Increased AMH plasma concentration and small antral follicles - Deslorelin slow release implants as a promising tool for preparation of mares for OPU?

Abstract

A major factor affecting oocyte yield during ovum pick-up (OPU) procedures in mares is the number of follicles present on the ovary and the associated number of recovered oocytes (Cuervo-Arango et al. Reproduction, Fertility and Development. 2019; 31(12): 1894-1903). The interest in manipulation of ovarian follicular populations in horses is increasing because of benefits with regard to the oocyte yield and the overall outcome for assisted reproductive procedures. This study aimed to evaluate effects of slow-release implants containing deslorelin acetate on antral follicle counts (AFC), follicular subpopulations of different size ranges and plasma anti-Mullerian hormone (AMH) and gonadotrophin concentration in mares. Haflinger mares (n=12) were synchronized with two injections of a PGF2α analog. One day after the second injection, mares were randomly assigned to the slow-release implant (DES, 9.4mg deslorelin implant, n=6) or the control group (CON, deslorelin short-acting formulation, 1.25mg, n=6). Transrectal examination of the genital tract and collection of blood samples for the analysis of progesterone, AMH and gonadotrophins were performed until day 90 and on days of AFC assessment. AFC and follicle numbers according to size groups were determined on days 0, 10, 25, 50, 90, 180, 270 and 360 after implant insertion. The interval from implant insertion to the first spontaneous ovulation was prolonged in group DES (23.8±10.5 days) compared to CON(17.0±3.9 days) (p<0.05). Treatment influenced concentrations of LH and FSH over time (p<0.05) with the concentration of LH being greater in DES than in CON mares on days 30 and 55 and of FSH on day 15 after implant insertion, respectively (p<0.05). AMH concentration was greater in group DES from day 30 onwards (interaction treatment x time p<0.05). The AFC and the number of follicles with 5 to 10, 10 to 15 and 15 to 20 mm in diameter changed over time (p<0.05). A time x treatment interaction was demonstrated for follicles of 10 to 15 mm in diameter with more follicles of this size present in DES than in CON mares on day 90 (p<0.05). In group DES, concentration of AMH and the AFC (r=0.623, p<0.001) as well as concentration of AMH and the number of follicles 5 to 20 mm in diameter (r=0.442, p<0.01) were positively correlated whereas there was no such correlation in group CON. In conclusion, long-acting deslorelin implants induce changes in follicular development reflected in AMH plasma concentration. These changes are present from day 30 after implant insertion, suggesting that not only direct effects of deslorelin on gonadotrophin secretion but also the reduction of FSH suppressing factors from follicles such as inhibin may be involved. Treatment of mares with 9.4mg deslorelin slow-release implants might thus be a promising tool as preparation for OPU.