Vagus nerve stimulation (VNS) is effective in a rat model of antidepressant action

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.

Investigation of non-invasive focused ultrasound efficacy on depressive-like behavior in hemiparkinsonian rats

Depression is a common non-motor symptom in Parkinson's disease (PD) that includes anhedonia and impacts quality of life but is not effectively treated with conventional antidepressants clinically. Vagus nerve stimulation improves treatment-resistant depression in the general population, but research about its antidepressant efficacy in PD is limited. Here, we administered peripheral non-invasive focused ultrasound to hemiparkinsonian ('PD') and non-parkinsonian (sham) rats to mimic vagus nerve stimulation and assessed its antidepressant-like efficacy. Following 6-hydroxydopamine (6-OHDA) lesion, akinesia-like immobility was assessed in the limb-use asymmetry test, and despair- and anhedonic-like behaviors were evaluated in the forced swim test and sucrose preference test, respectively. After, tyrosine hydroxylase immuno-staining was employed to visualize and quantify dopaminergic degeneration in the substantia nigra pars compacta, ventral tegmental area, and striatum. We found that PD rats exhibited akinesia-like immobility and > 90% reduction in tyrosine hydroxylase immuno-staining ipsilateral to the lesioned side. PD rats also demonstrated anhedonic-like behavior in the sucrose preference test compared to sham rats. No 6-OHDA lesion effect on immobility in the forced swim test limited conclusions about the efficacy of ultrasound on despair-like behavior. However, ultrasound improved anhedonic-like behavior in PD rats and this efficacy was sustained through the end of the 1-week recovery period. The greatest number of animals demonstrating increased sucrose preference was in the PD group receiving ultrasound. Our findings here are the first to posit that peripheral non-invasive focused ultrasound to the celiac plexus may improve anhedonia in PD with further investigation needed to reveal its potential for clinical applicability.

Fractal Microelectrodes for More Energy-Efficient Cervical Vagus Nerve Stimulation

Vagus nerve stimulation (VNS) has the potential to treat various peripheral dysfunctions, but the traditional cuff electrodes for VNS are susceptible to off-target effects. Microelectrodes may enable highly selective VNS that can mitigate off-target effects, but they suffer from the increased impedance. Recent studies on microelectrodes with non-Euclidean geometries have reported higher energy efficiency in neural stimulation applications. These previous studies use electrodes with mm/cm-scale dimensions, mostly targeted for myelinated fibers. This study evaluates fractal microelectrodes for VNS in a rodent model (N = 3). A thin-film device with fractal and circle microelectrodes is fabricated to compare their neural stimulation performance on the same radial coordinate of the nerve. The results show that fractal microelectrodes can activate C-fibers with up to 52% less energy (p = 0.012) compared to circle microelectrodes. To the best of the knowledge, this work is the first to demonstrate a geometric advantage of fractal microelectrodes for VNS in vivo.

Vagus nerve-dependent effects of fluoxetine on anxiety- and depression-like behaviors in mice

The vagus nerve is a major pathway in the body that is responsible for regulating the activity of the parasympathetic nervous system, which plays an important role in mood disorders including anxiety and depression. Fluoxetine, also known as Prozac, is widely used to treat depression. Nevertheless, there are few studies on the vagus nerve-mediated action of fluoxetine. In this study, we aimed to investigate the vagus nerve-dependent actions of fluoxetine in mice with restraint stress-induced or antibiotics-induced anxiety- and depression-like behaviors. Compared to sham operation, vagotomy alone did not exhibit significant effects on behavioral changes and serotonin-related biomarkers in mice not exposed to stress, antibiotics, or fluoxetine. Oral administration of fluoxetine significantly alleviated anxiety- and depression-like behaviors. However, celiac vagotomy significantly attenuated the anti-depressive effects of fluoxetine. The vagotomy also inhibited the effect of fluoxetine to attenuate restraint stress- or cefaclor-induced reduction in serotonin levels and Htr1a mRNA expression in the hippocampus. These findings suggest that the vagus nerve may regulate the efficacy of fluoxetine for depression.

The Use of the Velocity Selective Recording Technique to Reveal the Excitation Properties of the Ulnar Nerve in Pigs

Decoding information from the peripheral nervous system via implantable neural interfaces remains a significant challenge, considerably limiting the advancement of neuromodulation and neuroprosthetic devices. The velocity selective recording (VSR) technique has been proposed to improve the classification of neural traffic by combining temporal and spatial information through a multi-electrode cuff (MEC). Therefore, this study investigates the feasibility of using the VSR technique to characterise fibre type based on the electrically evoked compound action potentials (eCAP) propagating along the ulnar nerve of pigs in vivo. A range of electrical stimulation parameters (amplitudes of 50 μA-10 mA and pulse durations of 100 μs, 500 μs, 1000 μs, and 5000 μs) was applied on a cutaneous and a motor branch of the ulnar nerve in nine Danish landrace pigs. Recordings were made with a 14 ring MEC and a delay-and-add algorithm was used to convert the eCAPs into the velocity domain. The results revealed two fibre populations propagating along the cutaneous branch of the ulnar nerve, with mean velocities of 55 m/s and 21 m/s, while only one dominant fibre population was found for the motor branch, with a mean velocity of 63 m/s. Because of its simplicity to provide information on the fibre selectivity and direction of propagation of nerve fibres, VSR can be implemented to advance the performance of the bidirectional control of neural prostheses and bioelectronic medicine applications.

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.

Identification of SSRI-evoked antidepressant sensory signals by decoding vagus nerve activity

The vagus nerve relays mood-altering signals originating in the gut lumen to the brain. In mice, an intact vagus is required to mediate the behavioural effects of both intraluminally applied selective serotonin reuptake inhibitors and a strain of Lactobacillus with antidepressant-like activity. Similarly, the prodepressant effect of lipopolysaccharide is vagus nerve dependent. Single vagal fibres are broadly tuned to respond by excitation to both anti- and prodepressant agents, but it remains unclear how neural responses encode behaviour-specific information. Here we demonstrate using ex vivo experiments that for single vagal fibres within the mesenteric neurovascular bundle supplying the mouse small intestine, a unique neural firing pattern code is common to both chemical and bacterial vagus-dependent antidepressant luminal stimuli. This code is qualitatively and statistically discernible from that evoked by lipopolysaccharide, a non-vagus-dependent antidepressant or control non-antidepressant Lactobacillus strain and are not affected by sex status. We found that all vagus dependent antidepressants evoked a decrease in mean spike interval, increase in spike burst duration, decrease in gap duration between bursts and increase in intra-burst spike intervals. Our results offer a novel neuronal electrical perspective as one explanation for mechanisms of action of gut-derived vagal dependent antidepressants. We expect that our ex vivo individual vagal fibre recording model will improve the design and operation of new, extant electroceutical vagal stimulation devices currently used to treat major depression. Furthermore, use of this vagal antidepressant code should provide a valuable screening tool for novel potential oral antidepressant candidates in preclinical animal models.

Vagus Nerve Stimulation Transiently Mitigates Chemotherapy-Induced Peripheral Neuropathy in Rats

Background

Chemotherapy-induced peripheral neuropathy is a severe side effect of chemotherapeutic agents. Vagus nerve stimulation attenuates neuroinflammation by activating the cholinergic anti-inflammatory pathway and thus may attenuate CIPN.

Methods

Adult male Sprague-Dawley rats received intraperitoneal paclitaxel injection (2 mg/kg) every other day for a total of 4 injections. Three weeks later, the left cervical vagus nerve was exposed under general anesthesia, and the rats randomly received 20-min stimulation (1 V, 2 ms, 5 Hz, 30 s ON/5 min OFF) or sham stimulation. Heat and mechanical pain sensitivity was evaluated using Hargreaves and von Frey tests before and after treatment (n=12 per group per time point). Additionally, rats receiving paclitaxel or saline but no surgery were included. Expression of representative pro- and anti-inflammatory cytokines in dorsal root ganglia was assessed by Western blotting assays and immunohistochemistry.

Results

Paclitaxel significantly reduced the sensitivity for heat (withdrawal latency: paclitaxel 6.16 ± 0.54 s vs saline 9.93 ± 0.78 s, p<0.001) and mechanical pain (withdrawal frequency: paclitaxel 32.22 ± 15.51% vs saline 3.33 ± 4.92%, p<0.001). Compared with sham-stimulated rats, rats receiving vagus nerve stimulation had significantly higher sensitivity for heat (withdrawal latency: VNS 10.28 ± 1.15 s vs sham 6.27 ± 0.56 s, p<0.001) and mechanical pain (withdrawal frequency: VNS 10.00 ± 9.54% vs Sham 31.67 ± 18.99%, p=0.003) on +1 day, but not 7 days later (withdrawal latency: VNS 6.97 ± 1.13 s vs Sham 6.23 ± 0.79 s, p=0.080; withdrawal frequency: VNS 21.67 ± 11.93% vs Sham 23.33 ± 7.79%, p=0.689). Western blotting assays and immunohistochemistry revealed that interleukin-10 level was elevated in the dorsal root ganglia of rats receiving vagus nerve stimulation while no apparent changes in NF-κB or TNF-α levels were observed.

Conclusion

Vagus nerve stimulation could transiently attenuate paclitaxel-induced hyperalgesia in rats. Future studies are needed to investigate whether stimulation with different protocols could achieve durable effects.

Can selective serotonin reuptake inhibitors have a neuroprotective effect during COVID-19?

The absence of a specific treatment for SARS-CoV-2 infection led to an intense global effort in order to find new therapeutic interventions and improve patient outcomes. One important feature of COVID-19 pathophysiology is the activation of immune cells, with consequent massive production and release of inflammatory mediators that may cause impairment of several organ functions, including the brain. In addition to its classical role as a neurotransmitter, serotonin (5-hydroxytryptamine, 5-HT) has immunomodulatory properties, downregulating the inflammatory response by central and peripheral mechanisms. In this review, we describe the roles of 5-HT in the regulation of systemic inflammation and the potential benefits of the use of specific serotonin reuptake inhibitors as a coadjutant therapy to attenuate neurological complications of COVID-19.

Advances in understanding mechanisms and therapeutic targets to treat comorbid depression and cardiovascular disease

Chronic or repeated social stress exposure often precipitates the onset of depression and cardiovascular disease (CVD). Despite a clear clinical association between CVD and depression, the pathophysiology underlying these comorbid conditions is unclear. Chronic exposure to social stress can lead to immune system dysregulation, mitochondrial dysfunction, and vagal withdrawal. Further, regular physical exercise is well-known to exert cardioprotective effects, and accumulating evidence demonstrates the antidepressant effect of exercise. This review explores the contribution of inflammation, mitochondrial dysfunction, and vagal withdrawal to stress-induced depression and CVD. Evidence for therapeutic benefits of exercise, anti-inflammatory therapies, and vagus nerve stimulation are also reviewed. Benefits of targeted therapeutics of mitochondrial agents, anti-inflammatory therapies, and vagus nerve stimulation are discussed. Importantly, the ability of exercise to impact each of these factors is also reviewed. The current findings described here implicate a new direction for research, targeting the shared mechanisms underlying comorbid depression-CVD. This will guide the development of novel therapeutic strategies for the prevention and treatment of these stress-related pathologies, particularly within treatment-resistant populations.

The Long and Winding Road of Vagus Nerve Stimulation: Challenges in Developing an Intervention for Difficult-to-Treat Mood Disorders

PURPOSE

The Vagus Nerve Stimulation (VNS) Therapy System has been studied for more than 20 years in patients with severe, treatment-resistant, chronic mood disorder, i.e., difficult-to-treat depression (DTD). This review distills some of the implications of this research for future therapeutic trials in this population.

METHODS

A narrative review is provided on VNS in DTD. Protocols for a new, large, sham-controlled trial and a global, longitudinal observational study are described.

RESULTS

Following encouraging results in open studies, a randomized, masked, sham-controlled trial of VNS for DTD failed to demonstrate an effect on the primary outcome. The negative results may have been partly due to inadequate treatment duration (10 weeks). In long-term observational studies, adjunctive VNS, combined with treatment-as-usual (VNS+TAU), was administered to more than 1100 DTD patients and compared with TAU alone in more than 400 patients. VNS+TAU had superior antidepressant effects, but maximal symptom reduction was often observed after 12 months or longer of stimulation. VNS+TAU had also marked superiority in durability of benefit. Sustained levels of symptom reduction below the traditional cutoff for response (i.e. < 50%) were associated with improved quality of life.

LIMITATIONS

Most comparisons of VNS+TAU and TAU were derived from observational, open label studies.

CONCLUSION

The history of VNS in DTD has implications for interventional studies in this population, and perhaps other chronic medical disorders. The slow onset of benefit with VNS necessitates considerably longer controlled observation periods to establish efficacy. Durability of benefit should be routinely incorporated in efficacy assessment. New outcome metrics are needed to both categorically identify clinically meaningful benefit and to integrate information on symptom burden over time.

Oral selective serotonin reuptake inhibitors activate vagus nerve dependent gut-brain signalling

The vagus nerve can transmit signals to the brain resulting in a reduction in depressive behavior as evidenced by the long-term beneficial effects of electrical stimulation of the vagus in patients with intractable depression. The vagus is the major neural connection between gut and brain, and we have previously shown that ingestion of beneficial bacteria modulates behaviour and brain neurochemistry via this pathway. Given the high levels of serotonin in the gut, we considered if gut-brain signaling, and specifically the vagal pathway, might contribute to the therapeutic effect of oral selective serotonin reuptake inhibitors (SSRI). Mesenteric nerve recordings were conducted in mice after treatment with SSRI to ascertain if this class of drugs resulted in increased vagal excitability. Patch clamp recordings of enteric neurons were carried out to measure activity of primary afferent neurons in the gut in response to SSRI and to assess the importance of gut epithelium in transducing signal. The tail suspension test (TST) was used following 14d feeding of SSRI in vagotomised and surgical sham mice to measure depressive-like behaviour. Brain mRNA expression was examined via PCR and the intestinal microbiome was assessed. Mesenteric nerve recordings in BALB/c mice demonstrated that oral treatment with SSRI leads to a significant increase in vagal activity. This effect was not observed in mice treated with a representative noradrenaline-dopamine reuptake inhibitor. It is known that signals from the gut can be transmitted to the vagus via the enteric nervous system. Exposure of the gut to SSRI increased the excitability of intrinsic primary afferent neurons in the myenteric plexus, through an intestinal epithelium dependent mechanism, and alpha-diversity of gut microbiota was altered. Critically, blocking vagal signaling from gut to brain, via subdiaphragmatic vagotomy, abolished the antidepressive effects of oral SSRI treatment as determined by the tail suspension test. This work suggests that vagus nerve dependent gut-brain signaling contributes to the effects of oral SSRI and further, highlights the potential for pharmacological approaches to treatment of mood disorders that focus on vagal stimulation and may not even require therapeutic agents to enter the circulation.

Exercise activates vagal induction of dopamine and attenuates systemic inflammation

Physical exercise is one of the most important factors improving quality of life, but it is not feasible for patients with morbidity or limited mobility. Most previous studies focused on high-intensity or long-term exercise that causes metabolic stress or physiological adaption, respectively. Here, we studied how moderate-intensity swimming affects systemic inflammation in 6-8 week old C57BL/6J male mice during endotoxemia. One-hour swimming prevented hypokalemia, hypocalcemia, attenuated serum levels of inflammatory cytokines, increased anti-inflammatory cytokines but affected neither IL6 nor glycemia before or after the endotoxic challenge. Exercise attenuated serum TNF levels by inhibiting its production in the spleen through a mechanism mediated by the subdiaphragmatic vagus nerve but independent of the splenic nerve. Exercise increased serum levels of dopamine, and adrenalectomy prevented the potential of exercise to induce dopamine and to attenuate serum TNF levels. Dopaminergic agonist type-1, fenoldopam, inhibited TNF production in splenocytes. Conversely, dopaminergic antagonist type-1, butaclamol, attenuated exercise control of serum TNF levels. These results suggest that vagal induction of dopamine may contribute to the anti-inflammatory potential of physical exercise.

Medial septal cholinergic mediation of hippocampal theta rhythm induced by vagal nerve stimulation

BACKGROUND

Electrical vagal nerve stimulation (VNS) has been used for years to treat patients with drug-resistant epilepsy. This technique also remains under investigation as a specific treatment of patients with Alzheimer's disease. Recently we discovered that VNS induced hippocampal formation (HPC) type II theta rhythm, which is involved in memory consolidation. In the present study, we have extended our previous observation and addressed the neuronal substrate and pharmacological profile of HPC type II theta rhythm induced by VNS in anesthetized rats.

METHODS

Male Wistar rats were implanted with a VNS cuff electrode around the left vagus nerve, a tungsten microelectrode for recording the HPC field activity, and a medial septal (MS) cannula for the injection of a local anesthetic, procaine, and muscarinic agents. A direct, brief effect of VNS on the HPC field potential was evaluated before and after medial-septal drug injection.

RESULTS

Medial septal injection of local anesthetic, procaine, reversibly abolished VNS-induced HPC theta rhythm. With the use of cholinergic muscarinic agonist and antagonists, we demonstrated that medial septal M1 receptors are involved in the mediation of the VNS effect on HPC theta field potential.

CONCLUSION

The MS cholinergic M1 receptor mechanism integrates not only central inputs from the brainstem synchronizing pathway, which underlies the production of HPC type II theta rhythm, but also the input from the vagal afferents in the brain stem.

Can prebiotics assist in the management of cognition and weight gain in schizophrenia?

Schizophrenia is among the top half of the 25 leading causes of disabilities worldwide with a 10-20 year decrease in life expectancy. Ineffective pharmacotherapy in the management of cognitive deficits and weight gain are known to be significant contributors; therefore interventions that may mitigate one, or both, of these parameters would be highly beneficial. Manipulation of the gut microbiome using dietary supplements such as prebiotics may be one such intervention. Preclinical studies have shown that a 2-4 week dietary supplementation with a prebiotic has beneficial effects on learning and memory, and prevents pro-inflammatory signals that are detrimental to cognitive processes. Furthermore, prebiotics influence metabolism, and in obesity they increase the expression of anorexigenic gut hormones such as peptide tyrosine tyrosine, glucagon-like peptide 1 and leptin, as well as decrease levels of orexigenic hormones such as ghrelin. Despite compelling evidence for the pro-cognitive and neuroprotective effects of prebiotics in rodents, their ability to alleviate cognitive deficits or enhance cognition needs to be evaluated in humans. Here we suggest that important symptoms associated with schizophrenia, such as cognitive impairment and weight gain, may benefit from concurrent prebiotic therapy.

The development of neural stimulators: a review of preclinical safety and efficacy studies

OBJECTIVE

Given the rapid expansion of the field of neural stimulation and the rigorous regulatory approval requirements required before these devices can be applied clinically, it is important that there is clarity around conducting preclinical safety and efficacy studies required for the development of this technology.

APPROACH

The present review examines basic design principles associated with the development of a safe neural stimulator and describes the suite of preclinical safety studies that need to be considered when taking a device to clinical trial.

MAIN RESULTS

Neural stimulators are active implantable devices that provide therapeutic intervention, sensory feedback or improved motor control via electrical stimulation of neural or neuro-muscular tissue in response to trauma or disease. Because of their complexity, regulatory bodies classify these devices in the highest risk category (Class III), and they are therefore required to go through a rigorous regulatory approval process before progressing to market. The successful development of these devices is achieved through close collaboration across disciplines including engineers, scientists and a surgical/clinical team, and the adherence to clear design principles. Preclinical studies form one of several key components in the development pathway from concept to product release of neural stimulators. Importantly, these studies provide iterative feedback in order to optimise the final design of the device. Key components of any preclinical evaluation include: in vitro studies that are focussed on device reliability and include accelerated testing under highly controlled environments; in vivo studies using animal models of the disease or injury in order to assess efficacy and, given an appropriate animal model, the safety of the technology under both passive and electrically active conditions; and human cadaver and ex vivo studies designed to ensure the device's form factor conforms to human anatomy, to optimise the surgical approach and to develop any specialist surgical tooling required.

SIGNIFICANCE

The pipeline from concept to commercialisation of these devices is long and expensive; careful attention to both device design and its preclinical evaluation will have significant impact on the duration and cost associated with taking a device through to commercialisation. Carefully controlled in vitro and in vivo studies together with ex vivo and human cadaver trials are key components of a thorough preclinical evaluation of any new neural stimulator.

Male rats with same-sex preference show higher immobility in the forced swim test, but similar effects of fluoxetine and desipramine than males that prefer females

Sex preference in male rats is partly determined by the organizational action of estradiol. Thus, several paradigms have used aromatase inhibitors to manipulate sex preference. We recently showed that a subpopulation of male rats prenatally treated with letrozole (0.56 μg/kg, G10-G22), a non-steroidal third generation aromatase inhibitor, had same-sex preference, female sexual behavior (including lordosis and proceptivity) and penile erections when exposed to other males. These males, in addition, displayed high levels of experimental anxiety in the plus maze test and were insensitive to the anxiogenic-like acute effect of FLX (10 mg/kg). The two main purposes of the present work were: a) to study the behavioral profile of males displaying same-sex preference in the forced swim test (FST), and b) to analyze if the antidepressant-like effect of the subchronic treatment with FLX (10 mg/kg, 3 times) or desipramine (DMI, 10 mg/kg, 3 times) vary according to sex preference. Males treated prenatally with letrozole with same-sex preference showed more immobility and less active behaviors (swimming and climbing) in the FST than males with female preference. Subchronic treatment with FLX and DMI reduced immobility when compared to saline controls, while FLX increased swimming and DMI increased climbing behavior. Treatments were equally effective in males with preference for other males and those that preferred females. These results indicate that an association exists between prenatal letrozole treatment, same-sex preference and immobility in the FST.

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.

The antidepressant-like effect of vagus nerve stimulation is mediated through the locus coeruleus

It has been shown that vagus nerve stimulation (VNS) has an antidepressant-like effect in the forced swim test. The mechanism of action underlying this effect is incompletely understood, but there is evidence suggesting that the locus coeruleus (LC) may play an important role. In this study, noradrenergic LC neurons were selectively lesioned to test their involvement in the antidepressant-like effect of VNS in the forced swim test. Forced swim test behavior was assessed in rats that were subjected to VNS or sham treatment. In half of the VNS-treated animals, the noradrenergic neurons from the LC were lesioned using the selective neurotoxin DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride], yielding three experimental arms: sham, VNS and DSP-4-VNS (n = 8 per group). Furthermore, the open field test was performed to evaluate locomotor activity. A dopamine-β-hydroxylase immunostaining was performed to confirm lesioning of noradrenergic LC neurons. VNS significantly reduced the percentage of immobility time in the forced swim test compared to sham treatment (median: 56%, interquartile range: 41% vs. median: 75%, interquartile range: 12%). This antidepressant-like effect of VNS could not be demonstrated in the DSP-4-VNS group (median: 79%, interquartile range: 33%). Locomotor activity in the open field test was not different between the three treatment arms. The absence of hippocampal dopamine-β-hydroxylase immunostaining in the DSP-4-treated rats confirmed the lesioning of noradrenergic neurons originating from the brainstem LC. The results of this study demonstrate that the noradrenergic neurons from the LC play an important role in the antidepressant-like effect of VNS.

Vagal pathways for microbiome-brain-gut axis communication

There is now strong evidence from animal studies that gut microorganism can activate the vagus nerve and that such activation plays a critical role in mediating effects on the brain and behaviour. The vagus appears to differentiate between non-pathogenic and potentially pathogenic bacteria even in the absence of overt inflammation and vagal pathways mediate signals that can induce both anxiogenic and anxiolytic effects, depending on the nature of the stimulus. Certain vagal signals from the gut can instigate an anti-inflammatory reflex with afferent signals to the brain activating an efferent response, releasing mediators including acetylcholine that, through an interaction with immune cells, attenuates inflammation. This immunomodulatory role of the vagus nerve may also have consequences for modulation of brain function and mood.What is currently lacking are relevant data on the electrophysiology of the system. Certainly, important advances in our understanding of the gut-brain and microbiome- gut-brain axis will come from studies of how distinct microbial and nutritional stimuli activate the vagus and the nature of the signals transmitted to the brain that lead to differential changes in the neurochemistry of the brain and behaviour.Understanding the induction and transmission of signals in the vagus nerve may have important implications for the development of microbial-or nutrition based therapeutic strategies for mood disorders.

Deep brain stimulation in rats: different targets induce similar antidepressant-like effects but influence different circuits

Recent studies in patients with treatment-resistant depression have shown similar results with the use of deep brain stimulation (DBS) in the subcallosal cingulate gyrus (SCG), ventral capsule/ventral striatum (VC/VS) and nucleus accumbens (Acb). As these brain regions are interconnected, one hypothesis is that by stimulating these targets one would just be influencing different relays in the same circuitry. We investigate behavioral, immediate early gene expression, and functional connectivity changes in rats given DBS in homologous regions, namely the ventromedial prefrontal cortex (vmPFC), white matter fibers of the frontal region (WMF) and nucleus accumbens. We found that DBS delivered to the vmPFC, Acb but not WMF induced significant antidepressant-like effects in the FST (31%, 44%, and 17% reduction in immobility compared to controls). Despite these findings, stimulation applied to these three targets induced distinct patterns of regional activity and functional connectivity. While animals given vmPFC DBS had increased cortical zif268 expression, changes after Acb stimulation were primarily observed in subcortical structures. In animals receiving WMF DBS, both cortical and subcortical structures at a distance from the target were influenced by stimulation. In regard to functional connectivity, DBS in all targets decreased intercorrelations among cortical areas. This is in contrast to the clear differences observed in subcortical connectivity, which was reduced after vmPFC DBS but increased in rats receiving Acb or WMF stimulation. In conclusion, results from our study suggest that, despite similar antidepressant-like effects, stimulation of the vmPFC, WMF and Acb induces distinct changes in regional brain activity and functional connectivity.

Effects of electroconvulsive seizures on depression-related behavior, memory and neurochemical changes in Wistar and Wistar-Kyoto rats

BACKGROUND

Investigations in healthy outbred rat strains have shown a potential role for brain-derived neurotrophic factor (BDNF) and the hypothalamic-pituitary-adrenal (HPA) axis in the antidepressant and memory side effects of electroconvulsive therapy (ECT, or ECS in animals). The Wistar-Kyoto (WKY) rat strain is used as a genetic model of depression yet no studies to date have directly compared the impact of ECS on the WKY strain to its healthy outbred control (Wistar).

OBJECTIVE

The objective of this study is to examine behavioral (antidepressant and retrograde memory) and neurochemical (BDNF and HPA axis) changes immediately (1day) and at a longer delay (7days) after repeated ECS (5 daily administrations) in WKY and Wistar rats.

METHODS

Male Wistar and WKY rats received 5days of repeated ECS or sham treatment and were assessed 1 and 7days later for 1) depression-like behavior and mobility; 2) retrograde memory; and 3) brain BDNF protein, brain corticotropin-releasing factor (CRF) and plasma corticosterone levels.

RESULTS

Both strains showed the expected antidepressant response and retrograde memory impairments at 1day following ECS, which were sustained at 7days. In addition, at 1day after ECS, Wistar and WKY rats showed similar elevations in brain BDNF and extra-hypothalamic CRF and no change in plasma corticosterone. At 7days after ECS, Wistar rats showed sustained elevations of brain BDNF and CRF, whereas WKY rats showed a normalization of brain BDNF, despite sustained elevations of brain CRF.

CONCLUSIONS

The model of 5 daily ECS was effective at eliciting behavioral and neurochemical changes in both strains. A temporal association was observed between brain CRF levels, but not BDNF, and measures of antidepressant effectiveness of ECS and retrograde memory impairments suggesting that extra-hypothalamic CRF may be a potential important contributor to these behavioral effects after repeated ECS/ECT.

Therapeutic modalities for treatment resistant depression: focus on vagal nerve stimulation and ketamine

Treatment resistant depression (TRD) is a global health concern affecting a large proportion of depressed patients who then require novel therapeutic options. One such treatment option that has received some attention in the past several years is vagal nerve stimulation (VNS). The present review briefly describes the relevance of this treatment in the light of other existing pharmacological and non-pharmacological options. It then summarizes clinical findings with respect to the efficacy of VNS. The anatomical rationale for its efficacy and other potential mechanisms of its antidepressant effects as compared to those employed by classical antidepressant drugs are discussed. VNS has been approved in some countries and has been used for patients with TRD for quite some time. A newer, fast-acting, non-invasive pharmacological option called ketamine is currently in the limelight with reference to TRD. This drug is currently in the investigational phase but shows promise. The clinical and preclinical findings related to ketamine have also been summarized and compared with those for VNS. The role of neurotrophin factors, specifically brain derived neurotrophic factor and its receptor, in the beneficial effects of both VNS and ketamine have been highlighted. It can be concluded that both these therapeutic modalities, while effective, need further research that can reveal specific targets for intervention by novel drugs and address concerns related to side-effects, especially those seen with ketamine.

Biomechanical and functional variation in rat sciatic nerve following cuff electrode implantation

BACKGROUND

Nerve cuff electrodes are commonly and successfully used for stimulating peripheral nerves. On the other hand, they occasionally induce functional and morphological changes following chronic implantation, for reasons not always clear. We hypothesize that restriction of nerve mobility due to cuff implantation may alter nerve conduction.

METHODS

We quantified acute changes in nerve-muscle electrophysiology, using electromyography, and nerve kinematics in anesthetized Sprague Dawley rat sciatic nerves during controlled hindlimb joint movement. We compared electrophysiological and biomechanical response in uncuffed nerves and those secured within a cuff electrode using analysis of variance (ANOVA) and regression analysis.

RESULTS

Tethering resulting from cuff implantation resulted in altered nerve strain and a complex biomechanical environment during joint movement. Coincident with biomechanical changes, electromyography revealed significantly increased variability in the response of conduction latency and amplitude in cuffed, but not free, nerves following joint movement.

CONCLUSION

Our findings emphasize the importance of the mechanical interface between peripheral nerves and their devices on neurophysiological performance. This work has implications for nerve device design, implantation, and prediction of long-term efficacy.

Vagus Nerve Stimulation

The vagus nerve is a major component of the autonomic nervous system, has an important role in the regulation of metabolic homeostasis, and plays a key role in the neuroendocrine-immune axis to maintain homeostasis through its afferent and efferent pathways. Vagus nerve stimulation (VNS) refers to any technique that stimulates the vagus nerve, including manual or electrical stimulation. Left cervical VNS is an approved therapy for refractory epilepsy and for treatment resistant depression. Right cervical VNS is effective for treating heart failure in preclinical studies and a phase II clinical trial. The effectiveness of various forms of non-invasive transcutaneous VNS for epilepsy, depression, primary headaches, and other conditions has not been investigated beyond small pilot studies. The relationship between depression, inflammation, metabolic syndrome, and heart disease might be mediated by the vagus nerve. VNS deserves further study for its potentially favorable effects on cardiovascular, cerebrovascular, metabolic, and other physiological biomarkers associated with depression morbidity and mortality.

Vagus nerve stimulation for the treatment of depression and other neuropsychiatric disorders

Vagus nerve stimulation is an interesting new approach to treating neuropsychiatric diseases within the class of brain-stimulation devices sometimes labeled 'neuromodulators'. With vagus nerve stimulation, a battery-powered generator implanted in the chest wall connects to a wire wrapped around the vagus nerve in the neck, and sends intermittent pulses of electricity along the nerve directly into the brain. This mechanism takes advantage of the natural role of the vagus nerve in conveying information into the brain concerning homeostatic information (e.g., hunger, chest pain and respirations). Vagus nerve stimulation therapy is US FDA approved for the adjunctive treatment of epilepsy and has recently been FDA approved for the treatment of medication-resistant depression. Owing to its novel route into the brain, it has no drug-drug interactions or systemic side effects. This treatment also appears to have high long-term tolerability in patients, with low rates of patients relapsing on vagus nerve stimulation or becoming tolerant. However, alongside the excitement and enthusiasm for this new treatment, a lack of Class I evidence of efficacy in treating depression is currently slowing down adoption by psychiatrists. Much more research is needed regarding exactly how to refine and deliver the electrical pulses and how this differentially affects brain function in health and disease.

The role of electroconvulsive and neuromodulation therapies in the treatment of geriatric depression

Geriatric depression is associated with increased mortality because of suicide and decreases in functional and physical health. Many elders' depression is resistant to psychotherapy and medication and can become chronic. Electroconvulsive therapy (ECT) is increasingly used in the treatment of medication-resistant or life-threatening geriatric depression. Neuromodulation therapies (subconvulsive, focal, or subconvulsive and focal) are alternatives for the management of treatment-resistant depression in the elderly. Therapies that combine both strategies could be safer but may not be as effective as ECT. This review covers the evidence on the safety and efficacy of ECT and the neuromodulation therapies in geriatric depression.

Supraorbital stimulation does not induce an antidepressant-like response in rats

BACKGROUND

Neuromodulation therapies are currently being investigated as potential treatments for depression. One of these treatments involves the stimulation of supraorbital branches of the trigeminal nerve.

OBJECTIVE

To show that supraorbital stimulation is effective in preclinical models.

METHODS

Rats were given supraorbital stimulation at different settings in the forced swim test (FST) and open field.

RESULTS

Supraorbital stimulation did not induce an antidepressant-like response in rats undergoing the FST. This is in contrast to other neuromodulation treatments, such as deep brain stimulation, vagus nerve stimulation and electroconvulsive therapy, which are all effective in this paradigm.

CONCLUSIONS

Supraorbital stimulation was ineffective in rats undergoing the FST. Such findings do not invalidate results of recent clinical trials.

Preclinical studies modeling deep brain stimulation for depression

Deep brain stimulation (DBS) is currently being investigated for the treatment of depression. Results of early clinical trials have been very promising, but the mechanisms responsible for the effects of DBS are still unknown. This article reviews behavioral findings of stimulation applied to different brain targets in rodents, with a particular focus on the ventromedial prefrontal cortex. Mechanisms and substrates involved in the antidepressant-like effects of DBS, including the role of local tissue inactivation, the modulation of fiber pathways in the vicinity of the electrodes, as well as the importance of the serotonergic system and brain derived neurotrophic factor are discussed.

Mortality and suicide risk in treatment-resistant depression: an observational study of the long-term impact of intervention

Major depressive disorder is a common global disease that causes a significant societal burden. Most interventional studies of depression provide a limited assessment of the interventions on mortality and suicide risks. This study utilizes data from an observational registry of patients with major depressive disorder to determine the impact of intervention (vagus nerve stimulation or standard pharmacological/non-pharmacological therapy) and a latent factor, patient trajectory toward response, on mortality, suicide and suicidal ideation. A total of 636 patients were available for an intent-to-treat analysis of all-cause mortality, suicide and suicidal ideation. Patients treated with vagus nerve stimulation in addition to standard therapies experienced lower, but not statistically significant, all-cause mortality (vagus nerve stimulation 4.93 per 1,000 person-years vs. 10.02 per 1,000 patient years for treatment as usual) and suicide rates (vagus nerve stimulation 0.88 per 1,000 person-years vs. 1.61 per 1,000 patient years for treatment as usual). Treatment with vagus nerve stimulation produced a statistically lower relative risk of suicidal ideation 0.80, 95% confidence interval (0.68,0.95). Further, patients that responded to either treatment saw a 51% reduction in relative risk of suicidal behavior; relative risk and 95% confidence interval of 0.49 (0.41,0.58). In summary, we find that treatment with adjunctive vagus nerve stimulation can potentially lower the risk of all-cause mortality, suicide and suicide attempts.

Vagus nerve stimulation enhances perforant path-CA3 synaptic transmission via the activation of β-adrenergic receptors and the locus coeruleus

Vagus nerve stimulation (VNS) is an approved treatment for epilepsy and depression and has cognition-enhancing effects in patients with Alzheimer's disease. The hippocampus is widely recognized to be related to epilepsy, depression, and Alzheimer's disease. One possible mechanism of VNS involves its effect on the hippocampus; i.e. it increases the release of noradrenaline in the hippocampus. However, the effect of VNS on synaptic transmission in the hippocampus is unknown. To determine whether VNS modulates neurotransmission in the hippocampus, we examined the effects of VNS on perforant path (PP)-CA3 synaptic transmission electrophysiologically in anaesthetized rats. VNS induces a persistent enhancement of PP-CA3 field excitatory post-synaptic potentials (fEPSPs). Arc, an immediate early gene, was used to identify active brain regions after VNS. The locus coeruleus (LC), which contains the perikarya of noradrenergic projections, harboured more Arc-positive cells, as measured by in-situ hybridization, after 10-min VNS. In addition, electrical lesions of LC neurons or intraventricular administration of the β-adrenergic receptor antagonist timolol prevented the enhancement of PP-CA3 responses by VNS. In conclusion, the protracted increase in PP-CA3 synaptic transmission that is induced by VNS entails activation of the LC and β-adrenergic receptors. Our novel findings suggest that information from the periphery modulates synaptic transmission in the CA3 region of the hippocampus.

Vagal nerve stimulation rapidly activates brain-derived neurotrophic factor receptor TrkB in rat brain

Background

Vagal nerve stimulation (VNS) has been approved for treatment-resistant depression. Many antidepressants increase expression of brain-derived neurotrophic factor (BDNF) in brain or activate, via phosphorylation, its receptor, TrkB. There have been no studies yet of whether VNS would also cause phosphorylation of TrkB.

Methods

Western blot analysis was used to evaluate the phosphorylation status of TrkB in the hippocampus of rats administered VNS either acutely or chronically. Acute effects of VNS were compared with those caused by fluoxetine or desipramine (DMI) whereas its chronic effects were compared with those of sertraline or DMI.

Results

All treatments, given either acutely or chronically, significantly elevated phosphorylation of tyrosines 705 and 816 on TrkB in the hippocampus. However, only VNS increased the phosphorylation of tyrosine 515, with both acute and chronic administration causing this effect. Pretreatment with K252a, a nonspecific tyrosine kinase inhibitor, blocked the phosphorylation caused by acute VNS at all three tyrosines. Downstream effectors of Y515, namely Akt and ERK, were also phosphorylated after acute treatment with VNS, whereas DMI did not cause this effect.

Conclusion

VNS rapidly activates TrkB phosphorylation and this effect persists over time. VNS-induced phosphorylation of tyrosine 515 is distinct from the effect of standard antidepressant drugs.

Neurostimulation therapies for treatment resistant depression: a focus on vagus nerve stimulation and deep brain stimulation

Antidepressant treatments, including pharmacotherapy and psychotherapy, do not result in remission for the majority of patients with major depressive disorder. The high prevalence of treatment resistant depression (TRD) poses a significant issue for patients as well as both societal and economic costs. Due to the limited efficacy of existing therapies in this sub-population, alternative somatic treatments are being explored. Both vagus nerve stimulation (VNS) and deep brain stimulation (DBS) are neurostimulation treatments for TRD. While VNS has Food Drug Administration approval as an adjunctive therapy for MDD, DBS is still in the experimental stages. This article will review the evidence supporting the clinical utility of these therapies.

Behavioural effects of antidepressants are dependent and independent on the integrity of the dentate gyrus

The dentate gyrus (DG), a part of the hippocampal formation, is a candidate target of antidepressants and may play a role in the development of depressive syndrome; however, there is no direct neurobiological evidence supporting this theory. Here, we examined whether DG integrity is necessary for the behavioural effects of acute or chronic antidepressant treatment. Microinjection of colchicine into DG severely damaged the granule cells, as confirmed by morphological, electrophysiological, and behavioural analyses. Acute treatment with desipramine and fluoxetine decreased the immobility of saline-treated rats in the forced swimming test, whereas this decrease was inhibited in colchicine-treated rats. Chronic treatment with desipramine and fluoxetine also decreased the immobility of saline-treated rats; however, the extensive DG damage induced by colchicine had no effect on this decrease. In the novelty-suppressed feeding test, chronic treatment with desipramine and fluoxetine decreased the latency to feed in saline-treated rats while, once again, the extensive DG damage caused by colchicine had no effect on this decrease. Thus, we concluded that DG integrity was required for the behavioural effects of acute but not chronic antidepressant treatment; this disparity was not due to the time interval between surgery and behavioural tests. These findings indicate that treatment duration determines the influence of DG integrity on antidepressant effects.

Novel attempts to optimize vagus nerve stimulation parameters on serotonin neuronal firing activity in the rat brain

BACKGROUND

Vagus nerve stimulation (VNS) is indicated for treatment-resistant epilepsy and depression. Electrophysiologic recordings in the rat brain have shown that VNS promptly increases the firing rate of NE neurons and subsequently that of 5-HT neurons. Thus far, it appears that the standard stimulation parameters currently used in depressed patients produce an optimal activation of 5-HT neurons.

OBJECTIVE/HYPOTHESIS

This study was therefore aimed at investigating additional alterations of stimulation parameters to optimize VNS efficacy to further increase 5-HT neuronal activity.

METHODS

Rats were implanted with a VNS device and stimulated for 14 days using standard (0.25 mA/20 Hz/500 microseconds/30 seconds ON-5 minutes OFF, continuously) or various stimulation parameters: extension of the OFF period (30 seconds ON every 10 to 30 minutes), the OFF and ON periods, discontinuous stimulation (12 hours per day using standard parameters), and burst stimulation modes. Rat dorsal raphe 5-HT neurons were recorded under chloral hydrate anesthesia.

RESULTS

Both 12-hour stimulation periods for 14 days, and the 30-second stimulation every 10 or 15 minutes significantly increased the firing activity of 5-HT neurons to the same extent as standard parameters while the 30-minute intervals were ineffective. Stimulations in a burst mode and the pseudo-one-pulse stimulations also significantly increased 5-HT neuronal activity.

CONCLUSIONS

These results indicate that less stimulation is sufficient to achieve the same VNS efficacy on 5-HT neuronal firing. These data may be relevant for patients using VNS because these new parameters could minimize or prevent side effects and increase battery life of the stimulator.