The effects of opiate agonist and antagonist on serum prolactin in primates: possible role for endorphins in prolactin regulation.

Intravenous administration of the opiate receptor antagonist naloxone produced a significant reduction in basal serum PRL concentrations in four male Macaca arctoides. Significant decreases from basal levels were found 15, 30, 45, 60, 90, 120, 150, and 180 min after the iv injection of 0.05 and 0.25 mg/kg naloxone. The iv administration of 0.4 mg/kg morphine produced rapid and significant increases in PRL levels, while 0.04 mg/kg morphine or saline produced no change. Both the dopamine receptor-stimulating agent apomorphine (0.15 mg/kg) and naloxone (0.25 mg/kg) decreased basal serum PRL and blocked the morphine-induced increases in serum PRL. These data support the hypothesis that endorphins are involved in the stimulation of PRL secretion.