Differential responses in prolactin levels induced by naloxone in humans

The plasma prolactin (PRL) response to the opiate antagonist naloxone was tested in drug-free healthy volunteers (10 men, 18 regularly menstruating women who were in the late follicular phase of their ovarian cycles, and seven post-menopausal women). Naloxone hydrochloride (2 mg intravenous bolus) and placebo (normal saline) were administered on two different days in a double-blind fashion. Blood samples were collected at - 15, 0, 30, 60, 120, 180 and 240 min after the injection. In the women of reproductive age, naloxone reduced plasma PRL concentrations, whereas in the post-menopausal women and the men, naloxone resulted in no significant change. However, in the post-menopausal women treated with estrogen (intramuscular 17-β-estradiol), the opiate antagonist was able to lower plasma PRL concentrations. Thus, it appears that opiate effects on PRL secretion are influenced by the gonadal steroid environment and that estrogens may play a modulating role.