Active immunization of mares against the recombinant human inhibin alpha-subunit

Four cycling non-lactating mares were immunized against the recombinant human inhibin alpha-subunit, two mares treated with vehicle only served as controls. Anti-inhibin antibody concentrations started to rise 10 days after the first antigen injection with further increases after booster injections, although antibody titres varied greatly between the treated mares. No anti-inhibin antibodies were detectable in the control animals. After booster injections there was an increasing incidence of irregular ovarian activity (delayed ovulations, anovulatory hemorrhagic follicles, follicle atresia, ovulation without oestrus behaviour) which appeared to be more frequent in the animals with high titres and low pre-ovulatory oestrogen/progesterone ratios. The treated mares tended to develop more medium-sized and large follicles than did the controls. However, only 33 (51.7%) out of 64 dominant follicles ovulated in the immunized mares, whereas 13 out of 16 dominant follicles (81.3%) ovulated in the control animals. Whenever oestrus was normal, the ovulation rate was 1.4 in treated mares vs. 1.0 in controls (p > 0.5). The concentrations of FSH and LH did not differ between the immunized mares and the controls throughout the experimental period. Gonadotrophin pulsatility during the luteal phase after the second booster injection was similar to the controls in two of the treated mares, however, no LH pulses were detectable in the other two treated animals. The present immunization protocol is not practical for the induction of multiple ovulations in the mare. From the endocrine data it appears that the effects of immunization against the inhibin alpha-subunit on ovarian function are mediated through local paracrine mechanisms rather than effects on the pituitary function. This view is supported by the presence of anti-inhibin antibodies in the developing follicles.