Effect of GnRH Immunization on Testicular Function in Colts

Ten Australian Stock Horses colts (five yearling and five 3-year old colts) of which 2 yearlings and 2 three year old colts served as control animals while 3 yearlings and 3 three year old colts received two GnRH immunizations within 4 weeks interval were used in this study. By the 5th to 6th week after immunization, the GnRH antibody titres in the plasma rose above 1:1000 and attained peak levels of 1:6500 by the 8th week and gradually declined to about 1:3000 by the 10th week in both the age groups. The testosterone and androstenedione concentrations of the control colts in both age groups were significantly greater (p<0.05) than that of the vaccinated groups. During the immunosuppression period, the vaccinated colts behaved like geldings. Semen could not be collected from 2 of the 3 three-year old vaccinated colts. The testicular dimensions, testicular weight, parenchymal weight, seminiferous tubule volumes, interstitial space volumes, Leydig cell volume, seminiferous tubule % of the control colts were significantly greater than those of the vaccinated colts in both the age groups. The 3-year old control colts had a significantly (p<0.05) greater % of Leydig cells than the control and vaccinated 1-year old colts. There was arrest of spermatogenesis with complete absence of sperm in the testes of the vaccinated colts while there was various stages of spermatogenesis in those of the control colts. Morphometric analysis demonstrated that the 3-year old colts had significantly (p<0.05) greater DSP/gm of testis and DSP/testis than those of the 1-year old control colts. This study elucidated that the GnRH immunization could suppress the testicular function of the 3-year old and yearling colts.