Immunolocalization of oxytocin and neurophysin in the mare uterus

The presence of oxytocin in the mare uterus was determined by radioimmunoassay of tissue extracts and uterine lavage fluid, and by immunohistochemistry. Samples were obtained from mares during the oestrous cycle and on day 14 of pregnancy. Immunoreactive oxytocin was detected in extracts of endometrium and myometrium, and in uterine lavage fluid. In tissue sections, the luminal epithelium, the epithelium of the superficial, but not deep, glands, blood vessel walls and myometrium all stained positively for oxytocin, whereas only the luminal epithelium and epithelium of the superficial glands stained positively for neurophysin. These results indicate that oxytocin is synthesized in the epithelium but not in the myometrium. Immunostaining for oxytocin was most intense in early pregnancy and least intense during mid- to late dioestrus. The great variation in staining density during different reproductive states indicates that uterine oxytocin may have an autocrine or paracrine role in controlling uterine function.