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.