Expression of auxiliary β subunits of sodium channels in primary afferent neurons and the effect of nerve injury

Abstract

Multiple voltage-gated sodium channels are the primary mediators of cell excitability. They are multimers that consist of the pore-forming α subunit and auxiliary β subunits. Although ion permeability and voltage sensing are primarily determined by the α subunit, β subunits are important modulators of sodium channel function. The purpose of this study was to assess the effect of axotomy on the expression of β subunits (β 1, β 2 and β 3) and coexpression of Na v1.3 and β 3 subunits in the dorsal root ganglion (DRG). We used sciatic nerve transection models or spared nerve injury (SNI) models in the rat. In reverse transcriptase–polymerase chain reaction analysis, there were no significant differences between contralateral and ipsilateral DRGs of β 1 and β 2 mRNA 3 days after axotomy. β 3 mRNA expression in ipsilateral DRGs increased significantly compared with contralateral DRGs 3 days after axotomy. In in situ hybridization histochemistry, β 1 mRNA was predominantly expressed in medium- to large-size neurons, whereas β 2 mRNA was expressed in small- to large-size neurons. There were no significant differences in β 1 and β 2 mRNA between contralateral and ipsilateral DRGs 3 days after axotomy. In contrast, β 3 mRNA was mainly expressed in small neurons and occasionally in medium- to large-size neurons, and β 3 mRNA expression in small c-type neurons in ipsilateral DRGs was increased significantly compared with contralateral DRGs. We examined β 3 mRNA expression with one of α subunits, Na v1.3-ir, in DRG neurons after axotomy using the double labeling method. We found a high percentage of coexpression in injured DRG neurons: 83.6±2.8% of neurons expressing β 3 mRNA were labeled for Na v1.3-ir; 70.1±3.1% of Na v1.3-ir neurons expressed β 3 mRNA. We also examined the expression of β 3 mRNA in DRG neurons in the SNI model, a neuropathic pain model. We used activating transcription factor 3 to identify axotomized neurons, and found that β 3 mRNA up-regulation occurred mainly in axotomized neurons in the neuropathic pain model. These data strongly suggest that β 3 expression in injured DRG neurons following axotomy might be an important pathomechanism of post-nerve injury pain in primary sensory neurons.