Efferent Vagal Research Articles

Structural covariance of the salience network associated with heart rate variability.

The salience network (SN) anchored by the anterior insula and cingulate is crucial for the autonomic control of body homeostasis. Resting-sate fMRI studies have linked intrinsic SN connectivity with multiple autonomic measures, but little is known about the structural organization of this system relating to cardiac vagal function. To address the above issue, the current study used covariance analysis of MRI-based gray matter volume (GMV) to map structural covariance of SN with two independent datasets, and then determined whether interregional structural connections within SN related to individual differences in vagally-mediated heart rate variability (HRV). The results showed a significant positive association between structural covariance of the SN and vagal component of HRV in two independent samples. More importantly, the conjunction and pooled data analysis revealed that structural correlation from left anterior insula to dorsal anterior cingulate cortex positively interacted with vagally-mediated HRV. The current results demonstrated a crucial role of the SN in the cortical modulation of efferent vagal activity to the heart, and provided new insight into structural neural network implicated in cardiac vagal control.

Targeting the cholinergic anti-inflammatory pathway with vagus nerve stimulation in patients with Covid-19?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), at the origin of the worldwide COVID-19 pandemic, is characterized by a dramatic cytokine storm in some critical patients with COVID-19. This storm is due to the release of high levels of pro-inflammatory cytokines such as interleukin (IL)-1 β, IL-6, tumor necrosis factor (TNF), and chemokines by respiratory epithelial and dendritic cells, and macrophages. We hypothesize that this cytokine storm and the worsening of patients’ health status can be dampened or even prevented by specifically targeting the vagal-driven cholinergic anti-inflammatory pathway (CAP). The CAP is a concept that involves an anti-inflammatory effect of vagal efferents by the release of acetylcholine (ACh). Nicotinic acetylcholine receptor alpha7 subunit (α7nAChRs) is required for ACh inhibition of macrophage-TNF release and cytokine modulation. Hence, targeting the α7nAChRs through vagus nerve stimulation (VNS) could be of interest in the management of patients with SARS-CoV-2 infection. Indeed, through the wide innervation of the organism by the vagus nerve, especially the lungs and gastrointestinal tract, VNS appears as a serious candidate for a few side effect treatment that could dampen or prevent the cytokine storm observed in COVID-19 patients with severe symptoms. Finally, a continuous vagal tone monitoring in patients with COVID-19 could be used as a predictive marker of COVID-19 illness course but also as a predictive marker of response to COVID-19 treatment such as VNS or others.

The Role of Serotonin in Singultus: A Review.

The use of dopamine receptor blockers for chronic singultus treatment is based—at least partially—on circular thinking: chlorpromazine is FDA-approved for hiccups, chlorpromazine is a neuroleptic, neuroleptics are dopamine receptor blockers, and therefore hiccup is due to dopaminergic dysfunction. Chlorpromazine interacts with high affinity with a multitude of receptors and ion channels. This promiscuity is the basis for many of the therapeutic effects and adverse drug reactions of this drug. While an involvement of dopamine is certain, it is by no means clear that dopaminergic dysfunction is the hallmark of singultus. The common denominator of most remedies for transient hiccup is their ability to activate the vagus nerve. Both afferent and efferent vagal activity and the central integration of the Xth cranial nerve function are modulated, inter alia, via serotonergic mechanisms; beneficial (therapeutic) effects for hiccup are to be expected from serotonin (5-HT) receptor subtype ligands that enhance vagal activity. Taken together, it appears that the ability to increase vagus output is mainly associated with 5-HT1A, 5-HT3, and 5-HT7 agonists and with 5-HT2C antagonists. The plausibility of the serotonergic singultus hypothesis is examined against available pharmacokinetic, pharmacodynamic, and clinical data for a number of drugs.

ACTH 1-24 and other melanocortins for COVID-19 treatment

To the Editor: In the April 09 issue of this Journal, Feldman et al. (1) commented on the “urgent need for trials of anti-tumor necrosis factor (TNF) therapy for COVID-19.” In both animal and human studies we have demonstrated the impressive effect of several melanocortin peptides (particularly ACTH 1-24 and alpha-MSH) in extremely severe conditions of shock, respiratory arrest, and cardiac arrest, while being almost completely devoid of cardiovascular effects under normal conditions (2–7). The in-depth studies on these novel, unknown activities of melanocortins and of the underlying mechanism(s) of action, involving groups of researchers from several Universities (Modena, Bologna, Messina, Pavia, Milano, Napoli, Uppsala), coordinated by one of us and financially supported by Italian Ministry of University and Research and by Ministry of Defense, disclosed a rather complex picture. In brief, in experimental conditions of tissue hypoperfusion/hypoxia, induced in anesthetized animals (hemorrhagic shock, respiratory arrest, cardiac arrest), the i.v. bolus injection of melanocortins (ACTH 1-24 and alpha-MSH the most effective) induces a dose-dependent reversal of hypotension and of respiratory depression, and the restoration of cardiac output. Adrenals are not involved, because the effect is the same either in intact or in adrenalectomized animals, and is obtained to the same degree with ACTH fragments with full adrenocorticotropic activity (ACTH 1–24) or with fragments practically devoid of such activity (alpha-MSH = ACTH 1–13). In fact, the above effects are mostly due to the activation of melanocortin receptors (predominantly MC1 and MC3) on immunocytes. This causes the inhibition of the nuclear factor kB-dictated massive production of several factors of the inflammatory response (cell adhesion molecules, chemokines, cytokines, and cytokines receptors) with consequent impressive reduction of free radicals in blood, decrease of liver TNF-alpha mRNA and of plasma levels of TNF-alpha, inhibition of inducible nitric oxide (NO) syntheses and normalization of NO blood levels, and reduced expression of adhesion molecules. Furthermore, melanocortins activate the so-called “cholinergic anti-inflammatory pathway” through binding prevalently to MC4 receptors in the dorsal motor nucleus of the vagus, activation of efferent vagus fibers, acetylcholine release in organs of the reticulum endothelial system (especially liver, spleen, gastrointestinal tract, heart) (cholinergic anti-inflammatory pathway, or “inflammatory reflex”), activation of alpha7-nicotinic receptors on tissue macrophages, inhibition of the production of pro-inflammatory cytokines (mainly TNF-alpha and other actors of the early “cytokine storm” and acute endotheliopathy), and, on the other hand, stimulation of the production of anti-inflammatory cytokines (8). Finally, melanocortins exert a functional antagonism against endogenous opioids, which are massively released in shock conditions and contribute to the circulatory disaster: opioids inhibit sympathetic outflow and noradrenalin release from sympathetic terminals, contribute to peripheral pooling of blood, decrease venous return, and reduce cardiac output. In clinical studies, the melanocortin ACTH 1–24 has been used, because marketed from several years for other clinical conditions and thus freely available also in off label pathologies, and completely devoid of acute toxicity up to the dose of 100 mg in adult subjects. After some anecdotal case reports showing that the i.v. bolus injection of 5–10 mg of ACTH 1–24, in addition to the conventional therapeutic protocol, produced a dramatic improvement in extremely critical conditions, a phase 2 clinical trial enrolling 32 subjects, all with type A aortic dissection complicated by aortic rupture and cardiac tamponade, and with clinical signs of severe shock, confirmed the life-saving effect of ACTH 1–24 (survival 1 month after surgery: 87% in the ACTH group, 47% in the standard treatment group) (9). Moreover, a randomized phase 3 clinical study conducted on 100 critically ill patients and emergency surgery, has shown that early treating with 10 mg bolus of tetracosactide, have reduced the mortality at 30 days and normalization of the cytokine overload at 18 hours after surgery, toward the expected mortality of > 20% and to observed 18.8% of the untreated (10). Furthermore, the use of melanocortins has had a cytokine contrasting action in the interstitial pneumonia of the Ebola virus as well as a drug authorized by EMA and AIFA for compassionate use in Italy for the Ebola outbreak (11). In conclusion, in view of all the above data and prompted by the considerations of Feldman et al., we suggest that the treatment protocol of COVID patients should include the twice daily i.v. bolus injection of 10 mg of ACTH 1–24 (or of other agonists at MC1, MC3, and MC4 receptors). In the case of ACTH 1–24, an anti-aldosterone diuretic should also be administered to avoid the fluid retention caused by the ACTH stimulated overproduction of aldosterone by the adrenals.

Chronic unilateral cervical vagotomy reduces renal inflammation, blood pressure, and renal injury in a mouse model of lupus.

Systemic lupus erythematosus (SLE) is characterized by hypertension that results from chronic renal inflammation and dysautonomia in the form of dampened vagal tone. In health, the vagus nerve regulates inflammatory processes through mechanisms like the cholinergic anti-inflammatory pathway; so in the case of SLE, reduced efferent vagus nerve activity may indirectly affect renal inflammation and therefore hypertension. In this study, we sought to investigate the impact of disrupting vagal neurotransmission on renal inflammation and hypertension in the setting of chronic inflammatory disease. Female SLE (NZBWF1) and control (NZW) mice were subjected to a right unilateral cervical vagotomy or sham surgery and 3 wk later were implanted with indwelling catheters to measure blood pressure. Indices of splenic and renal inflammation, as well as renal injury, were assessed. Unilateral vagotomy blunted SLE-induced increases in mean arterial pressure, albumin excretion rate, and glomerulosclerosis. This protection was associated with reduced splenic T cells and attenuated SLE-induced increases in renal proinflammatory mediators. In summary, these data indicate that unilateral vagotomy reduces renal inflammation and reduces blood pressure in SLE mice. The vagus nerves have myriad functions, and perhaps other neuroimmune interactions compensate for the ligation of one nerve.

Anodal block permits directional vagus nerve stimulation

Vagus nerve stimulation (VNS) is a bioelectronic therapy for disorders of the brain and peripheral organs, and a tool to study the physiology of autonomic circuits. Selective activation of afferent or efferent vagal fibers can maximize efficacy and minimize off-target effects of VNS. Anodal block (ABL) has been used to achieve directional fiber activation in nerve stimulation. However, evidence for directional VNS with ABL has been scarce and inconsistent, and it is unknown whether ABL permits directional fiber activation with respect to functional effects of VNS. Through a series of vagotomies, we established physiological markers for afferent and efferent fiber activation by VNS: stimulus-elicited change in breathing rate (ΔBR) and heart rate (ΔHR), respectively. Bipolar VNS trains of both polarities elicited mixed ΔHR and ΔBR responses. Cathode cephalad polarity caused an afferent pattern of responses (relatively stronger ΔBR) whereas cathode caudad caused an efferent pattern (stronger ΔHR). Additionally, left VNS elicited a greater afferent and right VNS a greater efferent response. By analyzing stimulus-evoked compound nerve potentials, we confirmed that such polarity differences in functional responses to VNS can be explained by ABL of A- and B-fiber activation. We conclude that ABL is a mechanism that can be leveraged for directional VNS.

Differential effects of vagus nerve stimulation strategies on glycemia and pancreatic secretions.

Despite advancements in pharmacotherapies, glycemia is poorly controlled in type 2 diabetic patients. As the vagus nerve regulates energy metabolism, here we evaluated the effect various electrical vagus nerve stimulation strategies have on glycemia and glucose‐regulating hormones, as a first step to developing a novel therapy of type 2 diabetes. Sprague–Dawley rats were anesthetized, the abdominal (anterior) vagus nerve implanted, and various stimulation strategies applied to the nerve: (a) 15 Hz; (b) 4 kHz, or 40 kHz and; (c) a combination of 15 Hz and 40 kHz to directionally activate afferent or efferent vagal fibers. Following a glucose bolus (500 mg/kg, I.V.), stimulation strategies were applied (60 min) and serial blood samples taken. No stimulation was used as a crossover control sequence. Applying 15 Hz stimulation significantly increased glucose (+2.9 ± 0.2 mM·hr, p = .015) and glucagon (+17.1 ± 8.0 pg·hr/ml, p = .022), compared to no stimulation. Application of 4 kHz stimulation also significantly increased glucose levels (+1.5 ± 0.5 mM·hr, p = .049), while 40 kHz frequency stimulation resulted in no changes to glucose levels but did significantly lower glucagon (−12.3 ± 1.1 pg·hr/ml, p = .0009). Directional afferent stimulation increased glucose (+2.4 ± 1.5 mM·hr) and glucagon levels (+39.5 ± 15.0 pg·hr/ml). Despite hyperglycemia resulting when VNS, aVNS, and 4 kHz stimulation strategies were applied, the changes in insulin levels were not significant (p ≥ .05). In summary, vagus nerve stimulation modulates glycemia by effecting glucagon and insulin secretions, and high‐frequency 40 kHz stimulation may have potential application for the treatment of type 2 diabetes.

AJP-Regulatory, Integrative and Comparative Physiology: Looking Toward the Future

<i>AJP-Regulatory, Integrative and Comparative Physiology</i>: Looking Toward the Future

Carotid body‐specific shRNA knockdown of PKCɛ blunts TRPV1‐dependent asthmatic bronchoconstriction

RationaleAggravated airway bronchoconstriction and mucus production in response to allergen and/or bradykinin (i.e., airway hyperresponsiveness) is a primary defining symptom of asthma. Recently, we discovered a critical role for the carotid bodies: inhaled allergen causes an increase in arterial plasma concentrations of asthmakines which excite the carotid bodies via a transient receptor potential vanilloid 1 channel (TRPV1) dependent mechanism. Elevated carotid body excitation in turn increases efferent vagal activity, resulting in heightened bronchoconstriction. While our data demonstrate that the carotid bodies are strongly excited by multiple Th‐2 cytokines, in the ovalbumin primed and sensitized Brown Norway rat model of asthma, the primary asthmakine is likely lysophosphatidic acid (LPA). However, the cellular link between LPA receptors and TRPV1 has not been identified. Here we show that the effect of LPA on the carotid bodies is likely mediated by PKCɛ‐dependent phosphorylation of TRPV1.MethodsLentivirus vectors containing shRNA to knockdown PKCɛ or scrambled sequence were injected bilaterally into the carotid bodies of asthmatic rats (n=7, each). The responses of carotid bodies to LPA (5μM), and their subsequent responses to TRPV1 blockade (AMG9810, 10μM) were tested using the en bloc perfused carotid body preparation. The effects of carotid body‐specific PKCɛ knockdown on changes in lung mechanics in response to inhaled bradykinin were tested using the Flexivent system.ResultsCarotid bodies from asthmatic rats injected with PKCɛ knockdown sequence mounted smaller responses to LPA than those injected with scrambled sequence. No difference in LPA response was detected between groups following TRPV1 blockade, suggesting the primary target for PKCɛ is TRPV1. Asthmatic rats with carotid bodies injected with PKCɛ knockdown sequence had reduced lung resistance in response to bradykinin at 1, 10 and 20 minutes. Similar results were obtained using a PKCɛ inhibitory peptide.ConclusionWe show for the first time that carotid body‐mediated bronchoconstriction in asthmatic rats occurs through LPA receptor activation of PKCɛ and subsequent TRPV1‐mediated carotid sinus nerve excitation. Targeting carotid body LPA receptors, PKCɛ and/or TRPV1 may provide a new therapeutic target to reduce asthmatic airway hyperresponsiveness.Support or Funding InformationCIHR, Alberta Innovates, Alberta Lung Association and the Parker B. Francis Foundation.

Re‐expression of REST in ventricular vagal neurons aggravates myocardial infarctionevoked ventricular arrhythmia in type 2 diabetes mellitus

Diabetes mellitus, especially type 2 diabetes mellitus (T2DM, 90 to 95% of diabetic population), is a major public health problem worldwide and has become a leading cause of mortality. Myocardial infarction (MI) is the most common cause of mortality in T2DM. Comparing with non‐diabetic patients, patients with T2DM are twice to die from MI. Withdrawal of cardiac vagal activity is considered to be an important trigger for MI‐induced ventricular arrhythmias in T2DM. Our previous study has demonstrated that N‐type Ca2+ currents and cell excitability of atrioventricular ganglion (AVG) neurons were reduced in T2DM rats. We have recently demonstrated that reduced expression of Cav2.2‐α in AVG neurons contributes to cardiac vagal dysfunction and ventricular arrhythmogenesis. The repressor element 1‐silencing transcription factor (REST) is a key factor to block Cav2.2‐α expression. Normally REST is not expressed in neuronal cells. Here, we test the hypothesis that re‐expression of REST inhibits N‐type Ca2+ channel function in AVG neurons and further reduces cardiac vagal activity and promotes MI‐evoked fatal ventricular arrhythmias in T2DM rats. Using reverse‐phase protein array, we found that there was no expression of REST in the AVG from sham rats, while REST expression was significantly increased in the AVG from T2DM rats. In vivo infection of lentiviral REST shRNA (2 μl, 1×107 TU/ml) to the AVG markedly reduced REST expression in the AVG from T2DM rats. REST shRNA also increased Cav2.2‐α expression in AVG neurons from T2DM rats. The results from whole‐cell patch clamp recording showed that REST shRNA markedly restored T2DM‐reduced N‐type Ca2+ currents and cell excitability in AVG neurons. Additionally, infection of REST shRNA to AVG neurons markedly restored cardiac vagal activity, as evidenced by blunted response of LVSP to left efferent vagal stimulation. Moreover, REST shRNA also decreased the inducibility of ventricular arrhythmia in anesthetized T2DM rats (0.67±0.6 in T2DM+REST group vs. 3.6±1.03 in T2DM group). The ECG data obtained from 24‐hour radiotelemetry recording in conscious rat demonstrated that REST shRNA significantly reduced the duration of MI‐evoked ventricular arrhythmia (39.39±13.03 sec/hour in T2DM+REST group vs. 64.26±13.17 sec/hour in T2DM group). Based on above data, we conclude that re‐expression of REST decreases Cav2.2‐α expression and N‐type Ca2+ currents in AVG neurons, and subsequently reduces cardiac vagal function and increases susceptibility to ventricular arrhythmias in the T2DM.Support or Funding InformationDHHS/NIH/NHLBI, 34‐5370‐2003‐001

Ameliorating effects of sacral neuromodulation on gastric and small intestinal dysmotility mediated via a sacral afferent-vagal efferent pathway.

In a recent study of sacral nerve stimulation (SNS) for colonic inflammation, a possible spinal-vagal pathway was implicated. The aim of this study was to provide evidence for such a pathway by investigating the effects of SNS on dysmotility of the stomach and duodenum that are not directly innervated by the sacral efferents. Twenty-seven rats were chronically implanted with wire electrodes for SNS and gastrointestinal slow waves. SNS was performed in several acute sessions to investigate its effects on gastric/duodenal slow waves and emptying/transit impaired by glucagon and rectal distention (RD). (a) SNS increased the percentage of normal gastric slow waves impaired by glucagon (from 53.9% to 77.0%, P<.0001) and RD (from 64% to 78%, P=.037). This improvement was abolished by atropine. (b) Similar effects were observed with SNS on duodenal slow waves, which was also blocked by atropine. (c) SNS normalized delayed gastric emptying induced by glucagon (control: 61.3%, glucagon: 44.3%, glucagon+SNS: 65.8%) and RD (control: 61.3%, RD: 46.7%, RD+SNS: 64.3%). It also normalized small intestinal transit delayed by RD (P=.001, RD+SNS vs RD; P=.9, RD+SNS vs control). (4) Both glucagon and RD induced an increase in the sympathovagal ratio (P=.007, glucagon vs baseline; P<.001, RD vs baseline) and SNS decreased the ratio (P=.006, glucagon+SNS vs glucagon; P=.04, RD+SNS vs RD). Neuromodulation of the sacral nerve improves gastric and small intestinal pacemaking activity and transit impaired by glucagon and RD by normalizing the sympathovagal balance via a retrograde neural pathway from the sacral nerve to vagal efferents.

Threshold and saturation pressures of baroreflex-mediated myocardial interstitial acetylcholine release in rats.

Cardiac microdialysis allows the assessment of cardiac efferent vagal nerve activity from myocardial interstitial acetylcholine (ACh) levels with minimal influence on the neural control of the heart; however, a total picture of the baroreflex-mediated myocardial interstitial ACh release including the threshold and saturation pressures has yet to be quantified. In eight anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release simultaneously with efferent sympathetic nerve activity (SNA) during a carotid sinus baroreceptor pressure input between 60 and 180mmHg. The baroreflex-mediated ACh release approximated a positive sigmoid curve, and its threshold and saturation pressures were not significantly different from those of an inverse sigmoid curve associated with the baroreflex-mediated SNA response (threshold: 94.3±8.6 vs. 99.3±6.0mmHg; saturation: 150.0±10.3 vs. 158.8±5.8mmHg). The sympathetic and vagal systems have certain levels of activities across most of the normal pressure range.

Autonomic Tone and Baroreflex Sensitivity during 70° Head-up Tilt in Yoga Practitioners.

Introduction:The intervention of yoga was shown to improve the autonomic conditioning in humans evident from the enhancement of parasympathetic activity and baroreflex sensitivity (BRS). From the documented health benefits of yoga, we hypothesized that the experience of yoga may result in adaptation to the orthostatic stress due to enhanced BRS.Aim:To decipher the effects of yoga in the modulation of autonomic function during orthostatic challenge.Materials and Methods:This was a comparative study design conducted in autonomic function test lab, of the Department of Physiology, All India Institute of Medical Sciences, New Delhi, India. Heart rate variability (HRV), blood pressure variability, and BRS were analyzed on forty naïve to yoga (NY) subjects and forty yoga practitioners with an average age of 31.08 ± 7.31 years and 29.93 ± 7.57 years, respectively. All participants were healthy. Seventy degrees head up tilt (HUT) was used as an intervention to evaluate the cardiovascular variability during orthostatic challenge.Results:During HUT, the R-R interval (P = 0.042), root mean square of succesive R-R interval differences (RMSSD) (P = 0.039), standard deviation of instantaneous beat-to-beat R-R interval variability (SD1) (P = 0.039) of HRV, and sequence BRS (P = 0.017) and α low frequency of spectral BRS (P = 0.002) were higher in the yoga group. The delta decrease in RRI (P = 0.033) and BRS (P < 0.01) was higher in the yoga group than the NY group.Conclusion:The efferent vagal activity and BRS were higher in yoga practitioners. The delta change (decrease) in parasympathetic activity and BRS was higher, with relatively stable systolic blood pressure indicating an adaptive response to orthostatic challenge by the yoga practitioners compared to the NY group.

Non-invasive stimulation of vagal afferents reduces gastric frequency

Metabolic feedback between the gut and the brain relayed via the vagus nerve contributes to energy homeostasis. We investigated in healthy adults whether non-invasive stimulation of vagal afferents impacts energy homeostasis via efferent effects on metabolism or digestion. In a randomized crossover design, we applied transcutaneous auricular vagus nerve stimulation (taVNS) while recording efferent metabolic effects using simultaneous electrogastrography (EGG) and indirect calorimetry. We found that taVNS reduced gastric myoelectric frequency (p = .008), but did not alter resting energy expenditure. We conclude that stimulating vagal afferents induces gastric slowing via vagal efferents without acutely affecting net energy expenditure at rest. Collectively, this highlights the potential of taVNS to modulate digestion by activating the dorsal vagal complex. Thus, taVNS-induced changes in gastric frequency are an important peripheral marker of brain stimulation effects.

The Vagus and Glossopharyngeal Nerves in Two Autonomic Disorders.

The glossopharyngeal and vagus cranial nerves provide the brainstem with sensory inputs from different receptors in the heart, lung, and vasculature. This afferent information is critical for the short-term regulation of arterial blood pressure and the buffering of emotional and physical stressors. Glossopharyngeal afferents supply the medulla with continuous mechanoreceptive signals from baroreceptors at the carotid sinus. Vagal afferents ascending from the heart supply mechanoreceptive signals from baroreceptors in different reflexogenic areas including the aortic arch, atria, ventricles, and pulmonary arteries. Ultimately, afferent information from each of these distinct pressure/volume baroreceptors is all relayed to the nucleus tractus solitarius, integrated within the medulla, and used to rapidly adjust sympathetic and parasympathetic activity back to the periphery. Lesions that selectively destroy the afferent fibers of the vagus and/or glossopharyngeal nerves can interrupt the transmission of baroreceptor signaling, leading to extreme blood pressure fluctuations. Vagal efferent neurons project back to the heart to provide parasympathetic cholinergic inputs. When activated, they trigger profound bradycardia, reduce myocardial oxygen demands, and inhibit acute inflammation. Impairment of the efferent vagal fibers seems to play a role in stress-induced neurogenic heart disease (i.e., takotsubo cardiomyopathy). This focused review describes: (1) the importance of the vagus and glossopharyngeal afferent neurons in regulating arterial blood pressure and heart rate, (2) how best to assess afferent and efferent cardiac vagal function in the laboratory, and (3) two clinical phenotypes that arise when the vagal and/or glossopharyngeal nerves do not survive development or are functionally impaired.

Simultaneous cardiac and respiratory inhibition during seizure precedes death in the DBA/1 audiogenic mouse model of SUDEP.

This study was designed to evaluate cardiac and respiratory dysfunction in a mouse model of sudden unexpected death in epilepsy i.e., SUDEP. We simultaneously monitored respiration via plethysmography and the electrocardiogram via telemetry before, during, and after an audiogenic seizure. DBA/1 mice responded to an acoustic stimulus with one or two cycles of circling and jumping before entering a clonic/tonic seizure. This was followed by death unless the mice were resuscitated by mechanical ventilation using room air. During the initial clonic phase, respiration declined and cardiac rhythm is slowed. By the tonic phase, respiration had ceased, atrial P-waves were absent or dissociated from the QRS complex, and heart rate had decreased from 771±11 to 252±16 bpm. Heart rate further deteriorated terminating in asystole unless the mice were resuscitated at the end of the tonic phase which resulted in abrupt recovery of P-waves and a return to normal sinus rhythm, associated with gasping. Interestingly, P-waves were preserved in the mice treated with methylatropine during the pre-ictal period (to block parasympathetic stimulation) and heart rate remained unchanged through the end of the tonic phase (765±8 vs. 748±21 bpm), but as in control, methylatropine treated mice died from respiratory arrest. These results demonstrate that a clonic/tonic seizure in the DBA/1 mouse results in abrupt and simultaneous respiratory and cardiac depression. Although death clearly results from respiratory arrest, our results suggest that seizure activates two central nervous system pathways in this model—one inhibits respiratory drive, whereas the other inhibits cardiac function via vagal efferents. The abrupt and simultaneous recovery of both respiration and cardiac function with mechanical ventilation within an early post-ictal timeframe shows that the vagal discharge can be rapidly terminated. Understanding the central mechanism associated with the abrupt cardiorespiratory dysfunction and equally abrupt recovery may provide clues for therapeutic targets for SUDEP.

Contrasting open-loop dynamic characteristics of sympathetic and vagal systems during baroreflex-mediated heart rate control in rats.

Although heart rate (HR) is governed by the sympathetic and parasympathetic nervous systems, a head-to-head comparison of the open-loop dynamic characteristics of the total arc from a baroreceptor pressure input to the HR response has yet to be performed. We estimated the transfer function from carotid sinus pressure input to the HR response (HCSP→HR) before and after bilateral vagotomy (n = 7) as well as before and after the administration of a β-blocker propranolol (n = 8) in anesthetized male Wistar-Kyoto rats. The carotid sinus pressure was perturbed according to a Gaussian white noise signal so that the input power spectra were relatively flat between 0.01 and 1 Hz. The gain plot of HCSP→HR was V-shaped. Vagotomy reduced the dynamic gain at 1 Hz (0.0598 ± 0.0065 to 0.0025 ± 0.0004 beats·min-1·mmHg-1, P < 0.001) but not at 0.01 or 0.1 Hz. β-Blockade reduced the dynamic gain at 0.01 Hz (0.247 ± 0.069 to 0.077 ± 0.017 beats·min-1·mmHg-1, P = 0.020) but not at 0.1 or 1 Hz. We also estimated the efferent limb transfer function from electrical vagal efferent stimulation to the HR response (HVN→HR) under β-blockade conditions. We associated the model parameters of HVN→HR with the mean HR and the standard deviation of HR so that HVN→HR could be estimated based only on the HR data. We finally estimated the neural arc transfer function from a pressure input to efferent vagal nerve activity by dividing HCSP→HR by HVN→HR. The mathematically determined vagal neural arc showed derivative characteristics with its phase near zero radians at the lowest frequency.

EA at PC6 Promotes Gastric Motility: Role of Brainstem Vagovagal Neurocircuits.

Background We aimed to assess whether electroacupuncture (EA) at PC6 affects gastric motility via the vagovagal reflex and if so whether brainstem vagovagal neurocircuits and related transmitters are involved. Methods Gastric motility was measured in male Sprague-Dawley (SD) rats by placing a small manometric balloon in the gastric antrum. The rats were subjected to control, sham surgery, vagotomy, sympathectomy, and microinjection group, including artificial cerebrospinal fluid, gamma-aminobutyric acid (GABA), and glutamic acid (L-Glu). The effect of EA at PC6 on gastric motility was measured. Moreover, electrophysiological testing was used to measure the effect of EA at PC6 on the parasympathetic and sympathetic nerves. In addition, artificial cerebrospinal fluid, L-Glu, and GABA have been microinjected into the dorsal motor nucleus of the vagus (DMV) to measure the changes in gastric motility and parasympathetic nerve discharge induced by EA at PC6. Key Results EA facilitated the gastric motility in control group. In the vagotomy group, gastric motility was not affected by EA at PC6. However, in the sympathectomy group, gastric motility was similar to control group. Acupuncture at PC6 increased parasympathetic nerve discharge but not sympathetic nerve discharge. Furthermore, the microinjection of L-Glu into the DMV increased gastric motility, although EA at PC6 showed no remarkable change in this group. The injection of GABA reduced gastric motility and parasympathetic nerve discharge, but EA at PC6 significantly increased gastric motility and the parasympathetic nerve discharge in this group. Conclusions and Inferences EA at PC6—primarily by inhibiting GABA transmission to DMV—reduced the inhibition of efferent vagal motor fibers and thus promoted efferent vagus nerve activity and increased gastric motility.

Intravenous ivabradine augments the dynamic heart rate response to moderate vagal nerve stimulation in anesthetized rats.

Ivabradine is a selective bradycardic agent that reduces the heart rate (HR) by inhibiting the hyperpolarization-activated cyclic nucleotide-gated channels. Although its cardiovascular effect is thought to be minimal except for inducing bradycardia, ivabradine could interact with cardiovascular regulation by the autonomic nervous system. We tested whether ivabradine modifies dynamic characteristics of peripheral vagal HR control. In anesthetized Wistar-Kyoto rats (n = 7), the right vagal nerve was sectioned and stimulated for 10 min according to a binary white noise sequence with a switching interval of 500 ms. The efferent vagal nerve stimulation (VNS) trials were performed using three different rates (10, 20, and 40 Hz), and were designated as V0-10, V0-20, and V0-40, respectively. The transfer function from the VNS to the HR was estimated before and after the intravenous administration of ivabradine (2 mg/kg). Ivabradine increased the asymptotic dynamic gain in V0-20 [from 3.88 (1.78-5.79) to 6.62 (3.12-8.31) beats·min-1·Hz-1, P < 0.01, median (range)] but not in V0-10 or V0-40. Ivabradine increased the corner frequency in V0-10 [from 0.032 (0.026-0.041) to 0.064 (0.029-0.090) Hz, P < 0.01] and V0-20 [from 0.040 (0.037-0.056) to 0.068 (0.051-0.100) Hz, P < 0.01] but not in V0-40. In conclusion, ivabradine augmented the dynamic HR response to moderate VNS. At high VNS, however, ivabradine did not significantly augment the dynamic HR response, possibly because ivabradine reduced the baseline HR and limited the range for the bradycardic response to high VNS.NEW & NOTEWORTHY Ivabradine is considered to be a pure bradycardic agent that has little effect on cardiovascular function except inducing bradycardia. The present study demonstrated that ivabradine interacts with the dynamic vagal heart rate control in a manner that augments the heart rate response to moderate-intensity efferent vagal nerve stimulation.

Vagal mechanisms as neuromodulatory targets for the treatment of metabolic disease.

With few effective treatments available, the global rise of metabolic diseases, including obesity, type 2 diabetes mellitus, and cardiovascular disease, seems unstoppable. Likely caused by an obesogenic environment interacting with genetic susceptibility, the pathophysiology of obesity and metabolic diseases is highly complex and involves crosstalk between many organs and systems, including the brain. The vagus nerve is in a key position to bidirectionally link several peripheral metabolic organs with the brain and is increasingly targeted for neuromodulation therapy to treat metabolic disease. Here, we review the basics of vagal functional anatomy and its implications for vagal neuromodulation therapies. We find that most existing vagal neuromodulation techniques either ignore or misinterpret the rich functional specificity of both vagal efferents and afferents as demonstrated by a large body of literature. This lack of specificity of manipulating vagal fibers is likely the reason for the relatively poor beneficial long‐term effects of such therapies. For these therapies to become more effective, rigorous validation of all physiological endpoints and optimization of stimulation parameters as well as electrode placements will be necessary. However, given the large number of function‐specific fibers in any vagal branch, genetically guided neuromodulation techniques are more likely to succeed.