Background & Aims

The excitability in small sensory fibers is altered in neuropathic pain conditions [1,2]. Sodium channels play a key role in shaping the excitability of small fibers and there is a subset of sodium channels (Nav 1.7 – Nav1.9) that are preferentially expressed in small fibers. However, these alterations are not readily assessed in patients. Our research group has previously developed the Perception Threshold Tracking (PTT) technique to indirectly measure the excitability of small sensory nerve fibers [3-8]. A customized electrode is used for preferential small fiber activation. An indirect measurement of the excitability can be obtained by comparing the perception thresholds to different electrical pulse shapes. The PTT technique has been used to describe generic excitability properties of the small sensory nerve fibers. The aim of this project was to further develop the PTT technique by creating a protocol for Sodium Channel Excitability of Nociceptor Testing (SCENT).

Methods

The SCENT protocol has been derived from an in silico model, which predicts the perception thresholds for different pulse shapes of transcutaneous electrical stimulation. The in silico model consists of two-parts: a finite element model (COMSOL) and a multi-compartmental nerve fiber model (NEURON). The protocol has been optimized to identify up-regulation and down-regulation of the maximum conductance of Nav 1.7 – Nav1.9. The SCENT protocol was validated by an in vitro patch-clamp experiment on wild type Nav1.7 channels expressed in HEK293 cells (n=16). The Nav1.7 with the L823R mutation was also expressed in HEK293 cells (n=51). We analyzed the in vitro data using analyses of variance with repeated measurements. Finally, the SCENT protocol’s perception thresholds were estimated in nine healthy subjects with the adaptive Psi method (LabBench).

Results

The SCENT protocol consists of five different pulse shapes composed by ramp and square pulses preceded by a conditioning pre-pulse. The computational model predicted that individual alterations of sodium channel subtypes can be classified on a population level (n=30) with an accuracy of 96% (±19%, SD). On an individual level, the computational model predicts that the SCENT protocol can identify alterations in Nav 1.7 – Nav1.9 with a 75% (±41%, SD) accuracy. The result from the in vitro experiment showed that the measured Nav1.7 current was highly consistent with the current predicted by the in silico model. The SCENT protocol showed that the peak current and the area current generated by the mutant Nav1.7 (L823R) was significantly different from the currents generated by the wildtype (p<0.001).

Conclusions

A SCENT protocol to identify abnormal alterations of the three sodium channel isoforms (Nav1.7-Nav1.9) has been derived. The SCENT protocol has been validated in an in vitro experiment as well as by the PTT technology.

References

1.J. Serra, et. al. “Microneurographic identification of spontaneous activity in C-nociceptors in neuropathic pain states in humans and rats,” Pain, vol. 153, no. 42-55 , pp. :42-55. , 2012.
2.Kleggetveit IP, Namer B, Schmidt R, Helås T, Rückel M, Ørstavik K, Schmelz M, High spontaneous activity of C-nociceptors in painful polyneuropathy.
Jørum E., Pain. 2012, 153:2040-2047
3.K. Hennings, et. al., “Membrane properties in small cutaneous nerve fibers in humans,” Muscle Nerve, no. 55, pp. 195-201, 2017.
4.R. Hugosdottir, et. al., “Preferential activation of small cutaneous fibers through small pin electrode also depends on the shape of a long duration electrical current,” BMC Neurosci., vol. 2, p. 48, 2019.
5. J. Tigerholm, et. al., “Small and large cutaneous fibers display different excitability properties to slowly increasing ramp pulses.,” J Neurophysiol., vol. 124, pp. 883-894, 2020.
6.J. Tigerholm, , et. al., “From Perception Threshold to Ion Channels-A Computational Study,” Biophys J, vol. 117, pp. 281-295, 2019.
7.A. Poulsen, et. al., “Novel surface electrode design for preferential activation of cutaneous nociceptor,” J Neural Eng. , p. 19, 2022.
8.M. Krabsmark Borbjerg, et. al., “The stability of perception threshold tracking for long session evaluation of A?- and A?-fiber function,” Muscle Nerve, vol. 68, pp. 743-749, 2023.

Presenting Author

Jenny Tigerholm

Poster Authors

Jenny Tigerholm

PhD

Uniklinik RWTH Aachen University

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Carsten Mørch

Aalborg University

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Aida Hejlskov Poulsen

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Aylin Kesdo?an

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Sofie Rämisch

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Angelika Lampert Professor MD

Institute of Neurophysiology, University Hospital RWTH Aachen; Scientific Center for Neuropathic Pai

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Ole Kæseler Andersen

Integrative Neuroscience group, Center for Neuroplasticity and Pain (CNAP), Aalborg University

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Jenny Tigerholm

Lead Author

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