Immunohistochemical characterization of the intrinsic cardiac neural plexus in whole-mount mouse heart preparations

Pulmonary Vein Ganglia Are Remodeled in the Diabetic Heart

Background Cardiac autonomic neuropathy is thought to cause adverse cardiovascular effects in diabetes mellitus. Pulmonary vein ganglia ( PVG ), which have been implicated in normal and abnormal heart rhythm regulation, have not been fully investigated in type 1 diabetes mellitus (T1D). We examined the functional and anatomical effects of T1D on PVG and studied the details of T1D-induced remodeling on the PVG structure and function. Methods and Results We used a mouse model of T1D (Akita mouse), immunofluorescence, isolated Langendorff-perfused hearts, and mathematical simulations to explore the effects of T1D on PVG . Whole-mount atrial immunofluorescence of choline acetyltransferase and tyrosine hydroxylase labeling showed that sympathetic and parasympathetic somas of the PVG neurons were significantly hypotrophied in T1D hearts versus wild type. Stimulation of PVG in isolated Langendorff-perfused hearts caused more pronounced P-P interval prolongation in wild type compared with Akita hearts. Propranolol resulted in a comparable P-P prolongation in both phenotypes, and atropine led to more pronounced P-P interval shortening in wild type compared with Akita hearts. Numerical modeling using network simulations revealed that a decrease in the sympathetic and parasympathetic activities of PVG in T1D could explain the experimental results. Conclusions T1D leads to PVG remodeling with hypotrophy of sympathetic and parasympathetic cell bodies and a concomitant decrease in the PVG sympathetic and parasympathetic activities.

The local repolarization heterogeneity in the murine pulmonary veins myocardium contributes to the spatial distribution of the adrenergically induced ectopic foci

An atrial tachyarrhythmias is predominantly triggered by a proarrhythmic activity originate from the pulmonary veins (PV) myocardial sleeves; sympathetic or adrenergic stimulation facilitates PV proarrhythmia. In the present study the electrophysiological inhomogeneity, spatiotemporal characteristics of the adrenergically induced ectopic firing and sympathetic nerves distribution have been investigated in a murine PV myocardium to clarify mechanisms of adrenergic PV ectopy. Electrically paced murine PV demonstrate atrial-like pattern of conduction and atrial-like action potentials (AP) with longest duration in the mouth of PV. The application of norepinephrine (NE), agonists of α- and β-adrenergic receptors (ARs) or intracardiac nerves stimulation induced spontaneous AP in a form of periodical bursts or continuous firing. NE- or ARs agonists-induced SAP originated from unifocal ectopic foci with predominant localization in the region surrounding PV mouth, but not in the distal portions of a murine PV myocardium. A higher level of catecholamine content and catecholamine fiber network density was revealed in the PV myocardial sleeves relative to LA appendage. However, no significant local variation of catecholamine content and fiber density was observed in the murine PV. In conclusion, PV mouth region appear to be a most susceptible to adrenergic proarrhythmia in mice. Intrinsic spatial heterogeneity of AP duration can be considered as a factor influencing localization of the ectopic foci in PV.

The expressions of some metabolic hormones (leptin, ghrelin and obestatin) in the tissues of sheep tongue

In this study, we aimed to observe the localization and expression of peptide hormones-leptin, ghrelin and obestatin-in the sheep tongue by immunohistochemistry. For that purpose, tongues of ten adult sheep were used. Leptin expression of moderate intensity was observed in the basal and parabasal epithelial cells of the luminal epithelium, and leptin was strongly expressed in the taste buds of the circumvallate and fungiform papillae and in von Ebner's glands. Ghrelin was primarily expressed in some of the skeletal muscle cells and the smooth muscle cells of the middle layer of blood vessels. A strong expression was observed in the epithelial cells lining the base of the groove surrounding the circumvallate papillae. Obestatin expression was particularly strong in the epithelial cells of the salivary ducts. It was also stronger in the von Ebner's glands than in the seromucous glands. Leptin, ghrelin and obestatin were shown to be produced at varying levels in different cell types, including epithelial, stromal and skeletal muscle cells, as well as in ganglion neurons, neural plexuses and blood vessels in the sheep tongue. Cellular localization and expression of these peptide hormones have not been investigated in many species including sheep.

Transmural Autonomic Regulation of Cardiac Contractility at the Intact Heart Level

The relationship between cardiac excitability and contractility depends on when Ca²⁺ influx occurs during the ventricular action potential (AP). In mammals, it is accepted that Ca²⁺ influx through the L-type Ca²⁺ channels occurs during AP phase 2. However, in murine models, experimental evidence shows Ca²⁺ influx takes place during phase 1. Interestingly, Ca²⁺ influx that activates contraction is highly regulated by the autonomic nervous system. Indeed, autonomic regulation exerts multiple effects on Ca²⁺ handling and cardiac electrophysiology. In this paper, we explore autonomic regulation in endocardial and epicardial layers of intact beating mice hearts to evaluate their role on cardiac excitability and contractility. We hypothesize that in mouse cardiac ventricles the influx of Ca²⁺ that triggers excitation–contraction coupling (ECC) does not occur during phase 2. Using pulsed local field fluorescence microscopy and loose patch photolysis, we show sympathetic stimulation by isoproterenol increased the amplitude of Ca²⁺ transients in both layers. This increase in contractility was driven by an increase in amplitude and duration of the L-type Ca²⁺ current during phase 1. Interestingly, the β-adrenergic increase of Ca²⁺ influx slowed the repolarization of phase 1, suggesting a competition between Ca²⁺ and K⁺ currents during this phase. In addition, cAMP activated L-type Ca²⁺ currents before SR Ca²⁺ release activated the Na⁺-Ca²⁺ exchanger currents, indicating Ca_v1.2 channels are the initial target of PKA phosphorylation. In contrast, parasympathetic stimulation by carbachol did not have a substantial effect on amplitude and kinetics of endocardial and epicardial Ca²⁺ transients. However, carbachol transiently decreased the duration of the AP late phase 2 repolarization. The carbachol-induced shortening of phase 2 did not have a considerable effect on ventricular pressure and systolic Ca²⁺ dynamics. Interestingly, blockade of muscarinic receptors by atropine prolonged the duration of phase 2 indicating that, in isolated hearts, there is an intrinsic release of acetylcholine. In addition, the acceleration of repolarization induced by carbachol was blocked by the acetylcholine-mediated K⁺ current inhibition. Our results reveal the transmural ramifications of autonomic regulation in intact mice hearts and support our hypothesis that Ca²⁺ influx that triggers ECC occurs in AP phase 1 and not in phase 2.

Identification of peripheral neural circuits that regulate heart rate using optogenetic and viral vector strategies

Heart rate is under the precise control of the autonomic nervous system. However, the wiring of peripheral neural circuits that regulate heart rate is poorly understood. Here, we develop a clearing-imaging-analysis pipeline to visualize innervation of intact hearts in 3D and employed a multi-technique approach to map parasympathetic and sympathetic neural circuits that control heart rate in mice. We identify cholinergic neurons and noradrenergic neurons in an intrinsic cardiac ganglion and the stellate ganglia, respectively, that project to the sinoatrial node. We also report that the heart rate response to optogenetic versus electrical stimulation of the vagus nerve displays different temporal characteristics and that vagal afferents enhance parasympathetic and reduce sympathetic tone to the heart via central mechanisms. Our findings provide new insights into neural regulation of heart rate, and our methodology to study cardiac circuits can be readily used to interrogate neural control of other visceral organs.

The wiring of peripheral neural circuits that regulate heart rate is poorly understood. In this study, authors used tissue clearing for high-resolution characterization of nerves in the heart in 3D and transgenic and novel viral vector approaches to identify peripheral parasympathetic and sympathetic neuronal populations involved in heart rate control in mice.

Sudden Heart Rate Reduction Upon Optogenetic Release of Acetylcholine From Cardiac Parasympathetic Neurons in Perfused Hearts

The balance of sympathetic and parasympathetic tone provides exquisite control of heart rate and contractility and has also been shown to modulate coronary flow and inflammation. Understanding how autonomic balance is altered by cardiac disease is an active area of research, and developing new ways to control this balance provides insights into disease therapies. However, achieving acute neuron-specific stimulation of autonomic neurons can be difficult in experiments that measure the acute effects of nerve stimulation on the heart. Conventional electrical and pharmacological approaches can be spatially and temporally non-selective. Cell-specific expression of light-activated channels (channelrhodopsin, ChR2) is a powerful approach that enables control of the timing and distribution of cellular stimulation using light. We present such an optogenetic approach where parasympathetic cardiac neurons are selectively photoactivated at high temporal precision to initiate cholinergic-mediated slowing of heart rate. Mice were crossbred to express ChR2 in peripheral cholinergic neurons using Cre-Lox recombination driven by a choline acetyltransferase (ChAT) promoter. Hearts from adult mice were excised, perfused, and the epicardium was illuminated (peak 460–465 nm) to photoactivate ChR2. In one set of studies, hearts were illuminated using a large-field LED light source. In other studies, a micro LED was placed on the right atrium to selectively illuminate the junction of the superior vena cava (SVC) and right atrium. The ECG was acquired before, during, and after tissue illumination to measure changes in heart rate. Upon illumination, hearts exhibited sudden and dramatic reductions in heart rate with restoration of normal heart rate after cessation of illumination. Delays in atrioventricular conduction were also observed. Heart rate reductions at the highest irradiance levels were similar to heart rate reductions caused by application of bethanechol (10 μM) or acetylcholine (800 μM). Atropine (50 nM) completely blocked the effect of ChR2 photoactivation, confirming cholinergic mediation. Optogenetic activation of intrinsic parasympathetic neurons reduced heart rate in an immediate, dose-dependent fashion, resembling the slowing of sinus rate in response to acetylcholine. Our results demonstrate a new approach for controlling parasympathetic modulation of cardiac function by selectively activating the endogenous release of acetylcholine from intrinsic cardiac cholinergic neurons.

Key Message: Optogenetic photoactivation of intrinsic cardiac neurons provides immediate, tissue-specific stimulation with minimal cross-reactivity. Our results demonstrate that selective expression of channelrhodopsin within cardiac cholinergic neurons enables photoactivated release of acetylcholine, thereby instantaneously slowing sinus rate and altering atrioventricular conduction. This provides for in-depth examination of the endogenous interplay between cardiac autonomic neurons and the functional outcomes of downstream post-synaptic receptor activation.

Ganglion cardiacum or juxtaductal body of human fetuses

The ganglion cardiacum or juxtaductal body is situated along the left recurrent laryngeal nerve in the aortic window and is an extremely large component of the cardiac nerve plexus. This study was performed to describe the morphologies of the ganglion cardiacum or juxtaductal body in human fetuses and to compare characteristics with intracardiac ganglion. Ganglia were immunostained in specimens from five fetuses of gestational age 12–16 weeks and seven fetuses of gestational age 28–34 weeks. Many ganglion cells in the ganglia were positive for tyrosine hydroxylase (TH; sympathetic nerve marker) and chromogranin A, while a few neurons were positive for neuronal nitric oxide synthase (NOS; parasympathetic nerve marker) or calretinin. Another ganglion at the base of the ascending aorta carried almost the same neuronal populations, whereas a ganglion along the left common cardinal vein contained neurons positive for chromogranin A and NOS but no or few TH-positive neurons, suggesting a site-dependent difference in composite neurons. Mixtures of sympathetic and parasympathetic neurons within a single ganglion are consistent with the morphology of the cranial base and pelvic ganglia. Most of the intracardiac neurons are likely to have a non-adrenergic non-cholinergic phenotype, whereas fewer neurons have a dual cholinergic/noradrenergic phenotype. However, there was no evidence showing that chromogranin A- and/or calretinin-positive cardiac neurons corresponded to these specific phenotypes. The present study suggested that the ganglion cardiacum was composed of a mixture of sympathetic and parasympathetic neurons, which were characterized the site-dependent differences in and near the heart.

The non-neuronal cholinergic system in the heart: A comprehensive review

The autonomic influences on the heart have a ying-yang nature, albeit oversimplified, the interplay between the sympathetic and parasympathetic system (known as the cholinergic system) is often complex and remain poorly understood. Recently, the heart has been recognized to consist of neuronal and non-neuronal cholinergic system (NNCS). The existence of cardiac NNCS has been confirmed by the presence of cholinergic markers in the cardiomyocytes, which are crucial for synthesis (choline acetyltransferase, ChAT), storage (vesicular acetylcholine transporter, VAChT), reuptake of choline for synthesis (high-affinity choline transporter, CHT1) and degradation (acetylcholinesterase, AChE) of acetylcholine (ACh). The non-neuronal ACh released from cardiomyocytes is believed to locally regulate some of the key physiological functions of the heart, such as regulation of heart rate, offsetting hypertrophic signals, maintenance of action potential propagation as well as modulation of cardiac energy metabolism via the muscarinic ACh receptor in an auto/paracrine manner. Apart from this, several studies have also provided evidence for the beneficial role of ACh released from cardiomyocytes against cardiovascular diseases such as sympathetic hyperactivity-induced cardiac remodeling and dysfunction as well as myocardial infarction, confirming the important role of NNCS in disease prevention. In this review, we aim to provide a fundamental overview of cardiac NNCS, and information about its physiological role, regulatory factors as well as its cardioprotective effects. Finally, we propose the different approaches to target cardiac NNCS as an adjunctive treatment to specifically address the withdrawal of neuronal cholinergic system in cardiovascular disease such as heart failure.

Limulus and heart rhythm

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cell-derived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

Atrial Innervation Patterns of Intrinsic Cardiac Autonomic Nerves

BACKGROUND

Although ganglionated plexi (GPs) are important in the pathogenesis of arrhythmia, their patterns of atrial innervation have remained unclear. We investigated patterns of GP innervation to cardiac atria and the neuroanatomical interconnections among GPs in an animal model.

METHODS

Atrial innervation by GPs was evaluated in 10 mongrel dogs using a retrograde neuronal tracer (cholera toxin subunit B [CTB] conjugated with fluorescent dyes). In Experiment 1, CTB was injected into the atria. In Experiment 2, CTB was injected into the major GP, including the anterior right GP (ARGP), inferior right GP (IRGP), superior left GP (SLGP), and ligament of Marshall (LOM). After 7 days, the GPs were examined for the presence of tracer-positive neurons.

RESULTS

GPs in either right or left-side were innervating to both the same and opposite sides of the atrium. In quantitative analysis, right-sided GPs, especially ARGP, showed numerical predominance in atrial innervation. Based on the proportion of CTB-labeled ganglion in each GP, atrial innervation by GPs showed a tendency of laterality. In Experiment 2, CTB that was injected to a particular GP widely distributed in different GP. ARGP projected the largest number of innervating neurons to the IRGP, SLGP and LOM.

CONCLUSION

This study demonstrated that GPs project axons widely to both the same and opposite sides of atria. ARGP played a dominant role in atrial innervation. Furthermore, there were numerous neuroanatomical interconnections among GPs. These findings about neuronal innervation and interconnections of GPs could offer useful information for understanding intrinsic cardiac nervous system neuroanatomy.

Electrophysiological effects of nicotinic and electrical stimulation of intrinsic cardiac ganglia in the absence of extrinsic autonomic nerves in the rabbit heart

Background

The intrinsic cardiac nervous system is a rich network of cardiac nerves that converge to form distinct ganglia and extend across the heart and is capable of influencing cardiac function.

Objective

The goals of this study were to provide a complete picture of the neurotransmitter/neuromodulator profile of the rabbit intrinsic cardiac nervous system and to determine the influence of spatially divergent ganglia on cardiac electrophysiology.

Methods

Nicotinic or electrical stimulation was applied at discrete sites of the intrinsic cardiac nerve plexus in the Langendorff-perfused rabbit heart. Functional effects on sinus rate and atrioventricular conduction were measured. Immunohistochemistry for choline acetyltransferase (ChAT), tyrosine hydroxylase, and/or neuronal nitric oxide synthase (nNOS) was performed using whole mount preparations.

Results

Stimulation within all ganglia produced either bradycardia, tachycardia, or a biphasic brady-tachycardia. Electrical stimulation of the right atrial and right neuronal cluster regions produced the largest chronotropic responses. Significant prolongation of atrioventricular conduction was predominant at the pulmonary vein-caudal vein region. Neurons immunoreactive (IR) only for ChAT, tyrosine hydroxylase, or nNOS were consistently located within the limits of the hilum and at the roots of the right cranial and right pulmonary veins. ChAT-IR neurons were most abundant (1946 ± 668 neurons). Neurons IR only for nNOS were distributed within ganglia.

Conclusion

Stimulation of intrinsic ganglia, shown to be of phenotypic complexity but predominantly of cholinergic nature, indicates that clusters of neurons are capable of independent selective effects on cardiac electrophysiology, therefore providing a potential therapeutic target for the prevention and treatment of cardiac disease.

A brain within the heart: A review on the intracardiac nervous system

Cardiac function is under the control of the autonomic nervous system, composed by the parasympathetic and sympathetic divisions, which are finely tuned at different hierarchical levels. While a complex regulation occurs in the central nervous system involving the insular cortex, the amygdala and the hypothalamus, a local cardiac regulation also takes place within the heart, driven by an intracardiac nervous system. This complex system consists of a network of ganglionic plexuses and interconnecting ganglions and axons. Each ganglionic plexus contains numerous intracardiac ganglia that operate as local integration centres, modulating the intricate autonomic interactions between the extrinsic and intracardiac nervous systems. Herein, we summarize the current understanding on the intracardiac nervous system, and acknowledge its role in the pathophysiology of cardiovascular diseases.

Reduced N-Type Ca2+ Channels in Atrioventricular Ganglion Neurons Are Involved in Ventricular Arrhythmogenesis

Background

Attenuated cardiac vagal activity is associated with ventricular arrhythmogenesis and related mortality in patients with chronic heart failure. Our recent study has shown that expression of N‐type Ca²⁺ channel α‐subunits (Ca_v2.2‐α) and N‐type Ca²⁺ currents are reduced in intracardiac ganglion neurons from rats with chronic heart failure. Rat intracardiac ganglia are divided into the atrioventricular ganglion (AVG) and sinoatrial ganglion. Ventricular myocardium receives projection of neuronal terminals only from the AVG. In this study we tested whether a decrease in N‐type Ca²⁺ channels in AVG neurons contributes to ventricular arrhythmogenesis.

Methods and Results

Lentiviral Ca_v2.2‐α shRNA (2 μL, 2×10⁷ pfu/mL) or scrambled shRNA was in vivo transfected into rat AVG neurons. Nontransfected sham rats served as controls. Using real‐time single‐cell polymerase chain reaction and reverse‐phase protein array, we found that in vivo transfection of Ca_v2.2‐α shRNA decreased expression of Ca_v2.2‐α mRNA and protein in rat AVG neurons. Whole‐cell patch‐clamp data showed that Ca_v2.2‐α shRNA reduced N‐type Ca²⁺ currents and cell excitability in AVG neurons. The data from telemetry electrocardiographic recording demonstrated that 83% (5 out of 6) of conscious rats with Ca_v2.2‐α shRNA transfection had premature ventricular contractions (P<0.05 versus 0% of nontransfected sham rats or scrambled shRNA‐transfected rats). Additionally, an index of susceptibility to ventricular arrhythmias, inducibility of ventricular arrhythmias evoked by programmed electrical stimulation, was higher in rats with Ca_v2.2‐α shRNA transfection compared with nontransfected sham rats and scrambled shRNA‐transfected rats.

Conclusions

A decrease in N‐type Ca²⁺ channels in AVG neurons attenuates vagal control of ventricular myocardium, thereby initiating ventricular arrhythmias.

Variable expression of GFP in different populations of peripheral cholinergic neurons of ChATBAC-eGFP transgenic mice

Immunohistochemistry is used widely to identify cholinergic neurons, but this approach has some limitations. To address these problems, investigators developed transgenic mice that express enhanced green fluorescent protein (GFP) directed by the promoter for choline acetyltransferase (ChAT), the acetylcholine synthetic enzyme. Although, it was reported that these mice express GFP in all cholinergic neurons and non-neuronal cholinergic cells, we could not detect GFP in cardiac cholinergic nerves in preliminary experiments. Our goals for this study were to confirm our initial observation and perform a qualitative screen of other representative autonomic structures for the presences of GFP in cholinergic innervation of effector tissues. We evaluated GFP fluorescence of intact, unfixed tissues and the cellular localization of GFP and vesicular acetylcholine transporter (VAChT), a specific cholinergic marker, in tissue sections and intestinal whole mounts. Our experiments identified two major tissues where cholinergic neurons and/or nerve fibers lacked GFP: 1) most cholinergic neurons of the intrinsic cardiac ganglia and all cholinergic nerve fibers in the heart and 2) most cholinergic nerve fibers innervating airway smooth muscle. Most cholinergic neurons in airway ganglia stained for GFP. Cholinergic systems in the bladder and intestines were fully delineated by GFP staining. GFP labeling of input to ganglia with long preganglionic projections (vagal) was sparse or weak, while that to ganglia with short preganglionic projections (spinal) was strong. Total absence of GFP might be due to splicing out of the GFP gene. Lack of GFP in nerve projections from GFP-positive cell bodies might reflect a transport deficiency.

Cardiac Innervation and the Autonomic Nervous System in Sudden Cardiac Death

Neural remodeling in the autonomic nervous system contributes to sudden cardiac death. The fabric of cardiac excitability and propagation is controlled by autonomic innervation. Heart disease predisposes to malignant ventricular arrhythmias by causing neural remodeling at the level of the myocardium, the intrinsic cardiac ganglia, extracardiac intrathoracic sympathetic ganglia, extrathoracic ganglia, spinal cord, and the brainstem, as well as the higher centers and the cortex. Therapeutic strategies at each of these levels aim to restore the balance between the sympathetic and parasympathetic branches. Understanding this complex neural network will provide important therapeutic insights into the treatment of sudden cardiac death.

Fix the problem - A practical guide to whole-mount immunohistochemistry of teased nerve fibres

BACKGROUND

Immunohistochemical staining of entire nerve fibres allows for studying the molecular composition of functional fibre subunits and may add to the diagnostic value of nerve fibre teasing.

NEW METHOD

In this study, we established a sealed-slide method for reproducible immunostaining of deep axoplasmic proteins in permanently straightened nerve fibres.

RESULTS

Immunostaining of teased nerve fibres very much is facilitated by tip-fixation with biocompatible glass adhesives. Antibody penetration in fresh nerves can be achieved by thermic and chemical permeabilisation while enzymatic digestion allows for sufficient permeability after aldehyde fixation.

COMPARISON WITH EXISTING METHODS

The methods recommended herein are easy to perform and represent a reliable and reproducible way to whole mount immunostaining.

CONCLUSIONS

Sealed-slide immunostaining of tip-fixed and permeabilised nerve biopsies will help to validate neurophysiological abnormalities and to screen for target molecules and predictive markers of peripheral nerve disorders such as in inherited neuropathies and Guillain-Barré syndrome.

Cardiac neuroanatomy - Imaging nerves to define functional control

The autonomic nervous system regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Further understanding of the interplay between the autonomic nervous system and cardiovascular system holds promise for the development of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. In this review, we detail the evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy.

Nicotinic acetylcholine receptor-mediated protection of the rat heart exposed to ischemia reperfusion

Reperfusion injury following acute myocardial infarction is associated with significant morbidity. Activation of neuronal or non-neuronal cholinergic pathways in the heart has been shown to reduce ischemic injury and this effect has been attributed primarily to muscarinic acetylcholine receptors. In contrast, the role of nicotinic receptors, specifically alpha-7 subtype (α7nAChR) in the myocardium remains unknown which offers an opportunity to potentially repurpose several agonists/modulators that are currently under development for neurologic indications. Treatment of ex vivo and in vivo rat models of cardiac ischemia/reperfusion (I/R) with a selective α7nAChR agonist (GTS21) showed significant increases in left ventricular developing pressure, and rates of pressure development without effects on heart rate. These positive functional effects were blocked by co-administration with methyllycaconatine (MLA), a selective antagonist of α7nAChRs. In vivo, delivery of GTS21 at the initiation of reperfusion, reduced infarct size by 42% (p<0.01) and decreased tissue reactive oxygen species (ROS) by 62% (p<0.01). Flow cytometry of MitoTracker Red stained mitochondria showed that mitochondrial membrane potential was normalized in mitochondria isolated from GTS21 treated compared to untreated I/R hearts. Intracellular ATP concentration in cultured cardiomyocytes exposed to hypoxia/reoxygenation was reduced (p<0.001), but significantly increased to normoxic levels with GTS21 treatment, and this was abrogated by MLA pretreatment. Activation of stress-activated kinases, JNK and p38MAPK, were significantly reduced by GTS21 in I/R. We conclude that targeting myocardial 17nAChRs in I/R may provide therapeutic benefit by improving cardiac contractile function through a mechanism that preserves mitochondrial membrane potential, maintains intracellular ATP and reduces ROS generation, thus limiting infarct size.

The role of neuropeptides in adverse myocardial remodeling and heart failure

In addition to traditional neurotransmitters of the sympathetic and parasympathetic nervous systems, the heart also contains numerous neuropeptides. These neuropeptides not only modulate the effects of neurotransmitters, but also have independent effects on cardiac function. While in most cases the physiological actions of these neuropeptides are well defined, their contributions to cardiac pathology are less appreciated. Some neuropeptides are cardioprotective, some promote adverse cardiac remodeling and heart failure, and in the case of others their functions are unclear. Some have both cardioprotective and adverse effects depending on the specific cardiac pathology and progression of that pathology. In this review, we briefly describe the actions of several neuropeptides on normal cardiac physiology, before describing in more detail their role in adverse cardiac remodeling and heart failure. It is our goal to bring more focus toward understanding the contribution of neuropeptides to the pathogenesis of heart failure, and to consider them as potential therapeutic targets.

Disruption of cardiac cholinergic neurons enhances susceptibility to ventricular arrhythmias

The parasympathetic nervous system plays an important role in the pathophysiology of atrial fibrillation. Catheter ablation, a minimally invasive procedure deactivating abnormal firing cardiac tissue, is increasingly becoming the therapy of choice for atrial fibrillation. This is inevitably associated with the obliteration of cardiac cholinergic neurons. However, the impact on ventricular electrophysiology is unclear. Here we show that cardiac cholinergic neurons modulate ventricular electrophysiology. Mechanical disruption or pharmacological blockade of parasympathetic innervation shortens ventricular refractory periods, increases the incidence of ventricular arrhythmia and decreases ventricular cAMP levels in murine hearts. Immunohistochemistry confirmed ventricular cholinergic innervation, revealing parasympathetic fibres running from the atria to the ventricles parallel to sympathetic fibres. In humans, catheter ablation of atrial fibrillation, which is accompanied by accidental parasympathetic and concomitant sympathetic denervation, raises the burden of premature ventricular complexes. In summary, our results demonstrate an influence of cardiac cholinergic neurons on the regulation of ventricular function and arrhythmogenesis.

Catheter ablation is a common therapy for atrial fibrillation but disrupts cardiac cholinergic neurons. Here the authors report that cholinergic neurons innervate heart ventricles and show that their ablation leads to increased susceptibility to ventricular arrhythmias in mouse models and in patients.

Characterization of glutamatergic neurons in the rat atrial intrinsic cardiac ganglia that project to the cardiac ventricular wall

The intrinsic cardiac nervous system modulates cardiac function by acting as an integration site for regulating autonomic efferent cardiac output. This intrinsic system is proposed to be composed of a short cardio-cardiac feedback control loop within the cardiac innervation hierarchy. For example, electrophysiological studies have postulated the presence of sensory neurons in intrinsic cardiac ganglia (ICG) for regional cardiac control. There is still a knowledge gap, however, about the anatomical location and neurochemical phenotype of sensory neurons inside ICG. In the present study, rat ICG neurons were characterized neurochemically with immunohistochemistry using glutamatergic markers: vesicular glutamate transporters 1 and 2 (VGLUT1; VGLUT2), and glutaminase (GLS), the enzyme essential for glutamate production. Glutamatergic neurons (VGLUT1/VGLUT2/GLS) in the ICG that have axons to the ventricles were identified by retrograde tracing of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected in the ventricular wall. Co-labeling of VGLUT1, VGLUT2, and GLS with the vesicular acetylcholine transporter (VAChT) was used to evaluate the relationship between post-ganglionic autonomic neurons and glutamatergic neurons. Sequential labeling of VGLUT1 and VGLUT2 in adjacent tissue sections was used to evaluate the co-localization of VGLUT1 and VGLUT2 in ICG neurons. Our studies yielded the following results: (1) ICG contain glutamatergic neurons with GLS for glutamate production and VGLUT1 and 2 for transport of glutamate into synaptic vesicles; (2) atrial ICG contain neurons that project to ventricle walls and these neurons are glutamatergic; (3) many glutamatergic ICG neurons also were cholinergic, expressing VAChT; (4) VGLUT1 and VGLUT2 co-localization occurred in ICG neurons with variation of their protein expression level. Investigation of both glutamatergic and cholinergic ICG neurons could help in better understanding the function of the intrinsic cardiac nervous system.

Co-dependence of the neural and humoral pathways in the mechanism of remote ischemic conditioning

The cardioprotection afforded by remote ischaemic conditioning (RIC) is mediated via a complex mechanism involving sensory afferent nerves, the vagus nerve, and release of a humoral blood-borne factor. However, it is unknown whether release of the protective factor depends on vagal activation or occurs independently. This study aimed to evaluate the co-dependence of the neural and humoral pathways of RIC, focussing on the vagus nerve and intrinsic cardiac ganglia. In the first study, anesthetised rats received bilateral cervical vagotomy or sham-surgery immediately prior to RIC (4 × 5 min limb ischemia–reperfusion) or sham-RIC. Venous blood plasma was dialysed across a 12–14 kDa membrane and dialysate perfused through a naïve-isolated rat heart prior to 35-min left anterior descending ischemia and 60-min reperfusion. In the second study, anesthetised rats received RIC (4 × 5-min limb ischemia–reperfusion) or control (sham-RIC). Dialysate was prepared and perfused through a naïve-isolated rat heart in the presence of the ganglionic blocker hexamethonium or muscarinic antagonist atropine, prior to ischemia–reperfusion as above. Dialysate collected from RIC-treated rats reduced infarct size in naïve rat hearts from 40.7 ± 6.3 to 23.7 ± 3.1 %, p < 0.05. Following bilateral cervical vagotomy, the protection of RIC dialysate was abrogated (42.2 ± 3.2 %, p < 0.05 vs RIC dialysate). In the second study, the administration of 50-μM hexamethonium (45.8 ± 2.5 %) or 100-nM atropine (36.5 ± 3.4 %) abrogated the dialysate-mediated protection. Release of a protective factor following RIC is dependent on prior activation of the vagus nerve. In addition, this factor appears to induce cardioprotection via recruitment of intrinsic cardiac ganglia.

Constitutive Intracellular Na+ Excess in Purkinje Cells Promotes Arrhythmogenesis at Lower Levels of Stress Than Ventricular Myocytes From Mice With Catecholaminergic Polymorphic Ventricular Tachycardia

Supplemental Digital Content is available in the text.

Background—

In catecholaminergic polymorphic ventricular tachycardia (CPVT), cardiac Purkinje cells (PCs) appear more susceptible to Ca²⁺ dysfunction than ventricular myocytes (VMs). The underlying mechanisms remain unknown. Using a CPVT mouse (RyR2^{R4496C+/Cx40eGFP}), we tested whether PC intracellular Ca²⁺ ([Ca²⁺]_i) dysregulation results from a constitutive [Na⁺]_i surplus relative to VMs.

Methods and Results—

Simultaneous optical mapping of voltage and [Ca²⁺]_i in CPVT hearts showed that spontaneous Ca²⁺ release preceded pacing-induced triggered activity at subendocardial PCs. On simultaneous current-clamp and Ca²⁺ imaging, early and delayed afterdepolarizations trailed spontaneous Ca²⁺ release and were more frequent in CPVT PCs than CPVT VMs. As a result of increased activity of mutant ryanodine receptor type 2 channels, sarcoplasmic reticulum Ca²⁺ load, measured by caffeine-induced Ca²⁺ transients, was lower in CPVT VMs and PCs than respective controls, and sarcoplasmic reticulum fractional release was greater in both CPVT PCs and VMs than respective controls. [Na⁺]_i was higher in both control and CPVT PCs than VMs, whereas the density of the Na⁺/Ca²⁺ exchanger current was not different between PCs and VMs. Computer simulations using a PC model predicted that the elevated [Na⁺]_i of PCs promoted delayed afterdepolarizations, which were always preceded by spontaneous Ca²⁺ release events from hyperactive ryanodine receptor type 2 channels. Increasing [Na⁺]_i monotonically increased delayed afterdepolarization frequency. Confocal imaging experiments showed that postpacing Ca²⁺ spark frequency was highest in intact CPVT PCs, but such differences were reversed on saponin-induced membrane permeabilization, indicating that differences in [Na⁺]_i played a central role.

Conclusions—

In CPVT mice, the constitutive [Na⁺]_i excess of PCs promotes triggered activity and arrhythmogenesis at lower levels of stress than VMs.

Identifying the Source of a Humoral Factor of Remote (Pre)Conditioning Cardioprotection

Signalling pathways underlying the phenomenon of remote ischaemic preconditioning (RPc) cardioprotection are not completely understood. The existing evidence agrees that intact sensory innervation of the remote tissue/organ is required for the release into the systemic circulation of preconditioning factor(s) capable of protecting a transplanted or isolated heart. However, the source and molecular identities of these factors remain unknown. Since the efficacy of RPc cardioprotection is critically dependent upon vagal activity and muscarinic mechanisms, we hypothesized that the humoral RPc factor is produced by the internal organ(s), which receive rich parasympathetic innervation. In a rat model of myocardial ischaemia/reperfusion injury we determined the efficacy of limb RPc in establishing cardioprotection after denervation of various visceral organs by sectioning celiac, hepatic, anterior and posterior gastric branches of the vagus nerve. Electrical stimulation was applied to individually sectioned branches to determine whether enhanced vagal input to a particular target area is sufficient to establish cardioprotection. It was found that RPc cardioprotection is abolished in conditions of either total subdiaphragmatic vagotomy, gastric vagotomy or sectioning of the posterior gastric branch. The efficacy of RPc cardioprotection was preserved when hepatic, celiac or anterior gastric vagal branches were cut. In the absence of remote ischaemia/reperfusion, electrical stimulation of the posterior gastric branch reduced infarct size, mimicking the effect of RPc. These data suggest that the circulating factor (or factors) of RPc are produced and released into the systemic circulation by the visceral organ(s) innervated by the posterior gastric branch of the vagus nerve.

The nervous heart

Many cardiac electrophysiological abnormalities are accompanied by autonomic nervous system dysfunction. Here, we review mechanisms by which the cardiac nervous system controls normal and abnormal excitability and may contribute to atrial and ventricular tachyarrhythmias. Moreover, we explore the potential antiarrhythmic and/or arrhythmogenic effects of modulating the autonomic nervous system by several strategies, including ganglionated plexi ablation, vagal and spinal cord stimulations, and renal sympathetic denervation as therapies for atrial and ventricular arrhythmias.

Cholinergic activity as a new target in diseases of the heart

The autonomic nervous system is an important modulator of cardiac signaling in both health and disease. In fact, the significance of altered parasympathetic tone in cardiac disease has recently come to the forefront. Both neuronal and nonneuronal cholinergic signaling likely play a physiological role, since modulating acetylcholine (ACh) signaling from neurons or cardiomyocytes appears to have significant consequences in both health and disease. Notably, many of these effects are solely due to changes in cholinergic signaling, without altered sympathetic drive, which is known to have significant adverse effects in disease states. As such, it is likely that enhanced ACh-mediated signaling not only has direct positive effects on cardiomyocytes, but it also offsets the negative effects of hyperadrenergic tone. In this review, we discuss recent studies that implicate ACh as a major regulator of cardiac remodeling and provide support for the notion that enhancing cholinergic signaling in human patients with cardiac disease can reduce morbidity and mortality. These recent results support the idea of developing large clinical trials of strategies to increase cholinergic tone, either by stimulating the vagus or by increased availability of Ach, in heart failure.

The heart's 'little brain' controlling cardiac function in the rabbit

NEW FINDINGS

What is the topic of this review? The topic of the review is the intrinsic cardiac nervous system in the rabbit. What advances does it highlight? The anatomy of rabbit intrinsic ganglia is similar to that of other species, including humans. Immunohistochemistry confirms the presence of cholinergic and adrenergic neurones, with a striking arrangement of neuronal nitric oxide synthase-positive cell bodies. Activation of atrial ganglia produces effects on local and remote regions. Heart disease is a primary cause of mortality in the developed world, and it is well recognized that neural mechanisms play an important role in many cardiac pathologies. The role of extrinsic autonomic nerves has traditionally attracted the most attention. However, there is a rich intrinsic innervation of the heart, including numerous cardiac ganglia (ganglionic plexuses), that has the potential to affect cardiac function independently as well as to influence the actions of the extrinsic nerves. To investigate this, an isolated, perfused, innervated rabbit Langendorff heart preparation was considered the best option. Although ganglionic plexuses have been well described for several species, there was no full description of the anatomy and histochemistry of rabbit hearts. To this end, rabbit intrinsic ganglia were located using acetylcholinesterase histology (n = 33 hearts). This revealed six generalized ganglionic regions, defined as a left neuronal complex above the left pulmonary vein, a right neuronal complex around the base of right cranial vein, three scattered in the dorsal right atrium and a region containing numerous ventricular ganglia located on the conus arteriosus. Using immunohistochemistry, neurons were found to contain choline acetyltransferase or tyrosine hydroxylase and/or neuronal nitric oxide synthase in differing amounts (choline acetyltransferase > tyrosine hydroxylase > neuronal nitric oxide synthase). The function of rabbit intrinsic ganglia was investigated using a bolus application of nicotine or electrical stimulation at each of the above sites whilst measuring heart rate and atrioventricular conduction. Nicotine applied to different ganglionic plexuses caused a bradycardia, a tachycardia or a mixture of the two, with the right atrial plexus producing the largest chronotropic responses. Electrical stimulation at these sites induced only a bradycardia. Atrioventricular conduction was modestly changed by nicotine, the main response being a prolongation. Electrical stimulation produced significant prolongation of atrioventricular conduction, particularly when the right neuronal complex was stimulated. These studies show that the intrinsic plexuses of the heart are important and could be crucial for understanding impairments of cardiac function. Additionally, they provide a strong basis from which to progress using the isolated, innervated rabbit heart preparation.

A combined acetylcholinesterase and immunohistochemical method for precise anatomical analysis of intrinsic cardiac neural structures

A significant challenge when investigating autonomic neuroanatomy is being able to reliably obtain tissue that contains neuronal structures of interest. Currently, histochemical staining for acetylcholinesterase (AChE) remains the most feasible and reliable method to visualize intrinsic nerves and ganglia in whole organs. In order to precisely visualize and sample intrinsic cardiac nerves and ganglia for subsequent immunofluorescent labeling, we developed a modified histochemical AChE method using material from pig and sheep hearts. The method involves: (1) chemical prefixation of the whole heart, (2) short-term and weak histochemical staining for AChE in situ, (3) visual examination and extirpation of the stained neural structures from the whole heart, (4) freezing, embedding and cryostat sectioning of the tissue of interest, and (5) immunofluorescent labeling and microscopic analysis of neural structures. Firstly, our data demonstrate that this modified AChE protocol labeled intrinsic cardiac nerves as convincingly as our previously published data. Secondly, there was the added advantage that adrenergic, cholinergic and peptidergic neuropeptides, namely protein gene product 9.5 (PGP 9.5), neurofilament (NF), tyrosine hydroxylase (TH), vesicular monoamine transporter (VMAT2), neuronal nitric oxide synthase (nNOS), choline acetyltransferase (ChAT), calcitonin gene related peptide (CGRP), and substance P may be identified. Our method allows the precise sampling of neural structures including autonomic ganglia, intrinsic nerves and bundles of nerve fibers and even single neurons from the whole heart. This method saves time, effort and a substantial amount of antisera. Nonetheless, the proof of specific staining for many other autonomic neuronal markers has to be provided in subsequent studies.

Innervation of sinoatrial nodal cardiomyocytes in mouse. A combined approach using immunofluorescent and electron microscopy

Fluorescent immunohistochemistry on the cardiac conduction system in whole mount mouse heart preparations demonstrates a particularly dense and complex network of nerve fibres and cardiomyocytes which are positive to the hyperpolarization activated cyclic nucleotide-gated potassium channel 4 (HCN4-positive cardiomyocytes) in the sinoatrial node region and adjacent areas around the root of right cranial vein. The present study was designed to investigate the morphologic and histochemical pattern of nerve fibres and HCN4-positive cardiomyocytes using fluorescent techniques and/or electron microscopy. Adrenergic and cholinergic nerve fibres together with HCN4-positive cardiomyocytes were identified using primary antibodies for tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), and the HCN4 channel respectively. Amid HCN4-positive cardiomyocytes, fluorescence and electron microscopy data demonstrated a dense distribution of nerve fibres immunoreactive for ChAT and TH. In addition, novel electron microscopy data revealed that the mouse sinoatrial node contained exclusively unmyelinated nerve fibres, in which the majority of axons possess varicosities with clear mediatory vesicles that can be classified as cholinergic. Synapses occurred without any clear terminal connection with the effector cell, i.e. these synapes were of "en passant" type. In general, the morphologic pattern of innervation of mouse HCN4-positive cardiomyocytes identified using electron microscopy corresponds well to the dense network of nerve fibres demonstrated by fluorescent immunohistochemistry in mouse sinoatrial node and adjacent areas. The complex and extraordinarily dense innervation of HCN4-positive cardiomyocytes in mouse sinoatrial node underpins the importance of neural regulation for the cardiac conduction system. Based on the present observations, it is concluded that the occurrence of numerous nerve fibres nearby atrial cardiomyocytes serves as a novel reliable extracellular criterion for discrimination of SA nodal cardiomyocytes using electron microscopy.

SHP-2 deletion in postmigratory neural crest cells results in impaired cardiac sympathetic innervation

Autonomic innervation is an essential component of cardiovascular regulation that is first established from the neural crest (NC) lineage in utero and continues developing postnatally. Although in vitro studies have indicated that SH2-containing protein tyrosine phosphatase 2 (SHP-2) is a signaling factor critical for regulating sympathetic neuron differentiation, this has yet to be shown in the complex in vivo environment of cardiac autonomic innervation. Targeting SHP-2 within postmigratory NC lineages resulted in a fully penetrant mouse model of diminished sympathetic cardiac innervation and concomitant bradycardia. Immunohistochemistry of the sympathetic nerve marker tyrosine hydroxylase revealed a progressive loss of adrenergic ganglionic neurons and reduction of cardiac sympathetic axon density in Shp2 cKOs. Molecularly, Shp2 cKOs exhibit lineage-specific suppression of activated phospo-ERK1/2 signaling but not of other downstream targets of SHP-2 such as pAKT. Genetic restoration of the phosphorylated-extracellular signal-regulated kinase (pERK) deficiency via lineage-specific expression of constitutively active MEK1 was sufficient to rescue the sympathetic innervation deficit and its physiological consequences. These data indicate that SHP-2 signaling specifically through pERK in postmigratory NC lineages is essential for development and maintenance of sympathetic cardiac innervation postnatally.

Mechanisms underlying the autonomic modulation of ventricular fibrillation initiation--tentative prophylactic properties of vagus nerve stimulation on malignant arrhythmias in heart failure

Classical physiology teaches that vagal post-ganglionic nerves modulate the heart via acetylcholine acting at muscarinic receptors, whilst it is accepted that vagus nerve stimulation (VNS) slows heart rate, atrioventricular conduction and decreases atrial contraction; there is continued controversy as to whether the vagus has any significant direct effect on ventricular performance. Despite this, there is a significant body of evidence from experimental and clinical studies, demonstrating that the vagus nerve has an anti-arrhythmic action, protecting against induced and spontaneously occurring ventricular arrhythmias. Over 100 years ago Einbrodt first demonstrated that direct cervical VNS significantly increased the threshold for experimentally induced ventricular fibrillation. A large body of evidence has subsequently been collected supporting the existence of an anti-arrhythmic effect of the vagus on the ventricle. The development of prognostic indicators of heart rate variability and baroreceptor reflex sensitivity—measures of parasympathetic tone and reflex activation respectively—and the more recent interest in chronic VNS therapy are a direct consequence of the earlier experimental studies. Despite this, mechanisms underlying the anti-arrhythmic actions of the vagus nerve have not been fully characterised and are not well understood. This review summarises historical and recently published data to highlight the importance of this powerful endogenous protective phenomenon.

Morphological pattern of intrinsic nerve plexus distributed on the rabbit heart and interatrial septum

Although the rabbit is routinely used as the animal model of choice to investigate cardiac electrophysiology, the neuroanatomy of the rabbit heart is not well documented. The aim of this study was to examine the topography of the intrinsic nerve plexus located on the rabbit heart surface and interatrial septum stained histochemically for acetylcholinesterase using pressure-distended whole hearts and whole-mount preparations from 33 Californian rabbits. Mediastinal cardiac nerves entered the venous part of the heart along the root of the right cranial vein (superior caval vein) and at the bifurcation of the pulmonary trunk. The accessing nerves of the venous part of the heart passed into the nerve plexus of heart hilum at the heart base. Nerves approaching the heart extended epicardially and innervated the atria, interatrial septum and ventricles by five nerve subplexuses, i.e. left and middle dorsal, dorsal right atrial, ventral right and left atrial subplexuses. Numerous nerves accessed the arterial part of the arterial part of the heart hilum between the aorta and pulmonary trunk, and distributed onto ventricles by the left and right coronary subplexuses. Clusters of intrinsic cardiac neurons were concentrated at the heart base at the roots of pulmonary veins with some positioned on the infundibulum. The mean number of intrinsic neurons in the rabbit heart is not significantly affected by aging: 2200 ± 262 (range 1517-2788; aged) vs. 2118 ± 108 (range 1513-2822; juvenile). In conclusion, despite anatomic differences in the distribution of intrinsic cardiac neurons and the presence of well-developed nerve plexus within the heart hilum, the topography of all seven subplexuses of the intrinsic nerve plexus in rabbit heart corresponds rather well to other mammalian species, including humans.

Substance P in heart failure: the good and the bad

The tachykinin, substance P, is found primarily in sensory nerves. In the heart, substance P-containing nerve fibers are often found surrounding coronary vessels, making them ideally situated to sense changes in the myocardial environment. Recent studies in rodents have identified substance P as having dual roles in the heart, depending on disease etiology and/or timing. Thus far, these studies indicate that substance P may be protective acutely following ischemia-reperfusion, but damaging long-term in non-ischemic induced remodeling and heart failure. Sensory nerves may be at the apex of the cascade of events leading to heart failure, therefore, they make a promising potential therapeutic target that warrants increased investigation.

Calsequestrin 2 deletion causes sinoatrial node dysfunction and atrial arrhythmias associated with altered sarcoplasmic reticulum calcium cycling and degenerative fibrosis within the mouse atrial pacemaker complex1

AIMS

Loss-of-function mutations in Calsequestrin 2 (CASQ2) are associated with catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT patients also exhibit bradycardia and atrial arrhythmias for which the underlying mechanism remains unknown. We aimed to study the sinoatrial node (SAN) dysfunction due to loss of CASQ2.

METHODS AND RESULTS

In vivo electrocardiogram (ECG) monitoring, in vitro high-resolution optical mapping, confocal imaging of intracellular Ca(2+) cycling, and 3D atrial immunohistology were performed in wild-type (WT) and Casq2 null (Casq2(-/-)) mice. Casq2(-/-) mice exhibited bradycardia, SAN conduction abnormalities, and beat-to-beat heart rate variability due to enhanced atrial ectopic activity both at baseline and with autonomic stimulation. Loss of CASQ2 increased fibrosis within the pacemaker complex, depressed primary SAN activity, and conduction, but enhanced atrial ectopic activity and atrial fibrillation (AF) associated with macro- and micro-reentry during autonomic stimulation. In SAN myocytes, CASQ2 deficiency induced perturbations in intracellular Ca(2+) cycling, including abnormal Ca(2+) release, periods of significantly elevated diastolic Ca(2+) levels leading to pauses and unstable pacemaker rate. Importantly, Ca(2+) cycling dysfunction occurred not only at the SAN cellular level but was also globally manifested as an increased delay between action potential (AP) and Ca(2+) transient upstrokes throughout the atrial pacemaker complex.

CONCLUSIONS

Loss of CASQ2 causes abnormal sarcoplasmic reticulum Ca(2+) release and selective interstitial fibrosis in the atrial pacemaker complex, which disrupt SAN pacemaking but enhance latent pacemaker activity, create conduction abnormalities and increase susceptibility to AF. These functional and extensive structural alterations could contribute to SAN dysfunction as well as AF in CPVT patients.

Myths and realities of the cardiac vagus

There is continuing belief that cardiac parasympathetic postganglionic fibres are sparse or absent from the ventricles. This review of the literature shows that the supposition is a myth. Early studies considered that fine silver-stained fibres coursing amongst ventricle myocardial cells were most likely cardiac parasympathetic postganglionic fibres. The conclusions were later supported by acetyl cholinesterase staining using a method that appeared not to be associated with noradrenaline nerve fibres. The conclusion is critically examined in the light of several recent histological studies using the acetyl cholinesterase method and also a more definitive technique (CHAT), that suggest a widespread location of parasympathetic ganglia and a relatively dense parasympathetic innervation of ventricular muscle in a range of mammals including man. The many studies demonstrating acetylcholine release in the ventricle on vagal nerve stimulation and a high density of acetylcholine M2 receptors is in accord with this as are tests of ventricular performance from many physiological studies. Selective control of cardiac functions by anatomically segregated parasympathetic ganglia is discussed. It is argued that the influence of vagal stimulation on ventricular myocardial action potential refractory period, duration, force and rhythm is evidence that vagal fibres have close apposition to myocardial fibres. This is supported by clear evidence of accentuated antagonism between sympathetic activity and vagal activity in the ventricle and also by direct effects of vagal activity independent of sympathetic activity. The idea of differential control of atrial and ventricular physiology by vagal C and vagal B preganglionic fibres is examined as well as differences in chemical phenotypes and their function. The latter is reflected in medullary and supramedullary control. Reference is made to the importance of this knowledge to understanding the normal physiology of cardiac autonomic control and significance to pathology.

Remodelling of the intracardiac ganglia in diabetic Goto-Kakizaki rats: an anatomical study

Background

Although cardiac autonomic neuropathy is one of major complications of diabetes mellitus (DM), anatomical data on cardiac innervation of diabetic animal models is scant and controversial. We performed this study to check whether long-term diabetic state impacts the anatomy of intracardiac ganglia in Goto-Kakizaki (GK) rats, a genetic model of type 2 DM.

Methods

Twelve GK rats (276 ± 17 days of age; mean ± standard error) and 13 metabolically healthy Wistar rats (262 ± 5 days of age) as controls were used for this study. Blood glucose was determined using test strips, plasma insulin by radioimmunoassay. Intrinsic ganglia and nerves were visualized by acetylcholinesterase histochemistry on whole hearts. Ganglion area was measured, and the neuronal number was assessed according to ganglion area.

Results

The GK rats had significantly elevated blood glucose level compared to controls (11.0 ± 0.6 vs. 5.9 ± 0.1 mmol/l, p < 0.001), but concentration of plasma insulin did not differ significantly between the two groups (84.0 ± 9.8 vs. 67.4 ± 10.9 pmol/l, p = 0.17). The GK rats contained significantly fewer intracardiac ganglia, decreased total area of intracardiac ganglia (1.4 ± 0.1 vs. 2.2 ± 0.1 mm², p < 0.001) and smaller somata of ganglionic neurons. Mean total number of intracardiac neurons in GK rats was 1461 ± 62, while this number in control rats was higher by 39% and reached 2395 ± 110 (p < 0.001).

Conclusions

Results of our study demonstrate the decreased number of intracardiac neurons in GK rats compared to metabolically healthy Wistar rats of similar age. It is likely that the observed structural remodelling of intracardiac ganglia in GK rats is caused by a long-term diabetic state.

Nerves projecting from the intrinsic cardiac ganglia of the pulmonary veins modulate sinoatrial node pacemaker function

AIMS

Pulmonary vein ganglia (PVG) are targets for atrial fibrillation ablation. However, the functional relevance of PVG to the normal heart rhythm remains unclear. Our aim was to investigate whether PVG can modulate sinoatrial node (SAN) function.

METHODS AND RESULTS

Forty-nine C57BL and seven Connexin40+/EGFP mice were studied. We used tyrosine-hydroxylase (TH) and choline-acetyltransferase immunofluorescence labelling to characterize adrenergic and cholinergic neural elements. PVG projected postganglionic nerves to the SAN, which entered the SAN as an extensive, mesh-like neural network. PVG neurones were adrenergic, cholinergic, and biphenotypic. Histochemical characterization of two human embryonic hearts showed similarities between mouse and human neuroanatomy: direct neural communications between PVG and SAN. In Langendorff perfused mouse hearts, PVG were stimulated using 200-2000 ms trains of pulses (300 μs, 400 µA, 200 Hz). PVG stimulation caused an initial heart rate (HR) slowing (36 ± 9%) followed by acceleration. PVG stimulation in the presence of propranolol caused HR slowing (43 ± 13%) that was sustained over 20 beats. PVG stimulation with atropine progressively increased HR. Time-course effects were enhanced with 1000 and 2000 ms trains (P < 0.05 vs. 200 ms). In optical mapping, PVG stimulation shifted the origin of SAN discharges. In five paroxysmal AF patients undergoing pulmonary vein ablation, application of radiofrequency energy to the PVG area during sinus rhythm produced a decrease in HR similar to that observed in isolated mouse hearts.

CONCLUSION

PVG have functional and anatomical biphenotypic characteristics. They can have significant effects on the electrophysiological control of the SAN.

Neuroanatomy of the murine cardiac conduction system: a combined stereomicroscopic and fluorescence immunohistochemical study

The mouse heart is a popular model to study the function and autonomic control of the specialized cardiac conduction system (CCS). However, the precise identity and anatomical distribution of the intrinsic cardiac nerves that modulate the function of the mouse CCS have not been adequately studied. We aimed at determining the organization and distribution of the intrinsic cardiac nerves that supply the CCS of the mouse. In whole mouse heart preparations, intrinsic neural structures were revealed by histochemical staining for acetylcholinesterase (AChE). Adrenergic, cholinergic and peptidergic neural components were identified, respectively, by immunohistochemical labeling for tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), calcitonin gene related peptide (CGRP), substance P (SP), and protein gene product 9.5 (PGP 9.5). Myocytes of the CCS were identified by immunolabeling of hyperpolarization activated cyclic nucleotide-gated potassium channel 4 (HCN4). In addition, the presence of CCS myocytes in atypical locations was verified using fluorescent immunohistochemistry performed on routine paraffin sections. The results demonstrate that four microscopic epicardial nerves orientated toward the sinuatrial nodal (SAN) region derive from both the dorsal right atrial and right ventral nerve subplexuses. The atrioventricular nodal (AVN) region is typically supplied by a single intrinsic nerve derived from the left dorsal nerve subplexus at the posterior interatrial groove. SAN myocytes positive for HCN4 were widely distributed both on the medial, anterior, lateral and even posterior sides of the root of the right cranial (superior caval) vein. The distribution of HCN4-positive myocytes in the AVN region was also wider than previously considered. HCN4-positive cells and thin slivers of the AVN extended to the roots of the ascending aorta, posteriorly to the orifice of the coronary sinus, and even along both atrioventricular rings. Notwithstanding the fact that cholinergic nerve fibers and axons clearly predominate in the mouse CCS, adrenergic nerve fibers and axons are abundant therein as well. Altogether, these results provide new insight into the anatomical basis of the neural control of the mouse CCS.

Localization of Islet-1-positive cells in the healthy and infarcted adult murine heart

RATIONALE

The transcription factor Islet-1 is a marker of cardiovascular progenitors during embryogenesis. The isolation of Islet-1-positive (Islet-1(+)) cells from early postnatal hearts suggested that Islet-1 also marks cardiac progenitors in adult life.

OBJECTIVE

We investigated the distribution and identity of Islet-1(+) cells in adult murine heart and evaluated whether their number or distribution change with age or after myocardial infarction.

METHODS AND RESULTS

Distribution of Islet-1(+) cells in adult heart was investigated using gene targeted mice with nuclear β-galactosidase inserted into the Islet-1 locus. nLacZ-positive cells were only present in 3 regions of the adult heart: clusters in the interatrial septum and around the pulmonary veins, scattered within the wall of the great vessels, and a strictly delimited cluster between the right atrium and superior vena cava. Islet-1(+) cells in the first type of clusters coexpressed markers for parasympathetic neurons. Positive cells in the great arteries coexpressed smooth muscle actin and myosin heavy chain, indicating a smooth muscle cell identity. Very few Islet-1(+) cells within the outflow tract expressed the cardiomyocyte marker α-actinin. Islet-1(+) cells in the right atrium coexpressed the sinoatrial node pacemaker cell marker HCN4. Cell number and localization remained unchanged between 1 to 18 months of age. Consistently Islet-1 mRNA was detected in human sinoatrial node. Islet-1(+) cells could not be detected in the infarct zone 2 to 28 days after myocardial infarction, aside from 10 questionable cells in 1/13 hearts.

CONCLUSIONS

Our results identify Islet-1 as a novel marker of the adult sinoatrial node and do not provide evidence for Islet-1(+) cells to serve as cardiac progenitors.

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.