Short-term modulation of prolactin secretion by melatonin in anestrous ewes following dopamine- and opiate receptor blockade

This study rested the: hypothesis that the dopaminergic and opioidergic sys tems are not involved in the short-term stimulatory action of melatonin (ML T) on the secretion of prolactin in anestrous ewes. Thus. MLT should stimul ate prolactin release after blockade of either dopamine (DA) or opiate rece ptors with specific antagonists at the level of the pituitary gland and cen tral nervous system (CNS). respectively. During afternoon intracerebroventr icular (icv.) infusion of MLT, the mean plasma prolactin concentration incr eased significantly (P<0.001) as compared with the concentrations noted bef ore and during the infusion of the vehicle (veh.). As a result of subcutane ous (sc.) injection of sulpiride (SULP, DA antagonist), an increase in plas ma prolactin concentration was observed. followed by a gradual decrease dur ing the icy. infusion of the vehicle. MLT infused icy. significantly increa sed (P<0.001) the secretion of prolactin in SULP+MLT-treated ewes. as compa red with the concentration of prolactin noted during infusion of the vehicl e in SULP+veh.-treated ewes. Naloxone (NAL. opioid antagonist) infused icy. did not significantly affect the secretion of prolactin, however, a signif icant (P<0.01) increase in the concentration was observed after the infusio n. In MLT+NAL-treated awes, the plasma prolactin concentration increased si gnificantly (P<0.001) during the infusion, as compared with the concentrati on noted before and that in NAL-alone infused ewes. These results demonstra te that melatonin stimulates prolactin release after the pharmacological ex clusion of the dopaminergic input with the DA antagonist sulpiride and also despite the presence of DA activity in the hypothalamus after NAL treatmen t. Secondly, endogenous opioid peptides are not a major component of this m elatonin action.