Transcutaneous Vagal Nerve Stimulation (tVNS): a new neuromodulation tool in healthy humans?

Auricular Neuromodulation: The Emerging Concept beyond the Stimulation of Vagus and Trigeminal Nerves

Neuromodulation, thanks to intrinsic and extrinsic brain feedback loops, seems to be the best way to exploit brain plasticity for therapeutic purposes. In the past years, there has been tremendous advances in the field of non-pharmacological modulation of brain activity. This review of different neurostimulation techniques will focus on sites and mechanisms of both transcutaneous vagus and trigeminal nerve stimulation. These methods are scientifically validated non-invasive bottom-up brain modulation techniques, easily implemented from the outer ear. In the light of this, auricles could transpire to be the most affordable target for non-invasive manipulation of central nervous system functions.

Transcutaneous vagus nerve stimulation (tVNS) enhances divergent thinking

Creativity is one of the most important cognitive skills in our complex and fast-changing world. Previous correlative evidence showed that gamma-aminobutyric acid (GABA) is involved in divergent but not convergent thinking. In the current study, a placebo/sham-controlled, randomized between-group design was used to test a causal relation between vagus nerve and creativity. We employed transcutaneous vagus nerve stimulation (tVNS), a novel non-invasive brain stimulation technique to stimulate afferent fibers of the vagus nerve and speculated to increase GABA levels, in 80 healthy young volunteers. Creative performance was assessed in terms of divergent thinking (Alternate Uses Task) and convergent thinking tasks (Remote Associates Test, Creative Problem Solving Task, Idea Selection Task). Results demonstrate active tVNS, compared to sham stimulation, enhanced divergent thinking. Bayesian analysis reported the data to be inconclusive regarding a possible effect of tVNS on convergent thinking. Therefore, our findings corroborate the idea that the vagus nerve is causally involved in creative performance. Even thought we did not directly measure GABA levels, our results suggest that GABA (likely to be increased in active tVNS condition) supports the ability to select among competing options in high selection demand (divergent thinking) but not in low selection demand (convergent thinking).

Supplementation of gamma-aminobutyric acid (GABA) affects temporal, but not spatial visual attention

In a randomized, double-blind, and placebo-controlled experiment, the acute effects of gamma-aminobutyric acid (GABA) supplementation on temporal and spatial attention in young healthy adults were investigated. A hybrid two-target rapid serial visual presentation task was used to measure temporal attention and integration. Additionally, a visual search task was used to measure the speed and accuracy of spatial attention. While temporal attention depends primarily on the distribution of limited attentional resources across time, spatial attention represents the engagement and disengagement by relevant and irrelevant stimuli across the visual field. Although spatial attention was unaffected by GABA supplementation altogether, we found evidence supporting improved performance in the temporal attention task. The attentional blink was numerically, albeit not significantly, attenuated at Lag 3, and significantly fewer order errors were committed at Lag 1, compared to the placebo condition. No effect was found on temporal integration rates. Although there is controversy about whether oral GABA can cross the blood-brain barrier, our results offer preliminary evidence that GABA intake might help to distribute limited attentional resources more efficiently, and can specifically improve the identification and ordering of visual events that occur in close temporal succession.

Transcutaneous vagus nerve stimulation (tVNS) modulates flow experience

Flow has been defined as a pleasant psychological state that people experience when completely absorbed in an activity. Previous correlative evidence showed that the vagal tone (as indexed by heart rate variability) is a reliable marker of flow. So far, it has not yet been demonstrated that the vagus nerve plays a causal role in flow. To explore this we used transcutaneous vagus nerve stimulation (tVNS), a novel non-invasive brain stimulation technique that increases activation of the locus coeruleus (LC) and norepinephrine release. A sham/placebo-controlled, randomized cross-over within-subject design was employed to infer a causal relation between the stimulated vagus nerve and flow as measured using the Flow Short-Scale in 32 healthy young volunteers. In both sessions, while being stimulated, participants had to rate their flow experience after having performed a task for 30 min. Active tVNS, compared to sham stimulation, decreased flow (as indexed by absorption scores). The results can be explained by the network reset theory, which assumes that high-phasic LC activity promotes a global reset of attention over exploitation of the current focus of attention, allowing rapid behavioral adaptation and resulting in decreased absorption scores. Furthermore, our findings corroborate the hypothesis that the vagus nerve and noradrenergic system are causally involved in flow.

Cholinergic agonists reduce blood pressure in a mouse model of systemic lupus erythematosus

Increased inflammation arising from an abnormal immune response can damage healthy tissue and lead to disease progression. An important example of this is the accumulation of inflammatory mediators in the kidney, which can subsequently lead to hypertension and renal injury. The origin of this inflammation may involve neuro-immune interactions. For example, the novel vagus nerve-to-spleen mechanism known as the "cholinergic anti-inflammatory pathway" controls inflammation upon stimulation. However, if this pathway is dysfunctional, inflammation becomes less regulated and chronic inflammatory diseases such as hypertension may develop. Systemic lupus erythematosus (SLE) is an autoimmune disease with aberrant immune function, increased renal inflammation, and prevalent hypertension. We hypothesized that the cholinergic anti-inflammatory pathway is impaired in SLE and that stimulation of this pathway would protect from the progression of hypertension in SLE mice. Female SLE (NZBWF1) and control (NZW) mice were administered nicotine or vehicle for 7 days (2 mg/kg/day, subcutaneously) in order to stimulate the cholinergic anti-inflammatory pathway at the level of the splenic nicotinic acetylcholine receptor (α7-nAChR). Blood pressure was assessed posttreatment. Nicotine-treated SLE mice did not develop hypertension and this lower blood pressure (compared to saline-treated SLE mice) coincided with lower splenic and renal cortical expression of pro-inflammatory cytokines. These data provide evidence that the cholinergic anti-inflammatory pathway is impaired in SLE In addition, these data suggest that stimulation of the cholinergic anti-inflammatory pathway can protect the kidney by dampening inflammation and therefore prevent the progression of hypertension in the setting of SLE.

The Obesity-Impulsivity Axis: Potential Metabolic Interventions in Chronic Psychiatric Patients

Pathological impulsivity is encountered in a broad range of psychiatric conditions and is thought to be a risk factor for aggression directed against oneself or others. Recently, a strong association was found between impulsivity and obesity which may explain the high prevalence of metabolic disorders in individuals with mental illness even in the absence of exposure to psychotropic drugs. As the overlapping neurobiology of impulsivity and obesity is being unraveled, the question asked louder and louder is whether they should be treated concomitantly. The treatment of obesity and metabolic dysregulations in chronic psychiatric patients is currently underutilized and often initiated late, making correction more difficult to achieve. Addressing obesity and metabolic dysfunction in a preventive manner may not only lower morbidity and mortality but also the excessive impulsivity, decreasing the risk for aggression. In this review, we take a look beyond psychopharmacological interventions and discuss dietary and physical therapy approaches.

Comparing neurostimulation technologies in refractory focal-onset epilepsy

For patients with pharmacoresistant focal epilepsy in whom surgical resection of the epileptogenic focus fails or was not feasible in the first place, there were few therapeutic options. Increasingly, neurostimulation provides an alternative treatment strategy for these patients. Vagal nerve stimulation (VNS) is well established. Deep brain stimulation (DBS) and cortical responsive stimulation (CRS) are newer neurostimulation therapies with recently published long-term efficacy and safety data. In this literature review, we introduce these therapies to a non-specialist audience. Furthermore, we compare and contrast long-term (5-year) outcomes of newer neurostimulation techniques with the more established VNS. A search to identify all studies reporting long-term efficacy (>5 years) of VNS, CRS and DBS in patients with refractory focal/partial epilepsy was conducted using PubMed and Cochrane databases. The outcomes compared were responder rate, percentage seizure frequency reduction, seizure freedom, adverse events, neuropsychological outcome and quality of life. We identified 1 study for DBS, 1 study for CRS and 4 studies for VNS. All neurostimulation technologies showed long-term efficacy, with progressively better seizure control over time. Sustained improvement in quality of life measures was demonstrated in all modalities. Intracranial neurostimulation had a greater side effect profile compared with extracranial stimulation, though all forms of stimulation are safe. Methodological differences between the studies mean that direct comparisons are not straightforward. We have synthesised the findings of this review into a pragmatic decision tree, to guide the further management of the individual patient with pharmacoresistant focal-onset epilepsy.

Neuroendocrinological and Epigenetic Mechanisms Subserving Autonomic Imbalance and HPA Dysfunction in the Metabolic Syndrome

Impact of environmental stress upon pathophysiology of the metabolic syndrome (MetS) has been substantiated by epidemiological, psychophysiological, and endocrinological studies. This review discusses recent advances in the understanding of causative roles of nutritional factors, sympathomedullo-adrenal (SMA) and hypothalamic-pituitary adrenocortical (HPA) axes, and adipose tissue chronic low-grade inflammation processes in MetS. Disturbances in the neuroendocrine systems for leptin, melanocortin, and neuropeptide Y (NPY)/agouti-related protein systems have been found resulting directly in MetS-like conditions. The review identifies candidate risk genes from factors shown critical for the functioning of each of these neuroendocrine signaling cascades. In its meta-analytic part, recent studies in epigenetic modification (histone methylation, acetylation, phosphorylation, ubiquitination) and posttranscriptional gene regulation by microRNAs are evaluated. Several studies suggest modification mechanisms of early life stress (ELS) and diet-induced obesity (DIO) programming in the hypothalamic regions with populations of POMC-expressing neurons. Epigenetic modifications were found in cortisol (here HSD11B1 expression), melanocortin, leptin, NPY, and adiponectin genes. With respect to adiposity genes, epigenetic modifications were documented for fat mass gene cluster APOA1/C3/A4/A5, and the lipolysis gene LIPE. With regard to inflammatory, immune and subcellular metabolism, PPARG, NKBF1, TNFA, TCF7C2, and those genes expressing cytochrome P450 family enzymes involved in steroidogenesis and in hepatic lipoproteins were documented for epigenetic modifications.

Transcutaneous Vagus Nerve Stimulation Enhances Post-error Slowing

People tend to slow down after they commit an error, a phenomenon known as post-error slowing (PES). It has been proposed that slowing after negative feedback or unforeseen errors is linked to the activity of the locus coeruleus–norepinephrine (LC–NE) system, but there is little direct evidence for this hypothesis. Here, we assessed the causal role of the noradrenergic system in modulating PES by applying transcutaneous vagus nerve stimulation (tVNS), a new noninvasive and safe method to stimulate the vagus nerve and to increase NE concentrations in the brain. A single-blind, sham-controlled, between-group design was used to assess the effect of tVNS in healthy young volunteers (n = 40) during two cognitive tasks designed to measure PES. Results showed increased PES during active tVNS, as compared with sham stimulation. This effect was of similar magnitude for the two tasks. These findings provide evidence for an important role of the noradrenergic system in PES.

Neurotransmitters as food supplements: the effects of GABA on brain and behavior

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the human cortex. The food supplement version of GABA is widely available online. Although many consumers claim that they experience benefits from the use of these products, it is unclear whether these supplements confer benefits beyond a placebo effect. Currently, the mechanism of action behind these products is unknown. It has long been thought that GABA is unable to cross the blood–brain barrier (BBB), but the studies that have assessed this issue are often contradictory and range widely in their employed methods. Accordingly, future research needs to establish the effects of oral GABA administration on GABA levels in the human brain, for example using magnetic resonance spectroscopy. There is some evidence in favor of a calming effect of GABA food supplements, but most of this evidence was reported by researchers with a potential conflict of interest. We suggest that any veridical effects of GABA food supplements on brain and cognition might be exerted through BBB passage or, more indirectly, via an effect on the enteric nervous system. We conclude that the mechanism of action of GABA food supplements is far from clear, and that further work is needed to establish the behavioral effects of GABA.

RETRACTED: Transcutaneous vagus nerve stimulation (tVNS) enhances response selection during action cascading processes

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This retraction comes after a thorough and time-consuming analysis of the published article and the data it reports, following an investigation from the University of Leiden. On date 15th November 2021 European Neuropsychopharmacology received advice from University of Leiden, containing allegations of misconduct concerning the data reported in this article. Due to the nature of the concerns raised about this paper, the Editor-in-Chief examined all documents provided, all aspects of the peer review and editorial process and requested the view of each of the authors. The Editor-in-Chief wishes to acknowledge the co-operation of all parties. The Editor-in-Chief considers that there is a legitimate cause for concern regarding deletion of participants from datasets. The Editor-in-Chief commends the authors for their willingness in participating in this dialogue. However, given the concerns raised, the suggestion of the University of Leiden, and the view expressed by some of the authors, Retraction of this paper is inevitable. The Editor-in-Chief and the entire editorial board will use this case as a reminder to be as diligent as possible in the peer review process and to support transparency and openness in data sharing.

Transcutaneous Vagus Nerve Stimulation (tVNS) does not increase prosocial behavior in Cyberball

Emerging research suggests that individuals experience vicarious social pain (i.e., ostracism). It has been proposed that observing ostracism increases activity in the insula and in the prefrontal cortex (PFC), two key brain regions activated by directly experiencing ostracism. Here, we assessed the causal role of the insula and PFC in modulating neural activity in these areas by applying transcutaneous Vagus Nerve Stimulation (tVNS), a new non-invasive and safe method to stimulate the vagus nerve that has been shown to activate the insula and PFC. A single-blind, sham-controlled, within-subjects design was used to assess the effect of on-line (i.e., stimulation overlapping with the critical task) tVNS in healthy young volunteers (n = 24) on the prosocial Cyberball game, a virtual ball-tossing game designed to measure prosocial compensation of ostracism. Active tVNS did not increase prosocial helping behavior toward an ostracized person, as compared to sham (placebo) stimulation. Corroborated by Bayesian inference, we conclude that tVNS does not modulate reactions to vicarious ostracism, as indexed by performance in a Cyberball game.