Sequential estrogen and progesterone (P) followed by P withdrawal increases the level of oxytocin messenger ribonucleic acid in the hypothalamic paraventricular nucleus of the male rat.

We previously reported that alterations in gonadal steroid hormones can influence oxytocin (OT) messenger ribonucleic acid (mRNA) expression in the hypothalamic paraventricular nucleus (PVN) of the steroid-treated ovariectomized rat, the pregnant rat, and the lactating rat. Specifically, OT mRNA in the PVN of the female rat was increased by sequential estrogen and progesterone (P) followed by withdrawal of P. In this study we investigated whether this same steroid paradigm increases OT mRNA levels in the castrate rat. Castrate rats were administered either sequential estrogen and P followed by P withdrawal or no steroid treatment prior to sacrifice. PVN OT mRNA was measured by Northern blot hybridization and pituitary OT peptide content by radioimmunoassay (RIA). The steroid-treated rats had increased OT mRNA levels compared to the sham treated rats (p < 0.04), but pituitary OT peptide content was not significantly altered. We conclude that sequential estrogen and P followed by P withdrawal increases PVN OT mRNA in the castrate, as well as the ovariectomized, rat.