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Non-invasive stimulation of vagal afferents reduces gastric frequency

  • Vanessa Teckentrup
    Correspondence
    Corresponding author. Calwerstr. 14, 72076, Tübingen, Germany.
    Affiliations
    University of Tübingen, Department of Psychiatry and Psychotherapy, Germany
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  • Sandra Neubert
    Affiliations
    University of Tübingen, Department of Psychiatry and Psychotherapy, Germany
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  • João C.P. Santiago
    Affiliations
    University of Tübingen, Department of Medical Psychology and Behavioral Neurobiology, Germany

    German Center for Diabetes Research (DZD), Tübingen, Germany

    Institute for Diabetes Research and Metabolic Diseases of the Helmholtz Center Munich at the Eberhard Karls University Tübingen, Tübingen, Germany
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  • Manfred Hallschmid
    Affiliations
    University of Tübingen, Department of Medical Psychology and Behavioral Neurobiology, Germany

    German Center for Diabetes Research (DZD), Tübingen, Germany

    Institute for Diabetes Research and Metabolic Diseases of the Helmholtz Center Munich at the Eberhard Karls University Tübingen, Tübingen, Germany
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  • Martin Walter
    Affiliations
    University of Tübingen, Department of Psychiatry and Psychotherapy, Germany

    University of Magdeburg, Department of Psychiatry and Psychotherapy, Germany

    Leibniz Institute for Neurobiology, Magdeburg, Germany

    University of Jena, Department of Psychiatry and Psychotherapy, Germany
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  • Nils B. Kroemer
    Correspondence
    Corresponding author. Calwerstr. 14, 72076, Tübingen, Germany.
    Affiliations
    University of Tübingen, Department of Psychiatry and Psychotherapy, Germany
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Open AccessPublished:December 18, 2019DOI:https://doi.org/10.1016/j.brs.2019.12.018

      Highlights

      • Metabolic feedback via the vagus nerve contributes to energy homeostasis.
      • taVNS reduced gastric frequency without altering resting energy expenditure.
      • Gastric frequency could be a promising peripheral marker of brain stimulation.

      Abstract

      Metabolic feedback between the gut and the brain relayed via the vagus nerve contributes to energy homeostasis. We investigated in healthy adults whether non-invasive stimulation of vagal afferents impacts energy homeostasis via efferent effects on metabolism or digestion. In a randomized crossover design, we applied transcutaneous auricular vagus nerve stimulation (taVNS) while recording efferent metabolic effects using simultaneous electrogastrography (EGG) and indirect calorimetry. We found that taVNS reduced gastric myoelectric frequency (p = .008), but did not alter resting energy expenditure. We conclude that stimulating vagal afferents induces gastric slowing via vagal efferents without acutely affecting net energy expenditure at rest. Collectively, this highlights the potential of taVNS to modulate digestion by activating the dorsal vagal complex. Thus, taVNS-induced changes in gastric frequency are an important peripheral marker of brain stimulation effects.

      Graphical abstract

      Keywords

      Introduction

      Maintaining energy homeostasis is vital for organisms and necessitates a balance between energy intake and expenditure [
      • Waterson M.J.
      • Horvath T.L.
      Neuronal regulation of energy homeostasis: beyond the hypothalamus and feeding.
      ]. Achieving this balance requires vagal afferents to transmit information between peripheral organs and the dorsal vagal complex in the brain stem [
      • Gribble F.M.
      • Reimann F.
      Function and mechanisms of enteroendocrine cells and gut hormones in metabolism.
      ,
      • Small D.M.
      • DiFeliceantonio A.G.
      Processed foods and food reward.
      ,
      • de Lartigue G.
      Role of the vagus nerve in the development and treatment of diet-induced obesity.
      ,
      • Johnson R.L.
      • Wilson C.G.
      A review of vagus nerve stimulation as a therapeutic intervention.
      ,
      • Mizrahi M.
      • Ben Ya’acov A.
      • Ilan Y.
      Gastric stimulation for weight loss.
      ]. Invasive stimulation of the vagus nerve (VNS) as well as the more recent non-invasive transcutaneous auricular VNS (taVNS [
      • Fallgatter A.J.
      • Neuhauser B.
      • Herrmann M.J.
      • Ehlis A.-C.
      • Wagener A.
      • Scheuerpflug P.
      • et al.
      Far field potentials from the brain stem after transcutaneous vagus nerve stimulation.
      ,
      • Frangos E.
      • Ellrich J.
      • Komisaruk B.R.
      Non-invasive access to the vagus nerve central projections via electrical stimulation of the external ear: fMRI evidence in humans.
      ,
      • Yakunina N.
      • Kim S.S.
      • Nam E.-C.
      Optimization of transcutaneous vagus nerve stimulation using functional MRI.
      ]) impact energy homeostasis by modulating food intake, energy metabolism, and glycemic control [
      • Cork S.C.
      The role of the vagus nerve in appetite control: implications for the pathogenesis of obesity.
      ,
      • Ikramuddin S.
      • Blackstone R.P.
      • Brancatisano A.
      • Toouli J.
      • Shah S.N.
      • Wolfe B.M.
      • et al.
      Effect of reversible intermittent intra-abdominal vagal nerve blockade on morbid obesity: the ReCharge randomized clinical trial.
      ,
      • Shikora S.
      • Toouli J.
      • Herrera M.F.
      • Kulseng B.
      • Zulewski H.
      • Brancatisano R.
      • et al.
      Vagal blocking improves glycemic control and elevated blood pressure in obese subjects with type 2 diabetes mellitus.
      ]. In rodents, VNS triggered by phasic stomach contractions resulted in weight loss [
      • Yao G.
      • Kang L.
      • Li J.
      • Long Y.
      • Wei H.
      • Ferreira C.A.
      • et al.
      Effective weight control via an implanted self-powered vagus nerve stimulation device.
      ]. In humans, taVNS led to a decreased frequency and increased amplitude of gastric motility [
      • Hong G.-S.
      • Pintea B.
      • Lingohr P.
      • Coch C.
      • Randau T.
      • Schaefer N.
      • et al.
      Effect of transcutaneous vagus nerve stimulation on muscle activity in the gastrointestinal tract (transVaGa): a prospective clinical trial.
      ] pointing to metabolic effects of taVNS on energy homeostasis via digestion. Such metabolic effects have also been observed in VNS-induced increases in the activity of brown adipose tissue, which in turn increased the basal metabolic rate, a measure related to energy expenditure reflecting physiological homeostasis [
      • Vijgen G.H.E.J.
      • Bouvy N.D.
      • Leenen L.
      • Rijkers K.
      • Cornips E.
      • Majoie M.
      • et al.
      Vagus nerve stimulation increases energy expenditure: relation to brown adipose tissue activity.
      ]. Notably, dopamine has been suggested as a neuromodulator of energy homeostasis within the gut-brain axis [
      • Tellez L.A.
      • Medina S.
      • Han W.
      • Ferreira J.G.
      • Licona-Limón P.
      • Ren X.
      • et al.
      A gut lipid messenger links excess dietary fat to dopamine deficiency.
      ,
      • Han W.
      • Tellez L.A.
      • Perkins M.H.
      • Perez I.O.
      • Qu T.
      • Ferreira J.
      • et al.
      A neural circuit for gut-induced reward.
      ]. Afferently, stimulation of the vagal sensory ganglion in mice was found to induce dopamine release in the substantia nigra [
      • Han W.
      • Tellez L.A.
      • Perkins M.H.
      • Perez I.O.
      • Qu T.
      • Ferreira J.
      • et al.
      A neural circuit for gut-induced reward.
      ]. Efferently, dopamine administration to the dorsal vagal complex in rats modulated the upper gastrointestinal tract by reducing gastric tone and motility via DA2 receptors in the dorsal motor nucleus of the vagus [
      • Anselmi L.
      • Toti L.
      • Bove C.
      • Travagli R.A.
      Vagally mediated effects of brain stem dopamine on gastric tone and phasic contractions of the rat.
      ]. Thus, while vagal stimulation mostly targets afferent pathways, studies in rodents provide evidence for brain-mediated effects on downstream targets.
      Although there is preliminary evidence linking vagal signaling and energy homeostasis [
      • Hong G.-S.
      • Pintea B.
      • Lingohr P.
      • Coch C.
      • Randau T.
      • Schaefer N.
      • et al.
      Effect of transcutaneous vagus nerve stimulation on muscle activity in the gastrointestinal tract (transVaGa): a prospective clinical trial.
      ], efferent taVNS-induced effects on digestion and energy metabolism in healthy humans have not been conclusively demonstrated. We therefore investigated whether taVNS vs. sham changes electrogastrography (EGG) and indirect calorimetry as markers of energy homeostasis.

      Methods

      Participants and procedure

      We invited 22 participants (14 female, Mage ± SD = 23.3 ± 2.7 years, range: 19–29) for two consecutive days with a 30-min stimulation phase each. In a randomized crossover design, we measured EGG using four standard electrocardiogram electrodes connected to a BrainProducts BrainAmp DC EEG recording system. Electrodes were placed as previously described ([
      • Rebollo I.
      • Devauchelle A.-D.
      • Béranger B.
      • Tallon-Baudry C.
      Stomach-brain synchrony reveals a novel, delayed-connectivity resting-state network in humans.
      ], see SI). Resting energy expenditure (REE) was measured with the CareFusion Vmax ventilated hood system for indirect calorimetry (see SI). For administering taVNS, we used Cerbomed NEMOS following the protocol presented in Ref. [
      • Frangos E.
      • Ellrich J.
      • Komisaruk B.R.
      Non-invasive access to the vagus nerve central projections via electrical stimulation of the external ear: fMRI evidence in humans.
      ]. Briefly, the electrode was placed at the left cymba conchae (taVNS) or was turned upside down and placed at the earlobe (sham). The stimulation protocol of NEMOS is preset with a biphasic impulse frequency of 25 Hz with alternating intervals of 30 s stimulation on and 30 s off.
      After a resting period of at least 15 min, we recorded a 15-min baseline for both EGG and calorimetry. Next, we placed the taVNS device on the participants’ left ear according to the randomization protocol. The individual stimulation intensity was adjusted based on subjective pain thresholds using concurrent VAS ratings (for details, see Ref. [
      • Kuehnel A.
      • Teckentrup V.
      • Neuser M.P.
      • Huys Q.J.M.
      • Burrasch C.
      • Walter M.
      • et al.
      Stimulation of the vagus nerve reduces learning in a go/no-go reinforcement learning task.
      ]). We then recorded at least 30 min of EGG and calorimetry during active stimulation before the participant was debriefed.

      Data preprocessing and statistical analysis

      EGG data were preprocessed and inspected for muscle artifacts. We then identified the gastric peak frequency for baseline, taVNS and sham, respectively, based on spectral density for each EGG channel (see SI). One participant had to be excluded after quality control due to absence of visibly identifiable peaks in any channel in both sessions, leaving N = 21 for the statistical analysis.
      We calculated baseline-corrected delta mean gastric frequency (in mHz) by subtracting the individual session-specific baseline mean gastric frequency from the respective taVNS and sham mean gastric frequency. Next, we calculated the net effect of stimulation (interaction) by subtracting delta sham from delta taVNS. After preprocessing the calorimetry data (see SI), we calculated the same measures for REE (in kcal/day). For non-parametric inference, we bootstrapped the distribution of taVNS-induced changes in gastric frequency and REE, respectively, using 50,000 repetitions and calculated two-tailed p-values.

      Results

      We found that taVNS compared to sham led to a significant reduction in gastric myoelectric frequency (Fig. 1A; mean [95% bootstrap CI] Time × Stimulation: −2.24 mHz [-4.44, −0.72], pboot = .008). In contrast, we observed no significant effect of taVNS on resting energy expenditure (Fig. 1B; mean [95% bootstrap CI] Time × Stimulation: −3.69 kcal/day [-46.52, 42.31], pboot = .863).
      Fig. 1
      Fig. 1Gastric frequency is reduced after transcutaneous auricular vagus nerve stimulation (taVNS) compared to sham stimulation, but there is no change in resting energy expenditure (REE). The figure depicts A) changes (stimulation – baseline) in gastric frequency and B) changes (stimulation – baseline) in REE for taVNS and sham. It further depicts C) bootstrapped delta gastric frequency distributions (stimulation – baseline) and D) bootstrapped delta REE distributions (stimulation – baseline) as well as the interaction between Time (pre, post) and Stimulation (taVNS, sham) for E) EGG and F) REE. While the interaction is significant for gastric frequency with pboot = .008, the same term does not reach significance for REE with pboot = .863. Bootstrapped distributions are based on 50,000 iterations.

      Discussion

      In line with the hypothesized efferent effect, we found that taVNS alters a marker of energy homeostasis in humans. The observed taVNS-induced reduction in gastric frequency is well in line with previous findings linking VNS to altered energy homeostasis [
      • Yao G.
      • Kang L.
      • Li J.
      • Long Y.
      • Wei H.
      • Ferreira C.A.
      • et al.
      Effective weight control via an implanted self-powered vagus nerve stimulation device.
      ,
      • Hong G.-S.
      • Pintea B.
      • Lingohr P.
      • Coch C.
      • Randau T.
      • Schaefer N.
      • et al.
      Effect of transcutaneous vagus nerve stimulation on muscle activity in the gastrointestinal tract (transVaGa): a prospective clinical trial.
      ,
      • Han W.
      • Tellez L.A.
      • Perkins M.H.
      • Perez I.O.
      • Qu T.
      • Ferreira J.
      • et al.
      A neural circuit for gut-induced reward.
      ]. This efferent effect on gastric motility might be due to a taVNS-induced release of dopamine in the brain stem. Previous work has shown that elevated levels of brain stem dopamine lead to reduced food intake [
      • Kaplan J.M.
      • Södersten P.
      Apomorphine suppresses ingestive behaviour in chronic decerebrate rats.
      ] and gastric relaxation [
      • Valenzuela J.E.
      Dopamine as A Possible neurotransmitter in gastric relaxation.
      ]. Moreover, dopamine administration in the brain stem reduced gastric tone and motility which was abolished by vagotomy [
      • Anselmi L.
      • Toti L.
      • Bove C.
      • Travagli R.A.
      Vagally mediated effects of brain stem dopamine on gastric tone and phasic contractions of the rat.
      ]. Studies linking alterations in vagal signaling to the development of Parkinson’s disease [
      • Svensson E.
      • Horváth-Puhó E.
      • Thomsen R.W.
      • Djurhuus J.C.
      • Pedersen L.
      • Borghammer P.
      • et al.
      Vagotomy and subsequent risk of Parkinson’s disease.
      ,
      • Liu B.
      • Fang F.
      • Pedersen N.L.
      • Tillander A.
      • Ludvigsson J.F.
      • Ekbom A.
      • et al.
      Vagotomy and Parkinson disease: a Swedish register-based matched-cohort study.
      ] further support the assumption of afferent signaling between the gut and key dopaminergic brain regions along the vagal pathway. Therefore, taVNS-induced neuromodulation in the brain stem might lead to the observed slowing of gastric myoelectric frequency via the efferent vagal pathway.
      In contrast to chronic VNS in patients [
      • Vijgen G.H.E.J.
      • Bouvy N.D.
      • Leenen L.
      • Rijkers K.
      • Cornips E.
      • Majoie M.
      • et al.
      Vagus nerve stimulation increases energy expenditure: relation to brown adipose tissue activity.
      ], we did not find changes in energy expenditure during acute taVNS. This pattern indicates that compared to changes in digestion, taVNS-induced effects on energy expenditure may develop over longer time periods.
      In sum, we demonstrated that taVNS reduces gastric frequency without affecting REE. This shows that transcutaneous stimulation of vagal afferents can elicit efferent gastric effects through a feedback loop via the dorsal vagal complex. Thus, in light of the heterogeneous efferent effects of taVNS on electrocardiogram parameters [
      • De Couck M.
      • Cserjesi R.
      • Caers R.
      • Zijlstra W.P.
      • Widjaja D.
      • Wolf N.
      • et al.
      Effects of short and prolonged transcutaneous vagus nerve stimulation on heart rate variability in healthy subjects.
      ,
      • Bauer S.
      • Baier H.
      • Baumgartner C.
      • Bohlmann K.
      • Fauser S.
      • Graf W.
      • et al.
      Transcutaneous vagus nerve stimulation (tVNS) for treatment of drug-resistant epilepsy: a randomized, double-blind clinical trial (cMPsE02).
      ], the EGG may be a promising positive control measure for taVNS in healthy humans [
      • Teckentrup V.
      • Krylova M.
      • Jamalabadi H.
      • Neubert S.
      • Neuser M.P.
      • Hartig R.
      • et al.
      Brain signaling dynamics after vagus nerve stimulation.
      ].

      Author contributions

      NBK was responsible for the concept and design of the study. VT & JCPS collected data. NBK & VT conceived the method and processed the data. VT performed the data analysis and NBK & SN contributed to analyses. VT, SN & NBK wrote the manuscript. All authors contributed to the interpretation of findings, provided critical revision of the manuscript for important intellectual content and approved the final version for publication.

      Declaration of competing interest

      We wish to confirm that there are no known conflicts of interest associated with this work and there has been no significant financial support for this work that could have influenced its outcome.

      Acknowledgements

      We thank Caroline Burrasch, Franziska Müller, Moritz Herkner, and Julius Gervelmeyer for help with data acquisition. This study was supported by the University of Tübingen, Faculty of Medicine fortüne grant #2453-0-0. NBK received support from the Else Kröner-Fresenius-Stiftung, grant #2017-A67. JCPS and MH received support via grants from the German Federal Ministry of Education and Research (BMBF) to the German Center for Diabetes Research (DZD e.V.; 01GI0925).

      Appendix A. Supplementary data

      The following is the Supplementary data to this article:

      References

        • Waterson M.J.
        • Horvath T.L.
        Neuronal regulation of energy homeostasis: beyond the hypothalamus and feeding.
        Cell Metabol. 2015; 22: 962-970https://doi.org/10.1016/j.cmet.2015.09.026
        • Gribble F.M.
        • Reimann F.
        Function and mechanisms of enteroendocrine cells and gut hormones in metabolism.
        Nat Rev Endocrinol. 2019; 15: 226-237https://doi.org/10.1038/s41574-019-0168-8
        • Small D.M.
        • DiFeliceantonio A.G.
        Processed foods and food reward.
        Science. 2019; 363: 346-347https://doi.org/10.1126/science.aav0556
        • de Lartigue G.
        Role of the vagus nerve in the development and treatment of diet-induced obesity.
        J Physiol. 2016; 594: 5791-5815https://doi.org/10.1113/JP271538
        • Johnson R.L.
        • Wilson C.G.
        A review of vagus nerve stimulation as a therapeutic intervention.
        J Inflamm Res. 2018; 11: 203-213https://doi.org/10.2147/JIR.S163248
        • Mizrahi M.
        • Ben Ya’acov A.
        • Ilan Y.
        Gastric stimulation for weight loss.
        World J Gastroenterol. 2012; 18: 2309-2319https://doi.org/10.3748/wjg.v18.i19.2309
        • Fallgatter A.J.
        • Neuhauser B.
        • Herrmann M.J.
        • Ehlis A.-C.
        • Wagener A.
        • Scheuerpflug P.
        • et al.
        Far field potentials from the brain stem after transcutaneous vagus nerve stimulation.
        J Neural Transm. 2003; 110: 1437-1443https://doi.org/10.1007/s00702-003-0087-6
        • Frangos E.
        • Ellrich J.
        • Komisaruk B.R.
        Non-invasive access to the vagus nerve central projections via electrical stimulation of the external ear: fMRI evidence in humans.
        Brain Stimul. 2015; 8: 624-636https://doi.org/10.1016/j.brs.2014.11.018
        • Yakunina N.
        • Kim S.S.
        • Nam E.-C.
        Optimization of transcutaneous vagus nerve stimulation using functional MRI.
        Neuromodulation. 2017; 20: 290-300https://doi.org/10.1111/ner.12541
        • Cork S.C.
        The role of the vagus nerve in appetite control: implications for the pathogenesis of obesity.
        J Neuroendocrinol. 2018; 30e12643https://doi.org/10.1111/jne.12643
        • Ikramuddin S.
        • Blackstone R.P.
        • Brancatisano A.
        • Toouli J.
        • Shah S.N.
        • Wolfe B.M.
        • et al.
        Effect of reversible intermittent intra-abdominal vagal nerve blockade on morbid obesity: the ReCharge randomized clinical trial.
        J Am Med Assoc. 2014; 312: 915-922https://doi.org/10.1001/jama.2014.10540
        • Shikora S.
        • Toouli J.
        • Herrera M.F.
        • Kulseng B.
        • Zulewski H.
        • Brancatisano R.
        • et al.
        Vagal blocking improves glycemic control and elevated blood pressure in obese subjects with type 2 diabetes mellitus.
        J Obes. 2013; 2013: 245683https://doi.org/10.1155/2013/245683
        • Yao G.
        • Kang L.
        • Li J.
        • Long Y.
        • Wei H.
        • Ferreira C.A.
        • et al.
        Effective weight control via an implanted self-powered vagus nerve stimulation device.
        Nat Commun. 2018; 9: 5349https://doi.org/10.1038/s41467-018-07764-z
        • Hong G.-S.
        • Pintea B.
        • Lingohr P.
        • Coch C.
        • Randau T.
        • Schaefer N.
        • et al.
        Effect of transcutaneous vagus nerve stimulation on muscle activity in the gastrointestinal tract (transVaGa): a prospective clinical trial.
        Int J Colorectal Dis. 2018; https://doi.org/10.1007/s00384-018-3204-6
        • Vijgen G.H.E.J.
        • Bouvy N.D.
        • Leenen L.
        • Rijkers K.
        • Cornips E.
        • Majoie M.
        • et al.
        Vagus nerve stimulation increases energy expenditure: relation to brown adipose tissue activity.
        PLoS One. 2013; 8e77221https://doi.org/10.1371/journal.pone.0077221
        • Tellez L.A.
        • Medina S.
        • Han W.
        • Ferreira J.G.
        • Licona-Limón P.
        • Ren X.
        • et al.
        A gut lipid messenger links excess dietary fat to dopamine deficiency.
        Science. 2013; 341: 800-802https://doi.org/10.1126/science.1239275
        • Han W.
        • Tellez L.A.
        • Perkins M.H.
        • Perez I.O.
        • Qu T.
        • Ferreira J.
        • et al.
        A neural circuit for gut-induced reward.
        Cell. 2018; 175: 887-888https://doi.org/10.1016/j.cell.2018.10.018
        • Anselmi L.
        • Toti L.
        • Bove C.
        • Travagli R.A.
        Vagally mediated effects of brain stem dopamine on gastric tone and phasic contractions of the rat.
        Am J Physiol Gastrointest Liver Physiol. 2017; 313: G434-G441https://doi.org/10.1152/ajpgi.00180.2017
        • Rebollo I.
        • Devauchelle A.-D.
        • Béranger B.
        • Tallon-Baudry C.
        Stomach-brain synchrony reveals a novel, delayed-connectivity resting-state network in humans.
        Elife. 2018; 7https://doi.org/10.7554/eLife.33321
        • Kuehnel A.
        • Teckentrup V.
        • Neuser M.P.
        • Huys Q.J.M.
        • Burrasch C.
        • Walter M.
        • et al.
        Stimulation of the vagus nerve reduces learning in a go/no-go reinforcement learning task.
        bioRxiv. 2019; https://doi.org/10.1101/535260
        • Kaplan J.M.
        • Södersten P.
        Apomorphine suppresses ingestive behaviour in chronic decerebrate rats.
        Neuroreport. 1994; 5: 1839-1840https://doi.org/10.1097/00001756-199409080-00039
        • Valenzuela J.E.
        Dopamine as A Possible neurotransmitter in gastric relaxation.
        Gastroenterology. 1976; 71: 1019-1022https://doi.org/10.1016/S0016-5085(76)80051-0
        • Svensson E.
        • Horváth-Puhó E.
        • Thomsen R.W.
        • Djurhuus J.C.
        • Pedersen L.
        • Borghammer P.
        • et al.
        Vagotomy and subsequent risk of Parkinson’s disease.
        Ann Neurol. 2015; 78: 522-529https://doi.org/10.1002/ana.24448
        • Liu B.
        • Fang F.
        • Pedersen N.L.
        • Tillander A.
        • Ludvigsson J.F.
        • Ekbom A.
        • et al.
        Vagotomy and Parkinson disease: a Swedish register-based matched-cohort study.
        Neurology. 2017; 88: 1996-2002https://doi.org/10.1212/WNL.0000000000003961
        • De Couck M.
        • Cserjesi R.
        • Caers R.
        • Zijlstra W.P.
        • Widjaja D.
        • Wolf N.
        • et al.
        Effects of short and prolonged transcutaneous vagus nerve stimulation on heart rate variability in healthy subjects.
        Auton Neurosci. 2017; 203: 88-96https://doi.org/10.1016/j.autneu.2016.11.003
        • Bauer S.
        • Baier H.
        • Baumgartner C.
        • Bohlmann K.
        • Fauser S.
        • Graf W.
        • et al.
        Transcutaneous vagus nerve stimulation (tVNS) for treatment of drug-resistant epilepsy: a randomized, double-blind clinical trial (cMPsE02).
        Brain Stimul. 2016; 9: 356-363https://doi.org/10.1016/j.brs.2015.11.003
        • Teckentrup V.
        • Krylova M.
        • Jamalabadi H.
        • Neubert S.
        • Neuser M.P.
        • Hartig R.
        • et al.
        Brain signaling dynamics after vagus nerve stimulation.
        Neuroimage. 2019; (In-principle accepted stage 1 registered report)