Effects of immunization against luteinizing hormone-releasing hormone and treatment with trenbolone acetate on reproductive function of beef bulls and steers.

The objectives of this study were 1) to evaluate the ability of trenbolone acetate (TBA) administered in tandem with LHRH immunization to suppress reproductive function in bulls and 2) to examine the effects of LHRH and androgen (TBA) signaling on pituitary gland function. Forty-four Angus × Hereford crossbred calves (BW=225 ± 2 kg; age=187 ± 6 d) received castration, LHRH immunization, or TBA administration in a 2 × 2 × 2 factorial design. Treatment groups receiving LHRH immunization contained 6 animals, whereas other treatment groups contained 5 animals. Animals immunized against LHRH received a primary injection and 2 booster injections of ovalbumin-LHRH-7 fusion protein on d 0, 42, and 196, respectively. Animals treated with TBA were implanted on d 224. Serum LHRH antibodies increased (P<0.05) after each booster for immunized animals, but were negligible in nonimmunized animals throughout the experiment. Serum testosterone concentration (P<0.001) and scrotal circumference (P<0.05) were depressed in LHRH-immunized bulls compared with nonimmunized bulls by d 84 and 168 of the experiment, respectively. Treatment with TBA tended (P=0.08) to decrease serum testosterone concentrations of nonimmunized bulls. Weights of testes at slaughter were decreased (P<0.001) for LHRH-immunized (232 ± 41 g) compared with nonimmunized (752 ± 45 g) bulls, but did not differ (P=0.80) between TBA-implanted (500 ± 49 g) and nonimplanted bulls (484 ± 36 g). Both LHRH immunization and castration decreased pituitary gland stores of LH and FSH (P<0. 001). There was no effect (P>0.10) of TBA on pituitary gland FSH content and only a tendency (P=0.09) to increase pituitary gland LH content. Immunization against LHRH decreased expression of LH β-subunit and common α-subunit genes (P<0.001). Castration increased expression of LH β-subunit and common α-subunit genes (P=0.02). Treatment with TBA further suppressed (P=0.04) α-subunit mRNA expression in LHRH-immunized steers. In summary, LHRH immunization decreased synthesis and storage of LH and decreased storage, but not synthesis of FSH in bulls. The increased synthesis of LH and FSH in nonimmunized, but not LHRH-immunized steers suggests that castration removes the negative feedback on gonadotropin synthesis but that LHRH is still needed for release of these hormones. Androgen replacement with TBA did not restore the negative feedback control of gonadotropin synthesis.