Use of buserelin to induce ovulation in the cyclic mare

Inducing ovulation in a cyclic mare is often necessary. For this purpose, h CG has been used commonly, but the response can be reduced after successive administrations. The aims of this study were to test the effectiveness of buserelin in hastening ovulation in estrus mares, and its influence on fert ility; and to investigate the effect of treatment on LH secretion. Five cro ssover trials were designed to compare the effect of two treatments: busere lin (40 mug in 4 doses iv at 12 h intervals) vs placebo (Experiments 1 and 2); buserelin 40 mug tin 4 doses iv) vs 20 mug (Experiment 3); buserelin (4 doses of 20 mug iv) vs hCG(1 dose of 2,500 IU iv) (Experiment 4); or buser elin (3 doses of 13.3 mug at 6 h interval) vs hCG (Experiment 5). In Experi ment 2, blood samples were taken hourly until ovulation, for LH measurement s. In Experiment 1, buserelin treatment significantly hastened ovulation. R eduction of the dose by half(Experiment 3) did not alter the effectiveness. In Experiments 4 and 5, buserelin was as effective as hCG in inducing ovul ation between 24 and 48 h after initiation of treatment. Buserelin treatmen t induced a rise in LH concentration during the 48 h period of the experime nt, and LH concentrations before ovulation were significantly higher in bus erelin treated cycles than in placebo cycles. These experiments demonstrate d the usefulness of two new protocols of administration of buserelin, as an alternative to hCG for induction of ovulation. One hypothesis explaining t he mechanism of action is that the persistant rise in LH concentration coul d modify the ratio of biological/immunological LH, as it occurs physiologic ally, thereby hastening ovulation. (C) 2001 by Elsevier Science Inc.