The effects of transcutaneous auricular vagal nerve stimulation on cortical GABA-ergic and cholinergic circuits: a transcranial magnetic stimulation study.

Abstract

Neurophysiological evidence that transcutaneous auricular vagal nerve stimulation (taVNS) affects neuronal signaling at the cortical level is sparse. We used transcranial magnetic stimulation to assess the effect of taVNS on the excitability of intracortical GABA-ergic and cholinergic circuits. In this within-subject, double-blind study on 30 healthy participants, we used TMS paradigms to assess the effect of a single session of taVNS at 100Hz and sham earlobe VNS (sVNS) on short-interval intracortical inhibition (SICI) curve and short-latency afferent inhibition (SAI). Control experiment was performed on additional 15 participants using the same experimental settings, but delivering no stimulation (xVNS). Bayesian statistics were used to assess the differences, producing % values that reflect the certainty that the values of interest were decreased during or after stimulation compared to baseline. taVNS increased SICI (96.3%), whereas sVNS decreased SICI (1.2 %). SAI was not affected by taVNS, while it was decreased during sVNS (1.34% and 9.1 %, for interstimulus intervals 20 and 24ms, respectively). The changes in TMS parameters detected during sVNS were present in the same direction in the control experiment with no stimulation. Our study provides evidence that taVNS increases the activity of cortical GABA-A-ergic system, leaving cortical cholinergic circuits unaffected. Changes in intracortical cortical excitability during sVNS, which were also observed in the control experiment with no stimulation were likely the effect of expectation related to participation in an interventional study.