The effect of a GnRH antagonist on endocrine and seminal parameters in stallions

Relatively little is known about endocrine control of reproduction in the stallion, but gonadotropins are thought to be central in regulating spermatogenesis and libido. The ability to effectively antagonise GnRH, and thereby gonadotropins, is therefore important both in further investigations of hormonal control of reproduction in stallions, and for clinical applications. In the present study four pony stallions were treated with a potent GnRH antagonist, Antarelix. Their libido, seminal parameters, and hormonal profiles were compared with those recorded before administration of the antagonist. Plasma concentrations of gonadotropins, testosterone and estradiol decreased by 48 h after antagonist administration, with estradiol and FSH being most consistently suppressed, and remained at reduced concentrations for 4 weeks. Spermatozoal motility, numbers and morphology were not significantly affected by treatment, but increasing numbers of round spermatogenic cells were seen in the ejaculate as the trial progressed. Libido was assessed by the time taken for the stallions to regain an erection in the presence of a mare after ejaculation (refractory period). The refractory period increased significantly after treatment. When the stallions were castrated 8 weeks after antagonist treatment, histological evidence of testicular degeneration was present. We concluded that use of this antagonist showed promise as a valuable research tool in modulating changes in circulating hormone concentrations in stallions. Reversibility of the effects on libido and testicular changes need further investigation.