Background and objectives: Tramadol hydrochloride is known as a centrally acting analgesic drug, used for the treatment of moderate to severe pain. A local analgesic effect has been demonstrated, but its mechanism of action remains unclear.
Methods: In this study, we examined the effect of local, systemic and nerve block tramadol on the nociceptive flinching behavior elicited by injection of 50 microL of 1% formalin into the dorsal region of hind paw of rats. Nociceptive flinching behavior was observed for 60 minutes.
Results: Local tramadol in higher concentrations (2.5 and 5 mg) almost eliminated flinching behavior during the entire test. Systemic and neural block tramadol did not affect flinching behavior in phase I and partially decreased it in phase II.
Conclusions: Tramadol presented a local analgesic effect in formalin nociceptive flinching behavior that is different from its central analgesic effect. This analgesic effect, in this model, seems not to be linked to a local anesthetic like effect.