Contributions of Na V 1.8 and Na V 1.9 to excitability in human induced pluripotent stem-cell derived somatosensory neurons

The inhibition of voltage-gated sodium (Na V ) channels in somatosensory neurons presents a promising novel modality for the treatment of pain. However, the precise contribution of these channels to neuronal excitability, the cellular correlate of pain, is unknown; previous studies using genetic knockout models or pharmacologic block of Na V channels have identified general roles for distinct sodium channel isoforms, but have never quantified their exact contributions to these processes. To address this deficit, we have utilized dynamic clamp electrophysiology to precisely tune in varying levels of Na V 1.8 and Na V 1.9 currents into induced pluripotent stem cell-derived sensory neurons (iPSC-SNs), allowing us to quantify how graded changes in these currents affect different parameters of neuronal excitability and electrogenesis. We quantify and report direct relationships between Na V 1.8 current density and action potential half-width, overshoot, and repetitive firing. We additionally quantify the effect varying Na V 1.9 current densities have on neuronal membrane potential and rheobase. Furthermore, we examined the simultaneous interplay between Na V 1.8 and Na V 1.9 on neuronal excitability. Finally, we show that minor biophysical changes in the gating of Na V 1.8 can render human iPSC-SNs hyperexcitable, in a first-of-its-kind investigation of a gain-of-function Na V 1.8 mutation in a human neuronal background.