AXOTOMY INCREASES THE EXCITABILITY OF DORSAL-ROOT GANGLION-CELLS WITHUNMYELINATED AXONS

To better understand the neuronal mechanism of neuropathic pain, the e ffect of axotomy on the excitability of dorsal root ganglion (DRG) cel ls with unmyelinated axons (C cells) was investigated. Whole cell patc h-clamp recordings were performed on intact DRG cells with intact axon s or with axons transected 7-12 days earlier. C cells were identified by I) soma size, 2) action potential morphology, 3) conduction velocit y, and 4) in some cases, injection of Fast Blue into the injured nerve fibers. Axotomy reduced (more negative) action potential threshold bu t did not significantly change resting membrane potential, action pote ntial duration, or maximal depolarization rate. Axotomy significantly increased the peak sodium current measured under voltage-clamp conditi ons. In Fast Blue-labeled (injured) cells, the tetrodotoxin (TTX)-sens itive current was enhanced while the TTX-resistant current was reduced . These results suggest that axotomy increased the excitability of C c ells, possibly because of a preferential increase in expression of TTX -sensitive sodium currents.