Parity-associated alterations of medial preoptic opiate receptors in female rats.

Preoptic area opiate receptor density was measured by quantitative autoradiography using [3H]naloxone in female rats during their first and second pregnancies and lactations and in a separate group of ovariectomized, nulliparous animals. Opiate receptor density in the medial preoptic area (MPOA) was elevated on day 12 of gestation in both primigravid and multigravid rats when compared with ovariectomized subjects. MPOA receptor density was reduced in primiparous mothers on day 5 of lactation relative to pregnancy. In contrast, receptor density in the MPOA did not decline in multiparous (second lactation) rats relative to pregnancy levels. Opiate receptor density was significantly higher on day 5 of lactation in multiparous than in primiparous mothers. No difference in receptor density was detected in the adjacent lateral preoptic area among the treatment groups. An examination of hormone titers revealed that basal prolactin levels were significantly higher in primigravid than multigravid rats, and that during lactation prolactin titers were negatively correlated with MPOA opiate receptor density in the primiparous mothers. The data demonstrate that multiple pregnancies and lactations result in changes of MPOA opiate receptor density and of circulating hormone levels. The findings are discussed in terms of the concurrent changes in neural opiate sensitivity associated with multiparity.