EFFECT OF CHRONIC ADMINISTRATION OF GONADOTROPHIN RELEASING HORMONE AND THYROTROPHIN RELEASING HORMONE TO PUBERTAL BULLS ON PLASMA LUTEINIZING HORMONE, PROLACTIN AND TESTOSTERONE CONCENTRATIONS, THE NUMBER OF EPIDIDYMAL SPERM AND BODY WEIGHT

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Abstract
SUMMARY The experiment was designed to determine whether chronic administration of gonadotrophin releasing hormone (GnRH) plus thyrotrophin releasing hormone (TRH) to pubertal bulls would hasten sexual maturation. Nine bulls, three each at 4, 5 and 6 months of age were given i.m. injections of GnRH (0·4 μg/kg) plus TRH (2 μg/kg) three times daily (09.00, 16.00 and 23.00 h) for 30 days. Equivalent animals were given 0·9% saline vehicle as controls. At the outset of injections the 6-, 5- and 4-month-old bulls weighed 201, 157 and 169 kg respectively; equivalent weights at the final injection were 222, 186 and 198 kg. Plasma levels of LH, prolactin and testosterone were determined weekly around the injection given at 09.00 h. Twenty-four-hour plasma testosterone and LH profiles were obtained immediately after the last injection. The animals were unilaterally castrated at the end of this 24 h period and the second testis was removed 60 days later to assess testicular weights and epididymal sperm numbers. Surges of LH, testosterone and prolactin occurred throughout the experimental period in response to GnRH/TRH injection. The LH surge in response to the first injection was nearly fivefold larger than those measured at subsequent weekly intervals but prolactin and testosterone responses did not decline during the 28-day experimental period. Induced LH and prolactin peaks were larger than those occurring in the control bulls, but control animals produced an equivalent amount of testosterone. These bulls produced quasi-adult levels of testosterone even at 4 months of age. Chronic GnRH/TRH administration resulted in a depression of plasma testosterone, and a disruption of the normal episodic release pattern during the 24 h after the final treatment, but normal patterns returned within 60 days. Testicular weights and epididymal sperm numbers were not increased by chronic GnRH/TRH treatment.