HYPERPOLARIZING AFTERPOTENTIALS IN C-FIBERS AND LOCAL-ANESTHETIC EFFECTS OF CLONIDINE AND LIDOCAINE

Effects of clonidine and lidocaine on the hyperpolarizing afterpotential (HAP) and frequency-dependent block in C fibers were examined on desheathed rabbit vagus nerves, using the sucrose gap technique. A single action potential (AP) was followed by a fast and a slow HAP. Clonidine, at concentrations from 0.05 to 50 mu mol/l, decreased the fast HAP, while the AP amplitude was unchanged. At a 500 mu mol/l concentration of clonidine, the fast HAP amplitude was similar to control, the slow HAP was increased, and the AP amplitude decreased. Lidocaine at 500 mu mol/l delayed and broadened the HAP, making a distinction between fast and slow HAP impossible, and decreased and delayed the AP amplitude. In the presence of lidocaine (500 mu mol/l), clonidine at concentrations from 0.05 to 500 mu mol/l decreased the HAP amplitude, without modifying the lidocaine-induced shape of the HAP. The modifications of the HAP, however, do not contribute to the local anesthetic effects of clonidine, as the addition of clonidine (0.5 and 500 mu mol/l) to Locke or lidocaine (500 mu mol/l) solution does not enhance the frequency-dependent block (3 and 10Hz) observed with either Locke or lidocaine solution alone.