Vagus nerve stimulation enhances perforant path-CA3 synaptic transmission via the activation of β-adrenergic receptors and the locus coeruleus

Vagus nerve stimulation enhances remyelination and decreases innate neuroinflammation in lysolecithin-induced demyelination

BACKGROUND

Current treatments for Multiple Sclerosis (MS) poorly address chronic innate neuroinflammation nor do they offer effective remyelination. The vagus nerve has a strong regulatory role in inflammation and Vagus Nerve Stimulation (VNS) has potential to affect both neuroinflammation and remyelination in MS.

OBJECTIVE

This study investigated the effects of VNS on demyelination and innate neuroinflammation in a validated MS rodent model.

METHODS

Lysolecithin (LPC) was injected in the corpus callosum (CC) of 46 Lewis rats, inducing a demyelinated lesion. 33/46 rats received continuously-cycled VNS (cVNS) or one-minute per day VNS (1minVNS) or sham VNS from 2 days before LPC-injection until perfusion at 3 days post-injection (dpi) (corresponding with a demyelinated lesion with peak inflammation). 13/46 rats received cVNS or sham from 2 days before LPC-injection until perfusion at 11 dpi (corresponding with a partial remyelinated lesion). Immunohistochemistry and proteomics analyses were performed to investigate the extend of demyelination and inflammation.

RESULTS

Immunohistochemistry showed that cVNS significantly reduced microglial and astrocytic activation in the lesion and lesion border, and significantly reduced the Olig2+ cell count at 3 dpi. Furthermore, cVNS significantly improved remyelination with 57.4 % versus sham at 11 dpi. Proteomic gene set enrichment analyses showed increased activation of (glutamatergic) synapse pathways in cVNS versus sham, most pronounced at 3 dpi.

CONCLUSION

cVNS improved remyelination of an LPC-induced lesion. Possible mechanisms might include modulation of microglia and astrocyte activity, increased (glutamatergic) synapses and enhanced oligodendrocyte clearance after initial injury.

Bursts of vagus nerve stimulation paired with auditory rehabilitation fail to improve speech sound perception in rats with hearing loss

Hearing loss can lead to long-lasting effects on the central nervous system, and current therapies, such as auditory training and rehabilitation, show mixed success in improving perception and speech comprehension. Vagus nerve stimulation (VNS) is an adjunctive therapy that can be paired with rehabilitation to facilitate behavioral recovery after neural injury. However, VNS for auditory recovery has not been tested after severe hearing loss or significant damage to peripheral receptors. This study investigated the utility of pairing VNS with passive or active auditory rehabilitation in a rat model of noise-induced hearing loss. Although auditory rehabilitation helped rats improve their frequency discrimination, learn novel speech discrimination tasks, and achieve speech-in-noise performance similar to normal hearing controls, VNS did not enhance recovery of speech sound perception. These results highlight the limitations of VNS as an adjunctive therapy for hearing loss rehabilitation and suggest that optimal benefits from neuromodulation may require restored peripheral signaling.

Locus coeruleus features are linked to vagus nerve stimulation response in drug-resistant epilepsy

The locus coeruleus-norepinephrine system is thought to be involved in the clinical effects of vagus nerve stimulation. This system is known to prevent seizure development and induce long-term plastic changes, particularly with the release of norepinephrine in the hippocampus. However, the requisites to become responder to the therapy and the mechanisms of action are still under investigation. Using MRI, we assessed the structural and functional characteristics of the locus coeruleus and microstructural properties of locus coeruleus-hippocampus white matter tracts in patients with drug-resistant epilepsy responding or not to the therapy. Twenty-three drug-resistant epileptic patients with cervical vagus nerve stimulation were recruited for this pilot study, including 13 responders or partial responders and 10 non-responders. A dedicated structural MRI acquisition allowed in vivo localization of the locus coeruleus and computation of its contrast (an accepted marker of LC integrity). Locus coeruleus activity was estimated using functional MRI during an auditory oddball task. Finally, multi-shell diffusion MRI was used to estimate the structural properties of locus coeruleus-hippocampus tracts. These characteristics were compared between responders/partial responders and non-responders and their association with therapy duration was also explored. In patients with a better response to the therapy, trends toward a lower activity and a higher contrast were found in the left medial and right caudal portions of the locus coeruleus, respectively. An increased locus coeruleus contrast, bilaterally over its medial portions, correlated with duration of the treatment. Finally, a higher integrity of locus coeruleus-hippocampus connections was found in patients with a better response to the treatment. These new insights into the neurobiology of vagus nerve stimulation may provide novel markers of the response to the treatment and may reflect neuroplasticity effects occurring in the brain following the implantation.

Implications of microbe-derived ɣ-aminobutyric acid (GABA) in gut and brain barrier integrity and GABAergic signaling in Alzheimer's disease

The gut microbial ecosystem communicates bidirectionally with the brain in what is known as the gut-microbiome-brain axis. Bidirectional signaling occurs through several pathways including signaling via the vagus nerve, circulation of microbial metabolites, and immune activation. Alterations in the gut microbiota are implicated in Alzheimer's disease (AD), a progressive neurodegenerative disease. Perturbations in gut microbial communities may affect pathways within the gut-microbiome-brain axis through altered production of microbial metabolites including ɣ-aminobutyric acid (GABA), the primary inhibitory mammalian neurotransmitter. GABA has been shown to act on gut integrity through modulation of gut mucins and tight junction proteins and may be involved in vagus nerve signal inhibition. The GABAergic signaling pathway has been shown to be dysregulated in AD, and may be responsive to interventions. Gut microbial production of GABA is of recent interest in neurological disorders, including AD. Bacteroides and Lactic Acid Bacteria (LAB), including Lactobacillus, are predominant producers of GABA. This review highlights how temporal alterations in gut microbial communities associated with AD may affect the GABAergic signaling pathway, intestinal barrier integrity, and AD-associated inflammation.

Effect of transcutaneous cervical vagus nerve stimulation on declarative and working memory in patients with Posttraumatic Stress Disorder (PTSD): A pilot study

BACKGROUND

Posttraumatic stress disorder (PTSD) is associated with changes in multiple neurophysiological systems, including verbal declarative memory deficits. Vagus Nerve Stimulation (VNS) has been shown in preliminary studies to enhance function when paired with cognitive and motor tasks. The purpose of this study was to analyze the effect of transcutaneous cervical VNS (tcVNS) on attention, declarative and working memory in PTSD patients.

METHODS

Fifteen PTSD patients were randomly assigned to active tcVNS (N = 8) or sham (N = 7) stimulation in a double-blinded fashion. Memory assessment tests including paragraph recall and N-back tests were performed to assess declarative and working memory function when paired with active/sham tcVNS once per month in a longitudinal study during which patients self-administered tcVNS/sham twice daily.

RESULTS

Active tcVNS stimulation resulted in a significant improvement in paragraph recall performance following pairing with paragraph encoding for PTSD patients at two months (p < 0.05). It resulted in a 91 % increase in paragraph recall performance within group (p = 0.03), while sham tcVNS exhibited no such trend in performance improvement. In the N-back study, positive deviations in accuracy, precision and recall measures on different day visits (7,34,64,94) of patients with respect to day 1 revealed a pattern of better performance of the active tcVNS population compared to sham VNS which did not reach statistical significance.

LIMITATIONS

Our sample size was small.

CONCLUSIONS

These preliminary results suggest that tcVNS improves attention, declarative and working memory, which may improve quality of life and productivity for patients with PTSD. Future studies are required to confirm these results.

The microbiota-gut-brain axis in hippocampus-dependent learning and memory: current state and future challenges

A fundamental shift in neuroscience suggests bidirectional interaction of gut microbiota with the healthy and dysfunctional brain. This microbiota-gut-brain axis has mainly been investigated in stress-related psychopathology (e.g. depression, anxiety). The hippocampus, a key structure in both the healthy brain and psychopathologies, is implicated by work in rodents that suggests gut microbiota substantially impact hippocampal-dependent learning and memory. However, understanding microbiota-hippocampus mechanisms in health and disease, and translation to humans, is hampered by the absence of a coherent evaluative approach. We review the current knowledge regarding four main gut microbiota-hippocampus routes in rodents: through the vagus nerve; via the hypothalamus-pituitary-adrenal-axis; by metabolism of neuroactive substances; and through modulation of host inflammation. Next, we suggest an approach including testing (biomarkers of) the four routes as a function of the influence of gut microbiota (composition) on hippocampal-dependent (dys)functioning. We argue that such an approach is necessary to proceed from the current state of preclinical research to beneficial application in humans to optimise microbiota-based strategies to treat and enhance hippocampal-dependent memory (dys)functions.

Vagus nerve stimulation: mechanisms and factors involved in memory enhancement

Vagus nerve stimulation (VNS) has been recognized as a useful neuromodulation tool to target the central nervous system by electrical stimulation of peripheral nerves. Activation of the nucleus of the solitary tract (NTS) in the brainstem by vagal afferent nerve fibers allows for modulation of various higher order brain regions, including limbic and cerebral cortex structures. Along with neurological and psychiatric indications, clinical and preclinical studies suggest that VNS can improve memory. While the underlying mechanisms to improve memory with VNS involve brain areas, such as the prefrontal cortex and processes including alertness and arousal, here we focus on VNS-induced memory improvements related to the hippocampus, the main area implicated in memory acquisition. In addition, we detail research demonstrating that a targeted approach to VNS can modify memory outcomes and delve into the molecular mechanisms associated with these changes. These findings indicate that a greater understanding of VNS mechanisms while also considering stimulation parameters, administration site, timing in relation to training, and sex-specific factors, may allow for optimal VNS application to enhance memory.

Sex differences in autonomic responses to stress: implications for cardiometabolic physiology

Chronic stress is a significant risk factor for negative health outcomes. Furthermore, imbalance of autonomic nervous system control leads to dysregulation of physiological responses to stress and contributes to the pathogenesis of cardiometabolic and psychiatric disorders. However, research on autonomic stress responses has historically focused on males, despite evidence that females are disproportionality affected by stress-related disorders. Accordingly, this mini-review focuses on the influence of biological sex on autonomic responses to stress in humans and rodent models. The reviewed literature points to sex differences in the consequences of chronic stress, including cardiovascular and metabolic disease. We also explore basic rodent studies of sex-specific autonomic responses to stress with a focus on sex hormones and hypothalamic-pituitary-adrenal axis regulation of cardiovascular and metabolic physiology. Ultimately, emerging evidence of sex differences in autonomic-endocrine integration highlights the importance of sex-specific studies to understand and treat cardiometabolic dysfunction.

Vagus nerve stimulation-induced cognitive enhancement: Hippocampal neuroplasticity in healthy male rats

BACKGROUND

Vagus nerve stimulation (VNS) improves cognition in humans and rodents, but the effects of a single session of VNS on performance and plasticity are not well understood.

OBJECTIVE

Behavioral performance and hippocampal (HC) electrophysiology/neurotrophin expression were measured in healthy adult rats after VNS paired training to investigate changes in cognition and synaptic plasticity.

METHODS

Platinum/iridium electrodes were surgically implanted around the left cervical branch of the VN of anesthetized male Sprague-Dawley rats (N = 47). VNS (100 μs biphasic pulses, 30 Hz, 0.8 mA) paired Novel Object Recognition (NOR)/Passive Avoidance Task (PAT) were assessed 24 h after training and post-mortem tissue was collected 48 h after VNS (N = 28). Electrophysiology recordings were collected using a microelectrode array system to assess functional effects on HC slices 90 min after VNS (N = 19). Sham received the same treatment without VNS and experimenters were blinded.

RESULTS

Stimulated rats exhibited improved performance in NOR (p < 0.05, n = 12) and PAT (p < 0.05, n = 14). VNS enhanced long-term potentiation (p < 0.05, n = 7-12), and spontaneous spike amplitude (p < 0.05, n = 7-12) and frequency (p < 0.05, n = 7-12) in the CA1. Immunohistochemical analysis found increased brain-derived neurotrophic factor expression in the CA1 (p < 0.05, n = 8-9) and CA2 (p < 0.01, n = 7-8).

CONCLUSION

These findings suggest that our VNS parameters promote synaptic plasticity and target the CA1, which may mediate the positive cognitive effects of VNS. This study significantly contributes to a better understanding of VNS mediated HC synaptic plasticity, which may improve clinical utilization of VNS for cognitive enhancement.

Rapid Effects of Vagus Nerve Stimulation on Sensory Processing Through Activation of Neuromodulatory Systems

After sensory information is encoded into neural signals at the periphery, it is processed through multiple brain regions before perception occurs (i.e., sensory processing). Recent work has begun to tease apart how neuromodulatory systems influence sensory processing. Vagus nerve stimulation (VNS) is well-known as an effective and safe method of activating neuromodulatory systems. There is a growing body of studies confirming VNS has immediate effects on sensory processing across multiple sensory modalities. These immediate effects of VNS on sensory processing are distinct from the more well-documented method of inducing lasting neuroplastic changes to the sensory pathways through repeatedly delivering a brief VNS burst paired with a sensory stimulus. Immediate effects occur upon VNS onset, often disappear upon VNS offset, and the modulation is present for all sensory stimuli. Conversely, the neuroplastic effect of pairing sub-second bursts of VNS with a sensory stimulus alters sensory processing only after multiple pairing sessions, this alteration remains after cessation of pairing sessions, and the alteration selectively affects the response properties of neurons encoding the specific paired sensory stimulus. Here, we call attention to the immediate effects VNS has on sensory processing. This review discusses existing studies on this topic, provides an overview of the underlying neuromodulatory systems that likely play a role, and briefly explores the potential translational applications of using VNS to rapidly regulate sensory processing.

Vagus Nerve Stimulation as a Potential Therapy in Early Alzheimer's Disease: A Review

Cognitive dysfunction in Alzheimer's disease (AD) is caused by disturbances in neuronal circuits of the brain underpinned by synapse loss, neuronal dysfunction and neuronal death. Amyloid beta and tau protein cause these pathological changes and enhance neuroinflammation, which in turn modifies disease progression and severity. Vagal nerve stimulation (VNS), via activation of the locus coeruleus (LC), results in the release of catecholamines in the hippocampus and neocortex, which can enhance synaptic plasticity and reduce inflammatory signalling. Vagal nerve stimulation has shown promise to enhance cognitive ability in animal models. Research in rodents has shown that VNS can have positive effects on basal synaptic function and synaptic plasticity, tune inflammatory signalling, and limit the accumulation of amyloid plaques. Research in humans with invasive and non-invasive VNS devices has shown promise for the modulation of cognition. However, the direct stimulation of the vagus nerve afforded with the invasive procedure carries surgical risks. In contrast, non-invasive VNS has the potential to be a broadly available therapy to manage cognitive symptoms in early AD, however, the magnitude and specificity of its effects remains to be elucidated, and the non-inferiority of the effects of non-invasive VNS as compared with invasive VNS still needs to be established. Ongoing clinical trials with healthy individuals and patients with early AD will provide valuable information to clarify the potential benefits of non-invasive VNS in cognition and AD. Whether invasive or non-invasive VNS can produce a significant improvement on memory function and whether its effects can modify the progression of AD will require further investigation.

How Is the Norepinephrine System Involved in the Antiepileptic Effects of Vagus Nerve Stimulation?

Vagus Nerve Stimulation (VNS) is an adjunctive treatment for patients suffering from inoperable drug-resistant epilepsy. Although a complete understanding of the mediators involved in the antiepileptic effects of VNS and their complex interactions is lacking, VNS is known to trigger the release of neurotransmitters that have seizure-suppressing effects. In particular, norepinephrine (NE) is a neurotransmitter that has been associated with the clinical effects of VNS by preventing seizure development and by inducing long-term plastic changes that could restore a normal function of the brain circuitry. However, the biological requisites to become responder to VNS are still unknown. In this review, we report evidence of the critical involvement of NE in the antiepileptic effects of VNS in rodents and humans. Moreover, we emphasize the hypothesis that the functional integrity of the noradrenergic system could be a determining factor to obtain clinical benefits from the therapy. Finally, encouraging avenues of research involving NE in VNS treatment are discussed. These could lead to the personalization of the stimulation parameters to maximize the antiepileptic effects and potentially improve the response rate to the therapy.

Role of the Cholinergic Anti-Inflammatory Reflex in Central Nervous System Diseases

In several central nervous system diseases, it has been reported that inflammation may be related to the etiologic process, therefore, therapeutic strategies are being implemented to control inflammation. As the nervous system and the immune system maintain close bidirectional communication in physiological and pathological conditions, the modulation of inflammation through the cholinergic anti-inflammatory reflex has been proposed. In this review, we summarized the evidence supporting chemical stimulation with cholinergic agonists and vagus nerve stimulation as therapeutic strategies in the treatment of various central nervous system pathologies, and their effect on inflammation.

Acute vagus nerve stimulation enhances reversal learning in rats

Cognitive flexibility is a prefrontal cortex-dependent neurocognitive process that enables behavioral adaptation in response to changes in environmental contingencies. Electrical vagus nerve stimulation (VNS) enhances several forms of learning and neuroplasticity, but its effects on cognitive flexibility have not been evaluated. In the current study, a within-subjects design was used to assess the effects of VNS on performance in a novel visual discrimination reversal learning task conducted in touchscreen operant chambers. The task design enabled simultaneous assessment of acute VNS both on reversal learning and on recall of a well-learned discrimination problem. Acute VNS delivered in conjunction with stimuli presentation during reversal learning reliably enhanced learning of new reward contingencies. Enhancement was not observed, however, if VNS was delivered during the session but was not coincident with presentation of to-be-learned stimuli. In addition, whereas VNS delivered at 30 HZ enhanced performance, the same enhancement was not observed using 10 or 50 Hz. Together, these data show that acute VNS facilitates reversal learning and indicate that the timing and frequency of the VNS are critical for these enhancing effects. In separate rats, administration of the norepinephrine reuptake inhibitor atomoxetine also enhanced reversal learning in the same task, consistent with a noradrenergic mechanism through which VNS enhances cognitive flexibility.

Gray Matter Atrophy: The Impacts of Resective Surgery and Vagus Nerve Stimulation in Drug-Resistant Epilepsy

BACKGROUND

There is great concern for cognitive function after resective temporal lobe surgery for drug-resistant epilepsy. However, few studies have investigated postoperative anatomical changes, and the downstream effects of surgery are poorly understood. This study investigated volumetric changes after resective surgery and vagus nerve stimulation (VNS) for epilepsy.

METHODS

Preoperative and latest postoperative (mean, 28 months) structural T1 magnetic resonance imaging scans were retrospectively obtained for 43 patients: 27 temporal lobe resections (TLRs), 6 extratemporal lobe resections, and 10 VNS, undergoing surgery for drug-resistant epilepsy between 2012 and 2017. Automated volumetric analyses of predefined cortical gray matter and subcortical structures were performed. Preoperative and postoperative volumes were compared, and the effects of age, gender, operation type, resection laterality, selectivity, time since surgery, and seizure outcome on volumetric changes were analyzed.

RESULTS

After TLRs, there were reductions in contralateral hemispheric gray matter, temporal lobe, entorhinal cortex, parahippocampal, superior temporal, middle temporal, inferior temporal (P = 0.02), lingual, fusiform, precentral, paracentral, postcentral, pericalcarine gyri, and ipsilateral superior parietal gyrus. After VNS, there was bilateral atrophy in the thalamus, putamen, cerebellum, rostral anterior cingulate, posterior cingulate, medial orbitofrontal, paracentral, fusiform, and transverse temporal gyri. There was a significant effect of surgery type but no effect of age, gender, operation type, resection laterality, selectivity, time since surgery, and seizure outcome on contralateral hippocampal gray matter change.

CONCLUSION

This is the first study to demonstrate volumetric decreases in temporal and connected regions after TLRs and VNS. These results provide interesting insight into functional network changes.

Changes in Hippocampal Plasticity in Depression and Therapeutic Approaches Influencing These Changes

Depression is a common neurological disease that seriously affects human health. There are many hypotheses about the pathogenesis of depression, and the most widely recognized and applied is the monoamine hypothesis. However, no hypothesis can fully explain the pathogenesis of depression. At present, the brain-derived neurotrophic factor (BDNF) and neurogenesis hypotheses have highlighted the important role of plasticity in depression. The plasticity of neurons and glial cells plays a vital role in the transmission and integration of signals in the central nervous system. Plasticity is the adaptive change in the nervous system in response to changes in external signals. The hippocampus is an important anatomical area associated with depression. Studies have shown that some antidepressants can treat depression by changing the plasticity of the hippocampus. Furthermore, caloric restriction has also been shown to affect antidepressant and hippocampal plasticity changes. In this review, we summarize the latest research, focusing on changes in the plasticity of hippocampal neurons and glial cells in depression and the role of BDNF in the changes in hippocampal plasticity in depression, as well as caloric restriction and mitochondrial plasticity. This review may contribute to the development of antidepressant drugs and elucidating the mechanism of depression.

Identification of vagus nerve stimulation parameters affecting rat hippocampal electrophysiology without temperature effects

BACKGROUND

Recent experiments in rats have demonstrated significant effects of VNS on hippocampal excitability but were partially attributed to hypothermia, induced by the applied VNS parameters.

OBJECTIVE

To allow meaningful preclinical research on the mechanisms of VNS and translation of rodent results to clinical VNS trials, we aimed to identify non-hypothermia inducing VNS parameters that significantly affect hippocampal excitability.

METHODS

VNS was administered in cycles of 30 s including either 0.1, 0.16, 0.25, 0.5, 1.5, 3 or 7 s of VNS ON time (biphasic pulses, 250μs/phase, 1 mA, 30 Hz) and the effect of different VNS ON times on brain temperature was evaluated. VNS paradigms with and without hypothermia were compared for their effects on hippocampal neurophysiology in freely moving rats.

RESULTS

Using VNS parameters with an ON time/OFF time of up to 0.5 s/30 s did not cause hypothermia, while clear hypothermia was detected with ON times of 1.5, 3 and 7 s/30 s. Relative to SHAM VNS, the normothermic 0.5 s VNS condition significantly decreased hippocampal EEG power and changed dentate gyrus evoked potentials with an increased field excitatory postsynaptic potential slope and a decreased population spike amplitude.

CONCLUSION

VNS can be administered in freely moving rats without causing hypothermia, while profoundly affecting hippocampal neurophysiology suggestive of reduced excitability of hippocampal neurons despite increased synaptic transmission efficiency.

Electrical stimulation of cranial nerves in cognition and disease

The cranial nerves are the pathways through which environmental information (sensation) is directly communicated to the brain, leading to perception, and giving rise to higher cognition. Because cranial nerves determine and modulate brain function, invasive and non-invasive cranial nerve electrical stimulation methods have applications in the clinical, behavioral, and cognitive domains. Among other neuromodulation approaches such as peripheral, transcranial and deep brain stimulation, cranial nerve stimulation is unique in allowing axon pathway-specific engagement of brain circuits, including thalamo-cortical networks. In this review we amalgamate relevant knowledge of 1) cranial nerve anatomy and biophysics; 2) evidence of the modulatory effects of cranial nerves on cognition; 3) clinical and behavioral outcomes of cranial nerve stimulation; and 4) biomarkers of nerve target engagement including physiology, electroencephalography, neuroimaging, and behavioral metrics. Existing non-invasive stimulation methods cannot feasibly activate the axons of only individual cranial nerves. Even with invasive stimulation methods, selective targeting of one nerve fiber type requires nuance since each nerve is composed of functionally distinct axon-types that differentially branch and can anastomose onto other nerves. None-the-less, precisely controlling stimulation parameters can aid in affecting distinct sets of axons, thus supporting specific actions on cognition and behavior. To this end, a rubric for reproducible dose-response stimulation parameters is defined here. Given that afferent cranial nerve axons project directly to the brain, targeting structures (e.g. thalamus, cortex) that are critical nodes in higher order brain networks, potent effects on cognition are plausible. We propose an intervention design framework based on driving cranial nerve pathways in targeted brain circuits, which are in turn linked to specific higher cognitive processes. State-of-the-art current flow models that are used to explain and design cranial-nerve-activating stimulation technology require multi-scale detail that includes: gross anatomy; skull foramina and superficial tissue layers; and precise nerve morphology. Detailed simulations also predict that some non-invasive electrical or magnetic stimulation approaches that do not intend to modulate cranial nerves per se, such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS), may also modulate activity of specific cranial nerves. Much prior cranial nerve stimulation work was conceptually limited to the production of sensory perception, with individual titration of intensity based on the level of perception and tolerability. However, disregarding sensory emulation allows consideration of temporal stimulation patterns (axon recruitment) that modulate the tone of cortical networks independent of sensory cortices, without necessarily titrating perception. For example, leveraging the role of the thalamus as a gatekeeper for information to the cerebral cortex, preventing or enhancing the passage of specific information depending on the behavioral state. We show that properly parameterized computational models at multiple scales are needed to rationally optimize neuromodulation that target sets of cranial nerves, determining which and how specific brain circuitries are modulated, which can in turn influence cognition in a designed manner.

Vagus nerve stimulation and Neurotrophins: a biological psychiatric perspective

Since 2004, vagus nerve stimulation (VNS) has been used in treatment-resistant or treatment-intolerant depressive episodes. Today, VNS is suggested as possible therapy for a larger spectrum of psychiatric disorders, including schizophrenia, obsessive compulsive disorders, and panic disorders. Despite a large body of literature supports the application of VNS in patients' treatment, the exact mechanism of action of VNS remains not fully understood. In the present study, the major knowledges on the brain areas and neuronal pathways regulating neuroimmune and autonomic response subserving VNS effects are reviewed. Furthermore, the involvement of the neurotrophins (NTs) Nerve Growth Factor (NGF) and Brain Derived Neurotrophic Factor (BDNF) in vagus nerve (VN) physiology and stimulation is revised. The data on brain NGF/BDNF synthesis and in turn on the activity-dependent plasticity, connectivity rearrangement and neurogenesis, are presented and discussed as potential biomarkers for optimizing stimulatory parameters for VNS. A vagus nerve-neurotrophin interaction model in the brain is finally proposed as a working hypothesis for future studies addressed to understand pathophysiology of psychiatric disturbance.

Norepinephrine and serotonin are required for vagus nerve stimulation directed cortical plasticity

Vagus nerve stimulation (VNS) paired with forelimb training drives robust, specific reorganization of movement representations in the motor cortex. This effect is hypothesized to be mediated by VNS-dependent engagement of neuromodulatory networks. VNS influences activity in the locus coeruleus (LC) and dorsal raphe nucleus (DRN), but the involvement of these neuromodulatory networks in VNS-directed plasticity is unknown. We tested the hypothesis that cortical norepinephrine and serotonin are required for VNS-dependent enhancement of motor cortex plasticity. Rats were trained on a lever pressing task emphasizing proximal forelimb use. Once proficient, all rats received a surgically implanted vagus nerve cuff and cortical injections of either immunotoxins to deplete serotonin or norepinephrine, or vehicle control. Following surgical recovery, rats received half second bursts of 0.8 mA or sham VNS after successful trials. After five days of pairing intracortical microstimulation (ICMS) was performed in the motor cortex contralateral to the trained limb. VNS paired with training more than doubled cortical representations of proximal forelimb movements. Depletion of either cortical norepinephrine or serotonin prevented this effect. The requirement of multiple neuromodulators is consistent with earlier studies showing that these neuromodulators regulate synaptic plasticity in a complimentary fashion.

Hypothermia Masks Most of the Effects of Rapid Cycling VNS on Rat Hippocampal Electrophysiology

AIM. Vagus nerve stimulation (VNS) modulates hippocampal dentate gyrus (DG) electrophysiology and induces hypothermia in freely moving rats. This study evaluated whether hippocampal (CA1) electrophysiology is similarly modulated and to what extent this is associated with VNS-induced hypothermia. METHODS. Six freely moving rats received a first 4h session of rapid cycling VNS (7s on/18s off), while CA1 evoked potentials, EEG and core temperature were recorded. In a second 4h session, external heating was applied during the 3rd and 4thh of VNS counteracting VNS-induced hypothermia. RESULTS. VNS decreased the slope of the field excitatory postsynaptic potential (fEPSP), increased the population spike (PS) amplitude and latency, decreased theta (4-12Hz) and gamma (30-100Hz) band power and theta peak frequency. Normalizing body temperature during VNS through external heating abolished the effects completely for fEPSP slope, PS latency and gamma band power, partially for theta band power and theta peak frequency and inverted the effect on PS amplitude. CONCLUSIONS. Rapid cycle VNS modulates CA1 electrophysiology similarly to DG, suggesting a wide-spread VNS-induced effect on hippocampal electrophysiology. Normalizing core temperature elucidated that VNS-induced hypothermia directly influences several electrophysiological parameters but also masks a VNS-induced reduction in neuronal excitability.

Vagus nerve stimulation as a potential adjuvant to behavioral therapy for autism and other neurodevelopmental disorders

Background

Many children with autism and other neurodevelopmental disorders undergo expensive, time-consuming behavioral interventions that often yield only modest improvements. The development of adjunctive interventions that can increase the benefit of rehabilitation therapies is essential in order to improve the lives of individuals with neurodevelopmental disorders.

Main text

Vagus nerve stimulation (VNS) is an FDA approved therapy that is safe and effective in reducing seizure frequency and duration in individuals with epilepsy. Individuals with neurodevelopmental disorders often exhibit decreased vagal tone, and studies indicate that VNS can be used to overcome an insufficient vagal response. Multiple studies have also documented significant improvements in quality of life after VNS therapy in individuals with neurodevelopmental disorders. Moreover, recent findings indicate that VNS significantly enhances the benefits of rehabilitative training in animal models and patients, leading to greater recovery in a variety of neurological diseases. Here, we review these findings and provide a discussion of how VNS paired with rehabilitation may yield benefits in the context of neurodevelopmental disorders.

Conclusions

VNS paired with behavioral therapy may represent a potential new approach to enhance rehabilitation that could significantly improve the outcomes of individuals with neurodevelopmental disorders.

Parametric characterization of neural activity in the locus coeruleus in response to vagus nerve stimulation

Vagus nerve stimulation (VNS) has emerged as a therapy to treat a wide range of neurological disorders, including epilepsy, depression, stroke, and tinnitus. Activation of neurons in the locus coeruleus (LC) is believed to mediate many of the effects of VNS in the central nervous system. Despite the importance of the LC, there is a dearth of direct evidence characterizing neural activity in response to VNS. A detailed understanding of the brain activity evoked by VNS across a range of stimulation parameters may guide selection of stimulation regimens for therapeutic use. In this study, we recorded neural activity in the LC and the mesencephalic trigeminal nucleus (Me5) in response to VNS over a broad range of current amplitudes, pulse frequencies, train durations, inter-train intervals, and pulse widths. Brief 0.5s trains of VNS drive rapid, phasic firing of LC neurons at 0.1mA. Higher current intensities and longer pulse widths drive greater increases in LC firing rate. Varying the pulse frequency substantially affects the timing, but not the total amount, of phasic LC activity. VNS drives pulse-locked neural activity in the Me5 at current levels above 1.2mA. These results provide insight into VNS-evoked phasic neural activity in multiple neural structures and may be useful in guiding the selection of VNS parameters to enhance clinical efficacy.

Vagus Nerve Stimulation Applied with a Rapid Cycle Has More Profound Influence on Hippocampal Electrophysiology Than a Standard Cycle

Although vagus nerve stimulation (VNS) is widely used, therapeutic mechanisms and optimal stimulation parameters remain elusive. In the present study, we investigated the effect of VNS on hippocampal field activity and compared the efficiency of different VNS paradigms. Hippocampal electroencephalography (EEG) and perforant path dentate field-evoked potentials were acquired before and during VNS in freely moving rats, using 2 VNS duty cycles: a rapid cycle (7 s on, 18 s off) and standard cycle (30 s on, 300 s off) and various output currents. VNS modulated the evoked potentials, reduced total power of the hippocampal EEG, and slowed the theta rhythm. In the hippocampal EEG, theta (4-8 Hz) and high gamma (75-150 Hz) activity displayed strong phase amplitude coupling that was reduced by VNS. Rapid-cycle VNS had a greater effect than standard-cycle VNS on all outcome measures. Using rapid cycle VNS, a maximal effect on EEG parameters was found at 300 μA, beyond which effects saturated. The findings suggest that rapid-cycle VNS produces a more robust outcome than standard cycle VNS and support already existing preclinical evidence that relatively low output currents are sufficient to produce changes in brain physiology and thus likely also therapeutic efficacy.

Reorganization of Motor Cortex by Vagus Nerve Stimulation Requires Cholinergic Innervation

BACKGROUND

Vagus nerve stimulation (VNS) paired with forelimb training drives robust, specific reorganization of movement representations in the motor cortex. The mechanisms that underlie VNS-dependent enhancement of map plasticity are largely unknown. The cholinergic nucleus basalis (NB) is a critical substrate in cortical plasticity, and several studies suggest that VNS activates cholinergic circuitry.

OBJECTIVE

We examined whether the NB is required for VNS-dependent enhancement of map plasticity in the motor cortex.

METHODS

Rats were trained to perform a lever pressing task and then received injections of the immunotoxin 192-IgG-saporin to selectively lesion cholinergic neurons of the NB. After lesion, rats underwent five days of motor training during which VNS was paired with successful trials. At the conclusion of behavioral training, intracortical microstimulation was used to document movement representations in motor cortex.

RESULTS

VNS paired with forelimb training resulted in a substantial increase in the representation of proximal forelimb in rats with an intact NB compared to untrained controls. NB lesions prevent this VNS-dependent increase in proximal forelimb area and result in representations similar to untrained controls. Motor performance was similar between groups, suggesting that differences in forelimb function cannot account for the difference in proximal forelimb representation.

CONCLUSIONS

Together, these findings indicate that the NB is required for VNS-dependent enhancement of plasticity in the motor cortex and may provide insight into the mechanisms that underlie the benefits of VNS therapy.

Vagus Nerve Stimulation as a Tool to Induce Plasticity in Pathways Relevant for Extinction Learning

Extinction describes the process of attenuating behavioral responses to neutral stimuli when they no longer provide the reinforcement that has been maintaining the behavior. There is close correspondence between fear and human anxiety, and therefore studies of extinction learning might provide insight into the biological nature of anxiety-related disorders such as post-traumatic stress disorder, and they might help to develop strategies to treat them. Preclinical research aims to aid extinction learning and to induce targeted plasticity in extinction circuits to consolidate the newly formed memory. Vagus nerve stimulation (VNS) is a powerful approach that provides tight temporal and circuit-specific release of neurotransmitters, resulting in modulation of neuronal networks engaged in an ongoing task. VNS enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here, we provide a detailed protocol for the preparation of custom-made parts and the surgical procedures required for VNS in rats. Using this protocol we show how VNS can facilitate the extinction of conditioned fear responses in an auditory fear conditioning task. In addition, we provide evidence that VNS modulates synaptic plasticity in the pathway between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA), which is involved in the expression and modulation of extinction memory.

Vagus nerve stimulation enhances extinction of conditioned fear and modulates plasticity in the pathway from the ventromedial prefrontal cortex to the amygdala

Fearful experiences can produce long-lasting and debilitating memories. Extinction of the fear response requires consolidation of new memories that compete with fearful associations. Subjects with posttraumatic stress disorder (PTSD) show impaired extinction of conditioned fear, which is associated with decreased ventromedial prefrontal cortex (vmPFC) control over amygdala activity. Vagus nerve stimulation (VNS) enhances memory consolidation in both rats and humans, and pairing VNS with exposure to conditioned cues enhances the consolidation of extinction learning in rats. Here we investigated whether pairing VNS with extinction learning facilitates plasticity between the infralimbic (IL) medial prefrontal cortex and the basolateral complex of the amygdala (BLA). Rats were trained on an auditory fear conditioning task, which was followed by a retention test and 1 day of extinction training. Vagus nerve stimulation or sham-stimulation was administered concurrently with exposure to the fear-conditioned stimulus and retention of fear conditioning was tested again 24 h later. Vagus nerve stimulation-treated rats demonstrated a significant reduction in freezing after a single extinction training session similar to animals that received 5× the number of extinction pairings. To study plasticity in the IL-BLA pathway, we recorded evoked field potentials (EFPs) in the BLA in anesthetized animals 24 h after retention testing. Brief burst stimulation in the IL produced LTD in the BLA field response in fear-conditioned and sham-treated animals. In contrast, the same stimulation resulted in potentiation of the IL-BLA pathway in the VNS-treated group. The present findings suggest that VNS promotes plasticity in the IL-BLA pathway to facilitate extinction of conditioned fear responses (CFRs).

Locus Coeruleus Stimulation Facilitates Long-Term Depression in the Dentate Gyrus That Requires Activation of β-Adrenergic Receptors

Synaptic plasticity comprises a cellular mechanism through which the hippocampus most likely enables memory formation. Neuromodulation, related to arousal, is a key aspect in information storage. The activation of locus coeruleus (LC) neurons by novel experience leads to noradrenaline release in the hippocampus at the level of the dentate gyrus (DG). We explored whether synaptic plasticity in the DG is influenced by activation of the LC via electrical stimulation. Coupling of test-pulses that evoked stable basal synaptic transmission in the DG with stimulation of the LC induced β-adrenoreceptor-dependent long-term depression (LTD) at perforant path–DG synapses in adult rats. Furthermore, persistent LTD (>24 h) induced by perforant path stimulation also required activation of β-adrenergic receptors: Whereas a β-adrenergic receptor antagonist (propranolol) prevented, an agonist (isoproterenol) strengthened the persistence of LTD for over 24 h. These findings support the hypothesis that persistent LTD in the DG is modulated by β-adrenergic receptors. Furthermore, LC activation potently facilitates DG LTD. This suggests in turn that synaptic plasticity in the DG is tightly regulated by activity in the noradrenergic system. This may reflect the role of the LC in selecting salient information for subsequent synaptic processing in the hippocampus.

Facilitative effects of VNS on the motor threshold: implications for its antidepressive mode of action?

In the present study, the effects of vagus nerve stimulation (VNS) on the resting motor threshold (rMT) of patients treated with repetitive transcranial magnetic stimulation were evaluated. Patients showed a significant decrease in the rMT during VNS-on stimulation. VNS was the only significant factor affecting rMT changes and did not appear to be a static variable. Further studies should focus on the effect of VNS on neural neurogenesis in depressive disorders, and the effects of other treatment options for major depressive disorder on the rMT should also be determined.