Gonadotropin secretion in ovariectomized pony mares treated with dexamethasone or progesterone and subsequently with dihydrotestosterone

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Twelve long-term ovariectomized (OVX) pony mares were used to determine the effects of dexamethasone (DEX) or progesterone (PR) on concentrations of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in daily blood samples and after administration of gonadotropin releasing hormone (GnRH). All mares were subsequently administered dihydrotestosterone (DHT) to determine if DEX or PR treatment altered the FSH or LH response to this androgen. Daily blood sampling was started on day 1. After a pretreatment injection of GnRH on day 5, four mares were administered DEX at 125 μg/kg of body weight (BW), four mares were administered PR at 500 μg/kg of BW and four mares were administered vehicle. Injections were given subcutaneously in vegetable shortening daily through day 14. After a second injection of GnRH on day 15, all mares were administered DHT in shortening at 150 μg/kg of BW. Injections of DHT were given daily through day 24. A final injection of GnRH was given on day 25. Treatment of mares with DEX 1) reduced (P<.01) daily LH secretion and briefly increased (P<.05) daily FSH secretion and 2) increased (P<.01) the FSH response to exogenous GnRH. Treatment of mares with PR had no effect on daily LH secretion but increased (P<.05) daily FSH secretion and increased (P<.01) the FSH response to exogenous GnRH. Subsequent treatment of all mares with DHT (1) decreased (P<.01) daily LH and FSH secretion, 2) decreased (P<.01) the LH response to exogenous GnRH and 3) increased (P<.01) the FSH response to exogenous GnRH in all three groups. There was no interaction of previous DEX or PR treatment with DHT treatment. We conclude that DEX and PR have qualitatively similar effects on FSH secretion, whereas at the doses used, DEX was a potent inhibitor of daily LH secretion. Moreover, the effects of the nonaromatizable androgen, DHT, on LH and FSH secretion were similar to those reported previously for testosterone. © 1988.

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Domestic Animal Endocrinology

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