Distribution of muscarinic receptors and acetylcholinesterase in the rat heart

Potential effect of the non-neuronal cardiac cholinergic system on hepatic glucose and energy metabolism

The vagus nerve belongs to the parasympathetic nervous system, which is involved in the regulation of organs throughout the body. Since the discovery of the non-neuronal cardiac cholinergic system (NNCCS), several studies have provided evidence for the positive role of acetylcholine (ACh) released from cardiomyocytes against cardiovascular diseases, such as sympathetic hyperreactivity-induced cardiac remodeling and dysfunction as well as myocardial infarction. Non-neuronal ACh released from cardiomyocytes is believed to regulate key physiological functions of the heart, such as attenuating heart rate, offsetting hypertrophic signals, maintaining action potential propagation, and modulating cardiac energy metabolism through the muscarinic ACh receptor in an auto/paracrine manner. Moreover, the NNCCS may also affect peripheral remote organs (e.g., liver) through the vagus nerve. Remote ischemic preconditioning (RIPC) and NNCCS activate the central nervous system and afferent vagus nerve. RIPC affects hepatic glucose and energy metabolism through the central nervous system and vagus nerve. In this review, we discuss the mechanisms and potential factors responsible for NNCCS in glucose and energy metabolism in the liver.

The future of cardioneuroablation in cardiovascular medicine

INTRODUCTION

Cardioneuroablation is increasingly being utilized to improve outcomes in patients with vagally mediated bradyarrhythmias. However, there are still controversial issues in the field including patient selection, safety and efficacy, and procedural end-points.

AREAS COVERED

In this review, the current role of cardioneuroablation is summarized, and controversial issues related to the modality are discussed.

EXPERT OPINION

According to small open-label cohort studies, overall freedom from syncope recurrence was higher than 90% after cardioneuroablation in patients with vasovagal syncope (VVS). Use of the electrogram-based strategy or high-frequency stimulation demonstrate similar success rate except in procedures limited to the right atrium. Based on a recently published randomized controlled trial and metanalysis, it may be possible now to make a strong recommendation for cardioneuroablation in patients <40 years of age, and those with the cardioinhibitory or mixed type of VVS who continue to experience frequent and/or burdensome syncope recurrences. Considering patients with VVS are prone to significant placebo/expectation effect, sham-controlled trials may help to quantify the placebo effect. In well-selected patients with functional atrioventricular block and sinus bradycardia, may result in encouraging medium-term outcomes. However, functional bradycardia is identified in a minority of patients presenting with high-grade atrioventricular block or sinus node dysfunction.

Characteristic Effects of the Cardiac Non-Neuronal Acetylcholine System Augmentation on Brain Functions

Since the discovery of non-neuronal acetylcholine in the heart, this specific system has drawn scientific interest from many research fields, including cardiology, immunology, and pharmacology. This system, acquired by cardiomyocytes independent of the parasympathetic nervous system of the autonomic nervous system, helps us to understand unsolved issues in cardiac physiology and to realize that the system may be more pivotal for cardiac homeostasis than expected. However, it has been shown that the effects of this system may not be restricted to the heart, but rather extended to cover extra-cardiac organs. To this end, this system intriguingly influences brain function, specifically potentiating blood brain barrier function. Although the results reported appear to be unusual, this novel characteristic can provide us with another research interest and therapeutic application mode for central nervous system diseases. In this review, we discuss our recent studies and raise the possibility of application of this system as an adjunctive therapeutic modality.

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.

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.

Non-neuronal cholinergic machinery present in cardiomyocytes offsets hypertrophic signals

Recent work has provided compelling evidence that increased levels of acetylcholine (ACh) can be protective in heart failure, whereas reduced levels of ACh secretion can cause heart malfunction. Previous data show that cardiomyocytes themselves can actively secrete ACh, raising the question of whether this cardiomyocyte derived ACh may contribute to the protective effects of ACh in the heart. To address the functionality of this non-neuronal ACh machinery, we used cholinesterase inhibitors and a siRNA targeted to AChE (acetylcholinesterase) as a way to increase the availability of ACh secreted by cardiac cells. By using nitric oxide (NO) formation as a biological sensor for released ACh, we showed that cholinesterase inhibition increased NO levels in freshly isolated ventricular myocytes and that this effect was prevented by atropine, a muscarinic receptor antagonist, and by inhibition of ACh synthesis or vesicular storage. Functionally, cholinesterase inhibition prevented the hypertrophic effect as well as molecular changes and calcium transient alterations induced by adrenergic overstimulation in cardiomyocytes. Moreover, inhibition of ACh storage or atropine blunted the anti-hypertrophic action of cholinesterase inhibition. Altogether, our results show that cardiomyocytes possess functional cholinergic machinery that offsets deleterious effects of hyperadrenergic stimulation. In addition, we show that adrenergic stimulation upregulates expression levels of cholinergic components. We propose that this cardiomyocyte cholinergic signaling could amplify the protective effects of the parasympathetic nervous system in the heart and may counteract or partially neutralize hypertrophic adrenergic effects.

Cholinergic modulation of slow cortical rhythm in urethane-anesthetized rats

Slow cortical rhythm (SCR) is characterized by rhythmic cycling of active (UP) and silent (DOWN) states in cortical cells. In urethane anesthesia, SCR appears as alternation of almost isoelectrical EEG periods and low-frequency, high-amplitude large shifts with superimposed high-frequency activity in the local field potentials (LFPs). Dense cholinergic projection reaches the cortex from the basal forebrain (BF), and acetylcholine (ACh) has been demonstrated to play a crucial role in the regulation of cortical activity. In the present experiments, cholinergic drugs were administered topically to the cortical surface of urethane-anesthetized rats to examine the direct involvement of ACh and the BF cholinergic system in the SCR. SCR was recorded by a 16-pole vertical electrode array from the hindlimb area of the somatosensory cortex. Multiple unit activity (MUA) was recorded from layer V to VI in close proximity of the recording array. Neither a low dose (10 mM solution) of the muscarinic antagonist atropine or the nicotinic agonist nicotine (1 mM solution) had any effect on SCR. In contrast, the higher dose (100 mM solution) of atropine, the cholinergic agonist carbachol (32 mM solution), and the cholinesterase inhibitor physostigmine (13 mM solution) all decreased the number of UP states, delta power (0-3 Hz) and MUA. These results suggest that cholinergic system may influence SCR through muscarinic mechanisms during urethane anesthesia. Cholinergic activation obstructs the mechanisms responsible for local or global synchronization seen during SCR as this rhythm was disrupted or aborted. Muscarinic antagonism can evoke similar changes when high dose of atropine is applied.

Cholinoceptive and cholinergic properties of cardiomyocytes involving an amplification mechanism for vagal efferent effects in sparsely innervated ventricular myocardium

Our recent studies have shown that, as indicated by vagal stimulation, an acetylcholinesterase inhibitor donepezil, an anti-Alzheimer's disease drug, prevents progression of heart failure in rats with myocardial infarction, and activates a common cell survival signal shared by acetylcholine (ACh) in vitro. On the basis of this and evidence that vagal innervation is extremely poor in the left ventricle, we assessed the hypothesis that ACh is produced by cardiomyocytes, which promotes its synthesis via a positive feedback mechanism. Rat cardiomyocytes expressed choline acetyltransferase (ChAT) in the cytoplasm and vesicular acetylcholine transporter with the vesicular structure identified by immunogold electron microscopy, suggesting that cardiomyocytes possess components for ACh synthesis. Intracellular ACh in rat cardiomyocytes was identified with physostigmine or donepezil. However, with atropine, the basal ACh content was reduced. In response to exogenous ACh or pilocarpine, cardiomyocytes increased the transcriptional activity of the ChAT gene through a muscarinic receptor and ChAT protein expression, and, finally, the intracellular ACh level was upregulated by pilocarpine. Knockdown of ChAT by small interfering RNA accelerated cellular energy metabolism, which is suppressed by ACh. Although physostigmine had a minimal effect on the ChAT promoter activity by inhibiting acetylcholinesterase, donepezil resulted in elevation of the activity, protein expression and intracellular ACh level even in the presence of sufficient physostigmine. Orally administered donepezil in mice increased the ChAT promoter activity in a reporter gene-transferred quadriceps femoris muscle and the amount of cardiac ChAT protein. These findings suggest that cardiomyocytes possess an ACh synthesis system, which is positively modulated by cholinergic stimuli. Such an amplification system in cardiomyocytes may contribute to the beneficial effects of vagal stimulation on the ventricles.

Vagus nerve stimulation in the treatment of refractory epilepsy

Many patients with epilepsy suffer from persistent seizures despite maximal anti-epileptic drug therapy. Chronic, intermittent vagus nerve stimulation has been proven to be an effective option for many patients suffering from refractory seizures who are not candidates for surgical resection. Although only a small minority of patients will be entirely seizure-free, vagus nerve stimulation, as an adjunct to medical therapy, may result in significant improvements in quality of life. Vagus nerve stimulation is generally well-tolerated, as device implantation is associated with a low rate of perioperative complications, and the majority of side effects are stimulation-dependent and thus reversible.

In vitro sensitivity of cholinesterases and [3H]oxotremorine-M binding in heart and brain of adult and aging rats to organophosphorus anticholinesterases

Organophosphorus (OP) insecticides elicit toxicity via acetylcholinesterase inhibition, allowing acetylcholine accumulation and excessive stimulation of cholinergic receptors. Some OP insecticides bind to additional macromolecules including butyrylcholinesterase and cholinergic receptors. While neurotoxicity from OP anticholinesterases has been extensively studied, effects on cardiac function have received less attention. We compared the in vitro sensitivity of acetylcholinesterase, butyrylcholinesterase and [(3)H]oxotremorine-M binding to muscarinic receptors in the cortex and heart of adult (3 months) and aging (18 months) rats to chlorpyrifos, methyl parathion and their active metabolites chlorpyrifos oxon and methyl paraoxon. Using selective inhibitors, the great majority of cholinesterase in brain was defined as acetylcholinesterase, while butyrylcholinesterase was the major cholinesterase in heart, regardless of age. In the heart, butyrylcholinesterase was markedly more sensitive than acetylcholinesterase to inhibition by chlorpyrifos oxon, and butyrylcholinesterase in tissues from aging rats was more sensitive than enzyme from adults, possibly due to differences in A-esterase mediated detoxification. Relatively similar differences were noted in brain. In contrast, acetylcholinesterase was more sensitive than butyrylcholinesterase to methyl paraoxon in both heart and brain, but no age-related differences were noted. Both oxons displaced [(3)H]oxotremorine-M binding in heart and brain of both age groups in a concentration-dependent manner. Chlorpyrifos had no effect but methyl parathion was a potent displacer of binding in heart and brain of both age groups. Such OP and age-related differences in interactions with cholinergic macromolecules may be important because of potential for environmental exposures to insecticides as well as the use of anticholinesterases in age-related neurological disorders.

Vagus nerve stimulation for epilepsy and depression

Many patients with epilepsy suffer from persistent seizures despite maximal antiepileptic drug (AED) therapy. Chronic, intermittent vagus nerve stimulation (VNS) has proven to be a safe, effective option for patients suffering from refractory seizures who are not candidates for surgical resection. Although only a small minority of patients will be entirely seizure-free, VNS as an adjunct to medical therapy does appear to provide a significant amount of improvement in quality of life. Reports of antidepressant effects independent of seizure control, along with the use of multiple AEDs in the treatment of depression, has led to the investigation of VNS as a potential adjunctive treatment for major depressive disorder. Both the number of severely depressed patients refractory to available pharmacologic options and the need for repeated treatments and significant side effects associated with electroconvulsive therapy have heightened the interest in VNS for this patient population. Pilot studies of VNS for depression have shown impressive response rates; however, the effect appears to be gradual in onset, as demonstrated by the lack of a favorable response in a short-term, randomized controlled study. Investigation is thus needed to establish the potential role of VNS as an adjunctive treatment for severe depression.

Relative distribution densities of cholinergic and adrenoceptor structures in the central part of the sinoatrial node in rat heart

Characteristics of distribution of cholinergic and adrenoceptor structures along the sinoatrial node artery in rat heart were evaluated by autoradiography on semithin sections by determining the density of (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate binding sites. The relative density of binding sites for (3)H-dihydroalprenolol and (3)H-quinuclidinyl benzilate was minimum in the functional nucleus of the sinoatrial node and asymmetrically increased to maximum values to cranial (sharply) and caudal (smoothly) directions. The relative level of binding for (3)H-dihydroalprenolol in the perinodal atrial myocardium tissue was markedly lower than in the periarterial zone of the central part of the sinoatrial node and comparable to that for (3)H-quinuclidinyl benzilate.

Insulin regulates neuronal M2 muscarinic receptor function in the ileum of diabetic rats

Acetylcholine release from cholinergic nerves in the gastrointestinal tract is limited by neuronal M(2) muscarinic receptors. In diabetic animals, M(2) muscarinic receptor function in the ileum is increased, leading to decreased acetylcholine release and smooth muscle contraction in response to nerve stimulation. The mechanisms responsible for increased M(2) muscarinic receptor function are unknown but may contribute to the gastrointestinal dysmotility that occurs frequently in diabetics. In this study, we investigated whether insulin modulates M(2) muscarinic receptor function in the gastrointestinal tract of diabetic rats. M(2) muscarinic receptor function was tested by measuring the ability of an agonist, pilocarpine, to inhibit and an antagonist, methoctramine, to potentiate electrical field stimulation (EFS)-induced contraction of ileum in vitro. Insulin administration (0.2, 0.6, and 2 U s.c. daily for 7 days) reversed the diabetes-induced increase in M(2) muscarinic receptor function and restored normal contractions to EFS. Insulin had no effect on the function of postjunctional M(3) muscarinic receptors, determined by measuring contractile responses to acetylcholine. These data suggest that insulin tonically inhibits neuronal M(2) muscarinic receptors. Thus, loss of insulin removes this inhibition and increases M(2) muscarinic receptor function leading to decreased acetylcholine release and contraction to EFS. In nondiabetic rats, there was a trend that higher insulin doses (0.6 and 2 U) increased M(2) muscarinic receptor function, suggesting a bell-shaped concentration-response relationship for insulin. In conclusion, lack of insulin or excess insulin increases M(2) muscarinic receptor function in rat ileum. This mechanism may contribute to decreased acetylcholine release in the gastrointestinal tract of diabetics, resulting in dysmotility.

A comparative study on cardiac ganglia in midday gerbil, Egyptian spiny mouse, chinchilla laniger and pigeon

Using the thiocholine method and histological techniques, the topography and morphology of cardiac ganglia in midday gerbil, Egyptian spiny mouse, chinchilla laniger and pigeon were studied. The results demonstrated that cardiac ganglia in all investigated species are embedded in epicardial fat. They formed plexo-ganglionic structures. Each of them composed of many ganglia (from seven up to 36) different in size and shape, and interconnected by fascicles of nerve fibres. Comparative analysis showed that the density of neural network and cell aggregations was different in individual species. The richest plexo-ganglionic structure was in pigeon. It was organized in three plexo-ganglia with an average of 30 ganglia. The largest one was located along the anterior interventricular sulcus. The cardiac ganglia of investigated mammals were localized mainly on the epicardium of atria; in Egyptian spiny mouse and chinchilla laniger on the ventral surface of right atrium, but in midday gerbil on the dorsal surface of left atrium. Moreover, in midday gerbil and Egyptian spiny mouse the little plexo-ganglionic structure on the ventricle were noticed. Additionally, in midday gerbil the single nerve cells might be observed between cardiac muscle of atria. It can be said that, the strongly developed cardiac plexus in pigeon is probably connected with his behaviour and functional properties of the heart. The arrangement of neurones in cardiac ganglia of all examined mammals was uniform over the whole surface of the sections, while in the pigeon, neurones were located mainly in the peripheral part of the ganglion.

The role of neuronal nicotinic acetylcholine receptor subunits in autonomic ganglia: lessons from knockout mice

Neuronal nicotinic acetylcholine receptors (nAChR), composed of 12 subunits (alpha2-alpha10, beta2-beta4), are expressed in autonomic ganglia, playing a central role in autonomic transmission. The repertoire of nicotinic subunits in autonomic ganglia includes alpha3, alpha5, alpha7, beta2 and beta4 subunits. In the last 10 years, heterologous expression studies have revealed much about the nature of neuronal nAChRs. However, there is only limited understanding of subunit actions in autonomic system. Functional deletions of subunit by gene knockout in animals could overcome these limitations. We review recent studies on nAChRs on autonomic ganglia for physiological and pharmacological properties and potential locations of the subunits.

Chronic decentralization of the heart differentially remodels canine intrinsic cardiac neuron muscarinic receptors

The objective of the study was to determine if chronic interruption of all extrinsic nerve inputs to the heart alters cholinergic-mediated responses within the intrinsic cardiac nervous system (ICN). Extracardiac nerve inputs to the ICN were surgically interrupted (ICN decentralized). Three weeks later, the intrinsic cardiac right atrial ganglionated plexus (RAGP) was removed and intrinsic cardiac neuronal responses were evaluated electrophysiologically. Cholinergic receptor abundance was evaluated using autoradiography. In sham controls and chronic decentralized ICN ganglia, neuronal postsynaptic responses were mediated by acetylcholine, acting at nicotinic and muscarinic receptors. Muscarine- but not nicotine-mediated synaptic responses that were enhanced after chronic ICN decentralization. After chronic decentralization, muscarine facilitation of orthodromic neuronal activation increased. Receptor autoradiography demonstrated that nicotinic and muscarinic receptor density associated with the RAGP was unaffected by decentralization and that muscarinic receptors were tenfold more abundant than nicotinic receptors in the right atrial ganglia in each group. After chronic decentralization of the ICN, intrinsic cardiac neurons remain viable and responsive to cholinergic synaptic inputs. Enhanced muscarinic responsiveness of intrinsic cardiac neurons occurs without changes in receptor abundance.

Heart rate responses to a muscarinic agonist in rats with experimentally induced acute and subacute chagasic myocarditis

We administered arecoline to rats, with experimentally induced chagasic myocarditis, in order to study the sinus node sensitivity to a muscarinic agonist. Sixteen month old rats were inoculated with 200,000 T. cruzi parasites ("Y" strain). Between days 18 and 21 (acute stage), 8 infected rats and 8 age-matched controls received intravenous arecoline as a bolus injection at the following doses: 5. 0, 10.0, 20.0, 40.0, and 80.0 microg/kg. Heart rate was recorded before, during and after each dose of arecoline. The remaining 8 infected animals and 8 controls were subjected to the same experimental procedure during the subacute stage, i.e., days 60 to 70 after inoculation. The baseline heart rate, of the animals studied during the acute stage (349 +/- 68 bpm, mean +/- SD), was higher than that of the controls (250 +/- 50 bpm, p < 0.005). The heart rate changes were expressed as percentage changes over baseline values. A dose-response curve was constructed for each group of animals. Log scales were used to plot the systematically doubled doses of arecoline and the induced-heart rate changes. The slope of the regression line for the acutely infected animals (r = - 0.99, b =1.78) was not different from that for the control animals (r = - 0.97, b = 1.61). The infected animals studied during the subacute stage (r = - 0.99, b = 1.81) were also not different from the age-matched controls (r = - 0.99, b = 1.26, NS). Consequently, our results show no pharmacological evidence of postjunctional hypersensitivity to the muscarinic agonist arecoline. Therefore, these results indirectly suggest that the postganglionic parasympathetic innervation, of the sinus node of rats with autopsy proved chagasic myocarditis, is not irreversibly damaged by Trypanosoma cruzi.

Cardiac ganglia of Japanese quail--distribution and morphology

The cardiac ganglia in Japanese quail were studied with the use of histological, histochemical and ultrastructural techniques. Histological investigations revealed the presence of a number of cholinergic ganglia in the fatty tissue of the epicardium. They were grouped in plexo-ganglionic forms localised in three regions: 1) on the ventral surface of the cardiac atria, 2) on the ventral surface of the cardiac ventricle, 3) on the dorsal surface of the cardiac ventricle. These plexoganglia are structures composed of many ganglia differing in size (from 77 microns to 577 microns length and from 53 microns to 163 microns width), connected by fascicles of nerve fibres. The cells of cardiac ganglia have single, round or oval nuclei with one or several dense nucleoli. There were myelinated and unmyelinated fibres in the intercellular spaces. Rough endoplasmic reticulum (RER) and free ribosomes were localised mainly in the perinuclear part of the cytoplasm. In the peripheral part, RER was less abundant, but mitochondria were more numerous in this part of the cytoplasm. In the peripheral parts of the neurones, axo-somatic synapses were usually observed. Profiles of the end sections of axons contained two kinds of synaptic vesicles: small, agranular ones and among them large ones with a dense core.

Characterization of acetylcholinesterase and butyrylcholinesterase forms in normal and dystrophic Lama2dy mouse heart

In searching for possible differences in acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) forms of dystrophic heart, the properties of ChE species in normal (NH) and dystrophic Lama2dy mouse heart (DH) were investigated. BuChE predominated over AChE. Loosely- and tightly-bound ChEs were released with saline (extract S1) and saline-Triton X-100 buffers (S2). About 50% of AChE, and 25% of BuChE, in NH or DH was measured in S1, and the rest in S2. Asymmetric AChE forms A12 (15%) and A8 (11%), globular hydrophilic G(H)4 (8%), amphiphilic G(A)4 (15%), and G(A)2+G(A)1 (51%) AChE species, and BuChE forms G(H)4 (13%), G(A)4 (3%), and G(A)2+G(A)1 (84%) were identified in NH and DH. Most of the asymmetric and G(A)4 AChE species were bound to Triticum vulgaris (WGA) or Ricinus communis (RCA) agglutinins. About half of G(H)4 and G(A)2+G(A)1 AChE were bound to WGA, and less (10%) to RCA. Variable amounts of G(H)4+G(A)4 (60%), and G(A)2+G(A)1 (75%) BuChE bound to WGA, and 50 and 10% to RCA. The lack of structural differences between ChE species in NH and DH indicates that, in contrast to the ChE forms in mouse skeletal muscle, the biosynthesis of ChE components in heart is not disturbed by dystrophy.

Beta-adrenergic and muscarinic receptor mRNA accumulation in the sinoatrial node area of adult and senescent rat hearts

The sinoatrial (SA) node is the cardiac pacemaker and changes in its adrenergic-muscarinic phenotype have been postulated as a determinant of age-associated modifications in heart rate variability. To address this question, right atria were microdissected, the SA node area was identified by acetylcholinesterase staining, and, using a RT-PCR method, the accumulation of mRNA molecules encoding beta1- and beta2-adrenergic (beta1- and beta2-AR) and muscarinic (M2-R) receptor was quantified to define the proportion between beta-AR and M2-R mRNAs within the sinoatrial area of adult (3 months) and senescent (24 months) individual rat hearts. In adult hearts, the highest M2-R/beta-AR mRNA ratio was observed within the sinoatrial area compared with adjacent atrial myocardium, while in the senescent hearts, no difference was observed between sinoatrial and adjacent areas. This change was specific of the sinoatrial area since adult and senescent whole atrial or ventricular myocardium did not differ in their M2-R/beta-AR mRNA ratio, and was associated with a fragmentation of acetylcholinesterase staining of the senescent SA node. Quantitative changes in the expression of genes encoding proteins involved in heart rate regulation specifically affect the sinoatrial area of the senescent heart.

Muscarinic receptor activation modulates Ca2+ channels in rat intracardiac neurons via a PTX- and voltage-sensitive pathway

With use of the whole cell patch-clamp technique, effects of the potent muscarinic agonist oxotremorine methiodide (oxo-M) on voltage-activated Ca2+ channel currents were investigated in acutely dissociated adult rat intracardiac neurons. In all tested neurons oxo-M reversibly inhibited the peak Ba2+ current. Inhibition of the peak Ba2+ current by oxo-M was associated with slowing of activation kinetics and was concentration dependent. The concentration of oxo-M necessary to produce a half-maximal inhibition of current and the maximal inhibition were 40.8 nM and 75.9%, respectively. Inhibitory effect of oxo-M was completely abolished by atropine. Among different muscarinic receptor antagonists, methoctramine (100 and 300 nM) significantly antagonized the current inhibition by oxo-M, with a negative logarithm of dissociation constant of 8.3 in adult rat intracardiac neurons. Internal dialysis of neurons with guanosine 5'-(thio)triphosphate (GTPgammaS, 0.5 mM) could mimic the muscarinic inhibition of the peak Ba2+ current and significantly occlude inhibitory effects of oxo-M. In addition, the internal dialysis of guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS, 2 mM) also significantly reduced the muscarinic inhibition of the peak Ba2+ current by oxo-M. Inhibitory effects of oxo-M were significantly abolished by pertussis toxin (PTX, 200 and 400 ng/ml) but not by cholera toxin (400 ng/ml). Furthermore, the bath application of N-ethylmaleimide (50 microM) significantly reduced the inhibition of the peak Ba2+ current by oxo-M. The oxo-M shifted the activation curve derived from measurments of tail currents toward more positive potentials. A strong conditioning prepulse to +100 mV significantly relieved the muscarinic inhibition of peak Ba2+ currents by oxo-M and the GTPgammaS-induced current inhibition. In a series of experiments, changes in intracellular concentration of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid and protein kinase activities failed to mimic or occlude the current inhibition by oxo-M. The dihydropyridine antagonist nifedipine (10 microM) was not able to occlude any of the inhibitory effects of oxo-M, and oxo-M (3 microM) failed to reduce the slow tail currents induced by the L-type agonist methyl 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate (FPL 64176; 2 microM). However, omega-conotoxin (omega-CgTX) GVIA (1 microM) significantly occluded the muscarinic inhibition of the Ba2+ currents. In the presence of omega-CgTX GVIA (1 microM) and nifedipine (10 microM), oxo-M could further inhibit approximately 20% of the total Ca2+ current. After complete removal of N-, Q-, and L-type currents with use of omega-CgTX GVIA, omega-agatoxin IVA, and nifedipine, 70% of the R-type current (approximately 6-7% of the total current) was inhibited by oxo-M (3 microM). In conclusion, the M2 muscarinic receptor activation selectively inhibits N-, Q-, and R-type Ca2+ channel currents, sparing L-type Ca2+ channel currents mainly via a PTX- and voltage-sensitive pathway in adult rat intracardiac neurons.

Expression of the cholinergic signal-transduction pathway components during embryonic rat heart development

BACKGROUND

Previous studies showed that acetylcholinesterase (AChE) activity is present in the downstream (arterial) part of the embryonic chick and rat heart, but its functional significance was unclear. To establish whether other components of a cholinergic signal-transduction pathway are present in the embryonic heart, we localised the mRNAs encoding choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and the muscarinic receptor isoforms (mAChRs; m1-m5).

METHODS

Messenger RNA detection and localisation by in situ hybridisation and reverse transcriptase-polymerase chain reaction were employed.

RESULTS

Expression of ChAT and AChE mRNAs was observed from 15 embryonic days onward in the neural tissue covering the dorsocranial wall of the atria. Muscarinic receptors (m1, m2, m4) were observed at the same localisation as AChE and ChAT mRNAs, both during embryogenesis and after birth. In addition, m1 and m4 mAChRs showed a low level of expression in the atrial myocardium during the fetal period. No expression of the m3 or the m5 mAChRs was observed in or near the embryonic hearts. ChAT, AChE, and mAChRs (m1, m2, m4) mRNAs always colocalised in the cardiac ganglia. However, none of these mRNAs was found at a detectable level in the outflow tract and/or the ventricular trabeculations.

CONCLUSIONS

The AChE activity in the arterial part of the embryonic heart is probably synthesised elsewhere and subserves a function different from the hydrolysis of locally produced acetylcholine.

Localization of muscarinic receptor mRNAs in rat heart and intrinsic cardiac ganglia by in situ hybridization

Although the heart is considered a relatively pure source of m2 muscarinic receptors, the possible expression of other muscarinic receptor genes at discrete sites within the myocardium or by intrinsic cardiac ganglia had not been evaluated. Accordingly, the present study used in situ hybridization histochemistry with 35S-labeled oligonucleotide probes to address this issue. Initial experiments demonstrated that the localization of m2 mRNA was similar to that reported for muscarinic receptors labeled with the nonselective muscarinic antagonist quinuclidinyl benzilate; however, there were two important exceptions. The conducting system contained less message than expected, whereas the intrinsic cardiac ganglia contained more. The mismatch between muscarinic receptor and m2 mRNA densities in the conducting system could not be explained by the local expression of other muscarinic receptor genes, since m1, m3, and m4 mRNAs were not detected at this or any other site within the myocardium. However, the presence of a high density of prejunctional muscarinic receptors in the conducting system would be consistent with such a mismatch. Surprisingly, the intrinsic cardiac ganglia contained more than four times as much m2 mRNA as found in the atria. This level of message may be necessary for the production of prejunctional receptors on cholinergic nerve fibers within the heart and receptors localized to the ganglion cell bodies. The ganglia also contained smaller amounts of m1 and m4 mRNAs. These observations suggest that prejunctional muscarinic receptors could have a prominent role in regulating cholinergic neurotransmission in the conducting system and that multiple muscarinic receptors are present in the intrinsic cardiac ganglia.

Gross and microscopic anatomy of the canine intrinsic cardiac nervous system

BACKGROUND

A three-dimensional description of the distribution and organization of the canine intrinsic cardiac nervous system was developed in order to characterize its full extent physiologically.

METHODS

The anatomy of the canine intrinsic cardiac nervous system was investigated in 67 mongrel dogs by means of visualization following methylene blue staining as well as by light and electron microscopic analyses.

RESULTS

Collections of ganglia associated with nerves, i.e., ganglionated plexuses, were identified in specific locations in epicardial fat and cardiac tissue. Distinct epicardial ganglionated plexuses were consistently observed in four atrial and three ventricular regions, with occasional neurons being located throughout atrial and ventricular tissues. One ganglionated plexus extended from the ventral to dorsal surfaces of the right atrium. Another ganglionated plexus, with three components, was identified in fat on the left atrial ventral surface. A ganglionated plexus was located on the mid-dorsal surface of the two atria, extending ventrally in the interatrial septum. A fourth atrial ganglionated plexus was located at the origin of the inferior vena cava extending to the dorsal caudal surface of the two atria. On the cranial surface of the ventricles a ganglionated plexus that surrounded the aortic root was identified. This plexus extended to the right and left main coronary arteries and origins of the ventral descending and circumflex coronary arteries. Two other ventricular ganglionated plexuses were identified adjacent to the origins of the right and left marginal coronary arteries. Intrinsic cardiac ganglia ranged in size from ones comprising one or a few neurons along the course of a nerve to ones as large as 1 x 3 mm estimated to contain a few hundred neurons. Intrinsic cardiac neuronal somata varied in size and shape, up to 36% containing multiple nucleoli. Electron microscopic examination demonstrated typical autonomic neurons and satellite cells in intrinsic cardiac ganglia. Many of their axon profiles contained large numbers of clear, round, and dense-core vesicles. Asymmetrical axodendritic synapses were common.

CONCLUSIONS

The canine intrinsic cardiac nervous system contains a variety of neurons interconnected via plexuses of nerves, the distribution of which is wider than previously assumed.

Co-expression of four muscarinic receptor genes by the intrinsic neurons of the rat and guinea-pig heart

Expression of the messenger RNAs encoding the five different muscarinic acetylcholine receptor subtypes was examined in intracardiac neurons from the rat and guinea-pig heart by in situ hybridization techniques. Newborn guinea-pig intracardiac neurons were studied in dissociated cell culture preparations employing both 35S- and digoxigenin-labelled oligonucleotide probes specific for the m1, m2, m3, m4 or m5 muscarinic receptor messenger RNAs. When 35S-tailed oligonucleotides were used, all intracardiac neurons in culture were found to express m1, m2, m3 and m4, but not m5 messenger RNAs. However after hybridization with digoxigenin-tailed probes, only m1 and m2 transcripts were detected. This may reflect differences in the sensitivity of the two techniques. Further to these experiments, intracardiac ganglia in sections of adult rat heart were studied employing m1-, m2-, m3- or m4-specific, 35S-labelled oligonucleotides, and again, all intracardiac neurons expressed messenger RNA for each of these four muscarinic receptor subtypes. Atrial myocytes in culture were only labelled by [35S]- and digoxigenin-tailed m2 oligonucleotides. No other heart cell type seen expressed messenger RNA for any of the muscarinic receptors. The expression of four different muscarinic receptor transcripts by intrinsic neurons of the heart provides the molecular basis for the diverse muscarinic actions observed in these and other autonomic ganglia.

Synaptic transmission in rat cardiac neurones

Intracellular recordings of spontaneous synaptic activity and synaptic responses to fibre tract stimulation were taken from neurones of ganglia isolated from the left atrium and interatrial septum of the rat. In six out of 57 neurones studied, spontaneous fast excitatory postsynaptic potentials (EPSPs) were recorded. Single stimulation of fibre tracts approaching the ganglion resulted in an all-or-none response consisting of an EPSP, from which an action potential abruptly appeared. This response disappeared in Ca(2+)-free/high-Mg2+ solution, indicating that it was orthodromic in origin. EPSPs were markedly exaggerated and prolonged by neostigmine (1-5 microM). EPSPs produced by high-frequency (0.1-20 Hz) fibre tract stimulation were markedly attenuated when compared with responses to single fibre tract stimulation, although they usually remained suprathreshold for spike initiation. High concentrations of hexamethonium (1 mM) and d-tubocurarine (300 microM) failed to inhibit responses to single fibre tract stimulation, although they completely abolished responses to high-frequency stimulation. Responses to single fibre tract stimulation were abolished by trimetaphan (greater than or equal to 100 microM). No slow synaptic responses were detected during single or high-frequency fibre tract stimulation. All cardiac neurones that responded orthodromically were highly excitable: they had a short post-spike after-hyperpolarization (AHP) and responded with multiple firing to prolonged membrane depolarization. It is concluded that cardiac neurones, in the region of the heart studied here, receive single 'strong' cholinergic inputs from some fibre tracts approaching the ganglion that elicit EPSPs accompanied by spikes. EPSPs are rather resistant to ganglion-blocking agents and subject to frequency modulation.

Acetylcholine receptors in rat cardiac neurones

Membrane potential changes produced by acetylcholine (ACh), and their underlying mechanisms, were studied in neurones of isolated cardiac ganglia of the rat by means of intracellular microelectrodes. Five components of membrane potential change could be detected in cardiac neurones following 1-5 s micro-application of ACh: (i) fast depolarization resulting from an activation of nonselective cationic conductance; (ii) slow depolarization associated with a decreased membrane conductance, presumably for potassium ions; slow hyperpolarization which consisted of (iii) early and (iv) late parts resulting from an activation of calcium-sensitive potassium current and from inhibition of steady-state inward current, respectively; and (v) delayed slow hyperpolarization associated with an increased conductance, most likely for potassium ions. Components (i), (iii) and (iv) persisted in the presence of atropine and were inhibited by nicotinic antagonists. Thus they were due to activation of nicotinic ACh receptors. However, the sensitivity of component (i) to ganglion-blocking agents appeared to be rather low: IC50s for inhibiting (i) were 226 +/- 34.2 microM, 31.2 +/- 4.31 microM and 15.3 +/- 3.27 microM for hexamethonium, d-tubocurarine, and trimetaphan, respectively. Components (ii) and (v) were abolished by atropine (1 microM) and mimicked by muscarine (component (ii) also persisted in d-tubocurarine), hence they resulted from activation of muscarinic ACh receptors. It is concluded that cardiac neurones are endowed with both nicotinic and muscarinic ACh receptors. Their activation leads to membrane depolarization and discharges followed by hyperpolarization and inhibition of discharges.

Gross and microscopic anatomy of the vagal innervation of the rat heart

Intrinsic cardiac ganglia and their vagal innervation are described from gross and microscopic dissections and functional studies in the anesthetized, open-chest, adult rat. Dissecting microscope sketches of the ventral and dorsal aspects of the rat heart provide gross descriptions of the anatomical course of the vagal cardiac nerves. Histological sectioning of adipose tissue packets surrounding the terminal endings of vagal branches distributed to the roots of the great cardiac vessels (aorta, pulmonary artery, precaval veins) revealed clusters of autonomic ganglia. These packets or "fat pads" were located: (1) along the dorsal surface of the right precava and extending medially toward the aortic root, (2) deep to the aortic arch, (3) in the angle between the root of the left precava and the pulmonary artery on the superior-dorsal surface of the left atrium, and (4) in the rostro-dorsal interatrial septum. Vagal distributions of small terminal branches were traced to each of these pads, which contained numerous autonomic ganglia. Electrical excitation of right or left cervical vagus elicited varying degrees of sinus slowing, slowing of A-V conduction, and suppression in atrial contractile force. Very small quantities (0.5 mg in 10 microliters saline) of the ganglionic blocking agent, hexamethonium (C6) were injected selectively into a single fat pad, followed by repetition of right or left vagal stimulation, with careful analysis of changes in heart rate (paced and unpaced), A-V conduction, and contractile force.

Identification and dissociation of cardiovascular neurons from the medulla for patch clamp analysis

This study describes a preparation that will enable us to study, using voltage clamp techniques, ionic currents from dissociated cardiovascular neurons that have retained their anatomical and functional identity of the intact animal. To identify dispersed preganglionic cardiac motoneurons various fluorescent dyes (rhodamine, fluorogold, microspheres, bizbenzimide and dextrans) were examined to determine which can be absorbed by preganglionic cardiac motorneuron nerve terminals (without surgical penetration of cardiac tissue), transported retrogradely to their soma in the medulla and retained during dissociation of the neurons. Rhodamine fulfilled these criteria. Dissociated preganglionic cardiac motorneurons had resting membrane potentials of -52.4 +/- 3 mV and input resistances of 236 +/- 71 M omega (mean +/- S.E.M., n = 10). Depolarizing voltage steps to -50 mV or above evoked a tetrodotoxin (TTX) sensitive inward sodium current followed by a biphasic outward current.

Brain natriuretic peptide-like immunoreactive innervation of the cardiovascular and cerebrovascular systems in the rat

Atrial natriuretic peptide is a potent dilator of aorta and renal and cerebral arteries and inhibits sympathetic tone in the heart in several mammalian species. We examined the possibility that a molecule related to porcine brain natriuretic peptide (pBNP), which acts at the same receptor sites as atrial natriuretic peptide, might provide an alternative source of natriuretic peptide to the cardiovascular system in the rat. An antiserum against pBNP demonstrated profuse immunoreactive innervation of the heart, cerebrovascular tree, and renal arteries. pBNP-like immunoreactive fibers ran in bundles along the surface of the heart, innervating the atria most heavily and penetrating the ventricular myocardium along the coronary arteries. There was greater density of innervation of the right side of the heart compared with the left, particularly in the ventricles, suggesting a parasympathetic origin. The entire cerebrovascular tree was innervated by immunoreactive pBNP fibers, with the densest concentration of immunoreactive fibers along the surface of the internal carotid, middle cerebral, posterior communicating, and anterior cerebral arteries. The proximal renal arteries were not innervated, but as they approached the kidney, they were invested by bundles of immunoreactive pBNP fibers. These axons followed the major branches of the renal artery into the kidney parenchyma, running along the surface of the arterioles up to their entrance into the renal glomeruli. No immunoreactive innervation of the aorta or proximal brachiocephalic, subclavian, or carotid arteries was seen. A substance related to pBNP may serve as a neuromodulator regulating cardiac output as well as blood flow in certain vascular beds.

Changes in the expression of acetylcholinesterase molecular forms during rat heart development

The development of the molecular forms of acetylcholinesterase was studied in rat heart during the perinatal period. In this study, the activity of acetylcholinesterase increased both per unit wet weight and per unit protein from post-conception day 14 to day 42. Additionally, the activity of atrial acetylcholinesterase per unit wet weight increased more rapidly after birth than that of ventricular acetylcholinesterase. The percent contribution of the various molecular forms to the total acetylcholinesterase pool in heart changed dramatically from the fetal to the neonatal period. This switch primarily consisted of a decrease in the ratio of the asymmetric to globular forms. Thus, the specific activity of globular forms increased while that of the asymmetric forms remained relatively stable. When the atria and ventricles were examined separately at 19 days post-conception, the percent contribution of the individual molecular forms in the two cardiac areas was different. The atria contained a pool of acetylcholinesterase forms similar to postnatal heart while the ventricles contained a pool of acetylcholinesterase forms with a lower globular to asymmetric ratio. Finally, this study showed that greater than half of the acetylcholinesterase pool was inhibited by incubating hearts from fetal rats with echothiopate iodide, suggesting that a large portion of acetylcholinesterase catalytic sites are externalized in fetal heart.

Acetylcholinesterase molecular forms in muscle and non-muscle cells of rat heart

Experiments were performed to determine the cellular associations of the molecular forms of acetylcholinesterase (AChE) in adult rat heart. For this purpose, a cardiac muscle and a non-muscle fraction were isolated from rat heart ventricles after perfusion with collagenase and hyaluronidase, extracts of these fractions were subjected to ultracentrifugation on linear density gradients of sucrose (5-20%), and fractions of these gradients were analyzed for AChE activity. The results show that only globular AChE molecular forms were present in isolated cardiac muscle cells. Globular AChE forms were also present in the non-muscle cells fraction but in different proportions. The proportions of globular AChE forms plus the high specific activity of choline acetyltransferase in the non-muscle cell fraction suggest that this fraction contains cholinergic nerve fragments. The results of this study also show that asymmetric AChE is released during the perfusion of heart with the digestive enzymes, which suggests that asymmetric AChE is bound to the extracellular matrix of heart.

Regional distribution of the molecular forms of acetylcholinesterase in adult rat heart

Acetylcholinesterase (AChE), the enzyme that degrades acetylcholine, exists as a multiple molecular forms that differ in their quaternary structure and mode of attachment to the cell surface. The distribution of the individual molecular forms of AChE in various cardiac regions with distinct anatomical characteristics was investigated. The results confirmed those of others by showing that the total pool of cardiac AChE had a nonuniform distribution in heart that paralleled the distribution of choline acetyltransferase. The rank order of this distribution was right atrial appendage greater than interatrial septum greater than left atrial appendage = right ventricle = interventricular septum greater than left ventricle. Velocity sedimentation in sucrose gradients of extracts from selected cardiac areas showed that four molecular forms were present in all areas but that the proportions of these forms differed as a function of area. The right and left ventricular walls, the apical portion of the interventricular septum, and the left atrial appendage contained G1 and G4 (globular) AChE in near-equal proportions, but in the basal portion of interventricular septum, the contribution of G4 AChE was greater than that of G1 AChE. The right atrial appendage and the interatrial septum had the largest amount of activity attributable to G4 AChE and the lowest amount attributable to G1 AChE. In all cardiac regions, A12 (asymmetric) AChE comprised 8-10% of the total AChE pool.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of cell and tissue culture techniques in the study of regulatory peptides

Cell and tissue culture preparations have a number of general advantages for the study of biological processes: cells are more accessible for study, diffusion delays and barriers to applied substances are minimised, the humoral and cellular components of the culture environment can be controlled and progressive changes in intracellular and intercellular events can be directly monitored. These significant advantages mean that culture preparations can provide unique opportunities for investigation of the properties and functions of regulatory peptides. Culture preparations also have disadvantages and not all cultures are suitable for use in all types of experiments; therefore, the choice of preparation must be made accordingly. Here we describe different types of culture preparation and give examples where cultures have been used to examine peptide synthesis, storage, secretion and receptor localisation, as well as the short-term and trophic actions of regulatory peptides.

The negative inotropic effect of acetylcholine on ferret ventricular myocardium

1. The effects of acetylcholine (ACh) on developed tension and intracellular Ca2+ concentration (as measured with aequorin) were studied in ferret papillary muscles, and on twitch shortening, the action potential and membrane currents in ferret ventricular myocytes. 2. Addition of ACh to ferret papillary muscles resulted in decreases in developed tension and the intracellular Ca2+ transient, both of which then partially recovered in the continued presence of ACh ('fade' of the response). On wash-off of ACh both developed tension and the intracellular Ca2+ transient increased above control ('rebound') before returning to control values. 3. Addition of ACh to ferret ventricular myocytes resulted in a membrane hyperpolarization of 2 +/- 0.5 mV (mean +/- S.E.M.; n = 9), a decrease in action potential duration to 23 +/- 6% of control and a decrease in twitch shortening to 31 +/- 5% of control. In the continued presence of ACh these responses to ACh faded. Thirty seconds after the maximal effect of ACh, action potential duration had partially recovered to 34 +/- 6% of control and twitch shortening to 46 +/- 7% of control. 4. The effects of ACh on twitch shortening could be mimicked under voltage clamp by varying voltage clamp pulse duration to simulate the ACh-induced changes in action potential duration. 5. When ACh was applied during a train of voltage clamp pulses of constant duration, 81% of the cells showed less than a 20% decrease in Ca2+ current and twitch shortening. However in 19% of the cells twitch shortening and the apparent Ca2+ current decreased by more than 30%. 6. In the 81% of cells, the normal decrease in twitch shortening was wholly the result of the shortening of the action potential. This in turn was the result of an increase in an outward background current which increased the rate of repolarization during the action potential. The ACh-induced background current reversed at -89 +/- 2 mV and showed inward-going rectification; these properties suggest that it was carried by K+. 7. In the 19% of cells, the normal decrease in twitch shortening was only partly the result of the shortening of the action potential (due to both the increase in outward background current as well as the apparent decrease in Ca2+ current). In these cells the decrease in twitch shortening may also have been partly the direct result of the apparent decrease of Ca2+ current.