Ondansetron does not block tramadol-induced analgesia in mice.

Tramadol is a weak opioid agonist and an inhibitor of the reuptake of noradrenaline and serotonin. This study was undertaken to assess a possible pharmacological interaction of ondansetron. a serotonin-3 (5-hydroxytryptamine-3. 5-HT 3 ) antagonist, and tramadol in an animal model for acute pain. Sixty-three male albino mice were randomly given saline, tramadol (10, 20, and 40 mg kg -1 ), ondanselron (1, 2, and 4 mg kg -1 ). or ondansetron (1, 2. and 4 mg kg -1 ) and tramadol (20 mg kg 1 . given 10 min after ondansetron injection) intraperitoneally. Each mouse was assessed twice for tail-flick latency before saline or drug administration and 15, 30, 60, 90, and 120 min thereafter. Tramadol (10 mg kg -1 ) had no effect on pain threshold levels of mice. but tramadol doses of 20 or 40 mg kg -1 increased pain threshold levels in a dose-dependent manner (p 0.05 for 1. 2. and 4 mg kg ') were similar. Our results indicate that ondansetron-a 5-HT 3 selective antagonist-does not decrease the analgesic effectiveness of tramadol in mice. which may be the result of different mechanisms involving 5-HT 3 receptors.