Stimulation of the production of progesterone by the corpus luteum of the ewe by the perfusion of melatonin in vivo and by treatment of granulosa cells with melatonin in vitro.

The ability of melatonin to stimulate luteal steroidogenesis directly in the ewe was investigated by using a perfusion cannula system in vivo and luteinised granulosa cells in vitro. Exposure to human chorionic gonodotrophin (hCG) or ovine luteinising hormone (oLH), as positive controls, significantly stimulated progesterone secretion by the perfused corpus luteum within 20 minutes and by granulosa cells after two days of culture. Melatonin perfused through the corpus luteum at a physiological concentration, significantly stimulated progesterone secretion but the response was not as rapid as that to hCG. The significant stimulation of progesterone by melatonin in the perfusion system was maintained for between 30 and 60 minutes. Melatonin significantly stimulated progesterone production by granulosa cells in vitro after two days of culture, but the degree of response depended on the size of the follicles from which the cells were obtained. These results demonstrate that melatonin can act directly on the corpus luteum to increase progesterone production, and that its mechanism of action appears to be different from that of hCG or oLH in the ewe. This action of melatonin may be related to the reported improved luteal function late in the breeding season after the prolonged exposure of ewes to melatonin.