Influence of Testicular Steroids on Thyrotropin Releasing Hormone-induced Prolactin Release in Mature Rams

¹ Roman L. Hruska U.S. Meat Animal Research Center, U.S. Department of Agriculture, Clay Center, Nebraska

Prolactin (PRL) concentrations were measured in serum of intact rams, castrate rams, and steroid-treated castrate rams to determine whether testicular steroids influence PRL secretion in this species. Testosterone was administered to castrate rams by subdermal Silastic implants providing serum testosterone concentrations similar to those found in intact rams; estradiol was administered similarly to castrate rams, providing a ten-fold elevation of serum estradiol. Testosterone and estradiol implants decreased serum LH to concentrations found in intact rams, whereas estradiol, but not testosterone, effectively increased basal concentrations of PRL. This effect was most apparent when animals were exposed to short photoperiods (8 hours light:16 hours darkness). When 5 µg of thyrotropin releasing hormone were injected into these animals, peak PRL concentrations were lowest in castrate rams and highest in estradiol-treated castrate rams. PRL responses in testosterone-treated castrate rams and intact rams to the releasing hormone were intermediate. Results for peak PRL concentrations and area under the PRL response curves were similar. In conclusion, testicular steroids are suggested to play an important role in the regulation of PRL secretion in mature rams.

     Key words: testicular steroids, prolactin, prolactin secretion

Submitted on October 15, 1979
Revised on December 5, 1979
Accepted on December 15, 1979