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 enhance d 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) SIL ASTIC capsules to ovariectomized rats is followed by removal of P. Lon g term and sustained E(2) combined with abrupt P withdrawal mimics the gonadal steroid hormone pattern of late gestation in the rat (22). Us ing 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. Furthe rmore, we show that P replacement in primiparous rats during late preg nancy blunts the increase in OT mRNA normally observed at the end of g estation. Our results support a role for E(2) priming and P withdrawal in the enhanced accumulation of OT mRNA in the hypothalamus of the fe male rat.