Effect of labor induction on the expression of oxytocin receptor, cytochrome P450 aromatase, and estradiol receptor in the reproductive tract of the late-pregnant ewe

In this study, we investigated the timing of changes in aromatase, estradiol receptor, and oxytocin receptor expression in ovine uterine and placental tissues before parturition. Labor was induced by betamethasone injection into the fetus on Days 130-132 of pregnancy. Tissue samples were collected at injection and then every 14 h until labor (56 h) from four ewes at each time point. Samples were analyzed for aromatase, estradiol receptor, and oxytocin receptor expression by in situ hybridization; for oxytocin binding to its receptor using a specific antagonist; and for estradiol receptor quantitation by immunocytochemistry. Aromatase mRNA expression increased by 14 h postinjection (p < 0.02) in the fetal villi and remained high until labor. Expression of estradiol and oxytocin receptor mRNAs was unchanged in myometrium but increased in the endometrial luminal epithelium by 28 h (p < 0.05) and remained high until labor. Estradiol receptor protein concentration increased modestly at labor while oxytocin receptor binding in the luminal epithelium changed in parallel to the mRNA concentration. In conclusion: 1) induction of aromatase may facilitate the expression of endometrial estradiol and oxytocin receptors in the placentome, 2) changes in endometrial rather than myometrial oxytocin receptor may be important in inducing parturition, and 3) the transcription of estradiol receptor and oxytocin receptor in the uterine epithelium are positively correlated during parturition.