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Androgen and Estradiol Effects on Gonadotropin Secretion and Response to GnRH in Ovariectomized Pony Mares

Louisiana Stare University Agricultural Center^,3, Baton Rouge 70803

Abstract

In Exp. 1, 16 long-term ovariectomized pony mares were used to determine the effects of treatment with estradiol benzoate (EB) and dihydrotestosterone (DHT) benzoate alone, and in combination, on secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in daily blood samples and after three consecutive injections of gonadotropin releasing hormone (GnRH). Administration of EB alone, or in combination with DHT, every other day for 11 d reduced (P<.05) concentrations of FSH and increased (P<.05) concentrations of LH in daily blood samples, and increased (P<,05) the secretion of both gonadotropins after administration of GnRH. Treatment with DHT alone had no effect (P>.10) on LH or FSH concentrations in daily blood samples and no effect on the LH response to exogenous GnRH. There was no interaction (P>.10) between DHT and EB treatment for any hormonal characteristic. In Exp. 2, the control mares and mares treated with DHT in Exp. 1 were equally allotted to treatment with vehicle or testosterone propionate (TP) every other day for six injections, and then GnRH was administered as in Exp. 1. Treatment with TP had no effect (P>.10) on LH or FSH concentrations in daily blood samples but increased (P<.05) the FSH response to exogenous GnRH, confirming our findings in previous experiments. It is concluded that the TP-induced stimulation of FSH secretion after exogenous GnRH in ovariectomized mares may involve estrogens produced from aromatization of the injected androgen. However, because there are differences between TP and EB effects on LH and FSH secretion, it is likely that some of the effects of TP are mediated by testosterone itself or via some other metabolite(s) other than DHT or estradiol.

¹ Address reprint requests to D. L. Thompson, Jr.

² Dept. of Vet. Sci.

³ Dept. of Anim. Sci., Louisiana Agr. Exp. Sta.