The Effects of Transcutaneous Vagus Nerve Stimulation on Functional Connectivity Within Semantic and Hippocampal Networks in Mild Cognitive Impairment

Influence of a 2-week transcutaneous auricular vagus nerve stimulation on memory: findings from a randomized placebo controlled trial in non-clinical adults

PURPOSE

Memory plays an essential role in daily life and is one of the first functions to deteriorate in cognitive impairment and dementia. Transcutaneous vagus nerve stimulation (tVNS) is a promising therapeutic method; however, its ability to enhance memory is underexplored, especially considering long-term stimulation. We aimed to investigate the effect of a 2-week course of auricular tVNS (taVNS) on memory in a non-clinical population.

METHODS

This single-blind randomized placebo-wait-list controlled trial recruited 76 participants (30 men; mean age 48.32 years) and randomized them into four groups: early active/sham taVNS and late active/sham taVNS. Participation in the study lasted 4 weeks; early groups underwent 2 weeks intervention immediately following the first study site visit (days 0-13) and late groups 2 weeks after the first study site visit (days 14-27). Active and sham taVNS included 2 weeks of daily 4-h neurostimulation at the tragus or earlobe, respectively. To assess memory, we used the Rey Auditory Verbal Learning Test.

RESULTS

Two weeks of active taVNS, but not sham taVNS, improved immediate recall and short-term memory score both in early and late groups. Furthermore, the improvements persisted over subsequent follow-up in early active taVNS. Importantly, the effect of active taVNS was superior to sham for immediate recall in both early and late groups. There were no statistical differences in delayed recall.

CONCLUSION

Our findings suggest that taVNS has potential to improve memory, particularly immediate recall, and may be an effective method in preventing memory loss and mitigating cognitive aging.

Transcutaneous vagus nerve stimulation: a new strategy for Alzheimer's disease intervention through the brain-gut-microbiota axis?

Transcutaneous vagus nerve stimulation (tVNS) is an emerging non-invasive technique designed to stimulate branches of the vagus nerve distributed over the body surface. Studies suggest a correlation between the brain-gut-microbiota (BGM) axis and the pathogenesis of Alzheimer's disease (AD). The BGM axis represents a complex bidirectional communication system, with the vagus nerve being a crucial component. Therefore, non-invasive electrical stimulation of the vagus nerve might have the potential to modify-most of the time probably in a non-physiological way-the signal transmission within the BGM axis, potentially influencing the progression or symptoms of AD. This review explores the interaction between percutaneous vagus nerve stimulation and the BGM axis, emphasizing its potential effects on AD. It examines various aspects, such as specific brain regions, gut microbiota composition, maintenance of intestinal environmental homeostasis, inflammatory responses, brain plasticity, and hypothalamic-pituitary-adrenal (HPA) axis regulation. The review suggests that tVNS could serve as an effective strategy to modulate the BGM axis and potentially intervene in the progression or treatment of Alzheimer's disease in the future.

Preliminary evidence of transcutaneous vagus nerve stimulation effects on sleep in veterans with post-traumatic stress disorder

Sleep problems are common among veterans with post-traumatic stress disorder and closely associated with hyperarousal symptoms. Transcutaneous vagus nerve stimulation (tVNS) may have potential to improve sleep quality in veterans with PTSD through effects on brain systems relevant to hyperarousal and sleep-wake regulation. The current pilot study examines the effect of 1 h of tVNS administered at "lights out" on sleep architecture, microstructure, and autonomic activity. Thirteen veterans with PTSD completed two nights of laboratory-based polysomnography during which they received 1 h of either active tVNS (tragus) or sham stimulation (earlobe) at "lights out" with randomised order. Sleep staging and stability metrics were derived from polysomnography data. Autonomic activity during sleep was assessed using the Porges-Bohrer method for calculating respiratory sinus arrhythmia (RSAP-B ). Paired t-tests revealed a small decrease in the total sleep time (d = -0.31), increase in N3 sleep (d = 0.23), and a small-to-moderate decrease in REM sleep (d = -0.48) on nights of active tVNS relative to sham stimulation. tVNS was also associated with a moderate reduction in cyclic alternating pattern (CAP) rate (d = -0.65) and small-to-moderate increase in RSAP-B during NREM sleep. Greater NREM RSAP-B was associated with a reduced CAP rate and NREM alpha power. This pilot study provides preliminary evidence that tVNS may improve sleep depth and stability in veterans with PTSD, as well as increase parasympathetically mediated nocturnal autonomic activity. These results warrant continued investigation into tVNS as a potential tool for treating sleep disturbance in veterans with PTSD.

Autonomic nervous system dysfunction throughout menopausal transition: A potential mechanism underpinning cardiovascular and cognitive alterations during female ageing

Cardiovascular diseases (CVD) and neurodegenerative disorders, such as Alzheimer's disease (AD), are highly prevalent conditions in middle-aged women that severely impair quality of life. Recent evidence suggests the existence of an intimate cross-talk between the heart and the brain, resulting from a complex network of neurohumoral circuits. From a pathophysiological perspective, the higher prevalence of AD in women may be explained, at least in part, by sex-related differences in the incidence/prevalence of CVD. Notably, the autonomic nervous system, the main heart-brain axis physiological orchestrator, has been suggested to play a role in the incidence of adverse cardiovascular events in middle-aged women because of decreases in oestrogen-related signalling during transition into menopause. Despite its overt relevance for public health, this hypothesis has not been thoroughly tested. Accordingly, in this review, we aim to provide up to date evidence supporting how changes in circulating oestrogen levels during transition to menopause may trigger autonomic dysfunction, thus promoting cardiovascular and cognitive decline in women. A main focus on the effects of oestrogen-mediated signalling at CNS structures related to autonomic regulation is provided, particularly on the role of oestrogens in sympathoexcitation. Improving the understanding of the contribution of the autonomic nervous system on the development, maintenance and/or progression of both cardiovascular and cognitive dysfunction during the transition to menopause should help improve the clinical management of elderly women, with the outcome being an improved life quality during the natural ageing process.

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.

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.

Impact of transcutaneous vagus nerve stimulation on healthy cognitive and brain aging

Evidence for clinically meaningful benefits of transcutaneous vagus nerve stimulation (VNS) has been rapidly accumulating over the past 15  years. This relatively novel non-invasive brain stimulation technique has been applied to a wide range of neuropsychiatric disorders including schizophrenia, obsessive compulsive disorder, panic disorder, post-traumatic stress disorder, bipolar disorder, and Alzheimer's disease. More recently, non-invasive forms of VNS have allowed for investigations within healthy aging populations. These results offer insight into protocol considerations specific to older adults and how to translate those results into effective clinical trials and, ultimately, effective clinical care. In this review, we characterize the possible mechanisms by which non-invasive VNS may promote healthy aging (e.g., neurotransmitter effects, inflammation regulation, functional connectivity changes), special considerations for applying non-invasive VNS in an older adult population (e.g., vagus nerve changes with age), and how non-invasive VNS may be used in conjunction with existing behavioral interventions (e.g., cognitive behavioral therapy, cognitive training) to promote healthy emotional and cognitive aging.