Control of follicular development and luteal function in the mare: Effectsof a GnRH antagonist

Control of the equine estrous cycle was studied by suppressing gonadotropin secretion by administration of a GnRH antagonist to cyclic pony nares. Fou r mares received vehicle (control cycle) or a GnRH antagonist, Antarelix(TM ) (100 mug/kg) on Day 8 of diestrus, and blood samples were collected at 15 -min intervals from 0 to 16 h 24 to 36 h and daily until the next ovulation . Ovarian activity was monitored by transrectal ultrasonography, and measur ement of plasma concentrations of progesterone and estradiol. Antagonist tr eatment eliminated large diestrous pulses of LH. Progesterone concentration s had fallen significantly in all mares by the day after treatment and, in three of the four mares, remained low until luteolysis. However timing of l uteolysis (ie., progesterone concentrations <1 ng/mL) was not affected by a ntagonist treatment. The preovulatory surges of estradiol and LH were signi ficantly delayed in the treatment cycle, as was the appearance of a preovul atory follicle >30 mm. Cycle length was significantly longer during the tre atment than the control cycle. These results show that treatment of diestro us mares with a GnRH antagonist attenuated progesterone secretion, indicati ng a role for LH in control of CL function in the mare, and delayed ovulati on presumably because of lack of gonadotropic support. (C) 2000 by Elsevier Science Inc.