Objective: To determine the effect of single long-acting doses of GnRH antagonists on reproductive function in a sheep model.
Design: Observational, model study.
Setting: University-affiliated research unit.
Animal(s): Nine intact mature Merino sheep in experiment 1 and 12 mature Merino-crossed ewes with the ovary autotransplanted to the neck in experiment 2.
Intervention(s): Synchronization of estrous cycle either with intravaginal progestins or prostaglandin F2alpha analogues and treatment with a single dose of GnRH antagonist; evaluation of reproductive activity, plasma sampling, and ovarian ultrasonography.
Main outcome measure(s): Determination of estrus behavior; plasma concentrations of P, FSH, LH, and inhibin A; and number and size of ovarian follicles.
Result(s): In both experiments, the concentrations of FSH and LH were suppressed when compared with those in control ewes. In experiment 1, the ovulatory cycles were suppressed for > or = 55 days in treated sheep. In experiment 2, there were no follicles sized > or = 5 mm in treated ewes for 50 days.
Conclusion(s): The suppression of the development of large follicles for > or = 30 days after a single injection of a long-acting GnRH antagonist provides a novel convenient method of pretreatment before COS.