EFFECT OF ACTIVE IMMUNIZATION OF HEIFERS AGAINST INHIBIN ON PLASMA FSH CONCENTRATIONS, OVARIAN FOLLICULAR DEVELOPMENT AND OVULATION RATE

To study the effects of immunoneutralization of endogenous inhibin on gonadotrophin secretion and ovarian function, prepubertal heifers (n=6) were actively immunized against a synthetic peptide replica of the N-terminal sequence of bovine inhibin a subunit (bI-alpha(1-29)Tyr30) coupled to ovalbumin. In contrast to ovalbumin-immunized controls (n=6), bI-alpha(1-29)Tyr30-immunized heifers had detectable inhibin antibody titres (% binding to I-125-labelled bovine inhibin at 1:2000 dilution of plasma) of 17+/-3% (S.E.M.) at puberty, rising to 31+/-5% by the end of the study period 7 months later. Neither age (immunized: 295+/-8 days; controls: 300+/-5 days) nor body weight (immunized: 254+/-13 kg; controls 251+/-9 kg) at onset of puberty differed between groups. Although the difference did not reach statistical significance, mean plasma FSH concentrations recorded in inhibin-immunized heifers remained 35-40% higher than in controls throughout the 12-week period leading up to puberty (P=0.14) and during nine successive oestrous cycles studied after puberty (P=0.10). Plasma LH concentrations did not differ between groups at any time during the study. Inhibin immunization had no effect on oestrous cycle length (immunized: 19.8+/-0.5 days; controls: 19.9+/-0.5 days). However, in comparison with controls, inhibin-immunized heifers had more medium sized (greater-than-or-equal-to 0.5 to <1 cm diameter) follicles during both the preovulatory (95%, P<0.001) and post-ovulatory (110%, P<0.05) waves of follicular growth and more large (greater-than-or-equal-to 1 cm diameter) follicles during the preovulatory wave (49%, P<0.05). In addition, the number of corpora lutea observed during the post-ovulatory phase of each cycle was significantly greater in the inhibin-immunized group (43%, P<0.01), as was the recorded incidence of cycles with multiple ovulations (19/56 in the inhibin-immunized group compared with 0/54 in controls; P<0.001). All six inhibin-immunized heifers had at least one cycle with multiple ovulation whereas none of the control heifers did so. These results support the conclusion that immunoneutralization of endogenous inhibin using a synthetic peptide-based vaccine can enhance ovarian follicular development and ovulation rate in heifers. Whether this ovarian response is dependent upon the expected increase in secretion of FSH remains to be established.