GnRH analogs induce a LH peak and increase pregnancy per timed-AI in ewes

To date, there have been no studies testing the capacity of GnRH analogs and respective doses to induce a LH peak in sheep. In this sense, the present study aimed to evaluate the capacity of different synthetic forms and doses of GnRH in inducing LH release in sheep, and the effect of GnRH administration at timed artificial insemination (TAI) on pregnancy per timed-AI. In experiment 1, ewes (n = 40) received an intravaginal device (IVD) of medroxyprogesterone acetate (MPA; 60 mg) for 7 d and prostaglandin F-2 alpha analog on Day 5. On Day 7, the ewes were allocated randomly into one of eight groups (n = 5/group), which received a GnRH analog at a specific dose, as follows: lecirelin (12.5 or 25 mu g), gonadorelin (50 or 100 mu g), buserelin acetate (4.2 or 8.4 mu g), or deslorelin (375 or 750 mu g). Blood samples for LH determination were obtained at 0, 2, 4, and 6 h after GnRH and the IVDs were removed after the last blood collection. The maximal LH concentration induced by gonadorelin at doses of 50 mu g and 100 mu g (12.0 +/- 2.4 ng/mL and 28.6 +/- 7.1 ng/mL, respectively) was lower (P < 0.05) than serum LH induced by 8.4 mu g of buserelin (78.9 +/- 12.9 ng/mL), 375 mu g and 750 mu g of deslorelin (75.6 +/- 7.4 ng/mL and 72.1 +/- 10.6 ng/mL, respectively) and 12.5 mu g and 25 mu g of lecirelin (73.3 +/- 17.8 ng/mL and 61.6 +/- 5.9 ng/mL, respectively). However, the maximal LH concentration induced by 4.2 mu g of buserelin (49.4 +/- 5.9 ng/mL) was similar (P > 0.05) to the 100 mu g of gonadorelin. The total release of LH (area under the curve - AUC) after treatment with 50 mu g of gonadorelin (31.7 +/- 5.9 ng h/mL) was lower (P < 0.05) than after other agonists. In a second experiment, 330 ewes were treated with IVD containing MPA for 7 d. Simultaneously with IVD removal, 250 mu g of cloprostenol and 200 IU of eCG were administered. Then, ewes were assigned randomly to either no further treatment (control); or to receive 4.2 mu g of buserelin acetate (GnRH group) at cervical TAI, which was performed with fresh semen 54 h after IVD withdrawal in all the animals. Higher pregnancy per timed-AI was observed for GnRH (50.3 %) compared to control (40.7 %). We conclude that buserelin acetate (8.4 mu g), lecirelin (12.5 and 25 mu g) and deslorelin (375 and 750 mu g) induced a greater stimulatory effect on LH secretion than gonadorelin treatment. Furthermore, buserelin acetate treatment at TAI increased pregnancy per timed-AI in ewes previously treated with MPA and eCG.