Continuous low-level vagus nerve stimulation reduces stellate ganglion nerve activity and paroxysmal atrial tachyarrhythmias in ambulatory canines

Role of neural modulation in the pathophysiology of atrial fibrillation

Atrial-fibrillation (AF) is the most common clinically encountered arrhythmia affecting over 1 per cent of population in the United States and its prevalence seems to be moving only in forward direction. A recent systemic review estimates global prevalence of AF to be 596.2 and 373.1 per 100,000 population in males and females respectively. Multiple mechanisms have been put forward in the pathogenesis of AF, however; multiple wavelet hypothesis is the most accepted theory so far. Similar to the conduction system of the heart, a neural network exists which surrounds the heart and plays an important role in formation of the substrate of AF and when a trigger is originated, usually from pulmonary vein sleeves, AF occurs. This neural network includes ganglionated plexi (GP) located adjacent to pulmonary vein ostia which are under control of higher centers in normal people. When these GP become hyperactive owing to loss of inhibition from higher centers e.g. in elderly, AF can occur. We can control these hyperactive GP either by stimulating higher centers and their connections, e.g. vagus nerve stimulation or simply by ablating these GP. This review provides detailed information about the different proposed mechanisms underlying AF, the exact role of autonomic neural tone in the pathogenesis of AF and the possible role of neural modulation in the treatment of AF.

Atrial remodeling and atrial fibrillation: recent advances and translational perspectives

Atrial fibrillation (AF) is the most common sustained arrhythmia in clinical practice. AF and its complications are responsible for important population morbidity and mortality. Presently available therapeutic approaches have limited efficacy and nontrivial potential to cause adverse effects. Thus, new mechanistic knowledge is essential for therapeutic innovation. Atrial arrhythmogenic remodeling, defined as any change in atrial structure or function that promotes atrial arrhythmias, is central to AF. Remodeling can be due to underlying cardiac conditions, systemic processes and conditions such as aging, or AF itself. Recent work has underlined the importance of remodeling in AF, provided new insights into basic mechanisms, and identified new biomarker/imaging approaches to follow remodeling processes. The importance of intracellular Ca(2+) handling abnormalities has been highlighted, both for the induction of triggered ectopic activity and for the activation of Ca(2+)-related cell signaling that mediates profibrillatory remodeling. The importance of microRNAs, which are a new class of small noncoding sequences that regulate gene expression, has emerged in both electrical and structural remodeling. Remodeling related to aging, cardiac disease, and AF itself is believed to underlie the progressive nature of the arrhythmia, which contributes to the complexities of long-term management. New tools that are being developed to quantify remodeling processes and monitor their progression include novel biomarkers, imaging modalities to quantify/localize fibrosis, and noninvasive monitoring/mapping to better characterize the burden of AF and identify arrhythmic sources. This report reviews recent advances in the understanding of the basic pathophysiology of atrial remodeling and potential therapeutic implications.

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.

Intermittent vagal nerve stimulation alters the electrophysiological properties of atrium in the myocardial infarction rat model

Intermittent vagal nerve stimulation (VNS) has emerged as a potential therapy to treat cardiovascular diseases by delivering electrical stimulation to the vagus nerves. The purpose of this study was to investigate the electrophysiological changes in the atrium resulting from long-term intermittent VNS therapy in the chronic myocardial infarction (MI) rat model. MI was induced via left anterior descending coronary artery (LAD) ligation in male Sprague-Dawley rats, randomized into two groups: MI (implanted with nonfunctional VNS stimulators) and MI-VNS (implanted with functional VNS stimulators and received chronic intermittent VNS treatment) groups. Further, a sham group was used as control in which MI was not performed and received nonfunctional VNS stimulators. At 12 weeks, optical mapping of right atrium (RA) of sinus rhythm was performed. Our results demonstrated that chronic MI changed the electrical properties of the atrium action potentials and resulted in reduced action potential duration at 50% (APD50) and 80% (APD80) repolarization. Chronic right cervical VNS restored the APD back to healthy heart APD values. Additionally, APD heterogeneity index increased as a result of the chronic MI. Chronic VNS was not found to alter this increase. By calculating PR intervals from weekly ECG recordings of anaesthetized rats, we demonstrated that chronic MI and intermittent VNS did not affect the AV conduction time from the atria to the ventricles. From our study, we conclude the MI decreased the APD and increased APD spatial dispersion. VNS increased the APD back to healthy normal values but did change the APD spatial dispersion and the electrical conduction in the RA.

Autonomic nerve activity and blood pressure in ambulatory dogs

BACKGROUND

The relationship between cardiac autonomic nerve activity and blood pressure (BP) changes in ambulatory dogs is unclear.

OBJECTIVE

The purpose of this study was to test the hypotheses that simultaneous termination of stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA) predisposes to spontaneous orthostatic hypotension and that specific β₂-adrenoceptor blockade prevents the hypotensive episodes.

METHODS

We used a radiotransmitter to record SGNA, VNA, and BP in eight ambulatory dogs. Video imaging was used to document postural changes.

RESULTS

Of these eight dogs, five showed simultaneous sympathovagal discharges in which the minute-by-minute integrated SGNA correlated with integrated VNA in a linear pattern (group 1). In these dogs, abrupt termination of simultaneous SGNA-VNA at the time of postural changes (as documented by video imaging) was followed by abrupt (>20 mm Hg over four beats) drops in BP. Dogs without simultaneous on/off firing (group 2) did not have drastic drops in pressure. ICI-118,551 (ICI, a specific β₂-blocker) infused at 3 µg/kg/h for 7 days significantly increased BP from 126 mm Hg (95% confidence interval 118-133) to 133 mm Hg (95% confidence interval 125-141; P = .0001). The duration of hypotension (mean systolic BP <100 mm Hg) during baseline accounted for 7.1% of the recording. The percentage was reduced by ICI to 1.3% (P = .01).

CONCLUSION

Abrupt simultaneous termination of SGNA-VNA was observed at the time of orthostatic hypotension in ambulatory dogs. Selective β₂-adrenoceptor blockade increased BP and reduced the duration of hypotension in this model.

The Role of the Atrial Neural Network In Atrial Fibrillation: The Metastatic Progression Hypothesis

With the advent of catheter ablation of atrial fibrillation (AF) there has been acceleration in our understanding of the mechanisms underlying the etiology of this common clinical arrhythmia. In this regard, the role of the intrinsic cardiac autonomic nervous system in the initiation and maintenance of AF began to receive attention in numerous experimental and clinical investigations. Up to now, the focus has been on the large ganglionated plexi (GP) which are located in the posterior left atrium mainly at the pulmonary vein-atrial junctions. As long term outcomes have been reported and single procedures have indicated diminished success rates particularly for persistent/long standing persistent AF, emphasis has begun to shift away from the pulmonary vein isolation (PVI) alone as well as GP ablation with or without PVI. An understanding of the atrial substrate represented by the extensions of the intrinsic cardiac autonomic system constituting the atrial neural network is beginning to evolve. In this review, the contribution of the intrinsic cardiac autonomic nervous system to the etiology of AF is addressed, particularly in regard to the greater prevalence of AF in the elderly. In addition, we emphasize the involvement of the atrial neural network in the "metastatic" progression of paroxysmal to persistent and long standing persistent forms of AF.

Low-amplitude, left vagus nerve stimulation significantly attenuates ventricular dysfunction and infarct size through prevention of mitochondrial dysfunction during acute ischemia-reperfusion injury

BACKGROUND

Right cervical vagus nerve stimulation (VNS) provides cardioprotective effects against acute ischemia-reperfusion injury in small animals. However, inconsistent findings have been reported.

OBJECTIVE

To determine whether low-amplitude, left cervical VNS applied either intermittently or continuously imparts cardioprotection against acute ischemia-reperfusion injury.

METHODS

Thirty-two isoflurane-anesthetized swine (25-30 kg) were randomized into 4 groups: control (sham operated, no VNS), continuous-VNS (C-VNS; 3.5 mA, 20 Hz), intermittent-VNS (I-VNS; continuously recurring cycles of 21-second ON, 30-second OFF), and I-VNS + atropine (1 mg/kg). Left cervical VNS was applied immediately after left anterior descending artery occlusion (60 minutes) and continued until the end of reperfusion (120 minutes). The ischemic and nonischemic myocardium was harvested for cardiac mitochondrial function assessment.

RESULTS

VNS significantly reduced infarct size, improved ventricular function, decreased ventricular fibrillation episodes, and attenuated cardiac mitochondrial reactive oxygen species production, depolarization, and swelling, compared with the control group. However, I-VNS produced the most profound cardioprotective effects, particularly infarct size reduction and decreased ventricular fibrillation episodes, compared to both I-VNS + atropine and C-VNS. These beneficial effects of VNS were abolished by atropine.

CONCLUSIONS

During ischemia-reperfusion injury, both C-VNS and I-VNS provide significant cardioprotective effects compared with I-VNS + atropine. These beneficial effects were abolished by muscarinic blockade, suggesting the importance of muscarinic receptor modulation during VNS. The protective effects of VNS could be due to its protection of mitochondrial function during ischemia-reperfusion.

Effects of low-level autonomic stimulation on prevention of atrial fibrillation induced by acute electrical remodeling

Background. Rapid atrial pacing (RAP) can induce electrical and autonomic remodeling and facilitate atrial fibrillation (AF). Recent reports showed that low-level vagosympathetic nerve stimulation (LLVNS) can suppress AF, as an antiarrhythmic effect. We hypothesized that LLVNS can reverse substrate heterogeneity induced by RAP. Methods and Results. Mongrel dogs were divided into (LLVNS+RAP) and RAP groups. Electrode catheters were sutured to multiple atrial sites, and LLVNS was applied to cervical vagosympathetic trunks with voltage 50% below the threshold slowing sinus rate by ⩽30 msec. RAP induced a significant decrease in effective refractory period (ERP) and increase in the window of vulnerability at all sites, characterized by descending and elevated gradient differences towards the ganglionic plexi (GP) sites, respectively. The ERP dispersion was obviously enlarged by RAP and more significant when the ERP of GP-related sites was considered. Recovery time from AF was also prolonged significantly as a result of RAP. LLVNS could reverse all these changes induced by RAP and recover the heterogeneous substrate to baseline. Conclusions. LLVNS can reverse the electrical and autonomic remodeling and abolish the GP-central gradient differences induced by RAP, and thus it can recover the homogeneous substrate, which may be the underlying mechanism of its antiarrhythmic effect.

Adjunctive renal sympathetic denervation to modify hypertension as upstream therapy in the treatment of atrial fibrillation (H-FIB) study: clinical background and study design

BACKGROUND

Hypertension is the most important risk factor directly attributable to the high prevalence of atrial fibrillation (AF), and is one of the few modifiable risk factors for AF. Activation and overactivity of the sympathetic nervous system (SNS) have been implicated in the pathogenesis of both essential hypertension and AF. Catheter-based renal sympathetic denervation (RSDN) appears to be an effective adjunctive treatment for refractory hypertension, and may be beneficial in other conditions characterized by SNS overactivity, such as left ventricular hypertrophy and atrial arrhythmias.

OBJECTIVE

The H-FIB study is a multicenter prospective, double-blind, randomized (1:1) controlled trial. The primary efficacy endpoint is antiarrhythmic drug-free freedom from AF recurrence through 12 months.

METHODS

Patients with a history of significant hypertension who are receiving treatment with at least one antihypertensive agent who are planned for a first time ablation for symptomatic paroxysmal or persistent AF will be randomized to either AF ablation alone (control group) or AF ablation + RSDN (study group).

CONCLUSIONS

H-FIB is a multicenter, randomized trial that will test the hypothesis that adjunctive renal sympathetic denervation, at the time of AF ablation, will increase the freedom from recurrent AF.

Low-level vagus nerve stimulation upregulates small conductance calcium-activated potassium channels in the stellate ganglion

BACKGROUND

Small conductance calcium-activated potassium (SK) channels are responsible for afterhyperpolarization that suppresses nerve discharges.

OBJECTIVES

To test the hypothesis that low-level vagus nerve stimulation (LL-VNS) leads to the upregulation of SK2 proteins in the left stellate ganglion.

METHODS

Six dogs (group 1) underwent 1-week LL-VNS of the left cervical vagus nerve. Five normal dogs (group 2) were used as controls. SK2 protein levels were examined by using Western blotting. The ratio between SK2 and glyceraldehydes-3-phosphate-dehydrogenase levels was used as an arbitrary unit (AU).

RESULTS

We found higher SK2 expression in group 1 (0.124 ± 0.049 AU) than in group 2 (0.085 ± 0.031 AU; P<.05). Immunostaining showed that the density of nerve structures stained with SK2 antibody was also higher in group 1 (11,546 ± 7,271 μm(2)/mm(2)) than in group 2 (5321 ± 3164 μm(2)/mm(2); P<.05). There were significantly more ganglion cells without immunoreactivity to tyrosine hydroxylase (TH) in group 1 (11.4%±2.3%) than in group 2 (4.9% ± 0.7%; P<.05). The TH-negative ganglion cells mostly stained positive for choline acetyltransferase (95.9% ± 2.8% in group 1 and 86.1% ± 4.4% in group 2; P = .10). Immunofluorescence confocal microscopy revealed a significant decrease in the SK2 staining in the cytosol but an increase in the SK2 staining on the membrane of the ganglion cells in group 1 compared to group 2.

CONCLUSIONS

Left LL-VNS results in the upregulation of SK2 proteins, increased SK2 protein expression in the cell membrane, and increased TH-negative (mostly choline acetyltransferase-positive) ganglion cells in the left stellate ganglion. These changes may underlie the antiarrhythmic efficacy of LL-VNS in ambulatory dogs.

Effect of the stellate ganglion on atrial fibrillation and atrial electrophysiological properties and its left-right asymmetry in a canine model

OBJECTIVE

To investigate the effect of the stellate ganglion (SG) and its left-right asymmetry on atrial fibrillation (AF) inducibility, AF duration and atrial electrophysiological properties.

METHODS

Sixteen adult mongrel dogs were randomly assigned to three groups. The control group (n=4) underwent 6 h rapid atrial pacing (RAP) only; the right SG (RSG) group (n=6) underwent 6 h RSG stimulation plus RAP; and the left SG (LSG) group (n=6) underwent 6 h LSG stimulation plus RAP. AF induction rate, AF duration, effective refractory period (ERP) and dispersion of ERP (dERP) were measured.

RESULTS

In the RSG group, the induction rate of AF was significantly increased in sites in the right atrium (RA) compared with baseline (P<0.05). In the LSG group, the induction rate of AF was significantly increased (P<0.05) compared with baseline in the left atrium (LA), left superior pulmonary vein and left inferior pulmonary vein, respectively. Compared with RSG stimulation, right stellate ganglionectomy markedly decreased the AF induction rate of the RA (P<0.05). Compared with LSG stimulation, left stellate ganglionectomy markedly decreased the AF induction rate of the LA, the left superior pulmonary vein and the left inferior pulmonary vein (P<0.05). In the RSG group, the ERP was significantly shortened (P<0.05) and the dERP was significantly increased (P<0.05) in RA sites (P<0.05). The ERP was significantly shortened in the LSG group (P<0.05). The dERP was significantly increased (P<0.05) in LA and pulmonary vein sites (P<0.05).

CONCLUSIONS

Unilateral electrical stimulation of the SG in combination with RAP can successfully establish a canine model of acute AF mediated by excessive sympathetic activity. SG stimulation facilitates AF induction and aggravates electrical remodelling in sites in the atrium and pulmonary vein. Inhibiting sympathetic nerve activation through unilateral stellate ganglionectomy can reduce AF initiation.

Inhibition of atrial fibrillation by low-level vagus nerve stimulation: the role of the nitric oxide signaling pathway

PURPOSE

We examined the role of the phosphatidylinositol-3 kinase (PI3K)/nitric oxide (NO) signaling pathway in low-level vagus nerve stimulation (LLVNS)-mediated inhibition of atrial fibrillation (AF).

METHODS

In 17 pentobarbital anesthetized dogs, bilateral thoracotomies allowed the attachment of electrode catheters to the superior and inferior pulmonary veins and atrial appendages. Rapid atrial pacing (RAP) was maintained for 6 h. Each hour, programmed stimulation was used to determine the window of vulnerability (WOV), a measure of AF inducibility, at all sites. During the last 3 h, RAP was overlapped with right LLVNS (50 % below that which slows the sinus rate). In group 1 (n = 7), LLVNS was the only intervention, whereas in groups 2 (n = 6) and 3 (n = 4), the NO synthase inhibitor N (G)-nitro-L-arginine methyl ester (L-NAME) and the PI3K inhibitor wortmannin, respectively, were injected in the right-sided ganglionated plexi (GP) during the last 3 h. The duration of acetylcholine-induced AF was determined at baseline and at 6 h. Voltage-sinus rate curves were constructed to assess GP function.

RESULTS

LLVNS significantly decreased the acetylcholine-induced AF duration by 8.2 ± 0.9 min (p < 0.0001). Both L-NAME and wortmannin abrogated this effect. The cumulative WOV (the sum of the individual WOVs) decreased toward baseline with LLVNS (p < 0.0001). L-NAME and wortmannin blunted this effect during the fifth (L-NAME only, p < 0.05) and the sixth hour (L-NAME and wortmannin, p < 0.05). LLVNS suppressed the ability of GP stimulation to slow the sinus rate, whereas L-NAME and wortmannin abolished this effect.

CONCLUSION

The anti-arrhythmic effects of LLVNS involve the PI3K/NO signaling pathway.

Spinal cord stimulation protects against atrial fibrillation induced by tachypacing

BACKGROUND

Spinal cord stimulation (SCS) has been shown to modulate atrial electrophysiology and confer protection against ischemia and ventricular arrhythmias in animal models.

OBJECTIVE

To determine whether SCS reduces the susceptibility to atrial fibrillation (AF) induced by tachypacing (TP).

METHODS

In 21 canines, upper thoracic SCS systems and custom cardiac pacing systems were implanted. Right atrial and left atrial effective refractory periods were measured at baseline and after 15 minutes of SCS. Following recovery in a subset of canines, pacemakers were turned on to induce AF by alternately delivering TP and searching for AF. Canines were randomized to no SCS therapy (CTL) or intermittent SCS therapy on the initiation of TP (EARLY) or after 8 weeks of TP (LATE). AF burden (percent AF relative to total sense time) and AF inducibility (percentage of TP periods resulting in AF) were monitored weekly. After 15 weeks, echocardiography and histology were performed.

RESULTS

Effective refractory periods increased by 21 ± 14 ms (P = .001) in the left atrium and 29 ± 12 ms (P = .002) in the right atrium after acute SCS. AF burden was reduced for 11 weeks in EARLY compared with CTL (P <.05) animals. AF inducibility remained lower by week 15 in EARLY compared with CTL animals (32% ± 10% vs 91% ± 6%; P <.05). AF burden and inducibility were not significantly different between LATE and CTL animals. There were no structural differences among any groups.

CONCLUSIONS

SCS prolonged atrial effective refractory periods and reduced AF burden and inducibility in a canine AF model induced by TP. These data suggest that SCS may represent a treatment option for AF.

Surgical Atrial Fibrillation Ablation: An Electrophysiologist's Perspective

The experience and insight obtained during surgical ablation of all types of arrhythmias was formative for electrophysiology and catheter ablation. The early surgical ablation experience provided proof of concept as well direct operative observation of anatomy and pathophysiologic mechanisms. For atrial fibrillation (AF), surgical ablation anticipated many of the problems that catheter ablation subsequently encountered, although these lessons were not promptly appreciated. Rather than competition, greater cooperation and communication between surgeons and electrophysiologists in the future would be more likely to enhance understanding of the underlying pathophysiology of AF.

Neural mechanisms of atrial fibrillation

PURPOSE OF REVIEW

The autonomic nerve system is a potentially potent modulator of the initiation and perpetuation of atrial fibrillation. This review will briefly summarize the neural mechanisms of atrial fibrillation.

RECENT FINDINGS

Complex interactions exist between the sympathetic and parasympathetic nervous system on the atrial electrophysiologic properties. Direct autonomic recordings in canine models demonstrated simultaneous sympathovagal discharges are the most common triggers of paroxysmal atrial tachycardia and paroxysmal atrial fibrillation. Also, intrinsic cardiac autonomic nerve can serve as a sole triggering factor for the initiation of atrial fibrillation. Modulation of autonomic nervous system (ANS) by electrical stimulation has been tried as a treatment strategy clinically and experimentally. Recent studies showed that autonomic nervous system modulation can suppress the stellate ganglion nerve activity and reduce the incidence of paroxysmal atrial tachyarrhythmias in ambulatory dogs.

SUMMARY

The autonomic nerve system influences the initiation and perpetuation of atrial fibrillation. Scientific advances toward a better understanding of the complex interrelationships of the various components of the ANS will hopefully lead to improvement of treatments for this common arrhythmia.

Antiarrhythmic effects of vasostatin-1 in a canine model of atrial fibrillation

BACKGROUND

We examined the antiarrhythmic effects of vasostatin-1, a recently identified cardioregulatory peptide, in canine models of atrial fibrillation (AF).

METHODS AND RESULTS

In 13 pentobarbital-anesthetized dogs bilateral thoracotomies allowed the attachment of multielectrode catheters to superior and inferior pulmonary veins and atrial appendages (AA). Rapid atrial pacing (RAP) was maintained for 6 hours. Each hour, programmed stimulation was performed to determine the window of vulnerability (WOV), a measure of AF inducibility, at all sites. During the last 3 hours, vasostatin-1, 33 nM, was injected into the anterior right (AR) ganglionated plexus (GP) and inferior right (IR) GP every 30 minutes (n = 6). Seven dogs underwent 6 hours of RAP only (controls). At baseline, acetylcholine, 100 mM, was applied on the right AA and AF duration was recorded before and after injection of vasostatin-1, 33 nM, into the ARGP and IRGP. In separate experiments (n = 8), voltage-sinus rate response curves (surrogate for GP function) were constructed by applying high-frequency stimulation to the ARGP with incremental voltages with or without vasostatin-1. Vasostatin-1 significantly decreased the duration of acetylcholine-induced AF (11.0 ± 4.1 vs 5.5 ± 2.6 min, P = 0.02). The cumulative WOV (the sum of individual WOVs) significantly increased (P < 0.0001) during the first 3 hours and decreased toward baseline in the presence of vasostatin-1 (P < 0.0001). Cumulative WOV in controls steadily increased. Vasostatin-1 blunted the slowing of sinus rate with increasing stimulation voltage of ARGP.

CONCLUSIONS

Vasostatin-1 suppresses AF inducibility, likely by inhibiting GP function. These data may provide new insights into the role of peptide neuromodulators for AF therapy.

Circulation: Arrhythmia and Electrophysiology Editors' Picks

The following articles are being highlighted as part of Circulation: Arrhythmia and Electrophysiology's Topic Review series. This series will summarize the most important articles, as selected by the editors, published in Circulation: Arrhythmia and Electrophysiology, Circulation, and the other Circulation subspecialty journals. The studies included in this article represent the most read articles published on the topic of bradyarrhythmias in 2010 and 2011.

Neuromodulation targets intrinsic cardiac neurons to attenuate neuronally mediated atrial arrhythmias

Our objective was to determine whether atrial fibrillation (AF) results from excessive activation of intrinsic cardiac neurons (ICNs) and, if so, whether select subpopulations of neurons therein represent therapeutic targets for suppression of this arrhythmogenic potential. Trains of five electrical stimuli (0.3-1.2 mA, 1 ms) were delivered during the atrial refractory period to mediastinal nerves (MSN) on the superior vena cava to evoke AF. Neuroanatomical studies were performed by injecting the neuronal tracer DiI into MSN sites that induced AF. Functional studies involved recording of neuronal activity in situ from the right atrial ganglionated plexus (RAGP) in response to MSN stimulation (MSNS) prior to and following neuromodulation involving either preemptive spinal cord stimulation (SCS; T(1)-T(3), 50 Hz, 200-ms duration) or ganglionic blockade (hexamethonium, 5 mg/kg). The tetramethylindocarbocyanine perchlorate (DiI) neuronal tracer labeled a subset (13.2%) of RAGP neurons, which also colocalized with cholinergic or adrenergic markers. A subset of DiI-labeled RAGP neurons were noncholinergic/nonadrenergic. MSNS evoked an ∼4-fold increase in RAGP neuronal activity from baseline, which SCS reduced by 43%. Hexamethonium blocked MSNS-evoked increases in neuronal activity. MSNS evoked AF in 78% of right-sided MSN sites, which SCS reduced to 33% and hexamethonium reduced to 7%. MSNS-induced bradycardia was maintained with SCS but was mitigated by hexamethonium. We conclude that MSNS activates subpopulations of intrinsic cardiac neurons, thereby resulting in the formation of atrial arrhythmias leading to atrial fibrillation. Stabilization of ICN local circuit neurons by SCS or the local circuit and autonomic efferent neurons with hexamethonium reduces the arrhythmogenic potential.