Preoptic area opioids and opiate receptors increase during pregnancy and decrease during lactation.

Opiate receptor and endogenous opioid content were determined in pregnant, lactating, ovariectomized, and ovariectomized and subsequently estradiol- and progesterone-treated adult female rats. Levels of estradiol and progesterone produced by Silastic capsules implanted in animals of the ovariectomized, hormone-treated group were similar to natural levels of those hormones induced during pregnancy. Quantitative receptor autoradiography and radioimmunoassay were used to determine [3H]naloxone binding density and immunoreactive beta-endorphin content, respectively, in the preoptic area of the hypothalamus. Both opiate receptor binding density and beta-endorphin content in the preoptic area varied in the same direction in all experimental groups. The highest levels of both were observed during pregnancy and the lowest levels during lactation. Ovariectomy without subsequent hormone treatment produced intermediate levels of both opiate receptor and beta-endorphin. Ovariectomy with experimentally-induced estradiol and progesterone levels similar to those of pregnancy produced opiate receptor density and beta-endorphin content similar to those observed in pregnant animals. These data suggest that gonadal steroids are capable of altering function of the endogenous opiate system in the preoptic area. Moreover, preoptic area levels of opioids and opiate receptors are normally elevated during pregnancy and reduced during lactation. Since opiates are known to disrupt ongoing maternal behavior, a reduction of preoptic opiate function during lactation may be required to promote normal maternal behavior. The specific preoptic region involved in opiate regulation of maternal behavior may be illustrated by the zone of opiate receptor alteration observed herein.