Fig. 10. The effects of an intrathecal injection of ketanserin (20 μg) on systemic tramadol-induced ( A ) and O-desmethyltramadol (M1)-induced ( B ) thermal antihyperalgesia. Ketanserin was intrathecally given 30 min after subcutaneous administration of tramadol and M1. The results are presented as mean ± SEM. Paw withdrawal latencies at 60 min after tramadol ( C ) or M1 ( D ) administration were converted to % reversal of hyperalgesia to generate the dose-response curve. N = 8 per group.