Fig. 3. The effects of an intrathecal injection of SB-258719 (20 μg) on systemic tramadol-induced (  A ) and O-desmethyltramadol (M1)-induced (  B ) antinociception. SB-258719 was intrathecally given 30 min after subcutaneous administration of tramadol and M1. The results are presented as mean ± SEM. Tail-flick latencies at the time of peak blocking effects (30 min) were converted to % antinociception to generate the dose-response curve of tramadol (  C ) and M1 (  D ). N = 8 per group. * Differences corresponding to the dose of tramadol and M1 alone , P < 0.05 (Bonferroni  post hoc test).