Projections of intrinsic cardiac neurons to different targets in the guinea-pig heart

The Intrinsic Cardiac Nervous System and Its Role in Cardiac Pacemaking and Conduction

The cardiac autonomic nervous system (CANS) plays a key role for the regulation of cardiac activity with its dysregulation being involved in various heart diseases, such as cardiac arrhythmias. The CANS comprises the extrinsic and intrinsic innervation of the heart. The intrinsic cardiac nervous system (ICNS) includes the network of the intracardiac ganglia and interconnecting neurons. The cardiac ganglia contribute to the tight modulation of cardiac electrophysiology, working as a local hub integrating the inputs of the extrinsic innervation and the ICNS. A better understanding of the role of the ICNS for the modulation of the cardiac conduction system will be crucial for targeted therapies of various arrhythmias. We describe the embryonic development, anatomy, and physiology of the ICNS. By correlating the topography of the intracardiac neurons with what is known regarding their biophysical and neurochemical properties, we outline their physiological role in the control of pacemaker activity of the sinoatrial and atrioventricular nodes. We conclude by highlighting cardiac disorders with a putative involvement of the ICNS and outline open questions that need to be addressed in order to better understand the physiology and pathophysiology of the ICNS.

Angiotensinergic Innervation of the Human Right Atrium: Implications for Cardiac Reflexes

BACKGROUND

The right atrium is densely innervated and provides sensory input to important cardiocirculatory reflexes controlling cardiac output and blood pressure. Its angiotensin (Ang) II-expressing innervation may release Ang II as a neuropeptide cotransmitter to modulate reflexes but has not yet been characterized.

METHODS

Intraoperative surgical biopsies from human right atria (n = 7) were immunocytologically stained for Ang II, tyrosine hydroxylase (TH), and synaptophysin (SYN). Tissue angiotensins were extracted and quantified by radioimmunoassay.

RESULTS

Angiotensinergic fibers were frequent in epicardial nerves and around vessels with variable TH co-localization (none to >50%/bundle). Fibers were also widely distributed between cardiomyocytes and in the endocardium where they were typically nonvaricose, TH/SYN-negative and usually accompanied by varicose catecholaminergic fibers. In the endocardium, some showed large varicosities and were partially TH or SYN-positive. A few endocardial regions showed scattered nonvaricose Ang fibers ending directly between endothelial cells. Occasional clusters of thin varicose terminals co-localizing SYN or TH were located underneath, or protruded into, the endothelium. Endocardial density of Ang and TH-positive fibers was 30-300 vs. 200-450/mm2. Atrial Ang II, III, and I concentrations were 67, 16, and 5 fmol/g (median) while Ang IV and V were mostly undetectable.

CONCLUSIONS

The human right atrium harbors an abundant angiotensinergic innervation and a novel potential source of atrial Ang II. Most peripheral fibers were noncatecholaminergic afferents or preterminal vagal efferents and a minority was presumably sympathetic. Neuronal Ang II release from these fibers may modulate cardiac and circulatory reflexes independently from plasma and tissue Ang II sources.

Intrinsic regulation of sinoatrial node function and the zebrafish as a model of stretch effects on pacemaking

Excitation of the heart occurs in a specialised region known as the sinoatrial node (SAN). Tight regulation of SAN function is essential for the maintenance of normal heart rhythm and the response to (patho-)physiological changes. The SAN is regulated by extrinsic (central nervous system) and intrinsic (neurons, peptides, mechanics) factors. The positive chronotropic response to stretch in particular is essential for beat-by-beat adaptation to changes in hemodynamic load. Yet, the mechanism of this stretch response is unknown, due in part to the lack of an appropriate experimental model for targeted investigations. We have been investigating the zebrafish as a model for the study of intrinsic regulation of SAN function. In this paper, we first briefly review current knowledge of the principal components of extrinsic and intrinsic SAN regulation, derived primarily from experiments in mammals, followed by a description of the zebrafish as a novel experimental model for studies of intrinsic SAN regulation. This mini-review is followed by an original investigation of the response of the zebrafish isolated SAN to controlled stretch. Stretch causes an immediate and continuous increase in beating rate in the zebrafish isolated SAN. This increase reaches a maximum part way through a period of sustained stretch, with the total change dependent on the magnitude and direction of stretch. This is comparable to what occurs in isolated SAN from most mammals (including human), suggesting that the zebrafish is a novel experimental model for the study of mechanisms involved in the intrinsic regulation of SAN function by mechanical effects.

The living aortic valve: From molecules to function

The aortic valve lies in a unique hemodynamic environment, one characterized by a range of stresses (shear stress, bending forces, loading forces and strain) that vary in intensity and direction throughout the cardiac cycle. Yet, despite its changing environment, the aortic valve opens and closes over 100,000 times a day and, in the majority of human beings, will function normally over a lifespan of 70–90 years. Until relatively recently heart valves were considered passive structures that play no active role in the functioning of a valve, or in the maintenance of its integrity and durability. However, through clinical experience and basic research the aortic valve can now be characterized as a living, dynamic organ with the capacity to adapt to its complex mechanical and biomechanical environment through active and passive communication between its constituent parts. The clinical relevance of a living valve substitute in patients requiring aortic valve replacement has been confirmed. This highlights the importance of using tissue engineering to develop heart valve substitutes containing living cells which have the ability to assume the complex functioning of the native valve.

Inotropic effect of dopamine on rat heart during postnatal ontogeny

We studied the effect of dopamine in concentrations of 10(-9), 10(-8), 10(-7), 10(-6), and 10(-5) M on contraction strength of isolated myocardial strips from the right atrium and right ventricle of rats aging 21, 42, 56, and 100 days. Dopamine in a concentration of 10(-9) M had a positive inotropic effect in rats of various ages. Increasing the concentration of dopamine to 10(-7)-10(-5) M was accompanied by a negative inotropic effect on the heart.

Regional cardiac ganglia projections in the guinea pig heart studied by postmortem DiI tracing

Our purpose was to identify and localize intrinsic cardiac ganglia innervating distinct regions of the heart using postmortem tracing of nerve projections with DiI, a method not previously used to study the intrinsic cardiac nervous system. We also investigated the possibility of collateral innervation of myocardium and intrinsic ganglia. In isolated paraformaldehyde-fixed guinea pig hearts, crystals of DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were inserted into the posterior ventricular myocardium below the atrioventricular groove, the right atrium, or the left ventricular septum. Hearts were placed in the dark at 37 degrees C for 2-14 weeks to allow DiI diffusion within neuronal membranes. Labeled neurons were observed in intracardiac ganglia after at least 4 weeks of dye exposure. Labeling was restricted to the inferior-most ganglia (those near the atrioventricular groove) when DiI was inserted into the posterior ventricular myocardium and to ganglia near the sinus node after right atrial DiI placement. Application of DiI to the left ventricular septum resulted in neuron labeling in ganglia primarily in the interatrial septum near the atrioventricular node. After 8 weeks, DiI-labeled nerve fibers and varicosities were seen surrounding unlabeled neurons in some ganglia, suggesting that axons terminating in or passing through the DiI application site in posterior ventricular tissue had collateral branches innervating these ganglia. These results indicate that intrinsic innervation of major cardiac subdivisions is accomplished by regionally segregated cardiac ganglia. Also, tracing with DiI has provided evidence for collateral nerve projections that could be the substrate for novel intracardiac regulatory circuits.

Heterogenous changes in neuropeptide Y, norepinephrine and epinephrine concentrations in the hearts of diabetic rats

The changes in concentrations of neuropeptide Y (NPY), norepinephrine and epinephrine were investigated in the rat hearts 1, 2, 4, 6, 9 and 12 months after administration of streptozotocin (STZ; 65 mg/kg i.v.). About 30% of diabetic animals displayed symptoms of partial spontaneous recovery, i.e. decreasing blood glucose levels and increasing insulin concentrations in the plasma and pancreas. NPY concentrations in the atria of diabetic rats did not differ from those in age-matched control rats 1, 2, 4, 6 months in the right atria and even 9 months after STZ in the left atria. However, uncompensated diabetes led to a significant decrease in NPY levels 9 and 12 months after STZ administration in the right and left atria, respectively. In the ventricles, NPY concentrations were significantly decreased 6 months after the onset of diabetes. Interestingly, partial spontaneous recovery of diabetes was associated with increased NPY levels in the atria. Myocardial norepinephrine concentrations increased 1 month after STZ and then declined reaching approximately 60% of the respective control values 12 months after the onset of the disease. Partial spontaneous recovery of diabetes had no effect on norepinephrine concentrations. Myocardial epinephrine concentrations did not differ from those found in controls till month 9 of the disease and they became significantly lower at month 12. Partial recovery of diabetes resulted in epinephrine concentrations not differing from the control values at month 12 of diabetes. Regarding to preferential localization of norepinephrine in the sympathetic postganglionic fibers and that of NPY also in intrinsic ganglion neurons, intrinsic neuronal circuits seem to be less susceptible to STZ-induced damage than extrinsic nerves and they might be able to recover after amelioration of diabetes.

Pituitary adenylate cyclase activating polypeptide (PACAP) decreases neuronal somatostatin immunoreactivity in cultured guinea-pig parasympathetic cardiac ganglia

Postganglionic parasympathetic neurons in guinea-pig cardiac ganglia exhibit choline acetyltransferase (ChAT)-immunoreactivity, and a large fraction (60%) of the ChAT-positive cardiac neurons co-express somatostatin-immunoreactivity. This co-expression remained when the cardiac ganglia explants were maintained in culture for 72 h (40% somatostatin-immunoreactive). The guinea-pig cardiac ganglia neurons express the high affinity pituitary adenylate cyclase activating polypeptide (PACAP)-selective PAC1 receptor, and treatment of the ganglia explants with 20 nM PACAP27 for 72 h to evaluate PACAP regulation of somatostatin expression revealed a dramatic 85% decrease in the number of somatostatin-IR neurons (6% somatostatin-IR neurons) compared with untreated control explant preparations. The decrease in percentage of somatostatin-IR neurons by PACAP27 was time- and concentration-dependent, and selective for PACAP27; PACAP38 and vasoactive intestinal polypeptide were less effective. PACAP6-38, a PACAP antagonist, eliminated the PACAP27-induced change in somatostatin positive neurons. The PACAP-mediated decrease in somatostatin-IR neurons was eliminated in calcium-deficient solutions and by the addition of nifedipine, indicating a requirement for calcium influx through L-type calcium channels. The addition of either the calmodulin inhibitor N-(4-aminobutyl)-1-naphthalenesulfonamide or the MEK inhibitor PD98059, also eliminated the PACAP27-induced decrease in somatostatin-IR cells. The PACAP27-mediated effect on somatostatin expression was not affected by inhibitors of protein kinase A or phospholipase C, but was reduced by the adenylyl cyclase inhibitor SQ22356, suggesting cAMP involvement. Semiquantitative and quantitative reverse transcription PCR prosomatostatin transcript measurements showed that cardiac ganglia prosomatostatin mRNA levels were not diminished by chronic PACAP27 exposure despite the dramatic decrement in somatostatin-expressing neurons. Neuronal peptide-IR content represents a balance between production and secretion. These results suggested that one of the primary effects of PACAP exposure may be enhanced levels of neuropeptide release that exceeded production levels, resulting in somatostatin depletion and a decrement in the number of identifiable somatostatin-expressing cardiac neurons.

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.

Effect of stimulating non-myelinated vagal axons on atrio-ventricular conduction and left ventricular function in anaesthetized rabbits

It has previously been demonstrated in several species that stimulation of myelinated vagal efferent fibres evokes slowing of heart rate and atrio-ventricular (A-V) conduction and a decreased ventricular contractility but recruitment of non-myelinated fibres did not further increase the response. Only in rabbits was a significant bradycardia evoked on recruiting non-myelinated fibres. However, if stimulating myelinated fibres produced a near maximal response, then effects of further activation of non-myelinated fibres may have been missed. Indeed, selective stimulation of non-myelinated fibres alone now has been shown to evoke a slowing of heart rate independent of the effects of myelinated fibres. In the present study we tested in rabbits whether selective stimuli are also capable of slowing A-V conduction and changing ventricular contractility. In rabbits pretreated with the beta 1-adrenoceptor antagonist atenolol, ECG, arterial blood pressure, left ventricular pressure and dP/dt were recorded before and during stimulation of non-myelinated vagal efferent fibres using an anodal block technique (J. Physiol. 273 (1977) 539). R-R interval and A-V conduction times were computed off-line. Stimulation of non-myelinated vagal fibres increased R-R interval by 97.7 +/- 18.8 ms from a baseline of 315.3 +/- 7.7 ms, increased A-V conduction time by 9.9 +/- 1.1 ms from a baseline of 81.9 +/- 3.1 ms and decreased left ventricular dP/dtmax by 2486 +/- 362 mmHg s-1 from a baseline of 11,186 +/- 795 mmHg s-1. When hearts were paced at a rate about 10% higher than normal, A-V conduction time still increased by 13.3 +/- 1.9 ms from a baseline of 104.2 +/- 3.6 ms and dP/dtmax still fell by 2300 +/- 188 mmHg s-1 from a baseline of 11,200 +/- 777 mmHg s-1. Ganglionic blockade with hexamethonium (15-20 mg kg-1) always abolished the evoked increases in A-V conduction time, whilst there was still an increase in R-R interval in seven of the 12 animals tested. The data demonstrate that non-myelinated vagal efferent fibres can modulate chronotropic, dromotropic and inotropic actions on the heart.

Pathway specific expression of neuropeptides and autonomic control of the vasculature

In this article, we review the immunohistochemical evidence for the pathway-specific expression of co-existing neuropeptides in autonomic vasomotor neurons, and examine the functional significance of these expression patterns for the autonomic regulation of the vasculature. Most final motor neurons in autonomic vasomotor pathways contain neuropeptides in addition to non-peptide co-transmitters such as catecholamines, acetylcholine and nitric oxide. Neuropeptides also occur in preganglionic vasomotor neurons. The precise combinations of neuropeptides expressed by neurons in vasomotor pathways vary with species, vascular bed, and the level within the vascular bed. This applies to both vasoconstrictor and vasodilator pathways. There is a similar degree of variation in the expression of neuropeptide receptors in the vasculature. Consequently, the contributions of different peptides to autonomic vasomotor control are closely matched to the functional requirements of specific vascular beds. This arrangement allows for a high degree of precision in vascular control in normal conditions and has the potential for considerable plasticity under pathophysiological conditions.

Contribution of the cervical sympathetic ganglia to the innervation of the pharyngeal arch arteries and the heart in the chick embryo

In the chick heart, sympathetic innervation is derived from the sympathetic neural crest (trunk neural crest arising from somite level 10-20). Since the trunk neural crest gives rise to sympathetic ganglia of their corresponding level, it suggests that the sympathetic neural crest develops into cervical ganglia 4-14. We therefore tested the hypothesis that, in addition to the first thoracic ganglia, the cervical ganglia might contribute to cardiac innervation as well. Putative sympathetic nerve connections between the cervical ganglia and the heart were demonstrated using the differentiation markers tyrosine hydroxylase and HNK-1. In addition, heterospecific transplantation (quail to chick) of the cardiac and trunk neural crest was used to study the relation between the sympathetic neural crest and the cervical ganglia. Quail cells were visualized using the quail nuclear antibody QCPN. The results by immunohistochemical study show that the superior and the middle cervical ganglia and possibly the carotid paraganglia contribute to the carotid nerve. This nerve subsequently joins the nodose ganglion of the vagal nerve via which it contributes to nerve fibers in cardiac vagal branches entering the arterial and venous pole of the heart. In addition, the carotid nerve contributes to nerve fibers connected to putative baro- and chemoreceptors in and near the wall of pharyngeal arch arteries suggesting a role of the superior and middle cervical ganglia and the paraganglia of the carotid plexus in sensory afferent innervation. The lower cervical ganglia 13 and 14 contribute predominantly to nerve branches entering the venous pole via the anterior cardinal veins. We did not observe a thoracic contribution. Heterospecific transplantation shows that the cervical ganglia 4-14 as well as the carotid paraganglia are derived from the sympathetic neural crest. The cardiac neural crest does not contribute to the neurons of the cervical ganglia. We conclude that the cervical ganglia contribute to cardiac innervation which explains the contribution of the sympathetic neural crest to the innervation of the chick heart.

3-D characterization of ganglion cells of the terminal nerve plexus of rat atrioventricular junction

Three-dimensional (3-D) morphology of neurons of the terminal nerve plexus of the atrioventricular junction was examined in a scanning electron microscope. Distributions of different cell types encountered as well as their relations to different structures of the atrioventricular specialized tissue were also studied. Most neurons were found disseminated in a thin connective tissue layer separating different segments of the atrioventricular conductive tissue from the interventricular septum. Sometimes, they formed small pluricellular ganglia (up to 5 neurons) but, frequently, they occurred isolated in the terminal ramifications of the intramural nerve plexus of specialized tissue. Some intranodal neurons could also be identified. According to their 3-D morphology, nerve cells of the perinodal ganglionated plexus could be divided into three categories: (1) Large unipolar neurons were scattered throughout the atrioventricular junction. Their long and thin axonal projections were often directed towards the interventricular septum. (2) Large pseudounipolar or bipolar neurons were located at a few specific loci, namely all along the bundle of His and its bifurcation into the right and left bundle branches. Frequently, they occurred solitary and immersed amongst strands of surrounding muscle cells. Only occasional synaptic impacts could be identified on the surface of neuronal bodies of these bipolar neurons. On the other hand, their dendritic varicosities were richly innervated. Due to their irregular shape, intimate association with muscular elements and their topographical superposition with occasional spindle-like structures, these nerve cells recall prospective sensory neurons involved in integration of mechanical and neural stimuli to the heart. (3) Small multipolar interneurons could be identified in the retronodal ganglion and within right and left bundle branches. The present description of morphological heterogeneity of intramural nerve cells agrees with recent morphological and functional classifications of autonomic neurons and supports the idea that, at the level of the atrioventricular junction, a self-governed neuronal network may be operating.

Heterogeneity of nicotinic receptor class and subunit mRNA expression among individual parasympathetic neurons from rat intracardiac ganglia

Neurons have the potential to form thousands of distinct neuronal nicotinic receptors from the eight alpha and three beta subunits that currently are known. In an effort to determine how much of this potential complexity is realized among individual neurons, we examined the nicotinic pharmacological and biophysical properties and receptor subunit mRNA expression patterns in individual neurons cultured from rat epicardial ganglia. Analysis of the whole-cell pharmacology of these neurons showed a diversity of responses to the agonists acetylcholine, nicotine, cytisine, and 1,1-dimethyl-4-phenylpiperazinium, suggesting that a heterogeneous population of nicotinic receptor classes, or subtypes, is expressed by individual neurons. Single-channel analysis demonstrated three distinct conductances (18, 24, and 31 pS), with patches from different neurons containing different combinations of these channel classes. We used single-cell RT-PCR to examine nicotinic acetylcholine receptor (nAChR) subunit mRNA expression by individual neurons. Although mRNAs encoding all eight neuronal nAChR subunits for which we probed (alpha 2-alpha 5, alpha 7, beta 2-beta 4) were present in multicellular cultures, we found that individual epicardial neurons express distinct subsets of these nAChR subunit mRNAs. These results suggest that individual epicardial neurons express distinct arrays of nAChR subunits and that these subunits may assemble into functional receptors with distinct and variable subunit composition. This variable receptor subunit expression provides an explanation for the diversity of pharmacological and single-channel responses we have observed in individual neurons.

Distribution of PGP 9.5, TH, NPY, SP and CGRP immunoreactive nerves in the rat and guinea pig atrioventricular valves and chordae tendineae

The distribution of nerves immunoreactive to protein gene product 9.5 (PGP 9.5), tyrosine hydroxylase (TH), neuropeptide Y (NPY), substance P (SP) and calcitonin gene related peptide (CGRP) antisera was investigated in the atrioventricular valves of the Sprague-Dawley rat and the Dunkin-Hartley guinea pig using confocal and epifluoresence microscopy. No major differences were noted between the innervation of the mitral and tricuspid valves in either species. For all antisera the staining was more extensive in the guinea pig valves. Two distinct nerve plexuses separated by a 'nearly nerve free' zone were identified in both species with each antiserum tested. This was most apparent on the anterior cusp of the mitral valve. The major nerve plexus extends from the atrioventricular ring through the basal, intermediate and distal zones of the valves towards the free edge of the valve cusp. These nerve bundles, arranged as primary, secondary and tertiary components, ramify to the free edge of the valve and extend to the attachment of the chordae. They do not contribute to the innervation of the chordae tendineae. The second, minor chordal plexus, runs from the papillary muscles through the chordae tendineae and passes parallel to the free edge of the cusp. The nerves of this minor plexus are interchordal, branching to terminate mainly in the distal zone, free edge of the valve cusp and adjacent chordae tendineae. Some interchordal nerve fibres loop from a papillary muscle up through a chorda, along the free edge and pass down an adjacent chorda into another papillary muscle. The nerve fibres of the major and minor plexuses intermingle although no evidence was found for interconnectivity between them. In the distal zone between the major plexus which extends from the base of the valve and the minor chordal plexus there is a zone completely free of nerves staining with antisera to TH and NPY. Occasional nerves which stained positive for PGP 9.5, SP and CGRP immunoreactivities crossed this 'nearly nerve free zone' passing either from the chordal/free edge nerves to the intermediate and basal zones or vice versa. An additional small nerve plexus which displayed immunoreactivity to CGRP antiserum extended from the atrioventricular ring into the basal zone of the valve cusp. Not all chordae tendineae displayed immunoreactive nerve fibres. It is concluded that the innervation patterns of the sensory and sympathetic neurotransmitters and neuropeptides examined in the atrioventricular valves of the rat and guinea pig are ubiquitous in nature. The complexity of the terminal innervation network of the mammalian atrioventricular valves and chordae tendineae may contribute to the complex functioning of these valves in the cardiac cycle.

Vagal afferent innervation of the atria of the rat heart reconstructed with confocal microscopy

We have used confocal microscopy to analyze the vagal afferent innervation of the rat heart. Afferents were labeled by injecting 1,1'-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI) into the nodose ganglia of animals with prior supranodose de-efferentations, autonomic ganglia were stained with Fluoro-gold, and tissues were examined in whole mounts. Distinctively different fiber specializations were observed in the epi-, myo-, and endocardium: Afferents to the epicardium formed complexes associated with cardiac ganglia. These ganglia consisted of four major ganglionated plexuses, two on each atrium, at junctions of the major vessels with the atria. Ganglionic locations and sizes (left > right) were consistent across animals. In addition to principal neurons (PNs), significant numbers of small intensely fluorescent (SIF) cells were located in each of these plexuses, and vagal afferents provided dense pericellular varicose endings around the SIF cells in each ganglionic plexus, with few if any terminations on PNs. In the myocardium, vagal afferents formed close contacts with cardiac muscles, including conduction fibers. In the endocardium, vagal fibers formed "flower-spray" and "end-net" terminals in connective tissue. With three-dimensional reconstruction of confocal optical sections, a novel polymorphism was seen: Some fibers had one or more collaterals ending as endocardial flower sprays and other collaterals ending as myocardial intramuscular endings. Some unipolar or pseudounipolar neurons within each cardiac ganglionic plexus were retrogradely labeled from the nodose ganglia. In conclusion, vagal afferents form a heterogeneity of differentiated endings in the heart, including structured elements which may mediate chemoreceptor function, stretch reception, and local cardiac reflexes.