Alterations in gonadotrophin secretion and ovarian sensitivity to gonadotrophins induced by active immunization against androstenedione

Immunization against 5alpha-androstane-3alpha,17beta-diol affects ovarian folliculogenesis in Merino ewes

The 5alpha-reduced androgens have been implicated as antagonists of follicular development. In this experiment, we examined the effect of active immunization against 5alpha-reduced androgen on follicular development in ewes. During the breeding season, cyclic Merino ewes were either actively immunized three times against 5alpha-androstane-3alpha,17beta-diol (3alpha-diol) or served as controls. Six to nine weeks after the last immunization, they were treated with PGF(2alpha) analog (PG, 125mg cloprostenol i.m.) and luteolysis was induced. Fourteen days after the PG treatment, the ewes were either killed (mid-luteal phase) or treated a second time with PG and killed 24h later (early follicular phase). At slaughter, blood samples were collected and ovaries recovered. All CL and follicles larger than 2mm were dissected and their size and appearance were recorded. Follicular fluid was collected and concentrations of estradiol-17beta (E(2)), progesterone (P), androstenedione (A(4)), testosterone (T), dihydrotestosterone (DHT), 5alpha-androstane-3alpha-ol,17beta-one (androsterone: 3alpha-ol) and 3alpha-diol were determined by RIA. Immunization induced antibodies primarily to DHT and its 5alpha-reduced substrates 3alpha-diol and 3alpha-ol but not to E(2), P, A(4) or T. Immunization increased ovulation rate, size of ovulatory follicles and weight of CL. Immunization appeared to increase ovulation rate by decreasing the incidence of atresia in large preovulatory follicles. Regardless of their physiological status follicles contained only low levels of DHT; 3alpha-ol and 3alpha-diol were not detected in most follicles. Immunization did not appear to affect levels of DHT or other steroids in the follicular fluid. In conclusion, the induction of antibodies to 5alpha-reduced androgens increases ovulation rate by enhancing follicular viability during the preovulatory period in ewes. However, this effect is not brought about by the direct immune-neutralization of DHT or its 5alpha-reduced substrates 3alpha-ol and 3alpha-diol at the ovarian level.

Evidence that the conceptus contributes to the inhibition of follicular growth in the ewe

The present study was designed to determine the contribution of the gravid uterus and the conceptus in the inhibition of antral follicular growth. All antral follicles were counted and measured on day 48 post mating in ovaries of pregnant ewes (Group 1, n = 5) and in ovaries of ewes from which the uterus (Group 2, n = 5) or the conceptus (Group 3, n = 5) had been surgically removed on days 32-33 post mating. The secretory activity of the corpora lutea was maintained after removal of the uterus or the conceptus until slaughter of ewes. No difference was found between the three groups of ewes in the mean (+/-SEM) total number (non-atretic and atretic) of follicles per ewe, being 59.8 +/- 7.6, 35.0 +/- 7.6 and 50.8 +/- 4.6, respectively for Groups 1, 2 and 3. The mean (+/-SEM) number of follicles 2 mm or more in diameter was greater (P < 0.01) in Group 2 ewes (3.4 +/- 0.5) and Group 3 ewes (4.2 +/- 0.5) than in Group 1 ewes (1.6 +/- 0.4). Of all follicles counted which were 2 mm or larger, 90% in Group 1 were atretic, this value being higher (P < 0.05) than in Group 2 (68%) and Group 3 (55%). The mean (+/-SEM) diameter of the largest (F1) and second largest (F2) follicles was greater (P < 0.01) in Group 2 (F1, 4.0 +/- 0.4; F2, 3.2 +/- 0.4) and in Group 3 (F1, 3.9 +/- 0.2; F2, 3.3 +/- 0.2) than in Group 1 (F1, 2.6 +/- 0.1; F2, 2.2 +/- 0.1). The mean diameter (+/-SEM) of the largest non-atretic follicles was greater (P < 0.01) in Group 2 (4.0 +/- 0.4) and Group 3 (3.9 +/- 0.2) than in Group 1 (1.7 +/- 0.3). In conclusion, the results from this in vivo experiment provide evidence that the conceptus prevents the development of large healthy follicles by inducing atresia during the first trimester of pregnancy. The inhibition of follicular growth would reduce oestradiol production and permit the maintenance of luteal function during early pregnancy.

Immunological approaches to fertility regulation in domestic livestock

Antibodies to hormones have been used extensively to study endocrine regulation of reproduction and in recent times they have been commercially exploited as therapeutic agents to manipulate reproduction. Despite the release of two commercial products, little is known about the mechanism of action of immunoneutralization. The presence of hormone-binding antibodies in the circulation and the detection of hormone antibody complexes in blood has led investigators to suggest a vascular site of action for immunoneutralization. However, a vascular site of action is difficult to reconcile with some of the known effects of immunoneutralization to the boar taint steroid (5 alpha-androst-16-en-3-one) to androstenedione and to gonadotrophin-releasing hormone. In this paper we review some of our recent research into the mechanism of action of androstenedione immunoneutralization. We suggest that hormonal immunoneutralization of androstenedione may take place in the extracellular compartment or within the plasma membrane where antibodies to androstenedione may bind to a specific steroid-binding protein in the plasma membrane of granulosa cells.