Oxytocin and oxytocin receptor gene expression in the reproductive tract of the pregnant cow: rescue of luteal oxytocin production at term.

The peptide hormone oxytocin is produced both in the hypothalamus and in certain peripheral organs. The extent of extra-hypothalamic hormone synthesis in the pregnant cow has not previously been examined. In this study we have analyzed different tissues from the pregnant bovine reproductive tract and corpus luteum for the presence of mRNA encoding the oxytocin peptide as well as the oxytocin receptor. In uterine tissues oxytocin mRNA could only be detected sporadically with the help of a reverse transcription-polymerase chain reaction method, implying only very low levels of expression. The caruncles showed a consistently low level of oxytocin gene expression, which appeared up-regulated at term. However, in the corpus luteum there was a significant level of oxytocin gene expression at term, particularly following the onset of labor. The transcript levels were sufficiently high to be measurable by both RNase protection assay and by Northern hybridization; these levels imply a rescue of the oxytocin gene expression seen in the corpora lutea of cyclic and early pregnant cows. At the peptide level this expression was confirmed by immunohistochemistry. A sensitive RNase protection assay was developed to detect transcripts encoding the oxytocin receptor. Transcripts were detected in most uterine tissues, including the caruncles, with highest levels in the endometrium and myometrium at term. No transcripts could be detected in the corpus luteum at any stage of pregnancy, nor in the amnion. The results suggest the possibility of local, paracrine effects of oxytocin within the uterus of the pregnant cow. The rescue of luteal oxytocin at term could act to supplement the circulating hormone of pituitary origin.