The effect of transcutaneous vagus nerve stimulation on cortical excitability

There is great interest about the therapeutic potentialities of transcutaneous vagus nerve stimulation (tVNS) applied to neuropsychiatric disorders. However, the mechanisms of action of tVNS and its impact on cortical excitability are unclear. To this regard, transcranial magnetic stimulation (TMS) can be useful because it is able of evaluating non-invasively excitatory and inhibitory circuitry of the human cortex. Aim of the present study is to investigate the effects of tVNS on cerebral cortex excitability in healthy volunteers by means of TMS. Ten healthy subjects participated in this randomized placebo-controlled double-blind study. Real tVNS was administered at left external acoustic meatus, while sham stimulation was performed at left ear lobe, both of them for 60 min. We evaluated motor thresholds, motor evoked potential amplitude, recruitment curves, and short-interval intracortical inhibition (SICI) in right and left motor cortex. Such parameters were evaluated before and 60 min after the exposure to tVNS, for both the real and the sham stimulation. Cardiovascular parameters were monitored during the stimulation. A generalized linear model for repeated measures was implemented to assess the effect of time and stimulation type on cardiovascular and neurophysiological variables. SICI, a double-pulse TMS paradigm informative of GABA-A activity, was significantly increased in right motor cortex after real tVNS. Other neurophysiological parameters, as well as cardiovascular variables, remained unchanged. Our findings confirm that tVNS is a safe and effective way to stimulate vagus nerve and provide innovative data about the possible mechanisms of action that supports the potential therapeutic application of this technique.