90 Effect of the gonadotrophin-releasing hormone antagonist on the sheep follicular population

The present study was designed to study the follicular population dynamics followed or not by treatment with different doses of the GnRH antagonist in sheep. A total of 18 ewes were submitted to short-term oestrus synchronization protocol (Oliveira et al. 2009 Proc. Braz. Congr. Anim. Reprod.). The animals were 2 or 3 years old, multiparous, and had a body score of 3 to 3.5. On Day 7 after ovulation of synchronized oestrus, females were randomly divided into groups (n = 6/group) according to the dose of the gonadotrophin-releasing hormone (GnRH) antagonist (Firmagon®, Ferring Pharmaceuticals) subcutaneously administered: G-control: placebo treatment (administration of saline solution); G-lower dose: 215 µg/kg; and G-higher dose: 235 µg/kg of bodyweight. B-mode ultrasound exams of the ovaries were conducted daily from 1 day before treatment with GnRH antagonist until the females showed oestrous behaviour. Ultrasound equipment (MyLab Vet®, Esaote) was used coupled to a transrectal linear transducer with a frequency of 6 and 8 MHz to assess the ovarian population. Data were compared between groups, evaluation days, and their interaction by ANOVA with post hoc using Tukey’s test (P   0.05) between the studied effects (treatments and evaluation days). The number of small follicles (2–3.49 mm) was higher (P = 0.0002) in the G-lower dose (5.4 ± 0.4) compared with the G-control (4.1 ± 0.3) and G-higher dose (3.5 ± 0.2). The number of large follicles (≥4.5 mm) was lower (P = 0.01) in the G-higher dose (0.2 ± 0.0) compared with the G-control (0.5 ± 0.1) and G-lower dose (0.4 ± 0.1). The number of medium follicles (3.5– 4.49 mm) and the average diameter of the follicles in the 3 categories of diameter did not differ (P > 0.05) between groups. The number of medium follicles differed (P = 0.0131) between Days 8 and 15 after synchronized oestrus ovulation. The number of large follicles on Day 6 differed (P = 0.0002) of Days 8, 9, 10, 11, 12, 16, and 17. The average diameter of medium follicles differed (P = 0.0095) between Days 8 and 10. The number of small follicles and the average diameter of small and large follicles did not differ (P > 0.05) between days. In conclusion, the administration of the GnRH antagonist at a higher dose in sheep suppressed the development of large tertiary or antral follicles, whereas at a lower dose, it led to an increase in the population of small follicles.