Deslorelin Slow-Release Implants Delay Ovulation and Increase Plasma AMH Concentration and Small Antral Follicles in Haflinger Mares

Abstract

There is an increasing interest in the manipulation of ovarian follicular populations in large domestic animals because this could prove beneficial for assisted reproductive techniques such as ovum pick-up (OPU). The aim of the present study was to evaluate the effects of deslorelin slow-release implants (SRI) on the interovulatory interval, antral follicle count (AFC), number of follicles of different size ranges and plasma anti-Muellerian hormone (AMH) concentration in mares. To synchronize their estrous cycles, Haflinger mares (n = 12) were treated twice with a PGF2α analogue. One day after the second injection (day 0), mares received a 9.4 mg deslorelin SRI (group DES, n = 6) or 1.25 mg deslorelin in a short-acting formulation (CON; n = 6), respectively. Regular transrectal ultrasonography of the genital tract was performed and blood samples were collected for the analysis of progesterone, AMH and gonadotrophins. The interval from implant insertion to the first spontaneous ovulation was 23.8 ± 10.5 days in group DES compared to 17.0 ± 3.9 days in group CON (p < 0.05). For the concentrations of LH, FSH and AMH, interactions between time and treatment were detected (p < 0.05). The AFC and the mean 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 (p < 0.05). The changes in this follicular subpopulation were reflected by increased plasma AMH concentration in group DES. In conclusion, 9.4 mg deslorelin implants show minor effects with regard to estrus suppression in mares, whereas the changes in the subpopulation of small ovarian follicles could be a promising tool for preparation of mares for OPU.

Sustained‐release deslorelin acetate implants disrupt oestrous cyclicity in the mare

“…Worth noting is that persistent corpora lutea are common in mares, affecting 6.1% of interovulatory intervals 16 and becoming more frequent as mares approach the autumn transition 17,18 . Persistent corpora lutea were not reported in the studies by Kaps et al 4,5 …”

“…In a number of species, chronic exposure to GnRH agonists down‐regulates the hypothalamic–pituitary–gonadal axis and inhibits reproductive function 3 . Recently, Kaps et al reported transient effects on ovarian function in Shetland mares administered one implant of either Suprelorin‐6 or Suprelorin‐12 4 and in Haflinger mares administered one implant of Suprelorin‐12 5 …”

“…3 Recently, Kaps et al reported transient effects on ovarian function in Shetland mares administered one implant of either Suprelorin-6 or Suprelorin-12 4 and in Haflinger mares administered one implant of Suprelorin-12. 5 The aim of the current study was to investigate the effects of up to three implants of Suprelorin-12 on oestrous cyclicity in domestic mares. Ethical approval was granted by the Animal Ethics Committee, James Cook University (A2462).…”

“…13,14 Few studies have investigated the effects of prolonged exposure to higher doses of GnRH agonists in mares. [4][5][6]15 Fitzgerald et al 15 administered the GnRH-agonist goserelin acetate for 28 days via biodegradable depots. Thirteen of 16 mares demonstrated interovulatory intervals in excess of 30 days.…”

Suppression of reproductive behaviour and gonadal function in female horses—An update

“…In contrast, treatment of cyclic mares of different breeds did not only result in a transient down‐regulation of pituitary GnRH receptors but was also associated with inhibition of ovulation and oestrus behaviour. The effect was dose‐related with spontaneous ovulation being delayed on average 40 days in Shetland mares treated with one 9.4 mg deslorelin implant, 20 days in Shetland mares treated with one 4.7 mg deslorelin implant (Kaps, Okada, Gautier, Aurich, Scarlet, et al., 2021 ) and <10 days in Haflinger mares treated with one 9.4 mg implant (Kaps, et al., 2021 ). Slow‐release deslorelin implants are thus not a reliable tool for long‐term suppression of ovarian function in horses, but may be worth considering when short time effects are desired.…”