2003 Volume 50 Issue 5 Pages 579-587
Oxytocin receptor (OTR) mRNA levels in the uterus dramatically increase in the near term human and rat. Estrogen is believed to be a potent stimulator of OTR mRNA expression. However, estrogen does not stimulate rat OTR mRNA expression on day 18 of pregnancy or in progesterone-treated rats. Thus, the regulation of uterine responsiveness to estrogen in the near term rat appears to be an important mediator of estrogen action. To determine the effect of altering uterine responsiveness to estrogen on OTR induction, uterine ERα and ER β mRNA levels were examined by competitive RT-PCR in pregnant and parturient rats, progesterone-treated ovariectomized (OVX) virgin rats and OVX pregnant rats. In pregnant and parturient rats, OTR mRNA levels were highest at 2200-2230 h on day 21 of pregnancy (P21pm) and during labor when compared with other groups. ERα mRNA levels significantly increased during labor compared with days 15-21 of pregnancy. Compared with control animals, ERα mRNA levels decreased significantly in OVX virgin rats implanted with tubes containing progesterone for one week; 24 h following the removal of the progesterone tubes, ERα mRNA levels were found to be similar to control levels. Estrogen treatment following OVX on day 18 of pregnancy caused increased OTR mRNA levels, whereas ovariectomy alone increased ERβ mRNA but not ERα mRNA. Results from the present study suggest that ERα and ERβ mRNA expressions are differentially regulated in the rat uterus. Moreover, during spontaneous labor our findings appear to suggest that ERα plays a more prominent role than ERβ in mediating estrogen action in the induction of uterine OTR mRNA before labor.
