Hormonal influences on the duration of postpartum maternal responsiveness in the rat.

These studies investigated the role of gestational hormones in controlling the duration of the postpartum period of elevated maternal responsiveness in rats. In the first study, females had pregnancies terminated by Caesarean section, and had ovaries removed on day 16 or 22 of pregnancy. Removal of the ovaries did not prevent the sustaining of an elevated maternal responsiveness for 7 days after pregnancy termination on either day. In the second study, changes in concentrations of estrogen and progesterone during pregnancy, and following parturition, were determined by radioimmunoassay. Levels of these hormones postpartum were found to decline to virgin levels by 7 days postpartum and could not therefore explain the elevated maternal behavior seen at this time. In the third study, ovariectomized virgins were given a pregnancy-mimicking schedule of estradiol and progesterone, with and without prolactin, and were tested for maternal behavior 2 and 7 days later. A 22-day estradiol-progesterone schedule induced rapid maternal behavior within 2 days, but did not sustain it for 7 days. Adding prolactin to the 22-day estradiol-progesterone schedule sustained some components of maternal behavior, notably nest-building and retrieval, but not maternal behavior as a whole.