Comparative study of voltage-gated sodium channel α-subunits in non-overlapping four neuronal populations in the rat dorsal root ganglion.

Voltage-gated sodium channel α-subunit (Nav) is the major determinant of neuronal electrophysiological characters. In order to compare the composition of Navs among neurochemically different neurons in the rat dorsal root ganglion (DRG), we examined the expression of Nav transcripts in four non-overlapping neuronal populations, with (+) or without (-) N52 immunoreactivity, a marker of neurons with myelinated axons, and TrkA mRNA identified by in situ hybridization histochemistry. Both N52-/TrkA+ and N52-/TrkA- populations had high levels of signals for Nav1.7, Nav1.8, and Nav1.9 mRNAs, but rarely expressed Nav1.1 or Nav1.6. There was no significant difference in these signals, suggesting that C-fiber peptidergic and non-peptidergic neurons have similar electrophysiological characters with regard to sodium currents. N52+/TrkA+ neurons (putative A-fiber nociceptors) had similar high levels of signals for Nav1.7 and Nav1.8, but a significantly lower level of Nav1.9 signals, as compared to N52- neurons. Although, almost no N52+/TrkA- neurons had Nav1.8 or Nav1.9, half of this population expressed Nav1.7 at similar levels to other three populations and the other half completely lacked this channel. These data suggest that Nav1.8 is a common channel for both C- and A-fiber nociceptors, and Nav1.9 is rather selective for C-fiber nociceptors. Nav1.7 is the most universal channel while some functionally unknown N52+/TrkA- subpopulation selectively lacks it.