Luteinizing Hormone Release by Gonadotropin Releasing Hormone Before and After Castration in Bulls 1

Abstract

Summary

Six 9-mo-old Holstein bulls were used to compare the magnitude of LH release in response to gonadotropin releasing hormone administered prior to and after castration and following testosterone replacement. In addition, effects of castration and testosterone replacement on patterns of episodic LH release were investigated. GnRH caused release of LH before and after castration, but the magnitude of LH increase in response to GnRH increased 2.5-fold at 14 days postcastration relative to precastration changes. Testosterone replacement did not reduce the magnitude of LH response to GnRH to precastration levels. LH concentration in serum collected at hourly intervals prior to castration averaged 1.1 ng/ml, only 17% of the comparable value at 21 days after castration (6.7 ng/ml). The number of episodic increases in serum LH concentration during a 24-hr period prior to castration averaged 3.7 and increased to 6.5 at 21 days postcastration. Testosterone replacement restored neither the average number nor the magnitude of LH increase to levels characteristic of the precastration period. We conclude that LH release in response to GnRH is increased after castration and the increase is not reversed by testosterone. In addition, LH is released episodically in bulls, and peaks of LH normally are closely followed by increased testosterone in serum. Castration increases the frequency but not the magnitude of LH increase and testosterone does not restore frequency to precastration levels.