Anodal Transcutaneous Spinal Direct Current Stimulation (tsDCS) Selectively Inhibits the Synaptic Efficacy of Nociceptive Transmission at Spinal Cord Level

Abstract

Recently studies have aimed at developing transcutaneous spinal direct current stimulation (tsDCS) as a non-invasive technique to modulate spinal function in humans. Independent studies evaluating its after-effects on nociceptive or non-nociceptive somatosensory responses have reported comparable effects suggesting that tsDCS impairs axonal conduction of both the spino-thalamic and the medial lemniscus tracts. The present study aimed to better understand how tsDCS affects, in humans, the spinal transmission of nociceptive and non-nociceptive somatosensory inputs. We compared the after-effects of anodal low-thoracic, anodal cervical and sham tsDCS on the perception and brain responses elicited by laser stimuli selectively activating Aδ-thermonociceptors of the spinothalamic system and vibrotactile stimuli selectively activating low-threshold Aβ-mechanoreceptors of the lemniscal system, delivered to the hands and feet. Low-thoracic tsDCS selectively and significantly affected the LEP-N2 wave elicited by nociceptive stimulation of the lower limbs, without affecting the LEP-N2 wave elicited by nociceptive stimulation of the upper limbs, and without affecting the SEP-N2 wave elicited by vibrotactile stimulation of either limb. This selective and segmental effect indicates that the neuromodulatory after-effects of tsDCS cannot be explained by anodal blockade of axonal conduction and, instead, are most probably due to a segmental effect on the synaptic efficacy of the local processing and/or transmission of nociceptive inputs in the dorsal horn.

Keywords: EEG; Transcutaneous spinal direct current stimulation; laser-evoked potentials; nociception; somatosensory system.

Copyright © 2018 IBRO. Published by Elsevier Ltd. All rights reserved.

Figures

Figure 3.1

Experimental design. Intensity of perception and ERPs elicited by non-nociceptive and nociceptive stimulation of the hands and feet were recorded before and immediately after anodal cervical tsDCS and sham low-thoracic tsDCS, anodal low-thoracic tsDCS and sham cervical tsDCS, and sham cervical and low-thoracic tsDCS, in three separate experimental sessions. The cervical anode electrode was located above the spinous process of C7. The low-thoracic anode electrode was located below the spinous process of T10. The reference cathode electrode was located on the right shoulder (deltoid muscle). Nociceptive stimuli consisted of short lasting laser pulses delivered on the hand and feet dorsum. Non-nociceptive stimuli were short lasting mechanical vibration delivered on the palmar side of the third fingertip and hallux.

Figure 3.2

Effects of tsDCS on perceived intensity elicited by somatosensory stimuli. Differences in the intensity of the percept (numerical rating scale [NRS] after minus before tsDCS) elicited by nociceptive laser stimulation (panel A) and non-nociceptive vibrotactile stimulation (panel B) of the hand and foot are for cervical tsDCS, low-thoracic tsDCS and sham tsDCS. Subject-level differences (after minus before tsDCS) are represented by the connected lines and black dots. Box plots represent the group-level average ± SD.

Figure 3.3

Grand-average waveforms of the ERPs elicited by nociceptive laser stimulation delivered on the hands. ERPs waveforms obtained after stimulation of the hands are shown before (blue) and after (red) applying cervical, low-thoracic and sham tsDCS. The early latency LEP-N1 component is shown at the contralateral temporal electrode (Tc) vs. Fz (panel A). The LEP-N2 and LEP-P2 components are shown at Cz vs. M1M2 (panel B). The scalp topographies of the different LEP components are shown in the blue (before tsDCS) and red (after tsDCS) frames. Box plots represent the group-level average ± SD of the difference (Δ after - before tsDCS) in magnitude of the three LEP components.

Figure 3.4

Grand-average waveforms of the ERPs elicited by nociceptive laser stimulation delivered on the feet. ERPs waveforms obtained after stimulation of the feet are shown before (blue) and after (red) applying cervical, low-thoracic and sham tsDCS. The early latency LEP-N1 component is shown at the contralateral temporal electrode (Tc) vs. Fz (panel A). The LEP-N2 and LEP-P2 components are shown at Cz vs. M1M2 (panel B). The scalp topographies of the different LEP components are shown in the blue (before tsDCS) and red (after tsDCS) frames. Box plots represent the group-level average ± SD of the difference (Δ after - before tsDCS) in magnitude of the three LEP components.

Figure 3.5

Effect of cervical tsDCS, low-thoracic tsDCS and sham tsDCS on the magnitude of LEP-N1 (panel A), LEP-N2 (panel B) and LEP-P2 (panel C) components of the ERPs elicited by nociceptive stimulation of the hands and feet. Subject-level differences are represented by the connecting lines and dots. Box plots represent the group-level average ± SD.

Figure 3.6

Grand-average waveforms of the ERPs elicited by non-nociceptive vibrotactile stimulation. ERPs waveforms obtained after stimulation of the hands (panel A) and feet (panel B) are shown before (blue) and after (red) applying cervical, low-thoracic and sham tsDCS. The SEP-N2 and SEP-P2 components are shown at Cz vs. M1M2 with their respective scalp topographies before (blue frame) and after (red frame) tsDCS. Box plots represent the group-level average ± SD of the difference (Δ after - before tsDCS) in magnitude of the two SEP components.

Figure 3.7

Effect of cervical tsDCS, low-thoracic tsDCS and sham tsDCS on the magnitude of SEP-N2 (panel A) and SEP-P2 (panel B) components of non-nociceptive ERPs elicited by non-nociceptive stimulation of the hands and feet. Subject-level differences (Δ after - before tsDCS) are represented by connecting lines and dots. Box plots represent the group-level average ± SD.