ACTIVE IMMUNIZATION OF MARES AGAINST THE RECOMBINANT HUMAN INHIBIN ALPHA-SUBUNIT

Four cycling non-lactating mares were immunized against the recombinan t human inhibin alpha-subunit, two mares treated with vehicle only ser ved 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 t he treated mares. No anti-inhibin antibodies were detectable in the co ntrol animals. After booster injections there was an increasing incide nce of irregular ovarian activity (delayed ovulations, anovulatory hem orrhagic follicles, follicle atresia, ovulation without oestrus behavi our) which appeared to be more frequent in the animals with high titre s and low pre-ovulatory oestrogen/progesterone ratios. The treated mar es tended to develop more medium-sized and large follicles than did th e controls. However, only 33 (51.7%) out of 64 dominant follicles ovul ated in the immunized mares, whereas 13 out of 16 dominant follicles ( 81.3%) ovulated in the control animals. Whenever oestrus was normal, t he 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 immuni zed mares and the controls throughout the experimental period. Gonadot rophin pulsatility during the luteal phase after the second booster in jection was similar to the controls in two of the treated mares, howev er, no LH pulses were detectable in the other two treated animals. The present immunization protocol is not practical for the induction of m ultiple ovulations in the mare. From the endocrine data it appears tha t the effects of immunization against the inhibin alpha-subunit on ova rian function are mediated through local paracrine mechanisms rather t han effects on the pituitary function. This view is supported by the p resence of anti-inhibin antibodies in the developing follicles.