Thoracic spinal cord and cervical vagosympathetic neuromodulation obtund nodose sensory transduction of myocardial ischemia

Cardiac Neuroanatomy and Fundamentals of Neurocardiology

Cardiac control is mediated via nested-feedback reflex control networks involving the intrinsic cardiac ganglia, intra-thoracic extra-cardiac ganglia, spinal cord, brainstem, and higher centers. This control system is optimized to respond to normal physiologic stressors; however, it can be catastrophically disrupted by pathologic events such as myocardial ischemia. In fact, it is now recognized that cardiac disease progression reflects the dynamic interplay between adverse remodeling of the cardiac substrate coupled with autonomic dysregulation. With advances in understanding of this network dynamic in normal and pathologic states, neuroscience-based neuromodulation therapies can be devised for the management of acute and chronic cardiac pathologies.

Real-time in vivo thoracic spinal glutamate sensing during myocardial ischemia

In the spinal cord, glutamate serves as the primary excitatory neurotransmitter. Monitoring spinal glutamate concentrations offers valuable insights into spinal neural processing. Consequently, spinal glutamate concentration has the potential to emerge as a useful biomarker for conditions characterized by increased spinal neural network activity, especially when uptake systems become dysfunctional. In this study, we developed a multichannel custom-made flexible glutamate-sensing probe for the large-animal model that is capable of measuring extracellular glutamate concentrations in real time and in vivo. We assessed the probe's sensitivity and specificity through in vitro and ex vivo experiments. Remarkably, this developed probe demonstrates nearly instantaneous glutamate detection and allows continuous monitoring of glutamate concentrations. Furthermore, we evaluated the mechanical and sensing performance of the probe in vivo, within the pig spinal cord. Moreover, we applied the glutamate-sensing method using the flexible probe in the context of myocardial ischemia-reperfusion (I/R) injury. During I/R injury, cardiac sensory neurons in the dorsal root ganglion transmit excitatory signals to the spinal cord, resulting in sympathetic activation that potentially leads to fatal arrhythmias. We have successfully shown that our developed glutamate-sensing method can detect this spinal network excitation during myocardial ischemia. This study illustrates a novel technique for measuring spinal glutamate at different spinal cord levels as a surrogate for the spinal neural network activity during cardiac interventions that engage the cardio-spinal neural pathway.NEW & NOTEWORTHY In this study, we have developed a new flexible sensing probe to perform an in vivo measurement of spinal glutamate signaling in a large animal model. Our initial investigations involved precise testing of this probe in both in vitro and ex vivo environments. We accurately assessed the sensitivity and specificity of our glutamate-sensing probe and demonstrated its performance. We also evaluated the performance of our developed flexible probe during the insertion and compared it with the stiff probe during animal movement. Subsequently, we used this innovative technique to monitor the spinal glutamate signaling during myocardial ischemia and reperfusion that can cause fatal ventricular arrhythmias. We showed that glutamate concentration increases during the myocardial ischemia, persists during the reperfusion, and is associated with sympathoexcitation and increases in myocardial substrate excitability.

Real-time in vivo thoracic spinal glutamate sensing reveals spinal hyperactivity during myocardial ischemia

Myocardial ischemia-reperfusion (IR) can cause ventricular arrhythmias and sudden cardiac death via sympathoexcitation. The spinal cord neural network is crucial in triggering these arrhythmias and evaluating its neurotransmitter activity during IR is critical for understanding ventricular excitability control. To assess the real-time in vivo spinal neural activity in a large animal model, we developed a flexible glutamate-sensing multielectrode array. To record the glutamate signaling during IR injury, we inserted the probe into the dorsal horn of the thoracic spinal cord at the T2-T3 where neural signals generated by the cardiac sensory neurons are processed and provide sympathoexcitatory feedback to the heart. Using the glutamate sensing probe, we found that the spinal neural network was excited during IR, especially after 15 mins, and remained elevated during reperfusion. Higher glutamate signaling was correlated with the reduction in the cardiac myocyte activation recovery interval, showing higher sympathoexcitation, as well as dispersion of the repolarization which is a marker for increased risk of arrhythmias. This study illustrates a new technique for measuring the spinal glutamate at different spinal cord levels as a surrogate for the spinal neural network activity during cardiac interventions that engage the cardio-spinal neural pathway.

Graphical abstract

Autonomic nervous system and cardiac neuro-signaling pathway modulation in cardiovascular disorders and Alzheimer's disease

The heart is a functional syncytium controlled by a delicate and sophisticated balance ensured by the tight coordination of its several cell subpopulations. Accordingly, cardiomyocytes together with the surrounding microenvironment participate in the heart tissue homeostasis. In the right atrium, the sinoatrial nodal cells regulate the cardiac impulse propagation through cardiomyocytes, thus ensuring the maintenance of the electric network in the heart tissue. Notably, the central nervous system (CNS) modulates the cardiac rhythm through the two limbs of the autonomic nervous system (ANS): the parasympathetic and sympathetic compartments. The autonomic nervous system exerts non-voluntary effects on different peripheral organs. The main neuromodulator of the Sympathetic Nervous System (SNS) is norepinephrine, while the principal neurotransmitter of the Parasympathetic Nervous System (PNS) is acetylcholine. Through these two main neurohormones, the ANS can gradually regulate cardiac, vascular, visceral, and glandular functions by turning on one of its two branches (adrenergic and/or cholinergic), which exert opposite effects on targeted organs. Besides these neuromodulators, the cardiac nervous system is ruled by specific neuropeptides (neurotrophic factors) that help to preserve innervation homeostasis through the myocardial layers (from epicardium to endocardium). Interestingly, the dysregulation of this neuro-signaling pathway may expose the cardiac tissue to severe disorders of different etiology and nature. Specifically, a maladaptive remodeling of the cardiac nervous system may culminate in a progressive loss of neurotrophins, thus leading to severe myocardial denervation, as observed in different cardiometabolic and neurodegenerative diseases (myocardial infarction, heart failure, Alzheimer's disease). This review analyzes the current knowledge on the pathophysiological processes involved in cardiac nervous system impairment from the perspectives of both cardiac disorders and a widely diffused and devastating neurodegenerative disorder, Alzheimer's disease, proposing a relationship between neurodegeneration, loss of neurotrophic factors, and cardiac nervous system impairment. This overview is conducive to a more comprehensive understanding of the process of cardiac neuro-signaling dysfunction, while bringing to light potential therapeutic scenarios to correct or delay the adverse cardiovascular remodeling, thus improving the cardiac prognosis and quality of life in patients with heart or neurodegenerative disorders.

Spinal Cord Stimulation Attenuates Neural Remodeling, Inflammation, and Fibrosis After Myocardial Infarction

OBJECTIVES

Spinal cord stimulation (SCS) is an established neuromodulation method that regulates the cardiac autonomic system. However, the biological mechanisms of the therapeutic effects of SCS after myocardial infarction (MI) remain unclear.

MATERIALS AND METHODS

Twenty-five rabbits were divided into five groups: SCS-MI (voltage: 0.5 v; pulse width: 0.2 ms; 50 Hz; ten minutes on and 30 minutes off; two weeks; n = 5), MI (n = 5), sham SCS-MI (voltage: 0 v; two weeks; n = 5), sham MI (n = 5), and blank control (n = 5) groups. MI was induced by permanent left anterior descending artery ligation. SCS-MI and sham SCS-MI rabbits received the corresponding interventions 24 hours after MI. Autonomic remodeling was evaluated using enzyme-linked immunosorbent assay and immunohistochemistry. Inflammation and myocardial fibrosis were assessed using immunohistochemistry, quantitative polymerase chain reaction, hematoxylin and eosin staining, Masson staining, and Western blot.

RESULTS

SCS improved the abnormal systemic autonomic activity. Cardiac norepinephrine decreased after MI (p < 0.01) and did not improve with SCS. Cardiac acetylcholine increased with SCS compared with the MI group (p < 0.05). However, no difference was observed between the MI and blank control groups. Growth-associated protein 43 (p < 0.001) and tyrosine hydroxylase (p < 0.001) increased whereas choline acetyltransferase (p < 0.05) decreased in the MI group compared with the blank control group. These changes were attenuated with SCS. SCS inhibited inflammation, decreased the ratio of phosphorylated-Erk to Erk (p < 0.001), and increased the ratio of phosphorylated-STAT3 to STAT3 (p < 0.001) compared with the MI group. Myocardial fibrosis was also attenuated by SCS.

CONCLUSIONS

SCS improved abnormal autonomic activity after MI, leading to reduced inflammation, reactivation of STAT3, and inhibition of Erk. Additionally, SCS attenuated myocardial fibrosis. Our results warrant future studies of biological mechanisms of the therapeutic effects of SCS after MI.

Advances in Our Clinical Understanding of Autonomic Regulation Therapy Using Vagal Nerve Stimulation in Patients Living With Heart Failure

The ANTHEM-HF, INOVATE-HF, and NECTAR-HF clinical studies of autonomic regulation therapy (ART) using vagus nerve stimulation (VNS) systems have collectively provided dose-ranging information enabling the development of several working hypotheses on how stimulation frequency can be utilized during VNS for tolerability and improving cardiovascular outcomes in patients living with heart failure (HF) and reduced ejection fraction (HFrEF). Changes in heart rate dynamics, comprising reduced heart rate (HR) and increased HR variability, are a biomarker of autonomic nerve system engagement and cardiac control, and appear to be sensitive to VNS that is delivered using a stimulation frequency that is similar to the natural operating frequency of the vagus nerve. Among prior studies, the ANTHEM-HF Pilot Study has provided the clearest evidence of autonomic engagement with VNS that was delivered using a stimulation frequency that was within the operating range of the vagus nerve. Achieving autonomic engagement was accompanied by improvement from baseline in six-minute walk duration (6MWD), health-related quality of life, and left ventricular EF (LVEF), over and above those achieved by concomitant guideline-directed medical therapy (GDMT) administered to counteract harmful neurohormonal activation, with relative freedom from deleterious effects. Autonomic engagement and positive directional changes have persisted over time, and an exploratory analysis suggests that improvement in autonomic tone, symptoms, and physical capacity may be independent of baseline NT-proBNP values. Based upon these encouraging observations, prospective, randomized controlled trials examining the effects on symptoms and cardiac function as well as natural history have been warranted. A multi-national, large-scale, randomized, controlled trial is well underway to determine the outcomes associated with ART using autonomic nervous system engagement as a guide for VNS delivery.

Optical vagus nerve modulation of heart and respiration via heart-injected retrograde AAV

Vagus nerve stimulation has shown many benefits for disease therapies but current approaches involve imprecise electrical stimulation that gives rise to off-target effects, while the functionally relevant pathways remain poorly understood. One method to overcome these limitations is the use of optogenetic techniques, which facilitate targeted neural communication with light-sensitive actuators (opsins) and can be targeted to organs of interest based on the location of viral delivery. Here, we tested whether retrograde adeno-associated virus (rAAV2-retro) injected in the heart can be used to selectively express opsins in vagus nerve fibers controlling cardiac function. Furthermore, we investigated whether perturbations in cardiac function could be achieved with photostimulation at the cervical vagus nerve. Viral injection in the heart resulted in robust, primarily afferent, opsin reporter expression in the vagus nerve, nodose ganglion, and brainstem. Photostimulation using both one-photon stimulation and two-photon holography with a GRIN-lens incorporated nerve cuff, was tested on the pilot-cohort of injected mice. Changes in heart rate, surface electrocardiogram, and respiratory responses were observed in response to both one- and two-photon photostimulation. The results demonstrate feasibility of retrograde labeling for organ targeted optical neuromodulation.

Autonomic Modulation for Cardiovascular Disease

Dysfunction of the autonomic nervous system has been implicated in the pathogenesis of cardiovascular disease, including congestive heart failure and cardiac arrhythmias. Despite advances in the medical and surgical management of these entities, progression of disease persists as does the risk for sudden cardiac death. With improved knowledge of the dynamic relationships between the nervous system and heart, neuromodulatory techniques such as cardiac sympathetic denervation and vagal nerve stimulation (VNS) have emerged as possible therapeutic approaches for the management of these disorders. In this review, we present the structure and function of the cardiac nervous system and the remodeling that occurs in disease states, emphasizing the concept of increased sympathoexcitation and reduced parasympathetic tone. We review preclinical evidence for vagal nerve stimulation, and early results of clinical trials in the setting of congestive heart failure. Vagal nerve stimulation, and other neuromodulatory techniques, may improve the management of cardiovascular disorders, and warrant further study.

Thoracic spinal cord neuromodulation obtunds dorsal root ganglion afferent neuronal transduction of the ischemic ventricle

Aberrant afferent signaling drives adverse remodeling of the cardiac nervous system in ischemic heart disease. The study objective was to determine whether thoracic spinal dorsal column stimulation (SCS) modulates cardiac afferent sensory transduction of the ischemic ventricle. In anesthetized canines (n = 16), extracellular activity generated by 62 dorsal root ganglia (DRG) soma (T1-T3), with verified myocardial ischemic (MI) sensitivity, were evaluated with and without 20-min preemptive SCS (T1-T3 spinal level; 50 Hz, 90% motor threshold). Transient MI was induced by 1-min coronary artery occlusion (CAO) of the left anterior descending (LAD) or circumflex (LCX) artery, randomized as to sequence. LAD and LCX CAO activated cardiac-related DRG neurons (LAD: 0.15 ± 0.04-1.05 ± 0.20 Hz, P < 0.00002; LCX: 0.08 ± 0.02-1.90 ± 0.45 Hz, P < 0.0003). SCS decreased basal neuronal activity of neurons that responded to LAD (0.15 ± 0.04 to 0.02 ± 0.01 Hz, P < 0.006) and LCX (0.08 ± 0.02 to 0.02 ± 0.01 Hz, P < 0.003). SCS suppressed responsiveness to transient MI (LAD: 1.05 ± 0.20-0.03 ± 0.01 Hz; P < 0.0001; LCX: 1.90 ± 0.45-0.03 ± 0.01 Hz; P < 0.001). Suprathreshold SCS (1 Hz) did not activate DRG neurons antidromically (n = 10 animals). Ventricular fibrillation (VF) was associated with a rapid increase in DRG activity to a maximum of 4.39 ± 1.07 Hz at 20 s after VF induction and a return to 90% of baseline within 10 s thereafter. SCS obtunds the capacity of DRG ventricular neurites to transduce the ischemic myocardium to second-order spinal neurons, a mechanism that would blunt reflex sympathoexcitation to myocardial ischemic stress, thereby contributing to its capacity to cardioprotect.NEW & NOTEWORTHY Aberrant afferent signaling drives adverse remodeling of the cardiac nervous system in ischemic heart disease. This study determined that thoracic spinal column stimulation (SCS) obtunds the capacity of dorsal root ganglia ventricular afferent neurons to transduce the ischemic myocardium to second-order spinal neurons, a mechanism that would blunt reflex sympathoexcitation to myocardial ischemic stress. This modulation does not reflect antidromic actions of SCS but likely reflects efferent-mediated changes at the myocyte-sensory neurite interface.

Premature ventricular contractions activate vagal afferents and alter autonomic tone: implications for premature ventricular contraction-induced cardiomyopathy

Mechanisms behind development of premature ventricular contraction (PVC)-induced cardiomyopathy remain unclear. PVCs may adversely modulate the autonomic nervous system to promote development of heart failure. Afferent neurons in the inferior vagal (nodose) ganglia transduce cardiac activity and modulate parasympathetic output. Effects of PVCs on cardiac parasympathetic efferent and vagal afferent neurotransmission are unknown. The purpose of this study was to evaluate effects of PVCs on vagal afferent neurotransmission and compare these effects with a known powerful autonomic modulator, myocardial ischemia. In 16 pigs, effects of variably coupled PVCs on heart rate variability (HRV) and vagal afferent neurotransmission were evaluated. Direct nodose neuronal recordings were obtained in vivo, and cardiac-related afferent neurons were identified based on their response to cardiovascular interventions, including ventricular chemical and mechanical stimuli, left anterior descending (LAD) coronary artery occlusion, and variably coupled PVCs. On HRV analysis before versus after PVCs, parasympathetic tone decreased (normalized high frequency: 83.6 ± 2.8 to 72.5 ± 5.3; P < 0.05). PVCs had a powerful impact on activity of cardiac-related afferent neurons, altering activity of 51% of neurons versus 31% for LAD occlusion (P < 0.05 vs. LAD occlusion and all other cardiac interventions). Both chemosensitive and mechanosensitive neurons were activated by PVCs, and their activity remained elevated even after cessation of PVCs. Cardiac afferent neural responses to PVCs were greater than any other intervention, including ischemia of similar duration. These data suggest that even brief periods of PVCs powerfully modulate vagal afferent neurotransmission, reflexly decreasing parasympathetic efferent tone.NEW & NOTEWORTHY Premature ventricular contractions (PVCs) are common in many patients and, at an increased burden, are known to cause heart failure. This study determined that PVCs powerfully modulate cardiac vagal afferent neurotransmission (exerting even greater effects than ventricular ischemia) and reduce parasympathetic efferent outflow to the heart. PVCs activated both mechano- and chemosensory neurons in the nodose ganglia. These peripheral neurons demonstrated adaptation in response to PVCs. This study provides additional data on the potential role of the autonomic nervous system in PVC-induced cardiomyopathy.