INCREASED ACCUMULATION OF OXYTOCIN MESSENGER-RIBONUCLEIC-ACID IN THE HYPOTHALAMUS OF THE FEMALE RAT - INDUCTION BY LONG-TERM ESTRADIOL AND PROGESTERONE ADMINISTRATION AND SUBSEQUENT PROGESTERONE WITHDRAWAL

To examine a possible role for gonadal steroid hormones in the enhanced accumulation of hypothalamic oxytocin (OT) messenger RNA (mRNA) and peptide in late pregnancy, we used an established model (22) in which sequential administration of estradiol (E(2)) and progesterone (P) SILASTIC capsules to ovariectomized rats is followed by removal of P. Long term and sustained E(2) combined with abrupt P withdrawal mimics the gonadal steroid hormone pattern of late gestation in the rat (22). Using this paradigm, we demonstrate that OT mRNA is increased in the rat hypothalamus after long term P treatment, but only in the presence of E(2) and only when P capsules are removed 48 h before killing. Furthermore, we show that P replacement in primiparous rats during late pregnancy blunts the increase in OT mRNA normally observed at the end of gestation. Our results support a role for E(2) priming and P withdrawal in the enhanced accumulation of OT mRNA in the hypothalamus of the female rat.