Proceedings of the National Academy of Sciences of the United States of America •
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.
Mis MA, Tyagi S, Akin EJ, Ghovanloo MR, Zhao P , et al.
Neurobiology of pain (Cambridge, Mass.) •
Gain-of-function mutations which enhance activation of Na1.7, a widely expressed sodium channel in nociceptors, cause human pain disorders including inherited erythromelalgia (IEM). IEM is characterized by attacks of burning pain in distal extremities triggered by warmth, with cooling of affected limbs providing temporary relief. We investigated the behaviour of the IEM-linked L858F mutant Na1.7 channel at physiological normal skin temperature (NST, 33-35 °C) in IB4-negative DRG sensory neurons known to include thermosensors. Using voltage-clamp recordings at NST we found that the Na1.7-L858F mutant channel shows the characteristic hyperpolarizing shift in activation as has been previously found in recordings at room temperature, and that the current density of the L858F channels is significantly larger than that of WT channels. Using a live-cell optical pulse-chase imaging methodology at NST we observed that accelerated forward-trafficking significantly increases membrane insertion of mutant channels in IB4 neurons. Current-clamp recordings at NST show increased firing of IB4 neurons that express the L858F mutant channel, consistent with increased trafficking of the channel at this physiological temperature. Our findings identify enhanced trafficking and membrane insertion of the L858F mutant channels at normal skin temperature in IB4 neurons as an additional mechanism underlying IEM-related neuronal hyperexcitability.