Melatonin modulation of prolactin and gonadotrophin secretion. Systems ancient and modern

Recent studies in sheep indicate that the pineal melatonin signal which transduces effects of photoperiod acts at separate sites in the pituitary gland and brain to regulate seasonality in prolactin (PRL) and gonadotrophin secretion. The pituitary gland is the proposed site for control of PRL based on the observation that hypothalamo-pituitary disconnected (HPD) rams continue to show normal patterns of PRL secretion in response to changes in photoperiod or treatment with melatonin. Lactotrophs do not express melatonin receptors, thus this pituitary effect is assumed to be mediated by cells in the pars tuberalis via "tuberalin". The mediobasal hypothalamus (MBH) is the putative target for gonadotrophin control since: i) gonadotrophin secretion is dependent on pulsatile GnRH secretion from the MBH, ii) local administration of melatonin in the MBH, but not in other areas of the brain and pituitary gland, readily reactivates GnRH-induced LH and FSH secretion in photo-inhibited rams; and iii) treatment of HPD rams with a chronic pulsatile infusion of GnRH stimulates gonadotrophin secretion irrespective of photoperiod. Complementary studies conducted by others in the Syrian hamster, have shown that lesions in the MBH block the action of melatonin on gonadotrophin but not on prolactin secretion; this supports the "dual-site hypothesis". Since all photoperiodic mammals are essentially similar in hyper-secreting PRL under long days, the pituitary control mechanism for PRL is regarded as conserved (ancient) with the pleiotrophic actions of PRL inducing a summer physiology (e.g. growth of summer pelage). In contrast, the variation between species in the timing of the gonadal cycle indicates that evolution has independently modified the melatonin-sensitive neural circuits in the MBH to permit the species-specific timing of the mating season.