Passive immunization of rams (Ovis aries) against GnRH:: effects on antibody titer, serum concentrations of testosterone, and sexual behavior

The effect of immunoneutralization of gonadotropin-releasing hormone (GnRH) on serum concentrations of testosterone and sexual behavior was evaluated in sexually mature male sheep. In Experiment 1, GnRH1 rams (n = 16) were passively immunized against GnRH (300 ml antiserum), control rams were either passively immunized against keyhole limpet hemocyanin (KLH, n = 15) or surgically castrated (Wethers1, n = 4). Sexual performance of the rams was assessed weekly for 3 weeks before and 6 weeks after immunization, using ovarihystertoniized ewes actively immunized against GnRH. Experiment 2 evaluated the effects of repeated immunization. Rams were immunized with two aliquots (400 and 300 ml, respectively) of anti-GnRH sera (GnRH, n = 5) or normal sheep serum (NSS, n = 4), 2 weeks apart. Surgically castrated animals were used as a second control group (Wethers2). Administration of anti-GnRH sera, but neither anti-KLH nor NSS sera, resulted in marked reduction (P < 0.05) in serum concentrations of testosterone. Sexual behavior was not consistently affected by administration of one aliquot of anti-GnRH sera, however repeated immunizations resulted in more persistent reduction in serum concentrations of testosterone and more consistent suppression of sexual behavior. (C) 2002 Elsevier Science B.V. All rights reserved.