Moderate increases in peripheral blood estradiol concentration in the adult ram do not directly inhibit testosterone secretion

Abstract

Two experiments were conducted in July with adult Dorset × Leicester × Suffolk rams to determine whether increases of 150 or 300% in estradiol (E2) concentration in peripheral blood (from 6.3 ± 0.8 pg/mL in control rams) would affect testosterone secretion directly as well as indirectly via the hypothalamic–pituitary axis. After 4 days of estradiol treatment (experiment 1) provided with subcutaneous polydimethylsiloxane implants filled with crystalline estradiol, luteinizing hormone (LH) and testosterone secretions were reduced by 50% (p < 0.05) in both groups of rams because of subtle decreases in pulse frequencies and amplitudes. Estradiol treatments were also associated with decreases in mean follicle-stimulating hormone (FSH) concentration (30–50% in both groups, p < 0.05) and increases in mean prolactin concentration (35% in low-E2 group; 105% in high-E2 group, p < 0.05), but testicular responsiveness to an LH challenge (single intravenous dose, 10 μg NIH-LH-S25) remained normal. When along with estradiol treatment, 10-μg doses LH were given every 80 min (experiment 2), testosterone secretion increased by 265% (p < 0.05) in both treated and control rams. Relative to day −1, secretion on day 4 was characterized by higher (p < 0.05) pulse frequencies and baseline concentrations and lower (p < 0.05) pulse amplitudes; values for all characteristics were similar to those for Dorset × Leicester × Suffolk rams in the breeding season. Interestingly, the decreases in mean FSH concentration brought about by estradiol and (or) LH treatments were not any greater than in experiment 1, and estradiol's ability to elevate mean prolactin concentration was blocked completely. We conclude that moderate increases in estradiol concentration in peripheral blood of rams suppress testosterone secretion only by impairing LH release.Key words: estradiol, hypothalamic–pituitary axis, testes, ram.