Tetrodotoxin-resistant Na+ spikes of C fibers have at least two subtypes in the isolated bullfrog sciatic nerve

The C fiber compound action potentials (C-CAPs) were studied for their properties with the vaseline gap method from the isolated bullfrog sciatic nerve, Two components (the fast and the slow C-CAPs) were found to be Na+ dependent and terrodotoxin-resistant. The conduction velocities of the fast and the slow C-CAPs were 0.61 +/- 0.06 m/s (n = 13) and 0.42 +/- 0.05 m/s (n = 8), respectively; their time constants were 11.4 +/- 1.7 ms and 16.3 +/- 1.7 ms, respectively, with both parameters being significantly different at P < 0.01. The slow C-CAP had about 1.3 times higher threshold than that of the fast C-CAP, with a significant difference (P < 0.01). They showed differential sensitivity to lidocaine and Cd2+. Capsaicin reduced the amplitudes of both fast and slow C-CAPs, but not abolished. These findings indicate that C fibers have at least two subtypes with different properties. (C) 1996 Elsevier Science Ireland Ltd.