Effects of dihydrotestosterone administration with and without estradiol pretreatment on gonadotropin secretion in ovariectomized pony mares.

Twenty ovariectomized pony mares were used to determine if dihydrotestosterone propionate (DHTP) administration, with or without estradiol benzoate (EB) pretreatment, would have the same effects on follicle stimulating hormone (FSH) and luteinizing hormone (LH) secretion as testosterone propionate (TP) administration. All mares were given an initial injection of gonadotropin releasing hormone (GnRH) to characterize their LH and FSH response, and then two groups of mares (n = 4/group) were administered EB (22 micrograms/kg of body weight), two groups were administered vehicle (safflower oil) and a fifth group was administered TP (175 micrograms/kg of body weight) daily for 10 days. Following a second injection of GnRH, one group of EB-treated mares and one group of oil-treated mares were administered DHTP (175 micrograms/kg of body weight) daily for 10 days; the other EB- and oil-treated mares were administered oil and the TP-treated mares were continued on the same dose of TP for 10 days. A final injection of GnRH was then given. Treatment with EB increased (P less than .01) concentrations of LH in daily blood samples and increased (P less than .05) the LH response to exogenous GnRH. Administration of TP or DHTP reduced (P less than .05) both daily LH concentrations and the LH response to exogenous GnRH. Concentrations of FSH in daily blood samples were reduced (P less than .05) and the FSH response to exogenous GnRH was increased (P less than .05) by administration of EB alone, DHTP alone or TP.(ABSTRACT TRUNCATED AT 250 WORDS)