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

Abstract

Voltage-gated sodium channels (VGSC) are responsible for the fast upstroke of an action potential (AP). In the peripheral nervous system, e.g., in sensory neurons responsible for detection of potentially painful stimuli, subthreshold depolarizations support the initiation of an AP. Nav1.7 was suggested to be a subthreshold channel, and recent studies from human cells assign it a role as threshold channel. We examine the biophysical properties of heterologously expressed sensory neuron VGSCs Nav1.1-Nav1.5 and Nav1.6-Nav1.8 with focus on the subthreshold depolarization phase of APs in manual patch clamp. VGSCs were similar in their biophysical properties; except for Nav1.8 which gated at more depolarized and Nav1.5 at more hyperpolarized potentials. For Nav1.1, Nav1.2 and Nav1.8, but not the remaining tested VGSCs, a positive correlation between ramp current and window currents was detected. During AP clamp Nav1.7 and Nav1.3 generated currents with the largest area under the curve compared to the other channels tested here. Experimental results were fitted by a modified Hodgkin-Huxley model and a simple AP was simulated. We adjusted the VGSC composition according to published RNAseq data to resemble a putative silent nociceptor and an Aδ-fiber. In both fiber types, the amount of Nav1.7 seems to strongly influence whether a neuron fires repetitively or not. In summary, we provide a comprehensive, comparable set of patch-clamp data of most of the VGSCs relevant for nociception, which can be used as basis for computational modelling, disease modelling and the development of adapted, targeted future pain medication.