Clonidine enhances the effects of lidocaine on C-fiber action potential.

We examined local anesthetic effects of clonidine and its interaction with lidocaine with regard to tonic inhibition of the C-fiber action potential (AP) on the isolated, desheathed rabbit vagus nerve by the sucrose gap method. Clonidine and lidocaine at 500 microM concentrations caused a comparable degree of C-fiber inhibition, corresponding to an AP area under the curve of 75.8% +/- 9.4% (mean +/- SE) and 82.2% +/- 5.9% of control, respectively. Concentrations of clonidine less than 500 microM did not inhibit C-fiber AP. Clonidine, added in concentrations of 500 nM, 500 microM, and 5 mM to a 500 microM lidocaine perfusion, caused a significant decrease in fiber blockade of 18%, 20%, and 54%, respectively, as compared with clonidine added to Locke perfusion (P less than 0.05). The sodium channel blocker tetrodotoxin (3 microM) decreased the AP area to 9.3% +/- 1.3% of control. The remaining tetrodotoxin-resistant AP was almost completely blocked by clonidine (500 microM) and lidocaine (500 microM), indicating a higher susceptibility of tetrodotoxin-resistant fibers to the two drugs than the C-fiber population as a whole. The enhancing effect of a low dose of clonidine (500 nM) on lidocaine-induced (500 microM) inhibition of C-fiber AP might explain the clinical observation that clonidine, at approximately 1000-fold lower concentrations than lidocaine, prolongs the action of lidocaine in peripheral nerve block.