Preliminary findings of cerebral responses on transcutaneous vagal nerve stimulation on experimental heat pain
Transcutaneous vagal nerve stimulation (TVNS) is a promising complementary method of pain relief. However, the neural networks associated with its analgesic effects are still to be elucidated. Therefore, we conducted two functional magnetic resonance imaging (fMRI) sessions, in a randomized order, with twenty healthy subjects who were exposed to experimental heat pain stimulation applied to the right forearm using a Contact Heat-Evoked Potential Stimulator. While in one session TVNS was administered bilaterally to the concha auriculae with maximal, non-painful intensity, the stimulation device was switched off in the other session (placebo condition). Pain thresholds were measured before and after each session. Heat stimulation elicited fMRI activation in cerebral pain processing regions. Activation magnitude in the secondary somatosensory cortex, posterior insula, anterior cingulate and caudate nucleus was associated with heat stimulation without TVNS. During TVNS, this association was only seen for the right anterior insula. TVNS decreased fMRI signals in the anterior cingulate cortex in comparison with the placebo condition; however, there was no relevant pain reducing effect over the group as a whole. In contrast, TVNS compared to the placebo condition showed an increased activation in the primary motor cortex, contralateral to the site of heat stimulation, and in the right amygdala. In conclusion, in the protocol used here, TVNS specifically modulated the cerebral response to heat pain, without having a direct effect on pain thresholds.
KeywordsTranscutaneous vagal nerve stimulation (TVNS) Thermal pain Functional MRI Placebo stimulation
The authors thank Vasyl Gizhko from the Department of Experimental Physics, University of Kiev, Ukraine, for his design and tests of the stimulation electrode; MTR GmbH, Germany for providing the TENS device; Dr. Konrad Meissner for providing the CHEPS thermode and the volunteers, who participated in this investigation. We would like to thank Henriette Hacker and Dr. Mike Cummings for carefully rechecking the manuscript for style and spelling mistakes.
Compliance with Ethical Standards
All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, and the applicable revisions at the time of the investigation. Informed consent was obtained from all patients for being included in the study
Conflict of interest
Taras Usichenko, René Laqua, Bianca Leutzow and Martin Lotze declare that he/she has no conflict of interest.
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