Prolactin response to breast stimulation in lactating women is not mediated by endogenous opioids.

Several reports have shown that the prolactin response to suckling in rats can be blunted by administration of the opiate antagonist naloxone. In order to investigate whether the prolactin response to breast stimulation in women is similarly affected by naloxone, nine healthy lactating women participated in 10 studies. Each woman served as her own control and was studied on two occasions, receiving pretreatment with either saline solution or naloxone. Prolactin was measured in the baseline state and for 60 minutes after the onset of a 20-minute period of nipple stimulation by use of the Egnell mechanical breast pump. Neither baseline nor stimulated prolactin values were different by paired t test. Thus, in contrast to rats, an opioid pathway does not appear to be involved in the prolactin response to suckling in humans.