Biophysical dissection of nociceptor hyperexcitability caused by a Nav1.8 gain-of-function mutation linked to severe pain

Voltage-gated sodium channel Nav1.8 is highly expressed in nociceptors, where it plays a critical role in sustaining repetitive action potential (AP) firing. Gain-of-function Nav1.8 mutations that increase nociceptor excitability have been identified in patients with painful peripheral neuropathy, but the biophysical mechanisms by which they confer nociceptor hyperexcitability are incompletely understood. Here we carry out a high-resolution dissection of the functional consequences of a Nav1.8 mutation (G1662S) identified in human subjects with severe neuropathic pain, using dynamic clamp modeling in small dorsal root ganglion (DRG) neurons. While Nav1.8WT/GS conductance did not alter resting membrane potential, rheobase, or single AP threshold, it produced a marked hyperexcitability during repetitive firing. Nav1.8WT/GS neurons generated nearly twice as many APs as wild-type controls in response to suprathreshold depolarization, an effect attributable to increased sodium charge transfer across successive spikes. Charge analysis revealed that the GS mutation disproportionately enhanced suprathreshold sodium influx, supporting greater AP fidelity without adaptation. Biophysical dissection showed that this excitability phenotype arises from frequency-dependent mechanisms: at lower firing frequencies, the depolarizing shift in steady-state inactivation increases channel availability and contributes to G1662S-mediated hyperexcitability, whereas at higher firing frequencies both the depolarized voltage-dependence of inactivation and the accelerated recovery from inactivation further sustain G1662S hyperexcitability. Together, these properties enable Nav1.8WT/GS neurons to maintain enhanced firing across a broad range of frequencies, in contrast to wild-type nociceptors that typically adapt faster. These findings provide mechanistic insight into Nav1.8-driven hyperexcitability and highlight Nav1.8 as a therapeutic target for genetic and acquired pain syndromes.