Intrinsic Cardiac Neurons Research Articles

Measure of synchrony in the activity of intrinsic cardiac neurons

Recent multielectrode array recordings in ganglionated plexi of canine atria have opened the way to the study of population dynamics of intrinsic cardiac neurons. These data provide critical insights into the role of local processing that these ganglia play in the regulation of cardiac function. Low firing rates, marked non-stationarity, interplay with the cardiovascular and pulmonary systems and artifacts generated by myocardial activity create new constraints not present in brain recordings for which almost all neuronal analysis techniques have been developed. We adapted and extended the jitter-based synchrony index (SI) to (1) provide a robust and computationally efficient tool for assessing the level and statistical significance of SI between cardiac neurons, (2) estimate the bias on SI resulting from neuronal activity possibly hidden in myocardial artifacts, (3) quantify the synchrony or anti-synchrony between neuronal activity and the phase in the cardiac and respiratory cycles. The method was validated on firing time series from a total of 98 individual neurons identified in 8 dog experiments. SI ranged from −0.14 to 0.66, with 23 pairs of neurons with SI > 0.1. The estimated bias due to artifacts was typically <1%. Strongly cardiovascular- and pulmonary-related neurons (SI > 0.5) were found. Results support the use of jitter-based SI in the context of intrinsic cardiac neurons.

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.

Dynamic remodeling of the guinea pig intrinsic cardiac plexus induced by chronic myocardial infarction

Myocardial infarction (MI) is associated with remodeling of the heart and neurohumoral control systems. The objective of this study was to define time-dependent changes in intrinsic cardiac (IC) neuronal excitability, synaptic efficacy, and neurochemical modulation following MI. MI was produced in guinea pigs by ligation of the coronary artery and associated vein on the dorsal surface of the heart. Animals were recovered for 4, 7, 14, or 50 days. Intracellular voltage recordings were obtained in whole mounts of the cardiac neuronal plexus to determine passive and active neuronal properties of IC neurons. Immunohistochemical analysis demonstrated an immediate and persistent increase in the percentage of IC neurons immunoreactive for neuronal nitric oxide synthase. Examination of individual neuronal properties demonstrated that after hyperpolarizing potentials were significantly decreased in both amplitude and time course of recovery at 7 days post-MI. These parameters returned to control values by 50 days post-MI. Synaptic efficacy, as determined by the stimulation of axonal inputs, was enhanced at 7 days post-MI only. Neuronal excitability in absence of agonist challenge was unchanged following MI. Norepinephrine increased IC excitability to intracellular current injections, a response that was augmented post-MI. Angiotensin II potentiation of norepinephrine and bethanechol-induced excitability, evident in controls, was abolished post-MI. This study demonstrates that MI induces both persistent and transient changes in IC neuronal functions immediately following injury. Alterations in the IC neuronal network, which persist for weeks after the initial insult, may lead to alterations in autonomic signaling and cardiac control.

Network interactions within the canine intrinsic cardiac nervous system: implications for reflex control of regional cardiac function

The aims of the study were to determine how aggregates of intrinsic cardiac (IC) neurons transduce the cardiovascular milieu versus responding to changes in central neuronal drive and to determine IC network interactions subsequent to induced neural imbalances in the genesis of atrial fibrillation (AF). Activity from multiple IC neurons in the right atrial ganglionated plexus was recorded in eight anaesthetized canines using a 16-channel linear microelectrode array. Induced changes in IC neuronal activity were evaluated in response to: (1) focal cardiac mechanical distortion; (2) electrical activation of cervical vagi or stellate ganglia; (3) occlusion of the inferior vena cava or thoracic aorta; (4) transient ventricular ischaemia, and (5) neurally induced AF. Low level activity (ranging from 0 to 2.7 Hz) generated by 92 neurons was identified in basal states, activities that displayed functional interconnectivity. The majority (56%) of IC neurons so identified received indirect central inputs (vagus alone: 25%; stellate ganglion alone: 27%; both: 48%). Fifty per cent transduced the cardiac milieu responding to multimodal stressors applied to the great vessels or heart. Fifty per cent of IC neurons exhibited cardiac cycle periodicity, with activity occurring primarily in late diastole into isovolumetric contraction. Cardiac-related activity in IC neurons was primarily related to direct cardiac mechano-sensory inputs and indirect autonomic efferent inputs. In response to mediastinal nerve stimulation, most IC neurons became excessively activated; such network behaviour preceded and persisted throughout AF. It was concluded that stochastic interactions occur among IC local circuit neuronal populations in the control of regional cardiac function. Modulation of IC local circuit neuronal recruitment may represent a novel approach for the treatment of cardiac disease, including atrial arrhythmias.

Network interactions within the intrinsic cardiac nervous system: Implications for reflex control of regional cardiac function

ObjectiveTo determine how aggregates of intrinsic cardiac (IC) neurons transduce the cardiovascular milieu vs respond to changes in central drive. Also to determine how IC network interactions, subsequent to induced neural imbalances, subserve the genesis of atrial fibrillation (AF).MethodsActivity from multiple IC neurons within the right atrial ganglionated plexus was recorded from anesthetized canines. Induced changes in IC neuronal activity were evaluated in response to: (1) cardiac touch; (2) electrical activation of the cervical vagus or stellate ganglia; (3) occlusion of the inferior vena cava or thoracic aorta; (4) focal left ventricular ischemia and (5) neurally induced atrial arrhythmias (AF).ResultsThe majority of IC neurons were local circuit in nature and in basal states displayed low level functional interconnectivity. The majority of IC neurons received indirect central inputs (vagus and stellate) and a lesser proportion transduced the cardiac milieu including responding to multimodal stressors applied to the great vessels and heart. In response to mediastinal nerve stimulation most IC neurons became excessive excited and their functional interconnectivity enhanced; such network behavior preceding and continuing throughout AF.ConclusionStochastic interactions among IC neuronal populations underlie control of regional cardiac function. (Supported by HL71830)

A non-cardiomyocyte autonomous mechanism of cardioprotection involving the SLO1 BK channel

Opening of BK-type Ca2+ activated K+ channels protects the heart against ischemia-reperfusion (IR) injury. However, the location of BK channels responsible for cardioprotection is debated. Herein we confirmed that openers of the SLO1 BK channel, NS1619 and NS11021, were protective in a mouse perfused heart model of IR injury. As anticipated, deletion of the Slo1 gene blocked this protection. However, in an isolated cardiomyocyte model of IR injury, protection by NS1619 and NS11021 was insensitive to Slo1 deletion. These data suggest that protection in intact hearts occurs by a non-cardiomyocyte autonomous, SLO1-dependent, mechanism. In this regard, an in-situ assay of intrinsic cardiac neuronal function (tachycardic response to nicotine) revealed that NS1619 preserved cardiac neurons following IR injury. Furthermore, blockade of synaptic transmission by hexamethonium suppressed cardioprotection by NS1619 in intact hearts. These results suggest that opening SLO1 protects the heart during IR injury, via a mechanism that involves intrinsic cardiac neurons. Cardiac neuronal ion channels may be useful therapeutic targets for eliciting cardioprotection.

Endogenous VIP May Contribute to Vagally Induced Electrophysiological Changes in Canine Atria

Background: There has been increasing evidence that complex interactions among the various components of intracardiac neural network play an important role in atrial fibrillation (AF). Perfusion of vasoactive intestine polypeptide (VIP), a neural polypeptide, co-released with acetylcholine from intrinsic cardiac neurons during vagal stimulation, was shown to shorten the action potential duration (APD), decrease the intraatrial conduction velocity (CV), and promote induction of AF. However, the effect of endogenous VIP remains unclear. Methods: In 6 isolated arterially perfused canine left atria, high-resolution optical mapping techniques with di-4-ANEPPS and blebbistatin were used to measure APD and CV during fat-pad ganglion plexus stimulation (GPS, 30Hz, 10.2±2.3Volt validated with blockage of atrioventricular conduction), at during H9935, a VIP antagonist (1 μM) perfusion. The atria were paced at 200beats per minute. Metoprolol (1.8μM) was used to block the sympathetic effects. Results: Average APD was shorter (21%) during GPS compared to the baseline (100±8ms vs. 126±7ms, p<0.05), and average CV was slower than baseline (87±10cm/sec vs. 103±13cm/sec, p<0.05), which recovered within 2 min (APD: 128±8ms, p<0.05; CV: 105±13cm/sec, p<0.05). With H9335, the APD shortening effect (17%) of GPS persisted (GPS, 105±14ms, vs. 127±8ms at baseline and 125±10ms after recovery, p<0.05) with a trend towards being less pronounced as compared to GPS effect without H9335 (−18±6ms vs. −29±3ms, p=0.07). However, the GPS-induced CV slowing was abolished by H9335 (97±14cm/sec, vs. 103±15cm/sec at baseline and 101±16cm/sec after recovery, p=0.23). The effects of endogenous VIP were eliminated by atropine during GPS, indicating VIP release from intrinsic cardiac neurons may be mediated by muscarinic recpetors. Conclusion: Neuronally released VIP could contribute vagal effects and is primarily responsible for CV slowing during GPS. Neuronal release of VIP is likely mediated by muscarinic receptors in the intrinsic cardiac neurons.

VIP Enhances Acetylcholine-Activated Potassium Current via a cAMP/PKA Transduction Pathway

Background: Recent studies have indicated that various components of cardiac nerve network may play important role in atrial fibrillation (AF). Vagal stimulation shortens atrial effective refractory period, predominantly via the activation of the acetylcholine-activated potassium channel (IKACh) that shortens action potential duration and increases the vulnerability to AF. Vasoactive intestine polypeptide (VIP), a neural polypeptide co-released with acetylcholine (ACh) largely from intrinsic cardiac neurons during vagal stimulation, was shown to contribute to the vulnerability to AF. However, the underlying mechanism remains unclear. The aim of this study is to investigate the effect of VIP on IKACh in the presence of ACh in rat atrial myocytes.Methods: The effects of VIP on IKACh in isolated adult rat atrial myocytes were investigated with voltage patch-clamp recording techniques. IKACh were recorded as an inwardly rectifying current activated by muscarinic agonist carbachol (CCH, 1μM). Furthermore, the post-synaptic signal pathway was investigated with Rp-cAMP (10 μM), a competitive antagonist of cyclic AMP (cAMP), H-89 (0.1μM), PKA inhibitor, and forskolin (0.1μM), an activator of adenylyl cyclase.Results: VIP (1μM) potentiates IKACh at a holding potential of −115 mV (VIP: −19.34 ± 5.22 pA/pF, n=12, vs. baseline: −15.85 ± 2.66 pA/pF, n=14, p<0.05). The enhancement of IKACh by VIP could be mitigated by Rp-cAMP (VIP plus Rp-cAMP: −8.24 ± 1.44 pA/pF n= 6; p<0.05 vs. baseline), and could be inhibited by H-89 (VIP plus H-89: −13.41 ± 2.95 pA/pF n=6; p=0.59 vs. baseline). Furthermore, these enhancements could be mimicked by an activator of adenylyl cyclase, forskolin (−27.35 ± 4.82pA/pF n= 5; p<0.01 vs. baseline).Conclusion: VIP enhanced IKACh in atrial myocytes which might contribute to vagal-induced APD shortening and is likely mediated by a cAMP-PKA dependent pathway.

Remodeling of intrinsic cardiac neurons: effects of β-adrenergic receptor blockade in guinea pig models of chronic heart disease

Chronic heart disease induces remodeling of cardiac tissue and associated neuronal components. Treatment of chronic heart disease often involves pharmacological blockade of adrenergic receptors. This study examined the specific changes in neuronal sensitivity of guinea pig intrinsic cardiac neurons to autonomic modulators in animals with chronic cardiac disease, in the presence or absence of adrenergic blockage. Myocardial infarction (MI) was produced by ligature of the coronary artery and associated vein on the dorsal surface of the heart. Pressure overload (PO) was induced by a banding of the descending dorsal aorta (∼20% constriction). Animals were allowed to recover for 2 wk and then implanted with an osmotic pump (Alzet) containing either timolol (2 mg·kg(-1)·day(-1)) or vehicle, for a total of 6-7 wk of drug treatment. At termination, intracellular recordings from individual neurons in whole mounts of the cardiac plexus were used to assess changes in physiological responses. Timolol treatment did not inhibit the increased sensitivity to norepinephrine seen in both MI and PO animals, but it did inhibit the stimulatory effects of angiotensin II on the norepinephrine-induced increases in neuronal excitability. Timolol treatment also inhibited the increase in synaptically evoked action potentials observed in PO animals with stimulation of fiber tract bundles. These results demonstrate that β-adrenergic blockade can inhibit specific aspects of remodeling within the intrinsic cardiac plexus. In addition, this effect was preferentially observed with active cardiac disease states, indicating that the β-receptors were more influential on remodeling during dynamic disease progression.

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.

Angiotensin II potentiates adrenergic and muscarinic modulation of guinea pig intracardiac neurons

The intrinsic cardiac plexus represents a major peripheral integration site for neuronal, hormonal, and locally produced neuromodulators controlling efferent neuronal output to the heart. This study examined the interdependence of norepinephrine, muscarinic agonists, and ANG II, to modulate intrinsic cardiac neuronal activity. Intracellular voltage recordings from whole-mount preparations of the guinea pig cardiac plexus were used to determine changes in active and passive electrical properties of individual intrinsic cardiac neurons. Application of either adrenergic or muscarinic agonists induced changes in neuronal resting membrane potentials, decreased afterhyperpolarization duration of single action potentials, and increased neuronal excitability. Adrenergic responses were inhibited by removal of extracellular calcium ions, while muscarinic responses were inhibited by application of TEA. The adrenergic responses were heterogeneous, responding to a variety of receptor-specific agonists (phenylephrine, clonidine, dobutamine, and terbutaline), although α-receptor agonists produced the most frequent responses. Application of ANG II alone produced a significant increase in excitability, while application of ANG II in combination with either adrenergic or muscarinic agonists produced a much larger potentiation of excitability. The ANG II-induced modulation of firing was blocked by the angiotensin type 2 (AT(2)) receptor inhibitor PD 123319 and was mimicked by the AT(2) receptor agonist CGP-42112A. AT(1) receptor blockade with telmasartin did not alter neuronal responses to ANG II. These data demonstrate that ANG II potentiates both muscarinically and adrenergically mediated activation of intrinsic cardiac neurons, doing so primarily via AT(2) receptor-dependent mechanisms. These neurohumoral interactions may be fundamental to regulation of neuronal excitability within the intrinsic cardiac nervous system.

Neuromodulation of the intrinsic cardiac nervous system attenuates the formation of neurally‐induced atrial arrhythmias

The objective was to determine whether neurally‐induced atrial fibrillation (AF) differentially activates intrinsic cardiac neurons (ICN) and whether electrical or pharmacological modulation of ICN activity can reduce the arrhythmogenic potential. Trains of 5 electrical stimuli (1ms) were delivered during the atrial refractory period to mediastinal nerves (MSNS) on the right atria at the pericardial reflection to evoked AF. Recordings of neuronal activity were obtained from right atrial ganglionated plexus (RAGP) in response to MSNS prior to and following neuromodulation involving either pre‐emptive spinal cord stimulation (SCS) or ganglionic blockade (hexamethonium). MSNS evoked a ~4‐fold increase in RAGP neuronal activity, which SCS reduced by 43%. Hexamethonium blocked the MSNS evoked increase in RAGP neuronal activation. MSNS evoked atrial fibrillation/flutter in 78% of right‐sided nerve sites stimulated, which SCS reduced to 33% and hexamethonium reduced to 7%. MSNS induced bradycardia was maintained with SCS, but mitigated by hexamethonium. We conclude that MSNS activates subpopulations of intrinsic cardiac neurons resulting in the formation of atrial arrhythmias leading to atrial fibrillation. Stabilization of ICN local circuit neurons with SCS or local circuit and autonomic efferent projections with hexamethonium reduces the arrhythmogenic potential. (HL71830)

The cardiac neuronal hierarchy and susceptibility to arrhythmias

The cardiac neuronal hierarchy and susceptibility to arrhythmias

Localization of multiple neurotransmitters in surgically derived specimens of human atrial ganglia

Dysfunction of the intrinsic cardiac nervous system is implicated in the genesis of atrial and ventricular arrhythmias. While this system has been studied extensively in animal models, far less is known about the intrinsic cardiac nervous system of humans. This study was initiated to anatomically identify neurotransmitters associated with the right atrial ganglionated plexus (RAGP) of the human heart. Biopsies of epicardial fat containing a portion of the RAGP were collected from eight patients during cardiothoracic surgery and processed for immunofluorescent detection of specific neuronal markers. Colocalization of markers was evaluated by confocal microscopy. Most intrinsic cardiac neuronal somata displayed immunoreactivity for the cholinergic marker choline acetyltransferase and the nitrergic marker neuronal nitric oxide synthase. A subpopulation of intrinsic cardiac neurons also stained for noradrenergic markers. While most intrinsic cardiac neurons received cholinergic innervation evident as punctate immunostaining for the high affinity choline transporter, some lacked cholinergic inputs. Moreover, peptidergic, nitrergic, and noradrenergic nerves provided substantial innervation of intrinsic cardiac ganglia. These findings demonstrate that the human RAGP has a complex neurochemical anatomy, which includes the presence of a dual cholinergic/nitrergic phenotype for most of its neurons, the presence of noradrenergic markers in a subpopulation of neurons, and innervation by a host of neurochemically distinct nerves. The putative role of multiple neurotransmitters in controlling intrinsic cardiac neurons and mediating efferent signaling to the heart indicates the possibility of novel therapeutic targets for arrhythmia prevention.

Remodeling of the guinea pig intrinsic cardiac plexus with chronic pressure overload

Chronic pressure overload (PO) is associated with cardiac hypertrophy and altered autonomic control of cardiac function, in which the latter may involve adaptations in central and/or peripheral cardiac neural control mechanisms. To evaluate the specific remodeling of the intrinsic cardiac nervous system following pressure overload, the descending thoracic aorta artery of the guinea pig was constricted approximately 20%, and the animals recovered for 9 wk. Thereafter, atrial neurons of the intrinsic cardiac plexus were isolated for electrophysiological and immunohistochemical analyses. Intracellular voltage recordings from intrinsic cardiac neurons demonstrated no significant changes in passive membrane properties or action potential depolarization compared with age-matched controls and sham-operated animals, but afterhyperpolarization duration was increased in PO animals. Neuronal excitability, as determined by the number of action potentials produced with depolarizing stimuli, was differentially increased in phasic neurons derived from PO animals in response to exogenously applied histamine compared with sham and age-matched controls. Conversely, pituitary adenylate cyclase-activating polypeptide-induced increases in intrinsic cardiac neuron evoked AP frequency were similar between control and PO animals. Immunohistochemical analysis demonstrated a twofold increase in the percentage of neurons immunoreactive for neuronal nitric oxide synthase in PO animals compared with control. The density of mast cells within the intrinsic cardiac plexus from PO animals was also increased twofold compared with preparations from control animals. These results indicate that congestive heart failure associated with chronic pressure overload induces a differential remodeling of intrinsic cardiac neurons and upregulation of neuronal responsiveness to specific neuromodulators.

Differential Activation of Guinea Pig Intrinsic Cardiac Neurons by the PAC1 Agonists Maxadilan and Pituitary Adenylate Cyclase-Activating Polypeptide 27 (PACAP27)

Pituitary adenylate cyclase-activating polypeptide (PACAP) evokes tachycardia followed by a larger cholinergic bradycardia in isolated guinea pig hearts. We used the selective PAC1 receptor agonist maxadilan and vasoactive intestinal polypeptide (VIP) to test the hypothesis that PACAP27-evoked tachycardia and bradycardia are mediated by VPAC and PAC1 receptors, respectively. Chronotropic actions of these peptides were evaluated in isolated perfused hearts. Direct neuronal actions were determined by intracellular voltage recordings from cholinergic neurons in atrial ganglion whole mounts. Administration of 1 nmol of PACAP27 to isolated hearts evoked typical biphasic rate responses, whereas 1 nmol of maxadilan caused only a minor rate decrease. Desensitization with VIP eliminated the positive chronotropic effect of PACAP27 selectively. Local application of PACAP27 to cardiac neurons frequently evoked slow depolarization and caused prolonged increase of neuronal excitability. Maxadilan rarely affected membrane potential but consistently increased excitability. VIP had no effect on excitability and evoked depolarization in only a few neurons. Because maxadilan increased neuronal excitability but did not trigger action potentials as PACAP often does, we evaluated the interaction of maxadilan with substance P (SP) in isolated hearts. SP depolarizes cardiac neurons more consistently than PACAP, often triggers neuronal action potentials, and causes bradycardia but does not increase neuronal excitability. Maxadilan had a persistent effect to augment negative chronotropic responses to SP. These findings support our hypothesis that PACAP evokes tachycardia and bradycardia through VPAC and PAC1 receptors, respectively. They also suggest that maxadilan and PACAP27 differ in activating PAC1 receptors on cardiac neurons and/or stimulating downstream signaling mechanisms.

P3.6 Metabolic inhibition increases intracellular calcium, alters the electrophysiological properties and ganglionic transmission in rat intrinsic cardiac ganglion neurones in vitro

P3.6 Metabolic inhibition increases intracellular calcium, alters the electrophysiological properties and ganglionic transmission in rat intrinsic cardiac ganglion neurones in vitro

Enhancement of Ih, but not inhibition of IM, is a key mechanism underlying the PACAP-induced increase in excitability of guinea pig intrinsic cardiac neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) increases excitability of guinea pig cardiac neurons, an effect mediated by PACAP-selective PAC(1) receptors. In dissociated guinea pig cardiac neurons, PACAP causes a positive shift of the voltage dependence of activation of the hyperpolarization-activated nonselective cation current (I(h)). This observation suggested that an enhancement of I(h) contributed to the increase in excitability in neurons within whole-mount cardiac ganglia preparations. To evaluate the role of I(h) in the PACAP-induced increase in excitability, we compared the increase in action potentials generated by 10 nM PACAP in control neurons and in neurons treated with ZD7288 (10 or 100 muM) or CsCl (2 or 2.5 mM), drugs known to inhibit I(h). In control cells exposed to PACAP, 1-s depolarizing current pulses elicited multiple action potential firing in 79% of the neurons. In ZD7288- or CsCl-containing solutions, the 10 nM PACAP-induced increase in excitability was markedly suppressed, with 7% and 21% of the neurons generating multiple action potentials, respectively. Prior results indicated that PACAP initiates depolarization by activating an inward current, which is separate from its enhancement of I(h). Here, we show that a PACAP-induced depolarization was comparable in control neurons and neurons bathed in a CsCl-containing solution, an observation indicating that CsCl did not interfere with activation of the PAC(1) receptor by PACAP. Additional experiments indicated that pretreatment with the putative M current (I(M)) inhibitor 1 mM BaCl(2), but not 10 microM XE991, initiated multiple firing in a majority of neurons, with resting potentials maintained at approximately -60 mV. Furthermore, in Ba(2+)-treated cells, 10 nM PACAP increased the number of action potentials generated. Our results indicate that PACAP enhancement of I(h), rather than inhibition of I(M) and other 1 mM Ba(2+)-sensitive K(+) currents, is a key ionic mechanism contributing to the peptide-induced increase in excitability for neurons within whole-mount cardiac ganglia preparations.

Spinal cord stimulation (SCS) stabilizes intrinsic cardiac neurons to reduce neurally induced atrial fibrillation

ObjectiveTo determine the contribution of discrete neuronal inputs to the intrinsic cardiac nervous system for targeted reduction of the atrial arrhythmogenic substrate.MethodsIn 15 anesthetized dogs, trains of electrical stimuli (1 mA, 1ms) were delivered during the atrial refractory period to mediastinal nerves associated with the superior vena cava to induce atrial tachyarrhythmias. Neuronal activity was recorded in the right atrial ganglionated plexus (RAGP). For SCS, the dorsal aspect of the T1‐T3 spinal cord was stimulated electrically (50 Hz; 0.2 ms; 90% of motor threshold).ResultsMediastinal nerve stimulation caused an abrupt increase in RAGP neuronal activity (~5 fold) that was maintained for the duration of transient neurally‐induced atrial fibrillation. SCS reduced basal neuronal activity and mitigated any mediastinal nerve‐induced increases in RAGP neuronal activity. Prior to SCS, mediastinal nerve stimulation evoked atrial fibrillation/atrial flutter (AF/Afl) from 95% of sites. After SCS, atrial rhythms induced by mediastinal nerve stimulation were 38% AF/Afl, 56% bradycardia/sinus rhythm, and 6% sinus tachycardia. SCS likewise reduced dispersion in early pacemaker breakthrough loci secondary to mediastinal nerve stimulation.ConclusionsNeuromodulation with dorsal cranial thoracic SCS stabilizes elements within the intrinsic cardiac nervous system to reduce the atrial arrhythmogenic substrate to its excessive neural inputs. (Supported by HL71830)

Chronic myocardial infarction induces phenotypic and functional remodeling in the guinea pig cardiac plexus

Chronic myocardial infarction (CMI) is associated with remodeling of the ventricle and evokes adaption in the cardiac neurohumoral control systems. To evaluate the remodeling of the intrinsic cardiac nervous system following myocardial infarction, the dorsal descending coronary artery was ligated in the guinea pig heart and the animals were allowed to recover for 7-9 wk. Thereafter, atrial neurons of the intrinsic cardiac plexus were isolated for electrophysiological and immunohistochemical analyses. Intracellular voltage recordings from intrinsic cardiac neurons demonstrated no significant changes in passive membrane properties or action potential configuration compared with age-matched controls and sham-operated animals. The intrinsic cardiac neurons from chronic infarcted hearts did demonstrate an increase in evoked action potential (AP) frequency (as determined by the number of APs produced with depolarizing stimuli) and an increase in responses to exogenously applied histamine compared with sham and age-matched controls. Conversely, pituitary adenylate cyclase-activating polypeptide (PACAP)-induced increases in intrinsic cardiac neuron-evoked AP frequency were similar between control and CMI animals. Immunohistochemical analysis demonstrated a threefold increase in percentage of neurons immunoreactive for neuronal nitric oxide synthase (NOS) in CMI animals compared with control and the additional expression of inducible NOS by some neurons, which was not evident in control animals. Finally, the density of mast cells within the intrinsic cardiac plexus was increased threefold in preparations from CMI animals. These results indicate that CMI induces a differential remodeling of intrinsic cardiac neurons and functional upregulation of neuronal responsiveness to specific neuromodulators.