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

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.