220 ENDOCRINE AND OVARIAN FUNCTION AFTER GNRH ANTAGONIST TREATMENTS IN GOATS

Abstract

In goats, as in other mammals, the use of treatment with GnRH antagonists (GnRHa) inhibits gonadotrophin secretion, causing a suppression of the growth of large ovarian follicles. Thus, GnRHa treatment could be useful to decrease the effects of dominant follicles prior to ovarian stimulation, increasing the number of gonadotrophin-responsive follicles at the start of FSH treatments and improving the ovarian response in terms of transferable embryos. However, in goats, the beneficial effects of this treatment is annulled by a high number of unfertilised ova and degenerated embryos (2003, Cognie et al., Theriogenology 59, 171–188), which suggests deficiencies in oocyte developmental competence per se or induced by endocrine or follicular alterations during the peri-ovulatory period. We have tested whether these failures can be related to a prolongation of gonadotrophin down-regulation and/or alterations in follicular function after cessation of the antagonist, during the period of administration of the superovulatory treatment, around 4 days after the end of GnRHa treatment. A total of 15 does received 45-mg FGA intravaginal sponges (Chronogest^®, Intervet Int, H), the first group of 10 females were treated with daily injections of 0.5 mg of the GnRHa Teverelix (Antarelix™, Zentaris, G) for 6 days from Day 5 of sponge insertion, while five does acted as controls receiving saline. Endocrine and ovarian function were monitored daily from Day −5 to Day 4 (Day 0 = day of last GnRHa injection). Pituitary activity was determined by measuring plasma FSH and LH, and follicular activity by ultrasonographic monitoring of all >2 mm follicles and by assessing plasma inhibin A levels. During GnRH antagonist treatment, the mean plasma LH concentration was lower in treated than control goats (0.5 ± 0.2 v. 0.7 ± 0.5 ng/mL, P < 0.0005); however, the FSH levels remained unaffected (0.8 ± 0.4 v. 0.8 ± 0.5 ng/mL). In this period, treated does also showed an increase in the number of small follicles 2–3 mm in size (10.7 ± 0.7 v. 8.4 ± 0.6, P < 0.05), and a decrease in both the number of follicles >4 mm in size (5.0 ± 0.3 v. 6.8 ± 0.5, P < 0.005) and the secretion of inhibin A (120.9 ± 10.7 v. 151.6 ± 12.6 pg/mL, P < 0.05). After GnRHa treatment, LH levels increased in treated goats from the day after the last Teverelix injection (Day 1), so that LH levels were the same as controls on Day 3 (0.6 ± 0.1 v. 0.6 ± 0.2 ng/mL). However, there were even greater numbers of small follicles than during the period of GnRHa treatment (15.4 ± 0.6 in treated v. 8.9 ± 0.7 in control, P < 0.0005). Moreover, the number of follicles >4 mm in size and the secretion of inhibin A remained lower in treated goats (3.9 ± 0.3 follicles and 84.4 ± 7.0 pg/mL v. 5.4 ± 0.5 follicles, P < 0.05 and 128.9 ± 14.2 pg/mL, P < 0.05). These results indicate that pituitary secretion of gonadotrophins is restored shortly after the end of GnRHa treatment, but the number of follicles and the secretion of inhibin A are affected. This may be relevant to the failures in ovulation and/or fertilization reported for superovulatory protocols with GnRHa pre-treatments in goats.