The circannual rhythm controlling ovarian function in the ewe: Possible neural elements

Three distinct endogenously-generated rhythms can contribute to the annual periodicity in fertility seen in seasonal breeders: an ultradian, a circadian, and a circannual rhythm. The roles of an ultradian rhythm in GnRH secretion and a circadian rhythm in photosensitivity have been extensively studied in a number of species (1-4). In contrast, much less data are available about possible circannual rhythms. This paucity of information reflects the difficulties inherent in studying a rhythm that approximates a year, and the absence of such rhythms in short-lived animals that are convenient models of seasonal breeding (e.g., hamsters). However, circannual rhythms appear to play an important role in the seasonal reproductive patterns of some long-lived animals (1,5,6). One mammal in which there is now compelling evidence for such an endogenously-generated rhythm is the sheep. Moreover, recent work has begun to identify some of the neural elements that comprise the circannual rhythm controlling fertility in the ewe. In this paper, we will first review the evidence that a circannual rhythm controls ovarian function in the ewe and then describe three neural systems that participate in this rhythm.