Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content

These experiments tested the hypothesis that administration of steroid hormones to ovariectomized (OVX) mares during the vernal transition to the breeding season would influence LH and FSH secretion. Circulating gonadotropin concentrations, response to exogenous GnRH, and pituitary gonadotropin content were monitored. Experiments 1 and 2 were conducted, beginning 10 March, and 3 February, respectively, utilizing a total of 30 long-term OVX pony mares. In experiment 1, mares were administered vehicle (n = 5) or estradiol-17 beta (E2, n = 5, 5 mg/3 ml sesame oil,) twice daily for 16 days. Blood samples were collected daily for assessment of circulating LH and FSH concentrations. On Day 10 of treatment, 400 microgram GnRH were administered to all mares. LH increased significantly over days of treatment in the estradiol-treated group, but pituitary response to GnRH tended to be less than in control mares. Circulating FSH tended to decline over days of treatment in estradiol-treated mares, and the pituitary response to GnRH was significantly reduced. Pituitary LH, but not FSH, was increased on Day 16 of treatment with estradiol. In experiment 2, 20 OVX mares received, twice daily, vehicle (n = 5), E2, n = 5; 5 mg), progesterone (P4, n = 5; 100 mg), or progesterone plus estradiol (P4/E2, n = 5; 100 + 5 mg). Treatment continued for 14 days. GnRH (100 microgram) challenges were administered on Days 6 and 13 of treatment. Circulating LH was elevated by Day 13, but not by Day 6, in estradiol-treated mares; circulating FSH was not affected by steroid treatment. Estradiol increased LH response to GnRH somewhat on Day 6 but not on Day 13, reflecting the increased baseline LH concentrations prior to GnRH administration on Day 13. Progesterone reduced the response to GnRH extensively on Day 13. Progesterone plus estradiol markedly reduced GnRH response on Day 6 and essentially eliminated the response to GnRH on Day 13. Pituitary LH but not FSH was increased on Day 14 of treatment with estradiol. These data indicate that estrogen treatment was associated with a small, apparently transient, increase in pituitary response to GnRH, and a more prominent increase in pituitary LH content and circulating LH.