Prevention of the Polycystic Ovarian Phenotype and Characterization of Ovulatory Capacity in the Estrogen Receptor-α Knockout Mouse

Ovarian-derived estradiol plays a critical endocrine role in the regulation of gonadotropin synthesis and secretion from the hypothalamic-pituitary axis. In turn, several para/autocrine effects of estrogen within the ovary are known, including increased ovarian weight, stimulation of granulosa cell growth, augmentation of FSH action, and attenuation of apoptosis. The estrogen receptor-a (ERa) is present in all three components of the hypothalamic-pituitaryovarian axis of the mouse. In contrast, estrogen receptor-b (ERb )i s easily detectable in ovarian granulosa cells but is low to absent in the pituitary of the adult mouse. This distinct expression pattern for the two ERs suggests the presence of separate roles for each in the regulation of ovarian function. Herein, we definitively show that a lack of ERa in the hypothalamic-pituitary axis of the ERa-knockout (aERKO) mouse results in chronic elevation of serum LH and is the primary cause of the ovarian phenotype of polycystic follicles and anovulation. Prolonged treatment with a GnRH antagonist reduced serum LH levels and prevented the aERKO cystic ovarian phenotype. To investigate a direct role for ERa within the ovary, immature aERKO females were stimulated to ovulate with exogenous gonadotropins. Ovulatory capacity in the immature aERKO female was reduced compared with age-matched wild-type (14.5 6 2.9 vs. 40.6 6 2.6 oocytes/animal, respectively); however, oocytes collected from the aERKO were able to undergo successful in vitro fertilization. A similar discrepancy in oocyte yield was observed after superovulation of peripubertal (42 days) wild-type and aERKO females. In addition, ovaries from immature superovulated aERKO females possessed several ovulatory but unruptured follicles. Investigations of the possible reasons for the reduced number of ovulations in the aERKO included ribonuclease protection assays to assess the mRNA levels of several markers of follicular maturation and ovulation, including ERb, LHreceptor, cyclin-D2, P450-side chain cleavage enzyme, prostaglandin synthase-2, and progesterone receptor. No marked differences in the expression pattern for these mRNAs during the superovulation regimen were observed in the immature aERKO ovary compared with that of the wild-type. Serum progesterone levels just before ovulation were slightly lower in the aERKO compared with wild-type. These studies indicate that treatment of aERKO females with a GnRH antagonist decreased the serum LH levels to within the wild-type range and concurrently prevented development of the characteristic ovarian phenotype of cystic and hemorrhagic follicles. Furthermore, a lack of functional ERa within the ovary had no effect on the regulation of several genes required for follicular maturation and ovulation. However, the reduced numbers of ovulations following the administration of exogenous gonadotropins in the aERKO suggests an intraovarian role for ERa in follicular development and ovulation. (Endocrinology 140: 5855‐5865, 1999)