The coincidence of light and melatonin with a specific phase of the circadian pacemaker is important for the timing of seasonal breeding in the ewe

The timing of reproductive activity in seasonal breeding sheep relies on da ily photoperiodic signals being relayed to provide information on the time of year. Although light and melatonin are involved, the exact mechanism is not understood. In this experiment, three groups of 6 Romney Marsh ewes, a highly seasonal breed, were provided with 8 weeks of short nights (9.6-9.8 h,by artificially advancing dawn) around the winter solstice, near the end of their natural breeding season. One group of animals was infused to a phy siological level with melatonin for 5 h during the afternoon prior to the o nset of dark, while a second group was identically infused but for 5 h Dom the time of lights on. A third group received the short-night treatment onl y. Following the short-night treatment, all groups were exposed to long nig hts (> 14 h, by delaying dawn) until the summer solstice. Ovarian activity, assessed by progesterone monitoring twice weekly, showed that the noninfus ed and the morning-infused groups displayed renewed reproductive activity i n response to the short-night/long-night treatment. There was no renewed ov arian activity in the afternoon-infused group, indicating that the time of day that melatonin is present, rather than the duration of melatonin exposu re, is an important signal. in the control of reproductive timing. Measurem ents of a marker of the endogenous circadian pacemaker, by melatonin measur ements under acutely extended darkness, revealed that the short-night treat ments phase advanced the onset of the pacemaker in all groups such that the afternoon phase of the pacemaker was coincident with Light. The results pr ovide strong support for the model that proposes that an afternoon-located sensitive phase of the pacemaker is responsible for the relay of photoperio dic signals in the timing control of seasonal breeding. The model proposes that the reproductive axis be primed during short nights when the sensitive phase is coincident with light in the afternoon so ovarian activity can be induced when the sensitive phase is located within the longer nights of au tumn and coincident with endogenous melatonin.