LOCALIZATION OF OXYTOCIN RECEPTOR MESSENGER-RNA IN THE OVINE UTERUS DURING THE ESTROUS-CYCLE AND EARLY-PREGNANCY

A synthetic 45-mer oligonucleotide corresponding to part of the ovine endometrial oxytocin receptor cDNA was hybridized to sections of ovine uterus collected from 40 ewes at different stages during the oestrous cycle, the first 3 weeks of pregnancy and seasonal anoestrus. The qua ntity of oxytocin receptor mRNA was measured as the optical density (O D) value on autoradiographs using image analysis. Message first appear ed in the luminal epithelium on days 14-15 of the cycle, increasing to a peak OD of 0.48 at oestrus and decreasing again between days 2 and 5. Oxytocin receptor mRNA in the superficial glands, deep glands and c aruncular stroma increased between day 15 and oestrus to peak OD value s of 0.17, 0.11 and 0.11 respectively, declining again by day 2 and re aching basal values (OD<0.015) by day 5. Hybridization to the myometri um tended to rise from a mean OD value of 0.01 on days 2-15 to a peak of 0.03 +/- 0.01 (mean +/- S.E.M.) on days 0-1, but the change was not significant. In pregnant ewes there was no detectable oxytocin recept or mRNA on days 14-15 in any region, but hybridization to the luminal epithelium was present in two of three ewes on day 21. In anoestrous e wes oxytocin receptor mRNA concentrations in all areas of the endometr ium were approximately half those measured at oestrus. Optical density readings for oxytocin receptor mRNA in the various uterine compartmen ts were compared with measurements of oxytocin receptors in the same r egions as assessed by binding studies using the I-125-labelled oxytoci n antagonist d(CH2)(5)[Tyr(Me)(2),Thr(4),Tyr-NH29]-vasotocin (I-125-la belled OTA). In the endometrium, receptor mRNA and I-125-labelled OTA binding patterns changed in parallel, and both sets of measurements we re significantly correlated (P<0.01). In the myometrium, a significant increase in I-125-labelled OTA binding occurred at oestrus; this was not accompanied by a similar increase in oxytocin receptor mRNA hybrid ization. This study helps to confirm that the previously identified cD NA clone is derived from the ovine oxytocin receptor, as patterns of o xytocin receptor mRNA expression in the endometrium closely resembled those of oxytocin binding. Maximum expression and binding both occurre d at oestrus, suggesting that regulation of the oxytocin receptor gene in the uterus occurs principally at the transcriptional, rather than at the translational, level. Failure to detect a significant increase in myometrial mRNA expression at oestrus may indicate that the endomet rial and myometrial oxytocin receptors are of different isoforms.