Fig. 5. The effects of an intrathecal injection of ondansetron (20 μg) on systemic tramadol-induced ( A ) and O-desmethyltramadol (M1)-induced ( B ) antinociception. Ondansetron was intrathecally given 30 min after subcutaneous administration of tramadol and M1. The results are presented as mean ± SEM. Tail-flick latencies at 60 min after tramadol ( C ) and M1 ( D ) administration were converted to % antinociception to generate the dose-response curve. N = 8 per group.