PREOVULATORY FOLLICLE DEVELOPMENT AND LUTEAL FUNCTION IN EWES IMMUNIZED AGAINST A SYNTHETIC PEPTIDE SEQUENCE OF THE ALPHA-SUBUNIT OF BOVINE INHIBIN

Immunization against inhibin consistently results in an increase in ovulation rate in sheep, but the effects that this treatment has on follicle development are unknown. In order to determine the influence of inhibin, parameters of follicle development were assessed in ewes that had been actively immunized against a synthetic peptide homologous to the N-terminal sequence (alpha-1-29, Tyr30) of the alpha-subunit of bovine inhibin, a treatment that neutralizes the biological activity of endogenous inhibin. The final stages of preovulatory follicle development that culminate in ovulation were induced in seasonally anoestrous ewes, and follicles were recovered prior to the predicted time of ovulation. After priming with progestagen, inhibin-immunized and control ewes were treated with gonadotrophin-releasing hormone (GnRH) by continuous infusion (200 ng/h). The ovaries were recovered at slaughter 24 h after the start of GnRH treatment and all follicles greater-than-or-equal-to 2.0 mm diameter were dissected out and their capacity to produce oestradiol in vitro was assessed. Further groups of similarly treated animals were blood-sampled daily to determine luteal function following GnRH-induced ovulation. The ovaries were recovered from these ewes at slaughter 10 days after the start of GnRH treatment, the corpora lutea were dissected out and their progesterone content was assessed. There were more (P<0.01) follicles of 5-6 mm diameter (3.2 +/- 0.45 (S.E.M.) compared with 1.1+/-0.25 follicles/ewe) and more (P<0.001) follicles of > 6 mm diameter (2.8+/-0.56 compared with 0.9+/-0.17 follicles/ewe) in inhibin-immunized than in control ewes. In addition, the mean number of the antral follicles that were oestrogenic was greater (P<0.05) in immunized than in control ewes (2.8+/-0.66 compared with 1.3+/-0.25 follicles/ewe). In animals slaughtered 10 days after the start of GnRH treatment, mean ovulation rate was greater (3.17+/-0.65 and 1.14+/-0.14, P<0.01) in inhibin-immunized ewes. Although there was more (P<0.01) total luteal tissue/ewe in the immunized group, both the mean weight and progesterone content (ng/mg tissue) of individual corpora lutea were similar between treatment groups. Mean plasma progesterone levels increased earlier and reached higher (P<0.01) mean concentrations in immunized than in control ewes. These results demonstrate that immunization against inhibin increases the number of preovulatory follicles during the follicular phase, and that steroidogenesis within these follicles and the resultant corpora lutea appears to be normal.