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Various combinations of gonadotropin-releasing hormone (GnRH) antagonists and long-acting GnRH agonists have been assessed in several species to prevent the "flare-up" effect that agonists cause on the pituitary-gonadal axis. To determine the effect of a single administration of the GnRH antagonist acyline in anestrous GnRH agonist-implanted domestic bitches, 19 dogs (canis familiaris) were randomly assigned to receive either 10mg sc deslorelin acetate (DA; n=6) or DA combined with 330 microg/kg sc acyline within the first 48 h (DA & ACY; n=13). These bitches were examined daily for detection of posttreatment flare-up, manifested as an estrous response during the month after treatment. In the DA and DA&ACY groups, an estrous response was detected in 6 of 6 and 9 of 12 (P<0.5) of the bitches, starting 5.3+/-1.3 and 10.1+/-1.8 d (mean+/-SEM, P=0.5), respectively, after treatment. Based on serum progesterone concentrations, ovulation occurred in 6 of 6 and 5 of 9 of these bitches (P=0.1). None of the dogs had any local or systemic side effects related to the treatments. In five DA and six DA&ACY bitches that could be followed up after the trial, interestrus intervals were 385+/-22.5 and 330+/-69.1 d, respectively (P>0.1). It was concluded that the current antagonist protocol prevented initial ovarian stimulation in one quarter of the treated dogs, whereas the stimulation period was postponed and ovulation was inhibited in approximately half of the remainder.

Gonadotrophin-releasing hormone (GnRH) antagonists may have a future role in the control of canine reproductive function. In this study, the effects of a single dose of the potent GnRH antagonist, acyline, on serum concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) were evaluated in male dogs. Blood samples were drawn before (Day -1) and after (30, 60, and 90 min, 3, 6, 9, 12, and 24h, and 3, 6, 9, 14, 22, and 29 d) treatment with acyline (330 microg/kg, sc); serum concentrations of FSH, LH, and T varied throughout the study period (P<0.01, <0.05, and <0.01, respectively). Gonadotrophins decreased below pretreatment concentrations 60 min after injection, whereas T took 90 min to decrease below baseline (P>0.05). Follicle-stimulating hormone, LH and T decreased until Day 9, when they reached their nadir at 2.0 +/-1.1 ng/mL (P<0.01), 1.2+/-0.2 ng/mL (P>0.05), and 0.5+/-0.2 ng/mL (P<0.05), respectively. Both gonadotrophins and T began increasing on Day 14 after treatment, although FSH and T serum concentrations still remained below baseline on that day (P>0.05). Follicle-stimulating hormone and T rebounded above baseline on Day 29, whereas LH reached concentrations were similar to baseline at this time (P>0.05). No local or systemic side effects were detected in any dog following acyline treatment. In conclusion, a single acyline treatment safely and reversibly decreased serum gonadotrophin and T concentrations in dogs for 9 d.