Background: Lidocaine may be useful in the treatment of neuropathic pain states. The authors hypothesized that lidocaine would reduce tactile allodynia observed in a rat nerve injury model. In an effort to determine the site of drug action, effects after intravenous, intrathecal, and regional administration were compared.
Methods: Rats underwent ligation of the left fifth and sixth lumbar spinal nerves. The 50% thresholds (g) for left hind paw withdrawal of awake rats to von Frey hairs were documented before, during, and after intravenous administration of lidocaine at programmed/documented pseudo-steady-state plasma concentrations, and correlated with measured plasma concentrations. Responses to lidocaine application intrathecally and regionally to the injured nerves were also recorded.
Results: In rats with tactile allodynia, intravenous lidocaine yielded 66 +/- 11% of the maximal possible effect on thresholds (100% = normal threshold), versus -1.3 +/- 2.7% for saline infusion. Twenty-one days after lidocaine infusion, 30-40% of the maximal possible effect persisted. Threshold increases depended on plasma concentration, rather than quantity of drug administered: rats receiving 15 mg/kg with higher plasma concentrations (1.2 +/- 0.1 micrograms/ml) showed significant allodynia suppression throughout 7 days of follow-up, whereas rats receiving 15 mg/kg at a slower rate with lower plasma concentrations (0.6 +/- 0.1 microgram/ml) did not. The EC50 for acute allodynia suppression was 0.75 microgram/ml. No such allodynia suppression was seen after intrathecal or regional administration of lidocaine despite transient neural blockade.
Conclusions: Intravenous, but not intrathecal or regionally applied, lidocaine produces dose-dependent suppression of allodynia associated with nerve injury. The effects far outlast plasma concentrations of lidocaine. The mechanism of these prolonged effects is unknown.