Mechanisms of the anti-ischemic effect of vagus nerve stimulation in the gerbil hippocampus

Mechanisms of Vagus Nerve Stimulation in Improving Motor Dysfunction After Stroke

Patients with stroke would have persistent functional deficits despite undergoing physiotherapy and rehabilitation training. Recently, vagus nerve stimulation (VNS), a newly emerging neuroregulatory technique, has been shown to improve motor dysfunction after stroke. Evidence from clinical and preclinical studies has proven the safety, feasibility, and efficacy of invasive and noninvasive VNS. It has been reported that the positive effect may be related to anti-inflammatory effects, mediating neuroplasticity, increasing blood-brain barrier integrity, promoting angiogenesis and reducing spreading depolarization. However, the underlying mechanism remains poorly understood. In this review, we have summarized the potential molecular mechanisms by which VNS promotes stroke prognosis. We believe that VNS combined with upper-extremity rehabilitation can improve impairment and function among moderately to severely impaired stroke survivors. The applications and further exploration are discussed to provide new insights into this novel therapeutic technique.

A narrative review of vagus nerve stimulation in stroke

Stroke is a significant health concern impacting society and the health care system. Reperfusion therapy for acute ischemic stroke and standard rehabilitative therapies may not always be effective at improving post-stroke neurological function, and developing alternative strategies is particularly important. Vagus nerve stimulation (VNS) is a treatment option currently approved by the Food and Drug Administration (FDA) for intractable epilepsy, refractory depression, primary headache disorders, obesity, and moderate to severe upper-limb motor dysfunction in chronic ischemic stroke patients. Moreover, VNS has demonstrated potential efficacy in various conditions, including autoimmune diseases, disorders of consciousness, Alzheimer's disease, Parkinson's disease, traumatic brain injury, stroke, and other diseases. Although the popularity and application of VNS continue to increase rapidly, the field generally lacks a consensus on the optimal stimulation parameters. The stimulation parameters for VNS are directly related to the clinical outcome, and determining the optimal stimulation conditions for VNS has become an essential concern in its clinical application. This review summarizes the current evidence on VNS for stroke in preclinical models and clinical trials in humans, paying attention to the current types and stimulation parameters of VNS, highlighting the mechanistic pathways involved in the beneficial effects of VNS, critically evaluating clinical implementation challenges and proposing some suggestions for its future research directions. Achieving safe and effective clinical transformation of VNS requires further animal and clinical studies to determine the optimal stimulation parameters and therapeutic mechanisms.

Vagus nerve stimulation in cerebral stroke: biological mechanisms, therapeutic modalities, clinical applications, and future directions

Stroke is a major disorder of the central nervous system that poses a serious threat to human life and quality of life. Many stroke victims are left with long-term neurological dysfunction, which adversely affects the well-being of the individual and the broader socioeconomic impact. Currently, post-stroke brain dysfunction is a major and difficult area of treatment. Vagus nerve stimulation is a Food and Drug Administration-approved exploratory treatment option for autism, refractory depression, epilepsy, and Alzheimer's disease. It is expected to be a novel therapeutic technique for the treatment of stroke owing to its association with multiple mechanisms such as altering neurotransmitters and the plasticity of central neurons. In animal models of acute ischemic stroke, vagus nerve stimulation has been shown to reduce infarct size, reduce post-stroke neurological damage, and improve learning and memory capacity in rats with stroke by reducing the inflammatory response, regulating blood-brain barrier permeability, and promoting angiogenesis and neurogenesis. At present, vagus nerve stimulation includes both invasive and non-invasive vagus nerve stimulation. Clinical studies have found that invasive vagus nerve stimulation combined with rehabilitation therapy is effective in improving upper limb motor and cognitive abilities in stroke patients. Further clinical studies have shown that non-invasive vagus nerve stimulation, including ear/cervical vagus nerve stimulation, can stimulate vagal projections to the central nervous system similarly to invasive vagus nerve stimulation and can have the same effect. In this paper, we first describe the multiple effects of vagus nerve stimulation in stroke, and then discuss in depth its neuroprotective mechanisms in ischemic stroke. We go on to outline the results of the current major clinical applications of invasive and non-invasive vagus nerve stimulation. Finally, we provide a more comprehensive evaluation of the advantages and disadvantages of different types of vagus nerve stimulation in the treatment of cerebral ischemia and provide an outlook on the developmental trends. We believe that vagus nerve stimulation, as an effective treatment for stroke, will be widely used in clinical practice to promote the recovery of stroke patients and reduce the incidence of disability.

Combination of Acute and Maintenance Esketamine Treatment With Adjunctive Long-Term Vagus Nerve Stimulation in Difficult-to-Treat Depression

INTRODUCTION

The antidepressant effect of N-methyl-D-aspartate antagonists is often short lasting, raising the question of the best maintenance strategy, which has remained unanswered. Vagus nerve stimulation (VNS) as a treatment option for refractory and chronic major depression was shown to reduce the need for maintenance treatment sessions in patients receiving electroconvulsive therapy. To our knowledge, there are no published data on the combination of VNS and esketamine in the literature.

MATERIALS AND METHODS

This is a naturalistic prospective and retrospective observational study in patients treated with long-term VNS owing to difficult-to-treat depression. These patients also have received esketamine maintenance sessions in addition to short-term treatment. We have investigated the need for maintenance esketamine sessions per month after VNS implantation (number of sessions/number of months between visits), the change in depression severity (mean Montgomery-Asberg Depression Rating Scale [MADRS] score), and the number of hospitalizations per month (number of hospitalizations/number of postoperative observation months). Follow-up visits have been scheduled every three months after VNS implantation (follow-up period 12-24 months, mean 17).

RESULTS

All patients (n = 8, mean age 53.1 years) had severe difficult-to-treat depression (DTD) (mean MADRS at baseline 30.9). Mean number of hospitalizations per month decreased from 0.17 to 0.11 after VNS implantation (p = 0.041, T = 2.030, df = 7). Mean MADRS at 12 months was 18.3 (40.8% MADRS reduction, p = 0.008, T = 3.146, df = 7). Six of eight patients were offered maintenance esketamine treatment. Mean number of esketamine treatment sessions per month and case decreased from 2.3 at the six-month visit to 0.8 at 12 months (p = 0.076, T = 1,690, df = 5) after VNS implantation. Termination of maintenance esketamine was possible in four cases after a mean of 11.5 months.

CONCLUSIONS

Combination of esketamine and VNS was effective in patients with DTD to relieve disease severity and reduce hospitalizations. The need for esketamine treatment sessions decreased after 6 months of VNS. No safety concerns arose in this study regarding the combination treatment.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT03320304.

Vagus nerve stimulation as immunomodulatory therapy for stroke: A comprehensive review

Stroke is a devastating cerebrovascular pathology with high morbidity and mortality. Inflammation plays a central role in the pathophysiology of stroke. Vagus nerve stimulation (VNS) is a promising immunomodulatory method that has shown positive effects in stroke treatment, including neuroprotection, anti-apoptosis, anti-inflammation, antioxidation, reduced infarct volume, improved neurological scores, and promotion of M2 microglial polarization. In this review, we summarize the current knowledge about the vagus nerve's immunomodulatory effects through the cholinergic anti-inflammatory pathway (CAP) and provide a comprehensive assessment of the available experimental literature focusing on the use of VNS in stroke treatment.

Vagus Nerve Stimulation in Ischemic Stroke

PURPOSE OF REVIEW

Vagus nerve stimulation (VNS) has emerged as a potential therapeutic approach for neurological and psychiatric disorders. In recent years, there has been increasing interest in VNS for treating ischemic stroke. This review discusses the evidence supporting VNS as a treatment option for ischemic stroke and elucidates its underlying mechanisms.

RECENT FINDINGS

Preclinical studies investigating VNS in stroke models have shown reduced infarct volumes and improved neurological deficits. Additionally, VNS has been found to reduce reperfusion injury. VNS may promote neuroprotection by reducing inflammation, enhancing cerebral blood flow, and modulating the release of neurotransmitters. Additionally, VNS may stimulate neuroplasticity, thereby facilitating post-stroke recovery. The Food and Drug Administration has approved invasive VNS (iVNS) combined with rehabilitation for ischemic stroke patients with moderate to severe upper limb deficits. However, iVNS is not feasible in acute stroke due to its time-sensitive nature. Non-invasive VNS (nVNS) may be an alternative approach for treating ischemic stroke. While the evidence from preclinical studies and clinical trials of nVNS is promising, the mechanisms through which VNS exerts its beneficial effects on ischemic stroke are still being elucidated. Therefore, further research is needed to better understand the efficacy and underlying mechanisms of nVNS in ischemic stroke. Moreover, large-scale randomized clinical trials are necessary to determine the optimal nVNS protocols, assess its long-term effects on stroke recovery and outcomes, and identify the potential benefits of combining nVNS with other rehabilitation strategies.

Systemic immune responses after ischemic stroke: From the center to the periphery

Ischemic stroke is a leading cause of disability and death. It imposes a heavy economic burden on individuals, families and society. The mortality rate of ischemic stroke has decreased with the help of thrombolytic drug therapy and intravascular intervention. However, the nerve damage caused by ischemia-reperfusion is long-lasting and followed by multiple organ dysfunction. In this process, the immune responses manifested by systemic inflammatory responses play an important role. It begins with neuroinflammation following ischemic stroke. The large number of inflammatory cells released after activation of immune cells in the lesion area, along with the deactivated neuroendocrine and autonomic nervous systems, link the center with the periphery. With the activation of systemic immunity and the emergence of immunosuppression, peripheral organs become the second “battlefield” of the immune response after ischemic stroke and gradually become dysfunctional and lead to an adverse prognosis. The purpose of this review was to describe the systemic immune responses after ischemic stroke. We hope to provide new ideas for future research and clinical treatments to improve patient outcomes and quality of life.

Neuropsychiatric Treatments for Epilepsy: Nonpharmacological Approaches

Neuropsychiatric conditions are frequently found in patients with epilepsy (PWE). These entities can be as disabling as epilepsy resulting in a significant negative impact on the quality of life of this population if not addressed and treated appropriately. In this article, we provide an overview of non-pharmacological treatments currently available to these patients-and review their effect on mood and anxiety disorders as well as epilepsy. These treatment strategies will allow the practitioner to optimize clinical care during the initial evaluation, which begins with the recognition of the neuropsychiatric condition followed by the appropriate individualized psychotherapeutic approach and/or neuromodulation therapy. To plan a comprehensive treatment for PWE, practitioners must be familiar with these therapeutic tools. Additional clinical research is needed to further create a multidisciplinary team in the assessment and management of neuropsychiatric disorders in PWE.

Transcutaneous vagus nerve stimulation (tVNS) in stroke: the evidence, challenges and future directions

Stroke is one of the leading causes of death and disability globally. A significant proportion of stroke survivors are left with long term neurological deficits that have a detrimental effect on personal wellbeing and wider socioeconomic impacts. As such, there is an unmet need for novel therapies that improve neurological recovery after stroke. Invasive vagus nerve stimulation (VNS) paired with rehabilitation has been shown to improve upper limb motor function in chronic stroke. However, invasive VNS requires a surgical procedure and therefore may not be suitable for all stroke patients. Non-invasive, transcutaneous VNS (tVNS) via auricular vagus nerve stimulation in the ear (taVNS) and cervical vagus nerve stimulation in the neck (tcVNS) have been shown to activate similar vagal nerve projections in the central nervous system to invasive VNS. A number of pre-clinical studies indicate that tVNS delivered in acute middle cerebral artery occlusion reduces infarct size through anti-inflammatory effects, reduced excitotoxicity and increased blood-brain barrier integrity. Longer term effects of tVNS in stroke that may mediate neuroplasticity include microglial polarisation, angiogenesis and neurogenesis. Pilot clinical trials of taVNS indicate that taVNS paired with rehabilitation may improve upper limb motor and sensory function in patients with chronic stroke. In this review, we summarise and critically appraise the current pre-clinical and clinical evidence, outline the major ongoing clinical trials and detail the challenges and future directions regarding tVNS in acute and chronic stroke.

Regulation of Blood Flow in the Cerebral Posterior Circulation by Parasympathetic Nerve Fibers: Physiological Background and Possible Clinical Implications in Patients With Vertebrobasilar Stroke

Posterior circulation involves the vertebrobasilar arteries, which supply oxygen and glucose to vital human brainstem structures and other areas. This complex circulatory- perfusion system is not homogenous throughout the day; rather, its hemodynamic changes rely on physiological demands, ensuring brainstem perfusion. This dynamic autoregulatory pattern maintains cerebral perfusion during blood pressure changes. Accumulative evidence suggests that activity within the autonomic nervous system is involved in the regulation of cerebral blood flow. Neither the sympathetic nor parasympathetic nervous systems work independently. Functional studies have shown a tight and complicated cross talk between these systems. In pathological processes where sympathetic stimulation is present, systemic vasoconstriction is followed, representing the most important CNS parasympathetic trigger that will promote local vasodilation. Stroke is a clear example of this process. The posterior circulation is affected in 30% of strokes, causing high morbidity and mortality outcomes. Currently, the management of ischemic stroke is focused on thrombolytic treatment and endovascular thrombectomy within an overall tight 4.5 to 6 h ischemic time window. Therefore, the autonomic nervous system could represent a potential therapeutic target to modulate reperfusion after cerebral ischemia through vasodilation, which could potentially decrease infarct size and increase the thrombolytic therapeutic ischemic window. In addition, shifting the autonomic nervous system balance toward its parasympathetic branch has shown to enhance neurogenesis and decrease local inflammation. Regretfully, the vast majority of animal models and human research on neuromodulation during brain ischemia have been focused on anterior circulation with disappointing results. In addition, the source of parasympathetic inputs in the vertebrobasilar system in humans is poorly understood, substantiating a gap and controversy in this area. Here, we reviewed current available literature regarding the parasympathetic vascular function and challenges of its stimulation in the vertebrobasilar system.

Blocking the Hepatic Branch of the Vagus Aggravates Hepatic Ischemia-Reperfusion Injury via Inhibiting the Expression of IL-22 in the Liver

Liver ischemia-reperfusion injury (IRI) is an inevitable process during liver transplantation, hemorrhagic shock, resection, and other liver surgeries. It is an important cause of postoperative liver dysfunction and increased medical costs. The protective effects of the vagus nerve on hepatic IRI have been reported, but the underlying mechanism has not been fully understood. We established a hepatic vagotomy (Hv) mouse model to study the effect of the vagus on liver IRI and to explore the underlying mechanism. Liver IRI was more serious in mice with Hv, which showed higher serum ALT and AST activities and histopathological changes. Further experiments confirmed that Hv significantly downregulated the expression of IL-22 protein and mRNA in the liver, blocking the activation of the STAT3 pathway. The STAT3 pathway in the livers of Hv mice was significantly activated, and liver injury was clearly alleviated after treatment with exogenous IL-22 recombinant protein. In conclusion, Hv can aggravate hepatic IRI, and its mechanism may be related to inhibition of IL-22 expression and downregulation of the STAT3 pathway in the liver.

Vagus Nerve Stimulation Alleviates Hepatic Ischemia and Reperfusion Injury by Regulating Glutathione Production and Transformation

Inflammation and oxidative stress are pivotal mechanisms for the pathogenesis of ischemia and reperfusion injury (IRI). Vagus nerve stimulation (VNS) may participate in maintaining oxidative homeostasis and response to external stimulus or injury. We investigated whether the in vivo VNS can protect the liver from IRI. In this study, hepatic IRI were induced by ligating the vessels supplying the left and middle lobes of the liver, which underwent 1 h occlusion followed with 24 h reperfusion. VNS was initiated 15 min after ischemia and continued 30 min. Hepatic function, histology, and apoptosis rates were evaluated after 24 h reperfusion. Compared with the IRI group, VNS significantly improved hepatic function. The protective effect was accompanied by a reduction in histological damage in the ischemic area, and the apoptosis rate of hepatocytes has considerable reduction. To find the underlying mechanism, proteomic analysis was performed and differential expression of glutathione synthetase (GSS) and glutathione S-transferase (GST) was observed. Subsequently, test results indicated that VNS upregulated the expression of mRNA and protein of GSS and GST. Meanwhile, VNS increased the plasma levels of glutathione and glutathione peroxidases. We found that VNS alleviated hepatic IRI by upregulating the antioxidant glutathione via the GSS/glutathione/GST signaling pathway.

Effect of Electrical Vagus Nerve Stimulation on Cerebral Blood Flow and Neurological Outcome in Asphyxial Cardiac Arrest Model of Rats

BACKGROUND

Vagus nerve stimulation (VNS) during post-resuscitation may increase recovery of cerebral blood flow (CBF) and reduce neurological injury.

OBJECTIVE

This study was designed to investigate the effect of electrical VNS on neurological outcomes following cardiac arrest (CA).

METHODS

Male Sprague-Dawley rats (n = 48) were subjected to the asphyxial CA model and blindly allocated to the VN isolation (CA + VN isolation) or VNS group (CA + VNS group). Cardiopulmonary resuscitation was initiated 450 s after pulseless electrical arrest, and the left cervical vagus nerve was electrically stimulated (0.05 mA, 1 Hz) for 3 h in the CA + VNS group. The neurological deficit score (NDS) and overall performance category (OPC) were assessed at 24 h after resuscitation, and histological injury of the hippocampus was evaluated. Independent experiments were performed to evaluate the effect of VNS on global cortical CBF after resuscitation using laser speckle Doppler imaging through a thinned skull window from pre-arrest to 6 h after resuscitation.

RESULTS

The baseline characteristics were not significantly different between the two groups. The NDS was significantly higher, and the OPC was substantially lower in the CA + VNS group (p = 0.022 and p = 0.049, respectively) supported by decrease in histological injury of the hippocampal CA1 region. CBF in the early period of post-return of spontaneous circulation (ROSC) was significantly higher in the CA + VNS group (p < 0.05 at post-ROSC 2 h and 4 h), and 4-hydroxynonenal was significantly lower in the CA + VNS group (p = 0.026).

CONCLUSIONS

VNS improved cerebral perfusion and neurological outcomes at 24 h after ROSC in an asphyxial CA model of rats.

Autonomic Disturbances in Acute Cerebrovascular Disease

Autonomic disturbances often occur in patients with acute cerebrovascular disease due to damage of the central autonomic network. We summarize the structures of the central autonomic network and the clinical tests used to evaluate the functions of the autonomic nervous system. We review the clinical and experimental findings as well as management strategies of post-stroke autonomic disturbances including electrocardiographic changes, cardiac arrhythmias, myocardial damage, thermoregulatory dysfunction, gastrointestinal dysfunction, urinary incontinence, sexual disorders, and hyperglycemia. The occurrence of autonomic disturbances has been associated with poor outcomes in stroke patients. Autonomic nervous system modulation appears to be an emerging therapeutic strategy for stroke management in addition to treatments for sensorimotor dysfunction.

Vagus Nerve Stimulation and Other Neuromodulation Methods for Treatment of Traumatic Brain Injury

The objective of this paper is to review the current literature regarding the use of vagus nerve stimulation (VNS) in preclinical models of traumatic brain injury (TBI) as well as discuss the potential role of VNS along with alternative neuromodulation approaches in the treatment of human TBI. Data from previous studies have demonstrated VNS-mediated improvement following TBI in animal models. In these cases, VNS was observed to enhance motor and cognitive recovery, attenuate cerebral edema and inflammation, reduce blood brain barrier breakdown, and confer neuroprotective effects. Yet, the underlying mechanisms by which VNS enhances recovery following TBI remain to be fully elucidated. Several hypotheses have been offered including: a noradrenergic mechanism, reduction in post-TBI seizures and hyper-excitability, anti-inflammatory effects, attenuation of blood-brain barrier breakdown, and cerebral edema. We present other potential mechanisms by which VNS acts including enhancement of synaptic plasticity and recruitment of endogenous neural stem cells, stabilization of intracranial pressure, and interaction with the ghrelin system. In addition, alternative methods for the treatment of TBI including deep brain stimulation, transcranial magnetic stimulation, transcranial direct current stimulation, and focused ultrasound stimulation are discussed. Although the primary source data show that VNS improves TBI outcomes, it remains to be determined if these findings can be translated to clinical settings.

Vagus nerve stimulation inhibits cortical spreading depression

Vagus nerve stimulation has recently been reported to improve symptoms of migraine. Cortical spreading depression is the electrophysiological event underlying migraine aura and is a trigger for headache. We tested whether vagus nerve stimulation inhibits cortical spreading depression to explain its antimigraine effect. Unilateral vagus nerve stimulation was delivered either noninvasively through the skin or directly by electrodes placed around the nerve. Systemic physiology was monitored throughout the study. Both noninvasive transcutaneous and invasive direct vagus nerve stimulations significantly suppressed spreading depression susceptibility in the occipital cortex in rats. The electrical stimulation threshold to evoke a spreading depression was elevated by more than 2-fold, the frequency of spreading depressions during continuous topical 1 M KCl was reduced by ∼40%, and propagation speed of spreading depression was reduced by ∼15%. This effect developed within 30 minutes after vagus nerve stimulation and persisted for more than 3 hours. Noninvasive transcutaneous vagus nerve stimulation was as efficacious as direct invasive vagus nerve stimulation, and the efficacy did not differ between the ipsilateral and contralateral hemispheres. Our findings provide a potential mechanism by which vagus nerve stimulation may be efficacious in migraine and suggest that susceptibility to spreading depression is a suitable platform to optimize its efficacy.

Cholinergic Pathway Suppresses Pulmonary Innate Immunity Facilitating Pneumonia After Stroke

BACKGROUND AND PURPOSE

Temporary immunosuppression has been identified as a major risk factor for the development of pneumonia after acute central nervous system injury. Although overactivation of the sympathetic nervous system was previously shown to mediate suppression of systemic cellular immune responses after stroke, the role of the parasympathetic cholinergic anti-inflammatory pathway in the antibacterial defense in lung remains largely elusive.

METHODS

The middle cerebral artery occlusion model in mice was used to examine the influence of the parasympathetic nervous system on poststroke immunosuppression. We used heart rate variability measurement by telemetry, vagotomy, α7 nicotinic acetylcholine receptor-deficient mice, and parasympathomimetics (nicotine, PNU282987) to measure and modulate parasympathetic activity.

RESULTS

Here, we demonstrate a rapidly increased parasympathetic activity in mice after experimental stroke. Inhibition of cholinergic signaling by either vagotomy or by using α7 nicotinic acetylcholine receptor-deficient mice reversed pulmonary immune hyporesponsiveness and prevented pneumonia after stroke. In vivo and ex vivo studies on the role of α7 nicotinic acetylcholine receptor on different lung cells using bone marrow chimeric mice and isolated primary cells indicated that not only macrophages but also alveolar epithelial cells are a major cellular target of cholinergic anti-inflammatory signaling in the lung.

CONCLUSIONS

Thus, cholinergic pathways play a pivotal role in the development of pulmonary infections after acute central nervous system injury.

Electrical stimulation of the vagus nerve protects against cerebral ischemic injury through an anti-infammatory mechanism

Vagus nerve stimulation exerts protective effects against ischemic brain injury; however, the underlying mechanisms remain unclear. In this study, a rat model of focal cerebral ischemia was established using the occlusion method, and the right vagus nerve was given electrical stimulation (constant current of 0.5 mA; pulse width, 0.5 ms; frequency, 20 Hz; duration, 30 seconds; every 5 minutes for a total of 60 minutes) 30 minutes, 12 hours, and 1, 2, 3, 7 and 14 days after surgery. Electrical stimulation of the vagus nerve substantially reduced infarct volume, improved neurological function, and decreased the expression levels of tumor necrosis factor-α and interleukin-6 in rats with focal cerebral ischemia. The experimental findings indicate that the neuroprotective effect of vagus nerve stimulation following cerebral ischemia may be associated with the inhibition of tumor necrosis factor-α and interleukin-6 expression.

Autonomic dysfunction in acute ischemic stroke: an underexplored therapeutic area?

Impaired autonomic function, characterized by a predominance of sympathetic activity, is common in patients with acute ischemic stroke. This review describes methods to measure autonomic dysfunction in stroke patients. It summarizes a potential relationship between ischemic stroke-associated autonomic dysfunction and factors that have been associated with worse outcome, including cardiac complications, blood pressure variability changes, hyperglycemia, immune depression, sleep disordered breathing, thrombotic effects, and malignant edema. Involvement of the insular cortex has been suspected to play an important role in causing sympathovagal imbalance, but its exact role and that of other brain regions remain unclear. Although sympathetic overactivity in patients with ischemic stroke appears to be a negative prognostic factor, it remains to be seen whether therapeutic strategies that reduce sympathetic activity or increase parasympathetic activity might improve outcome.

Vagus nerve stimulation in ischemic stroke: old wine in a new bottle

Vagus nerve stimulation (VNS) is currently Food and Drug Administration-approved for treatment of both medically refractory partial-onset seizures and severe, recurrent refractory depression, which has failed to respond to medical interventions. Because of its ability to regulate mechanisms well-studied in neuroscience, such as norepinephrine and serotonin release, the vagus nerve may play an important role in regulating cerebral blood flow, edema, inflammation, glutamate excitotoxicity, and neurotrophic processes. There is strong evidence that these same processes are important in stroke pathophysiology. We reviewed the literature for the role of VNS in improving ischemic stroke outcomes by performing a systematic search for publications in Medline (1966–2014) with keywords “VNS AND stroke” in subject headings and key words with no language restrictions. Of the 73 publications retrieved, we identified 7 studies from 3 different research groups that met our final inclusion criteria of research studies addressing the role of VNS in ischemic stroke. Results from these studies suggest that VNS has promising efficacy in reducing stroke volume and attenuating neurological deficits in ischemic stroke models. Given the lack of success in Phase III trials for stroke neuroprotection, it is important to develop new therapies targeting different neuroprotective pathways. Further studies of the possible role of VNS, through normally physiologically active mechanisms, in ischemic stroke therapeutics should be conducted in both animal models and clinical studies. In addition, recent advent of a non-invasive, transcutaneous VNS could provide the potential for easier clinical translation.

Ablation of the sphenopalatine ganglion does not attenuate the infarct reducing effect of vagus nerve stimulation

Electrical stimulation of the cervical vagus nerve reduces infarct size by approximately 50% after cerebral ischemia in rats. The mechanism of ischemic protection by vagus nerve stimulation (VNS) is not known. In this study, we investigated whether the infarct reducing effect of VNS was mediated by activation of the parasympathetic vasodilator fibers that originate from the sphenopalatine ganglion (SPG) and innervate the anterior cerebral circulation. We examined the effects of electrical stimulation of the cervical vagus nerve in two groups of rats: one with and one without SPG ablation. Electrical stimulation was initiated 30 min after induction of ischemia, and lasted for 1h. Measurement of infarct size 24h later revealed that the volume of ischemic damage was smaller in those animals that received VNS treatment (41.32±2.07% vs. 24.19±2.62% of the contralateral hemispheric volume, n=6 in both; p<0.05). SPG ablation did not abolish this effect; the reduction in infarct volume following VNS was 58% in SPG-damaged animals, 41% in SPG-intact animals (p>0.05). In both SPG-intact and SPG-damaged animals VNS treatment resulted in better motor outcome (p<0.05 vs. corresponding controls for both). Our findings show that VNS can protect the brain against acute ischemic injury, and that this effect is not mediated by SPG projections.

Effect of vagus nerve stimulation during transient focal cerebral ischemia on chronic outcome in rats

The aim of this study was to investigate the effect of vagus nerve stimulation (VNS) on infarct volume and neurological recovery up to 3 weeks following transient focal cerebral ischemia. Transient ischemia was produced by filament occlusion of the proximal middle cerebral artery (MCA) in rats. The right vagus nerve was stimulated starting 30 min after MCA occlusion and consisted of 30-sec pulse trains (20 Hz) delivered to the animal's right vagus nerve every 5 min for a total period of 60 min (n = 10). All the procedures were duplicated, but no stimulus was delivered, in a control group (n = 10). Neurological evaluations were performed in all animals at 24 hr, 48 hr, 1 week, 2 weeks, and 3 weeks after MCA occlusion; animals were euthanized; and neuronal damage was evaluated in hematoxylin-eosin-stained sections. The ischemic lesion volume was smaller in the VNS-treated animals in comparison with the nonstimulated group (P < 0.02). Although the functional score in both treated and untreated groups improved over the 3-week observation period (P < 0.001), there was still a statistically significant improvement reszulting from VNS treatment compared with control animals (P < 0.05). Cerebral blood flow changes in the MCA territory during ischemia did not differ between the VNS-treated animals (31.9% ± 10.4% of baseline) and control animals (29.9% ± 9.1%; P = 0.6). Stimulation of the vagus nerve for only a brief period early in ischemia provides neuroprotection in transient ischemia, with neuroprotection persisting for at least 3 weeks.

The parasympathetic nervous system in the quest for stroke therapeutics

Stroke is a devastating neurovascular disease with limited therapeutic options. The pathogenesis of stroke involves complex interrelated molecular mechanisms including excitotoxicity, oxidative and nitrosative stress, cortical spreading depolarizations, inflammation, necrosis, and apoptosis. Successful development of stroke therapeutics depends on understanding these molecular mechanisms and how to counteract them to limit tissue damage during stroke. Activation of the parasympathetic nervous system (PNS) has been shown to antagonize a multiplicity of pathologic mechanisms. Elements of parasympathetic activation such as vagus nerve stimulation have already been used successfully in treating brain disorders such as epilepsy and depression. This review discusses the anatomical basis and molecular mechanisms involved in activation of the PNS, and assesses the strength of available evidence for the further development of this modality into a stroke therapy.

The effect of right vagus nerve stimulation on focal cerebral ischemia: an experimental study in the rat

BACKGROUND

The aim of this study was to determine the effect of vagus nerve stimulation (VNS) on infarct size after transient and after permanent focal cerebral ischemia in rats and to test the hypothesis that VNS-induced neuroprotection is due to changes in cerebral blood flow.

METHODS

Ischemia was produced by either temporary proximal middle cerebral artery occlusion (TMCAO) or permanent distal middle cerebral artery occlusion (PMCAO). Stimulating electrodes were implanted on the cervical part of the right vagus nerve, and electrical stimulation was initiated 30 minutes after the induction of ischemia and delivered for 30 seconds every 5 minutes for 1 hour. All the procedures were duplicated but no stimulus was delivered in control groups. Cerebral blood flow in the MCA territory was continuously monitored with laser speckle contrast imaging. A neurologic evaluation was undertaken after 24 hours of ischemia, and animals were euthanized and neuronal damage evaluated.

RESULTS

Ischemic lesion volume was smaller in VNS-treated animals in both the temporary and permanent ischemic groups (P<.01). VNS-treated animals in TMCAO had better functional scores at 24 hours as compared with control animals (P<.01), but there were no statistically significant differences in the neurobehavioral scores in PMCAO (P=.089). Cerebral blood flow changes in the MCA territory during ischemia did not differ between the VNS-treated animals and control animals in either group.

CONCLUSIONS

VNS offers neuroprotection against stroke in both temporary and permanent ischemia. Although the precise mechanism of this effect remains to be determined, alterations in cerebral blood flow do not appear to play a role. VNS could readily be translated to clinical practice.