Effects of melatonin on luteinizing hormone secretion in anestrous ewes following dopamine and opiate receptor blockade

In the present investigation we have examined the ability of melatonin to modify the pulsatile LH secretion induced by treatment with a DA antagonist (sulpiride, SULP) or opioid antagonist (naloxone, NAL) in intact mid-anestrous ewes. The experimental design comprised the following treatments—in experiment 1: (1) intracerebroventricular (i.c.v.) infusion of vehicle (control I); (2) pretreatment with SULP (0.6 mg/kg subcutaneously) and then i.c.v. infusion of vehicle (SULP+veh.); (3) pretreatment with SULP and then i.c.v. infusion of melatonin (SULP+MLT, 100 μg per 100 μl/h, total 400 μg). In experiment 2: (4) i.c.v. infusion of vehicle (control II); (5) i.c.v. infusion of NAL (NAL-alone, 100 μg per 100 μl/h, total 300 μg); (6) i.c.v. infusion of NAL in combination with MLT (NAL+MLT, 100 μg+100 μg per 100 μl/h). All infusions were performed during the afternoon hours. Pretreatment with SULP induced a significant ( P