DOES MELATONIN ACT ON DOPAMINERGIC PATHWAYS IN THE MEDIOBASAL HYPOTHALAMUS TO MEDIATE EFFECTS OF PHOTOPERIOD ON PROLACTIN SECRETION IN THE RAM

Previous studies have shown that the chronic administration of melaton in in the mediobasal hypothalamus (MBH) using micro-implants in Soay r ams housed under long days causes a sustained decrease in the secretio n of prolactin as occurs in response to short days. The purpose of thi s study was to investigate whether hypothalamic dopaminergic (DA) syst ems acting through D-2 receptors may be involved in this melatonin-ind uced effect. Groups of Soay rams living under long days were treated i n the MBH with micro-implants containing bromocriptine (BROM, DA D-2 r eceptor agonist), or sulpiride (SULP, DA D-2 receptor antagonist), giv en alone or in combination with melatonin (MEL), to establish whether the DA drugs would mimic or negate the effects of melatonin. A control group (C) received empty microimplants or no treatment. The micro-imp lants were bilateral and were left in place for 14 weeks; the trial co ntinued for a total of 28 weeks (14-week implant period and 14-week po stimplant period) while the animals remained under long days. The abil ity of the microimplants to release BROM and SULP for 14 weeks was con firmed by incubating implants in vitro and testing for the presence of the compounds using a pituitary cell bioassay. MEL in the MBH induced a marked decrease in the blood plasma concentrations of prolactin dur ing the implant period and an increase during the postimplant period ( MEL vs. C, p < 0.001). BROM given alone induced a sustained decrease i n the plasma concentrations of prolactin (less marked than MEL), while SULP caused an increase (BROM and SULP vs. C, p < 0.001); the effects were restricted to the implant period. BROM given in combination with MEL produced the same effect as MEL alone during both the implant and postimplant periods, while SULP given with MEL produced the same effe ct as MEL during the implant period, but impaired the increase in plas ma concentrations of prolactin during the postimplant period (MEL + SU LP vs. MEL, p < 0.001). There were changes in growth and moulting of t he pelage correlated with the marked changes in the secretion of prola ctin induced by MEL, but not related to the lesser effects of BROM and SULP. In conclusion, the long-term effects of the D-2 agonist and ant agonist are consistent with the inhibitory role of hypothalamic DA pat hways in the homeostatic regulation of prolactin secretion. The inhibi tory effect of the D-2 agonist did not mimic that of MEL in the MBH, t hus it is unlikely that the short day MEL signal operates primarily th rough a hypothalamic DA system to inhibit the secretion of prolactin. However, since the administration of the D-2 antagonist in the MBH did influence the response to MEL, it is probable that DA pathways are in volved in relaying the effects of MEL on the long-term cycle in the se cretion of prolactin in the ram.