Effects of vagus nerve stimulation on extinction of conditioned fear and post-traumatic stress disorder symptoms in rats

Ascending Vagal Sensory and Central Noradrenergic Pathways Modulate Retrieval of Passive Avoidance Memory in Male Rats

Visceral feedback from the body is often subconscious, but plays an important role in guiding motivated behaviors. Vagal sensory neurons relay "gut feelings" to noradrenergic (NA) neurons in the caudal nucleus of the solitary tract (cNTS), which in turn project to the anterior ventrolateral bed nucleus of the stria terminalis (vlBNST) and other hypothalamic-limbic forebrain regions. Prior work supports a role for these circuits in modulating memory consolidation and extinction, but a potential role in retrieval of conditioned avoidance remains untested. To examine this, adult male rats underwent passive avoidance conditioning. We then lesioned gut-sensing vagal afferents by injecting cholecystokinin-conjugated saporin toxin (CSAP) into the vagal nodose ganglia (Experiment 1), or lesioned NA inputs to the vlBNST by injecting saporin toxin conjugated to an antibody against dopamine-beta hydroxylase (DSAP) into the vlBNST (Experiment 2). When avoidance behavior was later assessed, rats with vagal CSAP lesions or NA DSAP lesions displayed significantly increased conditioned passive avoidance. These new findings support the view that gut vagal afferents and the cNTSNA-to-vlBNST circuit play a role in modulating the expression/retrieval of learned passive avoidance. Overall, our data suggest a dynamic modulatory role of vagal sensory feedback to the limbic forebrain in integrating interoceptive signals with contextual cues that elicit conditioned avoidance behavior.

Transcutaneous auricular vagus nerve stimulation to acutely reduce emotional vulnerability and improve emotional regulation in borderline personality disorder (tVNS-BPD): study protocol for a randomized, single-blind, sham-controlled trial

BACKGROUND

Borderline personality disorder (BPD) is considered a disorder of emotion regulation resulting from the expression of a biologically determined emotional vulnerability (that is, heightened sensitivity to emotion, increased emotional intensity/reactivity, and a slow return to emotional baseline) combined with exposure to invalidating environments. Vagal tone has been associated with activity in cortical regions involved in emotion regulation and a lower resting state of vagal tone has been observed in BPD patients relative to healthy controls. Non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) has been shown to reduce temper outbursts in adults with Prader-Willi Syndrome, to enhance recognition of emotions in healthy students, and to improve depressive and anxiety symptoms. Furthermore, a single session of taVNS has been shown to acutely alter the recognition of facial expressions of negative valence in adolescents with MDD and increase emotion recognition in controls. However, the effect of taVNS on emotional vulnerability and regulation in individuals diagnosed with BPD has not been investigated. Our aims are to determine if taVNS is effective in acutely reducing emotional vulnerability and improve emotional regulation in BPD patients.

METHODS

Forty-two patients will be randomized to a single session of taVNS or sham-taVNS while going through an affect induction procedure. It will consist of the presentation of one neutral and three negative affect-evoking 4-min-long videos in sequence, each of which is followed by a 4-min post-induction period during which participants will rate the quality and intensity of their current self-reported emotions (post-induction ratings) and the perceived effectiveness in managing their emotions during the video presentation. The rating of the current self-reported emotions will be repeated after every post-induction period (recovery ratings). Mixed models with individuals as random effect will be used to investigate the ratings at each stage of the study, taking into account the repeated measures of the same individuals at baseline, pre-induction, post-induction, and recovery.

DISCUSSION

The study has potential to yield new insights into the role of vagal tone in emotion dysregulation in BPD and offer preliminary data on the effectiveness of taVNS as a possible non-invasive brain stimulation to treat a core symptom of BPD.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05892900. Retrospectively registered on Jun 07, 2023.

REM disruption and REM Vagal Activity Predict Extinction Recall in Trauma-Exposed Individuals

Accumulating evidence suggests that rapid eye movement sleep (REM) supports the consolidation of extinction memory. REM is disrupted in PTSD, and REM abnormalities after traumatic events increase the risk of developing PTSD. Therefore, it was hypothesized that abnormal REM in trauma-exposed individuals may pave the way for PTSD by interfering with the processing of extinction memory. In addition, PTSD patients display reduced vagal activity. Vagal activity contributes to the strengthening of memories, including fear extinction memory, and recent studies show that the role of vagus in memory processing extends to memory consolidation during sleep. Therefore, it is plausible that reduced vagal activity during sleep in trauma-exposed individuals may be an additional mechanism that impairs extinction memory consolidation. However, to date, the contribution of sleep vagal activity to the consolidation of extinction memory or any emotional memory has not been investigated. To test these hypotheses, we examined the association of extinction memory with REM characteristics and REM vagal activity (indexed as heart rate variability) in a large sample of trauma-exposed individuals (n=113). Consistent with our hypotheses, REM disruption was associated with poorer physiological and explicit extinction memory. Furthermore, higher vagal activity during REM was associated with better explicit extinction memory, and physiological extinction memory in males. These findings support the notion that abnormal REM may contribute to PTSD by impairing the consolidation of extinction memory and indicate the potential utility of interventions that target REM sleep characteristics and REM vagal activity in fear-related disorders.

An ascending vagal sensory-central noradrenergic pathway modulates retrieval of passive avoidance memory

Background

Visceral feedback from the body is often subconscious, but plays an important role in guiding motivated behaviors. Vagal sensory neurons relay "gut feelings" to noradrenergic (NA) neurons in the caudal nucleus of the solitary tract (cNTS), which in turn project to the anterior ventrolateral bed nucleus of the stria terminalis (vlBNST) and other hypothalamic-limbic forebrain regions. Prior work supports a role for these circuits in modulating memory consolidation and extinction, but a potential role in retrieval of conditioned avoidance remains untested.

Results

To examine this, adult male rats underwent passive avoidance conditioning. We then lesioned gut-sensing vagal afferents by injecting cholecystokinin-conjugated saporin toxin (CSAP) into the vagal nodose ganglia (Experiment 1), or lesioned NA inputs to the vlBNST by injecting saporin toxin conjugated to an antibody against dopamine-beta hydroxylase (DSAP) into the vlBNST (Experiment 2). When avoidance behavior was later assessed, rats with vagal CSAP lesions or NA DSAP lesions displayed significantly increased conditioned passive avoidance.

Conclusions

These new findings support the view that a gut vagal afferent-to-cNTSNA-to-vlBNST circuit plays a role in modulating the expression/retrieval of learned passive avoidance. Overall, our data suggest a dynamic modulatory role of vagal sensory feedback to the limbic forebrain in integrating interoceptive signals with contextual cues that elicit conditioned avoidance behavior.

The gut-brain axis and cognitive control: A role for the vagus nerve

Survival requires the integration of external information and interoceptive cues to effectively guide advantageous behaviors, particularly foraging and other behaviors that promote energy acquisition and consumption. The vagus nerve acts as a critical relay between the abdominal viscera and the brain to convey metabolic signals. This review synthesizes recent findings from rodent models and humans revealing the impact of vagus nerve signaling from the gut on the control of higher-order neurocognitive domains, including anxiety, depression, reward motivation, and learning and memory. We propose a framework where meal consumption engages gastrointestinal tract-originating vagal afferent signaling that functions to alleviate anxiety and depressive-like states, while also promoting motivational and memory functions. These concurrent processes serve to favor the encoding of meal-relevant information into memory storage, thus facilitating future foraging behaviors. Modulation of these neurocognitive domains by vagal tone is also discussed in the context of pathological conditions, including the use of transcutaneous vagus nerve stimulation for the treatment of anxiety disorders, major depressive disorder, and dementia-associated memory impairments. Collectively, these findings highlight the contributions of gastrointestinal vagus nerve signaling to the regulation of neurocognitive processes that shape various adaptive behavioral responses.

Stress-induced vagal activity influences anxiety-relevant prefrontal and amygdala neuronal oscillations in male mice

The vagus nerve crucially affects emotions and psychiatric disorders. However, the detailed neurophysiological dynamics of the vagus nerve in response to emotions and its associated pathological changes remain unclear. In this study, we demonstrated that the spike rates of the cervical vagus nerve change depending on anxiety behavior in an elevated plus maze test, and these changes were eradicated in stress-susceptible male mice. Furthermore, instantaneous spike rates of the vagus nerve were negatively and positively correlated with the power of 2-4 Hz and 20-30 Hz oscillations, respectively, in the prefrontal cortex and amygdala. The oscillations also underwent dynamic changes depending on the behavioral state in the elevated plus maze, and these changes were no longer observed in stress-susceptible and vagotomized mice. Chronic vagus nerve stimulation restored behavior-relevant neuronal oscillations with the recovery of altered behavioral states in stress-susceptible mice. These results suggested that physiological vagal-brain communication underlies anxiety and mood disorders.

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.

Noninvasive Vagal Nerve Stimulation for Opioid Use Disorder

Background

Opioid Use Disorder (OUD) is an escalating public health problem with over 100,000 drug overdose-related deaths last year most of them related to opioid overdose, yet treatment options remain limited. Non-invasive Vagal Nerve Stimulation (nVNS) can be delivered via the ear or the neck and is a non-medication alternative to treatment of opioid withdrawal and OUD with potentially widespread applications.

Methods

This paper reviews the neurobiology of opioid withdrawal and OUD and the emerging literature of nVNS for the application of OUD. Literature databases for Pubmed, Psychinfo, and Medline were queried for these topics for 1982-present.

Results

Opioid withdrawal in the context of OUD is associated with activation of peripheral sympathetic and inflammatory systems as well as alterations in central brain regions including anterior cingulate, basal ganglia, and amygdala. NVNS has the potential to reduce sympathetic and inflammatory activation and counter the effects of opioid withdrawal in initial pilot studies. Preliminary studies show that it is potentially effective at acting through sympathetic pathways to reduce the effects of opioid withdrawal, in addition to reducing pain and distress.

Conclusions

NVNS shows promise as a non-medication approach to OUD, both in terms of its known effect on neurobiology as well as pilot data showing a reduction in withdrawal symptoms as well as physiological manifestations of opioid withdrawal.

Cerebral Organoid Research Ethics and Pinning the Tail on the Donkey

The risk of creating cerebral organoids/assembloids conscious enough to suffer is a recurrent concern in organoid research ethics. On one hand, we should, apparently, avoid discovering how to distinguish between organoids that it would be permissible (non-conscious) and impermissible (conscious) to use in research, since if successful we would create organoids that suffer. On the other, if we do not, the risk persists that research might inadvertently continue to cause organoids to suffer. Moreover, since modeling some brain disorders may require inducing stress in organoids, it is unclear how to eliminate the risk, if we want to develop effective therapies. We are committed to harm avoidance but hamstrung by a presumption that we should avoid research that might tell us clearly when suffering occurs. How can we negotiate this challenge and maximize the therapeutic benefits of cerebral organoid research? The author interrogates the challenge, suggesting a tentative way forward.

Noradrenergic α1, α2, and β1receptors mediate VNS-induced theta oscillations

Although vagal nerve stimulation (VNS) has been employed with success for almost four decades in many central nervous system disturbances, the physiological and pharmacological processes underlying this therapy are still unclear. Searching for central mechanisms of VNS is clinically limited. Hence, in many experiments, VNS technique is tested on the model of laboratory animals. In the present study we proceed with the experiments to verify some central effects of VNS. Specifically, we focussed on the hippocampal formation (HPC) noradrenergic profile which underlines the VNS-induced theta oscillations in anesthetized rats (Broncel et al., 2017; 2021). The effects of noradrenaline (NE) and selective noradrenergic α and β agonists and antagonists were tested in experiments organized in three stages. Initially, a nonspecific noradrenergic agonist, noradrenaline, was administrated. In the second stage, noradrenergic α and β agonists were applied. In the last stage, the administration of selected agonists was pretreated by specific antagonists. The results of the present study provide evidence that the selective activation of HPC α1, α2, and β1 noradrenergic receptors produce the inhibition of VNS-induced theta oscillations. Hippocampal β2 and β3 receptors were found not to be involved in the modulation of oscillations produced by the vagal nerve stimulation. The obtained outcomes are discussed in light of the effects of increased exogenous NE and induced release of endogenous NE.

The effectiveness of extinction training in male rats: Temporal considerations and brain mechanisms

The extinction of conditioned fear is frequently used in laboratories as a model for human exposure therapy and is crucial for studies of posttraumatic stress disorder (PTSD). However, the efficacy of specific protocols can vary greatly, and the underlying brain mechanisms are not sufficiently clarified. To address this issue, variable starting time (one or twenty-eight days after fear conditioning) and extinction protocols were used, and the efficacy and durability of fear extinction were also studied. Changes in the behavior, stress hormone levels and neuronal activation patterns of stressed rats were analyzed. Conditioned fear was rapidly and efficiently extinguished by all the protocols investigated. However, when these extinction protocols were initiated one day after fear training, conditioned fear relapsed spontaneously four weeks later. In contrast, when extinction trials were started 28 days after conditioning, no relapse occurred. Hormone measurements taken by the end of extinction trials indicated that adrenocorticotropin, but not corticosterone responses reflected behavioral extinction without any sign of relapse. The last extinction training increased the activation of the medial prefrontal cortex and decreased the activation of the central and medial amygdala when extinction began one day after fear conditioning. By contrast, the activation of the basolateral amygdala and the entire hippocampus decreased by the last training session when extinction started 28 days after fear conditioning. Our findings show that extinction training can extinguish remote fear memories more effectively than recent ones, and that the brain mechanisms underlying remote and recent fear memory extinction differ. Laboratory models should also focus on a later time point to increase their translational value.

Contemporary Approaches Toward Neuromodulation of Fear Extinction and Its Underlying Neural Circuits

Neuroscience and neuroimaging research have now identified brain nodes that are involved in the acquisition, storage, and expression of conditioned fear and its extinction. These brain regions include the ventromedial prefrontal cortex (vmPFC), dorsal anterior cingulate cortex (dACC), amygdala, insular cortex, and hippocampus. Psychiatric neuroimaging research shows that functional dysregulation of these brain regions might contribute to the etiology and symptomatology of various psychopathologies, including anxiety disorders and post traumatic stress disorder (PTSD) (Barad et al. Biol Psychiatry 60:322-328, 2006; Greco and Liberzon Neuropsychopharmacology 41:320-334, 2015; Milad et al. Biol Psychiatry 62:1191-1194, 2007a, Biol Psychiatry 62:446-454, b; Maren and Quirk Nat Rev Neurosci 5:844-852, 2004; Milad and Quirk Annu Rev Psychol 63:129, 2012; Phelps et al. Neuron 43:897-905, 2004; Shin and Liberzon Neuropsychopharmacology 35:169-191, 2009). Combined, these findings indicate that targeting the activation of these nodes and modulating their functional interactions might offer an opportunity to further our understanding of how fear and threat responses are formed and regulated in the human brain, which could lead to enhancing the efficacy of current treatments or creating novel treatments for PTSD and other psychiatric disorders (Marin et al. Depress Anxiety 31:269-278, 2014; Milad et al. Behav Res Ther 62:17-23, 2014). Device-based neuromodulation techniques provide a promising means for directly changing or regulating activity in the fear extinction network by targeting functionally connected brain regions via stimulation patterns (Raij et al. Biol Psychiatry 84:129-137, 2018; Marković et al. Front Hum Neurosci 15:138, 2021). In the past ten years, notable advancements in the precision, safety, comfort, accessibility, and control of administration have been made to the established device-based neuromodulation techniques to improve their efficacy. In this chapter we discuss ten years of progress surrounding device-based neuromodulation techniques-Electroconvulsive Therapy (ECT), Transcranial Magnetic Stimulation (TMS), Magnetic Seizure Therapy (MST), Transcranial Focused Ultrasound (TUS), Deep Brain Stimulation (DBS), Vagus Nerve Stimulation (VNS), and Transcranial Electrical Stimulation (tES)-as research and clinical tools for enhancing fear extinction and treating PTSD symptoms. Additionally, we consider the emerging research, current limitations, and possible future directions for these techniques.

Timing of vagus nerve stimulation during fear extinction determines efficacy in a rat model of PTSD

Studies have indicated that vagus nerve stimulation (VNS) enhances extinction learning in rodent models. Here, we investigated if pairing VNS with the conditioned stimulus is required for the enhancing effects of VNS. Adult Sprague-Dawley rats were exposed to intense stress followed by fear conditioning training to produce resistant fear. Rats were then implanted with a cuff electrode around the left vagus. After recovery, rats underwent extinction training paired with VNS (0.5 s, 0.8 mA, 100 µs, and 30 Hz) or with Sham VNS (0 mA). VNS rats were randomized into the following subgroups: During VNS (delivered during presentations of the conditioned stimulus, CS), Between VNS (delivered between CS presentations), Continuous VNS (delivered during the entire extinction session), and Dispersed VNS (delivered at longer inter-stimulation intervals across the extinction session). Sham VNS rats failed to extinguish the conditioned fear response over 5 days of repeated exposure to the CS. Rats that received Between or Dispersed VNS showed modest improvement in conditioned fear at the retention test. During and Continuous VNS groups displayed the greatest reduction in conditioned fear. These findings indicate that delivering VNS paired precisely with CS presentations or continuously throughout extinction promotes the maximum enhancement in extinction learning.

Involvement of the brain-heart axis in the link between PTSD and cardiovascular disease

Posttraumatic stress disorder (PTSD) has long been associated with a heightened risk of cardiovascular disease (CVD). A number of mechanisms have been implicated to underlie this brain-heart axis relationship, such as altered functioning of the autonomic nervous system and increased systemic inflammation. While neural alterations have repeatedly been observed in PTSD, they are rarely considered in the PTSD-CVD link. The brain-heart axis is a pathway connecting frontal and limbic brain regions to the brainstem and periphery via the autonomic nervous system and it may be a promising model for understanding CVD risk in PTSD given its overlap with PTSD neural deficits. We first provide a summary of the primary mechanisms implicated in the association between PTSD and CVD. We then review the brain-heart axis and its relevance to PTSD, as well as findings from PTSD trials demonstrating that a number of PTSD treatments have effects on areas of the brain-heart axis. Finally, we discuss sex considerations in the PTSD-CVD link. A critical next step in this study is to determine if PTSD treatments that affect the brain-heart axis (e.g., brain stimulation that improves autonomic function) also reduce the risk of CVD.

Role of noradrenergic arousal for fear extinction processes in rodents and humans

Fear extinction is a learning mechanism that is pivotal for the inhibition of fear responses towards cues or contexts that no longer predict the occurrence of a threat. Failure of fear extinction leads to fear expression under safe conditions and is regarded to be a cardinal characteristic of many anxiety-related disorders and posttraumatic stress disorder. Importantly, the neurotransmitter noradrenaline was shown to be a potent modulator of fear extinction. Rodent studies demonstrated that excessive noradrenaline transmission after acute stress opens a time window of vulnerability, in which fear extinction learning results in attenuated long-term extinction success. In contrast, when excessive noradrenergic transmission subsides, well-coordinated noradrenaline transmission is necessary for the formation of a long-lasting extinction memory. In addition, emerging evidence suggests that the neuropeptide corticotropin releasing hormone (CRF), which strongly regulates noradrenaline transmission under conditions of acute stress, also impedes long-term extinction success. Recent rodent work - using sophisticated methods - provides evidence for a hypothetical mechanistic framework of how noradrenaline and CRF dynamically orchestrate the neural fear and extinction circuitry to attenuate or to improve fear extinction and extinction recall. Accordingly, we review the evidence from rodent studies linking noradrenaline and CRF to fear extinction learning and recall and derive the hypothetical mechanistic framework of how different levels of noradrenaline and CRF may create a time window of vulnerability which impedes successful long-term fear extinction. We also address evidence from human studies linking noradrenaline and fear extinction success. Moreover, we accumulate emerging approaches to non-invasively measure and manipulate the noradrenergic system in healthy humans. Finally, we emphasize the importance of future studies to account for sex (hormone) differences when examining the interaction between fear extinction, noradrenaline, and CRF. To conclude, NA's effects on fear extinction recall strongly depend on the arousal levels at the onset of fear extinction learning. Our review aimed at compiling the available (mainly rodent) data in a neurobiological framework, suited to derive testable hypotheses for future work in humans.

How Daily Obstacles Affect Frontline Healthcare Professionals' Mental Health during Omicron: A Daily Diary Study of Handwashing Behavior

Based on coping theory, the current research examines how and why COVID-19 phobia affects frontline healthcare professionals' mental health, as well as their burnout and work-related stress. We focused on the mediating role of burnout and work-related stress in this study. In the current study, we also examined the moderating influence of healthcare professionals' handwashing behavior using the Hayes Process model. We employed a daily diary approach to collect data from respondents in Pakistan's frontline healthcare professionals (n = 79, 79 × 10 = 790) who were directly treating COVID-19 patients during the omicron wave. According to the findings of the study, COVID-19 phobia significantly disturbs healthcare professionals' mental health, as well as significantly strengthens burnout and work-related stress. The findings also demonstrated that burnout significantly negatively influences mental health. The mediation influence of burnout and work-related stress in the association between COVID-19 phobia and mental health has shown to be significant. The moderation analysis revealed that high handwashing behavior significantly buffers the negative impact of COVID-19 phobia, as well as the adverse effect of burnout on healthcare professionals' mental health. Moreover, our findings have theoretical and managerial implications, as well as new research directions for scholars to understand the adverse impact of daily obstacles on professionals' (nurses and doctors, etc.) mental health and work performance, as well as issues based on resource conversation philosophy.

Respiratory Sinus Arrhythmia Change during Trauma-Focused Cognitive-Behavioral Therapy: Results from a Randomized Controlled Feasibility Trial

Trauma-Focused Cognitive-Behavioral Therapy (TF-CBT) is a well-established treatment for pediatric posttraumatic stress disorder (PTSD). Animal-assisted therapy (AAT) has been proposed as an adjunct to TF-CBT that may improve treatment effects through enhanced targeting of affect regulation, as indexed by specific changes in the respiratory sinus arrhythmia (RSA). The current study reports results from a randomized controlled feasibility trial (N = 33; M_age = 11.79 [SD = 3.08]; 64% White; 67% female) that measured RSA during Sessions 1, 4, 8, and 12 of a twelve-session TF-CBT protocol and tested whether: 1) TF-CBT + AAT achieved higher average RSA amplitudes relative to TF-CBT alone, and 2) RSA regulation, defined as less variability around person-specific RSA slopes during treatment, explained variation in post-treatment PTSD symptoms. Multilevel modeling failed to support an effect for TF-CBT + AAT on RSA amplitudes (δ₀₀₁ = 0.08, p = 0.844). However, regardless of treatment condition, greater RSA withdrawal was observed within Sessions 4 (γ₁₁ = -.01, p < .001) and 12 (γ₁₃ = -.01, p = .015) relative to the Session 1 baseline. The average level of RSA amplitude in Session 8 was also significantly lower compared to Session 1 (γ₀₂ = -0.70, p = .046). Intraindividual regression models demonstrated that greater RSA regulation predicted improved PTSD symptoms at post-treatment after adjusting for pre-treatment levels (b₃ = 20.00, p = .012). These preliminary results offer support for future confirmatory trials testing whether affect regulation, as indexed by changes in RSA, is a mechanism of action for TF-CBT in the treatment of pediatric PTSD.

Neuroinflammation in Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a well-known mental illness, which is caused by various stressors, including memories of past physical assaults and psychological pressure. It is diagnosed as a mental and behavioral disorder, but increasing evidence is linking it to the immune system and inflammatory response. Studies on the relationship between inflammation and PTSD revealed that patients with PTSD had increased levels of inflammatory cytokine biomarkers, such as interleukin-1, interleukin-6, tumor necrosis factor-α, nuclear factor-κB, and C-reactive protein, compared with healthy controls. In addition, animal model experiments imitating PTSD patients suggested the role of inflammation in the pathogenesis and pathophysiology of PTSD. In this review, we summarize the definition of PTSD and its association with increased inflammation, its mechanisms, and future predictable diseases and treatment possibilities. We also discuss anti-inflammatory treatments to address inflammation in PTSD.

Understanding Mental Health Needs and Gathering Feedback on Transcutaneous Auricular Vagus Nerve Stimulation as a Potential PTSD Treatment among 9/11 Responders Living with PTSD Symptoms 20 Years Later: A Qualitative Approach

Posttraumatic stress disorder (PTSD) remains one of the most prevalent diagnoses of World Trade Center (WTC) 9/11 responders. Transcutaneous auricular vagus nerve stimulation (taVNS) is a potential treatment for PTSD, as it can downregulate activity in the brain, which is known to be related to stress responses and hyperarousal. To understand barriers and facilitators to engagement in mental health care and the feasibility and acceptability of using the taVNS device as a treatment for PTSD symptoms, a focus group was conducted among patients from the Queens WTC Health Program who had elevated symptoms of PTSD. The focus group discussion was recorded, transcribed, and analyzed. Three themes and subthemes emerged: (1) the continued prevalence of mental health difficulties and systematic challenges to accessing care; (2) positive reception toward the taVNS device as a potential treatment option, including a discussion of how to increase usability; and (3) feedback on increasing the feasibility and acceptance of the research methodology associated with testing the device in a pilot clinical trial. The findings highlight the need for additional treatment options to reduce PTSD symptoms in this population and provide key formative phase input for the pilot clinical trial of taVNS.

Common Cholinergic, Noradrenergic, and Serotonergic Drugs Do Not Block VNS-Mediated Plasticity

Vagus nerve stimulation (VNS) delivered during motor rehabilitation enhances recovery from a wide array of neurological injuries and was recently approved by the U.S. FDA for chronic stroke. The benefits of VNS result from precisely timed engagement of neuromodulatory networks during rehabilitative training, which promotes synaptic plasticity in networks activated by rehabilitation. Previous studies demonstrate that lesions that deplete these neuromodulatory networks block VNS-mediated plasticity and accompanying enhancement of recovery. There is a great deal of interest in determining whether commonly prescribed pharmacological interventions that influence these neuromodulatory networks would similarly impair VNS effects. Here, we sought to directly test the effects of three common pharmaceuticals at clinically relevant doses that target neuromodulatory pathways on VNS-mediated plasticity in rats. To do so, rats were trained on a behavioral task in which jaw movement during chewing was paired with VNS and received daily injections of either oxybutynin, a cholinergic antagonist, prazosin, an adrenergic antagonist, duloxetine, a serotonin-norepinephrine reuptake inhibitor, or saline. After the final behavioral session, intracortical microstimulation (ICMS) was used to evaluate reorganization of motor cortex representations, with area of cortex eliciting jaw movement as the primary outcome. In animals that received control saline injections, VNS paired with training significantly increased the movement representation of the jaw compared to naïve animals, consistent with previous studies. Similarly, none of the drugs tested blocked this VNS-dependent reorganization of motor cortex. The present results provide direct evidence that these common pharmaceuticals, when used at clinically relevant doses, are unlikely to adversely impact the efficacy of VNS therapy.

Daily Optogenetic Stimulation of the Left Infralimbic Cortex Reverses Extinction Impairments in Male Rats Exposed to Single Prolonged Stress

Post-traumatic stress disorder (PTSD) is associated with decreased activity in the prefrontal cortex. PTSD-like pathophysiology and behaviors have been observed in rodents exposed to a single prolonged stress (SPS) procedure. When animals are left alone for 7 days after SPS treatment, they show increased anxiety-like behavior and impaired extinction of conditioned fear, and reduced activity in the prefrontal cortex. Here, we tested the hypothesis that daily optogenetic stimulation of the infralimbic region (IL) of the medial prefrontal cortex (mPFC) during the 7 days after SPS would reverse SPS effects on anxiety and fear extinction. Male Sprague-Dawley rats underwent SPS and then received daily optogenetic stimulation (20 Hz, 2 s trains, every 10 s for 15 min/day) of glutamatergic neurons of the left or right IL for seven days. After this incubation period, rats were tested in the elevated plus-maze (EPM). Twenty-four hours after the EPM test, rats underwent auditory fear conditioning (AFC), extinction training and a retention test. SPS increased anxiety-like behavior in the EPM task and produced a profound impairment in extinction of AFC. Optogenetic stimulation of the left IL, but not right, during the 7-day incubation period reversed the extinction impairment. Optogenetic stimulation did not reverse the increased anxiety-like behavior, suggesting that the extinction effects are not due to a treatment-induced reduction in anxiety. Results indicate that increased activity of the left IL after traumatic experiences can prevent development of extinction impairments. These findings suggest that non-invasive brain stimulation may be a useful tool for preventing maladaptive responses to trauma.

Self-Administration of Right Vagus Nerve Stimulation Activates Midbrain Dopaminergic Nuclei

Background: Left cervical vagus nerve stimulation (l-VNS) is an FDA-approved treatment for neurological disorders including epilepsy, major depressive disorder, and stroke, and l-VNS is increasingly under investigation for a range of other neurological indications. Traditional l-VNS is thought to induce therapeutic neuroplasticity in part through the coordinated activation of multiple broadly projecting neuromodulatory systems in the brain. Recently, it has been reported that striking lateralization exists in the anatomical and functional connectivity between the vagus nerves and the dopaminergic midbrain. These emerging findings suggest that VNS-driven activation of this important plasticity-promoting neuromodulatory system may be preferentially driven by targeting the right, rather than the left, cervical nerve.

Objective: To compare the effects of right cervical VNS (r-VNS) vs. traditional l-VNS on self-administration behavior and midbrain dopaminergic activation in rats.

Methods: Rats were implanted with a stimulating cuff electrode targeting either the right or left cervical vagus nerve. After surgical recovery, rats underwent a VNS self-administration assay in which lever pressing was paired with r-VNS or l-VNS delivery. Self-administration was followed by extinction, cue-only reinstatement, and stimulation reinstatement sessions. Rats were sacrificed 90 min after completion of behavioral training, and brains were removed for immunohistochemical analysis of c-Fos expression in the dopaminergic ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), as well as in the noradrenergic locus coeruleus (LC).

Results: Rats in the r-VNS cohort performed significantly more lever presses throughout self-administration and reinstatement sessions than did rats in the l-VNS cohort. Moreover, this appetitive behavioral responding was associated with significantly greater c-Fos expression among neuronal populations within the VTA, SNc, and LC. Differential c-Fos expression following r-VNS vs. l-VNS was particularly prominent within dopaminergic midbrain neurons.

Conclusion: Our results support the existence of strong lateralization within vagal-mesencephalic signaling pathways, and suggest that VNS targeted to the right, rather than left, cervical nerve preferentially activates the midbrain dopaminergic system. These findings raise the possibility that r-VNS could provide a promising strategy for enhancing dopamine-dependent neuroplasticity, opening broad avenues for future research into the efficacy and safety of r-VNS in the treatment of neurological disease.

Vagus Nerve Stimulation as a Treatment for Fear and Anxiety in Individuals with Autism Spectrum Disorder

Anxiety disorders affect a large percentage of individuals who have an autism spectrum disorder (ASD). In children with ASD, excessive anxiety is also linked to gastrointestinal problems, self-injurious behaviors, and depressive symptoms. Exposure-based cognitive behavioral therapies are effective treatments for anxiety disorders in children with ASD, but high relapse rates indicate the need for additional treatment strategies. This perspective discusses evidence from preclinical research, which indicates that vagus nerve stimulation (VNS) paired with exposure to fear-provoking stimuli and situations could offer benefits as an adjuvant treatment for anxiety disorders that coexist with ASD. Vagus nerve stimulation is approved for use in the treatment of epilepsy, depression, and more recently as an adjuvant in rehabilitative training following stroke. In preclinical models, VNS shows promise in simultaneously enhancing consolidation of extinction memories and reducing anxiety. In this review, we will present potential mechanisms by which VNS could treat fear and anxiety in ASD. We also discuss potential uses of VNS to treat depression and epilepsy in the context of ASD, and noninvasive methods to stimulate the vagus nerve.

Vagal Nerve Stimulation: The Effect on the Brain Oscillatory Field Potential

More than thirty years of medical treatment with the use of vagal nerve stimulation (VNS) has shown that this therapeutic procedure works in a number of homeostatic disturbances. Although the clinical usage of VNS has a long history, our knowledge about the central mechanisms underlying this treatment is still limited. In the present paper we review the effects of VNS on brain oscillations as a possible electrophysiological bio-marker of VNS efficacy. The review was prepared mainly on the basis of data delivered from clinical observations and the outcomes of electrophysiological experiments conducted on laboratory animals that are available in PubMed. We consciously did not focus on epileptiform activity understood as a pathologic oscillatory activity, which was widely discussed in the numerous previously published reviews. The main conclusion of the present paper is that further, well-designed experiments on laboratory animals are absolutely necessary to address the electrophysiological issues. These will fill a number of gaps in our present knowledge of the central mechanisms underlying VNS therapy.

The Role of Context Conditioning in the Reinstatement of Responding to an Alcohol-Predictive Conditioned Stimulus

Re-exposure to an unconditioned stimulus (US) can reinstate extinguished conditioned responding elicited by a conditioned stimulus (CS). We tested the hypothesis that the reinstatement of responding to an appetitive CS is driven by an excitatory association formed between the US and the context that the US was ingested in during US re-exposure. Male, Long-Evans rats were acclimated to drinking alcohol (15%, v/v) in the home-cage, then trained to associate an auditory CS with an alcohol-US that was delivered into a fluid port for oral intake. During subsequent extinction sessions, the CS was presented as before, but without alcohol. After extinction, rats were re-exposed to alcohol as in training, but without the CS (alcohol re-exposure). 24h later at test, the CS was presented as in training, but without alcohol. First, we tested the effect of extinguishing the context-alcohol association, formed during alcohol re-exposure, on reinstatement. Conducting four context extinction sessions across four days (spaced extinction) after the alcohol re-exposure session did not impact reinstatement. However, four context extinction sessions conducted across two days (massed extinction) prevented reinstatement. Next, we conducted alcohol re-exposure in a context that either differed from, or was the same as, the test context. One alcohol re-exposure session in a different context did not affect reinstatement, however, three alcohol re-exposure sessions in a different context significantly reduced reinstatement during the first CS trial. These results partially support the view that a context-US association formed during US re-exposure drives the reinstatement of responding to an appetitive, alcohol-predictive CS.

Cortical Responses to Vagus Nerve Stimulation Are Modulated by Brain State in Nonhuman Primates

Vagus nerve stimulation (VNS) has been tested as therapy for several brain disorders and as a means to modulate cortical excitability and brain plasticity. Cortical effects of VNS, manifesting as vagal-evoked potentials (VEPs), are thought to arise from activation of ascending cholinergic and noradrenergic systems. However, it is unknown whether those effects are modulated by brain state at the time of stimulation. In 2 freely behaving macaque monkeys, we delivered short trains of 5 pulses to the left cervical vagus nerve at different frequencies (5-300 Hz) while recording local field potentials (LFPs) from sites in contralateral prefrontal, sensorimotor and parietal cortical areas. Brain states were inferred from spectral components of LFPs and the presence of overt movement: active awake, resting awake, REM sleep and NREM sleep. VNS elicited VEPs in all sampled cortical areas. VEPs comprised early (<70 ms), intermediate (70-250 ms) and late (>250 ms) components. The magnitude of the intermediate and late components was largest during NREM sleep and smallest during wakefulness, whereas that of the early component was not modulated by brain state. VEPs during NREM were larger for stimuli delivered at the depolarized phase of ongoing delta oscillations. Higher pulsing frequencies generated larger VEPs. These short VNS trains did not affect brain state transitions during wakefulness or sleep. Our findings suggest that ongoing brain state modulates the evoked effects of VNS on cortical activity. This has implications for the role of ongoing cortical activity and brain state in shaping cortical responses to peripheral stimuli, for the modulation of vagal interoceptive signaling by cortical activity, and for the dose calibration of VNS therapies.

Vagus Nerve Stimulation as a Potential Adjuvant to Rehabilitation for Post-stroke Motor Speech Disorders

Stroke often leaves lasting impairments affecting orofacial function. While speech therapy is able to enhance function after stroke, many patients see only modest improvements after treatment. This partial restoration of function after rehabilitation suggests that there is a need for further intervention. Rehabilitative strategies that augment the effects of traditional speech therapy hold promise to yield greater efficacy and reduce disability associated with motor speech disorders. Recent studies demonstrate that brief bursts of vagus nerve stimulation (VNS) can facilitate the benefits of rehabilitative interventions. VNS paired with upper limb rehabilitation enhances recovery of upper limb function in patients with chronic stroke. Animal studies reveal that these improvements are driven by VNS-dependent synaptic plasticity in motor networks. Moreover, preclinical evidence demonstrates that a similar strategy of pairing VNS can promote synaptic reorganization in orofacial networks. Building on these findings, we postulate that VNS-directed orofacial plasticity could target post-stroke motor speech disorders. Here, we outline the rationale for pairing VNS with traditional speech therapy to enhance recovery in the context of stroke of speech motor function. We also explore similar treatments that aim to enhance synaptic plasticity during speech therapy, and how VNS differs from these existing therapeutic strategies. Based on this evidence, we posit that VNS-paired speech therapy shows promise as a means of enhancing recovery after post-stroke motor speech disorders. Continued development is necessary to comprehensively establish and optimize this approach, which has the potential to increase quality of life for the many individuals suffering with these common impairments.

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.

Effects of transcutaneous auricular vagus nerve stimulation on reversal learning, tonic pupil size, salivary alpha-amylase, and cortisol

This study investigated whether transcutaneous auricular vagus nerve stimulation (taVNS) enhances reversal learning and augments noradrenergic biomarkers (i.e., pupil size, cortisol, and salivary alpha-amylase [sAA]). We also explored the effect of taVNS on respiratory rate and cardiac vagal activity (CVA). Seventy-one participants received stimulation of either the cymba concha (taVNS) or the earlobe (sham) of the left ear. After learning a series of cue-outcome associations, the stimulation was applied before and throughout a reversal phase in which cue-outcome associations were changed for some (reversal), but not for other (distractor) cues. Tonic pupil size, salivary cortisol, sAA, respiratory rate, and CVA were assessed at different time points. Contrary to our hypothesis, taVNS was not associated with an overall improvement in performance on the reversal task. Compared to sham, the taVNS group performed worse for distractor than reversal cues. taVNS did not increase tonic pupil size and sAA. Only post hoc analyses indicated that the cortisol decline was steeper in the sham compared to the taVNS group. Exploratory analyses showed that taVNS decreased respiratory rate but did not affect CVA. The weak and unexpected effects found in this study might relate to the lack of parameters optimization for taVNS and invite to further investigate the effect of taVNS on cortisol and respiratory rate.

Vagus nerve stimulation enhances fear extinction as an inverted-U function of stimulation intensity

Studies in rodents indicate that pairing vagus nerve stimulation (VNS) with extinction training enhances fear extinction. However, the role of stimulation parameters on the effects of VNS remains largely unknown. Identifying the optimal stimulation intensity is a critical step in clinical translation of neuromodulation-based therapies. Here, we sought to investigate the role of stimulation intensity in rats receiving VNS paired with extinction training in a rat model for Posttraumatic Stress Disorder (PTSD). Male Sprague-Dawley rats underwent single prolonged stress followed by a severe fear conditioning training and were implanted with a VNS device. After recovery, independent groups of rats were exposed to extinction training paired with sham (0 mA) or VNS at different intensities (0.4, 0.8, or 1.6 mA). VNS intensities of 0.4 mA or 0.8 mA decreased conditioned fear during extinction training compared to sham stimulation. Pairing extinction training with moderate VNS intensity of 0.8 mA produced significant reduction in conditioned fear during extinction retention when rats were tested a week after VNS-paired extinction. High intensity VNS at 1.6 mA failed to enhance extinction. These findings indicate that a narrow range of VNS intensities enhances extinction learning, and suggest that the 0.8 mA VNS intensity used in earlier rodent and human stroke studies may also be the optimal in using VNS as an adjuvant in exposure therapies for PTSD.

Electrical stimulation of the external ear acutely activates noradrenergic mechanisms in humans

BACKGROUND

Transcutaneous stimulation of the external ear is thought to recruit afferents of the auricular vagus nerve, providing a means to activate noradrenergic pathways in the central nervous system. Findings from human studies examining the effects of auricular stimulation on noradrenergic biomarkers have been mixed, possibly relating to the limited and variable parameter space explored to date.

OBJECTIVE

We tested the extent to which brief pulse trains applied to locations of auricular innervation (canal and concha) elicit acute pupillary responses (PRs) compared to a sham location (lobe). Pulse amplitude and frequency were varied systematically to examine effects on PR features.

METHODS

Participants (n = 19) underwent testing in three separate experiments, each with stimulation applied to a different external ear location. Perceptual threshold (PT) was measured at the beginning of each experiment. Pulse trains (∼600 ms) consisting of different amplitude (0.0xPT, 0.8xPT, 1.0xPT, 1.5xPT, 2.0xPT) and frequency (25 Hz, 300 Hz) combinations were administered during eye tracking procedures.

RESULTS

Stimulation to all locations elicited PRs which began approximately halfway through the pulse train and peaked shortly after the final pulse (≤1 s). PR size and incidence increased with pulse amplitude and tended to be greatest with canal stimulation. Higher pulse frequency shortened the latency of PR onset and peak dilation. Changes in pupil diameter elicited by pulse trains were weakly associated with baseline pupil diameter.

CONCLUSION

(s): Auricular stimulation elicits acute PRs, providing a basis to synchronize neuromodulator release with task-related neural spiking which preclinical studies show is a critical determinant of therapeutic effects. Further work is needed to dissociate contributions from vagal and non-vagal afferents mediating activation of the biomarker.

Systematic Review and Methodological Considerations for the Use of Single Prolonged Stress and Fear Extinction Retention in Rodents

Posttraumatic stress disorder (PTSD) is a mental health condition triggered by experiencing or witnessing a terrifying event that can lead to lifelong burden that increases mortality and adverse health outcomes. Yet, no new treatments have reached the market in two decades. Thus, screening potential interventions for PTSD is of high priority. Animal models often serve as a critical translational tool to bring new therapeutics from bench to bedside. However, the lack of concordance of some human clinical trial outcomes with preclinical animal efficacy findings has led to a questioning of the methods of how animal studies are conducted and translational validity established. Thus, we conducted a systematic review to determine methodological variability in studies that applied a prominent animal model of trauma-like stress, single prolonged stress (SPS). The SPS model has been utilized to evaluate a myriad of PTSD-relevant outcomes including extinction retention. Rodents exposed to SPS express an extinction retention deficit, a phenotype identified in humans with PTSD, in which fear memory is aberrantly retained after fear memory extinction. The current systematic review examines methodological variation across all phases of the SPS paradigm, as well as strategies for behavioral coding, data processing, statistical approach, and the depiction of data. Solutions for key challenges and sources of variation within these domains are discussed. In response to methodological variation in SPS studies, an expert panel was convened to generate methodological considerations to guide researchers in the application of SPS and the evaluation of extinction retention as a test for a PTSD-like phenotype. Many of these guidelines are applicable to all rodent paradigms developed to model trauma effects or learned fear processes relevant to PTSD, and not limited to SPS. Efforts toward optimizing preclinical model application are essential for enhancing the reproducibility and translational validity of preclinical findings, and should be conducted for all preclinical psychiatric research models.

Vagus nerve stimulation in brain diseases: Therapeutic applications and biological mechanisms

Brain diseases, including neurodegenerative, cerebrovascular and neuropsychiatric diseases, have posed a deleterious threat to human health and brought a great burden to society and the healthcare system. With the development of medical technology, vagus nerve stimulation (VNS) has been approved by the Food and Drug Administration (FDA) as an alternative treatment for refractory epilepsy, refractory depression, cluster headaches, and migraines. Furthermore, current evidence showed promising results towards the treatment of more brain diseases, such as Parkinson's disease (PD), autistic spectrum disorder (ASD), traumatic brain injury (TBI), and stroke. Nonetheless, the biological mechanisms underlying the beneficial effects of VNS in brain diseases remain only partially elucidated. This review aims to delve into the relevant preclinical and clinical studies and update the progress of VNS applications and its potential mechanisms underlying the biological effects in brain diseases.

Vagus nerve stimulation promotes extinction generalization across sensory modalities

Traumatic experiences involve complex sensory information, and individuals with trauma-related psychological disorders, such as posttraumatic stress disorder (PTSD), can exhibit abnormal fear to numerous different stimuli that remind them of the trauma. Vagus nerve stimulation (VNS) enhances extinction of auditory fear conditioning in rat models for PTSD. We recently found that VNS-paired extinction can also promote extinction generalization across different auditory cues. Here we tested whether VNS can enhance extinction of olfactory fear and promote extinction generalization across auditory and olfactory sensory modalities. Male Sprague Dawley rats were implanted with a stimulating cuff on the cervical vagus nerve. Rats then received two days of fear conditioning where olfactory (amyl acetate odor) and auditory (9 kHz tones) stimuli were concomitantly paired with footshock. Twenty-four hours later, rats were given three days of sham or VNS-paired extinction (5 stimulations, 30-sec trains at 0.4 mA) overlapping with presentation of either the olfactory or the auditory stimulus. Two days later, rats were given an extinction retention test where avoidance of the olfactory stimulus or freezing to the auditory stimulus were measured. VNS-paired with exposure to the olfactory stimulus during extinction reduced avoidance of the odor in the retention test. VNS-paired with exposure to the auditory stimulus during extinction also decreased avoidance of the olfactory cue, and VNS paired with exposure to the olfactory stimulus during extinction reduced freezing when the auditory stimulus was presented in the retention test. These results indicate that VNS enhances extinction of olfactory fear and promotes extinction generalization across different sensory modalities. Extinction generalization induced by VNS may therefore improve outcomes of exposure-based therapies.

Use of a physiological reflex to standardize vagal nerve stimulation intensity improves data reproducibility in a memory extinction assay

BACKGROUND

Modulating brainstem activity, via electrical vagus nerve stimulation (VNS), influences cognitive functions, including memory. However, controlling for changes in stimulus efficacy during chronic studies, and response variability between subjects, is problematic.

OBJECTIVE/HYPOTHESIS

We hypothesized that recruitment of an autonomic reflex, the Hering-Breuer reflex, would provide robust confirmation of VNS efficacy. We compared this to measurement of electrode resistance over time. We also examined whether VNS modulates contextual memory extinction.

METHODS

Electrodes for VNS and diaphragm electromyography recording were implanted into anesthetized Sprague Dawley rats. When conscious, we measured the electrode resistance as well as the minimum VNS current required to evoke the Hering-Breuer reflex, before, and after, an inhibitory avoidance assay - a two chamber, dark/light model, where the dark compartment was paired with an aversive foot shock. The extinction of this contextual memory was assessed in sham and VNS treated rats, with VNS administered for 30 s at 1.5 times the Hering-Breuer reflex threshold during extinction memory formation.

RESULTS

Assessment of VNS-evoked Hering-Breuer reflex successfully identified defective electrodes. VNS accelerated extinction memory and decreased multiple physiological metrics of fear expression. We observed an inverse relationship between memory extinction and respiratory rate during the behavioural assay. Additionally, no current - response relationship between VNS and extinction memory formation was established.

CONCLUSION

These data demonstrate that reliable, experimental VNS studies can be produced by verifying reflex initiation as a consequence of stimulation. Further, studies could be standardised by indexing stimulator efficacy to initiation of autonomic reflexes.

The vagus nerve mediates the suppressing effects of peripherally administered oxytocin on methamphetamine self-administration and seeking in rats

The neuropeptide oxytocin has emerged as a promising pharmacotherapy for methamphetamine (METH) addiction, and clinical trials of intranasal oxytocin are underway. However, there is debate as to how peripherally administered oxytocin alters brain signalling to modulate addiction processes. Interestingly, there is evidence for functional interactions between peripheral oxytocin administration and the vagus nerve. Therefore, this study investigated whether the effects of peripherally administered oxytocin require vagal signalling to reduce METH self-administration and reinstatement of METH-seeking behaviours. Male and female Sprague-Dawley rats underwent surgery for jugular catheterisation and either subdiaphragmatic vagotomy (SDV) or a sham operation. Rats were trained to self-administer METH, and the effect of peripherally administered oxytocin on METH intake was assessed. Rats then underwent extinction, and effects of oxytocin were assessed on cue- and METH-induced reinstatement of METH-seeking. Oxytocin treatment robustly attenuated METH intake in both sexes, and SDV entirely prevented the suppressant effect of oxytocin (0.3 mg/kg) on METH intake, and partially prevented the effects of 1 mg/kg oxytocin in both sexes. After extinction, SDV decreased the suppressing effects of oxytocin on cue- and METH-primed reinstatement in males, but not females. SDV was functionally confirmed by measuring food intake following administration of the vagal dependent peptide, cholecyostokin-8. Our data suggest that vagus nerve signalling is required for the inhibitory effects of peripherally administered oxytocin on METH self-administration and reinstatement, and that this vagal dependency is partially mediated by sex and drug withdrawal. This study has implications for the use of oxytocin as a therapy for METH use disorder for both sexes.

Sex-divergent long-term effects of single prolonged stress in adult rats

Single prolonged stress (SPS) is an experimental model that recapitulates in rodents some of the core symptoms of post-traumatic stress disorder (PTSD). Although women have a two-fold greater risk to develop PTSD, most preclinical studies have been carried out in males. Furthermore, the long-term effects of behavioral alterations induced by SPS have been rarely investigated. Here, we evaluated the long-term effects of SPS on PTSD-relevant behavioral domains in rats and whether these effects were sex-dependent. To this aim, separate cohorts of male and female adult rats were subjected to SPS and, 30 days later, long-term effects were assessed. We found that SPS exposure reduced locomotor activity in both sexes in an open field task. Males only showed increased anxiety-like behavior in the elevated plus maze and marble burying tests, enhanced acoustic startle response and impaired spatial memory retention while females were unaffected. SPS exposure did not alter auditory fear memory dynamics in males, but it did alter extinction retrieval in females. We provide the first evidence that SPS reproduces long-term emotional alterations in male, but not in female, rats which were observed 30 days following trauma exposure, thus resembling some of the hallmark symptoms of PTSD. Furthermore, our results show for the first time a long-term SPS-induced alteration of cued fear extinction in females. Our findings are relevant to future research on trauma-related disorders and may help develop sex-specific interventions to treat PTSD.

Transcutaneous vagal nerve stimulation blocks stress-induced activation of Interleukin-6 and interferon-γ in posttraumatic stress disorder: A double-blind, randomized, sham-controlled trial

Posttraumatic stress disorder (PTSD) is a highly disabling condition associated with alterations in multiple neurobiological systems, including increases in inflammatory function. Vagus nerve stimulation (VNS) decreases inflammation, however few studies have examined the effects of non-invasive VNS on physiology in human subjects, and no studies in patients with PTSD. The purpose of this study was to assess the effects of transcutaneous cervical VNS (tcVNS) on inflammatory responses to stress. Thirty subjects with a history of exposure to traumatic stress with (N ​= ​10) and without (N ​= ​20) PTSD underwent exposure to stressful tasks immediately followed by active or sham tcVNS and measurement of multiple biomarkers of inflammation (interleukin-(IL)-6, IL-2, IL-1β, Tumor Necrosis Factor alpha (TNFα) and Interferon gamma (IFNγ) over multiple time points. Stressful tasks included exposure to personalized scripts of traumatic events on day 1, and public speech and mental arithmetic (Mental Stress) tasks on days 2 and 3. Traumatic scripts were associated with a pattern of subjective anger measured with Visual Analogue Scales and increased IL-6 and IFNγ in PTSD patients that was blocked by tcVNS (p ​< ​.05). Traumatic stress had minimal effects on these biomarkers in non-PTSD subjects and there was no difference between tcVNS or sham. No significant differences were seen between groups in IL-2, IL-1β, or TNFα. These results demonstrate that tcVNS blocks behavioral and inflammatory responses to stress reminders in PTSD.

Effect of transcutaneous cervical vagus nerve stimulation on the pituitary adenylate cyclase-activating polypeptide (PACAP) response to stress: A randomized, sham controlled, double blind pilot study

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide that plays a key role in the neurobiology of the stress response, and prior studies suggest that its function is dysregulated in post-traumatic stress disorder (PTSD). Transcutaneous cervical vagus nerve stimulation (tcVNS) acts through PACAP and other neurobiological systems to modulate stress responses and/or symptoms of PTSD. In this pilot study, we examined the effects of tcVNS on PACAP in a three day chronic stress laboratory paradigm involving serial traumatic and mental stress exposures in healthy individuals with a history of exposure to psychological trauma (n ​= ​18) and patients with PTSD (n ​= ​12).

Methods

A total of 30 subjects with a history of exposure to psychological trauma experience were recruited (12 with PTSD diagnosis) for a three-day randomized double-blinded study of tcVNS or sham stimulation. Subjects underwent a protocol that included both personalized trauma recall and non-personalized mental stressors (public speaking, mental arithmetic) paired to tcVNS or sham stimulation over three days. Blood was collected at baseline and multiple time points after exposure to stressors. Linear mixed-effects models were used to assess changes in PACAP over time (in response to stressors) and its relation to active tcVNS or sham stimulation.

Results

PACAP blood levels increased over the course of three days for both active tcVNS and sham groups. This increase was statistically-significant in the sham group at the end of the second (Cohen's drm ​= ​0.35, p ​= ​0.04), and third days (drm ​= ​0.41, p ​= ​0.04), but not in the active tcVNS group (drm ​= ​0.21, drm ​= ​0.18, and p ​> ​0.20).

Conclusion

These pilot findings suggest tcVNS may attenuate this neurobiological stress-response. Larger studies are needed to investigate gender and interaction effects.

Transcutaneous cervical vagal nerve stimulation reduces sympathetic responses to stress in posttraumatic stress disorder: A double-blind, randomized, sham controlled trial

OBJECTIVE

Exacerbated autonomic responses to acute stress are prevalent in posttraumatic stress disorder (PTSD). The purpose of this study was to assess the effects of transcutaneous cervical VNS (tcVNS) on autonomic responses to acute stress in patients with PTSD. The authors hypothesized tcVNS would reduce the sympathetic response to stress compared to a sham device.

METHODS

Using a randomized double-blind approach, we studied the effects of tcVNS on physiological responses to stress in patients with PTSD (n = 25) using noninvasive sensing modalities. Participants received either sham (n = 12) or active tcVNS (n = 13) after exposure to acute personalized traumatic script stress and mental stress (public speech, mental arithmetic) over a three-day protocol. Physiological parameters related to sympathetic responses to stress were investigated.

RESULTS

Relative to sham, tcVNS paired to traumatic script stress decreased sympathetic function as measured by: decreased heart rate (adjusted β = -5.7%; 95% CI: ±3.6%, effect size d = 0.43, p < 0.01), increased photoplethysmogram amplitude (peripheral vasodilation) (30.8%; ±28%, 0.29, p < 0.05), and increased pulse arrival time (vascular function) (6.3%; ±1.9%, 0.57, p < 0.0001). Similar (p < 0.05) autonomic, cardiovascular, and vascular effects were observed when tcVNS was applied after mental stress or without acute stress.

CONCLUSION

tcVNS attenuates sympathetic arousal associated with stress related to traumatic memories as well as mental stress in patients with PTSD, with effects persisting throughout multiple traumatic stress and stimulation testing days. These findings show that tcVNS has beneficial effects on the underlying neurophysiology of PTSD. Such autonomic metrics may also be evaluated in daily life settings in tandem with tcVNS therapy to provide closed-loop delivery and measure efficacy.ClinicalTrials.gov Registration # NCT02992899.

Application of Noninvasive Vagal Nerve Stimulation to Stress-Related Psychiatric Disorders

BACKGROUND

Vagal Nerve Stimulation (VNS) has been shown to be efficacious for the treatment of depression, but to date, VNS devices have required surgical implantation, which has limited widespread implementation.

METHODS

New noninvasive VNS (nVNS) devices have been developed which allow external stimulation of the vagus nerve, and their effects on physiology in patients with stress-related psychiatric disorders can be measured with brain imaging, blood biomarkers, and wearable sensing devices. Advantages in terms of cost and convenience may lead to more widespread implementation in psychiatry, as well as facilitate research of the physiology of the vagus nerve in humans. nVNS has effects on autonomic tone, cardiovascular function, inflammatory responses, and central brain areas involved in modulation of emotion, all of which make it particularly applicable to patients with stress-related psychiatric disorders, including posttraumatic stress disorder (PTSD) and depression, since dysregulation of these circuits and systems underlies the symptomatology of these disorders.

RESULTS

This paper reviewed the physiology of the vagus nerve and its relevance to modulating the stress response in the context of application of nVNS to stress-related psychiatric disorders.

CONCLUSIONS

nVNS has a favorable effect on stress physiology that is measurable using brain imaging, blood biomarkers of inflammation, and wearable sensing devices, and shows promise in the prevention and treatment of stress-related psychiatric disorders.

GABAergic System in Stress: Implications of GABAergic Neuron Subpopulations and the Gut-Vagus-Brain Pathway

Stress can cause a variety of central nervous system disorders, which are critically mediated by the γ-aminobutyric acid (GABA) system in various brain structures. GABAergic neurons have different subsets, some of which coexpress certain neuropeptides that can be found in the digestive system. Accumulating evidence demonstrates that the gut-brain axis, which is primarily regulated by the vagus nerve, is involved in stress, suggesting a communication between the "gut-vagus-brain" pathway and the GABAergic neuronal system. Here, we first summarize the evidence that the GABAergic system plays an essential role in stress responses. In addition, we review the effects of stress on different brain regions and GABAergic neuron subpopulations, including somatostatin, parvalbumin, ionotropic serotonin receptor 5-HT3a, cholecystokinin, neuropeptide Y, and vasoactive intestinal peptide, with regard to signaling events, behavioral changes, and pathobiology of neuropsychiatric diseases. Finally, we discuss the gut-brain bidirectional communications and the connection of the GABAergic system and the gut-vagus-brain pathway.

Vagus Nerve Stimulation as a Gateway to Interoception

The last two decades have seen a growing interest in the study of interoception. Interoception can be understood as a hierarchical phenomenon, referring to the body-to-brain communication of internal signals, their sensing, encoding, and representation in the brain, influence on other cognitive and affective processes, and their conscious perception. Interoceptive signals have been notoriously challenging to manipulate in experimental settings. Here, we propose that this can be achieved through electrical stimulation of the vagus nerve (either in an invasive or non-invasive fashion). The vagus nerve is the main pathway for conveying information about the internal condition of the body to the brain. Despite its intrinsic involvement in interoception, surprisingly little research in the field has used Vagus Nerve Stimulation to explicitly modulate bodily signals. Here, we review a range of cognitive, affective and clinical research using Vagus Nerve Stimulation, showing that it can be applied to the study of interoception at each level of its hierarchy. This could have considerable implications for our understanding of the interoceptive dimension of cognition and affect in both health and disease, and lead to development of new therapeutic tools.

Non-invasive vagal nerve stimulation decreases brain activity during trauma scripts

Background:

Traumatic stress can have lasting effects on neurobiology and result in psychiatric conditions such as posttraumatic stress disorder (PTSD). We hypothesize that non-invasive cervical vagal nerve stimulation (nVNS) may alleviate trauma symptoms by reducing stress sympathetic reactivity. This study examined how nVNS alters neural responses to personalized traumatic scripts.

Methods:

Nineteen participants who had experienced trauma but did not have the diagnosis of PTSD completed this double-blind sham-controlled study. In three sequential time blocks, personalized traumatic scripts were presented to participants immediately followed by either sham stimulation (n = 8; 0–14 V, 0.2 Hz, pulse width = 5s) or active nVNS (n = 11; 0–30 V, 25 Hz, pulse width = 40 ms). Brain activity during traumatic scripts was assessed using High Resolution Positron Emission Tomography (HR-PET) with radiolabeled water to measure brain blood flow.

Results:

Traumatic scripts resulted in significant activations within the bilateral medial and orbital prefrontal cortex, premotor cortex, anterior cingulate, thalamus, insula, hippocampus, right amygdala, and right putamen. Greater activation was observed during sham stimulation compared to nVNS within the bilateral prefrontal and orbitofrontal cortex, premotor cortex, temporal lobe, parahippocampal gyrus, insula, and left anterior cingulate. During the first exposure to the trauma scripts, greater activations were found in the motor cortices and ventral visual stream whereas prefrontal cortex and anterior cingulate activations were more predominant with later script presentations for those subjects receiving sham stimulation.

Conclusion:

nVNS decreases neural reactivity to an emotional stressor in limbic and other brain areas involved in stress, with changes over repeated exposures suggesting a shift from scene appraisal to cognitively processing the emotional event.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Parametric characterization of the rat Hering-Breuer reflex evoked with implanted and non-invasive vagus nerve stimulation

Vagus nerve stimulation (VNS) has rapidly gained interest as a treatment for a variety of disorders. A number of methods have been employed to stimulate the vagus nerve, but the most common relies on a cuff electrode implanted around the cervical branch of the nerve. Recently, two non-invasive methods have increased in popularity: transcutaneous cervical VNS (tcVNS) and transcutaneous auricular VNS (taVNS). Despite promising clinical results, there has been little direct comparison of these methods to stimulation delivered via an implanted device. In this study, we directly compared both non-invasive strategies to stimulation with an implanted cuff electrode on activation of the Hering-Breuer (HB) reflex, a non-invasive biomarker of A-fiber activation in the vagus. Stimulation was delivered across a wide range of parameters using tcVNS, taVNS, and an implanted cuff electrode in female rats. Activation of the HB reflex, changes in heart rate, and neck muscle twitch force were recorded. Consistent with low thresholds reported in previous studies, we found that the threshold to activate the HB reflex using an implanted cuff electrode was 0.406 ± 0.066 mA. tcVNS was capable of activating the HB reflex, but the threshold was 34.18 ± 1.86 mA, over 15 fold higher than the stimulation intensity that caused twitching of the neck muscles (2.09 ± 0.16 mA). No activation of the HB reflex was observed with taVNS at any parameters. These results describe activation of the HB reflex with each strategy and provide initial evidence regarding differences in the activation of the vagus nerve with invasive and non-invasive methods.

Efficient parameters of vagus nerve stimulation to enhance extinction learning in an extinction-resistant rat model of PTSD

Vagus nerve stimulation (VNS) has shown promise as an adjuvant treatment for posttraumatic stress disorder (PTSD), as it enhances fear extinction and reduces anxiety symptoms in multiple rat models of this condition. Yet, identification of the optimal stimulation paradigm is needed to facilitate clinical translation of this potential therapy. Using an extinction-resistant rat model of PTSD, we tested whether varying VNS intensity and duration could maximize extinction learning while minimizing the total amount of stimulation. We confirmed that sham rats failed to extinguish after a week of extinction training. Delivery of the standard LONG VNS trains (30 s) at 0.4 mA enhanced extinction and reduced anxiety but did not prevent fear return. Increasing the intensity of LONG VNS trains to 0.8 mA prevented fear return and attenuated anxiety symptoms. Interestingly, delivering 1, 4 or 16 SHORT VNS bursts (0.5 s) at 0.8 mA during each cue presentation in extinction training also enhanced extinction. LONG VNS trains or multiple SHORT VNS bursts at 0.8 mA attenuated fear renewal and reinstatement, promoted extinction generalization and reduced generalized anxiety. Delivering 16 SHORT VNS bursts also facilitated extinction in fewer trials. This study provides the first evidence that brief bursts of VNS can enhance extinction training, reduce relapse and support symptom remission using much less VNS than previous protocols. These findings suggest that VNS parameters can be adjusted in order to minimize total charge delivery and maximize therapeutic effectiveness.

Vagus nerve stimulation produces immediate dose-dependent anxiolytic effect in rats

BACKGROUND

Chronic vagus nerve stimulation (VNS) attenuates anxiety in rats and humans. However, it is unclear whether VNS can promote acute anxiolytic effects. Here we examined short-term anxiolytic effects of VNS using single or multiple trains in rats submitted to a battery of tests.

METHODS

Three groups of rats were implanted with VNS cuffs and tested for anxiety using the elevated plus maze (EPM), novelty suppressed feeding (NSF), and acoustic startle response (ASR), after receiving either one or five VNS trains (0.4 mA/30 Hz/30‑sec), or sham stimulation.

RESULTS

Both single and multiple VNS trains reduced state anxiety as measured using the EPM, but only multiple trains reduced anxiety in the EPM and NSF. VNS did not decrease arousal as measured using the ASR.

LIMITATIONS

The anxiolytic effects of VNS may be differently influenced by test order or prior-exposure to stress. VNS did not affect startle responses in naïve rats but the present findings do not determine whether VNS would affect startle responses that are potentiated by fear or anxiety.

CONCLUSION

A single VNS train can produce an anxiolytic-like effect in the EPM minutes later, an effect that is not observed in the NSF. Delivering 5 VNS trains restores the immediate effects across tests of anxiety, indicating that more trains produce a more robust anxiolytic effect. The lack of effects on ASR suggests that VNS affects state anxiety but not baseline arousal in naïve rats. We suggest that the anxiolytic effect of VNS can increase tolerability and reduce dropout in exposure-based therapies.

Quantifying acute physiological biomarkers of transcutaneous cervical vagal nerve stimulation in the context of psychological stress

BACKGROUND

Stress is associated with activation of the sympathetic nervous system, and can lead to lasting alterations in autonomic function and in extreme cases symptoms of posttraumatic stress disorder (PTSD). Vagal nerve stimulation (VNS) is a potentially useful tool as a modulator of autonomic nervous system function, however currently available implantable devices are limited by cost and inconvenience.

OBJECTIVE

The purpose of this study was to assess the effects of transcutaneous cervical VNS (tcVNS) on autonomic responses to stress.

METHODS

Using a double-blind approach, we investigated the effects of active or sham tcVNS on peripheral cardiovascular and autonomic responses to stress using wearable sensing devices in 24 healthy human participants with a history of exposure to psychological trauma. Participants were exposed to acute stressors over a three-day period, including personalized scripts of traumatic events, public speech, and mental arithmetic tasks.

RESULTS

tcVNS relative to sham applied immediately after traumatic stress resulted in a decrease in sympathetic function and modulated parasympathetic/sympathetic autonomic tone as measured by increased pre-ejection period (PEP) of the heart (a marker of cardiac sympathetic function) of 4.2 ms (95% CI 1.6-6.8 ms, p < 0.01), decreased peripheral sympathetic function as measured by increased photoplethysmogram (PPG) amplitude (decreased vasoconstriction) by 47.9% (1.4-94.5%, p < 0.05), a 9% decrease in respiratory rate (-14.3 to -3.7%, p < 0.01). Similar effects were seen when tcVNS was applied after other stressors and in the absence of a stressor.

CONCLUSION

Wearable sensing modalities are feasible to use in experiments in human participants, and tcVNS modulates cardiovascular and peripheral autonomic responses to stress.