Effect of a GnRH Antagonist on Anti-Müllerian Hormone and Follicular Population in Anestrous Bitches

SIMPLE SUMMARY: This study was carried out to describe the effect of a new GnRH antagonist, acyline, on ovarian follicular numbers and the blood anti-Müllerian hormone (AMH) in non-cycling bitches. Acyline was serially administered for 60 days; then the females were followed up for an additional 45 days and spayed. Blood samples were taken and ovaries microscopically studied. None of the female dogs presented an estrus cycle during treatment. One bitch presented a normal cycle after it. The total number and proportions of ovarian follicles were normal. AMH diminished to very low values during treatment and rapidly increased after its effect. Acyline rapidly and reversibly prevented the initiation of cycling without affecting follicle count but diminishing AMH concentrations. AMH appears to be a functional biomarker to assess ovarian response to this inhibitory pharmacological protocol. ABSTRACT: The objective of this study was to describe the effect of the third-generation GnRH antagonist, acyline, on ovarian follicular population and serum anti-Müllerian hormone (AMH) concentrations in female dogs. Four late anestrous bitches were administered 330 μg/kg SC acyline every 10 days for 60 days and followed up for 45 days. Blood samples were drawn on days −1, 15, 30, 45, 60, 75, 90 and 105 for AMH determination. Then, the females were ovariectomized and the excised ovaries were gross and histologically evaluated. The total ovarian follicles were counted. None of the female dogs presented estrus during treatment. Only one bitch presented an ovulatory estrus 20 days after treatment. The total number of ovarian follicles in these bitches was 96,200.10 ± 26,125.12, with 84.13%, 11.36%, 7.8% and 0.01% corresponding to primordial, primary, secondary and antral structures, respectively. Pretreatment AMH concentrations were 0.62 ± 0.17 ng/mL. This hormone varied throughout the study period (p < 0.01), diminishing to nadir values during treatment to then rapidly recover after its effect (0.2 ± 0.05 vs. 0.67 ± 0.22 ng/mL; p < 0.01). Acyline rapidly and reversibly prevented the initiation of cycling without affecting follicle count but diminishing serum AMH concentrations.