UTERINE OXYTOCIN GENE-EXPRESSION .2. INDUCTION BY EXOGENOUS STEROID-ADMINISTRATION

As we have recently shown, the gene encoding the hypothalamic nonapept ide oxytocin (OT) is expressed in the rat endometrial epithelium durin g late pregnancy and the estrous phase of the estrous cycle. To invest igate the role of ovarian steroids in the regulation of uterine OT gen e expression, Silastic capsules containing estradiol or progesterone w ere implanted into immature ovariectomized rats. Exposure to estradiol alone for 2 days caused a significant rise in OT mRNA. Administration of progesterone alone was without effect. However, a strong synergism was observed when the two hormones were applied together; progesteron e potentiated the effect of estradiol by a factor of 7. In animals tre ated with steroids for 4 days, the removal of either the estradiol or progesterone capsule after day 2 led to a decrease in the total amount of OT mRNA accumulation, implying that the continued action of both s teroids was required to achieve maximal OT mRNA levels. Immunocytochem ical analysis demonstrated that the main site of steroid-induced uteri ne OT gene expression is the endometrial epithelium, the same site whe re endogenously induced OT gene expression occurs at the end of pregna ncy. The OT mRNA levels achieved after 4 days of treatment with both s teroids were comparable to those achieved at estrus or during pseudopr egnancy, but corresponded to less than 20% of the levels present in th e uterus on day 21 of pregnancy. These data suggest that in the uterus , the synergistic action of ovarian steroids represents an important, but probably not exclusive, regulator of OT gene expression.