Diversity of expression of the sensory neuron-specific TTX-resistant voltage-gated sodium ion channels SNS and SNS2

A simple and sensitive ultra-high performance liquid chromatography tandem mass spectrometry method for the quantitative analysis of VX-548 in monkey plasma: Method validation and application to pharmacokinetic study

VX-548 is an orally active and highly selective NaV 1.8 inhibitor that is undergoing development for the treatment of acute pain. The aim of this study was to develop a liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for the measurement of VX-548 in monkey plasma. VX-548 was extracted from the plasma using acetonitrile-mediated protein precipitation. The quantitative analysis was performed on a Thermo Vantage TSQ mass spectrometer with ibrutinib as an internal standard. Chromatography was performed on a Waters ACQUITY UPLC BEH C18 column with 0.1% aqueous formic acid and acetonitrile as mobile phase. The precursor-to-product ion transitions were m/z 474.2 > 165.0 and m/z 441.2 > 138.1 for VX-548 and internal standard, respectively. This developed method was successfully validated in the concentration range of 1-1000 ng/mL. The calibration curve showed excellent linearity with a correlation coefficient of >0.999. The precision expressed as relative standard deviation (RSD) was <8.4%, whereas the accuracy denoted as relative error (RE) ranged from -5.0% to 9.1%. The mean recovery was >84%. VX-548 was stable in monkey plasma after storage under certain conditions. The validated method was successfully applied to the pharmacokinetic study of VX-548 in monkey plasma after single oral (2 mg/kg) and intravenous (1 mg/kg) administrations.

A Narrative Review of the Dorsal Root Ganglia and Spinal Cord Mechanisms of Action of Neuromodulation Therapies in Neuropathic Pain

Neuropathic pain arises from injuries to the nervous system in diseases such as diabetes, infections, toxicity, and traumas. The underlying mechanism of neuropathic pain involves peripheral and central pathological modifications. Peripheral mechanisms entail nerve damage, leading to neuronal hypersensitivity and ectopic action potentials. Central sensitization involves a neuropathological process with increased responsiveness of the nociceptive neurons in the central nervous system (CNS) to their normal or subthreshold input due to persistent stimuli, leading to sustained electrical discharge, synaptic plasticity, and aberrant processing in the CNS. Current treatments, both pharmacological and non-pharmacological, aim to alleviate symptoms but often face challenges due to the complexity of neuropathic pain. Neuromodulation is emerging as an important therapeutic approach for the treatment of neuropathic pain in patients unresponsive to common therapies, by promoting the normalization of neuronal and/or glial activity and by targeting cerebral cortical regions, spinal cord, dorsal root ganglia, and nerve endings. Having a better understanding of the efficacy, adverse events and applicability of neuromodulation through pre-clinical studies is of great importance. Unveiling the mechanisms and characteristics of neuromodulation to manage neuropathic pain is essential to understand how to use it. In the present article, we review the current understanding supporting dorsal root ganglia and spinal cord neuromodulation as a therapeutic approach for neuropathic pain.

Inhibition and potentiation of the exercise pressor reflex by pharmacological modulation of TRPC6 in male rats

We determined the role played by the transient receptor potential canonical 6 (TRPC6) channel in evoking the mechanical component of the exercise pressor reflex in male decerebrated Sprague-Dawley rats. TRPC6 channels were identified by quadruple-labelled (DiI, TRPC6, neurofilament-200 and peripherin) immunohistochemistry in dorsal root ganglion (DRG) cells innervating the triceps surae muscles (n = 12). The exercise pressor reflex was evoked by statically contracting the triceps surae muscles before and after injection of the TRPC6 antagonist BI-749327 (n = 11; 12 μg kg-1 ) or SAR7334 (n = 11; 7 μg kg-1 ) or the TRPC6 positive modulator C20 (n = 11; 18 μg kg-1 ). Similar experiments were conducted while the muscles were passively stretched (n = 8-12), a manoeuvre that isolated the mechanical component of the reflex. Blood pressure, tension, renal sympathetic nerve activity (RSNA) and blood flow were recorded. Of the DRG cells innervating the triceps surae muscles, 85% stained positive for the TRPC6 antigen, and 45% of those cells co-expressed neurofilament-200. Both TRPC6 antagonists decreased the reflex pressor responses to static contraction (-32 to -42%; P < 0.05) and to passive stretch (-35 to -52%; P < 0.05), whereas C20 increased these responses (55-65%; P < 0.05). In addition, BI-749327 decreased the peak and integrated RSNA responses to both static contraction (-39 to -43%; P < 0.05) and passive stretch (-56 to -62%; P < 0.05), whereas C20 increased the RSNA to passive stretch only. The onset latency of the decrease or increase in RSNA occurred within 2 s of the onset of the manoeuvres (P < 0.05). Collectively, our results show that TRPC6 plays a key role in evoking the mechanical component of the exercise pressor reflex. KEY POINTS: The exercise pressor reflex plays a key role in the sympathetic and haemodynamic responses to exercise. This reflex is composed of two components, namely the mechanoreflex and the metaboreflex. The receptors responsible for evoking the mechanoreflex are poorly documented. A good candidate for this function is the transient receptor potential canonical 6 (TRPC6) channel, which is activated by mechanical stimuli and expressed in dorsal root ganglia of rats. Using two TRPC6 antagonists and one positive modulator, we investigated the role played by TRPC6 in evoking the mechanoreflex in decerebrated rats. Blocking TRPC6 decreased the renal sympathetic and the pressor responses to both contraction and stretch, the latter being a manoeuvre that isolates the mechanoreflex. In contrast, the positive modulator increased the pressor reflex to contraction and stretch, in addition to the sympathetic response to stretch. Our results provide strong support for a role played by the TRPC6 channel in evoking the mechanoreflex.

A journey from molecule to physiology and in silico tools for drug discovery targeting the transient receptor potential vanilloid type 1 (TRPV1) channel

The heat and capsaicin receptor TRPV1 channel is widely expressed in nerve terminals of dorsal root ganglia (DRGs) and trigeminal ganglia innervating the body and face, respectively, as well as in other tissues and organs including central nervous system. The TRPV1 channel is a versatile receptor that detects harmful heat, pain, and various internal and external ligands. Hence, it operates as a polymodal sensory channel. Many pathological conditions including neuroinflammation, cancer, psychiatric disorders, and pathological pain, are linked to the abnormal functioning of the TRPV1 in peripheral tissues. Intense biomedical research is underway to discover compounds that can modulate the channel and provide pain relief. The molecular mechanisms underlying temperature sensing remain largely unknown, although they are closely linked to pain transduction. Prolonged exposure to capsaicin generates analgesia, hence numerous capsaicin analogs have been developed to discover efficient analgesics for pain relief. The emergence of in silico tools offered significant techniques for molecular modeling and machine learning algorithms to indentify druggable sites in the channel and for repositioning of current drugs aimed at TRPV1. Here we recapitulate the physiological and pathophysiological functions of the TRPV1 channel, including structural models obtained through cryo-EM, pharmacological compounds tested on TRPV1, and the in silico tools for drug discovery and repositioning.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Optogenetic and Chemogenic Control of Pain Signaling: Molecular Markers

Pain is a complex experience that involves physical, emotional, and cognitive aspects. This review focuses specifically on the physiological processes underlying pain perception, with a particular emphasis on the various types of sensory neurons involved in transmitting pain signals to the central nervous system. Recent advances in techniques like optogenetics and chemogenetics have allowed researchers to selectively activate or inactivate specific neuronal circuits, offering a promising avenue for developing more effective pain management strategies. The article delves into the molecular targets of different types of sensory fibers such as channels, for example, TRPV1 in C-peptidergic fiber, TRPA1 in C-non-peptidergic receptors expressed differentially as MOR and DOR, and transcription factors, and their colocalization with the vesicular transporter of glutamate, which enable researchers to identify specific subtypes of neurons within the pain pathway and allows for selective transfection and expression of opsins to modulate their activity.

Neonatal injury modulates incisional pain sensitivity in adulthood: An animal study

Neonatal pain experiences including traumatic injury influences negatively on development of nociceptive circuit developments, resulting in persistent pain hypersensitivity in adults. However, the detailed mechanism is not yet well understood. In the present study, to clarify the pathogenesis of orofacial pain hypersensitivity associated with neonatal injury, the involvement of the voltage-gated sodium channel (Na_v) 1.8 and the C-C chemokine ligand 2 (CCL2)/C-C chemokine receptor 2 (CCR2) signaling in the trigeminal ganglion (TG) in facial skin incisional pain hypersensitivity was examined in 190 neonatal facial-injured and sham male rats. The whisker pad skin was incised on postnatal day 4 and week 7 (Incision-Incision group). Compared to the group without neonatal incision (Sham-Incision group), mechanical hypersensitivity in the whisker pad skin was enhanced in Incision-Incision group. The number of Na_v1.8-immunoreactive TG neurons and the amount of CCL2 expressed in the macrophages and satellite glial cells in the TG were increased on day 14 after re-incision in the Incision-Incision group, compared with Sham-Incision group. Blockages of Na_v1.8 in the incised region and CCR2 in the TG suppressed the enhancement of mechanical hypersensitivity in the Incision-Incision group. Administration of CCL2 into the TG enhanced mechanical hypersensitivity in the Sham-Sham, Incision-Sham and Sham-Incision group. Our results suggest that neonatal facial injury accelerates the TG neuronal hyperexcitability following orofacial skin injury in adult in association with Na_v1.8 overexpression via CCL2 signaling, resulting in the enhancement of orofacial incisional pain hypersensitivity in the adulthood.

Caldendrin represses neurite regeneration and growth in dorsal root ganglion neurons

Caldendrin is a Ca2+ binding protein that interacts with multiple effectors, such as the Cav1 L-type Ca2+ channel, which play a prominent role in regulating the outgrowth of dendrites and axons (i.e., neurites) during development and in response to injury. Here, we investigated the role of caldendrin in Cav1-dependent pathways that impinge upon neurite growth in dorsal root ganglion neurons (DRGNs). By immunofluorescence, caldendrin was localized in medium- and large- diameter DRGNs. Compared to DRGNs cultured from WT mice, DRGNs of caldendrin knockout (KO) mice exhibited enhanced neurite regeneration and outgrowth. Strong depolarization, which normally represses neurite growth through activation of Cav1 channels, had no effect on neurite growth in DRGN cultures from female caldendrin KO mice. Remarkably, DRGNs from caldendrin KO males were no different from those of WT males in terms of depolarization-dependent neurite growth repression. We conclude that caldendrin opposes neurite regeneration and growth, and this involves coupling of Cav1 channels to growth-inhibitory pathways in DRGNs of females but not males.

The Contribution of TSLP Activation to Hyperalgesia in Dorsal Root Ganglia Neurons of a Rat

Peripheral nerve injury involves divergent alterations within dorsal root ganglia (DRG) neurons sensitized by persistent inflammation. Thymic stromal lymphopoietin (TSLP) production is crucial in the development of chronic inflammatory responses. Herein, we investigate the changes of TSLP expression in rats’ DRG neurons between injured and uninjured sides in the same rat. Linalyl acetate (LA) was served as a TSLP inhibitor and given intraperitoneally. Rats were assigned to be group of chronic constriction injury (CCI) of the sciatic nerve and the group of CCI of the sciatic nerve administrated with LA. Over 14 days, the rats were measured for paw withdrawal thresholds. DRGs were collected to assess morphological changes via immunofluorescence study. After receiving CCI, the rats rapidly developed mechanical hyperalgesia. TSLP expression at DRG, on the ipsilateral injured side, was consistent with changes in pain behaviors. TSLP appeared in nerve fibers with both small diameters and large diameters. Additionally, TSLP was expressed mostly in transient receptor potential vanilloid-1 (TRPV1)-positive nociceptive neurons. Administration with LA can attenuate the pain behaviors and expression of TSLP in DRG neurons, and in apoptotic neurons at the injured side, but not in the contra-lateral uninjured side. Overall, these results imply that altered expressions of TSLP in nociceptive DRG neurons contributed to mechanical hyperalgesia in a CCI rat model.

A phase I, randomized, double-blind, placebo-controlled, single- and multiple dose escalation study evaluating the safety, pharmacokinetics and pharmacodynamics of VX-128, a highly selective Nav 1.8 inhibitor, in healthy adults

Selective inhibition of certain voltage‐gated sodium channels (Na_vs), such as Na_v1.8, is of primary interest for pharmacological pain research and widely studied as a pharmacological target due to its contribution to repetitive firing, neuronal excitability, and pain chronification. VX‐128 is a highly potent and selective Na_v1.8 inhibitor that was being developed as a treatment for pain. We evaluated the safety, tolerability, and pharmacokinetics of VX‐128 in healthy subjects in a single‐ and multiple‐ascending dose (MAD) first‐in‐human study. Pharmacodynamics were evaluated in the MAD part using a battery of evoked pain tests. Overall, single doses of VX‐128 up to 300 mg were well‐tolerated, although adverse effect (AE) incidence was higher in subjects receiving VX‐128 (41.7%) compared with placebo (25.0%). After multiple dosing of up to 10 days, skin rash events were observed at all dose levels (up to 100 mg once daily [q.d.]), in five of 26 (19.2%) subjects, including one subject receiving VX‐128 (100 mg q.d.) who had a serious AE of angioedema. A trend in pain tolerance were observed for cold pressor‐ and pressure pain, which was dose‐dependent for the latter. VX‐128 was rapidly absorbed (median time to maximum plasma concentration between 1 and 2 h) with a half‐life of ~80 h at 10 mg q.d., and approximately two‐fold accumulation ratio after 10 and 30 mg q.d. Although VX‐128, when given in a multiple dose fashion, resulted in early study termination due to tolerability issues, effects were observed on multiple pain tests that may support further investigation of Na_v1.8 inhibitors as pain treatments.

Distribution of Large and Small Dorsal Root Ganglion Neurons in Common Marmosets

The aim of this study was to elucidate the size and distribution of dorsal root ganglion (DRG) neurons in non-human primates and to compare them with those of rodent DRG neurons. By measuring the size of NeuN-, NF200-, and peripherin-positive DRG neurons in the lumbar spinal cord of rats and marmosets, we found that the cell size distribution pattern was comparable in both species, although DRG neurons in marmosets were larger than those of rodents. This is the first demonstration that DRG neurons in marmosets have a bimodal size distribution, which has been well established in rodents and humans.

Specific gene expression in unmyelinated dorsal root ganglion neurons in nonhuman primates by intra-nerve injection of AAV 6 vector

Adeno-associated virus 6 (AAV6) has been proposed as a potential vector candidate for specific gene expression in pain-related dorsal root ganglion (DRG) neurons, but this has not been confirmed in nonhuman primates. The aim of our study was to analyze the transduction efficiency and target specificity of this viral vector in the common marmoset by comparing it with those in the rat. When green fluorescent protein-expressing serotype-6 vector was injected into the sciatic nerve, the efficiency of gene expression in DRG neurons was comparable in both species. We found that the serotype-6 vector was largely specific to the pain-related ganglion neurons in the marmoset, as well as in the rat, whereas the serotype-9 vector resulted in contrasting effects in the two species. Neither AAV6 nor AAV9 resulted in DRG toxicity when administered via the sciatic nerve, suggesting this as a safer route of sensory nerve transduction than the currently used intrathecal or intravenous administrative routes. Furthermore, the AAV6 vector could be an optimal serotype for gene therapy for human chronic pain that has a minimal effect on other somatosensory functions of DRG neurons.

A Phase 1, Randomized, Double-Blind, Placebo-Controlled, Crossover Study to Evaluate the Pharmacodynamic Effects of VX-150, a Highly Selective NaV1.8 Inhibitor, in Healthy Male Adults

Objective

To evaluate the analgesic potential, safety, tolerability, and pharmacokinetics of VX-150, a pro-drug of a highly selective Na_V1.8 inhibitor, in healthy subjects.

Design

This was a randomized, double-blind, placebo-controlled, crossover study in healthy subjects.

Subjects

Twenty healthy male subjects with an age of 18–55 years, inclusive, were enrolled. Eligibility was based on general fitness, absence of current or previous medical conditions that could compromise subject safety, and a training assessment of pain tolerance across pain tests to exclude highly tolerant individuals whose tolerance could compromise the ability to detect analgesic responses. All dosed subjects completed the study.

Methods

Subjects were randomized 1:1 to one of two sequences receiving a single VX-150 dose and subsequently placebo, or vice versa, with at least 7 days between dosing. A battery of pain tests (pressure, electrical stair, [capsaicin-induced] heat, and cold pressor) was administered before dosing and repetitively up to 10 h after dosing, with blood sampling up to 24 h after dosing. Safety was monitored throughout the study. Data were analyzed with a repeated-measures mixed-effects model.

Results

VX-150 induced analgesia in a variety of evoked pain tests, without affecting subject safety. Significant effects were reported for the cold pressor and heat pain thresholds. Maximum median concentration for the active moiety was 4.30 µg/mL at 4 h after dosing.

Conclusion

Results of this proof-of-mechanism study are supportive of the potential of VX-150, a highly selective Na_V1.8 channel inhibitor, to treat various pain indications.

The neuronal potassium current IA is a potential target for pain during chronic inflammation

Voltage-gated ion channels play a key role in the action potential (AP) initiation and its propagation in sensory neurons. Modulation of their activity during chronic inflammation creates a persistent pain state. In this study, we sought to determine how peripheral inflammation caused by complete Freund's adjuvant (CFA) alters the fast sodium (INa ), L-type calcium (ICaL ), and potassium (IK ) currents in primary afferent fibers to increase nociception. In our model, intraplantar administration of CFA induced mechanical allodynia and thermal hyperalgesia at day 14 post-injection. Using whole-cell patch-clamp recording in dissociated small (C), medium (Aδ), and large-sized (Aβ) rat dorsal root ganglion (DRG) neurons, we found that CFA prolonged the AP duration and increased the amplitude of the tetrodotoxin-resistant (TTX-r) INa in Aβ fibers. In addition, CFA accelerated the recovery of INa from inactivation in C and Aδ nociceptive fibers but enhanced the late sodium current (INaL ) only in Aδ and Aβ neurons. Inflammation similarly reduced the amplitude of ICaL in each neuronal cell type. Fourteen days after injection, CFA reduced both components of IK (IKdr and IA ) in Aδ fibers. We also found that IA was significantly larger in C and Aδ neurons in normal conditions and during chronic inflammation. Our data, therefore, suggest that targeting the transient potassium current IA represents an efficient way to shift the balance toward antinociception during inflammation, since its activation will selectively decrease the AP duration in nociceptive fibers. Altogether, our data indicate that complex interactions between IK , INa , and ICaL reduce pain threshold by concomitantly enhancing the activity of nociceptive neurons and reducing the inhibitory action of Aβ fibers during chronic inflammation.

The α isoform of cGMP-dependent protein kinase 1 (PKG1α) is expressed and functionally important in intrinsic primary afferent neurons of the guinea pig enteric nervous system

BACKGROUND

Intrinsic primary afferent neurons (IPANs) enable the gut to manifest reflexes in the absence of CNS input. PKG1α is selectively expressed in a subset of neurons in dorsal root ganglia (DRG) and has been linked to nociception and long-term hyperexcitability.

METHODS

We used immunoblotting, immunocytochemistry, and in vitro assays of IPAN-dependent enteric functions to test hypotheses that subsets of primary neurons of the ENS and DRG share a reliance on PKG1α expression.

KEY RESULTS

PKG1α immunoreactivity was demonstrated in immunoblots from isolated myenteric ganglia. PKG1α, but not PKG1β, immunoreactivity, was coincident with that of neuronal markers (HuC/D; β3-tubulin) in both enteric plexuses. PKG1α immunoreactivity also co-localized with the immunoreactivities of the IPAN markers, calbindin (100%; myenteric plexus) and cytoplasmic NeuN (98 ± 1% submucosal plexus). CGRP-immunoreactive DRG neurons, identified as visceral afferents by retrograde transport, were PKG1α-immunoreactive. We used intraluminal cholera toxin to determine whether PKG1α was necessary to enable stimulation of the mucosa to activate Fos in enteric neurons. Tetrodotoxin (1.0 µM), low Ca²⁺ /high Mg²⁺ media, and the PKG inhibitor, N46 (100 µM), all inhibited Fos activation in myenteric neurons. N46 also concentration dependently inhibited peristaltic reflexes in isolated preparations of distal colon (IC₅₀  = 83.3 ± 1.3 µM).

CONCLUSIONS & INFERENCES

These data suggest that PKG1α is present and functionally important in IPANs and visceral afferent nociceptive neurons.

Longitudinal intravital imaging nerve degeneration and sprouting in the toes of spared nerve injured mice

Neuropathic pain is pain caused by damage to the somatosensory nervous system. Both degenerating injured nerves and neighboring sprouting nerves can contribute to neuropathic pain. However, the mesoscale changes in cutaneous nerve fibers over time after the loss of the parent nerve has not been investigated in detail. In this study, we followed the changes in nerve fibers longitudinally in the toe tips of mice that had undergone spared nerve injury (SNI). Nav1.8-tdTomato, Thy1-GFP and MrgD-GFP mice were used to observe the small and large cutaneous nerve fibers. We found that peripheral nerve plexuses degenerated within 3 days of nerve injury, and free nerve endings in the epidermis degenerated within 2 days. The timing of degeneration paralleled the initiation of mechanical hypersensitivity. We also found that some of the Nav1.8-positive nerve plexuses and free nerve endings in the fifth toe survived, and sprouting occurred mostly from 7 to 28 days. The timing of the sprouting of nerve fibers in the fifth toe paralleled the maintenance phase of mechanical hypersensitivity. Our results support the hypotheses that both injured and intact nerve fibers participate in neuropathic pain, and that, specifically, nerve degeneration is related to the initiation of evoked pain and nerve sprouting is related to the maintenance of evoked pain.

The Pivotal Potentials of Scorpion Buthus Martensii Karsch-Analgesic-Antitumor Peptide in Pain Management and Cancer

Scorpion Buthus martensii Karsch -analgesic-antitumor peptide (BmK AGAP) has been used to treat diseases like tetanus, tuberculosis, apoplexy, epilepsy, spasm, migraine headaches, rheumatic pain, and cancer in China. AGAP is a distinctive long-chain scorpion toxin with a molecular mass of 7142 Da and composed of 66 amino acids cross-linked by four disulfide bridges. Voltage-gated sodium channels (VGSCs) are present in excitable membranes and partakes in essential roles in action potentials generation as compared to the significant function of voltage-gated calcium channels (VGCCs). A total of nine genes (Nav1.1-Nav1.9) have been recognized to encode practical sodium channel isoforms. Nav1.3, Nav1.7, Nav1.8, and Nav1.9 have been recognized as potential targets for analgesics. Nav1.8 and Nav1.9 are associated with nociception initiated by inflammation signals in the neuronal pain pathway, while Nav1.8 is fundamental for neuropathic pain at low temperatures. AGAP has a sturdy inhibitory influence on both viscera and soma pain. AGAP potentiates the effects of MAPK inhibitors on neuropathic as well as inflammation-associated pain. AGAP downregulates the secretion of phosphorylated p38, phosphorylated JNK, and phosphorylated ERK 1/2 in vitro. AGAP has an analgesic activity which may be an effective therapeutic agent for pain management because of its downregulation of PTX3 via NF-κB and Wnt/beta-catenin signaling pathway. In cancers like colon cancer, breast cancer, lymphoma, and glioma, rAGAP was capable of blocking the proliferation. Thus, AGAP is a promising therapy for these tumors. Nevertheless, research is needed with other tumors.

Neurological Disturbances of Ciguatera Poisoning: Clinical Features and Pathophysiological Basis

Ciguatera fish poisoning (CFP), the most prevalent seafood poisoning worldwide, is caused by the consumption of tropical and subtropical fish contaminated with potent neurotoxins called ciguatoxins (CTXs). Ciguatera is a complex clinical syndrome in which peripheral neurological signs predominate in the acute phase of the intoxication but also persist or reoccur long afterward. Their recognition is of particular importance in establishing the diagnosis, which is clinically-based and can be a challenge for physicians unfamiliar with CFP. To date, no specific treatment exists. Physiopathologically, the primary targets of CTXs are well identified, as are the secondary events that may contribute to CFP symptomatology. This review describes the clinical features, focusing on the sensory disturbances, and then reports on the neuronal targets and effects of CTXs, as well as the neurophysiological and histological studies that have contributed to existing knowledge of CFP neuropathophysiology at the molecular, neurocellular and nerve levels.

Cadmium versus Lanthanum Effects on Spontaneous Electrical Activity and Expression of Connexin Isoforms Cx26, Cx36, and Cx45 in the Human Fetal Cortex

Electrical activity is important for brain development. In brain slices, human subplate neurons exhibit spontaneous electrical activity that is highly sensitive to lanthanum. Based on the results of pharmacological experiments in human fetal tissue, we hypothesized that hemichannel-forming connexin (Cx) isoforms 26, 36, and 45 would be expressed on neurons in the subplate (SP) zone. RNA sequencing of dissected human cortical mantles at ages of 17-23 gestational weeks revealed that Cx45 has the highest expression, followed by Cx36 and Cx26. The levels of Cx and pannexin expression between male and female fetal cortices were not significantly different. Immunohistochemical analysis detected Cx45- and Cx26-expressing neurons in the upper segment of the SP zone. Cx45 was present on the cell bodies of human SP neurons, while Cx26 was found on both cell bodies and dendrites. Cx45, Cx36, and Cx26 were strongly expressed in the cortical plate, where newborn migrating neurons line up to form cortical layers. New information about the expression of 3 "neuronal" Cx isoforms in each cortical layer/zone (e.g., SP, cortical plate) and pharmacological data with cadmium and lanthanum may improve our understanding of the cellular mechanisms underlying neuronal development in human fetuses and potential vulnerabilities.

Investigating stimulation parameters for preferential small-fiber activation using exponentially rising electrical currents

Electrical stimulation is widely used in pain research and profiling, but current technologies lack selectivity toward small sensory fibers. Pin electrodes deliver high current density in upper skin layers, and it has been proposed that slowly rising exponential pulses can elevate large-fiber activation threshold and thereby increase preferential small-fiber activation. Optimal stimulation parameters for the combined pin electrode and exponential pulse stimulation have so far not been established, which is the aim of this study. Perception thresholds were compared between pin and patch electrodes using single 1- to 100-ms exponential and rectangular pulses. Stimulus-response functions were evaluated for both pulse shapes delivered as single pulses and pulse trains of 10 Hz using intensities from 0.1 to 20 times perception threshold. Perception thresholds (mA) decreased when duration was increased for both electrodes with rectangular pulses and the pin electrode with exponential pulses. For the patch electrode, perception thresholds for exponential pulses decreased for durations ≤10 ms but increased for durations ≥15 ms, indicating accommodation of large fibers. Stimulus-response curves for single pulses were similar for the two pulse shapes. For pulse trains, the slope of the curve was higher for rectangular pulses. Maximal large-fiber accommodation to exponential pulses was observed for 100-ms pulses, indicating that 100-ms exponential pulses should be applied for preferential small-fiber activation. Intensity of 10 times perception threshold was sufficient to cause maximal pain ratings. The developed methodology may open new opportunities for using electrical stimulation paradigms for small-fiber stimulation and diagnostics.NEW & NOTEWORTHY Selective activation of small cutaneous nerve fibers is pivotal for investigations of the pain system. The present study demonstrated that patch electrode perception thresholds increase with increased duration of exponential currents from 20 to 100 ms. This is likely caused by large-fiber accommodation, which can be utilized to activate small fibers preferentially through small-diameter pin electrodes. This finding may be utilized in studies of fundamental pain mechanisms and, for example, in small-fiber neuropathy.

Preferential activation of small cutaneous fibers through small pin electrode also depends on the shape of a long duration electrical current

BACKGROUND

Electrical stimulation is widely used in experimental pain research but it lacks selectivity towards small nociceptive fibers. When using standard surface patch electrodes and rectangular pulses, large fibers are activated at a lower threshold than small fibers. Pin electrodes have been designed for overcoming this problem by providing a higher current density in the upper epidermis where the small nociceptive fibers mainly terminate. At perception threshold level, pin electrode stimuli are rather selectively activating small nerve fibers and are perceived as painful, but for high current intensity, which is usually needed to evoke sufficient pain levels, large fibers are likely co-activated. Long duration current has been shown to elevate the threshold of large fibers by the mechanism of accommodation. However, it remains unclear whether the mechanism of accommodation in large fibers can be utilized to activate small fibers even more selectively by combining pin electrode stimulation with a long duration pulse.

RESULTS

In this study, perception thresholds were determined for a patch- and a pin electrode for different pulse shapes of long duration. The perception threshold ratio between the two different electrodes was calculated to estimate the ability of the pulse shapes to preferentially activate small fibers. The perception threshold ratios were compared between stimulation pulses of 5- and 50 ms durations and shapes of: exponential increase, linear increase, bounded exponential, and rectangular. Qualitative pain perception was evaluated for all pulse shapes delivered at 10 times perception threshold. The results showed a higher perception threshold ratio for long duration 50 ms pulses than for 5 ms pulses. The highest perception threshold ratio was found for the 50 ms, bounded exponential pulse shape. Results furthermore revealed different strength-duration relation between the bounded exponential- and rectangular pulse shapes. Pin electrode stimulation at high intensity was mainly described as "stabbing", "shooting", and "sharp".

CONCLUSION

These results indicate that long duration pulses with a bounded exponential increase preferentially activate the small nociceptive fibers with a pin electrode and concurrently cause elevated threshold of large non-nociceptive fibers with patch electrodes.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

Protein kinase C-α upregulates sodium channel Nav1.9 in nociceptive dorsal root ganglion neurons in an inflammatory arthritis pain model of rat

Previous studies have found that increased expression of Nav1.9 and protein kinase C (PKC) contributes to pain hypersensitivity in a couple of inflammatory pain models. Here we want to observe if PKC can regulate the expression of Nav1.9 in dorsal root ganglion (DRG) in rheumatoid arthritis (RA) pain model. A chronic knee joint inflammation model was produced by intra-articular injection of the complete Freund's adjuvant (CFA) in rats. Nociceptive behaviors including mechanical, cold, and heat hyperalgesia were examined. The expression of Nav1.9 and PKCα in DRG was detected by a quantitative polymerase chain reaction, Western blot, and immunofluorescence. The in vitro and in vivo effects of a PKC activator (phorbol 12-myristate 13-acetate [PMA]) and a PKC inhibitor (GF-109203X) on the expression of Nav1.9 were examined. Moreover, the effects of PKC modulators on nociceptive behaviors were studied. Increased mechanical, heat, and cold sensitivity was observed 3 to 14 days after CFA injection. Parallel increases in messenger RNA and protein expression of Nav1.9 and PKCα were found. Immunofluorescence experiments found that Nav1.9 was preferentially colocalized with IB4+DRG neurons in RA rats. In cultured DRG neurons, PMA increased Nav1.9 expression while GF-109203X prevented the effect of PMA. PMA increased Nav1.9 expression in naïve rats while GF-109203X decreased Nav1.9 expression in RA rats. In naïve rats, PMA caused mechanical and cold hyperalgesia. On the other hand, GF-109203X attenuated mechanical and cold hyperalgesia in RA-pain model. Nav1.9 might be upregulated by PKCα in DRG, which contributes to pain hypersensitivity in CFA-induced chronic knee joint inflammation model of RA pain.

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.

Roles of inflammation, neurogenic inflammation, and neuroinflammation in pain

Inflammation is the body's response to injury and infection, involving a complex biological response of the somatosensory, immune, autonomic, and vascular systems. Inflammatory mediators such as prostaglandin, proinflammatory cytokines, and chemokines induce pain via direct activation of nociceptors, the primary sensory neurons that detect noxious stimuli. Neurogenic inflammation is triggered by nerve activation and results in neuropeptide release and rapid plasma extravasation and edema, contributing to pain conditions such as headache. Neuroinflammation is a localized inflammation in the peripheral nervous system (PNS) and central nervous system (CNS). A characteristic feature of neuroinflammation is the activation of glial cells in dorsal root ganglia, spinal cord, and brain which leads to the production of proinflammatory cytokines and chemokines in the PNS and CNS that drives peripheral sensitization and central sensitization. Here, we discuss the distinct roles of inflammation, neurogenic inflammation, and neuroinflammation in the regulation of different types of pain conditions, with a special focus on neuroinflammation in postoperative pain and opioid-induced hyperalgesia.

Agmatine preferentially antagonizes GluN2B-containing N-methyl-d-aspartate receptors in spinal cord

The role of the N-methyl-d-aspartate receptor (NMDAr) as a contributor to maladaptive neuroplasticity underlying the maintenance of chronic pain is well established. Agmatine, an NMDAr antagonist, has been shown to reverse tactile hypersensitivity in rodent models of neuropathic pain while lacking the side effects characteristic of global NMDAr antagonism, including sedation and motor impairment, indicating a likely subunit specificity of agmatine's NMDAr inhibition. The present study assessed whether agmatine inhibits subunit-specific NMDAr-mediated current in the dorsal horn of mouse spinal cord slices. We isolated NMDAr-mediated excitatory postsynaptic currents (EPSCs) in small lamina II dorsal horn neurons evoked by optogenetic stimulation of Nav1.8-containing nociceptive afferents. We determined that agmatine abbreviated the amplitude, duration, and decay constant of NMDAr-mediated EPSCs similarly to the application of the GluN2B antagonist ifenprodil. In addition, we developed a site-specific knockdown of the GluN2B subunit of the NMDAr. We assessed whether agmatine and ifenprodil were able to inhibit NMDAr-mediated current in the spinal cord dorsal horn of mice lacking the GluN2B subunit of the NMDAr by analysis of electrically evoked EPSCs. In control mouse spinal cord, agmatine and ifenprodil both inhibited amplitude and accelerated the decay kinetics. However, agmatine and ifenprodil failed to attenuate the decay kinetics of NMDAr-mediated EPSCs in the GluN2B-knockdown mouse spinal cord. The present study indicates that agmatine preferentially antagonizes GluN2B-containing NMDArs in mouse dorsal horn neurons. NEW & NOTEWORTHY Our study is the first to report that agmatine preferentially antagonizes the GluN2B receptor subunit of the N-methyl-d-aspartate (NMDA) receptor in spinal cord. The preferential targeting of GluN2B receptor is consistent with the pharmacological profile of agmatine in that it reduces chronic pain without the motor side effects commonly seen with non-subunit-selective NMDA receptor antagonists.

Topical Ambroxol 20% for the Treatment of Classical Trigeminal Neuralgia - A New Option? Initial Clinical Case Observations

BACKGROUND

Trigeminal neuralgia is difficult to treat and shows upregulation of sodium channels. The expectorant ambroxol acts as a strong local anesthetic, about 40 times stronger than lidocaine. It preferentially inhibits the channel subtype Na_v 1.8, expressed especially in nociceptive C-fibers. It seemed reasonable to try ambroxol for the treatment with neuropathic facial pain unresponsive to other standard options.

MATERIAL AND METHODS

Medical records of patients suffering from classical trigeminal neuralgia (n = 5) and successful pain reduction following topical ambroxol 20% cream in addition to standard treatment are reported.

RESULTS

All patients reported pain attacks with pain intensity between 4 and 10 NRS (numeric pain scale). In all cases they could be triggered, 3 patients reported additional spontaneous pain. Attacks were reduced in all 5 patients. Pain reduction achieved following ambroxol 20% cream was 2-8 points (NRS) earliest within 15-30 minutes and lasted for 4-6 hours mostly. This was reproducible in all cases; in one case pain was eliminated after 1 week. No patient reported side effects or skin changes; oral medication was reduced in 2 patients.

CONCLUSION

For the first time, a clinically significant pain relief following topical ambroxol 20% cream in patients with trigeminal neuralgia is reported. In view of the positive side effect profile, topical ambroxol for patients with such a highly impaired quality of life should be investigated further as a matter of urgency.

Somatosensory Neurons Enter a State of Altered Excitability during Hibernation

Hibernation in mammals involves prolonged periods of inactivity, hypothermia, hypometabolism, and decreased somatosensation. Peripheral somatosensory neurons play an essential role in the detection and transmission of sensory information to CNS and in the generation of adaptive responses. During hibernation, when body temperature drops to as low as 2°C, animals dramatically reduce their sensitivity to physical cues [1, 2]. It is well established that, in non-hibernators, cold exposure suppresses energy production, leading to dissipation of the ionic and electrical gradients across the plasma membrane and, in the case of neurons, inhibiting the generation of action potentials [3]. Conceivably, such cold-induced elimination of electrogenesis could be part of a general mechanism that inhibits sensory abilities in hibernators. However, when hibernators become active, the bodily functions-including the ability to sense environmental cues-return to normal within hours, suggesting the existence of mechanisms supporting basal functionality of cells during torpor and rapid restoration of activity upon arousal. We tested this by comparing properties of somatosensory neurons from active and torpid thirteen-lined ground squirrels (Ictidomys tridecemlineatus). We found that torpid neurons can compensate for cold-induced functional deficits, resulting in unaltered resting potential, input resistance, and rheobase. Torpid neurons can generate action potentials but manifest markedly altered firing patterns, partially due to decreased activity of voltage-gated sodium channels. Our results provide insights into the mechanism that preserves somatosensory neurons in a semi-active state, enabling fast restoration of sensory function upon arousal. These findings contribute to the development of strategies enabling therapeutic hypothermia and hypometabolism.

Quantitative Top-Down Mass Spectrometry Identifies Proteoforms Differentially Released during Mechanical Stimulation of Mouse Skin

Mechanotransduction refers to the processes whereby mechanical stimuli are converted into electrochemical signals that allow for the sensation of our surrounding environment through touch. Despite its fundamental role in our daily lives, the molecular and cellular mechanisms of mechanotransduction are not yet well-defined. Previous data suggest that keratinocytes may release factors that activate or modulate cutaneous sensory neuron terminals, including small molecules, lipids, peptides, proteins, and oligosaccharides. This study presents a first step toward identifying soluble mediators of keratinocyte-sensory neuron communication by evaluating the potential for top-down mass spectrometry to identify proteoforms released during 1 min of mechanical stimulation of mouse skin from naı̈ve animals. Overall, this study identified 47 proteoforms in the secretome of mouse hind paw skin, of which 14 were differentially released during mechanical stimulation, and includes proteins with known and previously unknown relevance to mechanotransduction. Finally, this study outlines a bioinformatic workflow that merges output from two complementary analysis platforms for top-down data and demonstrates the utility of this workflow for integrating quantitative and qualitative data.

Characterization of dural afferent neurons innervating cranial blood vessels within the dura in rats

Dural afferent neurons are implicated in primary headaches including migraine. Although a significant portion of primary afferent neurons innervating the dura are myelinated A-type neurons, previous electrophysiological studies have primarily characterized the functional properties of small-sized C-type sensory neurons. Here we show the functional characterization of dural afferent neurons identified with the fluorescent dye DiI. DiI-positive neurons were divided into three types: small-, medium-, and large-sized neurons, based on their diameter, area, and membrane capacitance. The immunoreactivity of NF200, a marker of A-type myelinated neurons, was detected in most large-sized, but it was also present in a limited number of small- and medium-sized DiI-positive neurons. Capsaicin, a transient receptor potential vanilloid 1 agonist, induced the membrane currents in most small- and medium-sized neurons, but not in large-sized DiI-positive neurons. Tetrodotoxin-resistant Na⁺ channels were expressed in almost all types of DiI-positive neurons. Mechanosensitive currents were detected from a majority of large-sized, and to a lesser extent, small- and medium-sized DiI-positive neurons. The results suggest that most dural afferent neurons are nociceptive, e.g., polymodal C-type for small- and medium-sized neurons, and high-threshold nociceptive A-type mechanoreceptors for large-sized neurons. We also found that DiI-positive neurons differed with respect to passive and active membrane properties, and that sumatriptan, a representative drug used for the acute treatment of migraine attack, inhibited voltage-gated Ca²⁺ currents in all types of DiI-positive neurons. The present results showing the nociceptive properties of dural afferent neurons would contribute to understand the pathophysiology of primary headaches.

Tuning in C-nociceptors to reveal mechanisms in chronic neuropathic pain

OBJECTIVE

Develop and validate a low-intensity sinusoidal electrical stimulation paradigm to preferentially activate C-fibers in human skin.

METHODS

Sinusoidal transcutaneous stimulation (4Hz) was assessed psychophysically in healthy volunteers (n = 14) and neuropathic pain patients (n = 9). Pursuing laser Doppler imaging and single nociceptor recordings in vivo in humans (microneurography) and pigs confirmed the activation of "silent" C-nociceptors. Synchronized C-fiber compound action potentials were evoked in isolated human nerve fascicles in vitro. Live cell imaging of L4 dorsal root ganglia in anesthetized mice verified the recruitment of small-diameter neurons during transcutaneous 4-Hz stimulation of the hindpaw (0.4mA).

RESULTS

Transcutaneous sinusoidal current (0.05-0.4mA, 4Hz) activated "polymodal" C-fibers (50% at ∼0.03mA) and "silent" nociceptors (50% at ∼0.04mA), intensities substantially lower than that required with transcutaneous 1-ms rectangular pulses ("polymodal" ∼3mA, "silent" ∼50mA). The stimulation induced delayed burning (nonpulsating) pain and a pronounced axon-reflex erythema, both indicative of C-nociceptor activation. Pain ratings to repetitive stimulation (1 minute, 4Hz) adapted in healthy volunteers by Numeric Rating Scale (NRS) -3 and nonpainful skin sites of neuropathic pain patients by NRS -0.5, whereas pain even increased in painful neuropathic skin by approximately NRS +2.

INTERPRETATION

Sinusoidal electrical stimulation at 4Hz enables preferential activation of C-nociceptors in pig and human skin that accommodates during ongoing (1-minute) stimulation. Absence of such accommodation in neuropathic pain patients suggest axonal hyperexcitability that could be predictive of alterations in peripheral nociceptor encoding and offer a potential therapeutic entry point for topical analgesic treatment. Ann Neurol 2018;83:945-957.

TRPs et al.: a molecular toolkit for thermosensory adaptations

The ability to sense temperature is crucial for the survival of an organism. Temperature influences all biological operations, from rates of metabolic reactions to protein folding, and broad behavioral functions, from feeding to breeding, and other seasonal activities. The evolution of specialized thermosensory adaptations has enabled animals to inhabit extreme temperature niches and to perform specific temperature-dependent behaviors. The function of sensory neurons depends on the participation of various types of ion channels. Each of the channels involved in neuronal excitability, whether through the generation of receptor potential, action potential, or the maintenance of the resting potential have temperature-dependent properties that can tune the neuron's response to temperature stimuli. Since the function of all proteins is affected by temperature, animals need adaptations not only for detecting different temperatures, but also for maintaining sensory ability at different temperatures. A full understanding of the molecular mechanism of thermosensation requires an investigation of all channel types at each step of thermosensory transduction. A fruitful avenue of investigation into how different molecules can contribute to the fine-tuning of temperature sensitivity is to study the specialized adaptations of various species. Given the diversity of molecular participants at each stage of sensory transduction, animals have a toolkit of channels at their disposal to adapt their thermosensitivity to their particular habitats or behavioral circumstances.

Tolerance to Morphine-Induced Inhibition of TTX-R Sodium Channels in Dorsal Root Ganglia Neurons Is Modulated by Gut-Derived Mediators

In the clinical setting, analgesic tolerance is a primary driver of diminished pain control and opioid dose escalations. Integral to this process are primary afferent sensory neurons, the first-order components of nociceptive sensation. Here, we characterize the factors modulating morphine action and tolerance in mouse small diameter dorsal root ganglia (DRG) neurons. We demonstrate that acute morphine inactivates tetrodotoxin-resistant (TTX-R) Na+ channels in these cells. Chronic exposure resulted in tolerance to this effect, which was prevented by treatment with oral vancomycin. Using colonic supernatants, we further show that mediators in the gut microenvironment of mice with chronic morphine exposure can induce tolerance and hyperexcitability in naive DRG neurons. Tolerance (but not hyperexcitability) in this paradigm was mitigated by oral vancomycin treatment. These findings collectively suggest that gastrointestinal microbiota modulate the development of morphine tolerance (but not hyperexcitability) in nociceptive primary afferent neurons, through a mechanism involving TTX-R Na+ channels.

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.

Expression and Role of Voltage-Gated Sodium Channels in Human Dorsal Root Ganglion Neurons with Special Focus on Nav1.7, Species Differences, and Regulation by Paclitaxel

Voltage-gated sodium channels (Navs) play an important role in human pain sensation. However, the expression and role of Nav subtypes in native human sensory neurons are unclear. To address this issue, we obtained human dorsal root ganglion (hDRG) tissues from healthy donors. PCR analysis of seven DRG-expressed Nav subtypes revealed that the hDRG has higher expression of Nav1.7 (~50% of total Nav expression) and lower expression of Nav1.8 (~12%), whereas the mouse DRG has higher expression of Nav1.8 (~45%) and lower expression of Nav1.7 (~18%). To mimic Nav regulation in chronic pain, we treated hDRG neurons in primary cultures with paclitaxel (0.1–1 μmol/L) for 24 h. Paclitaxel increased the Nav1.7 but not Nav1.8 expression and also increased the transient Na⁺ currents and action potential firing frequency in small-diameter (<50 μm) hDRG neurons. Thus, the hDRG provides a translational model in which to study “human pain in a dish” and test new pain therapeutics.

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

Antinociceptive Effects of AGAP, a Recombinant Neurotoxic Polypeptide: Possible Involvement of the Tetrodotoxin-Resistant Sodium Channels in Small Dorsal Root Ganglia Neurons

Antitumor-analgesic peptide (AGAP) is a novel recombinant polypeptide. The primary study showed that AGAP 1.0 mg/kg exhibited strong analgesic and antitumor effects. The tail vein administration of AGAP potently reduced pain behaviors in mice induced by intraplantar injection of formalin or intraperitoneal injection of acetic acid, without affecting basal pain perception. To further assess the mechanisms of AGAP, the effects of AGAP on sodium channels were assessed using the whole-cell patch clamp recordings in dorsal root ganglia (DRG) neurons. The results showed that AGAP (3–1000 nM) inhibited the sodium currents in small-diameter DRG neurons in a dose-dependent manner. 1000 nM AGAP could inhibit the current density-voltage relationship curve of sodium channels in a voltage-dependent manner and negatively shift the activation curves. 1000 nM AGAP could reduce the tetrodotoxin-resistant (TTX-R) sodium currents by 42.8% in small-diameter DRG neurons. Further analysis revealed that AGAP potently inhibited Na_V1.8 currents by 59.4%, and negatively shifted the activation and inactivation kinetics. 1000 nM AGAP also reduced the Na_V1.9 currents by 33.7%, but had no significant effect on activation and inactivation kinetics. Thus, our results demonstrated that submicromolar concentrations of AGAP inhibited TTX-R sodium channel in rat small-diameter DRG neurons. It is concluded that these new results may better explain, at least in part, the analgesic properties of this polypeptide.

Acid modulation of tetrodotoxin-sensitive Na+ channels in large-sized trigeminal ganglion neurons

Voltage-gated Na⁺ channels in primary afferent neurons can be divided into tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na⁺ channels. Although previous studies have shown the acid modulation of TTX-R Na⁺ channels, the effect of acidic pH on tetrodotoxin-sensitive (TTX-S) Na⁺ channels is still unknown. Here we report the effect of acidic pH on TTX-S Na⁺ channels expressed in large-sized trigeminal ganglion (TG) neurons using a whole-cell patch clamp technique. The application of acidic extracellular solution decreased the peak amplitude of TTX-S currents (I_Na) in a pH-dependent manner, but weak acid (≥pH 6.0) had no inhibitory effect on TTX-S I_Na. Acidic pH (pH 6.0) shifted both the activation and steady-state fast inactivation relationships of TTX-S Na⁺ channels toward depolarized potentials. However, acidic pH (pH 6.0) had no effect on use-dependent inhibition in response to high-frequency stimuli, development of inactivation, and accelerated the recovery from inactivation of TTX-S Na⁺ channels, suggesting that TTX-S Na⁺ channels in large-sized TG neurons are less sensitive to acidic pH. Given that voltage-gated Na⁺ channels play a pivotal role in the generation and conduction of action potentials in neural tissues, the insensitivity of TTX-S Na⁺ channels expressed in large-sized TG neurons to acidic pH would ensure transmission of innocuous tactile sensation from orofacial regions at acidic pH conditions.

SCN10A Mutation in a Patient with Erythromelalgia Enhances C-Fiber Activity Dependent Slowing

Gain-of-function mutations in the tetrodotoxin (TTX) sensitive voltage-gated sodium channel (Nav) Nav1.7 have been identified as a key mechanism underlying chronic pain in inherited erythromelalgia. Mutations in TTX resistant channels, such as Nav1.8 or Nav1.9, were recently connected with inherited chronic pain syndromes. Here, we investigated the effects of the p.M650K mutation in Nav1.8 in a 53 year old patient with erythromelalgia by microneurography and patch-clamp techniques. Recordings of the patient’s peripheral nerve fibers showed increased activity dependent slowing (ADS) in CMi and less spontaneous firing compared to a control group of erythromelalgia patients without Nav mutations. To evaluate the impact of the p.M650K mutation on neuronal firing and channel gating, we performed current and voltage-clamp recordings on transfected sensory neurons (DRGs) and neuroblastoma cells. The p.M650K mutation shifted steady-state fast inactivation of Nav1.8 to more hyperpolarized potentials and did not significantly alter any other tested gating behaviors. The AP half-width was significantly broader and the stimulated action potential firing rate was reduced for M650K transfected DRGs compared to WT. We discuss the potential link between enhanced steady state fast inactivation, broader action potential width and the potential physiological consequences.

Use dependence of peripheral nociceptive conduction in the absence of tetrodotoxin-resistant sodium channel subtypes

KEY POINTS

This study examines conduction in peripheral nerves and its use dependence in tetrodotoxin-resistant (TTXr) sodium channel (Nav 1.8, Nav 1.9) knockout and wildtype animals. We observed use-dependent decreases of single fibre and compound action potential amplitude in peripheral mouse C-fibres (wildtype). This matches the previously published hypothesis that increased Na/K-pump activity is not the underlying mechanism for use-dependent changes of neural conduction. Knocking out TTXr sodium channels influences use-dependent changes of conductive properties (action potential amplitude, latency, conduction safety) in the order Nav 1.8 KO > Nav 1.9KO > wildtype. This is most likely explained by different subsets of presumably (relatively) Nav 1.7-rich conducting fibres in knockout animals as compared to wildtypes, in combination with reduced per-pulse sodium influx.

ABSTRACT

Use dependency of peripheral nerves, especially of nociceptors, correlates with receptive properties. Slow inactivation of voltage-gated sodium channels has been discussed to be the underlying mechanism - pointing to a receptive class-related difference of sodium channel equipment. Using electrophysiological recordings of single unmyelinated cutaneous fibres and their compound action potential (AP), we evaluated use-dependent changes in mouse peripheral nerves, and the contribution of the tetrodotoxin-resistant (TTXr) sodium channels Nav 1.8 and Nav 1.9 to these changes. Nerve fibres were electrically stimulated using single or double pulses at 2 Hz. Use-dependent changes of latency, AP amplitude, and duration as well as the fibres' ability to follow the stimulus were evaluated. AP amplitudes substantially diminished in used fibres from C57BL/6 but increased in Nav 1.8 knockout (KO) mice, with Nav 1.9 KO in between. Activity-induced latency slowing was in contrast the most pronounced in Nav 1.8 KOs and the least in wildtype mice. The genotype was also predictive of how long fibres could follow the double pulsed stimulus with wildtype fibres blocking first and Nav 1.8 KO fibres enduring the longest. In contrast, changes in spike duration were less pronounced and displayed no significant tendency. Thus, all major measures of peripheral nerve accommodation (amplitude, latency and durability) depended on genotype. All use-dependent changes appeared in the order NaV 1.8 KO > NaV 1.9 KO > wildtype, which is most likely explained by the relative contribution of Nav 1.7 varying in the same order and the amounts of per-pulse sodium influx expected in the opposite order.

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.

Discovery and optimisation of potent and highly subtype selective Nav1.8 inhibitors with reduced cardiovascular liabilities

Potent and subtype selective Na_v1.8 inhibitors were designed and optimised for selectivity over hERG ion channel inhibition.

The role of Nav1.9 channel in the development of neuropathic orofacial pain associated with trigeminal neuralgia

BACKGROUND

Trigeminal neuralgia is accompanied by severe mechanical, thermal and chemical hypersensitivity of the orofacial area innervated by neurons of trigeminal ganglion (TG). We examined the role of the voltage-gated sodium channel subtype Nav1.9 in the development of trigeminal neuralgia.

RESULTS

We found that Nav1.9 is required for the development of both thermal and mechanical hypersensitivity induced by constriction of the infraorbital nerve (CION). The CION model does not induce change on Nav1.9 mRNA expression in the ipsilateral TG neurons when evaluated 9 days after surgery.

CONCLUSIONS

These results demonstrate that Nav1.9 channels play a critical role in the development of orofacial neuropathic pain. New routes for the treatment of orofacial neuropathic pain focussing on regulation of the voltage-gated Nav1.9 sodium channel activity should be investigated.

Trafficking regulates the subcellular distribution of voltage-gated sodium channels in primary sensory neurons

Voltage-gated sodium channels (Na_vs) comprise at least nine pore-forming α subunits. Of these, Na_v1.6, Na_v1.7, Na_v1.8 and Na_v1.9 are the most frequently studied in primary sensory neurons located in the dorsal root ganglion and are mainly localized to the cytoplasm. A large pool of intracellular Na_vs raises the possibility that changes in Na_v trafficking could alter channel function. The molecular mediators of Na_v trafficking mainly consist of signals within the Na_vs themselves, interacting proteins and extracellular factors. The surface expression of Na_vs is achieved by escape from the endoplasmic reticulum and proteasome degradation, forward trafficking and plasma membrane anchoring, and it is also regulated by channel phosphorylation and ubiquitination in primary sensory neurons. Axonal transport and localization of Na_vs in afferent fibers involves the motor protein KIF5B and scaffold proteins, including contactin and PDZ domain containing 2. Localization of Na_v1.6 to the nodes of Ranvier in myelinated fibers of primary sensory neurons requires node formation and the submembrane cytoskeletal protein complex. These findings inform our understanding of the molecular and cellular mechanisms underlying Na_v trafficking in primary sensory neurons.

Mucosal signaling in the bladder

The bladder mucosa is comprised of the multilayered urothelium, lamina propria (LP), microvasculature, and smooth muscle fibers (muscularis mucosae). The muscularis mucosae is not always present in the mucosa, and its presence is related to the thickness of the LP. Since there are no mucus secreting cells, "mucosa" is an imprecise term. Nerve fibers are present in the LP of the mucosa. Efferent nerves mediate mucosal contractions which can be elicited by electrical field stimulation (EFS) and various agonists. The source of mucosal contractility is unknown, but may arise from the muscularis mucosae or myofibroblasts. EFS also increases frequency of mucosal venule contractions. Thus, efferent neural activity has multiple effects on the mucosa. Afferent activity has been measured when the mucosa is stimulated by mechanical and stretch stimuli from the luminal side. Nerve fibers have been shown to penetrate into the urothelium, allowing urothelial cells to interact with nerves. Myofibroblasts are specialized cells within the LP that generate spontaneous electrical activity which then can modulate both afferent and efferent neural activities. Thus mucosal signaling is defined as interactions between bladder autonomic nerves with non-neuronal cells within the mucosa. Mucosal signaling is likely to be involved in clinical functional hypersensory bladder disorders (e.g. overactive bladder, urgency, urgency incontinence, bladder pain syndrome) in which mechanisms are poorly understood despite high prevalence of these conditions. Targeting aberrant mucosal signaling could represent a new approach in treating these disorders.

A 3.7 kb fragment of the mouse Scn10a gene promoter directs neural crest but not placodal lineage EGFP expression in a transgenic animal

Under physiological conditions, the voltage-gated sodium channel Nav1.8 is expressed almost exclusively in primary sensory neurons. The mechanism restricting Nav1.8 expression is not entirely clear, but we have previously described a 3.7 kb fragment of the Scn10a promoter capable of recapitulating the tissue-specific expression of Nav1.8 in transfected neurons and cell lines (Puhl and Ikeda, 2008). To validate these studies in vivo, a transgenic mouse encoding EGFP under the control of this putative sensory neuron specific promoter was generated and characterized in this study. Approximately 45% of dorsal root ganglion neurons of transgenic mice were EGFP-positive (mean diameter = 26.5 μm). The majority of EGFP-positive neurons bound isolectin B4, although a small percentage (∼10%) colabeled with markers of A-fiber neurons. EGFP expression correlated well with the presence of Nav1.8 transcript (95%), Nav1.8-immunoreactivity (70%), and TTX-R INa (100%), although not all Nav1.8-expressing neurons expressed EGFP. Several cranial sensory ganglia originating from neurogenic placodes, such as the nodose ganglion, failed to express EGFP, suggesting that additional regulatory elements dictate Scn10a expression in placodal-derived sensory neurons. EGFP was also detected in discrete brain regions of transgenic mice. Quantitative PCR and Nav1.8-immunoreactivity confirmed Nav1.8 expression in the amygdala, brainstem, globus pallidus, lateral and paraventricular hypothalamus, and olfactory tubercle. TTX-R INa recorded from EGFP-positive hypothalamic neurons demonstrate the usefulness of this transgenic line to study novel roles of Nav1.8 beyond sensory neurons. Overall, Scn10a-EGFP transgenic mice recapitulate the majority of the Nav1.8 expression pattern in neural crest-derived sensory neurons.