The discovery that oxytocin is synthesized and stored in corpora lutea of ruminants has fostered a renewed interest in the possible roles of oxytocin in ovarian function. In the present study we describe the distribution of binding sites for oxytocin in the guinea-pig ovary. Sections were reacted with a radioiodinated oxytocin antagonist (125I-labelled OTA) to yield autoradiograms on film. Specific binding sites for oxytocin were defined as those which bound 0·05 nmol 125I-labelled OTA/l and where 1 μmol non-radioactive oxytocin/1 displaced the radioactivity. 125I-Labelled OTA consistently labelled the ovarian stroma and the theca interna, but not the corpora lutea, the granulosa cells or the theca externa. The amount of 125I-labelled OTA bound to ovarian stroma and theca interna was high in animals killed during dioestrus, and low during and shortly after oestrus. These data suggest that the binding sites are regulated by steroid hormone levels and that in the guinea-pig ovary oxytocin could exert a role in follicular steroidogenesis, maturation or ovulation rather than in luteal function. Oxytocin-binding sites were also shown in the uterus but their numbers varied only slightly during the cycle.
Journal of Endocrinology (1991) 131, 421–426