Changes in oxytocin receptor in bovine preovulatory follicles between the gonadotropin surge and ovulation.

In cattle, production of oxytocin by granulosa cells of preovulatory follicles is induced by the LH/FSH surge and intrafollicular oxytocin increases dramatically toward the end of the interval between the surge and ovulation. We reported previously that oxytocin modulates steroid production by both theca and granulosa cells obtained from bovine preovulatory follicles, implying actions of oxytocin on both cell types of preovulatory follicles. The objective of the present study was to examine the temporal expression of oxytocin receptor mRNA and protein in both theca and granulosa cells of bovine periovulatory follicles. To induce luteal regression and initiate a follicular phase, heifers were injected with prostaglandin F(2alpha) on Day 6 or 7 of the estrous cycle and 36 h later, a GnRH analogue was administered to induce the LH/FSH surge. The periovulatory follicle was isolated at 0, 3.5, 12, or 24 h after GnRH injection. A significant increase in the levels of mRNA for oxytocin was detected in granulosa, but not theca, cells of periovulatory follicles at 12 and 24 h after GnRH injection, relative to time 0. In contrast, the levels of oxytocin receptor mRNA and specific binding sites for oxytocin in granulosa cells had decreased significantly at 12 and 24 h post-GnRH. In theca cells, the levels of oxytocin receptor mRNA were significantly lower at 12 and 24 h compared with values at 3.5 h, but specific binding of oxytocin to thecal cell membranes was not different at any time point. Immunopositive staining for oxytocin receptor was localized to both the theca and granulosa cell layer of periovulatory follicles at all four times of follicle isolation. These results suggest the direct action of oxytocin on both theca and granulosa cells of bovine periovulatory follicles through binding to its receptor, supporting the hypothesis that follicular oxytocin plays an important role(s) in the regulation of the final stage of follicular development. Down-regulation of oxytocin receptor mRNA and oxytocin binding may serve to temporally limit the actions of oxytocin on the preovulatory follicle.