The circadian timing system and reproduction in mammals

Circadian systems in a wide variety of organisms all appear to include thre e basic components: 1) biological oscillators that maintain a self-sustaine d circadian periodicity in the absence of environmental time cues; 2) input pathways that convey environmental information, especially light cues, tha t can entrain the circadian oscillations to local time; and 3) output pathw ays that drive overt circadian rhythms, such as the rhythms of locomotor ac tivity and a variety of endocrine rhythms. In mammals, the circadian system is employed in the regulation of reproductive physiology and behavior in t wo very important ways. 1) In some species, there is a strong circadian com ponent in the timing of ovulation and reproductive behavior, ensuring that these events will occur at a time when the animal is most likely to encount er a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms that are largely under photoperiod regulation; in these species, the circad ian system and the pineal gland are crucial components of the mechanism tha t is used to measure day length. The rhythm of pineal melatonin secretion i s driven by a neural pathway that includes the circadian oscillator(s) in t he suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, a nd the duration of each nocturnal episode of melatonin secretion is inverse ly related to day length. The pineal melatonin rhythm appears to serve as a n internal signal that represents day length and that is capable of regulat ing a variety of seasonal variations in physiology and behavior. (C) 1999 E lsevier Science Inc. All rights reserved.