Characterization of the unusual antinociceptive profile of tramadol in mice

The present study characterized the antinociceptive effects of tramadol with emphasis on the site of action, the possible development of antinociceptive tolerance, and possible cross‐tolerance to morphine. Both tramadol and morphine produced antinociception as assessed in male, ICR mice using the warm water (55°C) tail‐flick test. Intrathecal (i.t.) tramadol produced an antinociceptive response, but with a shallow dose‐response curve (DRC), but i.c.v. tramadol was ineffective up to toxic doses (> 380 nmol). Simultaneous co‐administration of tramadol by the i.c.v. and i.t. routes in a 1:1 fixed dose‐ratio resulted in a synergistic interaction. Construction of a naloxone dose‐response curve against an A₉₀ dose of agonist yielded an AD₅₀ (and 95% C.L.) of 0.19 (0.14–0.33) and 0.55 (0.38–0.74) μmol/kg against morphine and tramadol, respectively; both agonists were completely antagonized by naloxone. In mice pretreated twice daily (8:00 a.m. and 5:00 p.m.) with s.c. morphine the DRC for morphine was displaced 16‐fold to the right, indicating significant development of antinociceptive tolerance. The same pretreatment resulted in only a 3‐fold right‐ward shift in the tramadol DRC. Chronic twice‐daily (8:00 a.m. and 5:00 p.m.) pretreatment with tramadol resulted in only an approximate 3‐ and 4‐fold rightward displacement in the tramadol and morphine DRCs, respectively. These results suggest that tramadol has an unusual antinociceptive mechanism which may reflect a primary site of action at spinal sites with a synergistic spinal/supraspinal interaction. Further, the lack of complete cross‐tolerance between tramadol and morphine supports the suggestion of a non‐opioid mechanism for this compound, whereas the complete antagonism by naloxone apparently reflects the opioid component of its mechanism in this test. Finally, the minimal development of tolerance to tramadol antinociception reinforces the view of a favorable tolerance‐dependence profile for this compound. © 1993 Wiley‐Liss, Inc.