Nav1.8: Intrinsic limits on the functional effect of abrogation in DRG neurons

Voltage-gated sodium channel Nav1.8 plays a crucial role in regulating excitability of small dorsal root ganglion (DRG) neurons and is an emerging target for pain therapeutics. Using dynamic clamp, we systematically manipulated Nav1.8 conductance to assess its impact on action potential (AP) electrogenesis, rheobase, and repetitive firing in native rat DRG neurons and those expressing the gain-of-function Nav1.7L858H mutation which underlies inherited erythromelalgia, a human genetic pain disorder. Our findings reveal that the Nav1.8 contribution to net sodium current is highly correlated with AP voltage threshold. Nav1.8 conductance regulated AP overshoot and voltage threshold without significantly affecting undershoot or resting membrane potential. We identified two populations of wild-type DRG neurons: strong responders (50% of cells), which exhibited substantial rheobase modulation with alterations in Nav1.8 conductance, and weak responders (50% of cells), which remained largely unaffected. In hyperexcitable Nav1.7L858H-expressing neurons, partial Nav1.8 subtraction (50%) restored rheobase above control levels in 63% of cells. However, weak responders (37%) remained hyperexcitable. The effect of Nav1.8 subtraction in responsive neurons supports the conclusion that Nav1.8 inhibition can reduce neuropathic pain. However, the presence of weakly responsive DRG neurons suggests that other channels might need to be targeted for full pain relief.