SUPPRESSION OF TESTICULAR FUNCTION USING 2 DOSE-RATES OF A REVERSIBLEWATER-SOLUBLE GONADOTROPIN-RELEASING-HORMONE (GNRH) VACCINE IN COLTS

Objective To investigate the effect of two dose rates (200 and 400 ng) of a gonadotrophin releasing hormone (GnRH) vaccine on testicular fun ction. Design A vaccination dose rate experiment. Procedure Two inject ions were administered 4 weeks apart to six colts in each treatment gr oup. To maintain immunosuppression until the end of the breeding seaso n, a third injection was given if antibody titres fell below 1000. Res ults Effective antibody titres were present for 12 to 27 weeks. Testos terone concentrations decreased from 2.22 to 0.31 nmol/L 6 weeks after primary vaccination. Androstenedione concentrations decreased from 1. 78 to 0.28 nmol/L 5 weeks after vaccination. Testosterone and androste nedione concentrations above 0.69 and 0.87 nmol/L were attained 31 to 43 weeks after vaccination, Mean scrotal widths and lengths decreased over 29 weeks from 9.2 cm and 9.7 cm to 6.7 cm and 7.5 cm. At surgical castration these dimensions were 10.1 cm and 11.0 cm. Mean semen char acteristics before vaccination and after recovery were: gel-free volum e 16.5 and 13.5 mL, sperm concentration 295.5 x 10(6) and 315.6 x 10(6 )/mL, total sperm per ejaculate 4041 x 10(6) and 4557 x 10(6) and live normal spermatozoa 32% and 60%. Histologically, the testes showed act ive spermatogenesis. The mean testicular parenchyma weights for the 20 0 and 400 mg groups were 129.0 g and 109.8 g. Daily sperm production p er testis and per gram of testis for the 200 and 400 mg groups were 3. 7 x 10(8) and 2.8 x 10(6), and 2.3 x 10(8) and 2.0 x 10(6). Conclusion s Both dose rates suppressed testicular function. Data showed that the vaccine effects were reversible. Individual immune response was less varied in the 200 mg group. Further work is necessary to achieve a les s variable response in the immunosuppression of testicular function.