via an undefined secretory product, influenced reproductive physiology in some vertebrates
I recall being challenged after a presentation at a scientific meeting in San Francisco in 1967 by a well-known endocrinologist. He and other skeptics had misunderstood, that our study showed that pineal removal prevented rather than caused involution of the peripheral reproductive organs
It has been more than 50 years since the first report showing that surgical removal or sympathetic denervation of the pineal gland prevented short-day mediated regression of the peripheral reproductive organs of male and female Syrian hamsters. Soon after these findings were reported, it was observed that the neuroendocrine-reproductive axis of other hamster species remained highly functional after pinealectomy even when they were maintained under a reproductively-inhibitory photoperiod

Summary

INTRODUCTION

Even before melatonin was discovered in the bovine pineal gland by Lerner and colleagues in 1958 [1], there was evidence, albeit not very compelling, that the pineal, via an undefined secretory product, influenced reproductive physiology in some vertebrates. When it was first observed that there is a link between photoperiod length and reproduction, we predicted that seasonally-changing day lengths drive, via the pineal gland, the circannual reproductive cycles of this species [30] This was confirmed when Syrian hamsters were maintained under natural day lengths and temperature conditions throughout the winter months at a north latitude like that of their home range in the wild; during the short winter days, the reproductive organs of the intact hamsters collapsed while those that were pinealectomized remained functional, similar to animals during the long days of summer [42]. In a population of tammar wallabies on a military base in Australia that is artificially lit a night, birthing of the young has spread over a much longer period compared to the time of delivery of wallabies inhibiting a wilder environment where there was no artificial light contamination [74]

MECHANISMS GOVERNING SEASONAL REPRODUCTION

CONCLUDING REMARKS