Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

Effects of transient, persistent, and resurgent sodium currents on excitability and spike regularity in vestibular ganglion neurons

Vestibular afferent neurons occur as two populations, regular and irregular, that provide distinct information about head motions. Differences in spike timing regularity are correlated with the different sensory responses important for vestibular processing. Relative to irregular afferents, regular afferents have more sustained firing patterns in response to depolarizing current steps, are more excitable, and have different complements of ion channels. Models of vestibular regularity and excitability emphasize the influence of increased expression of low-voltage-activated potassium currents in irregular neurons. We investigated the potential impact of different modes of voltage-gated sodium (NaV) current (transient, persistent, and resurgent) in cell bodies from vestibular ganglion neurons (VGNs), dissociated and cultured overnight. We hypothesized that regular VGNs would show the greatest impact of persistent (non-inactivating) NaV currents and of resurgent NaV currents, which flow when NaV channels are blocked and then unblocked. Whole-cell patch clamp experiments showed that much of the NaV current modes is carried by NaV1.6 channels. With simulations, we detected little substantial effect in any model VGN of persistent or resurgent modes on regularity of spike timing driven by postsynaptic current trains. For simulated irregular neurons, we also saw little effect on spike rate or firing pattern. For simulated regular VGNs, adding resurgent current changed the detailed timing of spikes during a current step, while the small persistent conductance (less than10% of transient NaV conductance density) strongly depolarized resting potential, altered spike waveform, and increased spike rate. These results suggest that persistent and resurgent NaV current can have a greater effect on the regular VGNs than on irregular VGNs, where low-voltage-activated K conductances dominate.

Recent advances for using human induced-pluripotent stem cells as pain-in-a-dish models of neuropathic pain

Neuropathic pain is amongst the most common non-communicable disorders and the poor effectiveness of current treatment is an unmet need. Although pain is a universal experience, there are significant inter-individual phenotypic differences. Developing models that can accurately recapitulate the clinical pain features is crucial to better understand underlying pathophysiological mechanisms and find innovative treatments. Current data from heterologous expression systems that investigate properties of specific molecules involved in pain signaling, and from animal models, show limited success with their translation into the development of novel treatments for pain. This is in part because they do not recapitulate the native environment in which a particular molecule functions, and due to species-specific differences in the properties of several key molecules that are involved in pain signaling. The limited availability of post-mortem tissue, in particular dorsal root ganglia (DRG), has hampered research using human cells in pre-clinical studies. Human induced-pluripotent stem cells (iPSCs) have emerged as an exciting alternative platform to study patient-specific diseases. Sensory neurons that are derived from iPSCs (iPSC-SNs) have provided new avenues towards elucidating peripheral pathophysiological mechanisms, the potential for development of personalized treatments, and as a cell-based system for high-throughput screening for discovering novel analgesics. Nevertheless, reprogramming and differentiation protocols to obtain nociceptors have mostly yielded immature homogenous cell populations that do not recapitulate the heterogeneity of native sensory neurons. To close the gap between native human tissue and iPSCs, alternative strategies have been developed. We will review here recent developments in differentiating iPSC-SNs and their use in pre-clinical translational studies. Direct conversion of stem cells into the cells of interest has provided a more cost- and time-saving method to improve reproducibility and diversity of sensory cell types. Furthermore, multi-cellular strategies that mimic in vivo microenvironments for cell maturation, by improving cell contact and communication (co-cultures), reproducing the organ complexity and architecture (three-dimensional organoid), and providing iPSCs with the full spatiotemporal context and nutrients needed for acquiring a mature phenotype (xenotransplantation), have led to functional sensory neuron-like systems. Finally, this review touches on novel prospective strategies, including fluorescent-tracking to select the differentiated neurons of relevance, and dynamic clamp, an electrophysiological method that allows direct manipulation of ionic conductances that are missing in iPSC-SNs.

A Novel Spider Toxin Inhibits Fast Inactivation of the Nav1.9 Channel by Binding to Domain III and Domain IV Voltage Sensors

Venomous animals have evolved to produce peptide toxins that modulate the activity of voltage-gated sodium (Na_v) channels. These specific modulators are powerful probes for investigating the structural and functional features of Na_v channels. Here, we report the isolation and characterization of δ-theraphotoxin-Gr4b (Gr4b), a novel peptide toxin from the venom of the spider Grammostola rosea. Gr4b contains 37-amino acid residues with six cysteines forming three disulfide bonds. Patch-clamp analysis confirmed that Gr4b markedly slows the fast inactivation of Na_v1.9 and inhibits the currents of Na_v1.4 and Na_v1.7, but does not affect Na_v1.8. It was also found that Gr4b significantly shifts the steady-state activation and inactivation curves of Na_v1.9 to the depolarization direction and increases the window current, which is consistent with the change in the ramp current. Furthermore, analysis of Na_v1.9/Na_v1.8 chimeric channels revealed that Gr4b preferentially binds to the voltage-sensor of domain III (DIII VSD) and has additional interactions with the DIV VSD. The site-directed mutagenesis analysis indicated that N1139 and L1143 in DIII S3-S4 linker participate in toxin binding. In sum, this study reports a novel spider peptide toxin that may slow the fast inactivation of Na_v1.9 by binding to the new neurotoxin receptor site-DIII VSD. Taken together, these findings provide insight into the functional role of the Na_v channel DIII VSD in fast inactivation and activation.

The widely used antihistamine mepyramine causes topical pain relief through direct blockade of nociceptor sodium channels

Mepyramine, a first-generation antihistamine targeting the histamine H(1) receptor, was extensively prescribed to patients suffering from allergic reactions and urticaria. Serious adverse effects, especially in case of overdose, were frequently reported, including drowsiness, impaired thinking, convulsion, and coma. Many of these side effects were associated with the blockade of histaminergic or cholinergic receptors. Here we show that mepyramine directly inhibits a variety of voltage-gated sodium channels, including the Tetrodotoxin-sensitive isoforms and the main isoforms (Nav1.7, Nav1.8, and Nav1.9) of nociceptors. Estimated IC₅₀ were within the range of drug concentrations detected in poisoned patients. Mepyramine inhibited sodium channels through fast- or slow-inactivated state preference depending on the isoform. Moreover, mepyramine inhibited the firing responses of C- and Aβ-type nerve fibers in ex vivo skin-nerve preparations. Locally applied mepyramine had analgesic effects on the scorpion toxin-induced excruciating pain and produced pain relief in acute, inflammatory, and chronic pain models. Collectively, these data provide evidence that mepyramine has the potential to be developed as a topical analgesic agent.

Epigenetic Changes Governing Scn5a Expression in Denervated Skeletal Muscle

The SCN5A gene encodes the α-subunit of the voltage-gated cardiac sodium channel (Na_V1.5), a key player in cardiac action potential depolarization. Genetic variants in protein-coding regions of the human SCN5A have been largely associated with inherited cardiac arrhythmias. Increasing evidence also suggests that aberrant expression of the SCN5A gene could increase susceptibility to arrhythmogenic diseases, but the mechanisms governing SCN5A expression are not yet well understood. To gain insights into the molecular basis of SCN5A gene regulation, we used rat gastrocnemius muscle four days following denervation, a process well known to stimulate Scn5a expression. Our results show that denervation of rat skeletal muscle induces the expression of the adult cardiac Scn5a isoform. RNA-seq experiments reveal that denervation leads to significant changes in the transcriptome, with Scn5a amongst the fifty top upregulated genes. Consistent with this increase in expression, ChIP-qPCR assays show enrichment of H3K27ac and H3K4me3 and binding of the transcription factor Gata4 near the Scn5a promoter region. Also, Gata4 mRNA levels are significantly induced upon denervation. Genome-wide analysis of H3K27ac by ChIP-seq suggest that a super enhancer recently described to regulate Scn5a in cardiac tissue is activated in response to denervation. Altogether, our experiments reveal that similar mechanisms regulate the expression of Scn5a in denervated muscle and cardiac tissue, suggesting a conserved pathway for SCN5A expression among striated muscles.

Carbamazepine conquers spinal GAP43 deficiency and sciatic Nav1.5 upregulation in diabetic mice: novel mechanisms in alleviating allodynia and hyperalgesia

This work tested the role of carbamazepine in alleviating alloxan-induced diabetic neuropathy and the enhancement of spinal plasticity. Mice were randomized into four groups: normal, control, carbamazepine (25-mg/kg) and carbamazepine (50-mg/kg). Nine weeks after induction of diabetes, symptoms of neuropathy were confirmed and carbamazepine (or vehicle) was given every other day for five weeks. After completing the treatment period, mice were sacrificed and the pathologic features in the spinal cord and the sciatic nerves were determined. The spinal cords were evaluated for synaptic plasticity (growth associated protein-43, GAP43), microglia cell expression (by CD11b) and astrocyte expression (glial fibrillary acidic protein, GFAP). Further, sciatic nerve expression of Nav1.5 was measured. Results revealed that carbamazepine 50 mg/kg prolonged the withdrawal threshold of von-Frey filaments and increased the hot plate jumping time. Carbamazepine improved the histopathologic pictures of the sciatic nerves and spinal cords. Spinal cord of carbamazepine-treated groups had enhanced expression of GAP43 but lower content of CD11b and GFAP. Furthermore, specimens from the sciatic nerve indicated low expression of Nav1.5. In conclusion, this work provided evidence, for the first time, that the preventive effect of carbamazepine against diabetic neuropathy involves correction of spinal neuronal plasticity and glia cell expression.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Regional and Developmental Differences in Na+ Currents in Vestibular Primary Afferent Neurons

The vestibular system relays information about head position via afferent nerve fibers to the brain in the form of action potentials. Voltage-gated Na⁺ channels in vestibular afferents drive the initiation and propagation of action potentials, but their expression during postnatal development and their contributions to firing in diverse mature afferent populations are unknown. Electrophysiological techniques were used to determine Na⁺ channel subunit types in vestibular calyx-bearing afferents at different stages of postnatal development. We used whole cell patch clamp recordings in thin slices of gerbil crista neuroepithelium to investigate Na⁺ channels and firing patterns in central zone (CZ) and peripheral zone (PZ) afferents. PZ afferents are exclusively dimorphic, innervating type I and type II hair cells, whereas CZ afferents can form dimorphs or calyx-only terminals which innervate type I hair cells alone. All afferents expressed tetrodotoxin (TTX)-sensitive Na⁺ currents, but TTX-sensitivity varied with age. During the fourth postnatal week, 200–300 nM TTX completely blocked sodium currents in PZ and CZ calyces. By contrast, in immature calyces [postnatal day (P) 5–11], a small component of peak sodium current remained in 200 nM TTX. Application of 1 μM TTX, or Jingzhaotoxin-III plus 200 nM TTX, abolished sodium current in immature calyces, suggesting the transient expression of voltage-gated sodium channel 1.5 (Nav1.5) during development. A similar TTX-insensitive current was found in early postnatal crista hair cells (P5–9) and constituted approximately one third of the total sodium current. The Nav1.6 channel blocker, 4,9-anhydrotetrodotoxin, reduced a component of sodium current in immature and mature calyces. At 100 nM 4,9-anhydrotetrodotoxin, peak sodium current was reduced on average by 20% in P5–14 calyces, by 37% in mature dimorphic PZ calyces, but by less than 15% in mature CZ calyx-only terminals. In mature PZ calyces, action potentials became shorter and broader in the presence of 4,9-anhydrotetrodotoxin implicating a role for Nav1.6 channels in firing in dimorphic afferents.

Development and validation of an in vitro model system to study peripheral sensory neuron development and injury

The ability to discriminate between diverse types of sensation is mediated by heterogeneous populations of peripheral sensory neurons. Human peripheral sensory neurons are inaccessible for research and efforts to study their development and disease have been hampered by the availability of relevant model systems. The in vitro differentiation of peripheral sensory neurons from human embryonic stem cells therefore provides an attractive alternative since an unlimited source of biological material can be generated for studies that specifically address development and injury. The work presented in this study describes the derivation of peripheral sensory neurons from human embryonic stem cells using small molecule inhibitors. The differentiated neurons express canonical- and modality-specific peripheral sensory neuron markers with subsets exhibiting functional properties of human nociceptive neurons that include tetrodotoxin-resistant sodium currents and repetitive action potentials. Moreover, the derived cells associate with human donor Schwann cells and can be used as a model system to investigate the molecular mechanisms underlying neuronal death following peripheral nerve injury. The quick and efficient derivation of genetically diverse peripheral sensory neurons from human embryonic stem cells offers unlimited access to these specialised cell types and provides an invaluable in vitro model system for future studies.

Downregulation of adult and neonatal Nav1.5 in the dorsal root ganglia and axon of peripheral sensory neurons of rats with spared nerve injury

Previous studies demonstrated that Nav1.5 splice variants, including Nav1.5a and Nav1.5c, were expressed in dorsal root ganglia (DRG) neurons. However, since nine Nav1.5 isoforms have been identified, whether other Nav1.5 splice variants, especially the neonatal Nav1.5 splice variant, express in the DRG neurons is still unknown. In this study, we systematically investigated the expression of adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rats with spared nerve injury (SNI) by RT-PCR, DNA sequencing, restriction enzyme digestion, immunohistochemistry and immunofluorescence methods. The results demonstrated that both adult and neonatal Nav1.5 isoforms were expressed in the DRG neurons, but their expression ratio was ~2.5:1. In SNI rat models, the expression of both adult and neonatal Nav1.5 decreased by approximately a half in both mRNA and protein levels. In contrast, the expression of protein kinase C (PKC)-γ, one of the negative modulators for sodium currents, increased by ~1-fold. Taken together, this study first confirmed the expression of both adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rat, but their expression level decreased in pain models. The upregulation of PKC-γ may directly or indirectly downregulate the expression of Nav1.5 isoforms in SNI rat models, which may further involve in the pathological process of neuropathic pain.

Multiple Nav1.5 isoforms are functionally expressed in the brain and present distinct expression patterns compared with cardiac Nav1.5

It has previously been demonstrated that there are various voltage gated sodium channel (Nav) 1.5 splice variants expressed in brain tissue. A total of nine Nav1.5 isoforms have been identified, however, the potential presence of further Nav1.5 variants expressed in brain neurons remains to be elucidated. The present study systematically investigated the expression of various Nav1.5 splice variants and their associated electrophysiological properties in the rat brain tissue, via biochemical analyses and whole-cell patch clamp recording. The results demonstrated that adult Nav1.5 was expressed in the rat, in addition to the neonatal Nav1.5, Nav1.5a and Nav1.5f isoforms. Further studies indicated that the expression level ratio of neonatal Nav1.5 compared with adult Nav1.5 decreased from 1:1 to 1:3 with age development from postnatal (P) day 0 to 90. This differed from the ratios observed in the developing rat hearts, in which the expression level ratio decreased from 1:4 to 1:19 from P0 to 90. The immunohistochemistry results revealed that Nav1.5 immunoreactivity was predominantly observed in neuronal cell bodies and processes, whereas decreased immunoreactivity was detected in the glial components. Electrophysiological analysis of Nav1.5 in the rat brain slices revealed that an Na current was detected in the presence of 300 nM tetrodotoxin (TTX), however this was inhibited by ~1 µM TTX. The TTX-resistant Na current was activated at −40 mV and reached the maximum amplitude at 0 mV. The results of the present study demonstrated that neonatal and adult Nav1.5 were expressed in the rat brain and electrophysiological analysis further confirmed the functional expression of Nav1.5 in brain neurons.

Sodium Channel Nav1.8 Underlies TTX-Resistant Axonal Action Potential Conduction in Somatosensory C-Fibers of Distal Cutaneous Nerves

Voltage-gated sodium (Na_V) channels are responsible for the initiation and conduction of action potentials within primary afferents. The nine Na_V channel isoforms recognized in mammals are often functionally divided into tetrodotoxin (TTX)-sensitive (TTX-s) channels (Na_V1.1-Na_V1.4, Na_V1.6-Na_V1.7) that are blocked by nanomolar concentrations and TTX-resistant (TTX-r) channels (Na_V1.8 and Na_V1.9) inhibited by millimolar concentrations, with Na_V1.5 having an intermediate toxin sensitivity. For small-diameter primary afferent neurons, it is unclear to what extent different Na_V channel isoforms are distributed along the peripheral and central branches of their bifurcated axons. To determine the relative contribution of TTX-s and TTX-r channels to action potential conduction in different axonal compartments, we investigated the effects of TTX on C-fiber-mediated compound action potentials (C-CAPs) of proximal and distal peripheral nerve segments and dorsal roots from mice and pigtail monkeys (Macaca nemestrina). In the dorsal roots and proximal peripheral nerves of mice and nonhuman primates, TTX reduced the C-CAP amplitude to 16% of the baseline. In contrast, >30% of the C-CAP was resistant to TTX in distal peripheral branches of monkeys and WT and Na_V1.9^-/- mice. In nerves from Na_V1.8^-/- mice, TTX-r C-CAPs could not be detected. These data indicate that Na_V1.8 is the primary isoform underlying TTX-r conduction in distal axons of somatosensory C-fibers. Furthermore, there is a differential spatial distribution of Na_V1.8 within C-fiber axons, being functionally more prominent in the most distal axons and terminal regions. The enrichment of Na_V1.8 in distal axons may provide a useful target in the treatment of pain of peripheral origin.SIGNIFICANCE STATEMENT It is unclear whether individual sodium channel isoforms exert differential roles in action potential conduction along the axonal membrane of nociceptive, unmyelinated peripheral nerve fibers, but clarifying the role of sodium channel subtypes in different axonal segments may be useful for the development of novel analgesic strategies. Here, we provide evidence from mice and nonhuman primates that a substantial portion of the C-fiber compound action potential in distal peripheral nerves, but not proximal nerves or dorsal roots, is resistant to tetrodotoxin and that, in mice, this effect is mediated solely by voltage-gated sodium channel 1.8 (Na_V1.8). The functional prominence of Na_V1.8 within the axonal compartment immediately proximal to its termination may affect strategies targeting pain of peripheral origin.

Visceral and somatic pain modalities reveal NaV 1.7-independent visceral nociceptive pathways

KEY POINTS

Voltage-gated sodium channels play a fundamental role in determining neuronal excitability. Specifically, voltage-gated sodium channel subtype Na_V 1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for Na_V 1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality-specific manner and help to direct drug discovery efforts towards novel visceral analgesics.

ABSTRACT

Voltage-gated sodium channel Na_V 1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of Na_V 1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor-specific Na_V 1.7 knockout mouse (Na_V 1.7^Nav1.8 ) and selective small-molecule Na_V 1.7 antagonist PF-5198007. Na_V 1.7^Nav1.8 mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both Na_V 1.7^Nav1.8 and littermate controls. Loss, or blockade, of Na_V 1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve-gut preparations in mouse, or following antagonism of Na_V 1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage-gated sodium channel α subunits revealed Na_V 1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of Na_V 1.7 (in Na_V 1.8-expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective Na_V 1.7 antagonist PF-5198007. Our data demonstrate that Na_V 1.7 (in Na_V 1.8-expressing neurons) contributes to defined pain pathways in a modality-dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of Na_V 1.7 alone in the viscera may be insufficient in targeting chronic visceral pain.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Unusual Voltage-Gated Sodium Currents as Targets for Pain

Pain is a serious health problem that impacts the lives of many individuals. Hyperexcitability of peripheral sensory neurons contributes to both acute and chronic pain syndromes. Because voltage-gated sodium currents are crucial to the transmission of electrical signals in peripheral sensory neurons, the channels that underlie these currents are attractive targets for pain therapeutics. Sodium currents and channels in peripheral sensory neurons are complex. Multiple-channel isoforms contribute to the macroscopic currents in nociceptive sensory neurons. These different isoforms exhibit substantial variations in their kinetics and pharmacology. Furthermore, sodium current complexity is enhanced by an array of interacting proteins that can substantially modify the properties of voltage-gated sodium channels. Resurgent sodium currents, atypical currents that can enhance recovery from inactivation and neuronal firing, are increasingly being recognized as playing potentially important roles in sensory neuron hyperexcitability and pain sensations. Here we discuss unusual sodium channels and currents that have been identified in nociceptive sensory neurons, describe what is known about the molecular determinants of the complex sodium currents in these neurons. Finally, we provide an overview of therapeutic strategies to target voltage-gated sodium currents in nociceptive neurons.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Pattern of Functional TTX-Resistant Sodium Channels Reveals a Developmental Stage of Human iPSC- and ESC-Derived Nociceptors

Human pluripotent stem cells (hPSCs) offer the opportunity to generate neuronal cells, including nociceptors. Using a chemical-based approach, we generated nociceptive sensory neurons from HUES6 embryonic stem cells and retrovirally reprogrammed induced hPSCs derived from fibroblasts. The nociceptive neurons expressed respective markers and showed tetrodotoxin-sensitive (TTXs) and -resistant (TTXr) voltage-gated sodium currents in patch-clamp experiments. In contrast to their counterparts from rodent dorsal root ganglia, TTXr currents of hPSC-derived nociceptors unexpectedly displayed a significantly more hyperpolarized voltage dependence of activation and fast inactivation. This apparent discrepancy is most likely due to a substantial expression of the developmentally important sodium channel NAV1.5. In view of the obstacles to recapitulate neuropathic pain in animal models, our data advance hPSC-derived nociceptors as a better model to study developmental and pathogenetic processes in human nociceptive neurons and to develop more specific small molecules to attenuate pain.

Modeling pain in vitro using nociceptor neurons reprogrammed from fibroblasts

Reprogramming somatic cells from one cell fate to another can generate specific neurons suitable for disease modeling. To maximize the utility of patient-derived neurons, they must model not only disease-relevant cell classes, but also the diversity of neuronal subtypes found in vivo and the pathophysiological changes that underlie specific clinical diseases. We identified five transcription factors that reprogram mouse and human fibroblasts into noxious stimulus-detecting (nociceptor) neurons. These recapitulated the expression of quintessential nociceptor-specific functional receptors and channels found in adult mouse nociceptor neurons, as well as native subtype diversity. Moreover, the derived nociceptor neurons exhibited TrpV1 sensitization to the inflammatory mediator prostaglandin E2 and the chemotherapeutic drug oxaliplatin, modeling the inherent mechanisms underlying inflammatory pain hypersensitivity and painful chemotherapy-induced neuropathy. Using fibroblasts from patients with familial dysautonomia (hereditary sensory and autonomic neuropathy type III), we found that the technique was able to reveal previously unknown aspects of human disease phenotypes in vitro.

Acid modulation of tetrodotoxin-resistant Na⁺ channels in rat nociceptive neurons

Under pathological conditions including inflammation, ischemia and incision, extracellular pH falls down as low as 5.4. Although some mediators play pivotal roles in the development and maintenance of inflammatory hyperalgesia by affecting the functional properties of tetrodotoxin-resistant (TTX-R) Na(+) channels, the roles of tissue acidosis in nociceptive transmission mediated by TTX-R Na(+) channels are largely unknown. In the present study, we have investigated the effect of acidic pH on TTX-R Na(+) currents (I(Na)) in small-sized sensory neurons isolated from rat trigeminal ganglia using a whole-cell patch clamp technique. Acidic pH decreased the peak amplitude of TTX-R I(Na) in a pH-dependent manner, but weak acid (≥pH 6.0) had a minor inhibitory effect on the TTX-R I(Na). Acidic pH also significantly shifted both the activation and steady-state fast inactivation relationships toward depolarized potentials. In addition, acidic pH had little effect on the use-dependent inhibition, and significantly retarded the development of inactivation and accelerated the recovery from inactivation of TTX-R Na(+) channels. The results suggest that weak acid (≥pH 6.0) makes TTX-R Na(+) channels to be suitable for the repetitive activation at depolarized membrane potentials. Given that both tissue acidosis and inflammatory mediators in inflamed or injured tissues act synergistically to promote nociceptive transmission by affecting the functional properties of TTX-R Na(+) channels, these channels would be, at least in part, a good target to treat inflammatory pain.

Specialized functions of Nav1.5 and Nav1.9 channels in electrogenesis of myenteric neurons in intact mouse ganglia

Voltage-gated sodium (Nav) channels play a central role in gastrointestinal physiology because they transmit depolarizing impulses in enteric neurons, thereby enabling the coordination of intestinal motility. However, little is known about the ion channel machinery that specifies firing pattern of enteric neurons. Here, we used in situ patch-clamp recording of myenteric neurons from mice to define functionally the Nav channel subtypes responsible for the electrical signature of myenteric neurons. We found that mouse myenteric neurons exhibit two types of tetrodotoxin-resistant Na(+) currents: an early inactivating Na(+) current (INaT) and a persistent Na(+) current (INaP). INaT was encountered in all myenteric neurons, whereas INaP was preferentially found in Dogiel type II sensory neurons. Knock-out mouse studies, in combination with pharmacological assays, indicate that INaT is carried by the Scn5a-encoded "cardiac" Nav1.5, whereas INaP is attributed to the Scn11a-encoded Nav1.9. Current-clamp experiments show that Nav1.9 flows at subthreshold voltages, generating tonic firing. In addition, action potential (AP) clamp reveals that Nav1.5 contributes to the upstroke velocity of APs, whereas Nav1.9, which remains active during the falling phase, opposes AP repolarization. We developed a computational model of a Dogiel type II myenteric neuron that successfully reproduces all experimentally observed phenomena and highlights the differential roles of Nav1.5 and Nav1.9 in the control of excitability. Our data illustrate how excitability can be finely tuned to provide specific firing templates by the selective deployment of Nav1.5 and Nav1.9 isoforms. We propose that Nav-dependent ENS disorders of excitability may play important roles in the pathogenesis of digestive diseases.

Membrane properties and electrogenesis in the distal axons of small dorsal root ganglion neurons in vitro

Although it is generally thought that sensory transduction occurs at or close to peripheral nerve endings, with action potentials subsequently propagating along the axons of dorsal root ganglia (DRG) neurons toward the central nervous system, the small diameter of nociceptive axons and their endings have made it difficult to estimate their membrane properties and electrogenic characteristics. Even the resting potentials of nociceptive axons are unknown. In this study, we developed the capability to record directly with patch-clamp electrodes from the small-diameter distal axons of DRG neurons in vitro. We showed using current-clamp recordings that 1) these sensory axons have a resting potential of −60.2 ± 1 mV; 2) both tetrodotoxin (TTX)-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ channels are present and available for activation at resting potential, at densities that can support action potential electrogenesis in these axons; 3) TTX-sensitive channels contribute to the amplification of small depolarizations that are subthreshold with respect to the action potential in these axons; 4) TTX-R channels can support the production of action potentials in these axons; and 5) these TTX-R channels can produce repetitive firing, even at depolarized membrane potentials where TTX-S channels are inactivated. Finally, using voltage-clamp recordings with an action potential as the command, we confirmed the presence of both TTX-S and TTX-R channels, which are activated sequentially during action potential in these axons. These results provide direct evidence for the presence of TTX-S and TTX-R Na+ channels that are functionally available at resting potential and contribute to electrogenesis in small-diameter afferent axons.

Inhibition of voltage-gated Na(+) currents in sensory neurones by the sea anemone toxin APETx2

BACKGROUND AND PURPOSE

APETx2, a toxin from the sea anemone Anthropleura elegantissima, inhibits acid-sensing ion channel 3 (ASIC3)-containing homo- and heterotrimeric channels with IC(50) values < 100 nM and 0.1-2 µM respectively. ASIC3 channels mediate acute acid-induced and inflammatory pain response and APETx2 has been used as a selective pharmacological tool in animal studies. Toxins from sea anemones also modulate voltage-gated Na(+) channel (Na(v) ) function. Here we tested the effects of APETx2 on Na(v) function in sensory neurones.

EXPERIMENTAL APPROACH

Effects of APETx2 on Na(v) function were studied in rat dorsal root ganglion (DRG) neurones by whole-cell patch clamp.

KEY RESULTS

APETx2 inhibited the tetrodotoxin (TTX)-resistant Na(v) 1.8 currents of DRG neurones (IC(50) , 2.6 µM). TTX-sensitive currents were less inhibited. The inhibition of Na(v) 1.8 currents was due to a rightward shift in the voltage dependence of activation and a reduction of the maximal macroscopic conductance. The inhibition of Na(v) 1.8 currents by APETx2 was confirmed with cloned channels expressed in Xenopus oocytes. In current-clamp experiments in DRG neurones, the number of action potentials induced by injection of a current ramp was reduced by APETx2.

CONCLUSIONS AND IMPLICATIONS

APETx2 inhibited Na(v) 1.8 channels, in addition to ASIC3 channels, at concentrations used in in vivo studies. The limited specificity of this toxin should be taken into account when using APETx2 as a pharmacological tool. Its dual action will be an advantage for the use of APETx2 or its derivatives as analgesic drugs.

Whole-cell recording in isolated primary sensory neurons

The isolated sensory neuron in vitro is a powerful model with which to address a number of important neurobiological questions. Isolated neurons are relatively easy to prepare from both neonatal and adult animals and can be studied both acutely and after considerable time on culture. Intracellular recording is one of the most powerful ways to study these neurons. Methods are described for both the preparation of isolated sensory neurons in vitro as well as for recording major classes of ionic currents (Na(+), K(+), and Ca(2+)) from these neurons with whole cell voltage-clamp techniques. Methods are also provided for an initial characterization of active and passive electrophysiological properties of these neurons in current clamp as well as the use of perforated patch recording as a means to mitigate some of the limitations associated with conventional whole cell patch recording. The reader should be aware that the regulation of ion channels in sensory neurons may very subtle, requiring considerably more sophisticated protocols than have been provided here. The reader should also be aware that there is a tremendous heterogeneity among sensory neurons, which is both a curse and a blessing for those who wish to study them. Thus, the methods provided here should only be considered the starting point for a more detailed analysis of sensory neurons.

Exploring the obscure profiles of pharmacological binding sites on voltage-gated sodium channels by BmK neurotoxins

Diverse subtypes of voltage-gated sodium channels (VGSCs) have been found throughout tissues of the brain, muscles and the heart. Neurotoxins extracted from the venom of the Asian scorpion Buthus martensi Karsch (BmK) act as sodium channel-specific modulators and have therefore been widely used to study VGSCs. α-type neurotoxins, named BmK I, BmK αIV and BmK abT, bind to receptor site-3 on VGSCs and can strongly prolong the inactivation phase of VGSCs. In contrast, β-type neurotoxins, named BmK AS, BmK AS-1, BmK IT and BmK IT2, occupy receptor site-4 on VGSCs and can suppress peak currents and hyperpolarize the activation kinetics of sodium channels. Accumulating evidence from binding assays of scorpion neurotoxins on VGSCs, however, indicate that pharmacological sensitivity of VGSC subtypes to different modulators is much more complex than that suggested by the simple α-type and β-type neurotoxin distinction. Exploring the mechanisms of possible dynamic interactions between site 3-/4-specific modulators and region- and/or species-specific subtypes of VGSCs would therefore greatly expand our understanding of the physiological and pharmacological properties of diverse VGSCs. In this review, we discuss the pharmacological and structural diversity of VGSCs as revealed by studies exploring the binding properties and cross-competitive binding of site 3- or site 4-specific modulators in VGSC subtypes in synaptosomes from distinct tissues of diverse species.

An in vivo tethered toxin approach for the cell-autonomous inactivation of voltage-gated sodium channel currents in nociceptors

Understanding information flow in sensory pathways requires cell-selective approaches to manipulate the activity of defined neurones. Primary afferent nociceptors, which detect painful stimuli, are enriched in specific voltage-gated sodium channel (VGSC) subtypes. Toxins derived from venomous animals can be used to dissect the contributions of particular ion currents to cell physiology. Here we have used a transgenic approach to target a membrane-tethered isoform of the conotoxin MrVIa (t-MrVIa) only to nociceptive neurones in mice. T-MrVIa transgenic mice show a 44 +/- 7% reduction of tetrodotoxin-resistant (TTX-R) VGSC current densities. This inhibition is permanent, reversible and does not result in functional upregulation of TTX-sensitive (TTX-S) VGSCs, voltage-gated calcium channels (VGCCs) or transient receptor potential (TRP) channels present in nociceptive neurones. As a consequence of the reduction of TTX-R VGSC currents, t-MrVIa transgenic mice display decreased inflammatory mechanical hypersensitivity, cold pain insensitivity and reduced firing of cutaneous C-fibres sensitive to noxious cold temperatures. These data validate the use of genetically encoded t-toxins as a powerful tool to manipulate VGSCs in specific cell types within the mammalian nervous system. This novel genetic methodology can be used for circuit mapping and has the key advantage that it enables the dissection of the contribution of specific ionic currents to neuronal function and to behaviour.

Serotonin upregulates low- and high-threshold tetrodotoxin-resistant sodium channels in the same subpopulation of rat nociceptors

The modulation by serotonin (5-HT) of low- and high-threshold tetrodotoxin- (TTX) resistant Na(+) currents was studied in small-diameter (approximately 25 microm) acutely-isolated rat dorsal root ganglion (DRG) cells. Each DRG cell included in the study was classified as type 2 or non-type 2, based on expression of a low-threshold A-type K(+) current. When cells of either type were recorded from using a CsF based internal solution and a holding potential (HP) of -80 mV, the apparent threshold for activation of TTX-resistant Na(+) currents ranged from -75 to -60 mV. A 500 ms prepulse to -60 mV greatly suppressed currents evoked by test potentials (TPs) to -75 through -35 mV. A similar scenario was observed when the CsF based internal solution was changed for one that contained CsCl, except that the apparent threshold of activation was shifted by about +25 mV, and currents evoked by TPs to -55 to -35 mV in the absence of a prepulse were much smaller than their counterparts observed with the CsF internal. These data suggest two types of TTX-resistant Na(+) currents; one with a low-threshold for activation that is enhanced by the presence of fluoride inside the cell and is inactivated by holding at -60 mV, and one with a higher threshold for activation that is not inactivated by holding at -60 mV. In type 2 DRG cells, 10 microM 5-HT upregulated low-threshold currents evoked by TPs to -55 to -35 mV from HP -80 mV, as well as high-threshold currents evoked by more depolarized TPs from HP -60 mV. However, when cells were held at -60 mV, 5-HT did not upregulate currents evoked by TPs to -35 or -30 mV, suggesting that the low-threshold current was nearly completely inactivated. Previous studies have suggested that type 2 DRG cells are nociceptor cell bodies. If 5-HT produces similar effects in type 2 DRG cell peripheral receptors, the upregulation of the low-threshold currents may serve to lower the threshold for nociception, while the upregulation of the high-threshold current may strengthen nociceptive signals.

Effects of ranolazine, a novel anti-anginal drug, on ion currents and membrane potential in pituitary tumor GH(3) cells and NG108-15 neuronal cells

Ranolazine, a piperazine derivative, is currently approved for the treatment of chronic angina. However, its ionic mechanisms in other types of cells remain unclear, although it is thought to be a selective blocker of late Na(+) current. This study was conducted to evaluate the possible effects of ranolazine on Na(+) current (I(Na)), L-type Ca(2+) current (I(Ca,L)), inwardly rectifying K(+) current (I(K(IR))), delayed-rectifier K(+) current (I(K(DR))), and Ca(2+)-activated K(+) current (I(K(Ca))) in pituitary tumor (GH(3)) cells. Ranolazine depressed the transient and late components of I(Na) with different potencies. This drug exerted an inhibitory effect on I(K(IR)) with an IC(50) value of 0.92 microM, while it slightly inhibited I(K(DR)) and I(K(Ca)). It shifted the steady-state activation curve of I(K(IR)) to more positive potentials with no change in the gating charge of the channel. Ranolazine (30 microM) also reduced the activity of large-conductance Ca(2+)-activated K(+) channels in HEK293T cells expressing alpha-hSlo. Under current-clamp conditions, low concentrations (e.g., 1 microM) of ranolazine increased the firing of action potentials, while at high concentrations (>or=10 microM), it diminished the firing discharge. The exposure to ranolazine also suppressed I(Na) and I(K(IR)) effectively in NG108-15 neuronal cells. Our study provides evidence that ranolazine could block multiple ion currents such as I(Na) and I(K(IR)) and suggests that these actions may contribute to some of the functional activities of neurons and endocrine or neuroendocrine cells in vivo.

Pyramidal cells of rodent presubiculum express a tetrodotoxin-insensitive Na+ current

Presubicular neurons are activated physiologically by a specific preferred head direction. Here we show that firing in these neurones is characterized by action potentials with a large overshoot and a reduced firing frequency adaptation during repetitive firing. We found that a component of the sodium current of presubicular cells was not abolished by tetrodotoxin (TTX, 10 mum) and was activated at more depolarized voltages than TTX-sensitive currents. This inward current was completely abolished by the removal of external sodium, suggesting that sodium is the charge carrier of this TTX-insensitive (TTX-I) current. The channels responsible for the TTX-I sodium current seemed to be expressed at sites distant from the soma, giving rise to a voltage-dependent delay in current activation. The voltage required for half-maximal activation was 21 mV, and 36 mV for inactivation, which is similar to that reported for Na(V)1.8 sodium channels. However, the kinetics were considerably slower, with a time constant of current decay of 1.4 s. The current was not abolished in pyramidal cells from animals lacking either the Na(V)1.8 or the Na(V)1.9 subunit. This, possibly novel, TTX-I sodium current could contribute to the coding functions of presubicular neurons, specifically the maintained firing associated with signalling of a stable head position.

Increased peripheral nerve excitability and local NaV1.8 mRNA up-regulation in painful neuropathy

BACKGROUND

Neuropathic pain caused by peripheral nerve injury is a chronic disorder that represents a significant clinical challenge because the pathological mechanisms have not been fully elucidated. Several studies have suggested the involvement of various sodium channels, including tetrodotoxin-resistant NaV1.8, in affected dorsal root ganglion (DRG) neurons. We have hypothesized that altered local expression of NaV1.8 in the peripheral axons of DRG neurons could facilitate nociceptive signal generation and propagation after neuropathic injury.

RESULTS

After unilateral sciatic nerve entrapment injury in rats, compound action potential amplitudes were increased in both myelinated and unmyelinated fibers of the ipsilateral sciatic nerve. Tetrodotoxin resistance of both fiber populations and sciatic nerve NaV1.8 immunoreactivity were also increased. Further analysis of NaV1.8 distribution revealed that immunoreactivity and mRNA levels were decreased and unaffected, respectively, in the ipsilateral L4 and L5 DRG; however sciatic nerve NaV1.8 mRNA showed nearly an 11-fold ipsilateral increase. Nav1.8 mRNA observed in the sciatic nerve was likely of axonal origin since it was not detected in non-neuronal cells cultured from nerve tissue. Absence of changes in NaV1.8 mRNA polyadenylation suggests that increased mRNA stability was not responsible for the selective peripheral mRNA increase. Furthermore, mRNA levels of NaV1.3, NaV1.5, NaV1.6, NaV1.7, and NaV1.9 were not significantly different between ipsilateral and contralateral nerves. We therefore propose that selective NaV1.8 mRNA axonal transport and local up-regulation could contribute to the hyperexcitability of peripheral nerves in some neuropathic pain states.

CONCLUSION

Cuff entrapment injury resulted in significantly elevated axonal excitability and increased NaV1.8 immunoreactivity in rat sciatic nerves. The concomitant axonal accumulation of NaV1.8 mRNA may play a role in the pathogenesis of this model of neuropathic pain.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Comparative study of the distribution of the alpha-subunits of voltage-gated sodium channels in normal and axotomized rat dorsal root ganglion neurons

We compared the distribution of the alpha-subunit mRNAs of voltage-gated sodium channels Nav1.1-1.3 and Nav1.6-1.9 and a related channel, Nax, in histochemically identified neuronal subpopulations of the rat dorsal root ganglia (DRG). In the naïve DRG, the expression of Nav1.1 and Nav1.6 was restricted to A-fiber neurons, and they were preferentially expressed by TrkC neurons, suggesting that proprioceptive neurons possess these channels. Nav1.7, -1.8, and -1.9 mRNAs were more abundant in C-fiber neurons compared with A-fiber ones. Nax was evenly expressed in both populations. Although Nav1.8 and -1.9 were preferentially expressed by TrkA neurons, other alpha-subunits were expressed independently of TrkA expression. Actually, all IB4(+) neurons expressed both Nav1.8 and -1.9, and relatively limited subpopulations of IB4(+) neurons (3% and 12%, respectively) expressed Nav1.1 and/or Nav1.6. These findings provide useful information in interpreting the electrophysiological characteristics of some neuronal subpopulations of naïve DRG. After L5 spinal nerve ligation, Nav1.3 mRNA was up-regulated mainly in A-fiber neurons in the ipsilateral L5 DRG. Although previous studies demonstrated that nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF) reversed this up-regulation, the Nav1.3 induction was independent of either TrkA or GFRalpha1 expression, suggesting that the induction of Nav1.3 may be one of the common responses of axotomized DRG neurons without a direct relationship to NGF/GDNF supply.

Involvement of tetrodotoxin-resistant Na+ current and protein kinase C in the action of growth hormone (GH)-releasing hormone on primary cultured somatotropes from GH-green fluorescent protein transgenic mice

GHRH depolarizes the membrane of somatotropes, leading to an increase in intracellular free Ca2+ concentration and GH secretion. Na+ channels mediate the rapid depolarization during the initial phase of the action potential, and this regulates Ca2+ influx and GH secretion. GHRH increases a tetrodotoxin-sensitive somatotrope Na+ current that is mediated by cAMP. TTX-resistant (TTX-R) Na+ channels are abundant in sensory neurons and cardiac myocytes, but their occurrence and/or function in somatotropes has not been investigated. Here we demonstrate expression of TTX-R Na+ channels and a TTX-R Na+ current, using patch-clamp method, in green fluorescent protein-GH transgenic mouse somatotropes. GHRH (100 nm) increased the TTX-R Na+ current in a reversible manner. The GHRH-induced increase in TTX-R Na+ current was not affected by the cAMP antagonist Rp-cAMP or protein kinase A inhibitors KT5720 or H89. The TTX-R current was increased by 8-bromoadenosine-cAMP (cAMP analog), forskolin (adenylyl-cyclase activator), and 3-isobutyl-1-methylxanthine (phosphodiesterase inhibitor), but the additional, GHRH-induced increase in TTX-R Na+ currents was not affected. U-73122 (phospholipase C inhibitor) and protein kinase C (PKC) inhibitors, Gö-6983 and chelerythrine, blocked the effect of GHRH. PKC activators, phorbol dibutyrate and phorbol myristate acetate, increased the TTX-R Na+ current, but GHRH had no further effect on the current. Na+-free extracellular medium significantly reduced GHRH-stimulated GH secretion. We conclude that GHRH-induced increase in the TTX-R Na+ current in mouse somatotropes is mediated by the PKC system. An increase in the TTX-R Na+ current may contribute to the GHRH-induced exocytosis of GH granules from mouse somatotropes.

Low concentrations of tetrodotoxin interact with tetrodotoxin-resistant voltage-gated sodium channels

BACKGROUND AND PURPOSE

Tetrodotoxin (TTX) is used to distinguish between two classes of voltage-gated sodium channel (VGSC)--TTX sensitive (TTXS) and TTX resistant (TTXR). The resistance of TTXR VGSCs is thought to result from a low binding affinity of TTX, although at high TTX concentrations channel block does occur. Here, we show that, at concentrations below those which produce block, TTX can bind to TTXR VGSCs.

EXPERIMENTAL APPROACH

Whole-cell voltage clamp recordings were made from dissociated rat dorsal root ganglion neurones that expressed both TTXS and TTXR sodium currents. Voltage-gated calcium currents were blocked by 10 microM extracellular lanthanum chloride. TTXS, but not TTXR, current was suppressed by using a holding potential of -50 mV, and the effect of TTX on the isolated TTXR current was explored.

KEY RESULTS

Extracellular application of 0.5 microM TTX produced a 40% increase in TTXR current amplitude, a negative shift in the voltage-dependence of current activation (approximately -8 mV) and inactivation (approximately -10 mV) and increased rates of current activation and inactivation. The effect of TTX on current amplitude was dose-dependent (EC50 = 364 nM). Removal of lanthanum prevented the effect of TTX on TTXR current amplitude, whereas reducing extracellular calcium did not.

CONCLUSIONS AND IMPLICATIONS

The findings are consistent with an interpretation that TTX relieves a tonic block of the TTXR VGSC by lanthanum. We conclude that TTX binds to the TTXR VGSC at low concentrations, without blocking it. This appears to be the first demonstration of a clear distinction between binding affinity and blocking potency of a channel-blocking agent.

Ion channels in mammalian vestibular afferents may set regularity of firing

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Properties of wild-type and fluorescent protein-tagged mouse tetrodotoxin-resistant sodium channel (Na V 1.8) heterologously expressed in rat sympathetic neurons

The tetrodotoxin (TTX)-resistant Na(+) current arising from Na(V)1.8-containing channels participates in nociceptive pathways but is difficult to functionally express in traditional heterologous systems. Here, we show that injection of cDNA encoding mouse Na(V)1.8 into the nuclei of rat superior cervical ganglion (SCG) neurons results in TTX-resistant Na(+) currents with amplitudes equal to or exceeding the currents arising from natively expressing channels of mouse dorsal root ganglion (DRG) neurons. The activation and inactivation properties of the heterologously expressed Na(V)1.8 Na(+) channels were similar but not identical to native TTX-resistant channels. Most notably, the half-activation potential of the heterologously expressed Na(V)1.8 channels was shifted about 10 mV toward more depolarized potentials. Fusion of fluorescent proteins to the N- or C-termini of Na(V)1.8 did not substantially affect functional expression in SCG neurons. Unexpectedly, fluorescence was not concentrated at the plasma membrane but found throughout the interior of the neuron in a granular pattern. A similar expression pattern was observed in nodose ganglion neurons expressing the tagged channels. In contrast, expression of tagged Na(V)1.8 in HeLa cells revealed a fluorescence pattern consistent with sequestration in the endoplasmic reticulum, thus providing a basis for poor functional expression in clonal cell lines. Our results establish SCG neurons as a favorable surrogate for the expression and study of molecularly defined Na(V)1.8-containing channels. The data also indicate that unidentified factors may be required for the efficient functional expression of Na(V)1.8 with a biophysical phenotype identical to that found in sensory neurons.

Voltage-gated sodium channels in nociceptive versus non-nociceptive nodose vagal sensory neurons innervating guinea pig lungs

Lung vagal sensory fibres are broadly categorized as C fibres (nociceptors) and A fibres (non-nociceptive; rapidly and slowly adapting low-threshold stretch receptors). These afferent fibre types differ in degree of myelination, conduction velocity, neuropeptide content, sensitivity to chemical and mechanical stimuli, as well as evoked reflex responses. Recent studies in nociceptive fibres of the somatosensory system indicated that the tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSC) are preferentially expressed in the nociceptive fibres of the somatosensory system (dorsal root ganglia). Whereas TTX-R sodium currents have been documented in lung vagal sensory nerves fibres, a rigorous comparison of their expression in nociceptive versus non-nociceptive vagal sensory neurons has not been carried out. Using multiple approaches including patch clamp electrophysiology, immunohistochemistry, and single-cell gene expression analysis in the guinea pig, we obtained data supporting the hypothesis that the TTX-R sodium currents are similarly distributed between nodose ganglion A-fibres and C-fibres innervating the lung. Moreover, mRNA and immunoreactivity for the TTX-R VGSC molecules Na(V)1.8 and Na(V)1.9 were present in nearly all neurons. We conclude that contrary to findings in the somatosensory neurons, TTX-R VGSCs are not preferentially expressed in the nociceptive C-fibre population innervating the lungs.

A selective Nav1.8 sodium channel blocker, A-803467 [5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide], attenuates spinal neuronal activity in neuropathic rats

We have recently reported that systemic delivery of A-803467 [5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide], a selective Na(v)1.8 sodium channel blocker, reduces behavioral measures of chronic pain. In the current study, the effects of A-803467 on evoked and spontaneous firing of wide dynamic range (WDR) neurons were measured in uninjured and rats with spinal nerve ligations (SNLs). Administration of A-803467 (10-30 mg/kg i.v.) reduced mechanically evoked (10-g von Frey hair) and spontaneous WDR neuronal activity in SNL rats. In uninjured rats, A-803467 (20 mg/kg i.v.) transiently reduced evoked but not spontaneous firing of WDR neurons. The systemic effects of A-803467 in SNL rats were not altered by spinal transection or by systemic pretreatment with the transient receptor potential vanilloid type 1 (TRPV1) receptor agonist, resiniferatoxin, at doses that impair the function of TRPV1-expressing fibers. To determine sites of action, A-803467 was administered into spinal tissue, into the uninjured L4 dorsal root ganglion (DRG), or into the neuronal receptive field. Injections of A-803467 into the L4 DRG (30-100 nmol/1 mul) or into the hindpaw receptive field (300 nmol/50 mul) reduced evoked but not spontaneous WDR firing. In contrast, intraspinal (50-150 nmol/0.5 mul) injection of A-803467 decreased both evoked and spontaneous discharges of WDR neurons. Thus, Na(v)1.8 sodium channels on the cell bodies/axons within the L4 DRG as well as on peripheral and central terminals of primary afferent neurons regulate the inflow of low-intensity mechanical signals to spinal WDR neurons. However, Na(v)1.8 sodium channels on central terminals seem to be key to the modulation of spontaneous firing in SNL rats.

The roles of sodium channels in nociception: Implications for mechanisms of pain

Understanding the role of voltage-gated sodium channels in nociception may provide important insights into pain mechanisms. Voltage-gated sodium channels are critically important for electrogenesis and nerve impulse conduction, and a target for important clinically relevant analgesics such as lidocaine. Furthermore, within the last decade studies have shown that certain sodium channel isoforms are predominantly expressed in peripheral sensory neurons associated with pain sensation, and that the expression and functional properties of voltage-gated sodium channels in peripheral sensory neurons can be dynamically regulated following axonal injury or peripheral inflammation. These data suggest that specific voltage-gated sodium channels may play crucial roles in nociception. Experiments with transgenic mice lines have clearly implicated Na(v)1.7, Na(v)1.8 and Na(v)1.9 in inflammatory, and possibly neuropathic, pain. However the most convincing and perhaps most exciting results regarding the role of voltage-gated sodium channels have come out recently from studies on human inherited disorders of nociception. Point mutations in Na(v)1.7 have been identified in patients with two distinct autosomal dominant severe chronic pain syndromes. Electrophysiological experiments indicate that these pain-associated mutations cause small yet significant changes in the gating properties of voltage-gated sodium channels that are likely to contribute substantially to the development of chronic pain. Equally exciting, recent studies indicate that recessive mutations in Na(v)1.7 that eliminate functional current can result in an apparent complete, and possibly specific, indifference to pain in humans, suggesting that isoform specific blockers could be very effective in treating pain. In this review we will examine what is known about the roles of voltage-gated sodium channels in nociception.

Lidocaine Promotes the Trafficking and Functional Expression of Nav1.8 Sodium Channels in Mammalian Cells

Nociceptive neurons of the dorsal root ganglion (DRG) express a combination of rapidly gating TTX-sensitive and slowly gating TTX-resistant Na currents, and the channels that produce these currents have been cloned. The Nav1.7 and Nav1.8 channels encode for the rapidly inactivating TTX-sensitive and slowly inactivating TTX-resistant Na currents, respectively. Although the Nav1.7 channel expresses well in cultured mammalian cell lines, attempts to express the Nav1.8 channel using similar approaches has been met with limited success. The inability to heterologously express Nav1.8 has hampered detailed characterization of the biophysical properties and pharmacology of these channels. In this study, we investigated the determinants of Nav1.8 expression in tsA201 cells, a transformed variant of HEK293 cells, using a combination of biochemistry, immunochemistry, and electrophysiology. Our data indicate that the unusually low expression levels of Nav1.8 in tsA201 cells results from a trafficking defect that traps the channel protein in the endoplasmic reticulum. Incubating the cultured cells with the local anesthetic lidocaine dramatically enhanced the cell surface expression of functional Nav1.8 channels. The biophysical properties of the heterologously expressed Nav1.8 channel are similar but not identical to those of the TTX-resistant Na current of native DRG neurons, recorded under similar conditions. Our data indicate that the lidocaine acts as a molecular chaperone that promotes efficient trafficking and increased cell surface expression of Nav1.8 channels.

The sodium channel Nav1.5a is the predominant isoform expressed in adult mouse dorsal root ganglia and exhibits distinct inactivation properties from the full-length Nav1.5 channel

Nav1.5 is the principal voltage-gated sodium channel expressed in heart, and is also expressed at lower abundance in embryonic dorsal root ganglia (DRG) with little or no expression reported postnatally. We report here the expression of Nav1.5 mRNA isoforms in adult mouse and rat DRG. The major isoform of mouse DRG is Nav1.5a, which encodes a protein with an IDII/III cytoplasmic loop reduced by 53 amino acids. Western blot analysis of adult mouse DRG membrane proteins confirmed the expression of Nav1.5 protein. The Na+ current produced by the Nav1.5a isoform has a voltage-dependent inactivation significantly shifted to more negative potentials (by approximately 5 mV) compared to the full-length Nav1.5 when expressed in the DRG neuroblastoma cell line ND7/23. These results imply that the alternatively spliced exon 18 of Nav1.5 plays a role in channel inactivation and that Nav1.5a is likely to make a significant contribution to adult DRG neuronal function.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

Differential Slow Inactivation and Use-Dependent Inhibition of Nav1.8 Channels Contribute to Distinct Firing Properties in IB4+ and IB4− DRG Neurons

Nociceptive dorsal root ganglion (DRG) neurons can be classified into nonpeptidergic IB4+ and peptidergic IB4− subtypes, which terminate in different layers in dorsal horn and transmit pain along different ascending pathways, and display different firing properties. Voltage-gated, tetrodotoxin-resistant (TTX-R) Nav1.8 channels are expressed in both IB4+ and IB4− cells and produce most of the current underlying the depolarizing phase of action potential (AP). Slow inactivation of TTX-R channels has been shown to regulate repetitive DRG neuron firing behavior. We show in this study that use-dependent reduction of Nav1.8 current in IB4+ neurons is significantly stronger than that in IB4− neurons, although voltage dependency of activation and steady-state inactivation are not different. The time constant for entry of Nav1.8 into slow inactivation in IB4+ neurons is significantly faster and more Nav1.8 enter the slow inactivation state than in IB4− neurons. In addition, recovery from slow inactivation of Nav1.8 in IB4+ neurons is slower than that in IB4− neurons. Using current-clamp recording, we demonstrate a significantly higher current threshold for generation of APs and a longer latency to onset of firing in IB4+, compared with those of IB4− neurons. In response to a ramp stimulus, IB4+ neurons produce fewer APs and display stronger adaptation, with a faster decline of AP peak than IB4− neurons. Our data suggest that differential use-dependent reduction of Nav1.8 current in these two DRG subpopulations, which results from their different rate of entry into and recovery from the slow inactivation state, contributes to functional differences between these two neuronal populations.

The voltage-gated sodium channel Na(v)1.9 is an effector of peripheral inflammatory pain hypersensitivity

We used a mouse with deletion of exons 4, 5, and 6 of the SCN11A (sodium channel, voltage-gated, type XI, alpha) gene that encodes the voltage-gated sodium channel Na(v)1.9 to assess its contribution to pain. Na(v)1.9 is present in nociceptor sensory neurons that express TRPV1, bradykinin B2, and purinergic P2X3 receptors. In Na(v)1.9-/- mice, the non-inactivating persistent tetrodotoxin-resistant sodium TTXr-Per current is absent, whereas TTXr-Slow is unchanged. TTXs currents are unaffected by the mutation of Na(v)1.9. Pain hypersensitivity elicited by intraplantar administration of prostaglandin E2, bradykinin, interleukin-1beta, capsaicin, and P2X3 and P2Y receptor agonists, but not NGF, is either reduced or absent in Na(v)1.9-/- mice, whereas basal thermal and mechanical pain sensitivity is unchanged. Thermal, but not mechanical, hypersensitivity produced by peripheral inflammation (intraplanatar complete Freund's adjuvant) is substantially diminished in the null allele mutant mice, whereas hypersensitivity in two neuropathic pain models is unchanged in the Na(v)1.9-/- mice. Na(v)1.9 is, we conclude, an effector of the hypersensitivity produced by multiple inflammatory mediators on nociceptor peripheral terminals and therefore plays a key role in mediating peripheral sensitization.

Developmental changes in two voltage-dependent sodium currents in utricular hair cells

Two kinds of sodium current (I(Na)) have been separately reported in hair cells of the immature rodent utricle, a vestibular organ. We show that rat utricular hair cells express one or the other current depending on age (between postnatal days 0 and 22, P0-P22), hair cell type (I, II, or immature), and epithelial zone (striola vs. extrastriola). The properties of these two currents, or a mix, can account for descriptions of I(Na) in hair cells from other reports. The patterns of Na channel expression during development suggest a role in establishing the distinct synapses of vestibular hair cells of different type and epithelial zone. All type I hair cells expressed I(Na,1), a TTX-insensitive current with a very negative voltage range of inactivation (midpoint: -94 mV). I(Na,2) was TTX sensitive and had less negative voltage ranges of activation and inactivation (inactivation midpoint: -72 mV). I(Na,1) dominated in the striola at all ages, but current density fell by two-thirds after the first postnatal week. I(Na,2) was expressed by 60% of hair cells in the extrastriola in the first week, then disappeared. In the third week, all type I cells and about half of type II cells had I(Na,1); the remaining cells lacked sodium current. I(Na,1) is probably carried by Na(V)1.5 subunits based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. Na(V)1.5 was also localized to calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for I(Na,2).

Embryonic and larval expression of zebrafish voltage-gated sodium channel alpha-subunit genes

Whereas it is known that voltage-gated calcium channels play important roles during development, potential embryonic roles of voltage-gated sodium channels have received much less attention. Voltage-gated sodium channels consist of pore-forming alpha-subunits (Na(v)1) and auxiliary beta-subunits. Here, we report the embryonic and larval expression patterns for all eight members of the gene family (scna) coding for zebrafish Na(v)1 proteins. We find that each scna gene displays a distinct expression pattern that is temporally and spatially dynamic during embryonic and larval stages. Overall, our findings indicate that scna gene expression occurs sufficiently early during embryogenesis to play developmental roles for both muscle and nervous tissues.