Effect of melatonin on pituitary secretion of prolactin in vitro during delayed implantation and the periimplantation period in the spotted skunk.

The pars tuberalis of the western spotted skunk is the only region of the brain known to bind 2-[125I]iodomelatonin, thus suggesting that this region of the pituitary might be essential in mediating the effects of melatonin in this species. Female skunks were killed during short- and long-day photoperiods. Hemisections of the adenohypophysis, with and without the pars tuberalis were cultured in the presence and absence of 10 nM melatonin to determine whether the pars tuberalis mediates the previously observed suppressive effects of melatonin on the seasonal increase in plasma prolactin levels in this species. Pituitaries obtained during long-day photoperiods secreted significantly more prolactin than those obtained during the short-day photoperiod. Melatonin had no effect on pituitary response to gonadotropin releasing hormone and failed to inhibit prolactin secretion during the 48 h culture period. However, melatonin significantly reduced the inhibitory effects of dopamine on prolactin secretion. The latter response was observed in both photoperiods and in the presence and absence of the pars tuberalis. These results suggest that the pars tuberalis does not mediate the inhibitory effect of melatonin on prolactin secretion in the absence of the hypothalamus.