Vagal Signaling Research Articles

Neural Pathway for Gut Feelings: Vagal Interoceptive Feedback From the Gastrointestinal Tract Is a Critical Modulator of Anxiety-like Behavior

BackgroundAnxiety disorders are associated with an altered perception of the body’s internal state. Therefore, understanding the neuronal basis of interoception can foster novel anxiety therapies. In rodents, the feeding status bidirectionally modulates anxiety-like behavior but how the sensing of gastrointestinal state affects anxiety remains unclear. MethodsWe combined chemogenetics, neuropharmacology, and behavioral approaches in male and female rats to test whether vagal afferents terminating in the gastrointestinal tract mediate feeding-induced tuning of anxiety. Using saporin-based lesions and transcriptomics, we investigated the chronic impact of this gut-brain circuit on anxiety-like behavior. ResultsBoth feeding and selective chemogenetic activation of gut-innervating vagal afferents increased anxiety-like behavior. Conversely, chemogenetic inhibition blocked the increase in anxiety-like behavior induced by feeding. Using a selective saporin-based lesion, we demonstrate that the loss of gut-innervating vagal afferent signaling chronically reduces anxiety-like behavior in male rats but not in female rats. We next identify a vagal circuit that connects the gut to the central nucleus of the amygdala, using anterograde transsynaptic tracing from the nodose ganglia. Lesion of this gut-brain vagal circuit modulated the central amygdala transcriptome in both sexes but selectively affected a network of GABA (gamma-aminobutyric acid)-related genes only in males, suggesting a potentiation of inhibitory control. Blocking GABAergic signaling in the central amygdala re-established normal anxiety levels in male rats. ConclusionsVagal sensory signals from the gastrointestinal tract are critical for baseline and feeding-induced tuning of anxiety via the central amygdala in rats. Our results suggest vagal gut-brain signaling as a target to normalize interoception in anxiety disorders.

Gut Feelings: Vagal Stimulation Reduces Emotional Biases

The vagus nerve is a key physical constituent of the gut–brain axis. Increasing attention has recently been paid to the role that the gut, and the microorganisms inhabiting it, play in emotion and cognition. Animal studies have revealed the importance of the vagus nerve in mediating communication between the gut microbiome and the central nervous system, resulting in changes in emotional behaviour. This has renewed interest in understanding the role of vagal signalling in human emotion, particularly since human studies have also shown that alterations in gut microbiome composition can affect emotion. While stimulating the vagus nerve can help treat some cases of severe depression, here we investigate whether vagal afferent signalling can influence emotional processing in healthy subjects. We use the dot-probe task to determine the effect of transcutaneous vagus nerve stimulation on attentional biases towards emotional stimuli in 42 volunteers. Participants received both active and sham treatments using a within-subject design. We show that transcutaneous vagus nerve stimulation reduces the emotional bias towards faces expressing sadness and happiness, indicating a decrease in emotional reactivity. While our novel findings reveal the effect that vagal signalling can have on emotional biases in healthy subjects, future studies should seek to develop our understanding of the ways in which the microbiome interacts with, and stimulates, the vagus nerve. Since we find a reduction in emotional bias, most notably towards sadness, this may partly account for the effective use of vagus nerve stimulation in treatment-resistant depression. While its clinical application currently involves surgical stimulation, our results support the potential benefit of transcutaneous vagus nerve stimulation as a non-invasive, intermittent adjunctive therapy for patients with depression, given its frequent association with emotional biases.

Activation of dorsal motor nucleus cholinergic neurons ameliorates the severity of acute pancreatitis

Clusters of neuron cell bodies, termed nuclei, residing in the brainstem are the origin of cranial nerves which transmit action potentials controlling organ function. Neurons residing in the brainstem dorsal motor nucleus (DMN) project in the vagus nerve to communicate with the pancreas, liver, gastrointestinal tract and other organs. Cholinergic signaling in the vagus nerve‐mediated inflammatory reflex also controls immune system responses by inhibiting the production of cytokines in the spleen, although the function of DMN neurons in regulating pancreatic inflammation is not known. Here, we demonstrate that selective activation of cholinergic neurons in the DMN attenuates pancreatitis severity. Pancreatitis was induced with two intraperitoneal injections of the cholecystokinin analogue caerulein (50 mcg/kg), given one hour apart. A fiber‐optic cannula was inserted under stereotactic guidance into the DMN in transgenic mice expressing light‐responsive ion channel, channelrhodopsin, in cholinergic neurons under the choline acetyltransferase promoter. Animals were subjected to either optogenetic stimulation (473nm, 20 Hz, 25% duty cycle, 8‐12 mW total power) or sham stimulation (no light) for five minutes. Optogenetic stimulation but not sham stimulation of the cholinergic neurons in the DMN significantly attenuates serum amylase (4043mU/mL ± 264.4 vs. 2940mU/mL ± 170.9, p=0.0074; n=9‐11 mice/group) and pancreatic IL‐6 levels (1907pg/mg ± 879.3 vs. 1046pg/mg± 288.1, p=0.0330; n=9‐11 mice/group). The decreased severity of pancreatitis in optogenetically stimulated mice is further demonstrated by a significant improvement in pancreatic histologic severity score (6.86 ± 1.67 vs. 5.35 ± 1.44, p = 0.0462; n = 9‐11). Pharmacological blockade and surgical ablation of the vagus nerve signaling inhibit protective effects of DMN cholinergic neurons activation. Pre‐treatment with mecamylamine, a nicotinic receptor antagonist, attenuates the reduction in serum amylase following light stimulation (Sham stimulation vs. DMN vs DMN + Mecamylamine: 5047 ± 315.4 vs. 3769 ± 241.5 vs. 5620 ± 385.9, p = 0037; n = 10 mice/group). Additionally, mice with subdiaphragmatic vagotomy has no difference in serum amylase levels following optogenetic stimulation (2761mU/mL ± 787.3 vs. 2517mU/mL, p= 0.300; n=3), whereas sham surgery controls show a significant reduction in serum amylase levels (3224mU/mL ± 593.1 vs. 2087 mU/mL ± 951.2, p=0.0303; n=6 mice/group). These studies identify DMN as an important locus that regulates the severity of acute pancreatitis in vagus‐nerve‐dependent fashion, and provide new insights into the identity and central origin of the efferent vagus nerve fibers regulating acute pancreatitis.

An Underlying Mechanism by which Hepatic Steatosis Drives the Development of Hypertension

Hepatic steatosis, a consequence of obesity, is closely associated with metabolic and cardiovascular diseases, including hypertension. Even in patients that are not hypertensive, the amount of fat in the liver is positively associated with blood pressure. We hypothesized there was an underlying mechanism linking excess fat in the liver with increased blood pressure. We have previously established that liver lipid content is positively associated with hepatic production and release of the inhibitory neurotransmitter GABA, a process dependent on both the GABA shunt and electrogenic GABA transporters. Because GABA is co‐transported with 1‐2 net positive charges, hepatocyte depolarization, common in obesity, encourages hepatocyte GABA release. To investigate the effect of hepatocyte depolarization, we used an adeno‐associated virus to induce hepatocyte specific expression of an artificial chimeric channel that opens in the presence of an exogenous ligand causing depolarization. Inducing hepatocyte depolarization increases release of GABA and decreases hepatic vagal afferent nerve activity. We established that administering the depolarizing ligand acutely (15‐40 minutes after IP administration) increased systolic (30.08 ± 5.6 mmHg), diastolic (16.47 + 4.3 mmHg), and mean blood pressure (17.85 + 4.4 mmHg) measured via telemetry devices. In hepatic vagotomized mice hepatocyte depolarization had no effect on blood pressure, establishing the key role of afferent hepatic vagal signals in regulating blood pressure. Pharmacological inhibition of the GABA shunt by daily administration of ethanolamine‐O‐sulfate (4 days; 8 mg/mouse/day) limited liver slice GABA release and decreased systolic, diastolic, and mean (10 ± 3.07 mmHg; P < 0.05) blood pressure in diet‐induced obese mice during the first hour of the dark cycle. We subsequently knocked down GABA‐transaminase with bi‐weekly IP deliver of an anti‐sense oligonucleotide targeted to GABA‐transaminase (12.5 mg/kg). We have previously established that this decreases liver GABA‐transaminase mRNA expression by 97% without affecting mRNA expression in the pancreas or brain. GABA‐transaminase knockdown decreased 24h mean, systolic, and diastolic blood pressure (17 + 3, 20 +2, and 14 + 4 mmHg respectively) in obese, angiotensin II induced hypertensive (continuous delivery of angiotensin II 800 ng/kg/min by Alzet® osmotic minipump) male mice. Together this data supports a role of hepatocyte GABA production and release in the hypertension that accompanies obesity. Moreover, our results provide a mechanism by which hepatic lipid content can affect blood pressure, identifying potential targets for the treatment and prevention of obesity‐induced hypertension.

Altered Vagal Signaling and Its Pathophysiological Roles in Functional Dyspepsia.

The vagus nerve is crucial in the bidirectional communication between the gut and the brain. It is involved in the modulation of a variety of gut and brain functions. Human studies indicate that the descending vagal signaling from the brain is impaired in functional dyspepsia. Growing evidence indicate that the vagal signaling from gut to brain may also be altered, due to the alteration of a variety of gut signals identified in this disorder. The pathophysiological roles of vagal signaling in functional dyspepsia is still largely unknown, although some studies suggested it may contribute to reduced food intake and gastric motility, increased psychological disorders and pain sensation, nausea and vomiting. Understanding the alteration in vagal signaling and its pathophysiological roles in functional dyspepsia may provide information for new potential therapeutic treatments of this disorder. In this review, we summarize and speculate possible alterations in vagal gut-to-brain and brain-to-gut signaling and the potential pathophysiological roles in functional dyspepsia.

Divergent Adaptations in Autonomic Nerve Activity and Neuroimmune Signaling Associated With the Severity of Inflammation in Chronic Colitis.

The autonomic nervous system (ANS) is thought to play a critical role in the anti-inflammatory reflex pathway in acute colitis via its interaction with the spleen and colon. Inflammation in the intestine is associated with a blunting of vagal signaling and increased sympathetic activity. As a corollary, methods to restore sympatho-vagal balance are being investigated as therapeutic strategies for the treatment of intestinal inflammation. Nevertheless, it is indefinite whether these autonomic signaling adaptations in colitis are detrimental or beneficial to controlling intestinal inflammation. In this study, models of moderate and severe chronic colitis are utilized to resolve the correlations between sympatho-vagal signaling and the severity of intestinal inflammation. Spleens and colons were collected from Winnie (moderate colitis), Winnie-Prolapse (severe colitis), and control C57BL/6 mice. Changes to the size and histomorphology of spleens were evaluated. Flow cytometry was used to determine the expression of adrenergic and cholinergic signaling proteins in splenic B and T lymphocytes. The inflammatory profile of the spleen and colon was determined using a RT-PCR gene array. Blood pressure, heart rate, splanchnic sympathetic nerve and vagus nerve activity were recorded. Spleens and colons from Winnie and Winnie-Prolapse mice exhibited gross abnormalities by histopathology. Genes associated with a pro-inflammatory response were upregulated in the colons from Winnie and further augmented in colons from Winnie-Prolapse mice. Conversely, many pro-inflammatory markers were downregulated in the spleens from Winnie-Prolapse mice. Heightened activity of the splanchnic nerve was observed in Winnie but not Winnie-Prolapse mice. Conversely, vagal nerve activity was greater in Winnie-Prolapse mice compared with Winnie mice. Splenic lymphocytes expressing α1 and β2 adrenoreceptors were reduced, but those expressing α7 nAChR and producing acetylcholine were increased in Winnie and Winnie-Prolapse mice. Sympathetic activity may correlate with an adaptive mechanism to reduce the severity of chronic colitis. The Winnie and Winnie-Prolapse mouse models of moderate and severe chronic colitis are well suited to examine the pathophysiology of progressive chronic intestinal inflammation.

Abstract 14228: Mechanism of Atrial Fibrillation in an Animal Model of Inflammatory Bowel Disease

Introduction: Ulcerative colitis (UC) is an i nflammatory bowel disease with a relapsing and remitting course. UC has extraintestinal manifestations and UC patients exhibit twice the risk for atrial fibrillation (AF) during acute disease activity. The mechanism of AF due to UC remains to be determined. Hypothesis: During acute UC a dysregulation of vagal signaling increases a patients propensity for AF. Methods: Mice (male, C57BL/6) were treated with Dextran Sulfate Sodium (DSS: 3%) supplemented drinking water for 7days that disrupts intestinal barrier function and mimics the disease phenotype of acute UC. Changes in atrial electrophysiology were quantified in vivo (EKG), on the whole heart (Langendorff configuration) and the cellular level (Fluo-4AM) during peak inflammation (DSS A ) and remission (DSS R ). Results: DSS A mice exhibited an attenuated heart rate (HR) in vivo (CTL: 491 ± 11 bpm, n=10; DSS A : 448 ± 11 bpm, n=16; p<0.05) and in excised perfused hearts (Langendorff configuration: CTL: 321 ± 15 bpm, n = 3; DSS A : 256 ± 17 bpm, n = 4; p < 0.05). The P-wave duration was prolonged (in vivo, CTL: 25.5 ± 0.6 ms, n=10; DSS A : 30.2 ± 0.5 ms, n=16; p < 0.0001) and HR variability time-domain measure RR-corrected SDNN was attenuated (CTL: 11.3 ± 1.1 ms, n=10; DSS A : 8.2 ± 0.7 ms, n=16; p < 0.05). The latter is an indicator for an attenuated vagal tone in DSS A mice and coincided with an increased sensitivity to the vagal agonist Carbachol in-vivo (CCh 150 ng/g, change in HR: CTL: -3.8 ± 4.4 %, n=6; DSS A : -42.2 ± 4.8 %, n=10; p < 0.0001). After burst pacing DSS A hearts exhibited a prolonged RR-corrected sinus node recovery time (CTL: 11.2 ± 1.4 ms, n=3; DSS A : 25.5 ± 2.1 ms, n=4; p < 0.0001), an increased AF propensity, and prolonged duration of AF episodes (CTL: 0.8 ± 0.17 s, n=3; DSS A : 2.4 ± 0.31 s, n=4; p < 0.05) even in the absence of CCh. No changes in atrial myocyte Ca handling properties were determined. After remission HR, P-wave duration, and HRV in DSS R returned to control values. Conclusions: The data suggests that acute UC increases the risk for AF likely as a consequence of an attenuated sinus rhythm and atrial conduction velocity where vagal stimuli could exacerbate the risk for reentry of excitation. If these changes are the direct consequence of the attenuated vagal tone remains to be determined.

Secondhand Smoke Decreased Excitability and Altered Action Potential Characteristics of Cardiac Vagal Neurons in Mice.

Background: Secondhand smoke (SHS), a major indoor pollutant, is a significant risk factor for cardiovascular morbidity and mortality including arrhythmias and sudden cardiac death. Exposure to SHS can produce autonomic imbalance, as evidenced by reduced heart rate variability (HRV)—a clinical metric of cardiac vagal regulation. Currently, the mechanisms through which SHS changes the vagal preganglionic neuronal inputs to the heart to produce this remains unknown.Objectives: To characterize the effect of SHS on both the excitability and action potential (AP) characteristics of anatomically identified cardiac vagal neurons (CVNs) in the nucleus ambiguus and examine whether SHS alters small conductance calcium-activated potassium (SK) channel activity of these CVNs.Methods: Adult male mice were exposed to four weeks of filtered air or SHS (3 mg/m3) 6 h/day, 5 day/week. Using patch-clamp recordings on identified CVNs in brainstem slices, we determined neuronal excitability and AP characteristics with depolarizing step- and ramp-current injections.Results: Four weeks of SHS exposure reduced spiking responses to depolarizing current injections and increased AP voltage threshold in CVNs. Perfusion with apamin (20 nM) magnified these SHS-induced effects, suggesting reduced SK channel activity may serve to minimize the SHS-induced decreases in CVNs excitability. Medium afterhyperpolarization (a measurement of SK channel activity) was smaller in the SHS group, further supporting a lower SK channel activity. AP amplitude, rise rate, fast afterhyperpolarization amplitude (a measurement of voltage-gated channel activity), and decay rate were higher in the SHS group at membrane voltages more positive to 0 mV, suggesting altered inactivation properties of voltage-dependent channels underlying APs.Discussion: SHS exposure reduced neuronal excitability of CVNs with compensatory attenuation of SK channel activity and altered AP characteristics. Neuroplasticity of CVNs could blunt regulatory cardiac vagal signaling and contribute to the cardiovascular consequences associated with SHS exposure, including reduced HRV.

Reduced Liver-Specific PGC1a Increases Susceptibility for Short-Term Diet-Induced Weight Gain in Male Mice.

The central integration of peripheral neural signals is one mechanism by which systemic energy homeostasis is regulated. Previously, increased acute food intake following the chemical reduction of hepatic fatty acid oxidation and ATP levels was prevented by common hepatic branch vagotomy (HBV). However, possible offsite actions of the chemical compounds confound the precise role of liver energy metabolism. Herein, we used a hepatocyte PGC1a heterozygous (LPGC1a) mouse model, with associated reductions in mitochondrial fatty acid oxidation and respiratory capacity, to assess the role of liver energy metabolism in systemic energy homeostasis. LPGC1a male, but not female, mice had a 70% greater high-fat/high-sucrose (HFHS) diet-induced weight gain compared to wildtype (WT) mice (p < 0.05). The greater weight gain was associated with altered feeding behavior and lower activity energy expenditure during the HFHS diet in LPGC1a males. WT and LPGC1a mice underwent sham surgery or HBV to assess whether vagal signaling was involved in the HFHS-induced weight gain of male LPGC1a mice. HBV increased HFHS-induced weight gain (85%, p < 0.05) in male WT mice, but not LPGC1a mice. These data demonstrate a sex-specific role of reduced liver energy metabolism in acute diet-induced weight gain, and the need for a more nuanced assessment of the role of vagal signaling in short-term diet-induced weight gain.

Emerging roles for microglia and microbiota in the development of social circuits

Social withdrawal is a core component of the behavioral response to infection. This fact points to a deep evolutionary and biologic relationship between the immune system and the social brain. Indeed, a large body of literature supports such an intimate connection. In particular, immune activation during the perinatal period has been shown to have long-lasting consequences for social behavior, but the neuroimmune mechanisms by which this occurs are only partially understood. Microglia, the resident immune cells of the brain, influence the formation of neural circuits by phagocytosing synaptic and cellular elements, as well as by releasing chemokines and cytokines. Intriguingly, microbiota, especially those that reside within the gut, may also influence brain development via the release of metabolites that travel to the brain, by influencing vagal nerve signaling, or by modulating the host immune system. Here, I will review the work suggesting important roles for microglia and microbiota in social circuit formation during development. I will then highlight avenues for future work in this area, as well as technological advances that extend our capacity to ask mechanistic questions about the relationships between microglia, microbiota, and the social brain.

Simultaneous decoding of cardiovascular and respiratory functional changes from pig intraneural vagus nerve signals

Objective. Bioelectronic medicine is opening new perspectives for the treatment of some major chronic diseases through the physical modulation of autonomic nervous system activity. Being the main peripheral route for electrical signals between central nervous system and visceral organs, the vagus nerve (VN) is one of the most promising targets. Closed-loop VN stimulation (VNS) would be crucial to increase effectiveness of this approach. Therefore, the extrapolation of useful physiological information from VN electrical activity would represent an invaluable source for single-target applications. Here, we present an advanced decoding algorithm novel to VN studies and properly detecting different functional changes from VN signals. Approach. VN signals were recorded using intraneural electrodes in anaesthetized pigs during cardiovascular and respiratory challenges mimicking increases in arterial blood pressure, tidal volume and respiratory rate. We developed a decoding algorithm that combines discrete wavelet transformation, principal component analysis, and ensemble learning made of classification trees. Main results. The new decoding algorithm robustly achieved high accuracy levels in identifying different functional changes and discriminating among them. Interestingly our findings suggest that electrodes positioning plays an important role on decoding performances. We also introduced a new index for the characterization of recording and decoding performance of neural interfaces. Finally, by combining an anatomically validated hybrid neural model and discrimination analysis, we provided new evidence suggesting a functional topographical organization of VN fascicles. Significance. This study represents an important step towards the comprehension of VN signaling, paving the way for the development of effective closed-loop VNS systems.

Loss of vagal integrity disrupts immune components of the microbiota-gut-brain axis and inhibits the effect of Lactobacillus rhamnosus on behavior and the corticosterone stress response

The vagus nerve is one of the major signalling components between the gut microbiota and brain. However, the exact relationship between gut-brain signaling along the vagus and the effects of gut microbes on brain function and behaviour is unclear. In particular, the relationship between the vagus nerve and immune signaling, that also appears to play a critical role in microbiota-gut-brain communication, has not been delineated. The aim of the present study was to determine the effect of subdiaphragmatic vagotomy on peripheral and central immune changes associated with the anxiolytic actions of L.rhamnosus. Male mice underwent vagotomy or sham surgery, followed by administration of L.rhamnosus for 14 days. L.rhamnosus administration following sham surgery resulted in reduced anxiety-like behaviour, and an attenuation of the hypothalamic-pituitary-adrenal axis (HPA axis), as indicated by reduced plasma corticosterone after acute restraint stress. These effects were associated with an increase in splenic T regulatory cells and a decrease in activated microglia in the hippocampus. The anxiolytic effects, HPA modulation and increase in T regulatory cells were prevented by vagotomy, whereas vagotomy alone led to a significant increase in activated microglia in the hippocampus that was not altered with L.rhamnosus treatment. Thus, both microbe induced and constitutive vagal signaling influences critical immune components of the microbiota-gut-brain axis. These findings suggest that, rather than acting as a direct neural link to the central nervous system, the role of the vagus nerve in gut-microbe to brain signalling is as an integral component of a bi-directional neuroimmunoendocrine pathway.

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

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

First Encounters: Effects of the Microbiota on Neonatal Brain Development.

The microbiota plays important roles in host metabolism and immunity, and its disruption affects adult brain physiology and behavior. Although such findings have been attributed to altered neurodevelopment, few studies have actually examined microbiota effects on the developing brain. This review focuses on developmental effects of the earliest exposure to microbes. At birth, the mammalian fetus enters a world teeming with microbes which colonize all body sites in contact with the environment. Bacteria reach the gut within a few hours of birth and cause a measurable response in the intestinal epithelium. In adults, the gut microbiota signals to the brain via the vagus nerve, bacterial metabolites, hormones, and immune signaling, and work in perinatal rodents is beginning to elucidate which of these signaling pathways herald the very first encounter with gut microbes in the neonate. Neural effects of the microbiota during the first few days of life include changes in neuronal cell death, microglia, and brain cytokine levels. In addition to these effects of direct exposure of the newborn to microbes, accumulating evidence points to a role for the maternal microbiota in affecting brain development via bacterial molecules and metabolites while the offspring is still in utero. Hence, perturbations to microbial exposure perinatally, such as through C-section delivery or antibiotic treatment, alter microbiota colonization and may have long-term neural consequences. The perinatal period is critical for brain development and a close look at microbiota effects during this time promises to reveal the earliest, most primary effects of the microbiota on neurodevelopment.

New Insight into the Vagus Nerve Regulation of Glucose Homeostasis: A Role for Afferent Signaling and Skeletal Muscle Glucose Uptake

The autonomic nervous system (ANS) participates in maintaining glucose homeostasis, in part through vagus nerve signals between the liver and brain stem. However, the role of afferent and efferent vagus nerve signaling in the regulation of glucose homeostasis is unknown. Here we observed electrical stimulation of the cervical vagus nerve significantly reduced blood glucose levels from the baseline level in wildtype, fasted mice as compared with sham stimulated controls (change in blood glucose; sham stimulation vs. stimulated, -14.14 mg/dL ± 13.44 mg/dL vs. -27.14 mg/dL ± 18.54 mg/dL, p = 0.0218). Animals subjected to chemical ablation of the vagus nerve (vagotomy) proximal to the stimulating electrodes failed to respond to the glucose lowering effects of vagus nerve stimulation (change in blood glucose; proximal vagotomy vs. no vagotomy, -6.750 mg/dL ± 14.05 mg/dL vs. -25.22 mg/dL ± 19.14 mg/dL, p = 0.0243). This indicates that afferent vagus nerve signaling can decrease glucose levels. We also observed vagus nerve stimulation failed to mediate significant changes in circulating levels of insulin, glucagon, leptin, or GLP-1. Skeletal muscle glucose uptake is mediated by GLUT4 transporters, a major mechanism for maintaining glucose. To investigate whether skeletal muscle glucose uptake via GLUT4 is required for the glucose lowering effect of vagus nerve stimulation, we generated transgenic mice that lacked the GLUT4 transporter selectively in skeletal muscle. Vagus nerve stimulation resulted in decreases in circulating glucose levels in littermate controls, but not in animals with the GLUT4 transporter knocked out in the skeletal muscle (change in blood glucose; littermate controls vs. GLUT4 KO, -59.63 mg/dL ± 28.37 mg/dL vs. -36.67 mg/dL ± 15.63 mg/dL). Next, to test effects of vagus nerve stimulation in hyperglycemic conditions, obese ob/ob mice with baseline glucose greater than 250 mg/dL were subjected to vagus nerve stimulation. A significant decrease in blood glucose levels from baseline is observed in ob/ob mice as compared to sham stimulated controls (change in blood glucose; stimulated vs. sham stimulated, -84.72 mg/dL ± 27.41 mg/dL vs. -10.65 mg/dL ± 14.05 mg/dL, p = 0.003). Together these findings indicate afferent vagus nerve activity increases glucose uptake in skeletal muscle and lowers serum glucose.

IL‐1β‐Induced Thermoregulation and Vagus Nerve Activity is Mediated by Transient Receptor Potential Ankyrin 1

Intraperitoneal administration of interleukin-1β (IL-1β), a cytokine mediator of inflammation and injury, induces afferent vagus nerve signaling and brain-mediated sickness syndrome. However, the mechanism of IL-1β signaling through the vagus nerve is unknown. Here we show that transient receptor potential ankyrin 1 (TRPA1) expression in sensory vagus neurons is required to mediateIL-1β-induced thermoregulation and vagus nerve activation. We monitored body temperature in conscious and unrestrained mice by radiotelemetry using probes implanted into peritoneal cavity. Intraperitoneal administration of IL-1β induced a significant change in body temperature in wild type mice. However, this response was significantly impaired in TRPA1 knock out mice (TRPA1 KO) (wild type vs. TRPA1 KO; 1045 ± 35.29 AUC vs. 577.9 ± 26.13 AUC; t-test with Welch's correction; p<0.0001). To investigate whether selective TRPA1 expression in neurons is required for IL-1β-induced thermoregulation, we generated Syn-Cre/TRPA1fl/fl mice selectively devoid of TRPA1 expression in neurons. Selective ablation of TRPA1-expression in neurons significantly attenuates IL-1β-induced changes in body temperature (wild type vs. Syn-Cre/TRPA1fl/fl; 1082 ± 61.49 AUC vs. 775.8 ± 66.89 AUC; t-test with Welch's correction; p<0.01). We have previously shown IL-1β induces sensory signaling by recording vagus nerve activity. Accordingly, here we collected extra-neural recordings from the cervical vagus nerve and as expected observed administration of IL-1β induced a significant increase in vagus nerve activity (total spikes 5 minutes pre- and post-administration: 85.6 ± 0.17 spikes vs. 9998.0 ± 3617.0 spikes; t-test; p<0.01). Moreover, no significant changes in vagus nerve activity occurred in TRPA1 KO mice (pre vs post-administration: 867.3 ± 395.7 spikes vs. 1069.0 ± 455.8 spikes, t-test, N.S.), and in Syn-Cre/TRPA1fl/fl mice (pre vs post-administration: 2179.0 ± 558.8 spikes vs. 1996.0 ± 460.8 spikes; t-test; N.S.). Whole-cell patch-clamp recordings and calcium imaging revealed TRPA1 is required to mediate IL-1β-dependent activation of vagus sensory neurons. Together, these studies indicate IL-1β-induced thermoregulation and vagus nerve activation is mediated by TRPA1.

Vagus Nerve Stimulation and Brainstem Cholinergic Neurons Modulate Experimental Pancreatitis Severity

The nervous system maintains physiological homeostasis through neural reflex circuits that regulate organ function. A prototypical neural pathway is the inflammatory reflex, where action potentials originating in the cholinergic neurons residing in the brainstem dorsal motor nucleus (DMN) and transmitted in the vagus nerve inhibit production of inflammatory cytokines. Pancreatitis is an inflammatory condition leading to a significant risk of morbidity and mortality. Vagotomy or inhibition of vagus nerve mediated signaling result in an enhanced severity of experimental pancreatitis, although the function of vagus nerve signaling and DMN neurons in regulating inflammatory response and severity of pancreatitis is not known. Here, we show that vagus nerve stimulation (VNS) and optogenetic stimulation of DMN cholinergic neurons improve disease severity in acute pancreatitis assessed by serum amylase. Vagus nerve stimulation using a bipolar nerve cuff electrode (2 minutes, 300 mA, square, biphasic pulse, 250 uS phases, 50 uS interphase, 10 Hz) decreases circulating amylase levels as compared to sham stimulated controls (VNS vs sham: 3270 mU/mL ± 192.5 vs 4138 mU/mL ± 307.6, p = 0.0219; n = 10; mean ± S.E.M.). To selectively activate the cholinergic neurons in the DMN, we utilized ChAT-ChR2 transgenic mice that express the photoreactive ion channel, channel rhodopsin (ChR2), on cholinergic neurons, rendering them sensitive to 473 nm light. Selective optogenetic activation of DMN cholinergic neurons (5 minutes, 20 Hz, 25% duty cycle, 8-12 mW total power) significantly decreases circulating levels of amylase as compared to sham-stimulated controls (DMN vs sham stimulation: 2940 mU/mL ± 170.9 vs. 4043 mU/mL ± 264.4, p = 0.0074; n = 10 mice/group). Together, these results indicate activation of the inflammatory reflex, by stimulation of the cervical vagus nerve or DMN cholinergic neurons, is sufficient to attenuate the severity of caerulein-induced pancreatitis.

Tone it down: Vagal nerve activity is associated with pro-inflammatory and anti-viral factors in breast cancer – An exploratory study

In response to adverse social-environmental conditions, leukocytes gene expression profile is altered in a pattern recognized as the conserved transcriptional response to adversity (CTRA). This entails the up-regulated expression of pro-inflammatory genes and down-regulated expression of genes involved in type-I interferon (IFN) related anti-viral immunity. In contrast, vagal nerve activity is recognized as a significant anti-inflammatory modulator. In this work, we investigated the association between CTRA and vagal activity indicated by the standard deviation of all NN interval (SDNN), a measure of heart-rate variability, in breast cancer patients awaiting surgery (n = 16). This association was tested both at the molecular leukocyte transcription factor activity level, as well as at the cytokines serum levels. We found an association between higher SDNN and increased interferon (IFN) related anti-viral pathways, both on the leukocyte transcription factor level and serum protein level. Unexpectedly, we also found a positive correlation between higher SDNN and pro-inflammatory transcription factor activity and cytokine serum level, potentially suggesting that increased vagal activity was induced by increased inflammation, in the context of pre-surgical stress and the presence of malignant tissue. Transcription origin analysis (TOA) suggests a role for monocyte and B-cells in the anti-inflammatory and anti-metastatic effects induced by vagal nerve signaling. Larger prospective studies are needed to verify and elaborate on the results from this small cross-sectional study.

Vagal gut-brain signaling mediates amygdaloid plasticity, affect, and pain in a functional dyspepsia model.

Functional dyspepsia (FD) is associated with chronic gastrointestinal distress and with anxiety and depression. Here, we hypothesized that aberrant gastric signals, transmitted by the vagus nerve, may alter key brain regions modulating affective and pain behavior. Using a previously validated rat model of FD characterized by gastric hypersensitivity, depression-like behavior, and anxiety-like behavior, we found that vagal activity — in response to gastric distention — was increased in FD rats. The FD phenotype was associated with gastric mast cell hyperplasia and increased expression of corticotrophin-releasing factor (Crh) and decreased brain-derived neurotrophic factor genes in the central amygdala. Subdiaphragmatic vagotomy reversed these changes and restored affective behavior to that of controls. Vagotomy partially attenuated pain responses to gastric distention, which may be mediated by central reflexes in the periaqueductal gray, as determined by local injection of lidocaine. Ketotifen, a mast cell stabilizer, reduced vagal hypersensitivity, normalized affective behavior, and attenuated gastric hyperalgesia. In conclusion, vagal activity, partially driven by gastric mast cells, induces long-lasting changes in Crh signaling in the amygdala that may be responsible for enhanced pain and enhanced anxiety- and depression-like behaviors. Together, these results support a “bottom-up” pathway involving the gut-brain axis in the pathogenesis of both gastric pain and psychiatric comorbidity in FD.

Central and peripheral GLP-1 systems independently suppress eating.

The anorexigenic peptide glucagon-like peptide-1 (GLP-1) is secreted from gut enteroendocrine cells and brain preproglucagon (PPG) neurons, which respectively define the peripheral and central GLP-1 systems. PPG neurons in the nucleus tractus solitarii (NTS) are widely assumed to link the peripheral and central GLP-1 systems in a unified gut-brain satiation circuit. However, direct evidence for this hypothesis is lacking, and the necessary circuitry remains to be demonstrated. Here we show that PPGNTS neurons encode satiation in mice, consistent with vagal signalling of gastrointestinal distension. However, PPGNTS neurons predominantly receive vagal input from oxytocin receptor-expressing vagal neurons, rather than those expressing GLP-1 receptors. PPGNTS neurons are not necessary for eating suppression by GLP-1 receptor agonists, and concurrent PPGNTS neuron activation suppresses eating more potently than semaglutide alone. We conclude that central and peripheral GLP-1 systems suppress eating via independent gut-brain circuits, providing a rationale for pharmacological activation of PPGNTS neurons in combination with GLP-1 receptor agonists as an obesity treatment strategy.