Abstract:
Four experiments were carried out in Merino ewes during a period of 4 years todetermine the long-term effects of immunization against different syntheticpeptides mimicking the amine terminal of the a subunit of porcine inhibin.Peptides were conjugated to human serum albumin and 100-200 g emulsifiedin Freund’s complete adjuvant for the primary immunization. Usually twobooster injections were given at monthly intervals with 50-100 gconjugated peptide using either incomplete Freund’s adjuvant orMontanide : Marcol. In some experiments a further immunization was carried inthe next year. Blood samples were taken 10 days after each immunization,during the luteal phase, for estimation of gonadotrophin concentrations anddetermination of inhibin antibody titres. One day after blood samplingcloprostenol was used to induce luteolysis and laparoscopy was performed inthe subsequent oestrous cycle. Immunization of ewes with synthetic peptides1-32, 1-26, 7-26 and 8-30 resulted in large increasesin the ovulation rate (OR). An approximately two-fold increase in OR wasobserved following the first booster immunization with these peptides and athree- to five-fold increase after the second booster immunization.Immunization with these large peptides resulted in a sustained increase in ORfor a period of at least 1 year after the second booster immunization. Of theshorter peptides, peptides 10-26 and 13-26 gave a reasonableovulatory response, although it was more difficult to obtain a response withpeptides 1-16, 8-22, 13-25, 8-19 and 10-19;peptides 7-13 and 1-6 gave no response (but were examined for onebreeding season only). The smaller peptides led to lower inhibin antibodytitres that were not necessarily associated with increasedfollicle-stimulating hormone (FSH) or OR. More intensive blood sampling in oneexperiment showed that following primary immunization against peptide1-32 there was a transient increase in plasma FSH, which did not lead toan increased OR. Moreover, a prolonged period of raised FSH after the firstbooster was significantly correlated with increased OR. In these animalsantibody titres were only slightly increased after primary immunization, butafter the first booster immunization higher titres were observed that weresignificantly correlated with trough FSH values and the subsequent OR. Theseresults are interpreted as showing that (1) to obtain anincrease in OR peptides 1-32, 1-26 and 7-26 are suitable asimmunogens; (2) smaller peptides are less reliable,often require multiple injections, and the response may be delayed; and(3) an extended period of raised plasma FSH is needed togive a large ovulatory response.