Pineal and melatonin actions in reproductive endocrinology

Daylength is the major environmental variable governing seasonal breeding, and the pineal gland is needed for this response to occur in many photosensitive species. Although dependent upon the integrity of its peripheral sympathetic innervation, the pineal has no efferent projections, and therefore it affects neuroendocrine function by humoral means. Melatonin has been proposed as the pineal hormone; its timed administration replicate the effect of daylength on seasonal breeding in several mammals, including long-day, like the hamster, and short-day breeders, like the sheep. The pineal is regulated primarily by photo-periodic information attaining the gland through a polysynaptic pathway initiated in the retinohypothalamic tract. Secondary regulators of pineal secretory activity are several steroid and polypeptide hormones. Melatonin synthesis is maximal during nighttime while during daylight it is suppressed. Melatonin acts on specific receptor sites at hypothalamic and perhaps extrahypothalamic sites to affect LHRH release. Changes in calcium fluxes, cyclic nucleotide and prostaglandin synthesis may underlie the neuroendocrine activity of melatonin at hypothalamic sites.