EFFECT OF MELATONIN ON PITUITARY SECRETION OF PROLACTIN IN-VITRO DURING DELAYED IMPLANTATION AND THE PERIIMPLANTATION PERIOD IN THE SPOTTEDSKUNK

The pars tuberalis of the western spotted skunk is the only region of the brain known to bind 2-[I-125]iodomelatonin, thus suggesting that t his region of the pituitary might be essential in mediating the effect s 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 abse nce of 10 nM melatonin to determine whether the pars tuberalis mediate s the previously observed suppressive effects of melatonin on the seas onal increase in plasma prolactin levels in this species. Pituitaries obtained during long-day photoperiods secreted significantly more prol actin 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 peri od. 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 tuberali s. These results suggest that the pars tuberalis does not mediate the inhibitory effect of melatonin on prolactin secretion in the absence o f the hypothalamus. (C) 1993 Wiley-Liss, Inc.