Follicular growth and development and gonadotropin response of mares treated with dihydrotestosterone and estradiol benzoate

An experiment was conducted to determine the effects of dihydrotestosterone (DHT) and estradiol benzoate (EB) given simultaneously on concentrations of FSH and LH and the associated follicular activity, and to monitor the subsequent recovery of the gonadotropes and follicular growth following treatment with either DHT or EB. Twelve mares were assigned to daily treatments of DHT (150 Fg/kg BW) plus EB (22 Fg/kg BW) from d 0 through 30 (Phase I). Beginning on d 31, mares were randomly assigned to one of three treatment groups: 1) oil, 2) DHT (150 Fg/kg BW), or 3) EB (22 Fg/kg BW). On d 30, 44, and 58, GnRH was administered to determine the response of FSH and LH. In phase I, DHT plus EB decreased (P = 0.0001) concentrations of LH and FSH. In phase II (d 31 to 66), DHT decreased (P = 0.0001) LH and FSH concentrations. Estradiol benzoate decreased (P= 0.0001) concentrations of FSH but increased (P = 0.0001) concentrations of LH. There was a significant treatment x day x minute interaction of FSH (P = 0.0001) and LH (P = 0.0002) concentrations in response to administration of GnRH on d 30, 44, and 58. The number of medium follicles (P = 0.0001), large follicles (P = 0.0012) and the size of the largest follicle per day (P = 0.0001) decreased during phase I. During phase II, there was a treatment x day interaction (P = 0.0001) for medium follicles and large follicles. Control mares had more large follicles on d 57 through d 63 whereas EB treated mares had more large follicles on d 45, 49, and 51. Control mares had increased size of the largest follicles at d 57 to 63. Mares in the DHT and EB groups had prolonged (P = 0.01) ovulation intervals as compared with controls. It is concluded that the combination of DHT plus EB suppresses gonadotropin concentrations and the associated follicular activity in mares. Also, concentrations of both gonadotropins must be restored to normal for growth of an ovulatory size follicle and the subsequent ovulation of that follicle. v INTRODUCTION The estrous cycle of the mare has been a point of extensive research. It is generally accepted that FSH and LH are needed for normal growth and ovulation of ovarian follicles in mares. Concentrations of FSH and LH fluctuate throughout the estrous cycle. High concentrations of FSH are needed during diestrus for the growth of the preovulatory follicle. Increases in LH concentration are required in mares for the maturation and ovulation of the preovulatory follicle. Steroid hormones, produced from the ovaries, regulate the gonadotropins and the ovarian response to them. Garza et al. (1986b) reported that estradiol benzoate (EB) plus dihydrotestosterone (DHT) administered in combination to ovariectomized mares results in increased plasma concentrations of LH and increased LH response to exogenous GnRH. In contrast, these two steroids decreased plasma FSH concentrations and the FSH response to exogenous GnRH. Intact mares administered DHT alone were reported to have increased plasma FSH concentrations and no effect of LH concentrations (Thompson et al., 1983b), yet in ovariectomized mares, plasma FSH and LH concentrations decreased (Thompson et al., 1991). The secretion of FSH is increased in response to GnRH in DHT-treated mares, but LH secretion is decreased (Thompson et al., 1983b; 1991). Treatment with EB alone in intact mares is reported to decrease FSH concentrations and increase LH concentration after luteolysis (Burns and Douglas, 1981). Evans et al. (1982) reported that EB-treated mares had variable follicular growth and ovulation. The present experiment was conducted 1) to determine if the combination of DHT and EB given simultaneously would decrease the levels of LH and FSH and follicular activity to mimic that of an anestrous mare, and 2) to monitor the subsequent recovery of the gonadotropes and follicular growth following treatment with oil, DHT, or EB.