Cell type-specific changes of the membrane properties of peripherally- axotomized dorsal root ganglion neurons in a rat model of neuropathic pain

Recent evidence indicates that neuropathic pain from partial peripheral nerve injury is maintained by electrophysiologically abnormal signals from injured sensory neurons. To gain an insight into the mechanisms underlying this electrophysiological abnormality, we examined the effects of S₁ spinal nerve transection on the membrane properties of S₁ dorsal root ganglion neurons one to two weeks after injury. This injury produced significant action potential broadening [40% (1 ms) in C-, 149% (1.5 ms) in Aδ- and 84% (0.5 ms) in Aα/β-cells], which was primarily due to the enhancement of the 'shoulder' appearing on the falling phase of the action potential in C- and Aδ-cells and the emergence of a shoulder in Aα/β-cells, and significant cell-type specific changes in the time-course of the rising phase of the action potential: i.e. an increase in rise time (Aδ: 35%, 0.15 ms; Aα/β: 13%, 0.04 ms) and a decrease in the maximal rate of rise (Aδ: 17%, 77 V/s; Aα/β: 13%, 79 V/s). In addition, the nerve injury led to a significant reduction of the rheobase, an index of neuronal excitability, in all types of cells (by 41% in C-, 71% in Aδ- and 59% in Aα/β-cells). The reduction of rheobase in A-cells was associated with a concomitant increase in apparent input resistance (by 269% in Aδ- and 192% in Aα/β-cells), which was measured near the resting membrane potential. By contrast, the rheobase reduction in C-cells was associated with a concurrent depolarizing shift (~4 mV) of the resting membrane potential. The nerve injury-induced reduction of rheobase was not accompanied by related change in input resistance or threshold potential in any of the cell populations. The present results indicate that chronic peripheral axotomy of dorsal root ganglion neurons, which gives rise to neuropathic pain, produces profound changes in the action potential waveform of dorsal root ganglion neurons in a cell type-specific fashion. Furthermore, the results suggest that the axotomy increases the excitability of dorsal root ganglion neurons not by altering input resistance (i.e. leak conductance) or threshold potential, but by increasing apparent input resistance near the resting membrane potential in A-cells and decreasing the resting membrane potential in C-cells.