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Ann Neurol. 2018 May;83(5):945-957. doi: 10.1002/ana.25231. Epub 2018 May 11.

Tuning in C-nociceptors to reveal mechanisms in chronic neuropathic pain.

Author information

1
Department of Experimental Pain Research, Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany.
2
Department of Anesthesiology, Stanford University, Stanford, CA.
3
Department of Physiology I, Friedrich-Alexander-University Erlangen-Nuremberg, Nuremberg, Germany.
4
Centre for Pain Medicine, Swiss Paraplegic Centre, Nottwil, Switzerland.
5
London Pain Consortium, Neurorestoration Group, King's College London, London, United Kingdom.
6
Department of Anaesthesiology, Kantonsspital Lucerne, Lucerne, Switzerland.
7
Department Clinical Neurophysiology, Uppsala University, Uppsala, Sweden.

Abstract

OBJECTIVE:

Develop and validate a low-intensity sinusoidal electrical stimulation paradigm to preferentially activate C-fibers in human skin.

METHODS:

Sinusoidal transcutaneous stimulation (4Hz) was assessed psychophysically in healthy volunteers (n = 14) and neuropathic pain patients (n = 9). Pursuing laser Doppler imaging and single nociceptor recordings in vivo in humans (microneurography) and pigs confirmed the activation of "silent" C-nociceptors. Synchronized C-fiber compound action potentials were evoked in isolated human nerve fascicles in vitro. Live cell imaging of L4 dorsal root ganglia in anesthetized mice verified the recruitment of small-diameter neurons during transcutaneous 4-Hz stimulation of the hindpaw (0.4mA).

RESULTS:

Transcutaneous sinusoidal current (0.05-0.4mA, 4Hz) activated "polymodal" C-fibers (50% at ∼0.03mA) and "silent" nociceptors (50% at ∼0.04mA), intensities substantially lower than that required with transcutaneous 1-ms rectangular pulses ("polymodal" ∼3mA, "silent" ∼50mA). The stimulation induced delayed burning (nonpulsating) pain and a pronounced axon-reflex erythema, both indicative of C-nociceptor activation. Pain ratings to repetitive stimulation (1 minute, 4Hz) adapted in healthy volunteers by Numeric Rating Scale (NRS) -3 and nonpainful skin sites of neuropathic pain patients by NRS -0.5, whereas pain even increased in painful neuropathic skin by approximately NRS +2.

INTERPRETATION:

Sinusoidal electrical stimulation at 4Hz enables preferential activation of C-nociceptors in pig and human skin that accommodates during ongoing (1-minute) stimulation. Absence of such accommodation in neuropathic pain patients suggest axonal hyperexcitability that could be predictive of alterations in peripheral nociceptor encoding and offer a potential therapeutic entry point for topical analgesic treatment. Ann Neurol 2018;83:945-957.

PMID:
29659054
DOI:
10.1002/ana.25231

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