PARENT SESSION
OVARY - C

Wednesday, August 4, 2004
10:30 AM–12:30 PM
Buchanan Courtyard

(794) DEVELOPMENT OF OVARIAN FOLLICULAR CYSTS AFTER TREATMENT OF CATTLE WITH A GnRH RECEPTOR ANTAGONIST (ACYLINE) AND ESTRADIOL.

Gumen, Ahmet¹, Powell, Justin¹, Olbrich, Debra¹, Wiltbank, Milo^{1, 2}, ¹ Department of Dairy Science, Madison, WI² Endocrinology-Reproductive Physiology Program, Madison, WI

ABSTRACT- Many mammals, including cattle, can develop follicular cysts but the physiological mechanisms leading to this condition remain undefined. We hypothesized that an estradiol-induced GnRH surge would result in development of follicular cysts if the LH surge were prevented by treatment with a GnRH receptor antagonist (Acyline; GnRH-ANT). We further hypothesized that continuous estradiol (E2) exposure after this GnRH-ANT treatment, until a new estrogen-active follicle was present on the ovary, would increase the effectiveness of this treatment in inducing follicular cysts. A preliminary dose response study was performed and a dose of 10g GnRH-ANT per kg body weight was found to completely block GnRH action for 72 h. In the main study, heifers (n = 34) on D 7 of the estrous cycle were treated with PGF2. Heifers were randomly assigned to one of three treatments: untreated (Control), GnRH-ANT, and GnRH-ANT plus E2 implant for 12 d (GnRH-ANT+E2). Heifers in GnRH-ANT and GnRH-ANT+E2 groups were treated with GnRH-ANT 24 h after PGF2. All three groups were treated with 2 mg of estradiol benzoate (EB) 2 h after GnRH-ANT treatment. There was no difference in maximal follicle size at the time of EB injection among groups (13.9 ± 0.4 mm). After EB treatment all control heifers (n = 10) had an LH surge (peak = 19.6 ± 2.5 ng/ml), but an LH surge was not detected in any of the heifers treated with GnRH-ANT. All control heifers ovulated 2.3 ± 0.2 d after EB (ovulatory follicle diameter = 14.6 ± 0.5 mm), but none of the heifers treated with GnRH-ANT ovulated during the 7 d following EB treatment. However, 50% of the GnRH-ANT (6 of 12) and 8% of the GnRH-ANT+E2 (1 of 12) heifers subsequently ovulated (new follicle wave, 6.7 ± 0.3 d; ovulation, 13.0 ± 0.7 d after EB). The heifers that became anovular (50% of GnRH-ANT and 92% of GnRH-ANT+E2) developed large follicles (22.1 ± 0.7 mm) with high circulating estradiol concentrations (∼ 8 pg/ml). These results indicate that prevention of the LH surge and ovulation after an estradiol-induced GnRH surge can induce follicular cysts. Nevertheless, the greater rate of follicular cyst development during continuous estradiol exposure is consistent with the idea that estradiol from developing follicles may be important in maintaining unresponsiveness of the hypothalamus during development and maintenance of follicular cysts

KEY WORDS: follicular cysts, estradiol, anovulation, GnRH-receptor antagonist