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Transcutaneous vagus nerve stimulation (tVNS) as a potential therapeutic application for neurodegenerative disorders – A focus on dysautonomia in Parkinson's disease
The understandings of pathogenic processes in major neurodegenerative diseases has significantly advanced in recent years, with evidence showing pathological spread of intraneuronal proteinaceous inclusions as a fundamental factor. In Parkinson's disease (PD), the culprit protein has been identified as α-synuclein as the main component for mediating progressive neurodegeneration. With severe pathology evident in the autonomic nervous system prior to clinical manifestations of PD, pathogenic spread can occur from the peripheral nervous system through key nuclei, such as the anterior olfactory nucleus and dorsal motor nucleus of the glossopharyngeal and vagal nerves, gradually reaching the brainstem, midbrain and cerebral cortex. With this understanding and the proposed involvement of the vagus nerve in disease progression in PD, notably occurring prior to characterized clinical motor features, it raises intriguing questions as to whether vagal nerve pathology can be accurately detected, and importantly used as a reliable marker for determining early neurodegeneration. Along with this is the potential use of vagus nerve neuromodulation for treatment of early disease symptoms like dysautonomia, for modulating sympatho-vagal imbalances and easing severe comorbidities of the disease. In this article, we take a closer look at the pathogenic transmission processes in neurodegenerative disorders that impact the vagus nerve, and how vagus nerve neuromodulation can be potentially applied as a therapeutic approach for major neurodegenerative disorders.
The rapid and continued up-rise of patients suffering from neurological disorders is a prominent global health challenge that impacts the socio-economic status (
). In Europe, disorders of mood and dementia are the greatest cause of economic burden within brain diseases, from accounted figures for direct/other related health expenses and patient production losses (
). From a global perspective, the cost of care for dementia, most commonly caused by neurodegenerative disorder Alzheimer's disease (AD), reached over $800 billion USD in 2015 and is estimated to exceed $2 trillion USD by 2030 (
). With increased life expectancy of the global population, incidences of brain diseases are expected to surge, making these imminent health and economic challenges (
Although disease-modifying agents for debilitating neurodegenerative disorders are not yet available to cure patients, important steps have been made in recent years for uncovering the underlying pathological processes towards reaching this goal (
). With greater understanding of disease pathogenesis at this current time, advanced rehabilitation and treatment approaches can be adopted for effectively managing different clinical stages over the disease course. The complexity of having a diverse range of clinical symptoms that present in a progressive manner is likely to require a combinative approach with easy-to-apply treatments that can help achieve mosaic symptomatic management. In this article, we provide an overview of the current understanding of the underlying processes of neurodegeneration, with focus on Parkinson's Disease (PD) pathology, and specific involvement of the vagus nerve and the autonomic nervous system. We then discuss the clinically used non-invasive neuromodulation therapy, transcutaneous Vagus Nerve Stimulation (tVNS) for treatment of sympatho-vagal imbalance and its future use for neurodegenerative disorders, highlighting the clinical opportunities in this era of neuromodulation therapy.
2. Neurodegeneration
2.1 An insight into Parkinson's disease
Since the turn of the millennium, the incidence of PD has been projected to double by 2030 (
). Currently, PD is the 2nd most common neurodegenerative disorder, affecting 1% of the population over the age of 55 years and has highest prevalence in ages of over 85 years (
Prevalence of parkinsonism and Parkinson's disease in Europe: the EUROPARKINSON collaborative study. European Community concerted action on the epidemiology of Parkinson's disease.
). It remains important to recognize that in the years prior to the manifestation of these characteristic motor symptoms, which occurs only after approximately 70% of dopaminergic neurons have degenerated (
), the majority of patients (i.e. up to two-thirds) commonly suffer from non-motor symptoms, in a so-called ‘pre-motor’ stage of PD. This phase includes presentation of core symptoms such as olfactory dysfunction (anosmia), sleep abnormalities (REM sleep Behaviour Disorder, RBD), dysphagia, cardiac sympathetic denervation, cognitive impairment, anxiety and depression, pain, and gastrointestinal irregularities (constipation, reduced gastric motility), which are seen before any other neurological detriment (
). These non-motor symptoms arise from progressive neuronal loss of other important neurotransmitter systems, including noradrenergic, serotonergic and cholinergic neurons (
). This has been supported by clinical reports showing the presence of PD biomarkers, such as α-synuclein staining in bowel biopsy samples many years prior to presentation of PD motor signs (
As with other well-known α-synucleinopathies, such as Dementia with Lewy Bodies (DLB) and Multiple System Atrophy (MSA), PD is characterized by widespread clusters of aberrant forms of α-synuclein (
). In PD, these intraneuronal proteinaceous inclusions, known as Lewy Bodies (LB) are mainly composed of α-synuclein, a 14 kDa endogenous protein of 140 amino acid length, and can be found in perikaryal of neurons within the central and peripheral nervous systems. It is now established that the main ‘culprit’ linked to progressive neuronal degeneration in PD is α-synuclein, where the pathological structure is a misfolded conformation of oligomeric or fibrillary nature, of phosphorylated form (phosphor-Ser129) (
The neurodegenerative spread in a prionoid-like manner describes the transfer of α-synuclein-positive intracytoplasmic inclusions after initial seeding of pathogenic α-synuclein. In PD, pathological neuronal transmission was first indicated from post-mortem analysis showing host-to-graft transmission, where LB formations were found in embryonic mesencephalic neuron grafts in the striatum decades following brain implantation (
). Since then, pathogenic α-synuclein is found in monomeric, oligomeric and phosphor-Ser129 forms in PD patient cerebrospinal fluid and plasma, supporting a process of transmission across the nervous system in disease sufferers (
Post mortem cerebrospinal fluid a-synuclein levels are raised in multiple system atrophy and distinguish this from the other a-synucleinopathies, Parkinson's disease and dementia with lewy bodies.
). With its release into the extracellular space, pathogenic α-synuclein is then taken up by healthy neurons and through subsequent self-propagation, α-synuclein acts as a permissive template for misfolding of endogenous α-synuclein proteins (
). This propagates the spread of neurodegeneration across interconnected brain regions mediating widespread disruption of intracellular processes i.e. neurotransmission, mitochondrial activity, mitophagy, vesicular transport and protein degradation. Indeed, experimental models have demonstrated that the injection of α-synuclein preformed fibrils (PFFs) into striatum of mice causes accumulation of phosphor-Ser129 α-synuclein and dopaminergic cell death in the substantia nigra pars compacta (SNc), demonstrating retrograde transmission of pathogenic species in the brain (
). Since the original descriptions of a progressive spread of neurodegeneration in PD, prionoid-like mechanisms have now been proposed in other major neurodegenerative diseases including AD, Huntington's Disease and Amyotrophic Lateral Sclerosis (
Gastric alpha-synuclein immunoreactive inclusions in Meissner's and Auerbach's plexuses in cases staged for Parkinson's disease-related brain pathology.
) following their assessments of the regional distribution of α-synuclein immunoreactive structures in detailed post-mortem analyses. A ‘bottom-up’ approach was initially suggested, starting in the peripheral autonomic nervous system where topographic distribution of LBs were identified in the gut (
). It was also later suggested that the ‘unknown pathogen’ enters the human body through the nasal cavity and is swallowed, reaching the gastrointestinal (GI) tract. This was used to explain intrusion into the epithelial lining and anterior olfactory nucleus, before pathological spreading to the dorsal motor nucleus of the glosspharyngeal and vagal nerves. From the vagus nerve, the proposed non-random anterograde progression reaches the midbrain and cerebral cortex (
Gastric alpha-synuclein immunoreactive inclusions in Meissner's and Auerbach's plexuses in cases staged for Parkinson's disease-related brain pathology.
Gastric alpha-synuclein immunoreactive inclusions in Meissner's and Auerbach's plexuses in cases staged for Parkinson's disease-related brain pathology.
). The progressive pathogenic spread is suggested to move rostrally to brain regions including the medulla, pontine, tegmentum, midbrain and basal forebrain, before reaching the cerebral cortex (
). It has been shown in rodent experimental studies, the injection of specific viral vector (rAAV) expressing human wild-type α-synuclein into the left vagus nerve results in the expression of human α-synuclein in the medulla, before a caudo-rostral spread through brain regions (
). Moreover, an injection of pathological α-synuclein extract into the gut of rodents, from PD brain lysate samples or recombinant human α-synuclein fibrils, are found transported through the vagus nerve in a time-dependent manner (12, 48, 72 h after injection), reaching the DMV after 6 days (
Fig. 1The spread of Lewy Body pathology according to the Braak staging model. A. An illustration of Parkinson's disease progression in a stereotypic temporal pattern from trans-synaptic retrograde spread, starting from the peripheral enteric nervous system to the brainstem in the dorsal motor nucleus of the vagus nerve to areas including i. the medulla oblongata ii. pontine tegmentum iii. hypothalamus, thalamus, basal mid- and forebrain iv. mesocortex, allocortex. B. An illustration of the proposed bidirectional pathogenic transport of α-synuclein from gut to brain by the enteric nervous system to the vagus nerve, and/or non-vagus nerve pathways, such as bloodstream and lymphatic tissue.
Although Braak proposed 6 pathological stages (3 premotor stages with α-synuclein present in the ENS and lower brainstem, followed by 3 later stages with pathology affecting motor and cognitive areas), some experimental studies have reported discrepancies with this staging classification. For example, occurrence of a ‘top-down’ brain-to-gut transmission has been since proposed (
Nigral overexpression of a-synuclein in a rat Parkinson's disease model indicates alterations in the enteric nervous system and the gut microbiome.
). The similar descriptions, however, of an underlying non-random progressive development via vulnerable neuroanatomical regions that are likely mediated through thin, poorly myelinated axons are now conceived, greatly helping an understanding of the pathogenesis of certain neurodegenerative diseases (
For age-related neurodegenerative disorders such as AD and PD, notable pathology is observed with loss of neurons in the locus coeruleus of the brainstem, being one of the first areas affected. In early stages of AD, the locus coeruleus shows primary hallmarks being one of the first brainstem structures expressing neurofibrillary tangles (NFTs), which are the aggregates of microtubule-associated protein Tau, and can be found decades prior to any other established pathology (
). Interestingly for PD, α-synuclein is also identified in the locus coeruleus in early disease stages, which can be 10 years before clinical diagnosis (
) and is a brain region severely affected over the total disease time-course. Although a complete temporal relationship from (A) potential compromise of the GI tract, which increases in likelihood with aging (
), to (B) pathogenic entry and spread through the vagus nerve and (C) full clinical manifestation of PD, is not yet fully characterized, it is suggested that it may occur over a period of 10-20 years (
). At this time, however, from a more detailed understanding of some fundamental mechanisms in neurodegenerative disease progression, there are new opportunities now for developing strategies directed at alleviating symptoms at different disease stages.
2.4 Gut-brain connection – the vagus nerve
Using the Braak model for a framework in understanding neurodegeneration pathogenesis, the DMV represents the major neuroanatomical site where the pathogenic agent could enter the central nervous system. As the ENS provides bi-directional communication between the brain and the gut, mainly mediated through the vagus nerve, a compromised functionality of gut microbiome e.g. dysbiosis, has been suggested to increase the risk of developing certain neurological disorders such as anxiety, depression, autism, AD and PD (
The vagal nuclei that includes the nucleus ambiguous (NA), nucleus tractus solitarius (NTS) and DMV, are located in the caudal brainstem and give rise to vago-vagal neurocircuits (
). The DMV and NA mainly project efferent vagal motor fibres that modulate target organs in normal physiology. For the NTS, it receives a range of vagal afferents and integrates brainstem, limbic and hypothalamic sensory information, coordinating autonomic and visceral functions i.e. for gastric mobility. In PD, neurodegeneration affecting vagal neurocircuitry is seen with PD patients and may attribute to the delay of gastric emptying that can occur at all stages of the disease, with such symptoms appearing in up to 90% of PD patients (
Gastric alpha-synuclein immunoreactive inclusions in Meissner's and Auerbach's plexuses in cases staged for Parkinson's disease-related brain pathology.
). Interestingly, in large cohort studies conducted in Europe, full truncal vagotomy – that is the complete resection of the vagus nerve clinically used for in gastric disease treatment (
). The data showed that PD pathology and motor behaviour deficits induced by injection of α-synuclein fibrils into the muscularis layer of pylorus and duodenum of the gut in mice, are blocked following vagotomy and prevent a temporal stereotypic pattern of gut-to-brain propagation of the pathological phosphor-Ser129 α-synuclein (
). In a healthy state, the autonomic nervous system controls vital physiology for maintaining homeostasis, connecting to each major organ and providing regulation. Following LB pathology that affects peripheral ganglia of the autonomic nervous system (
), there is subsequent dysfunction which causes a distinct range of autonomic symptoms. Indeed, autonomic regulatory disorders are a prominent feature in PD, preceding motor symptoms and can dominate the clinical picture over the disease course (
). It is also worth highlighting that dysautonomia can be triggered and/or exacerbated by drug treatments for motor symptoms through effects on the autonomic nervous system. With overlapping features of autonomic failure, for example between PD and MSA, and the potential drug treatment induced side-effects, it remains difficult to separate PD from non-PD disorders in vivo (
) only based on autonomic features, requiring more detailed clinical evaluations for accurate disease diagnosis.
In PD, LB pathology can be found in the peripheral cardiovascular autonomic network, such as the superior sympathetic ganglia, which innervates the heart (
Comments on "Long-term exposure of Emicro-Pim1 transgenic mice to 898.4 MHz microwaves does not increase lymphoma incidence" by Utteridge et al., Radiat. Res. 158, 357-364 (2002).
) indicating features of autonomic failure. As well as the associated motor disability in PD patients, such non-motor symptoms can further increase the risk of falling and injury (
). The Heart Rate Variability (HRV) parameter analyses used to evaluate autonomic system activity has been studied in PD patients in comparison to control subjects, demonstrating differences in specific parameters. For example, the Low Frequency (LF)/High Frequency (HF) ratio (an index for sympatho-parasympathetic balance) is lower in early stage untreated PD (
) indicating a predominant parasympathetic drive to the heart, while there are no significant changes found in LF (0.004–0.15 Hz, sympathetic modulation of the sinoatrial node) or HF (0.15-0.4 Hz, parasympathethic vagal modulation). When comparing PD and MSA, patients show differences in the vagal tone, with changes associated with the natural circadian rhythm. More specifically, PD patients show a reduced vagal tone predominantly with sleep, while MSA patients show a depression of the sympathetic tone during daily activities (
), the advanced characterization of the pathophysiological basis of cardiovascular autonomic dysfunction across disease stages aids a basis for using target therapies and improving management strategies in patient care. A previous study analysed the time-domain analysis of HRV in PD patient groups across progressive stages by continuous ECG recordings (24-h spectral analysis). These groups included patients of a. early stage, minor impairment without L-DOPA drug treatment, b. mild impairment with L-DOPA, and c. advanced PD with motor fluctuations following L-DOPA treatment. It was found that in later stages of PD (groups b and c) there was decreased diurnal LF and LF/HF power compared to controls, and nocturnal vagal indicators (HF and pNN50) were reduced in the most advanced disease state (
It is important to note that the majority of clinical treatments to date for PD are for the alleviation of motor symptoms after clinical manifestation. In clinical neurology, detecting autonomic dysfunction in PD, which notably can occur years to decades prior to characteristic motor symptoms, may be fundamental for early detection of neurodegeneration. With potential disease modifying therapies on the horizon, it remains of vital importance for early stage detection given the best clinical outcomes would be expected with introduction at the earliest possible stages for slowing or changing the disease course. Although it is unlikely this could be resolved with a single screening tool, the regular assessments of cardiovascular autonomic dysfunction in PD could be advantageous for rapid and efficient screening, where recurrent use can also support the monitoring of disease progression.
4. Transcutaneous vagus nerve stimulation (tVNS) for neurodegenerative diseases
With complex multi-stage development in neurodegenerative diseases, it is conceivable that a combinative approach could best achieve effective symptomatic management and improve clinical prognosis. For PD, current treatment therapies remain largely unchanged with focus on motor symptom treatment using dopamine replacement drug therapies. Although these are highly effective for motor symptom relief during the initial years of treatment, the majority of PD patients inevitably experience drug-induced complications, such as motor fluctuations (
). This includes i. treatment with neurotrophic factors like glial cell line-derived neurotrophic factor, ii. deliverance of immunotherapeutic interventions such as monoclonal antibodies against pathogenic α-synuclein (
), iii. an intestinal gel pad for stable and continued release of L-DOPA, and iv. rehabilitation methods for drug-refractory complications, such as core posture control (
). The inclusion of therapies for reducing autonomic dysfunction could also provide a range of additional benefits, such as improved abnormal gut absorption rates for reaching greater effectiveness of drug treatments. A reduction of autonomic dysfunctions throughout the course of the disease, especially for treating sympatho-vagal dysfunction, is likely to substantially add to an overall clinical improvement. Given the involvement of the vagus nerve in the pathogenesis of PD, the effects of vagus nerve stimulation (VNS) on central nervous system neural circuitry, like the noradrenergic system, could potentially provide direct clinical benefits, as well as potential indirect benefits, for example in promoting neuroprotective mechanisms (
Impaired phasic discharge of locus coeruleus neurons based on persistent high tonic discharge-a new hypothesis with potential implications for neurodegenerative diseases.
A 5-year observational study of patients with treatment-resistant depression treated with vagus nerve stimulation or treatment as usual: comparison of response, remission, and suicidality.
). VNS can be achieved through non-invasive or invasive applications, with the latter requiring neurosurgery under general anesthesia. In this procedure, a spiral electrode is secured to the left cervical vagus nerve in the neck that is then connected via a subcutaneous cable to a pulse generator implanted into the chest cavity (
). Alternatively, non-invasive VNS can be achieved through stimulation of the auricular branch of the vagus nerve (ABVN) which is applied through the skin, and is more commonly known as transcutaneous vagus nerve stimulation (tVNS) (
). fMRI imaging studies have shown that tVNS activates key nuclei such as the NTS of the vagal nuclei complex, bilateral spinal trigeminal nucleus, dorsal raphe, locus coeruleus, parabrachialis nucleus, amygdala and nucleus accumbens (Fig. 2) (
Fig. 2A schematic diagram showing the effects of Vagus Nerve Stimulation (VNS). VNS activates ascending neural pathways leading to modulation of neural activity in the brainstem, midbrain and cortex, as shown from previous brain imaging studies. Activated regions include the dorsal motor nucleus of the vagus nerve, nucleus tractus solitarii, thalamus, hypothalamus, basal ganglia and forebrain regions.
). With a favorable risk-benefit profile from its non-surgical application and ease-of-use, clinical effectiveness is typically seen over several weeks to months, with high-level patience compliance being a major factor for producing a clinical benefit (
4.2 Treating neuropsychiatric comorbidities in neurodegenerative diseases
With an overall clinical view, the total burden of non-motor symptoms has been previously suggested to have greater impact on QoL than motor symptoms in early and advanced stages of PD (
). A previous study characterizing non-motor symptoms in over 100 drug-naïve PD patients (PRIAMO study), found that anxiety and depression occurred at the highest prevalence (66%) (
It is well documented that invasive VNS improves drug-resistant depression and has been approved in the USA for clinical use since 2005. A recent open-label 5-year observational study of 795 patients with major depression demonstrated that adjunct VNS provides significantly improved clinical outcomes in comparison to ‘treatment-as-usual’ patients, with a higher cumulative response rate (67.7% cf. 40.9%) and significantly higher remission rates (43.3% cf. 25.7%) (
A 5-year observational study of patients with treatment-resistant depression treated with vagus nerve stimulation or treatment as usual: comparison of response, remission, and suicidality.
). The application of tVNS in recent studies also shows efficacy in depression with improved symptoms of anxiety, psychomotor disability, sleep disturbance and hopelessness (
). Interestingly, short term use of tVNS over a 2-week period in MDD patients, demonstrates sustained clinical improvements in depression with >50% reduction in clinical ratings of Hamilton 17-item (HAMD) scores (
The specific changes in neural circuitry for achieving anti-depressant effects from clinical neuromodulation therapies remains to be fully elucidated (
). With VNS therapy, the anti-depressant effects in patients are likely attributed from afferent vagal fibres projecting to the NTS, resulting in modulated activity of key brain structures including the amygdala and insula, and other key limbic structures in mood regulation (Fig. 3) (
). Early studies using BOLD-fMRI by Kraus et al., (2007) showed that tVNS reduces activity in limbic and temporal brain structures, including the amygdala, hippocampus, parahippocampal gyrus, and middle and superior temporal gyrus, while elevating the activity in other brain regions including the insula, precentral gyrus and thalamus (
). These changes may be the main mechanisms to the mood enhancing effect caused by tVNS in depressed patients. A recent fMRI brain imaging study also demonstrated that the responsiveness of the left anterior insula following the first tVNS treatment may be a potential biomarker for predicting clinical improvements. Indeed, major depression patients that exhibited this marker reached a 42% decrease in HAMD scores following continued tVNS therapy (
). The mood enhancing effects of tVNS treatment has also been demonstrated in different age groups, with beneficial effects recently indicated in adolescents (
). In the latter age group, the authors proposed the responsiveness in subjects to tVNS therapy may be related to individuals with a higher baseline of LF/HF ratio, indicating a shift from sympathetic to parasympathetic prevalence (
Fig. 3The potential use of transcutaneous Vagus Nerve Stimulation (tVNS) for troublesome dysautonomia in neurodegenerative diseases such as Parkinson's Disease. tVNS activates the Auricular Branch of the Vagus Nerve (ABVN) in the ear, which may have potential therapeutic effects including neuropsychiatric symptoms, neuroprotection, orthostasis and orthostatic hypotension, gait dysfunction and bowel discomfort.
). So far, the application of non-invasive VNS (nVNS) has been conducted in a small pilot trial of PD patients for treatment of GI dysfunction. nVNS is utilized by application of a hand-held device placed at the left neck region (below the mandibular angle, medial to the sternocleidomastoid muscle and lateral to the larynx) for delivering a series of short electrical pulses through the skin. In a randomized pilot trial in PD patients (n = 19) receiving nVNS four times per day over a four week period, neurostimulation elicited modest improvements in GI symptoms (
). Further trials are now required with a larger number of patients that would help elucidate the potential benefit of VNS for GI disturbances in PD (Fig. 3).
4.4 Specific motor symptoms in PD
A previous clinical case report showed that invasive VNS can provide significant alleviation of PD motor symptoms (
). In a 64-year old female PD patient who presented colocalized symptoms of complex partial epilepsy and parkinsonism, the titration of VNS resolved slow resting tremor and improved bradykinesia, with a significant reduction in Unified Parkinson's Disease Rating Scale (UPDRS) scores. This also coincided with effective alleviation of epileptic seizures. Authors of this report also noted that PD symptoms were resolved before epilepsy, with improvements sustained even at 6 months follow-up (
In the approach to clinical treatment, certain parkinsonian motor features are unresponsive to the main treatment applications of dopamine replacement or deep brain stimulation, such as gain dysfunction (
), occurring from specific pathological degeneration. For example, gait disturbance has been attributed to cholinergic neuronal loss in the forebrain (nucleus of Meyert) and the Pedunculopontine Nucleus (PPN) of the brainstem, and remains difficult to clinically treat and manage (
). Following 2D spatiotemporal gait parameter analysis, and measurements of stride during walking, there were benefits seen in a small group of PD patients tested (n = 19). More specifically, acute nVNS (2 × 120 s application with a 15 min interval between treatments) improved the number steps taken while turning and reduced UPDRS III scores in FOG PD patients (
). In a larger trial, acute nVNS (1 × 120 s) in PD patients (n = 31) improved gait and also significantly reduced step length variability after a single treatment (
in PD patients (n = 33), it was found that repeated nVNS improved gait and promoted neuroplasticity. Following the application of daily nVNS (6 × 120 s) over one month, PD patients demonstrated improved motor function and gait, from measurements in walking speeds, stance, step-length, along with MDS-UPDRS III scores (
). Interestingly, the analysis of serum biomarkers after nVNS showed reduction in neuroinflammation (TNF-α) along with increased brain-derived neurotrophic factor (BDNF) in PD (
). Together, data from these preliminary studies support the utilization of VNS applications for treatment of gait and eliciting potential disease modifying effects (Fig. 3).
4.5 Rationale for VNS in neurodegenerative disorders for achieving potential disease modifying effects
Modulation of noradrenergic neurotransmission in the locus coeruleus from VNS is one of the key neural pathways modulated for eliciting therapeutic benefit in epilepsy (
). As the locus coeruleus is one of the first brain regions affected in the pathology of major age-related neurodegenerative disease including AD and PD, significant loss of locus coeruleus neurons is a prominent pathological hallmark. For instance in AD, extensive neuronal loss is identified specifically in the rostral and dorsal areas of the locus coeruleus, with pathology also in the locus coeruleus cortical projections (
Locus coeruleus volume and cell population changes during Alzheimer's disease progression: a stereological study in human postmortem brains with potential implication for early-stage biomarker discovery.
). In post-mortem studies of PD, LB pathology is identified in the locus coeruleus in the premotor stage (Braak II) prior to characteristic LB pathology in the SNc (Braak III) (
). While disease pathology of the locus coeruleus in AD and PD are typically seen prior to their characteristic pathological hallmarks, it is suggested that these neurons have higher vulnerability to neurodegeneration in age-related diseases. A contributing factor to the susceptibility of neurodegeneration of these neurons could be from excessive energy demands with high mitochondrial activity for endogenous pace-making functionality (
Impaired phasic discharge of locus coeruleus neurons based on persistent high tonic discharge-a new hypothesis with potential implications for neurodegenerative diseases.
). In addition, the phenotypic structure of noradrenergic neurons may also be a critical factor i.e. being long, extensively branched and thinly myelinated or unmyelinated axons (
), which may favor the early neurodegeneration in α-synucleinopathies. Interestingly, a recent study by Butkovich et al. (2020) identified the degeneration of the locus coeruleus in PD mice models as an early catalyst for neurodegenerative spread, with loss of central noradrenergic transmission contributing to chronic neuroinflammation in PD progression (
Transgenic mice expressing human a-synuclein in noradrenergic neurons develop locus ceruleus pathology and nonmotor features of Parkinson's disease.
). Dysfunction of locus coeruleus neurophysiology has also been suggested in PD pathophysiology, with impaired phasic discharge and a shift to persistent high tonic activity causing exacerbation of chronic neuroinflammation (
Impaired phasic discharge of locus coeruleus neurons based on persistent high tonic discharge-a new hypothesis with potential implications for neurodegenerative diseases.
). This is may be due to the findings that under normal physiology, locus coeruleus noradrenergic transmission elicits neuroprotective effects, with extra-synaptic release of noradrenaline mediating anti-inflammatory effects on surrounding neurons, glial cells and microvessels (
). Indeed, in healthy cells, the extra-synaptically released noradrenaline can mediate decreased inflammation caused by aberrant proteins like β-amyloid (
As neuromodulation therapy can be used to engage the locus coeruleus for increased noradrenergic release (Fig. 2), the subsequent effect may potentially provide neuronal protection against neurotoxicity and neuroinflammation (
Serotonergic and noradrenergic pathways are required for the anxiolytic-like and antidepressant-like behavioral effects of repeated vagal nerve stimulation in rats.
). It has been found that low frequency invasive VNS can mediate activation of systemic anti-inflammatory pathways, providing therapeutic benefit in patients suffering from rheumatoid arthritis and inflammatory bowel disorder (
). With specific relevance to neurodegenerative disorders, studies using the neurotoxin-induced experimental rat model of PD showed that invasive VNS reduced neuroinflammation and behavioral motor deficits (
). Moreover, the effects of VNS in this particular study extended to dampening pathological hallmarks, reducing the loss of dopamine neurons in the SNc and intrasomal α-synuclein accumulation (
), which importantly indicates the potential disease altering effects of VNS therapy.
It is also plausible that VNS therapy, which stimulates the NTS, could directly excite the locus coeruleus via the monosynaptic excitatory projections (
). The neuromodulation could therefore be used to elicit phasic release of noradrenaline in neurodegenerative diseases, where VNS induces significant increase in the percentage of locus coeruleus neurons to burst firing, as reported previously (
). With subsequent elevations in noradrenergic transmission, this could potentially promote endogenous anti-inflammatory and neuroprotective effects for combatting the neurodegenerative processes (Fig. 3).
4.6 Exploratory avenues or monitoring neurodegenerative diseases
In clinical neurology, a major task remains for establishing early detection in neurodegenerative disorders that can translate to a therapeutic window for implementing future disease-modifying therapies. The utilization of evoked potentials from vagal nuclei may help in early detection of degeneration of brainstem nuclei. So far, several preliminary studies have assessed brainstem activity with vagus somatosensory evoked potentials (VSEP) which are rapid, reproducible and do not require an invasive approach (
). VSEP are attained after stimulation of the ABVN, with measurements of far-field potentials taken from EEG electrodes. Although there has been no standardization characterized so far, pilot studies in AD patients (57–78 years of age) show VSEP have significantly longer latencies compared to healthy age-matched controls (
). The prolonged latencies are likely to originate from neurodegeneration of the vagal nuclei, which can be found present in AD but not in major depression (
). An ongoing observational study (the ‘Vogel’ study) is now observing 3 time points over a decade period in 604 subjects for the early detection of brainstem pathology with VSEP latencies in AD (
Near-infrared spectroscopy (NIRS) and vagus somatosensory evoked potentials (VSEP) in the early diagnosis of Alzheimer's disease: rationale, design, methods, and first baseline data of the vogel study.
), which could eventually help support early detection procedures. So far, studies utilizing VSEP in PD patients have reported contrasting data. It has been demonstrated that longer VSEP latencies can be found in PD indicating the potential use for identifying premotor manifestations (
), requiring further clinical investigations to elucidate the use of VSEP as a marker in PD.
Building up a clinical picture of early disease manifestation in neurodegenerative diseases, such as α-synucleinopathies, may be optimized with measurements of dysautonomia (Fig. 4). In combination with clinical signs such as sleep disorders like RBD, the reduction of HRV parameters could be used to detect cardiac autonomic denervation, with recurrent assessments for monitoring disease progression. It is known that people with RBD are at high risk of developing cognitive features of LB disorders, with 50% of individuals eventually developing parkinsonism (
). Moreover, people with RBD show reduction of HRV parameters (LF, HF power, and pNN50), which suggests the sympathetic and parasympathetic influence on cardiac function are reduced (
). It has been revealed that RBD sufferers have severe pathology in the autonomic nervous system and locus coeruleus i.e. noradrenergic denervation, without any detectable impairments in dopaminergic storage capacity, prior to any development of a severe parkinsonian syndrome (
Fig. 4The progressive spread of Lewy Body pathology in the brain from pre-clinical to clinical stages of Parkinson's disease (PD). The severity of midbrain dopaminergic neuron loss (depicted with a darkening colour) corresponds to the disease time-course with prominence in the clinical phase. Current symptomatic treatments are focused in the clinical phase, which includes dopaminergic drug replacement therapies (i.e. L-DOPA, dopamine D2 receptor agonists) and invasive neuromodulation (Deep Brain Stimulation, DBS), often followed by advanced PD motor symptom management. The proposal for tiered screening tools aims for early detection of PD from evaluations of 1. Related dysautonomia (i.e. anosmia, sleep abnormalities, anxiety, constipation, orthostasis, orthostatic hypotension) and vagus nerve neurophysiology (i.e. VSEP, HRV), that will be continually monitored for disease progression in clinical practice; 2. Autonomic nervous system imaging (123I-MIBG scintigraphy, CT assessment of cholinergic gut innervation integrity); and 3. Detailed imaging analyses and biomarker detection (11C-MeNER and 18F-DOPA PET imaging, CSF sampling). The integration of future treatments in pre-clinical stages aim to optimize treatment effects of slowing disease progression with disease modifying and neuroprotective therapies, as well as alleviation of PD dysautonomia at the pre-clinical stage (i.e. glial derived neurotrophic factors, immunotherapies and non-invasive VNS for treating sympatho-vagal imbalance).
A combined approach of detailed imaging for detection of early neurodegeneration would help support the implementation of early treatment and disease management (Fig. 4). The identification of behavioral deficits and the presence of dysautonomia, as well as the screening of the autonomic nervous system function and vagus nerve neurophysiology, can be supported with detailed imaging analysis from MRI and PET imaging, as well as cerebrospinal fluid sampling. The initial screening tools of the autonomic nervous system and the monitoring of physiological changes could be incorporated in routine examinations in aging populations, having the advantage of being both rapid and efficient. As such, structuring a multi-tiered approach may be most practical for examinations, with a first-tier screening criteria for dysautonomia followed by second-tier detailed imaging, such as 123I-MIBG cardiac scintigraphy for assessment of cardiac sympathetic innervation, and CT assessment of cholinergic gut innervation integrity (in the enteric and parasympathetic synapses) (
); to a third-tier for brain scans of 11C-methylreboxetine (MeNER) PET for assessing noradrenergic nerve terminals, and also 18F-Dihydroxyphenylalanine (DOPA) PET to assess nigrostriatal dopamine storage integrity (
). Although such a process remains to be established, combining these methods would ultimately increase the accuracy of early disease detection and provide an opportunity for implementing disease-modifying strategies for achieving better patient care and management (Fig. 4).
5. Conclusions
With a deeper understanding of neurodegenerative disease pathogenesis, new opportunities are available for establishing effective therapeutic and rehabilitation approaches to these disorders, for the complex range of disease symptoms that occur over a typically long progressive time course. From evidence of a severely affected autonomic nervous system in major neurodegenerative diseases, and considerations of pathological disease spread across the nervous system, we propose that vagus nerve neuromodulation for rebalancing sympatho-vagal abnormalities and brainstem neuro-signaling at an early stage, could provide improved clinical outcomes for disease sufferers, especially for troublesome dysautonomia. In addition, screening processes that are rapid and efficient for measuring the autonomic nervous system and brainstem integrity could be further employed to determine early neurodegeneration, and remain vital for enhancing the successes of any potential future disease-modifying agent or application.
Declaration of competing interest
D.K. has equity stake in Neuropix Company Ltd.
Acknowledgement
With special thanks to Professor Armin Bolz for his continued support.
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