c-Fos expression in rat brain stem and spinal cord in response to activation of cardiac ischemia-sensitive afferent neurons and electrostimulatory modulation

Spinal Cord Stimulation Reduces Ventricular Arrhythmias by Attenuating Reactive Gliosis and Activation of Spinal Interneurons

OBJECTIVES

This study investigated spinal cord neuronal and glial cell activation during cardiac ischemia-reperfusion (IR)-triggered ventricular arrhythmias and neuromodulation therapy by spinal cord stimulation (SCS).

BACKGROUND

Myocardial ischemia induces changes in cardiospinal neural networks leading to sudden cardiac death. Neuromodulation with SCS decreases cardiac sympathoexcitation; however, the molecular mechanisms remain unknown.

METHODS

Yorkshire pigs (n = 16) were randomized to Control, IR, or IR+SCS groups. A 4-pole SCS lead was placed in the T1-T4 epidural space with stimulation for 30 minutes before IR (50 Hz, 0.4-ms duration, 90% motor threshold). Cardiac electrophysiological mapping and Ventricular Arrhythmia Score (VAS) were recorded. Immunohistochemistry of thoracic spinal sections was used to map and identify Fos-positive neuronal and glial cell types during IR with and without SCS.

RESULTS

IR increased cardiac sympathoexcitation and arrhythmias (VAS = 6.2 ± 0.9) that were attenuated in IR + SCS (VAS = 2.8 ± 0.5; P = 0.017). IR increased spinal cellular Fos expression (#Fos+ cells Control = 23 ± 2 vs IR = 88 ± 5; P < 0.0001) in T1-T4, with the greatest increase localized to T3, and the greatest %Fos+ cells being microglia and astrocytes. Fos expression was attenuated by IR + SCS (62 ± 4; P < 0.01), primarily though a reduction in Fos+ microglia and astrocytes, as SCS also led to increase in Fos+ neurons in deep dorsal laminae.

CONCLUSIONS

In a porcine model, cardiac IR was associated with astrocyte and microglial cell activation. Our results suggest that preemptive thoracic SCS decreased IR-induced cardiac sympathoexcitation and ventricular arrhythmias through attenuation of reactive gliosis and activation of inhibitory interneurons in the dorsal horn of spinal cord.

Pleiotropic activity of nerve growth factor in regulating cardiac functions and counteracting pathogenesis

Cardiac innervation density generally reflects the levels of nerve growth factor (NGF) produced by the heart—changes in NGF expression within the heart and vasculature contribute to neuronal remodelling (e.g. sympathetic hyperinnervation or denervation). Its synthesis and release are altered under different pathological conditions. Although NGF is well known for its survival effects on neurons, it is clear that these effects are more wide ranging. Recent studies reported both in vitro and in vivo evidence for beneficial actions of NGF on cardiomyocytes in normal and pathological hearts, including prosurvival and antiapoptotic effects. NGF also plays an important role in the crosstalk between the nervous and cardiovascular systems. It was the first neurotrophin to be implicated in postnatal angiogenesis and vasculogenesis by autocrine and paracrine mechanisms. In connection with these unique cardiovascular properties of NGF, we have provided comprehensive insight into its function and potential effect of NGF underlying heart sustainable/failure conditions. This review aims to summarize the recent data on the effects of NGF on various cardiovascular neuronal and non‐neuronal functions. Understanding these mechanisms with respect to the diversity of NGF functions may be crucial for developing novel therapeutic strategies, including NGF action mechanism‐guided therapies.

Effect of myocardial ischemic preconditioning on ischemia-reperfusion stimulation-induced activation in rat thoracic spinal cord with functional MRI

BACKGROUND

Myocardial ischemia and reperfusion-evoked spinal reflexes involve nociceptive signals that trigger neuronal excitation through cardiac afferents, projecting into the thoracic spinal cord. Ischemic preconditioning (IPC) involves brief episodes of sublethal ischemia and reperfusion enhances the resistance of the myocardium to subsequent ischemic insults. This study investigated the effects of IPC on ischemia-reperfusion (I/R) stimulation-induced activation in the thoracic spinal cord of rats.

METHODS

A new remotely controlled I/R model was established. The infarct size was determined as a percentage of area at risk (IS/AAR) and arrhythmia scores were evaluated. Non-invasive in vivo fMRI was used for signal quantitative analysis of thoracic spinal spatiotemporal. The role of IPC on the excitability of substantia gelatinosa (SG) neurons was assessed by spinal patch clamp recording technique. The altered expressions of c-Fos, SP, and CGRP in T₄ segment were detected by immunohistochemical staining.

RESULTS

The novel I/R model was induced successfully and reliably utilized, and IPC treatment markedly reduced the myocardial infarct size. fMRI analysis revealed that IPC reduced the increased BOLD signals induced by prolonged ischemia-reperfusion. Patch clamp recording showed that IPC treatment reversed the enhanced excitability of SG neurons during I/R treatment. The results of immunofluorescent staining indicated that IPC mitigated the I/R-induced dramatic increase of c-Fos, and reduced the release of SP and CGRP in dorsal horns of spinal cord.

CONCLUSIONS

These results suggested that IPC suppressed neuronal activation induced by I/R stimuli in rat thoracic spinal cord using spinal cord fMRI and patch clamp recording techniques.

Progression of myocardial ischemia leads to unique changes in immediate-early gene expression in the spinal cord dorsal horn

The pathological consequences of ischemic heart disease involve signaling through the autonomic nervous system. Although early activation may serve to maintain hemodynamic stability, persistent aberrant sympathoexcitation contributes to the development of lethal arrhythmias and heart failure. We hypothesized that as the myocardium reacts and remodels to ischemic injury over time, there is an analogous sequence of gene expression changes in the thoracic spinal cord dorsal horn, the processing center for incoming afferent fibers from the heart to the central nervous system. Acute and chronic myocardial ischemia (MI) was induced in a large animal model of Yorkshire pigs, and the thoracic dorsal horn of treated pigs, along with control nonischemic pigs, was harvested for transcriptome analysis. We identified 32 differentially expressed genes between healthy and acute ischemia cohorts and 46 differentially expressed genes between healthy and chronic ischemia cohorts. The canonical immediate-early gene c-fos was upregulated after acute MI, along with fosB, dual specificity phosphatase 1 and 2 ( dusp1 and dusp2), and early growth response 2 (egr2). After chronic MI, there was a persistent yet unique activation of immediate-early genes, including fosB, nuclear receptor subfamily 4 group A members 1-3 ( nr4a1, nr4a2, and nr4a3), egr3, and TNF-α-induced protein 3 ( tnfaip3). In addition, differentially expressed genes from the chronic MI signature were enriched in pathways linked to apoptosis, immune regulation, and the stress response. These findings support a dynamic progression of gene expression changes in the dorsal horn with maturation of myocardial injury, and they may explain how early adaptive autonomic nervous system responses can maintain hemodynamic stability, whereas prolonged maladaptive signals can predispose patients to arrhythmias and heart failure. NEW & NOTEWORTHY Activation of the autonomic nervous system after myocardial injury can provide early cardiovascular support or prolonged aberrant sympathoexcitation. The later response can lead to lethal arrhythmias and heart failure. This study provides evidence of ongoing changes in the gene expression signature of the spinal cord dorsal horn as myocardial injury progresses over time. These changes could help explain how an adaptive nervous system response can become maladaptive over time.

BDNF - A key player in cardiovascular system

Neurotrophins (NTs) were first identified as target-derived survival factors for neurons of the central and peripheral nervous system (PNS). They are known to control neural cell fate, development and function. Independently of their neuronal properties, NTs exert unique cardiovascular activity. The heart is innervated by sensory, sympathetic and parasympathetic neurons, which require NTs during early development and in the establishment of mature properties, contributing to the maintenance of cardiovascular homeostasis. The identification of molecular mechanisms regulated by NTs and involved in the crosstalk between cardiac sympathetic nerves, cardiomyocytes, cardiac fibroblasts, and vascular cells, has a fundamental importance in both normal heart function and disease. The article aims to review the recent data on the effects of Brain-Derived Neurotrophic Factor (BDNF) on various cardiovascular neuronal and non-neuronal functions such as the modulation of synaptic properties of autonomic neurons, axonal outgrowth and sprouting, formation of the vascular and neural networks, smooth muscle migration, and control of endothelial cell survival and cardiomyocytes. Understanding these mechanisms may be crucial for developing novel therapeutic strategies, including stem cell-based therapies.

Cardiac sympatho-vagal balance and ventricular arrhythmia

A hallmark of cardiovascular disease is cardiac autonomic dysregulation. The phenotype of impaired parasympathetic responsiveness and sympathetic hyperactivity in experimental animal models is also well documented in large scale human studies in the setting of heart failure and myocardial infarction, and is predictive of morbidity and mortality. Despite advances in emergency revascularisation strategies for myocardial infarction, device therapy for heart failure and secondary prevention pharmacotherapies, mortality from malignant ventricular arrhythmia remains high. Patients at highest risk or those with haemodynamically significant ventricular arrhythmia can be treated with catheter ablation and implantable cardioverter defibrillators, but the morbidity and reduction in quality of life due to the burden of ventricular arrhythmia and shock therapy persists. Therefore, future therapies must aim to target the underlying pathophysiology that contributes to the generation of ventricular arrhythmia. This review explores recent advances in mechanistic research in both limbs of the autonomic nervous system and potential avenues for translation into clinical therapy. In addition, we also discuss the relationship of these findings in the context of the reported efficacy of current neuromodulatory strategies in the management of ventricular arrhythmia.

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We review advances in mechanistic research in the cardiac autonomic nervous system.

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This is discussed in relation to neuromodulatory therapy for ventricular arrhythmia.

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Neuromodulation therapies can influence both neurotransmitters and co-transmitters.

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This may therefore improve on conventional medical treatment.

Cardiac innervation and sudden cardiac death

Afferent and efferent cardiac neurotransmission via the cardiac nerves intricately modulates nearly all physiological functions of the heart (chronotropy, dromotropy, lusitropy, and inotropy). Afferent information from the heart is transmitted to higher levels of the nervous system for processing (intrinsic cardiac nervous system, extracardiac-intrathoracic ganglia, spinal cord, brain stem, and higher centers), which ultimately results in efferent cardiomotor neural impulses (via the sympathetic and parasympathetic nerves). This system forms interacting feedback loops that provide physiological stability for maintaining normal rhythm and life-sustaining circulation. This system also ensures that there is fine-tuned regulation of sympathetic-parasympathetic balance in the heart under normal and stressed states in the short (beat to beat), intermediate (minutes to hours), and long term (days to years). This important neurovisceral/autonomic nervous system also plays a major role in the pathophysiology and progression of heart disease, including heart failure and arrhythmias leading to sudden cardiac death. Transdifferentiation of neurons in heart failure, functional denervation, cardiac and extracardiac neural remodeling has also been identified and characterized during the progression of disease. Recent advances in understanding the cellular and molecular processes governing innervation and the functional control of the myocardium in health and disease provide a rational mechanistic basis for the development of neuraxial therapies for preventing sudden cardiac death and other arrhythmias. Advances in cellular, molecular, and bioengineering realms have underscored the emergence of this area as an important avenue of scientific inquiry and therapeutic intervention.

The excitatory synaptic transmission of the nucleus of solitary tract was potentiated by chronic myocardial infarction in rats

Angina pectoris is a common clinical symptom that often results from myocardial infarction. One typical characteristic of angina pectoris is that the pain does not match the severity of the myocardial ischemia. One possible explanation is that the intensity of cardiac nociceptive information could be dynamically regulated by certain brain areas. As an important nucleus for processing cardiac nociception, the nucleus of the solitary tract (NTS) has been studied to some extent. However, until now, the morphological and functional involvement of the NTS in chronic myocardial infarction (CMI) has remained unknown. In the present study, by exploring left anterior descending coronary artery ligation surgery, we found that the number of synaptophysin-immunoreactive puncta and Fos-immunoreactive neurons in the rat NTS two weeks after ligation surgery increased significantly. Excitatory pre- and postsynaptic transmission was potentiated. A bath application of a Ca²⁺ channel inhibitor GABApentin and Ca²⁺ permeable AMPA receptor antagonist NASPM could reverse the potentiated pre- and postsynaptic transmission, respectively. Meanwhile, rats with CMI showed significantly increased visceral pain behaviors. Microinjection of GABApentin or NASPM into the NTS decreased the CMI-induced visceral pain behaviors. In sum, our results suggest that the NTS is an important area for the process of cardiac afference in chronic myocardial infarction condition.

Neural Mechanisms That Underlie Angina-Induced Referred Pain in the Trigeminal Nerve Territory: A c-Fos Study in Rats

The present study was designed to determine whether the trigeminal sensory nuclear complex (TSNC) is involved in angina-induced referred pain in the trigeminal nerve territory and to identify the peripheral nerve conducting nociceptive signals that are input into the TSNC. Following application of the pain producing substance (PPS) infusion, the number of Fos-labeled cells increased significantly in the subnucleus caudalis (Sp5C) compared with other nuclei in the TSNC. The Fos-labeled cells in the Sp5C disappeared when the left and right cervical vagus nerves were sectioned. Lesion of the C1-C2 spinal segments did not reduce the number of Fos-labeled cells. These results suggest that the nociceptive signals that conduct vagal afferent fibers from the cardiac region are input into the Sp5C and then projected to the thalamus.

Relative contributions of the thalamus and the paraventricular nucleus of the hypothalamus to the cardiac sympathetic afferent reflex

The cardiac sympathetic afferent reflex (CSAR) is induced by stimulating the cardiac sympathetic afferents, which evokes increases in sympathetic outflow and arterial pressure. In the present study, we attempted to identify the contribution of thalamic and hypothalamic nuclei involved in the CSAR. First, we observed that there was an increase in the number of c-Fos-labeled cells in the paraventricular nucleus (PVN) (190 ± 18 vs. 101 ± 15; P < 0.05), the paraventricular nucleus of the thalamus (PVT) (239 ± 23 vs. 151 ± 15; P < 0.05), and the mediodorsal thalamic nucleus (MD) (92 ± 9 vs. 63 ± 6; P < 0.05) following epicardial application of bradykinin (BK) compared with the control group (P < 0.05). Second, using extracellular single-unit recording, we found 25% of spontaneously active neurons in the thalamus were stimulated by epicardial application of BK or capsaicin in intact rats. However, 24% of spontaneously active neurons in the thalamus were still stimulated by epicardial application of BK or capsaicin despite vagotomy and sinoaortic denervation. None of the neurons in the thalamus responded to baroreflex changes in arterial pressure, induced by intravenous injection of phenylephrine or sodium nitroprusside. The CSAR was inhibited by microinjection of muscimol or lidocaine into the PVN. However, it was not inhibited or blocked by microinjection of muscimol or lidocaine into the thalamus. Taken together, these data suggest that the thalamus, while activated, is not critical for autonomic adjustments in response to activation of the CSAR. On the other hand, the PVN is critically involved in the central pathway of the CSAR.

Substance P release in response to cardiac ischemia from rat thoracic spinal dorsal horn is mediated by TRPV1

Spinal cord stimulation (SCS) inhibits substance P (SP) release and decreases the expression of the transient receptor potential vanilloid 1 (TRPV1) in the spinal cord at thoracic 4 (T4) during cardiac ischemia in rat models (Ding et al., 2007). We hypothesized that activation of TRPV1 in the T4 spinal cord segment by intermittent occlusion of the left anterior descending coronary artery (CoAO) mediates spinal cord SP release. Experiments were conducted in urethane-anesthetized Sprague-Dawley male rats using SP antibody-coated microprobes to measure SP release at the central terminal endings of cardiac ischemic-sensitive afferent neurons (CISAN) in the spinal T4 dorsal horns. Vehicle, capsaicin (CAP; TRPV1 agonist) and capsazepine (CZP; TRPV1 antagonist) were injected into the left T4 prior to stimulation of CISAN by intermittent CoAO (with or without upper cervical SCS). CAP induced endogenous SP release from laminae I and II in the T4 spinal cord above baseline. Conversely, CZP injections significantly inhibited SP release from laminae I-VII in the T4 spinal cord segment below baseline. CZP also attenuated CoAO-induced SP release, while T4 injections of CZP with SCS completely restored SP release to basal levels during CoAO activation. CAP increased the number of c-Fos (a marker for CISAN activation) positive T4 dorsal horn neurons compared to sham-operated animals, while CZP (alone or during CoAO and SCS+CoAO) significantly reduced the number of c-Fos positive neurons. These results suggest that spinal release of the putative nociceptive transmitter SP occurs, at least in part, via a TRPV1 mechanism.

Activated cranial cervical cord neurons affect left ventricular infarct size and the potential for sudden cardiac death

To evaluate whether cervical spinal neurons can influence cardiac indices and myocyte viability in the acutely ischemic heart, the hearts of anesthetized rabbits subjected to 30 min of LAD coronary arterial occlusion (CAO) were studied 3h after reperfusion. Control animals were compared to those exposed to pre-emptive high cervical cord stimulation (SCS; the dorsal aspect of the C1-C2 spinal cord was stimulated electrically at 50 Hz; 0.2 ms; 90% of motor threshold, starting 15 min prior to and continuing throughout CAO). Four groups of animals were so tested: 1) neuroaxis intact; 2) prior cervical vagotomy; 3) prior transection of the dorsal spinal columns at C6; and 4) following pharmacological treatment [muscarinic (atropine) or adrenergic (atenolol, prazosin or yohimbine) receptor blockade]. Infarct size (IS) was measured by tetrazolium, expressed as percentage of risk zone. C1-C2 SCS reduced acute ischemia induced IS by 43%, without changing the incidence of sudden cardiac death (SCD). While SCS-induced reduction in IS was unaffected by vagotomy, it was no longer evident following transection of C6 dorsal columns or atropinization. Beta-adrenoceptor blockade eliminated ischemia induced SCD, while alpha-receptor blockade doubled its incidence. During SCS, myocardial ischemia induced SCD was eliminated following vagotomy while remaining unaffected by atropinization. These data indicate that, in contrast to thoracic spinal neurons, i) cranial cervical spinal neurons affect both adrenergic and cholinergic motor outflows to the heart such that ii) their activation modifies ventricular infarct size and lethal arrhythmogenesis.

Development, maturation, and transdifferentiation of cardiac sympathetic nerves

The heart is electrically and mechanically controlled as a syncytium by the autonomic nervous system. The cardiac nervous system comprises the sympathetic, parasympathetic, and sensory nervous systems that together regulate heart function on demand. Sympathetic electric activation was initially considered the main regulator of cardiac function; however, modern molecular biotechnological approaches have provided a new dimension to our understanding of the mechanisms controlling the cardiac nervous system. The heart is extensively innervated, although the innervation density is not uniform within the heart, being high in the subepicardium and the special conduction system. We and others showed previously that the balance between neural chemoattractants and chemorepellents determine cardiac nervous development, with both factors expressed in heart. Nerve growth factor is a potent chemoattractant synthesized by cardiomyocytes, whereas Sema3a is a neural chemorepellent expressed specifically in the subendocardium. Disruption of this well-organized molecular balance and innervation density can induce sudden cardiac death due to lethal arrhythmias. In diseased hearts, various causes and mechanisms underlie cardiac sympathetic abnormalities, although their detailed pathology and significance remain contentious. We reported that cardiac sympathetic rejuvenation occurs in cardiac hypertrophy and, moreover, interleukin-6 cytokines secreted from the failing myocardium induce cholinergic transdifferentiation of the cardiac sympathetic system via a gp130 signaling pathway, affecting cardiac performance and prognosis. In this review, we summarize the molecular mechanisms involved in sympathetic development, maturation, and transdifferentiation, and propose their investigation as new therapeutic targets for heart disease.

Implication of Substance P in myocardial contractile function during ischemia in rats

Evidence suggests that substance P (SP) participates in the pathology of acute myocardial ischemia and infarction but the profiles of the peptide in regulation of cardiac functions are still elusive. The aim of this study was to investigate the role of substance P in regulation of cardiac functions and its association with adrenergic mechanism in acute myocardial ischemia and infarction with rodent models. The experiments were carried out in Sprague-Dawley rats. SP and norepinephrine were significantly up-regulated in myocardium at 15min, 30min and 60min of coronary artery occlusion. Pretreatment of the rats with a specific antagonist of neurokinin-1 receptor, D-SP, significant increased+dp/dt and decreased -dp/dt, compared with the controls, pretreated with 0.9% saline. Pretreatment of the isolated CAO hearts with substance P (10(-7)mol/L) significantly increased left ventricular end diastolic pressure. SP producing no effects on cardiac functions when given alone to isolated (non-CAO) heart caused significant attenuation of the changes in the contractility and diastolic functions induced by norepinephrine, when given with norepinephrine. SP attenuated the increase in the activity of PKA provoked by norepinephrine in cultured myocytes. In conclusion, the findings may indicate SP regulates cardiac functions via modulation of adrenergic activity, through suppression of over-activation of PKA.

Upper thoracic postsynaptic dorsal column neurons conduct cardiac mechanoreceptive information, but not cardiac chemical nociception in rats

Postsynaptic dorsal column (PSDC) neurons transmit noxious visceral information from the lower thoracic and lumbosacral spinal cord. Cuneothalamic neurons in the PSDC pathway and upper thoracic (T(3)-T(4)) spinal neurons ascending through the ventrolateral funiculus (VLF) have been shown to transmit nociceptive cardiac information. Therefore, we hypothesized that upper thoracic PSDC neurons transmit noxious cardiac information. Neuronal responses to intrapericardially injected mechanical (1.0 ml saline) and noxious chemical (0.2 ml algogenic chemicals) stimuli were recorded from antidromically activated PSDC and VLF neurons in the T(3)-T(4) spinal cord of anesthetized Sprague-Dawley rats. Of the PSDC neurons, 43% responded to mechanical stimulation, but only one responded to noxious chemical stimuli. Fifty-eight percent of VLF neurons responded to mechanical stimulation and all responded to noxious chemical stimulation. Fluoro-Ruby (FR)-labeled PSDC neurons in the T(3)-T(4) spinal cord of Sprague-Dawley rats were processed for c-fos immunohistochemistry following intrapericardial stimulation with mechanical, chemical, or control stimuli. Sections were viewed under epifluorescence and light microscopy to detect FR-labeled neurons containing a c-fos immunoreactive (IR) nucleus. An average of 6 PSDC neurons per rat was found in the T(3) and T(4) spinal segments. The average number of c-fos-IR neurons per segment varied by type of stimulus: 12 (control), 67 (chemical) and 85 (mechanical) for T(3) and 8 (control), 37 (chemical) and 62 (mechanical) for T(4). None of the 200 PSDC neurons examined expressed c-fos-IR regardless of stimulus. Together, these results suggest that thoracic PSDC neurons transmit mechanical cardiac information, but they play a minimal role in cardiac nociception.

Cross-organ sensitization of thoracic spinal neurons receiving noxious cardiac input in rats with gastroesophageal reflux

Gastroesophageal reflux (GER) frequently triggers or worsens cardiac pain or symptoms in patients with coronary heart disease. This study aimed to determine whether GER enhances the activity of upper thoracic spinal neurons receiving noxious cardiac input. Gastric fundus and pyloric ligations as well as a longitudinal myelotomy at the gastroesophageal junction induced acute GER in pentobarbital-anesthetized, paralyzed, and ventilated male Sprague-Dawley rats. Manual manipulations of the stomach and lower esophagus were used as surgical controls in another group. At 4-9 h after GER surgery, extracellular potentials of single neurons were recorded from the T3 spinal segment. Intrapericardial bradykinin (IB) (10 microg/ml, 0.2 ml, 1 min) injections were used to activate cardiac nociceptors, and esophageal distensions were used to activate esophageal afferent fibers. Significantly more spinal neurons in the GER group responded to IB compared with the control group (69.1 vs. 38%, P < 0.01). The proportion of IB-responsive neurons in the superficial laminae of GER animals was significantly different from those in deeper layers (1/8 vs. 46/60, P < 0.01); no difference was found in control animals (7/25 vs. 20/46, P > 0.05). Excitatory responses of spinal neurons to IB in the GER group were greater than in the control group [32.4 +/- 3.5 impulses (imp)/s vs. 13.3 +/- 2.3 imp/s, P < 0.01]. Forty-five of 47 (95.7%) neurons responded to cardiac input and ED, which was higher than the control group (61.5%, P < 0.01). These results indicate that acute GER enhanced the excitatory responses of thoracic spinal neurons in deeper laminae of the dorsal horn to noxious cardiac stimulus.

Cardiac innervation and sudden cardiac death

The heart is extensively innervated and its performance is tightly controlled by the nervous system. Cardiac innervation density varies in diseased hearts leading to unbalanced neural activation and lethal arrhythmia. Diabetic sensory neuropathy causes silent myocardial ischemia, characterized by loss of pain perception during myocardial ischemia, which is a major cause of sudden cardiac death in diabetes mellitus (DM). Despite its clinical importance, the mechanisms underlying the control and regulation of cardiac innervation remain poorly understood.We found that cardiac innervation is determined by the balance between neural chemoattractants and chemorepellents within the heart. Nerve growth factor (NGF), a potent chemoattractant, is induced by endothelin-1 upregulation during development and is highly expressed in cardiomyocytes. By comparison, Sema3a, a neural chemorepellent, is highly expressed in the subendocardium of early stage embryos, and is suppressed during development. The balance of expression between NGF and Seme3a leads to epicardial-to-endocardial transmural sympathetic innervation patterning. We also found that downregulation of cardiac NGF leads to diabetic neuropathy, and that NGF supplementation rescues silent myocardial ischemia in DM. Cardiac innervation patterning is disrupted in Sema3a-deficient and Sema3a-overexpressing mice, leading to sudden death or lethal arrhythmias. The present review focuses on the regulatory mechanisms underlying cardiac innervation and the critical role of these processes in cardiac performance.

C2 spinal cord stimulation induces dynorphin release from rat T4 spinal cord: potential modulation of myocardial ischemia-sensitive neurons

During myocardial ischemia, the cranial cervical spinal cord (C1-C2) modulates the central processing of the cardiac nociceptive signal. This study was done to determine 1) whether C2 SCS-induced release of an analgesic neuropeptide in the dorsal horn of the thoracic (T4) spinal cord; 2) if one of the sources of this analgesic peptide was cervical propriospinal neurons, and 3) if chemical inactivation of C2 neurons altered local T4 substance P (SP) release during concurrent C2 SCS and cardiac ischemia. Ischemia was induced by intermittent occlusion of the left anterior descending coronary artery (CoAO) in urethane-anesthetized Sprague-Dawley rats. Release of dynorphin A (1-13), (DYN) and SP was determined using antibody-coated microprobes inserted into T4. SCS alone induced DYN release from laminae I-V in T4, and this release was maintained during CoAO. C2 injection of the excitotoxin, ibotenic acid, prior to SCS, inhibited T4 DYN release during SCS and ischemia; it also reversed the inhibition of SP release from T4 dorsal laminae during C2 SCS and CoAO. Injection of the kappa-opioid antagonist, nor-binaltorphimine, into T4 also allowed an increased SP release during SCS and CoAO. CoAO increased the number of Fos-positive neurons in T4 dorsal horns but not in the intermediolateral columns (IML), while SCS (either alone or during CoAO) minimized this dorsal horn response to CoAO alone, while inducing T4 IML neuronal recruitment. These results suggest that activation of cervical propriospinal pathways induces DYN release in the thoracic spinal cord, thereby modulating nociceptive signals from the ischemic heart.

The effect of the cholinergic anti-inflammatory pathway on experimental colitis

Inflammatory bowel diseases (IBD) are characterized by proinflammatory cytokines, tissue damage and loss of neuron in inflamed mucosa, which implies the cholinergic anti-inflammatory pathway may be destroyed during the process of inflammatory response. In the study, we identified the effect of cholinergic agonist as anabaseine (AN) and nicotinic receptor antagonist as chlorisondamine diiodide (CHD) on trinitrobenzene sulfonic acid (TNBS)-induced colitis, to investigate the potential therapeutic effect of the cholinergic anti-inflammatory pathway on IBD. Experimental colitis was induced by TNBS at day 1, 10 mug AN or 1.5 mug CHD was injected i.p. to mouse right after the induction of colitis, and repeated on interval day till the mice were sacrificed at day 8. Colonic inflammation was examined by histological analysis, myeloperoxidase (MPO) activity, and the production of tumour necrosis factor (TNF)-alpha in tissue. Lamina propria mononuclear cells (LPMC) were isolated, and NF-kappaB activation was detected by western blot. The mice with colitis treated by AN showed less tissue damage, less MPO activity, less TNF-alpha production in colon, and inhibited NF-kappaB activation in LPMC, compared with those mice with colitis untreated, whereas the mice with colitis treated by CHD showed the worst tissue damage, the highest MPO activity, the highest TNF-alpha level, and enlarged NF-kappaB activation in LPMC. Agonist of the cholinergic anti-inflammatory pathway inhibits colonic inflammatory response by downregulating the production of TNF-alpha, and inhibiting NF-kappaB activation, which suggests that modulating the cholinergic anti-inflammatory pathway may be a new potential management for IBD.

Modulation of cardiac ischemia-sensitive afferent neuron signaling by preemptive C2 spinal cord stimulation: effect on substance P release from rat spinal cord

The upper cervical spinal region functions as an intraspinal controller of thoracic spinal reflexes and contributes to neuronal regulation of the ischemic myocardium. Our objective was to determine whether stimulation of the C2 cervical spinal cord (SCS) of rats modified the input signal at the thoracic spinal cord when cardiac ischemia-sensitive (sympathetic) afferents were activated by transient occlusion of the left anterior descending coronary artery (CoAO). Changes in c-Fos expression were used as an index of neuronal activation within the spinal cord and brain stem. The pattern of substance P (SP) release, a putative nociceptive transmitter, was measured using antibody-coated microprobes. Two SCS protocols were used: reactive SCS, applied concurrently with intermittent CoAO and preemptive, sustained SCS starting 15 min before and continuing during the repeated intermittent CoAO. CoAO increased SP release from laminae I and II in the T4 spinal cord above resting levels. Intermittent SCS with CoAO resulted in greater levels of SP release from deeper laminae IV-VII in T4 than CoAO alone. In contrast, SP release from laminae I and II was inhibited when CoAO was applied during preemptive, sustained SCS. Preemptive SCS likewise reduced c-Fos expression in the T4 spinal cord (laminae I-V) and nucleus tractus solitarius but increased expression in the intermediolateral cell column of T4 compared with CoAO alone. These results suggest that preemptive SCS from the high cervical region modulates sensory afferent signaling from the ischemic myocardium.

The effect of high cervical spinal cord stimulation on the expression of SP, NK-1 and TRPV1 mRNAs during cardiac ischemia in rat

Spinal cord stimulation (SCS) is used to reduce angina that accompanies cardiac ischemia, but little is known about the molecular mechanisms mediating this effect. We studied the expression of SP, neurokinin-1 (NK-1) receptor, and transient receptor potential vanilloid type 1 (TRPV1) mRNA in the rat spinal cord at thoracic 4 (T4), cervical 2 (C2) and caudal brain stem by RT-PCR during intermittent occlusion of the left anterior descending coronary artery (CoAO), during sustained SCS by itself at the C2 spinal segment, and during sustained SCS plus intermittent CoAO. Only SP mRNA was increased significantly in T4 and brainstem during CoAO, while SCS decreased the mRNA levels of SP, NK-1 and TRPV1 significantly in T4 and the brainstem. SCS attenuated the increase of SP and TRPV1 mRNA levels at T4 level induced by intermittent CoAO when the stimulation was applied prior to the initiation of the cardiac ischemia. These results support the role for SP as a putative neurotransmitter for the myocardial ischemia-sensitive afferent neuron signal to the spinal level. They suggest that modification of the ischemic cardiac nociceptive afferent signal by SCS involves a change in SP and TRPV1 expression.

Characterization of thoracic spinal neurons with noxious convergent inputs from heart and lower airways in rats

Respiratory symptoms experienced in some patients with cardiac diseases may be due to convergence of noxious cardiac and pulmonary inputs onto neurons of the central nervous system. For example, convergence of cardiac and respiratory inputs onto single solitary tract neurons may be in part responsible for integration of regulatory and defensive reflex control. However, it is unknown whether inputs from the lungs and heart converge onto single neurons of the spinal cord. The present aim was to characterize upper thoracic spinal neurons responding to both noxious stimuli of the heart and lungs in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer bradykinin (BK, 10 microg/ml, 0.2 ml, 1 min) as a noxious cardiac stimulus. The lung irritant, ammonia, obtained as vapor over a 30% solution of NH(4)OH was injected into the inspiratory line of the ventilator (0.5-1.0 ml over 20 s). Intrapericardial bradykinin (IB) altered activity of 58/65 (89%) spinal neurons that responded to inhaled ammonia (IA). Among those cardiopulmonary convergent neurons, 81% (47/58) were excited by both IA and IB, and the remainder had complex response patterns. Bilateral cervical vagotomy revealed that vagal afferents modulated but did not eliminate responses of individual spinal neurons to IB and IA. The convergence of pulmonary and cardiac nociceptive signaling in the spinal cord may be relevant to situations where a disease process in one organ influences the behavior of the other.

Nerve Growth Factor Is Critical for Cardiac Sensory Innervation and Rescues Neuropathy in Diabetic Hearts

BACKGROUND

Molecular mechanisms regulating the cardiac sensory nervous system remain poorly understood. Cardiac sensory nerve impairment causes silent myocardial ischemia, a main cause of sudden death in diabetes mellitus (DM). The present study focused on the roles of nerve growth factor (NGF) in the regulation of the cardiac sensory nervous system and analyzed the mechanism of silent myocardial ischemia in DM.

METHODS AND RESULTS

We screened neurotrophic factors and found that cardiac sensory nerves developed in parallel with NGF synthesized in the heart. Cardiac nociceptive sensory nerves that were immunopositive for calcitonin gene-related peptide, dorsal root ganglia (DRG), and the dorsal horn were markedly retarded in NGF-deficient mice, whereas cardiac-specific overexpression of NGF rescued these deficits. DM was induced with streptozotocin in wild-type and transgenic mice overexpressing NGF in the heart. Downregulation of NGF, calcitonin gene-related peptide-immunopositive cardiac sensory denervation, and atrophic changes in DRG were observed in DM-induced wild-type mice, whereas these deteriorations were reversed in DM-induced NGF transgenic mice. Cardiac sensory function, measured by myocardial ischemia-induced c-Fos expression in DRG, was also downregulated by DM in the wild-type mice but not in NGF transgenic mice. Direct gene transfer of NGF in the diabetic rat hearts improved impaired cardiac sensory innervation and function, determined by electrophysiological activity of cardiac afferent nerves during myocardial ischemia.

CONCLUSIONS

These findings demonstrate that the development and regulation of the cardiac sensory nervous system are dependent on NGF synthesized in the heart and that DM-induced NGF reduction causes cardiac sensory neuropathy.

Left vagal stimulation induces dynorphin release and suppresses substance P release from the rat thoracic spinal cord during cardiac ischemia

Electrostimulatory forms of therapy can reduce angina that arises from activation of cardiac nociceptive afferent fibers during transient ischemia. This study sought to determine the effects of electrical stimulation of left thoracic vagal afferents (C(8)-T(1) level) on the release of putative nociceptive [substance P (SP)] and analgesic [dynorphin (Dyn)] peptides in the dorsal horn at the T(4) spinal level during coronary artery occlusion in urethane-anesthetized Sprague-Dawley rats. Release of Dyn and SP was measured by using antibody-coated microprobes. While Dyn and SP had a basal release, occlusion of the left anterior descending coronary artery only affected SP release, causing an increase from lamina I-VII. Left vagal stimulation increased Dyn release, inhibited basal SP release, and blunted the coronary artery occlusion-induced release of SP. Dyn release reflected activation of descending pathways in the thoracic spinal cord, because vagal afferent stimulation still increased the release of Dyn after bilateral dorsal rhizotomy of T(2)-T(5). These results indicate that electrostimulatory therapy, using vagal afferent excitation, may induce analgesia, in part, via inhibition of the release of SP in the spinal cord, possibly through a Dyn-mediated neuronal interaction.