Administration of long term estradiol and progesterone followed by progesterone withdrawal does not alter the plasma oxytocin secretory response to cholecystokinin or the pituitary oxytocin content in ovariectomized rats

The hormone oxytocin (OT) is important for several pre- and postpartum events, including uterine contractions at parturition, the induction of maternal behavior, and milk ejection during nursing. During late pregnancy, OT mRNA is increased in the paraventricular nucleus (PVN) due to high estrogen and declining progesterone levels. Administration of sequential estrogen and progesterone to, followed by withdrawal of progesterone from, an ovariectomized rat also increases OT mRNA. However, pituitary OT peptide is not affected. In the present experiment, we determined if this steroid exposure alters peripheral OT secretion during a provocative stimulus to OT release, such as cholecystokinin (CCK). Adult ovariectomized Sprague-Dawley rats were implanted on day 1 with either estrogen or empty silastic capsules, on day 3 with progesterone or empty capsules, and on day 14 progesterone or empty capsules were removed. Forty-eight hrs after removal of the progesterone capsules, plasma OT was measured before and afte...