Sex steroids modulate prolactin response to naloxone in postmenopausal women

To evaluate whether ovarian steroids modify the prolactin (PRL) response to opioid receptor blockade, the effects of naloxone infusion (1.6 mg/h for 4 h) on PRL secretion were studied in 5 postmenopausal women. Naloxone infusion was performed in basal conditions and after chronic oral treatment with conjugated estrogens (CE) (1.25 mg/day, for 20 days) or CE plus medroxyprogesterone acetate (MPA) (10 mg/day, for 20 days). Under basal conditions, 17 beta-estradiol, estrone, gonadotropin, and PRL plasma levels were in the normal range for postmenopausal women, and naloxone failed to affect PRL secretion. Naloxone induced a significant PRL increase after CE treatment alone (p less than 0.001) or in combination with MPA (p less than 0.001). The increase was significantly higher (p less than 0.05) after CE + MPA treatment than after CE treatment alone. These data suggest that steroids modulate the stimulatory effect of naloxone on PRL secretion in postmenopausal women.