Control of Neurotransmission by NaV1.7 in Human, Guinea Pig, and Mouse Airway Parasympathetic Nerves

Sensory neuron sodium channels as pain targets; from cocaine to Journavx (VX-548, suzetrigine)

Voltage-gated sodium channels underpin electrical signaling in sensory neurons. Their activity is an essential element in the vast majority of pain conditions, making them significant drug targets. Sensory neuron sodium channels play roles not only in afferent signaling but also in a range of efferent regulatory mechanisms. Side effects through actions on other cell types and efferent signaling are thus important issues to address during analgesic drug development. As an example, the human genetic evidence for NaV1.7 as an ideal pain target contrasts with the side effects of NaV1.7 antagonists. In this review, we describe the history and progress toward the development of useful analgesic drugs and the renewed focus on NaV1.8 as a key target in pain treatment. NaV1.8 antagonists alone or in combination with other analgesics are likely to provide new opportunities for pain relief for the vast number of people (about 33% of the population) impacted by chronic pain, particularly present in aging populations.

Interferential Current Stimulation Enhances Rectal Motor Activity: Insights from an Isolated Perfused Porcine Model

INTRODUCTION

 Interferential current (IFC) has been studied in several clinical trials for the treatment of bowel motility disorders, most often in children. However, only moderate effects are reported, and in contrast to IFC, the so-called placebo application is indiscernible. The mechanisms and neuroanatomic points of action remain elusive. Therefore, this therapy remains being questioned.

METHODS

 To gain objective experimental data about IFC stimulation, we examined this method ex vivo in an isolated perfused porcine rectum including the mesorectum. To elucidate the role of plexus nerve fibers and enteric ganglia, we performed IFC stimulation also in the presence of tetrodotoxin (TTX) or hexamethonium (HXN). We applied the commonly used stimulation modes with a beat frequency sweeping between 5 and 25 Hz (IFCd5-25) and 80 and 150 Hz (IFCd80-150). We monitored intraluminal pressure and motility by online barometry and video recording, respectively. Motor activity, reflected by changes in the intraluminal pressure (cm H2O·s-1) and longitudinal movements (pixels·s-1), was quantified over time as root mean squares (RMSs).

RESULTS

 After IFCd5-25, we observed a 30% increase in the rectal motility in the pressure changes which was sustained over 30 minutes post-stimulation (p < 0.02); only a minor effect was detected for IFCd80-150. Both TTX and HTX abolished the stimulation. This suggests neuronal modulation.

CONCLUSION

 IFCd5-25 stimulates rectal motor activity in the isolated perfused porcine rectum. Ganglia in the enteric nervous system are modulated to allow increased activity for at least 30 minutes. Therefore, the isolated porcine rectum is a suitable tool to study the effectiveness of various IFC settings in the rectum.

Sodium channels Nav1.7, Nav1.8 and pain; two distinct mechanisms for Nav1.7 null analgesia

Genetic deletion and pharmacological inhibition are distinct approaches to unravelling pain mechanisms, identifying targets and developing new analgesics. Both approaches have been applied to the voltage-gated sodium channels Nav1.7 and Nav1.8. Genetic deletion of Nav1.8 in mice leads to a loss of pain and antagonists are effective analgesics. The situation with Nav1.7 is more complex. Complete embryonic loss of Nav1.7 in humans or in mouse sensory neurons leads to anosmia as well as profound analgesia as a result of diminished neurotransmitter release. This is mediated by enhanced endogenous opioid signaling in humans and mice. In contrast, anosmia is opioid-independent. Sensory neuron excitability and autonomic function appear to be normal. Adult deletion of Nav1.7 in sensory neurons also leads to analgesia, but through diminished sensory and autonomic neuron excitability. There is no opioid component of analgesia or anosmia as shown by a lack of effect of naloxone. Pharmacological inhibition of Nav1.7 in mice and humans leads both to analgesia and dramatic side-effects on the autonomic nervous system with no therapeutic window. These data demonstrate that specific Nav1.7 channel blockers will fail as analgesic drugs. The viability of embryonic null mutants suggests that there are compensatory changes to replace the lost Nav1.7 channel. Here we show that sensory neuron sodium channels Nav1.1, Nav1.2 and β4 subunits detected by Mass Spectrometry are upregulated in Nav1.7 embryonic null neurons and, together with other proteome changes, potentially compensate for the loss of Nav1.7. Interestingly, many of the upregulated proteins are known to interact with Nav1.7.

Role of NaV1.7 in postganglionic sympathetic nerve function in human and guinea-pig arteries

NaV1.7 plays a crucial role in inducing and conducting action potentials in pain-transducing sensory nociceptor fibres, suggesting that NaV1.7 blockers could be effective non-opioid analgesics. While SCN9A is expressed in both sensory and autonomic neurons, its functional role in the autonomic system remains less established. Our single neuron rt-PCR analysis revealed that 82% of sympathetic neurons isolated from guinea-pig stellate ganglia expressed NaV1.7 mRNA, with NaV1.3 being the only other tetrodotoxin-sensitive channel expressed in approximately 50% of neurons. We investigated the role of NaV1.7 in conducting action potentials in postganglionic sympathetic nerves and in the sympathetic adrenergic contractions of blood vessels using selective NaV1.7 inhibitors. Two highly selective NaV1.7 blockers, GNE8493 and PF 05089771, significantly inhibited postganglionic compound action potentials by approximately 70% (P < 0.01), with residual activity being blocked by the NaV1.3 inhibitor, ICA 121431. Electrical field stimulation (EFS) induced rapid contractions in guinea-pig isolated aorta, pulmonary arteries, and human isolated pulmonary arteries via stimulation of intrinsic nerves, which were inhibited by prazosin or the NaV1 blocker tetrodotoxin. Our results demonstrated that blocking NaV1.7 with GNE8493, PF 05089771, or ST2262 abolished or strongly inhibited sympathetic adrenergic responses in guinea-pigs and human vascular smooth muscle. These findings support the hypothesis that pharmacologically inhibiting NaV1.7 could potentially reduce sympathetic and parasympathetic function in specific vascular beds and airways. KEY POINTS: 82% of sympathetic neurons isolated from the stellate ganglion predominantly express NaV1.7 mRNA. NaV1.7 blockers inhibit action potential conduction in postganglionic sympathetic nerves. NaV1.7 blockade substantially inhibits sympathetic nerve-mediated adrenergic contractions in human and guinea-pig blood vessels. Pharmacologically blocking NaV1.7 profoundly affects sympathetic and parasympathetic responses in addition to sensory fibres, prompting exploration into the broader physiological consequences of NaV1.7 mutations on autonomic nerve activity.

Action potential conduction in the mouse and rat vagus nerve is dependent on multiple voltage-gated sodium channels (NaV1s)

Action potential (AP) conduction depends on voltage-gated sodium channels, of which there are nine subtypes. The vagus nerve, comprising sensory afferent fibers and efferent parasympathetic fibers, provides autonomic regulation of visceral organs, but the voltage-gated sodium channels (NaV1) subtypes involved in its AP conduction are poorly defined. We studied the A- and C-waves of electrically stimulated compound action potentials (CAPs) of the mouse and rat vagus nerves with and without NaV1 inhibitor administration: tetrodotoxin (TTX), PF-05089771 (mouse NaV1.7), ProTX-II (NaV1.7), ICA-121341 (NaV1.1, NaV1.3, and NaV1.6), LSN-3049227 (NaV1.2, NaV1.6, and NaV1.7), and A-803467 (NaV1.8). We show that TTX-sensitive NaV1 channels are essential for all vagal AP conduction. PF-05089771 but not ICA-121341 inhibited the mouse A-wave, which was abolished by LSN-3049227, suggesting roles for NaV1.7 and NaV1.2. The mouse C-wave was abolished by LSN-3049227 and a combination of PF-05089771 and ICA-121341, suggesting roles for NaV1.7 and NaV1.6. The rat A-wave was inhibited by ProTX-II, ICA-121341, and a combination of these inhibitors but only abolished by LSN-3049227, suggesting roles for NaV1.7, NaV1.6, and NaV1.2. The rat C-wave was abolished by LSN-3049227 and a combination of ProTX-II and ICA-121341, suggesting roles for NaV1.7 and NaV1.6. A-803467 also inhibited the mouse and rat CAP suggesting a cooperative role for the TTX-resistant NaV1.8. Overall, our data demonstrate that multiple NaV1 subtypes contribute to vagal CAPs, with NaV1.7 and NaV1.8 playing predominant roles and NaV1.6 and NaV1.2 contributing to a different extent based on nerve fiber type and species. Inhibition of these NaV1 may impact autonomic regulation of visceral organs.NEW & NOTEWORTHY Distinct NaV1 channels are involved in action potential (AP) initiation and conduction from afferent terminals within specific organs. Here, we have identified the NaV1 necessary for AP conduction in the entire murine and rat vagus nerve. We show TTX-sensitive channels are essential for all AP conduction, predominantly NaV1.7 with NaV1.2 and NaV1.6 playing lesser roles depending on the species and fiber type. In addition, we show that NaV1.8 is also essential for most axonal AP conduction.

NaV1.7 channels are expressed in the lower airways of the human respiratory tract

Na_V channels expression have been reported in upper airways and tracheal smooth muscle cells controlling the generation and propagation of action potentials in the respiratory tract sensory neurons, but information about the presence of these proteins in the bronchioalveolar structures in human lungs was missing. The main objective covered in this work was to determine whether the Na_V1.7 channels are expressed in lower airways, and to identify the cellular identities expressing these proteins. We detected high levels of the mRNA coding for Na_V1.7 channels in isolated lung fibroblasts obtained from both normal lungs, and fibrotic lungs of patients with respiratory diseases. The protein was detected with two different antibodies in the bronchioalveolar tissue, alveolar endothelium, and capillary endothelium, in normal and pathologic lungs. These evidences are useful in the dissection of molecular mechanisms of pulmonary pathologies, and lead to consider the Na_V1.7 channels as potential therapeutic targets for the treatment of pulmonary diseases.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of NaV1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of NaV1.7 has been recently achieved, multiple NaV1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and NaV1.7 coverage among three types of voltage-gated sodium channel (VGSC) inhibitors, namely conventional VGSC inhibitors, sulphonamides and acyl sulphonamides, is discussed. By demonstrating the PK/PD discrepancy of preclinical studies versus in vivo models and clinical results, the potential reasons behind the disconnect between preclinical results and clinical outcomes are discussed together with strategies for developing ideal analgesic agents.

Cell specific regulation of NaV1.7 activity and trafficking in rat nodose ganglia neurons

The voltage-gated sodium NaV1.7 channel sets the threshold for electrogenesis. Mutations in the gene encoding human NaV1.7 (SCN9A) cause painful neuropathies or pain insensitivity. In dorsal root ganglion (DRG) neurons, activity and trafficking of NaV1.7 are regulated by the auxiliary collapsin response mediator protein 2 (CRMP2). Specifically, preventing addition of a small ubiquitin-like modifier (SUMO), by the E2 SUMO-conjugating enzyme Ubc9, at lysine-374 (K374) of CRMP2 reduces NaV1.7 channel trafficking and activity. We previously identified a small molecule, designated 194, that prevented CRMP2 SUMOylation by Ubc9 to reduce NaV1.7 surface expression and currents, leading to a reduction in spinal nociceptive transmission, and culminating in normalization of mechanical allodynia in models of neuropathic pain. In this study, we investigated whether NaV1.7 control via CRMP2-SUMOylation is conserved in nodose ganglion (NG) neurons. This study was motivated by our desire to develop 194 as a safe, non-opioid substitute for persistent pain, which led us to wonder how 194 would impact NaV1.7 in NG neurons, which are responsible for driving the cough reflex. We found functioning NaV1.7 channels in NG neurons; however, they were resistant to downregulation via either CRMP2 knockdown or pharmacological inhibition of CRMP2 SUMOylation by 194. CRMP2 SUMOylation and interaction with NaV1.7 was consered in NG neurons but the endocytic machinery was deficient in the endocytic adaptor protein Numb. Overexpression of Numb rescued CRMP2-dependent regulation on NaV1.7, rendering NG neurons sensitive to 194. Altogether, these data point at the existence of cell-specific mechanisms regulating NaV1.7 trafficking.

Neuro-Immune Regulation in Inflammation and Airway Remodeling of Allergic Asthma

Allergic asthma is a common chronic inflammation of the airways and causes airway remodeling eventually. For a long time, investigators have been focusing on the immunological mechanism of asthma. However, in recent years, the role of neuro-regulation in the occurrence of asthma has gradually attracted investigators’ attention. In this review, we firstly describe neuro-immune regulation in inflammation of allergic asthma from two aspects: innate immunity and adaptive immunity. Secondly, we introduce neuro-immune regulation in airway remodeling of asthma. Finally, we prospect the role of pulmonary neuroendocrine cells in the development of asthma. In general, the amount of researches is limited. Further researches on the neural regulation during the occurrence of asthma will help us clarify the mechanism of asthma more comprehensively and find more effective ways to prevent and control asthma.

Role of NaV 1.7 in action potential conduction along human bronchial vagal afferent C-fibres

BACKGROUND AND PURPOSE

The purpose of this study was to determine the role of NaV 1.7 in action potential conduction in C-fibres in the bronchial branches of the human vagus nerve.

EXPERIMENTAL APPROACH

Bronchial branches of the vagus nerve were dissected from human donor tissue. The C-wave of the electrically evoked compound action potential was quantified in the absence and presence of increasing concentrations of the selective NaV 1.7 blocking drugs, PF-05089771 and ST-2262, as well as the NaV 1.1, 1.2, and 1.3 blocking drug ICA121-431. The efficacy and potency of these inhibitors were compared to the standard NaV 1 blocker, tetrodotoxin. We then compared the relative potencies of the NaV 1 blockers in inhibiting the C-wave of the compound action potential, with their ability to inhibit parasympathetic cholinergic contraction of human isolated bronchi, a response previously shown to be strictly dependent on NaV 1.7 channels.

KEY RESULTS

The selective NaV 1.7 blockers inhibited the C-wave of the compound action potential with potencies similar to that observed in the NaV 1.7 bronchial contractions assay. Using rt-PCR, we noted that NaV 1.7 mRNA was strongly expressed and transported down the vagus nerve bundles.

CONCLUSIONS AND IMPLICATIONS

NaV 1.7 blockers can prevent action potential conduction in the majority of vagal C-fibres arising from human bronchi. Blockers of NaV 1.7 channels may therefore have value in inhibiting the responses to excessive airway C-fibre activation in inflammatory airway disease, responses that include coughing as well as reflex bronchoconstriction and secretions.

Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Contribution of tetrodotoxin-sensitive, voltage-gated sodium channels (NaV1) to action potential discharge from mouse esophageal tension mechanoreceptors

Action potentials depend on voltage-gated sodium channels (NaV1s), which have nine α subtypes. NaV1 inhibition is a target for pathologies involving excitable cells such as pain. However, because NaV1 subtypes are widely expressed, inhibitors may inhibit regulatory sensory systems. Here, we investigated specific NaV1s and their inhibition in mouse esophageal mechanoreceptors-non-nociceptive vagal sensory afferents that are stimulated by low threshold mechanical distension, which regulate esophageal motility. Using single fiber electrophysiology, we found mechanoreceptor responses to esophageal distension were abolished by tetrodotoxin. Single-cell RT-PCR revealed that esophageal-labeled TRPV1-negative vagal neurons expressed multiple tetrodotoxin-sensitive NaV1s: NaV1.7 (almost all neurons) and NaV1.1, NaV1.2, and NaV1.6 (in ∼50% of neurons). Inhibition of NaV1.7, using PF-05089771, had a small inhibitory effect on mechanoreceptor responses to distension. Inhibition of NaV1.1 and NaV1.6, using ICA-121341, had a similar small inhibitory effect. The combination of PF-05089771 and ICA-121341 inhibited but did not eliminate mechanoreceptor responses. Inhibition of NaV1.2, NaV1.6, and NaV1.7 using LSN-3049227 inhibited but did not eliminate mechanoreceptor responses. Thus, all four tetrodotoxin-sensitive NaV1s contribute to action potential initiation from esophageal mechanoreceptors terminals. This is different to those NaV1s necessary for vagal action potential conduction, as demonstrated using GCaMP6s imaging of esophageal vagal neurons during electrical stimulation. Tetrodotoxin-sensitive conduction was abolished in many esophageal neurons by PF-05089771 alone, indicating a critical role of NaV1.7. In summary, multiple NaV1 subtypes contribute to electrical signaling in esophageal mechanoreceptors. Thus, inhibition of individual NaV1s would likely have minimal effect on afferent regulation of esophageal motility.

Antitussive effects of NaV 1.7 blockade in Guinea pigs

Our previous studies implicated the voltage-gated sodium channel subtype NaV 1.7 in the transmission of action potentials by the vagal afferent nerves regulating cough and thus identified this channel as a rational therapeutic target for antitussive therapy. But it is presently unclear whether a systemically administered small molecule inhibitor of NaV 1.7 conductance can achieve therapeutic benefit in the absence of side effects on cardiovascular function, gastrointestinal motility or respiration. To this end, we have evaluated the antitussive effects of the NaV 1.7 selective blocker Compound 801 administered systemically in awake guinea pigs or administered topically in anesthetized guinea pigs. We also evaluated the antitussive effects of ambroxol, a low affinity NaV blocker modestly selective for tetrodotoxin resistant NaV subtypes. Both Compound 801 and ambroxol dose-dependently inhibited action potential conduction in guinea pig vagus nerves (assessed by compound potential), with ambroxol nearly 100-fold less potent than the NaV 1.7 selective Compound 801 in this and other NaV 1.7-dependent guinea pig and human tissue-based assays. Both drugs also inhibited citric acid evoked coughing in awake or anesthetized guinea pigs, with potencies supportive of an NaV 1.7-dependent mechanism. Notably, however, the antitussive effects of systemically administered Compound 801 were accompanied by hypotension and respiratory depression. Given the antitussive effects of topically administered Compound 801, we speculate that the likely insurmountable side effects on blood pressure and respiratory drive associated with systemic dosing make topical formulations a viable and perhaps unavoidable therapeutic strategy for targeting NaV 1.7 in cough.

The Changes in Expression of NaV1.7 and NaV1.8 and the Effects of the Inhalation of Their Blockers in Healthy and Ovalbumin-Sensitized Guinea Pig Airways

Background: The presented study evaluated the suppositional changes in the airway expression of Nav1.8 and Nav1.7 and their role in the airway defense mechanisms in healthy animals and in an experimental asthma model. Methods: The effects of the blockers inhalation on the reactivity of guinea pig airways, number of citric-acid-induced coughs and ciliary beating frequency (CBF) were tested in vivo. Chronic inflammation simulating asthma was induced by repetitive exposure to ovalbumin. The expression of Nav1.7 and Nav1.8 was examined by ELISA. Results: The Nav 1.8 blocker showed complex antitussive and bronchodilatory effects and significantly regulated the CBF in healthy and sensitized animals. The Nav1.7 blockers significantly inhibited coughing and participated in CBF control in the ovalbumin-sensitized animals. The increased expression of the respective ion channels in the sensitized animals corresponded to changes in CBF regulation. The therapeutic potency of the Nav1.8 blocker was evidenced in combinations with classic bronchodilators. Conclusion: The allergic-inflammation-upregulated expression of Nav1.7 and Nav1.8 and corresponding effects of blocker inhalation on airway defense mechanisms, along with the Nav1.8 blocker’s compatibility with classic antiasthmatic drugs, bring novel possibilities for the treatment of various respiratory diseases. However, the influence of the Nav1.8 blocker on CBF requires further investigation.

Voltage-gated sodium channels mediating conduction in vagal motor fibers innervating the esophageal striated muscle

The vagal motor fibers innervating the esophageal striated muscle are essential for esophageal motility including swallowing and vomiting. However, it is unknown which subtypes of voltage-gated sodium channels (NaV1s) regulate action potential conduction in these efferent nerve fibers. The information on the NaV1s subtypes is necessary for understanding their potential side effects on upper gut, as novel inhibitors of NaV1s are developed for treatment of pain. We used isolated superfused (35 °C) vagally-innervated mouse esophagus striated muscle preparation (mucosa removed) to measure isometric contractions of circular striated muscle evoked by electrical stimulation of the vagus nerve. NaV1 inhibitors were applied to the de-sheathed segment of the vagus nerve. Tetrodotoxin (TTX) applied to the vagus nerve completely abolished electrically evoked contractions. The selective NaV1.7 inhibitor PF-05089771 alone partially inhibited contractions and caused a >3-fold rightward shift in the TTX concentration-inhibition curve. The NaV1.1, NaV1.2 and NaV1.3 group inhibitor ICA-121431 failed to inhibit contractions, or to alter TTX concentration-inhibition curves in the absence or in the presence of PF-05089771. RT-PCR indicated lack of NaV1.4 expression in nucleus ambiguus and dorsal motor nucleus of the vagus nerve, which contain motor and preganglionic neurons projecting to the esophagus. We conclude that the action potential conduction in the vagal motor fibers to the esophageal striated muscle in the mouse is mediated by TTX-sensitive voltage gated sodium channels including NaV1.7 and most probably NaV1.6. The role of NaV1.6 is supported by ruling out other TTX-sensitive NaV1s (NaV1.1-1.4) in the NaV1.7-independent conduction.

Back to the future: re-establishing guinea pig in vivo asthma models

Research using animal models of asthma is currently dominated by mouse models. This has been driven by the comprehensive knowledge on inflammatory and immune reactions in mice, as well as tools to produce genetically modified mice. Many of the identified therapeutic targets influencing airway hyper-responsiveness and inflammation in mouse models, have however been disappointing when tested clinically in asthma. It is therefore a great need for new animal models that more closely resemble human asthma. The guinea pig has for decades been used in asthma research and a comprehensive table of different protocols for asthma models is presented. The studies have primarily been focused on the pharmacological aspects of the disease, where the guinea pig undoubtedly is superior to mice. Further reasons are the anatomical and physiological similarities between human and guinea pig airways compared with that of the mouse, especially with respect to airway branching, neurophysiology, pulmonary circulation and smooth muscle distribution, as well as mast cell localization and mediator secretion. Lack of reagents and specific molecular tools to study inflammatory and immunological reactions in the guinea pig has however greatly diminished its use in asthma research. The aim in this position paper is to review and summarize what we know about different aspects of the use of guinea pig in vivo models for asthma research. The associated aim is to highlight the unmet needs that have to be addressed in the future.

Stimulus intensity-dependent recruitment of NaV1 subunits in action potential initiation in nerve terminals of vagal C-fibers innervating the esophagus

We investigated voltage-gated sodium channel (Na_V1) subunits that regulate action potential initiation in the nerve terminals of vagal nodose C-fibers innervating the esophagus. Extracellular single fiber recordings were made from the nodose C-fibers, with mechanically sensitive nerve terminals in the isolated innervated guinea pig esophagus. Na_V1 inhibitors were selectively delivered to the tissue-containing nerve terminals. Graded esophageal distention was used for mechanical stimulation. The Na_V1.7 inhibitor PF-05089771 nearly abolished action potential initiation in response to low levels of esophageal distention but only partially inhibited the response to higher levels of esophageal distention. The PF-05089771-insensitive component of the response progressively increased (up to ≈50%) with increasing esophageal distention and was abolished by tetrodotoxin (TTX). In addition to Na_V1.7, nodose C-fiber [transient receptor potential channel-vanilloid subfamily member 1 (TRPV1)-positive] neurons retrogradely labeled from the esophagus expressed mRNA for multiple TTX-sensitive Na_V1s. The group Na_V1.1, Na_V1.2, and Na_V1.3 inhibitor ICA-121431 inhibited but did not abolish the PF-05089771-insensitive component of the response to high level of esophageal distention. However, combination of ICA-121431 with compound 801, which also inhibits Na_V1.7 and Na_V1.6, nearly abolished the response to the high level of esophageal distention. Our data indicate that the action potential initiation in esophageal nodose C-fibers evoked by low (innocuous) levels of esophageal distention is mediated by Na_V1.7. However, the response evoked by higher (noxious) levels of esophageal distention has a progressively increasing Na_V1.7-independent component that involves multiple TTX-sensitive Na_V1s. The stimulus intensity-dependent recruitment of Na_V1s may offer novel opportunities for strategic targeting of Na_V1 subunits for inhibition of nociceptive signaling in visceral C-fibers.NEW & NOTEWORTHY We report that pharmacologically distinguishable voltage-gated sodium channels (Na_V1) mediate action potential initiation at low (innocuous) versus high (noxious) intensity of esophageal distention in nerve terminals of vagal nodose C-fibers. Action potential initiation at low intensity is entirely dependent on Na_V1.7; however, additional tetrodotoxin (TTX)-sensitive Na_V1s are recruited at higher intensity of distention. This is the first demonstration that Na_V1s underlying action potential initiation in visceral C-fibers depend on the intensity of the stimulus.

Pathophysiological roles and therapeutic potential of voltage-gated ion channels (VGICs) in pain associated with herpesvirus infection

Herpesvirus is ranked as one of the grand old members of all pathogens. Of all the viruses in the superfamily, Herpes simplex virus type 1 (HSV-1) is considered as a model virus for a variety of reasons. In a permissive non-neuronal cell culture, HSV-1 concludes the entire life cycle in approximately 18–20 h, encoding approximately 90 unique transcriptional units. In latency, the robust viral gene expression is suppressed in neurons by a group of noncoding RNA. Historically the lesions caused by the virus can date back to centuries ago. As a neurotropic pathogen, HSV-1 is associated with painful oral lesions, severe keratitis and lethal encephalitis. Transmission of pain signals is dependent on the generation and propagation of action potential in sensory neurons. T-type Ca²⁺ channels serve as a preamplifier of action potential generation. Voltage-gated Na⁺ channels are the main components for action potential production. This review summarizes not only the voltage-gated ion channels in neuropathic disorders but also provides the new insights into HSV-1 induced pain.

Nuclear Factor-kappaB Gates Nav1.7 Channels in DRG Neurons via Protein-Protein Interaction

It is well known that nuclear factor-kappaB (NF-κB) regulates neuronal structures and functions by nuclear transcription. Here, we showed that phospho-p65 (p-p65), an active form of NF-κB subunit, reversibly interacted with Na_v1.7 channels in the membrane of dorsal root ganglion (DRG) neurons of rats. The interaction increased Na_v1.7 currents by slowing inactivation of Na_v1.7 channels and facilitating their recovery from inactivation, which may increase the resting state of the channels ready for activation. In cultured DRG neurons TNF-α upregulated the membrane p-p65 and enhanced Na_v1.7 currents within 5 min but did not affect nuclear NF-κB within 40 min. This non-transcriptional effect on Na_v1.7 may underlie a rapid regulation of the sensibility of the somatosensory system. Both NF-κB and Na_v1.7 channels are critically implicated in many physiological functions and diseases. Our finding may shed new light on the investigation into the underlying mechanisms.

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NF-κB p-p65 interacts with Na_v1.7 in the membrane of DRG neurons

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The interaction is reversible, depending on the cytoplasmic p-p65 content

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Reducing cytoplasmic p-p65 rapidly attenuates the interaction and Na_v1.7 currents

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The rapid effect on Na_v1.7 channels is independent of p-p65 nuclear translocation

Airway Innervation and Plasticity in Asthma

Airway nerves represent a mechanistically and therapeutically important aspect that requires better highlighting in the context of diseases such as asthma. Altered structure and function (plasticity) of afferent and efferent airway innervation can contribute to airway diseases. We describe established anatomy, current understanding of how plasticity occurs, and contributions of plasticity to asthma, focusing on target-derived growth factors (neurotrophins). Perspectives toward novel treatment strategies and future research are provided.

Different role of TTX-sensitive voltage-gated sodium channel (NaV 1) subtypes in action potential initiation and conduction in vagal airway nociceptors

KEY POINTS

The action potential initiation in the nerve terminals and its subsequent conduction along the axons of afferent nerves are not necessarily dependent on the same voltage-gated sodium channel (Na_V 1) subunits. The action potential initiation in jugular C-fibres within airway tissues is not blocked by TTX; nonetheless, conduction of action potentials along the vagal axons of these nerves is often dependent on TTX-sensitive channels. This is not the case for nodose airway Aδ-fibres and C-fibres, where both action potential initiation and conduction is abolished by TTX or selective Na_V 1.7 blockers. The difference between the initiation of action potentials within the airways vs. conduction along the axons should be considered when developing Na_V 1 blocking drugs for topical application to the respiratory tract.

ABSTRACT

The action potential (AP) initiation in the nerve terminals and its subsequent AP conduction along the axons do not necessarily depend on the same subtypes of voltage-gated sodium channels (Na_V 1s). We evaluated the role of TTX-sensitive and TTX-resistant Na_V 1s in vagal afferent nociceptor nerves derived from jugular and nodose ganglia innervating the respiratory system. Single cell RT-PCR was performed on vagal afferent neurons retrogradely labelled from the guinea pig trachea. Almost all of the jugular neurons expressed the TTX-sensitive channel Na_V 1.7 along with TTX-resistant Na_V 1.8 and Na_V 1.9. Tracheal nodose neurons also expressed Na_V 1.7 but, less frequently, Na_V 1.8 and Na_V 1.9. Na_V 1.6 were expressed in ∼40% of the jugular and 25% of nodose tracheal neurons. Other Na_V 1 α subunits were only rarely expressed. Single fibre recordings were made from the vagal nodose and jugular nerve fibres innervating the trachea or lung in the isolated perfused vagally-innervated preparations that allowed for selective drug delivery to the nerve terminal compartment (AP initiation) or to the desheathed vagus nerve (AP conduction). AP initiation in jugular C-fibres was unaffected by TTX, although it was inhibited by Na_V 1.8 blocker (PF-01247324) and abolished by combination of TTX and PF-01247324. However, AP conduction in the majority of jugular C-fibres was abolished by TTX. By contrast, both AP initiation and conduction in nodose nociceptors was abolished by TTX or selective Na_V 1.7 blockers. Distinction between the effect of a drug with respect to inhibiting AP in the nerve terminals within the airways vs. at conduction sites along the vagus nerve is relevant to therapeutic strategies involving inhaled Na_V 1 blocking drugs.

Voltage-Gated Sodium Channels Regulating Action Potential Generation in Itch-, Nociceptive-, and Low-Threshold Mechanosensitive Cutaneous C-Fibers

We evaluated the effect of voltage-gated sodium channel 1 (NaV1) blockers in three nonoverlapping C-fiber subtypes in the mouse skin: chloroquine (CQ)-sensitive C-fibers with high mechanical thresholds-itch C-fibers; second, CQ-insensitive, capsaicin-sensitive C-fibers with high mechanical thresholds-nociceptors; and CQ and capsaicin-insensitive C-fibers with a very low mechanical threshold-C-LTMs. NaV1-blocking drugs were applied to the nerve terminal receptive fields using an innervated isolated dorsal mouse skin-nerve preparation where the drugs are delivered into the skin intra-arterially. We combined these studies with an analysis of the mRNA expression of the α-subunits of NaV1 in individual dorsal root ganglia neurons labeled from the same region of the skin. Our results show that virtually all nociceptors and itch C-fibers expressed the tetrodotoxin (TTX)-resistant channels NaV1.8 and NaV1.9. However, TTX applied selectively into the skin abolished the action potential firing in response to mechanical stimulation in 75% of the itch C-fibers, 100% of the nociceptors, and 100% of C-LTMs. NaV1.7 was the most commonly expressed TTX-sensitive NaV1 in all three C-fiber subtypes innervating the dorsal skin. Selectively blocking NaV1.7 abolished responses in about 40% of itch C-fibers, 65% of nociceptors, but only 20% of C-LTMs. Blocking NaV1.8 alone had no affect on the firing sensitivity of the C-fibers. However, in itch and nociceptive C-fibers where the activation was not inhibited with a NaV1.7 blocker, adding the NaV1.8 blocker silenced action potential discharge.

Ozone-induced eosinophil recruitment to airways is altered by antigen sensitization and tumor necrosis factor-α blockade

Ozone is an atmospheric pollutant that causes lung inflammation and airway hyperresponsiveness. Ozone's effects occur in two distinct phases that are mediated by different populations of eosinophils. In the acute phase 1 day after exposure, mature airway-resident eosinophils alter parasympathetic nerve function that results in airway hyperresponsiveness. At this time point, the severity of hyperresponsiveness correlates with the number of eosinophils in close proximity to airway nerves, but not with eosinophils in bronchoalveolar lavage. Three days later, newly divided eosinophils are recruited to airways by a tumor necrosis factor-α-dependent mechanism. These new eosinophils paradoxically attenuate ozone-induced airway hyperresponsiveness. Ozone's effects on airway tissue eosinophils and nerve-associated eosinophils 3 days after exposure are unknown. Thus, we tested ozone's effects on eosinophils in airway subepithelium and around airway nerves 1 and 3 days after ozone in nonsensitized and ovalbumin-sensitized guinea pigs with or without the tumor necrosis factor-α antagonist, etanercept, and compared changes in eosinophils with ozone-induced airway hyperresponsiveness. More eosinophils were present in small, noncartilaginous airways and along small airway nerves compared to large cartilaginous airways in all treatment groups. The number of airway and nerve-associated eosinophils were unaffected 1 day after ozone exposure, whereas significantly fewer airway eosinophils were present 3 days later. Airway and nerve-associated eosinophils were also decreased in small airways 3 days after ozone in sensitized animals. These changes were blocked by etanercept. Airway eosinophils, but not nerve-associated or bronchoalveolar lavage eosinophils correlated with airway hyperresponsiveness 3 days after ozone. Our findings indicate ozone causes persistent alterations in airway eosinophils and reinforce the importance of characterizing eosinophils' effects within distinct airway compartments.

Newly divided eosinophils limit ozone-induced airway hyperreactivity in nonsensitized guinea pigs

Ozone causes vagally mediated airway hyperreactivity and recruits inflammatory cells, including eosinophils, to lungs, where they mediate ozone-induced hyperreactivity 1 day after exposure but are paradoxically protective 3 days later. We aimed to test the role of newly divided eosinophils in ozone-induced airway hyperreactivity in sensitized and nonsensitized guinea pigs. Nonsensitized and sensitized guinea pigs were treated with 5-bromo-2-deoxyuridine (BrdU) to label newly divided cells and were exposed to air or ozone for 4 h. Later (1 or 3 days later), vagally induced bronchoconstriction was measured, and inflammatory cells were harvested from bone marrow, blood, and bronchoalveolar lavage. Ozone induced eosinophil hematopoiesis. One day after ozone, mature eosinophils dominate the inflammatory response and potentiate vagally induced bronchoconstriction. However, by 3 days, newly divided eosinophils have reached the lungs, where they inhibit ozone-induced airway hyperreactivity because depleting them with antibody to IL-5 or a TNF-α antagonist worsened vagally induced bronchoconstriction. In sensitized guinea pigs, both ozone-induced eosinophil hematopoiesis and subsequent recruitment of newly divided eosinophils to lungs 3 days later failed to occur. Thus mature eosinophils dominated the ozone-induced inflammatory response in sensitized guinea pigs. Depleting these mature eosinophils prevented ozone-induced airway hyperreactivity in sensitized animals. Ozone induces eosinophil hematopoiesis and recruitment to lungs, where 3 days later, newly divided eosinophils attenuate vagally mediated hyperreactivity. Ozone-induced hematopoiesis of beneficial eosinophils is blocked by a TNF-α antagonist or by prior sensitization. In these animals, mature eosinophils are associated with hyperreactivity. Thus interventions targeting eosinophils, although beneficial in atopic individuals, may delay resolution of airway hyperreactivity in nonatopic individuals.