Administration of single short-acting doses of GnRH antagonist modifies pituitary and follicular function in sheep

Current study determined the effect of two different single subcutaneous doses (1.5 and 3 mg) of GnRH antagonist (GnRHa) on pituitary and follicular function in non-lactating cyclic ewes. Both doses abolished the pulsatile secretion of luteinizing hormone (LH) for at least 3 days and decreased mean LH concentration during 6 days (0.64 ± 0.09 for control and 0.54 ± 0.05, P < 0.005, and 0.46 ± 0.02, P < 0.00001, for 1.5 and 3 mg, respectively). Supply of GnRHa decreased the number of large dominant follicles, so the total number of smaller follicles, 2-3 mm in size, increased in both treated groups from day 0, reaching its maximum at day 2 in ewes treated with 1.5 mg (19.83 ± 1.05 versus 5.83 ± 0.50 in the control, P < 0.005) and at day 4 in sheep treated with 3 mg (18.67 ± 0.65 versus 5.50 ± 0.65 in the control, P < 0.0001). However, the analysis of follicular function in terms of inhibin A indicated a possible effect of the higher dose of GnRHa on follicular function. The pattern of inhibin secretion in the group treated with 3 mg of GnRHa decreased after the first 48 h, reaching its lowest value on day 4.5 (182.59 ± 3.75 to 140.28 ± 9.91 pg/ml, P < 0.05) concentration significant lower than control sheep (171.93 ± 6.21 pg/ml, P ± 0.01) or treated with 1.5 mg (168.04 ± 7.16 pg/ml, P ± 0.05). Hence, the use of 1.5 mg would be more suitable to induce the presence of a high number of follicles able to grow to preovulatory sizes.