Vagus nerve stimulation reduces cocaine seeking and alters plasticity in the extinction network

Vagus Nerve Stimulation (VNS) Modulates Synaptic Plasticity in the Infralimbic Cortex via Trk-B Receptor Activation to Reduce Drug-Seeking in Male Rats

Drugs of abuse cause changes in the prefrontal cortex (PFC) and associated regions that impair inhibitory control over drug-seeking. Breaking the contingencies between drug-associated cues and the delivery of the reward during extinction learning reduces relapse. Vagus nerve stimulation (VNS) has previously been shown to enhance extinction learning and reduce drug-seeking. Here we determined the effects of VNS-mediated release of brain-derived neurotrophic factor (BDNF) on extinction and cue-induced reinstatement in male rats trained to self-administer cocaine. Pairing 10 d of extinction training with VNS facilitated extinction and reduced drug-seeking behavior during reinstatement. Rats that received a single extinction session with VNS showed elevated BDNF levels in the medial PFC as determined via an enzyme-linked immunosorbent assay. Systemic blockade of tropomyosin receptor kinase B (TrkB) receptors during extinction, via the TrkB antagonist ANA-12, decreased the effects of VNS on extinction and reinstatement. Whole-cell recordings in brain slices showed that cocaine self-administration induced alterations in the ratio of AMPA and NMDA receptor-mediated currents in Layer 5 pyramidal neurons of the infralimbic cortex (IL). Pairing extinction with VNS reversed cocaine-induced changes in glutamatergic transmission by enhancing AMPAR currents, and this effect was blocked by ANA-12. Our study suggests that VNS consolidates the extinction of drug-seeking behavior by reversing drug-induced changes in synaptic AMPA receptors in the IL, and this effect is abolished by blocking TrkB receptors during extinction, highlighting a potential mechanism for the therapeutic effects of VNS in addiction.

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.

Vagus nerve stimulation (VNS) modulates synaptic plasticity in the rat infralimbic cortex via Trk-B receptor activation to reduce drug-seeking

Drugs of abuse cause changes in the prefrontal cortex (PFC) and associated regions that impair inhibitory control over drug-seeking. Breaking the contingencies between drug-associated cues and the delivery of the reward during extinction learning reduces relapse. Vagus nerve stimulation (VNS) has previously been shown to enhance extinction learning and reduce drug-seeking. Here we determined the effects of VNS-mediated release of brain-derived neurotrophic factor (BDNF) on extinction and cue-induced reinstatement in rats trained to self-administer cocaine. Pairing 10 days of extinction training with VNS facilitated extinction and reduced drug-seeking behavior during reinstatement. Rats that received a single extinction session with VNS showed elevated BDNF levels in the medial PFC as determined via an enzyme-linked immunosorbent assay (ELISA). Systemic blockade of Tropomyosin receptor kinase B (TrkB) receptors during extinction, via the TrkB antagonist ANA-12, decreased the effects of VNS on extinction and reinstatement. Whole-cell recordings in brain slices showed that cocaine self-administration induced alterations in the ratio of AMPA and NMDA receptor-mediated currents in layer 5 pyramidal neurons of the infralimbic cortex (IL). Pairing extinction with VNS reversed cocaine-induced changes in glutamatergic transmission by enhancing AMPAR currents, and this effect was blocked by ANA-12. Our study suggests that VNS consolidates extinction of drug-seeking behavior by reversing drug-induced changes in synaptic AMPA receptors in the IL, and this effect is abolished by blocking TrkB receptors during extinction, highlighting a potential mechanism for the therapeutic effects of VNS in addiction.

Regaining control over opioid use? The potential application of auricular transcutaneous vagus nerve stimulation to improve opioid treatment in China

Opioid use disorder (OUD) is a critical problem in China and is accompanied by depression and deficits in cognitive control. In China, the most successful intervention for OUD is the community drug rehabilitation where methadone maintenance treatment (MMT) plays a key role. Even though methadone for the treatment of OUD can be helpful, it can cause severe somatic side-effects, which limit its effectivity. Even worse, it can have detrimental effects on cognitive control, which is crucial to regain control over drug intake. Here, we consider the potential use of auricular transcutaneous vagus nerve stimulation (atVNS) as an addition to MMT for opioid withdrawal treatment. Compared to other non-invasive brain stimulation methods, atVNS also targets the locus coeruleus (LC) important for noradrenaline (NA) synthesis. NA is an essential neurotransmitter impacted in opioid withdrawal and also critically involved in cognitive control processes. Its ADD-ON to MMT might be a useful mean to improve mood and enhance cognitive control processes impacted in OUD. We discuss the translational advantages of atVNS in China such as the cultural acceptance of the modality of treatment similar to electroacupuncture. Additionally, the wearability of the ear electrode and at-home self-administration without intense medical supervision makes of atVNS a useful tool to enhance clinical and cognitive outcomes especially in everyday life situation. We discuss how atVNS can be integrated in tele-medical health approaches allowing that innovative treatments can widely be disseminated and continued even in situations of restricted medical access.

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.

Increasing coordination and responsivity of emotion-related brain regions with a heart rate variability biofeedback randomized trial

Heart rate variability is a robust biomarker of emotional well-being, consistent with the shared brain networks regulating emotion regulation and heart rate. While high heart rate oscillatory activity clearly indicates healthy regulatory brain systems, can increasing this oscillatory activity also enhance brain function? To test this possibility, we randomly assigned 106 young adult participants to one of two 5-week interventions involving daily biofeedback that either increased heart rate oscillations (Osc+ condition) or had little effect on heart rate oscillations (Osc- condition) and examined effects on brain activity during rest and during regulating emotion. While there were no significant changes in the right amygdala-medial prefrontal cortex (MPFC) functional connectivity (our primary outcome), the Osc+ intervention increased left amygdala-MPFC functional connectivity and functional connectivity in emotion-related resting-state networks during rest. It also increased down-regulation of activity in somatosensory brain regions during an emotion regulation task. The Osc- intervention did not have these effects. In this healthy cohort, the two conditions did not differentially affect anxiety, depression, or mood. These findings indicate that modulating heart rate oscillatory activity changes emotion network coordination in the brain.

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.

Acute Vagus Nerve Stimulation Facilitates Short Term Memory and Cognitive Flexibility in Rats

Vagus nerve stimulation (VNS) causes the release of several neuromodulators, leading to cortical activation and deactivation. The resulting preparatory cortical plasticity can be used to increase learning and memory in both rats and humans. The effects of VNS on cognition have mostly been studied either in animal models of different pathologies, and/or after extended VNS. Considerably less is known about the effects of acute VNS. Here, we examined the effects of acute VNS on short-term memory and cognitive flexibility in naïve rats, using three cognitive tasks that require comparatively brief (single session) training periods. In all tasks, VNS was delivered immediately before or during the testing phase. We used a rule-shifting task to test cognitive flexibility, a novel object recognition task to measure short-term object memory, and a delayed spontaneous alternation task to measure spatial short-term memory. We also analyzed exploratory behavior in an elevated plus maze to determine the effects of acute VNS on anxiety. Our results indicate that acute VNS can improve memory and cognitive flexibility relative to Sham-stimulation, and these effects are independent of unspecific VNS-induced changes in locomotion or anxiety.

Drugs and Bugs: The Gut-Brain Axis and Substance Use Disorders

Substance use disorders (SUDs) represent a significant public health crisis. Worldwide, 5.4% of the global disease burden is attributed to SUDs and alcohol use, and many more use psychoactive substances recreationally. Often associated with comorbidities, SUDs result in changes to both brain function and physiological responses. Mounting evidence calls for a precision approach for the treatment and diagnosis of SUDs, and the gut microbiome is emerging as a contributor to such disorders. Over the last few centuries, modern lifestyles, diets, and medical care have altered the health of the microbes that live in and on our bodies; as we develop, our diets and lifestyle dictate which microbes flourish and which microbes vanish. An increase in antibiotic treatments, with many antibiotic interventions occurring early in life during the microbiome's normal development, transforms developing microbial communities. Links have been made between the microbiome and SUDs, and the microbiome and conditions that are often comorbid with SUDs such as anxiety, depression, pain, and stress. A better understanding of the mechanisms influencing behavioral changes and drug use is critical in developing novel treatments for SUDSs. Targeting the microbiome as a therapeutic and diagnostic tool is a promising avenue of exploration. This review will provide an overview of the role of the gut-brain axis in a wide range of SUDs, discuss host and microbe pathways that mediate changes in the brain's response to drugs, and the microbes and related metabolites that impact behavior and health within the gut-brain axis.

Involvement of the Dorsal Vagal Complex in Alcohol-Related Behaviors

The neurobiological mechanisms that regulate the development and maintenance of alcohol use disorder (AUD) are complex and involve a wide variety of within and between systems neuroadaptations. While classic reward, preoccupation, and withdrawal neurocircuits have been heavily studied in terms of AUD, viable treatment targets from this established literature have not proven clinically effective as of yet. Therefore, examination of additional neurocircuitries not classically studied in the context of AUD may provide novel therapeutic targets. Recent studies demonstrate that various neuropeptides systems are important modulators of alcohol reward, seeking, and intake behaviors. This includes neurocircuitry within the dorsal vagal complex (DVC), which is involved in the control of the autonomic nervous system, control of intake of natural rewards like food, and acts as a relay of interoceptive sensory information via interactions of numerous gut-brain peptides and neurotransmitter systems with DVC projections to central and peripheral targets. DVC neuron subtypes produce a variety of neuropeptides and transmitters and project to target brain regions critical for reward such as the mesolimbic dopamine system as well as other limbic areas important for the negative reinforcing and aversive properties of alcohol withdrawal such as the extended amygdala. This suggests the DVC may play a role in the modulation of various aspects of AUD. This review summarizes the current literature on neurotransmitters and neuropeptides systems in the DVC (e.g., norepinephrine, glucagon-like peptide 1, neurotensin, cholecystokinin, thyrotropin-releasing hormone), and their potential relevance to alcohol-related behaviors in humans and rodent models for AUD research. A better understanding of the role of the DVC in modulating alcohol related behaviors may lead to the elucidation of novel therapeutic targets for drug development in AUD.

Looking beyond the opioid receptor: A desperate need for new treatments for opioid use disorder

The mainstay of treatment for opioid use disorder (OUD) is opioid agonist therapy (OAT), which modulates opioid receptors to reduce substance craving and use. OAT maintains dependence on opioids but helps reduce overdose and negative sequelae of substance abuse. Despite increasing availability of OAT, its effectiveness is limited by difficulty in initiating and maintaining patients on treatment. With the worsening opioid epidemic in the United States and rising overdose deaths, a more durable and effective treatment for OUD is necessary. This paper reviews novel treatments being investigated for OUD, including neuromodulatory interventions, psychedelic drugs, and other novel approaches. Neuromodulatory interventions can stimulate the addiction neural circuitry involving the dorsolateral prefrontal cortex and deeper mesolimbic structures to curb craving and reduce use, and multiple clinical trials for interventional treatment for OUD are currently conducted. Similarly, psychedelic agents are being investigated for efficacy in OUD specifically. There is a resurgence of interest in psychedelic agents' therapeutic potential, with evidence of improving mood symptoms and decreased substance use even after just one dose. Exact mechanism of their anti-addictive effect is not fully elucidated, but psychedelic agents do not maintain opioid dependence and some may even be helpful in abating symptoms of withdrawal. Other potential approaches for OUD include targeting different parts of the dopamine-dependent addiction pathway, identifying susceptible genes and modulating gene products, as well as utilizing vaccines as immunotherapy to blunt the addictive effects of substances. Much more clinical data are needed to support efficacy and safety of these therapies in OUD, but these proposed novel treatments look beyond the opioid receptor to offer hope for a more durably effective OUD treatment.

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.

Local activation of α2 adrenergic receptors is required for vagus nerve stimulation induced motor cortical plasticity

Vagus nerve stimulation (VNS) paired with rehabilitation training is emerging as a potential treatment for improving recovery of motor function following stroke. In rats, VNS paired with skilled forelimb training results in significant reorganization of the somatotopic cortical motor map; however, the mechanisms underlying this form of VNS-dependent plasticity remain unclear. Recent studies have shown that VNS-driven cortical plasticity is dependent on noradrenergic innervation of the neocortex. In the central nervous system, noradrenergic α2 receptors (α2-ARs) are widely expressed in the motor cortex and have been critically implicated in synaptic communication and plasticity. In current study, we examined whether activation of cortical α2-ARs is necessary for VNS-driven motor cortical reorganization to occur. Consistent with previous studies, we found that VNS paired with motor training enlarges the map representation of task-relevant musculature in the motor cortex. Infusion of α2-AR antagonists into M1 blocked VNS-driven motor map reorganization from occurring. Our results suggest that local α2-AR activation is required for VNS-induced cortical reorganization to occur, providing insight into the mechanisms that may underlie the neuroplastic effects of VNS therapy.

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.

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.

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.

Interoception and alcohol addiction: Vagotomy induces long-lasting suppression of relapse-type behavior

Drug addictions are chronic mental disorders characterized by compulsive drug seeking and drug use, despite their negative consequences. It is a priority to find therapeutic alternatives to prevent relapse, as there are still no treatments that can ensure abstinence. One of the neural systems implicated in the appearance of the states of discomfort that motivate relapse is the interoceptive system, which oversees our internal body states. However, less attention has been given to the peripheral components of the interoceptive system and their role in addictions. Within these pathways, the vagus nerve represents one of the main visceral afferents of the interoceptive system. We hypothesized that the interruption of visceral afferent pathways would decrease the motivational effects of the drug, thereby either decreasing or preventing drug cravings. To test this idea, we used rats of a high-alcohol-drinking line and measured the effect that vagus nerve resection had on the relapse-like alcohol drinking, expressed as the alcohol deprivation effect, a phenomenon that has been linked to addiction-related events such as alcohol cravings. We found that even though vagotomy completely eliminates the effect of alcohol deprivation, it has no impact on water consumption or animal weight. These results give us valuable information about the relationship between the autonomic nervous system and alcohol use disorders and allow us to propose new clinical research that might have translational options.

Transcranial magnetic stimulation, deep brain stimulation, and other forms of neuromodulation for substance use disorders: Review of modalities and implications for treatment

Given the high prevalence of individuals diagnosed with substance use disorder, along with the elevated rate of relapse following treatment initiation, investigating novel approaches and new modalities for substance use disorder treatment is of vital importance. One such approach involves neuromodulation which has been used therapeutically for neurological and psychiatric disorders and has demonstrated positive preliminary findings for the treatment of substance use disorder. The following article provides a review of several forms of neuromodulation which warrant consideration as potential treatments for substance use disorder. PubMed, PsycINFO, Ovid MEDLINE, and Web of Science were used to identify published articles and clinicaltrials.gov was used to identify currently ongoing or planned studies. Search criteria for Brain Stimulation included the following terminology: transcranial direct current stimulation, transcranial magnetic stimulation, theta burst stimulation, deep brain stimulation, vagus nerve stimulation, trigeminal nerve stimulation, percutaneous nerve field stimulation, auricular nerve stimulation, and low intensity focused ultrasound. Search criteria for Addiction included the following terminology: addiction, substance use disorder, substance-related disorder, cocaine, methamphetamine, amphetamine, alcohol, nicotine, tobacco, smoking, marijuana, cannabis, heroin, opiates, opioids, and hallucinogens. Results revealed that there are currently several forms of neuromodulation, both invasive and non-invasive, which are being investigated for the treatment of substance use disorder. Preliminary findings have demonstrated the potential of these various neuromodulation techniques in improving substance treatment outcomes by reducing those risk factors (e.g. substance craving) associated with relapse. Specifically, transcranial magnetic stimulation has shown the most promise with several well-designed studies supporting the potential for reducing substance craving. Deep brain stimulation has also shown promise, though lacks well-controlled clinical trials to support its efficacy. Transcranial direct current stimulation has also demonstrated promising results though consistently designed, randomized trials are also needed. There are several other forms of neuromodulation which have not yet been investigated clinically but warrant further investigation given their mechanisms and potential efficacy based on findings from other studied indications. In summary, given promising findings in reducing substance use and craving, neuromodulation may provide a non-pharmacological option as a potential treatment and/or treatment augmentation for substance use disorder. Further research investigating neuromodulation, both alone and in combination with already established substance use disorder treatment (e.g. medication treatment), warrants consideration.

Extinction Learning as a Potential Mechanism Linking High Vagal Tone with Lower PTSD Symptoms among Abused Youth

Childhood abuse is a potent risk factor for psychopathology, including posttraumatic stress disorder (PTSD). Research has shown high resting vagal tone, a measure of parasympathetic nervous system function, protects abused youth from developing internalizing psychopathology, but potential mechanisms explaining this effect are unknown. We explored fear extinction learning as a possible mechanism underlying the protective effect of vagal tone on PTSD symptoms among abused youth. We measured resting respiratory sinus arrhythmia (RSA) and skin conductance responses (SCR) during a fear conditioning and extinction task in youth with variability in abuse exposure (N = 94; aged 6-18 years). High RSA predicted lower PTSD symptoms and enhanced extinction learning among abused youths. In a moderated-mediation model, extinction learning mediated the association of abuse with PTSD symptoms only among youth with high RSA. These findings highlight extinction learning as a possible mechanism linking high vagal tone to decreased risk for PTSD symptoms among abused youth.

Electrical stimulation of cranial nerves in cognition and disease

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

Rapid and transient enhancement of thalamic information transmission induced by vagus nerve stimulation

OBJECTIVE

Vagus nerve stimulation (VNS) has been FDA-approved as a long-term, therapeutic treatment for multiple disorders, including pharmacoresistant epilepsy and depression. Here we elucidate the short-term effects of VNS on sensory processing.

APPROACH

We employed an information theoretic approach to examine the effects of VNS on thalamocortical transmission of sensory-related information along the somatosensory pathway.

MAIN RESULTS

We found that VNS enhanced the selectivity of the response of thalamic neurons to specific kinetic features in the stimuli, resulting in a significant increase in the efficiency and rate of stimulus-related information conveyed by thalamic spikes. VNS-induced improvements in thalamic sensory processing coincided with a decrease in thalamic burst firing. Importantly, we found VNS-induced enhancement of sensory processing had a rapid onset and offset, completely disappearing one minute after cessation of VNS. The timescales of these effects indicate against an underlying mechanism involving long-term neuroplasticity. We found several patterns of VNS (tonic, standard duty-cycle, and fast duty-cycle) all induced similar improvements in sensory processing. Under closer inspection we noticed that due to the fast timescale of VNS effects on sensory processing, standard duty-cycle VNS induced a fluctuating sensory processing state which may be sub-optimal for perceptual behavior. Fast duty-cycle VNS and continuous, tonic VNS induced quantitatively similar improvements in thalamic information transmission as standard duty-cycle VNS without inducing a fluctuating thalamic state. Further, we found the strength of VNS-induced improvements in sensory processing increased monotonically with amplitude and frequency of VNS.

SIGNIFICANCE

These results demonstrate, for the first time, the feasibility of utilizing specific patterns of VNS to rapidly improve sensory processing and confirm fast duty-cycle and tonic patterns as optimal for this purpose, while showing standard duty-cycle VNS causes non-optimal fluctuations in thalamic state.

The role of the gut microbiome in opioid use

Although the gut and brain are separate organs, they communicate with each other via trillions of intestinal bacteria that collectively make up one's gut microbiome. Findings from both humans and animals support a critical role of gut microbes in regulating brain function, mood, and behavior. Gut bacteria influence neural circuits that are notably affected in addiction-related behaviors. These include circuits involved in stress, reward, and motivation, with substance use influencing gut microbial abnormalities, suggesting significant gut-brain interactions in drug addiction. Given the overwhelming rates of opioid overdose deaths driven by abuse and addiction, it is essential to characterize mechanisms mediating the abuse potential of opioids. We discuss in this review the role of gut microbiota in factors that influence opioid addiction, including incentive salience, reward, tolerance, withdrawal, stress, and compromised executive function. We present clinical and preclinical evidence supporting a bidirectional relationship between gut microbiota and opioid-related behaviors by highlighting the effects of opioid use on gut bacteria, and the effects of gut bacteria on behavioral responses to opioids. Further, we discuss possible mechanisms of this gut-brain communication influencing opioid use. By clarifying the relationship between the gut microbiome and opioid-related behaviors, we improve understanding on mechanisms mediating reward-, motivation-, and stress-related behaviors and disorders, which may contribute to the development of effective, targeted therapeutic interventions in opioid dependence and addiction.

Vagus nerve stimulation during extinction learning reduces conditioned place preference and context-induced reinstatement of cocaine seeking

BACKGROUND

Drug use causes the formation of strong cue/reward associations which persist long after cessation of drug-taking and contribute to the long-term risk of relapse. Extinguishing these associations may reduce cue-induced craving and relapse. Previously, we found that pairing vagus nerve stimulation (VNS) with extinction of cocaine self-administration reduces cue-induced reinstatement; however, it remains unclear whether this was primarily caused by extinguishing the context, the instrumental response, or both.

OBJECTIVE

Hypothesis: We hypothesized that VNS can facilitate the extinction of both contextual cues and instrumental responding.

METHODS

Extinction of context was first tested using Pavlovian conditioned place preference (CPP). Next, the impact of VNS on the extinction of instrumental responding was assessed under ABA and AAA context conditions. In each extinction context separate groups of rats were either provided the opportunity to perform the instrumental response, or the levers were retracted for the duration of extinction training. Reinstatement was induced by reintroduction of the conditioned stimuli and/or the drug-paired context. Data were analyzed using one-way or two-way repeated measures ANOVAs.

RESULTS

VNS during extinction reduced reinstatement of CPP. VNS also reduced cue- and context-induced reinstatement of the instrumental response under both AAA and ABA conditions. The subjects' ability to engage with the lever during extinction was crucial for this effect. P values < 0.05 were considered significant.

CONCLUSIONS

Craving occurs in response to a range of conditioned stimuli and contexts; VNS may improve outcomes of behavioral therapy by facilitating extinction of both an instrumental response and/or contextual cues.

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

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

A potential role for the gut microbiome in substance use disorders

Pathological substance use disorders represent a major public health crisis with limited effective treatment options. While much work has been done to understand the neuronal signaling networks and intracellular signaling cascades associated with prolonged drug use, these studies have yielded few successful treatment options for substance use disorders. In recent years, there has been a growing interest to explore interactions between the peripheral immune system, the gut microbiome, and the CNS. In this review, we will present a summary of existing evidence, suggesting a potential role for gut dysbiosis in the pathogenesis of substance use disorders. Clinical evidence of gut dysbiosis in human subjects with substance use disorder and preclinical evidence of gut dysbiosis in animal models of drug addiction are discussed in detail. Additionally, we examine how changes in the gut microbiome and its metabolites may not only be a consequence of substance use disorders but may in fact play a role in mediating behavioral response to drugs of abuse. While much work still needs to be done, understanding the interplay of gut microbiome in substance use disorders may offer a promising avenue for future therapeutic development.

Role of prefrontal cortex in the extinction of drug memories

It has been recognized that drug addiction engages aberrant process of learning and memory, and substantial studies have focused on developing effective treatment to erase the enduring drug memories to reduce the propensity to relapse. Extinction, a behavioral intervention exposing the individuals to the drug-associated cues repeatedly, can weaken the craving and relapse induced by drug-associated cues, but its clinic efficacy is limited. A clear understanding of the neuronal circuitry and molecular mechanism underlying extinction of drug memory will facilitate the successful use of extinction therapy in clinic. As a key component of mesolimbic system, medial prefrontal cortex (mPFC) has received particular attention largely in that PFC stands at the core of neural circuits for memory extinction and manipulating mPFC influences extinction of drug memories and subsequent relapse. Here, we review the recent advances in both animal models of drug abuse and human addicted patients toward the understanding of the mechanistic link between mPFC and drug memory, with particular emphasis on how mPFC contributes to the extinction of drug memory at levels ranging from neuronal architecture, synaptic plasticity to molecular signaling and epigenetic regulation, and discuss the clinic relevance of manipulating the extinction process of drug memory to prevent craving and relapse through enhancing mPFC function.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Neuromodulation for substance addiction in human subjects: A review

Substance abuse is one of the most prevalent and costly health problems in the world today. Standard medical therapy is often not curative, and relapse is common. Research over the past several decades on the neural underpinnings of addiction has implicated a network of structures within the brain shown to be altered in patients with substance abuse. The field of neuromodulation aims to utilize this knowledge to treat dysfunctional circuits by targeting and modulating specific brain circuits. While invasive neuromodulation such as DBS and VNS have proven to be effective in treating movement disorders, OCD and epilepsy, there is increasing interest and data with regards to its potential application for the treatment of severe, intractable addiction. Several neuromodulatory techniques and brain targets are currently under investigation in patients with various substance abuse disorders. This review aims to summarize the current state of evidence for neurosurgical neuromodulation as a therapy for substance abuse and addiction, and to provide additional expert opinions as to the obstacles and future directions of this endeavor.

Reinstatement of contextual conditioned anxiety in virtual reality and the effects of transcutaneous vagus nerve stimulation in humans

Since exposure therapy for anxiety disorders incorporates extinction of contextual anxiety, relapses may be due to reinstatement processes. Animal research demonstrated more stable extinction memory and less anxiety relapse due to vagus nerve stimulation (VNS). We report a valid human three-day context conditioning, extinction and return of anxiety protocol, which we used to examine effects of transcutaneous VNS (tVNS). Seventy-five healthy participants received electric stimuli (unconditioned stimuli, US) during acquisition (Day1) when guided through one virtual office (anxiety context, CTX+) but never in another (safety context, CTX-). During extinction (Day2), participants received tVNS, sham, or no stimulation and revisited both contexts without US delivery. On Day3, participants received three USs for reinstatement followed by a test phase. Successful acquisition, i.e. startle potentiation, lower valence, higher arousal, anxiety and contingency ratings in CTX+ versus CTX-, the disappearance of these effects during extinction, and successful reinstatement indicate validity of this paradigm. Interestingly, we found generalized reinstatement in startle responses and differential reinstatement in valence ratings. Altogether, our protocol serves as valid conditioning paradigm. Reinstatement effects indicate different anxiety networks underlying physiological versus verbal responses. However, tVNS did neither affect extinction nor reinstatement, which asks for validation and improvement of the stimulation protocol.

Vagal Nerve Stimulation for Treatment-Resistant Depression

Major depressive disorder (MDD) is prevalent. Although standards antidepressants are more effective than placebo, up to 35% of patients do not respond to 4 or more conventional treatments and are considered to have treatment-resistant depression (TRD). Considerable effort has been devoted to trying to find effective treatments for TRD. This review focuses on vagus nerve stimulation (VNS), approved for TRD in 2005 by the Food and Drugs Administration. Stimulation is carried by bipolar electrodes on the left cervical vagus nerve, which are attached to an implanted stimulator generator. The vagus bundle contains about 80% of afferent fibers terminating in the medulla, from which there are projections to many areas of brain, including the limbic forebrain. Various types of brain imaging studies reveal widespread functional effects in brain after either acute or chronic VNS. Although more randomized control trials of VNS need to be carried out before a definitive conclusion can be reached about its efficacy, the results of open studies, carried out over period of 1 to 2 years, show much more efficacy when compared with results from treatment as usual studies. There is an increase in clinical response to VNS between 3 and 12 months, which is quite different from that seen with standard antidepressant treatment of MDD. Preclinically, VNS affects many of the same brain areas, neurotransmitters (serotonin, norepinephrine) and signal transduction mechanisms (brain-derived neurotrophic factor-tropomyosin receptor kinase B) as those found with traditional antidepressants. Nevertheless, the mechanisms by which VNS benefits patients nonresponsive to conventional antidepressants is unclear, with further research needed to clarify this.