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The GnRH antagonist, acyline, has not yet been investigated in the stallion. Our study aimed to: (1) evaluate the downregulation of the stallion hypothalamic-pituitary-gonadal axis by acyline through assessment of seminal parameters, testicular volume, and sexual behavior; (2) assess hormonal response of acyline-treated stallions to GnRH stimulation; and (3) verify reversibility after treatment. Stallions were assessed pretreatment and subsequently treated (every five days) for 50 days: acyline (n = 4; 330 μg/kg acyline) or control (n = 4, vehicle). The stallions were then monitored for 62 days after the last day of treatment. Treatment-induced declines (P < 0.05) in FSH, LH, testosterone, and estrone sulfate. Gonadotropins and testosterone returned to control values within 9 days, and estrone sulfate by 14 days, after discontinuation of treatment. Acyline-treated stallions failed to respond with a rise in FSH, LH, and testosterone after exogenous GnRH stimulation (gonadorelin) at Day 46 of treatment compared to pretreatment stimulation and control stallions. Decreases (P < 0.05) were observed in total sperm numbers and motility (week 2) in acyline-treated stallions, as well as total seminal plasma protein (week 2) and testicular volume (week 5). Over the course of the study, the time to erection, time to ejaculation, and number of mounts increased (P < 0.0001) across both groups of stallions; however, there was no effect of treatment or treatment by time interactions on these parameters. Testicular volume, and most seminal parameters regained normal levels within 62 days after treatment ended; on follow-up, sperm output of acyline-treated stallions was regained within 7 months after the end of experiment. In conclusion, acyline reversibly suppresses the stallion hypothalamic-pituitary-gonadal axis.

Anti-Müllerian hormone (AMH) plays a major role in sexual differentiation, Leydig cell differentiation, and folliculogenesis. In addition, AMH has clinical value in equine practice. In stallions, AMH can serve as an endocrine marker for equine cryptorchidism and as an immunohistochemical marker for Sertoli cell tumors. Considering that AMH is also an ovarian specific product, intact mares can be differentiated from ovariectomized mares. Peripheral AMH concentrations reflect the follicular population in mares, and therefore, are useful in the assessment of ovarian reserve and reproductive life-span of aged mares. Last, AMH is particularly suitable as a diagnostic marker for equine granulosa cell tumors.

Circulating anti-Müllerian hormone concentration (AMH) is positively correlated to the number of small growing follicles in the mare and may reflect ovarian function. Dynamics of AMH during immunocontraception have not previously been investigated. This study aimed to compare serum AMH in mares following treatment with native porcine zona pellucida (pZP), recombinant pZP3 and pZP4 (reZP) or gonadotrophin releasing hormone (GnRH) vaccines, and saline-treated controls. Stored sera collected during two previous studies examining ovarian activity in mares during zona pellucida (ZP) or GnRH immunocontraception were analysed for serum AMH. Data were compared among treatment groups using mixed-effects linear regression and one-way ANOVA with post hoc testing. Correlations between AMH and previously reported clinical variables were estimated using Spearman's rho. Mares immunized against GnRH showed variable but detectable AMH throughout successive breeding and non-breeding seasons that were not significantly different to unvaccinated control mares. Mares treated with pZP demonstrated marked, reversible suppression of AMH. Mares immunized using reZP showed an intermediate effect. In the ZP study, AMH was positively correlated to serum progesterone concentrations, mean ovarian volumes and antral follicle counts, whereas no correlations between AMH and serum progesterone concentrations, mean ovarian volumes, or the presence of one or more follicles ≥20 mm in diameter were detected in the GnRH study. In conclusion, marked suppression of AMH during pZP immunocontraception, but not during GnRH immunocontraception, suggested enhanced suppression of ovarian follicular development and, or follicular function during pZP immunocontraception. Serum AMH concentrations may provide a novel tool for the assessment of ovarian function during ZP-based immunocontraception.