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 pr egnancy, 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 pr ogesterone 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 dur ing a provocative stimulus to OT release, such as cholecystokinin (CCK ). Adult ovariectomized Sprague-Dawley rats were implanted on day 1 wi th either estrogen or empty silastic capsules, on day 3 with progester one or empty capsules, and on day 14 progesterone or empty capsules we re removed. Forty-eight hrs after removal of the progesterone capsules , plasma OT was measured before and after i.v. injection of 10 mu g/kg of CCK. At the completion of the study, pituitary glands were removed and OT peptide was measured. No significant differences were found be tween the sham and hormone-treated animals either in their basal or CC K-stimulated plasma OT levels or their pituitary content of OT peptide . Although sequential exposure to estradiol and progesterone followed by withdrawal of progesterone has been shown previously to increase PV N OT mRNA, neither pituitary OT immunoreactivity nor basal and CCK-sti mulated release of plasma OT is affected by this treatment. Although t he mechanism of this steroid effect is not yet understood, our observa tions suggest a unique action of gonadal steroids upon PVN OT neurons.