A Single Administration of the GnRH Antagonist Acyline Inhibits Basal and GnRH-Stimulated Serum Testosterone Concentrations in Male Dogs

Abstract

The objective of this study was to describe testosterone (T) response to GnRH challenge in antagonist-treated dogs over a 30-day period. Eight mongrel dogs were randomly assigned to either the GnRH antagonist acyline 330μg/kg sc (ACY; n=4) or a placebo group (PLA; n=4). The dogs were serially challenged with the GnRH agonist, buserelin 0.2μg/kg sc on days -1, 1, 3, 7, 10, 14, 21 and 30. On these days, blood samples for T determinations were collected before (-30min) and 60, 120 and 180min after the agonist injection. Basal (-30min) and post-GnRH agonist stimulation T values were compared by anova for repeated measures. Before treatments (day -1), there were no differences in basal T serum concentrations between groups (p>0.1). After treatments, basal T showed a significant interaction between treatment and day (p<0.05). Furthermore, when both groups were analysed independently, basal T varied in the ACY (p<0.01) but not in the PLA group (p>0.1). On day -1, before treatments, the stimulation tests had only a time effect (p=0.05) although on days 1 (p<0.01), 3 (p<0.01), 7 (p<0.01), 10 (p<0.01) and 14 (p<0.05), the response to the agonist differed between groups, becoming similar on days 21 (p>0.05) and 30 (p>0.05). It was concluded that, in dogs, a single administration of the GnRH antagonist prevented canine gonadal axis to physiologically respond to agonistic challenge during 14 days.