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16 stallions were castrated and 30 days later assigned to 1 of 4 treatments: testosterone propionate (T, 175 μg/kg BW), estradiol-17β-3-benzoate (E, 44 μg/kg BW), the combination of both steroids (TE) or control (safflower oil). These dosages were administered every other day for 18 days (low dose) and then were doubled (high dose). Treatment at the high dose was continued for 27 days and the geldings were sacrificed. Treatment at the low dose resulted in concentrations of testosterone and estradiol in serum similar to those in intact stallions during the breeding season. Concentrations of LH and FSH in peripheral serum increased by 4-10-fold after castration and stabilized in ~ 20 days. Concentrations of LH in serum were suppressed by T and even more rapidly by TE. Estradiol alone increased concentrations of LH in serum at the low dose. Compared with levels in control geldings, concentrations of LH in the pituitary glands were suppressed by ~ 50% in T or TE treated geldings, whereas they were increased by 10-fold in E treated geldings. Concentrations of FSH in serum were rapidly suppressed by E or TE. Testosterone alone suppressed concentrations of FSH in serum at the high dose. Compared with control geldings, all treatments resulted in elevated concentrations of FSH in the pituitary gland; the effect of T was ~ 3-fold greater than for E. The magnitude of increase in concentrations of LH and FSH in serum after an injection of GnRH generally reflected the concentrations of these hormones in the pituitary. However, E in combination with T decreased release of LH in response to GnRH, even though pituitary concentrations of LH were similar in T and TE treated geldings. Estradiol also decreased the release of FSH in response to GnRH in E treated geldings, compared with controls, even though pituitary concentrations of FSH were ~ 2-fold greater.

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Biology of Reproduction

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