Relationship between Serum Testosterone Concentration and Patterns of Luteinizing Hormone Secretion in Male Sheep*

To investigate the control of pulsatile LH secretion by testosterone in male sheep, castrated rams (wethers) were implanted at the time of castration with Silastic capsules filled with crystalline testosterone. The capsules were either 15 cm (0.5T capsule) or 30 cm (T capsule) in length, and the following treatments were imposed: no capsules (intact rams; control wethers); one 0.5T capsule; 2T capsules; 4T capsules. Testosterone and LH secretory patterns and LH responses to LHRH were characterized after 6 weeks of hormone treatment. Serum testosterone concentrations (nanograms per ml; mean ± SE) were as follows: rams, 2.25 ± 0.49; control wethers, 0.22 ± 0.01; 0.5T, 0.54 ± 0.1; 2T, 1.11 ± 0.08; 4T, 2.01 ± 0.09. Basal LH concentrations (nanograms per ml) and number of LH pulses per 24 h were, respectively: rams, 1.2 ± 0.1 and 3.3 ± 1.2; control wethers, 9.3 ± 3.6 and 29.5 ± 0.5; 0.5T, 14.6 ± 3.3 and 27.7 ± 1.8; 2T, 8.9 ± 0.6 and 20.3 ± 0.9; 4T, 1.3 ± 0.2 and 2.7 ± 1.8. Wethers with 2T implants showed a significant decrease in the frequency of rhythmic LH pulses relative to control wethers without a change in mean LH. A dramatic decrease in LH pulse frequency and basal concentrations occurred in wethers with 4T implants. Peak LH responses (nanograms per ml) to LHRH (5 ng/kg) were: rams, 10.8 ± 0.9; control wethers, 36.3 ± 5.8; 0.5T, 51.7 ± 4.0; 2T, 23.4 ± 7.3; 4T, 13.1 ± 2.3. This study demonstrates that testosterone feedback on LH secretion in male sheep is not an all-or-none phenomenon. Rather, different components of the LH secretory system are affected as serum testosterone is increased from castrate to intact levels. Concentrations of testosterone intermediate between those of wethers and rams cause a decrease in LH pulse frequency without an apparent effect on mean LH concentration. As testosterone approaches physiological concentrations there is a dramatic decrease in both LH pulse frequency and basal secretion to levels characteristic of rams. The results of this study suggest a negative feedback action of testosterone at both the hypothalamic and pituitary level in male sheep.