EFFECT OF OVARIAN-STEROID HORMONES AND THE PRESENCE OF THE FETUS ON OXYTOCIN GENE-EXPRESSION IN THE UTERUS

Oxytocin (OT) is a neurohypophysial hormone with potent stimulating ac tivity of the pregnant uterus, but its physiological role in parturiti on is still unclear. Recently, OT was found to be synthesized in the p regnant uterus, indicating that OT originating from the uterus, not fr om the posterior pituitary gland, may trigger the onset of labour. In order to define the factors responsible for the induction of uterine O T, the effect of ovarian steroid hormones and conceptus on the inducti on of OT mRNA in the rat uterus was examined by Northern and dot blot hybridization analysis. OT mRNA in the uterus started to increase on d ay 14 of pregnancy and showed very high levels at the time of parturit ion. Uterine OT mRNA was not altered by any steroid treatment, oestrad iol-17 beta (0.2 mu g), progesterone (4 mg) or both in combination, fo r 6 days. The gravid horn of the uterus had 3.6-fold as much OT mRNA a s the non-gravid horn on day 21 of pregnancy in hemipregnant rats with one ligated oviduct. The ovarian steroid hormones could not induce ac cumulation of OT mRNA in the uterus of ovariectomized rats, at least u nder the conditions used, but the presence of a conceptus may be criti cal for the very high levels of OT mRNA.