Uterine oxytocin gene expression. I. Induction during pseudopregnancy and the estrous cycle.

We have recently demonstrated that the gene encoding the hypothalamic peptide oxytocin (OT) is highly expressed in the rat endometrial epithelium during the last 4 days of pregnancy. Here, we show that uterine OT gene expression is also induced during the proestrous phase of the estrous cycle and after induction of pseudopregnancy. In mature female rats, OT mRNA levels increased more than 10-fold between diestrus and proestrus and remained elevated at estrus. The levels attained at estrus corresponded to about 1/20th of the levels present at term. In immature rats rendered pseudopregnant by treatment with pregnant mare serum and hCG, uterine OT mRNA levels rose steadily and reached a maximum on day 14 of pseudopregnancy, corresponding to about 1/8th of the levels observed on day 21 of normal pregnancy. Oil-induced decidualization of the left uterine horn prolonged pseudopregnancy and maintained OT mRNA levels in both uterine horns until day 19 of pseudopregnancy. These changes were tissue specific, as hypothalamic OT mRNA levels remained essentially unaffected. The present findings demonstrate that either spontaneous or induced changes in endogenous steroid levels are capable of eliciting important changes in uterine, but not hypothalamic, OT gene expression.