Effect of the gonadotropin surge on steroid receptor regulation in preovulatory follicles and newly formed corpora lutea in the cow.

The objective of the study was to characterize the mRNA expression patterns of specific steroid hormone receptors namely, estrogen receptors (ESRRA-estrogen related receptor alpha and ESRRB-estrogen related receptor beta) and progesterone receptors (PGR) in superovulation-induced bovine follicles during the periovulation and subsequent corpus luteum (CL) formation. The bovine ovaries (n = 5 cow / group), containing preovulatory follicles or early CL, were collected relative to injection of the gonadotropin-releasing hormone (GnRH) at (I) 0 h, (II) 4 h, (III) 10 h, (IV) 20 h, (V) 25 h (preovulatory follicles) and (VI) 60 h (CL, 2-3 days after induced ovulation). In this experiment, we analyzed the steroid receptor mRNA expression and their localization in the follicle and CL tissue. The high mRNA expression of ESRRA, ESRRB, and PGR analyzed in the follicles before ovulation is significantly reduced in the group of follicles during ovulation (25 h after GnRH), rising again significantly after ovulation in newly formed CL, only for ESRRA and PGR (P < 0.05). Immunohistochemically, the nuclei of antral follicles' granulosa cells showed a positive staining for ESRRA, followed by higher activity in the large luteal cells just after ovulation (early CL). In contrast, the lower PGR immunopresence in preovulatory follicles increased in both small and large luteal cell nuclei after follicle ovulation. Our results of steroid receptor mRNA expression in this experimentally induced gonadotropin surge provide insight into the molecular mechanisms of the effects of steroid hormones on follicular-luteal tissue in the period close to the ovulation and subsequent CL formation in the cow.