Sodium channels expressed in nociceptors contribute distinctly to action potential subthreshold phase, upstroke and shoulder

Voltage-gated sodium channels (VGSC) in the peripheral nervous system shape action potentials (AP) and thereby support the detection of sensory stimuli. Most of the nine mammalian VGSC subtypes are expressed in nociceptors, but predominantly, three are linked to several human pain syndromes: while Na v 1.7 is suggested to be a (sub-)threshold channel, Na v 1.8 is thought to support the fast AP upstroke. Na v 1.9, as it produces large persistent currents, is attributed a role in determining the resting membrane potential. We characterized gating of Na v 1.1-Na v 1.3 and Na v 1.5-Na v 1.9 in manual patch clamp with focus on the AP subthreshold depolarization phase. Na v 1.9 exhibited the most hyperpolarized activation while its fast inactivation resembled the depolarized inactivation of Na v 1.8. For some VGSCs (e.g., Na v 1.1 and Na v 1.2), a positive correlation between ramp current and window current was detected. Using a modified Hodgkin-Huxley model which accounts for the time needed for inactivation to occur, we used the acquired data to simulate two nociceptive nerve fiber types (an Aδ-and a mechano-insensitive C-nociceptor) containing VGSC conductances according to published human RNAseq data. Our simulations suggest that Na v 1.9 is supporting both the AP upstroke and its shoulder. A reduced threshold for AP generation was induced by enhancing Na v 1.7 conductivity or shifting its activation to more hyperpolarized potentials, as observed in Na v 1.7-related pain disorders. Here, we provide a comprehensive, comparative functional characterization of VGSCs relevant in nociception and describe their gating with Hodgkin-Huxley-like models, which can serve as a tool to study their specific contributions to AP shape and sodium channel-related diseases. Disclaimer Parts of this study were published as a preprint on bioRxiv: Köster, P.A., T. Stiehl, J. Tigerholm, A. Maxion, B. Namer, and A. Lampert. 2023. Biophysics of sodium channels during subthreshold depolarization in vitro and in silico . bioRxiv. doi.org/10.1101/2023.09.03.556095 (Preprint posted September 6, 2023) Summary Subthreshold gating of seven sodium channels (Na v 1.1-3, Na v 1.5-8) is determined by manual patch clamp and, together with Na v 1.9, integrated into a computer model of an Aδ-and a mechano-insensitive nociceptor (CMi). Simulations reveal contribution of Na v 1.9 to the action potential upstroke and shoulder and prove useful for Na v 1.7-related disease modelling.