The effects of cGnRH-II and lGnRH-III on gonadotropin secretion and testicular function

The objective of this experiment was to evaluate the effects of active immunization against two GnRH isoforms on gonadotropin secretion and testicular function in pigs. Synthetic cGnRH-II and lGnRH-III peptides, where the common pGlu-His-Trp-Ser sequence at the N-terminal was suppressed, were conjugated to BSA. Forty-eight male piglets were randomly assigned to 4 treatments. Pigs on treatment 1 were actively immunized against cGnRH-II, while pigs on treatment 2 were actively immunized against lGnRH-III. Pigs on treatment 3 were actively immunized against the carrier protein (BSA), and pigs on treatment 4 were castrated and actively immunized against BSA. The BSA conjugate was emulsified in Freund’s Incomplete Adjuvant and diethylaminoethyl-dextran. Primary immunization was given at 13 weeks of age (WOA), with booster immunizations given at 16, 19, and 23 WOA. Body weight (BW) and plasma samples were collected weekly beginning at 11 WOA. Treatments did not affect BW during the experimental period. Titers were significantly increased in animals immunized against cGnRH-II and lGnRH-III (P<0.01). Cross-reactivity of the antisera to mammalian GnRH or between cGnRH-II and lGnRH-III was minimal. Weekly FSH (P<0.03), LH (P<0.001), and testosterone (P<0.001) concentrations appear to be differentially regulated. At 26 WOA, pigs (n=3/trt) were randomly selected and serum samples were collected at 10 min intervals for 10 h. No significant differences on FSH and LH pulse frequency and amplitude were observed. At the end of the experiment, intact pigs were exsanguinated. Testes were immediately removed; Leydig cells were isolated and treated with 0, 1, and 10 ng/mL of LH. There was a LH x GnRH treatment effect on testosterone concentrations (P<0.01) indicating that Leydig cells were sensitive to the immunization protocol and LH doses. Taken together, these data suggest that gonadotropin secretion is differentially regulated in pigs immunized against GnRH isoforms. Additionally immunization against cGnRH-II and lGnRH-III significantly reduced the ability of Leydig cells to respond to LH challenges.