MELATONIN AND PHOTOPERIODIC TIME MEASUREMENT - SEASONAL BREEDING IN THE SHEEP

Well-established circadian physiology supports the view that photoperi odic time measurement utilizes the coincidence between the presence of light and a photosensitive phase of a 'biological clock' to alter rep roductive status-the so-called external coincidence model of seasonal breeding. In this review, we examine the mechanism whereby photoperiod interacts with presumed suprachiasmatic nuclei activity to allow endo genous melatonin to normally synchronize reproductive activity to the optimal time of year. The Romney Marsh sheep is particularly explored as an experimental model. It is suggested that the on/off activity of seasonal reproduction may be a robust mechanism able to be predictably manipulated by the judicious use of the light/dark cycle and exogenou s melatonin, but firmly based on circadian principles.