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Burning mouth syndrome (BMS) is a chronic idiopathic orofacial pain disorder, characterized by persistent burning sensations and pain without clear pathological causes. Recent research suggests that small fiber neuropathy (SFN) may play a significant role in the neuropathic pain and sensory disturbances associated with BMS. Following PRISMA guidelines, this systematic review aims to evaluate and synthesize current evidence supporting SFN's involvement in BMS. The protocol is registered in PROSPERO (CRD42024555839). The results show eight studies reported reductions in nerve fiber density in tongue biopsies (ranging from 30% to 60%), along with morphological changes indicative of small fiber damage. Additionally, an increase in TRPV1-positive, NGF-positive, and P2X3-positive fibers, overexpression of Nav1.7, and slight underexpression of Nav1.9 mRNA were observed in BMS patients. Quantitative Sensory Testing in seven studies revealed sensory abnormalities such as reduced cool detection and cold pain thresholds. Blink reflex and corneal confocal microscopy also indicated peripheral and central small fiber damage, along with increased artemin mRNA expression. The evidence strongly supports SFN as a key factor in the pathophysiology of BMS, particularly due to reductions in nerve fiber density and altered sensory thresholds. However, variability across studies highlights the need for larger, standardized research to establish causal relationships and guide therapeutic strategies. [ABSTRACT FROM AUTHOR]

• hNa v 1.9_C4 chimera in CHOK-1 cells encompasses the extracellular and transmembrane domain of hNa v 1.9. • Synthetic cannabinoid ACEA blocked hNa v 1.9_C4+ß1 currents causing a hyperpolarizing shift in inactivation. • hNa v 1.9_C4 channels display window currents, significantly reduced by ACEA. • Computationally, ACEA binds the local anaesthetic binding site. • The local anaesthetic profile of ACEA may be involved in cannabinoid analgesia. Cannabinoids regulate analgesia, which has aroused much interest in identifying new pharmacological therapies in the management of refractory pain. Voltage-gated Na+ channels (Na v s) play an important role in inflammatory and neuropathic pain. In particular, Na v 1.9 is involved in nociception and the understanding of its pharmacology has lagged behind because it is difficult to express in heterologous systems. Here, we utilized the chimeric channel hNa v 1.9_C4, that comprises the extracellular and transmembrane domains of hNa v 1.9, co-expressed with the ß1 subunit on CHO-K1 cells to characterize the electrophysiological effects of ACEA, a synthetic surrogate of the endogenous cannabinoid anandamide. ACEA induced a tonic block, decelerated the fast inactivation, markedly shifted steady-state inactivation in the hyperpolarized direction, decreasing the window current and showed use-dependent block, with a high affinity for the inactivated state (k i = 0.84 µM). Thus, we argue that ACEA possess a local anaesthetic-like profile. To provide a mechanistic understanding of its mode of action at the molecular level, we combined induced fit docking with Monte Carlo simulations and electrostatic complementarity. In agreement with the experimental evidence, our computer simulations revealed that ACEA binds Tyr1599 of the local anaesthetics binding site of the hNa v 1.9, contacting residues that bind cannabinol (CBD) in the Na v Ms channel. ACEA adopted a conformation remarkably similar to the crystallographic conformation of anandamide on a non-homologous protein, obstructing the Na+ permeation pathway below the selectivity filter to occupy a highly conserved binding pocket at the intracellular side. These results describe a mechanism of action, possibly involved in cannabinoid analgesia. [ABSTRACT FROM AUTHOR]

Background: Familial episodic pain syndrome type 3 (FEPS3) is an inherited disorder characterized by the early-childhood onset of severe episodic pain that primarily affects the distal extremities. As skin biopsy has revealed a reduction in intraepidermal nerve fiber density and degeneration of the unmyelinated axons, it remains unclear whether FEPS3 patients have pathological changes in the peripheral nerve. Methods: The clinical features of patients with FEPS3 were summarized in a large autosomal dominant family. Sural nerve biopsies were conducted in two patients. Whole exome sequencing (WES) was performed in the index patient. Sanger sequencing was used to analyze family co-segregation. Results: Fourteen members exhibited typical and uniform clinical phenotypes characterized by length-dependent and age-dependent severe episodic pain affecting the distal extremities, which can be relieved with anti-inflammatory medicine. The WES revealed a heterozygous mutation c.665G > A (p.R222H) in the SCN11A gene, which was co-segregated with the clinical phenotype in this family. A sural biopsy in patient V:1, who was experiencing episodic pain at 16 years old, showed normal structure, while the sural nerve in patient IV:1, whose pain attack had completely diminished at 42 years old, displayed a decrease of the density of unmyelinated axons with the axonal degeneration. Conclusions: The clinical phenotype of FEPS3 showed distinctive characteristics that likely arise from dysfunctional nociceptive neurons that lack detectable pathological alterations in the nerve fibers. Nevertheless, long-term dysfunction of the Nav1.9 channel may cause degeneration of the unmyelinated fibers in FEPS3 patient with pain remission. [ABSTRACT FROM AUTHOR]

Trigeminal neuralgia (TN) is a common type of peripheral neuralgia in clinical practice, which is usually difficult to cure. Common analgesic drugs are difficult for achieving the desired analgesic effect. Syb-prII-1 is a β-type scorpion neurotoxin isolated from the scorpion venom of Buthus martensi Karsch (BmK). It has an important influence on the voltage-gated sodium channel (VGSCs), especially closely related to Nav1.8 and Nav1.9. To explore whether Syb-prII-1 has a good analgesic effect on TN, we established the Sprague Dawley (SD) rats' chronic constriction injury of the infraorbital nerve (IoN-CCI) model. Behavioral, electrophysiological, Western blot, and other methods were used to verify the model. It was found that Syb-prII-1 could significantly relieve the pain behavior of IoN-CCI rats. After Syb-prII-1 was given, the phosphorylation level of the mitogen-activated protein kinases (MAPKs) pathway showed a dose-dependent decrease after IoN-CCI injury. Moreover, Syb-prII-1(4.0 mg/kg) could significantly change the steady-state activation and inactivation curves of Nav1.8. The steady-state activation and inactivation curves of Nav1.9 were similar to those of Nav1.8, but there was no significant difference. It was speculated that it might play an auxiliary role. The binding mode, critical residues, and specific interaction type of Syb-prII-1 and VSD2rNav1.8 were clarified with computational simulation methods. Our results indicated that Syb-prII-1 could provide a potential treatment for TN by acting on the Nav1.8 target. [ABSTRACT FROM AUTHOR]

Abstract: Human NaV1.9 (hNaV1.9), encoded by SCN11A, is preferentially expressed in nociceptors, and its mutations have been linked to pain disorders. NaV1.9 could be a promising drug target for pain relief. However, the modulation of NaV1.9 activity has remained elusive. Here, we identified a new candidate NaV1.9-interacting partner, protein arginine methyltransferase 7 (PRMT7). Whole-cell voltage-clamp recordings showed that coelectroporation of human SCN11A and PRMT7 in dorsal root ganglion (DRG) neurons of Scn11a-/- mice increased the hNaV1.9 current density. By contrast, a PRMT7 inhibitor (DS-437) reduced mNaV1.9 currents in Scn11a+/+ mice. Using the reporter molecule CD4, we observed an increased distribution of hLoop1 on the cell surface of PRMT7-overexpressing HKE293T cells. Furthermore, we found that PRMT7 mainly binds to residues 563 to 566 within the first intracellular loop of hNaV1.9 (hLoop1) and methylates hLoop1 at arginine residue 519. Moreover, overexpression of PRMT7 increased the number of action potential fired in DRG neurons of Scn11a+/+ mice but not Scn11a-/- mice. However, DS-437 significantly inhibited the action potential frequency of DRG neurons and relieved pain hypersensitivity in Scn11aA796G/A796G mice. In summary, our observations revealed that PRMT7 modulates neuronal excitability by regulating NaV1.9 currents, which may provide a potential method for pain treatment. [ABSTRACT FROM AUTHOR]

Familial episodic pain syndrome (FEPS) is an autosomal-dominant inherited disorder characterized by paroxysmal pain episodes. FEPS appears in early childhood, gradually disappearing with age, and pain episodes can be triggered by fatigue, bad weather, and cold temperatures. Several gain-of-function variants have been reported for SCN9A, SCN10A, or SCN11A, which encode the voltage-gated sodium channel α subunits Nav1.7, Nav1.8, and Nav1.9, respectively. In this study, we conducted genetic analysis in a four-generation Japanese pedigree. The proband was a 7-year-old girl, and her brother, sister, mother, and grandmother were also experiencing or had experienced pain episodes and were considered to be affected. The father was unaffected. Sequencing of SCN9A, SCN10A, and SCN11A in the proband revealed a novel heterozygous variant of SCN11A: g.38894937G>A (c.2431C>T, p.Leu811Phe). This variant was confirmed in other affected members but not in the unaffected father. The affected residue, Leu811, is located within the DII/S6 helix of Nav1.9 and is important for signal transduction from the voltage-sensing domain and pore opening. On the other hand, the c.2432T>C (p.Leu811Pro) variant is known to cause congenital insensitivity to pain (CIP). Molecular dynamics simulations showed that p.Leu811Phe increased the structural stability of Nav1.9 and prevented the necessary conformational changes, resulting in changes in the dynamics required for function. By contrast, CIP-related p.Leu811Pro destabilized Nav1.9. Thus, we speculate that p.Leu811Phe may lead to current leakage, while p.Leu811Pro can increase the current through Nav1.9. [ABSTRACT FROM AUTHOR]

Chronic pain is a global problem affecting up to 20% of the world's population and has a significant economic, social and personal cost to society. Sensory neurons of the dorsal root ganglia (DRG) detect noxious stimuli and transmit this sensory information to regions of the central nervous system (CNS) where activity is perceived as pain. DRG neurons express multiple voltage-gated sodium channels that underlie their excitability. Research over the last 20 years has provided valuable insights into the critical roles that two channels, NaV1.7 and NaV1.9, play in pain signalling in man. Gain of function mutations in NaV1.7 cause painful conditions while loss of function mutations cause complete insensitivity to pain. Only gain of function mutations have been reported for NaV1.9. However, while most NaV1.9 mutations lead to painful conditions, a few are reported to cause insensitivity to pain. The critical roles these channels play in pain along with their low expression in the CNS and heart muscle suggest they are valid targets for novel analgesic drugs. [ABSTRACT FROM AUTHOR]

Bipolar disorder (BD) is a complex neuropsychiatric disorder characterized by recurrent mania and depression episodes and requiring lifelong treatment with mood stabilizing drugs. Several lines of evidence, including with BD patient iPSC‐derived neurons, suggest that neuronal hyperexcitability may underlie the key clinical symptoms of BD. Indeed, higher mRNA levels of SCN11A, coding for the voltage‐gated sodium channel NaV1.9 implicated in nociception, were detected in iPSC‐derived neurons from BD patients, and were normalized by in vitro lithium. Here we studied SCN11A expression in peripheral blood mononuclear cells (PBMCs) from well‐phenotyped female BD patients and controls and evaluated their association with several clinical sub‐phenotypes. We observed higher mRNA levels of SCN11A in PBMCs from female BD patients with no records of alcohol dependence (p =.0050), no records of psychosis (p =.0097), or no records of suicide attempts (p =.0409). A trend was observed for higher SCN11A expression (FD = 1.91; p =.052) in BD PBMCs compared with controls. Datamining of published postmortem gene expression datasets indicated higher SCN11A expression in dorsolateral prefrontal cortex and orbitofrontal cortex tissues from BD patients compared with controls. Higher phenotype‐associated expression levels in PBMC from BD patients were also observed for ID2 (alcohol dependence, suicide attempts) and HDGFRP3 (seasonal BD pattern). Our findings suggest that higher PBMC SCN11A expression levels may be associated with certain behavioral BD sub‐phenotypes, including lack of alcohol dependence and psychosis, among BD patients. The NaV1.9 voltage‐gated sodium channel thus deserves consideration as a tentative phenotype modifier in BD. [ABSTRACT FROM AUTHOR]

Voltage-activated Na+ (Na V) channels are crucial contributors to rapid electrical signaling in the human body. As such, they are among the most targeted membrane proteins by clinical therapeutics and natural toxins. Several of the nine mammalian Na V channel subtypes play a documented role in pain or other sensory processes such as itch, touch, and smell. While causal relationships between these subtypes and biological function have been extensively described, the physiological role of Na V 1.9 is less understood. Yet, mutations in Na V 1.9 can cause striking disease phenotypes related to sensory perception such as loss or gain of pain and chronic itch. Here, we explore our current knowledge of the mechanisms by which Na V 1.9 may contribute to pain and elaborate on the challenges associated with establishing links between experimental conditions and human disease. This review also discusses the lack of comprehensive insights into Na V 1.9-specific pharmacology, an unfortunate situation since modulatory compounds may have tremendous potential in the clinic to treat pain or as precision tools to examine the extent of Na V 1.9 participation in sensory perception processes. [ABSTRACT FROM AUTHOR]

Pain is a distressing physical and emotional experience associated with actual or potential tissue injury, or an experience described in terms of such injury. The primary function of nociceptors, such as some dorsal root ganglion (DRG) neurons, is to transduce noxious sensory modalities, e.g. mechanical pressure, cold and heat, into electrical impulses and to transmit these to processing centres in the central nervous system (CNS). Modality-specific pain pathways have been identified through in vivo deletion of voltage-gated Na+ channels in mouse DRG neurons. Deletion of Nav1.8 channels has been shown to result in loss of mechanosensory and cold-induced pain, but not-heat induced pain, whereas deletion of Nav1.7 channels has been seen to abolish responses to noxious heat and mechanical stimuli. The present review constitutes an attempt to elucidate the mechanisms through which voltage-gated Na+ channels are involved in modality-specific pain pathways. It has been found that Nav1.8 and Nav1.9 channels are resistant to slow inactivation upon cooling, maintaining activity even though channels on other sensory afferents may be inactivated. Nav1.7 channel activity is reported to be coupled to substance P release into the dorsal horn of dorsal root ganglion (DRG) neurons in heat-specific pain pathways. Recent research has also offered insight into the role of Nav1.7 and Nav1.9 mutations in pain-related conditions, e.g. inherited erythromelalgia and cold-aggravated pain, respectively, as these influence kinetic parameters, such as open state probability. Therefore, voltage-gated Na+ channels appear to be playing an important role in segregating modality-specific pain pathways. The identification of markers for mechanisms implicated in the activation of these pathways could potentially pave the way towards the development of more effective analgesics. [ABSTRACT FROM AUTHOR]

Background Visceral pain is a common symptom for patients with gastrointestinal ( GI) disease. It is unpleasant, debilitating, and represents a large unmet medical need for effective clinical treatments. Recent studies have identified NaV1.9 as an important regulator of afferent sensitivity in visceral pain pathways to mechanical and inflammatory stimuli, suggesting that NaV1.9 could represent an important therapeutic target for the treatment of visceral pain. This potential has been highlighted by the identification of patients who have an insensitivity to pain or painful neuropathies associated with mutations in SCN11A, the gene encoding voltage-gated sodium channel subtype 1.9 (NaV1.9). Purpose Here, we address the role of NaV1.9 in visceral pain and what known human NaV1.9 mutants can tell us about NaV1.9 function in gut physiology and pathophysiology. [ABSTRACT FROM AUTHOR]

Activation of visceral nociceptors by inflammatory mediators contributes to visceral hypersensitivity and abdominal pain associated with many gastrointestinal disorders. Purine and pyrimidine nucleotides (e.g., ATP and UTP) are strongly implicated in this process following their release from epithelial cells during mechanical stimulation of the gut, and from immune cells during inflammation. Actions of ATP are mediated through both ionotropic P2X receptors and metabotropic P2Y receptors. P2X receptor activation causes excitation of visceral afferents; however, the impact of P2Y receptor activation on visceral afferents innervating the gut is unclear. Here we investigate the effects of stimulating P2Y receptors in isolated mouse colonic sensory neurons, and visceral nociceptor fibers in mouse and human nerve-gut preparations. Additionally, we investigate the role of Navl .9 in mediating murine responses. The application of UTP (P2Y2 and P2Y4 agonist) sensitized colonic sensory neurons by increasing action potential firing to current injection and depolarizing the membrane potential. The application of ADP (P2Y2,, P2Y12, and P2Y13 agonist) also increased action potential firing, an effect blocked by the selective P2Y, receptor antagonist MRS2500. UTP or ADP stimulated afferents, including mouse and human visceral nociceptors, in nerve-gut preparations. P2Y! and P2Y2 transcripts were detected in 80% and 56% of retrogradely labeled colonic neurons, respectively. Nav1.9 transcripts colocalized in 86% of P2Y1,-positive and 100% of P2Y2-positive colonic neurons, consistent with reduced afferent fiber responses to UTP and ADP in Nav1.9-/- mice. These data demonstrate that P2Y receptor activation stimulates mouse and human visceral nociceptors, highlighting P2Y-dependent mechanisms in the generation of visceral pain during gastrointestinal disease. [ABSTRACT FROM AUTHOR]

Neuropathic pain is a global medical concern, characterized by spontaneous pain, heat hyperalgesia and mechanical allodynia. The condition has been associated with alterations in the voltage-gated sodium channels, Nav1.8 and Nav1.9, in nociceptive neurons termed nociceptors. However, an explanation for the contribution of these channels to the phenotype observed in neuropathic pain remains to be elucidated. The changes induced by chronic constriction injury (CCI) to Nav1.8 and Nav1.9 mRNA and protein levels, as well as electrical currents in injured and contralateral non-injured dorsal root ganglion (DRG) neurons are described in the present study. A marked downregulation was observed for each Nav isoform transcript and protein expressed in injured neurons with the exception of the Nav1.9 protein, which exhibited no change, while in contralateral non-injured neurons, the levels of protein and mRNA remained unchanged. Nav isoform functional analysis was then performed in L4-6 DRG neurons 14 days after CCI. The Nav1.8 current density was markedly decreased in injured DRG neurons following CCI. The voltage-dependent activation of the Nav1.8 channel in these neurons was shifted to depolarized potentials by 5.3 mV, while it was shifted to hyperpolarized potentials by 10 mV for inactivation. The electrophysiological function of Nav1.9 was not affected by CCI. The present study demonstrated that ectopic discharge following CCI, which was likely induced by a positive shift in the Nav1.8 current inactivation curve in injured neurons, enhanced the excitability of the neurons by facilitating tetrodotoxin-resistant sodium channels into the fast inactivation state and did not occur as a result of a compensatory redistribution in the contralateral uninjured neurons. [ABSTRACT FROM AUTHOR]

In the central auditory system, cartwheel cells (CWCs) are a group of interneurons in the dorsal cochlear nucleus (DCN). While other DCN neurons respond to stimuli with a simple discharge pattern of single action potentials (SAPs), CWCs respond with complex action potentials (CAPs), consisting of SAPs superimposed on a slow depolarization. The CAPs in CWCs may participate in various auditory or non-auditory signaling processing but its intrinsic mechanisms are largely unknown. In the present study, in vitro whole-cell current clamp recordings on neonatal mice brain slices were used to demonstrate that CWCs respond to brief voltage stimulation with CAPs. Western blotting and immunohistochemistry were also utilized to demonstrate that Nav1.9 was expressed in the CWCs. Finally, when Nav1.9 was genetically silenced, CWCs responded to voltage stimulation with SAPs, not CAPs. The results strongly suggested that Nav1.9 was expressed and functionally contributed to the signaling processing in the central auditory pathway. [ABSTRACT FROM AUTHOR]

Abstract: Many veterans of the 1991 Gulf War (GW) returned from that conflict with a widespread chronic pain affecting deep tissues. Recently, we have shown that a 60day exposure to the insecticides permethrin, chlorpyrifos, and pyridostigmine bromide (NTPB) had little influence on nociceptor action potential forming Nav1.8, but increased Kv7 mediated inhibitory currents 8weeks after treatment. Using the same exposure regimen, we used whole cell patch methods to examine whether the influences of NTPB could be observed on Nav1.9 expressed in muscle and vascular nociceptors. During a 60day exposure to NTPB, rats exhibited lowered muscle pain thresholds and increased rest periods, but these measures subsequently returned to normal levels. Eight and 12weeks after treatments ceased, DRG neurons were excised from the sensory ganglia. Whole cell patch studies revealed little change in voltage dependent activation and deactivation of Nav1.9, but significant increases in the amplitude of Nav1.9 were observed 8weeks after exposure. Cellular studies, at the 8week delay, revealed that NTPB also significantly prolonged action potential duration and afterhyperpolarization (22°C). Acute application of permethrin (10μM) also increased the amplitude of Nav1.9 in skin, muscle and vascular nociceptors. In conclusion, chronic exposure to Gulf War agents produced long term changes in the amplitude of Nav1.9 expressed in muscle and vascular nociceptors. The reported increases in Kv7 amplitude may have been an adaptive response to increased Nav1.9, and effectively suppressed behavioral pain measures in the post treatment period. Factors that alter the balance between Nav1.9 and Kv7 could release spontaneous discharge and produce chronic deep tissue pain. [Copyright &y& Elsevier]

Background Voltage-gated sodium channels Nav1.8 and Nav1.9 are expressed preferentially in small diameter sensory neurons, and are thought to play a role in the generation of ectopic activity in neuronal cell bodies and/or their axons following peripheral nerve injury. The expression of Nav1.8 and Nav1.9 has been quantified in human lingual nerves that have been previously injured inadvertently during lower third molar removal, and any correlation between the expression of these ion channels and the presence or absence of dysaesthesia investigated. Results Immunohistochemical processing and quantitative image analysis revealed that Nav1.8 and Nav1.9 were expressed in human lingual nerve neuromas from patients with or without symptoms of dysaesthesia. The level of Nav1.8 expression was significantly higher in patients reporting pain compared with no pain, and a significant positive correlation was observed between levels of Nav1.8 expression and VAS scores for the symptom of tingling. No significant differences were recorded in the level of expression of Nav1.9 between patients with or without pain. Conclusions These results demonstrate that Nav1.8 and Nav1.9 are present in human lingual nerve neuromas, with significant correlations between the level of expression of Nav1.8 and symptoms of pain. These data provide further evidence that changes in expression of Nav1.8 are important in the development and/or maintenance of nerve injury-induced pain, and suggest that Nav1.8 may be a potential therapeutic target. [ABSTRACT FROM AUTHOR]

Abstract: Voltage-gated Na+ channels (Nav) are the targets of a variety of scorpion toxins. Here, we investigated the effects of Amm VIII, a toxin isolated from the venom of the scorpion Androctonus mauretanicus mauretanicus, on pain-related behaviours in mice. The effects of Amm VIII were compared with the classic scorpion α-toxin AaH II from Androctonus australis. Contrary to AaH II, intraplantar injection of Amm VIII at relatively high concentrations caused little nocifensive behaviours. However, Amm VIII induced rapid mechanical and thermal pain hypersensitivities. We evaluated the toxins’ effects on Nav currents in nociceptive dorsal root ganglion (DRG) neurons and immortalized DRG neuron-derived F11 cells. Amm VIII and AaH II enhanced tetrodotoxin-sensitive (TTX-S) Nav currents in DRG and F11 cells. Both toxins impaired fast inactivation and negatively shifted activation. AaH II was more potent than Amm VIII at modulating TTX-S Nav currents with EC50 of 5nM and 1μM, respectively. AaH II and Amm VIII also impaired fast inactivation of Nav1.7, with EC50 of 6.8nM and 1.76μM, respectively. Neither Nav1.8 nor Nav1.9 was affected by the toxins. AaH II and Amm VIII reduced first spike latency and lowered action potential threshold. Amm VIII was less efficient than AaH II in increasing the gain of the firing frequency-stimulation relationship. In conclusion, our data show that Amm VIII, although less potent than AaH II, acts as a gating-modifier peptide reminiscent of classic α-toxins, and suggest that its hyperalgesic effects can be ascribed to gain-of-function of TTX-S Na+ channels in nociceptors. [Copyright &y& Elsevier]

Background: Amitriptyline (AMI) is tricyclic antidepressant that has been widely used to manage various chronic pains such as migraines. Its efficacy is attributed to its blockade of voltage-gated sodium channels (VGSCs). However, the effects of AMI on the tetrodotoxin-resistant (TTX-r) sodium channel Nav1.9 currents have been unclear to present. Results: Using a whole-cell patch clamp technique, this study showed that AMI efficiently inhibited Nav1.9 currents in a concentration-dependent manner and had an IC50 of 15.16 μM in acute isolated trigeminal ganglion (TG) neurons of the rats. 10 μM AMI significantly shifted the steady-state inactivation of Nav1.9 channels in the hyperpolarizing direction without affecting voltage-dependent activation. Surprisingly, neither 10 nor 50 μM AMI caused a use-dependent blockade of Nav1.9 currents elicited by 60 pulses at 1 Hz. Conclusion: These data suggest that AMI is a state-selective blocker of Nav1.9 channels in rat nociceptive trigeminal neurons, which likely contributes to the efficacy of AMI in treating various pains, including migraines. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium channels (VGSC) are the primary mediators of electrical signal amplification and propagation in excitable cells. VGSC subtypes are diverse, with different biophysical and pharmacological properties, and varied tissue distribution. Altered VGSC expression and/or increased VGSC activity in sensory neurons is characteristic of inflammatory and neuropathic pain states. Therefore, VGSC modulators could be used in prospective analgesic compounds. VGSCs have specific binding sites for four conotoxin families: μ-, μO-, δ- and í-conotoxins. Various studies have identified that the binding site of these peptide toxins is restricted to well-defined areas or domains. To date, only the μ- and μO-family exhibit analgesic properties in animal pain models. This review will focus on conotoxins from the μ- and μO-families that act on neuronal VGSCs. Examples of how these conotoxins target various pharmacologically important neuronal ion channels, as well as potential problems with the development of drugs from conotoxins, will be discussed. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of low-threshold tetrodotoxin-resistant (TTX-r) Na+ current and its co-expression with high-threshold TTX-r Na+ current were studied in randomly selected acutely dissociated rat trigeminal ganglion (non-identified TG cells) and TG cells serving the temporomandibular joint (TMJ-TG cells). Conditions previously shown to enhance NaV1.9 channel-mediated currents (holding potential (HP) −80mV, 130-mM fluoride internally) were employed to amplify the low-threshold Na+ current. Under these conditions, detectable low-threshold Na+ current was exhibited by 16 out of 21 non-identified TG cells (average, 1810±358pA), and by nine of 14 TMJ-TG cells (average, 959±525pA). The low-threshold Na+ current began to activate around −55mV and was inactivated by holding TG cells at −60mV and delivering 40-ms test potentials (TPs) to 0mV. The inactivation was long lasting, recovering only 8±3% over a 5-min period after the HP was returned to −80mV. Following low-threshold Na+ current inactivation, high-threshold TTX-r Na+ current, evoked from HP −60mV, was observed. High-threshold Na+ current amplitude averaged 16,592±3913pA for TPs to 0mV, was first detectable at an average TP of −34±1.3mV, and was ½ activated at −7.1±2.3mV. In TG cells expressing prominent low-threshold Na+ currents, changing the external solution to one containing 0mM Na+ reduced the amount of current required to hold the cells at −80mV through −50mV, the peak effect being observed at HP −60mV. TG cells recorded from with a more physiological pipette solution containing chloride instead of fluoride exhibited small low-threshold Na+ currents, which were greatly increased upon superfusion of the TG cells with the adenylyl cyclase (AC) activator forskolin. These data suggest two hypotheses: (1) low- and high-threshold NaV1.9 and NaV1.8 channels, respectively, are frequently co-expressed in TG neurons serving the TMJ and other structures, and (2), NaV1.9 channel-mediated currents are small under physiological conditions, but may be enhanced by inflammatory mediators that increase AC activity, and may mediate an inward leak that depolarizes TG neurons, increasing their excitability. [Copyright &y& Elsevier]

Human voltage-activated sodium (Nav) channels are adept at rapidly transmitting electrical signals across long distances in various excitable tissues. As such, they are amongst the most widely targeted ion channels by drugs and animal toxins. Of the nine isoforms, Nav1.8 and Nav1.9 are preferentially expressed in DRG neurons where they are thought to play an important role in pain signaling. Although the functional properties of Nav1.8 have been relatively well characterized, difficulties with expressing Nav1.9 in established heterologous systems limit our understanding of the gating properties and toxin pharmacology of this particular isoform. This review summarizes our current knowledge of the role of Nav1.8 and Nav1.9 in pain perception and elaborates on the approaches used to identify molecules capable of influencing their function. [ABSTRACT FROM AUTHOR]

Suwanchai A, Theerapiboon U, Chattipakorn N, Chattipakorn SC. NaV1.8, but not NaV1.9, is upregulated in the inflamed dental pulp tissue of human primary teeth. International Endodontic Journal, 45, 372-378, 2012. Abstract Aim To investigate alterations in NaV1.8 and NaV1.9 expression within inflamed dental pulp tissue of human primary teeth. Methodology Dental pulp tissue obtained from both normal and inflamed pulps in primary teeth as well as pulps from normal and inflamed permanent teeth was used. The quantity of NaV1.8 and NaV1.9 expression in the dental pulp tissue was investigated using Western blot analysis. General neuron marker (PGP9.5) was used to quantify for neural density, and an increase in metalloproteinase-9 was used to indicate pulpal inflammation in inflamed teeth. Statistically significant differences for each determined parameter between normal and inflamed teeth of both primary and permanent teeth were tested using the Mann-Whitney rank sum test. Results There was no significant difference in neural density of normal and inflamed dental pulp tissue, although degrees of inflammation were increased in the inflamed dental pulp of both permanent and primary teeth ( P < 0.05). NaV1.8 and NaV1.9 expression in inflamed pulps of permanent teeth increased significantly compared with normal permanent teeth ( P < 0.05). However, only NaV1.8 expression was increased significantly in the inflamed dental pulp of primary teeth ( P < 0.05). Conclusions NaV1.8 alone may be the therapeutic target for treatment of painful pulpitis in primary teeth. [ABSTRACT FROM AUTHOR]

Abstract: The effects of forskolin on low-threshold tetrodotoxin-resistant (TTX-r) Na+ currents was studied in small diameter (average ≈ 25 μm) dorsal root ganglion (DRG) cells. All DRG cells included in the study were categorized as type-2 or non-type-2 based on the expression of a low-threshold A-current. In all type-2 and some non-type-2 DRG cells held at −80 mV, the adenylyl cyclase (AC) activator forskolin (10 μM) up-regulated TTX-r Na+ currents evoked with steps to −55 mV through −35 mV (low-threshold current). Up-regulation of low-threshold current by forskolin was mimicked by the protein kinase A (PKA) agonist Sp-cAMPs and the inflammatory mediator serotonin, and blocked by the PKA antagonist Rp-cAMPs. Forskolin-induced up-regulation of low-threshold current evoked from a holding potential of −60 mV was blocked by 40 ms steps to 0 mV, which presumably induced a long lasting inactivation of the low-threshold channels. Reducing to 3 ms the duration of steps to 0 mV, significantly increased the number of DRG cells where low-threshold current was up-regulated by forskolin, presumably by reducing the long-lasting inactivation of the low-threshold channels. In the same cells, high-threshold current, evoked by 40 ms or 3 ms steps to 0 mV, was consistently up-regulated by forskolin. The selective NaV1.8 channel blocker A-803467 markedly blocked high-threshold current but not low-threshold current. The different voltage protocols observed to activate and inactivate the low- and high-threshold currents, and the observation that A-803467 blocked high- but not low-threshold current suggests that the two currents were mediated by different channels, possibly NaV1.8 and NaV1.9, respectively. Inflammatory mediators may simultaneously up-regulate NaV1.8 and NaV1.9 channels in the same nociceptor via a AC/PKA signaling pathway, increasing nociceptor signaling strength, and lowering nociceptor threshold, respectively. [Copyright &y& Elsevier]

Animal models have provided useful insights into the development and treatment of neuropathic pain. New genetic data from both human studies and transgenic mouse models suggest that specific voltage-gated sodium channel subtypes are associated with specific types of pain and, as such, may be useful analgesic drug targets for a variety of pain types including neuropathic pain. Global voltagegated sodium channel blockers such as lidocaine have proven efficacy in treating pain but can be limited by adverse effects when administered systemically. Selective sodium channel blockers targeting channels at the periphery (Nav1.7, Nav1.8, and Nav1.9) could potentially reduce the side effect profile. Individual isoforms of voltage-gated sodium channels have been linked to particular types of pain. Nav1.7 is a useful target for ameliorating acute mechanical pain and inflammatory pain, and strong evidence also suggests that Nav1.9 could be targeted for treating inflammatory pain. Selective blockers of Nav1.8 could also have clinical benefit for visceral pain. Although there is no association between a single sodium channel isoform and neuropathic pain, combined blockade of peripherally expressed isoforms Nav1.7, Nav1.8, and Nav1.9 may prove useful. [ABSTRACT FROM AUTHOR]

Abstract: The modulation by serotonin (5-HT) of low- and high-threshold tetrodotoxin- (TTX) resistant Na+ currents was studied in small-diameter (≈25 μm) 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 μM 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. [Copyright &y& Elsevier]

Abstract: The tetrodotoxin-resistant (TTX-R) voltage-gated Na+ channels Nav1.8 and Nav1.9 are expressed by a subset of primary sensory neurons and have been implicated in various pain states. Although recent studies suggest involvement of TTX-R Na+ channels in sensory synaptic transmission and spinal pain processing, it remains unknown whether TTX-R Na+ channels are expressed and function presynaptically. We examined expression of TTX-R channels at sensory synapses formed between rat dorsal root ganglion (DRG) and spinal cord (SC) neurons in a DRG/SC co-culture system. Immunostaining showed extensive labeling of presynaptic axonal boutons with Nav1.8- and Nav1.9-specific antibodies. Measurements using the fluorescent Na+ indicator SBFI demonstrated action potential–induced presynaptic Na+ entry that was resistant to tetrodotoxin (TTX) but was blocked by lidocaine. Furthermore, presynaptic [Ca2+]i elevation in response to a single action potential was not affected by TTX in TTX-resistant DRG neurons. Finally, glutamatergic synaptic transmission was not inhibited by TTX in more than 50% of synaptic pairs examined; subsequent treatment with lidocaine completely blocked these TTX-resistant excitatory postsynaptic currents. Taken together, these results provide evidence for presynaptic expression of functional TTX-R Na+ channels that may be important for shaping presynaptic action potentials and regulating transmitter release at the first sensory synapse. [Copyright &y& Elsevier]

Abstract: Neuropathic pain resulting from chronic constriction injury (CCI) is critically linked to sensitization of peripheral nociceptors. Voltage gated sodium channels are major contributors to this state and their expression can be upregulated by nerve growth factor (NGF). We have previously demonstrated that neurotrophin-3 (NT-3) acts antagonistically to NGF in modulation of aspects of CCI-induced changes in trkA-associated nociceptor phenotype and thermal hyperalgesia. Thus, we hypothesized that exposure of neurons to increased levels of NT-3 would reduce expression of Nav1.8 and Nav1.9 in DRG neurons subject to CCI. In adult male rats, Nav1.8 and Nav1.9 mRNAs are expressed at high levels in predominantly small to medium size neurons. One week following CCI, there is reduced incidence of neurons expressing detectable Nav1.8 and Nav1.9 mRNA, but without a significant decline in mean level of neuronal expression, and similar findings observed immunohistochemically. There is also increased accumulation/redistribution of channel protein in the nerve most apparent proximal to the first constriction site. Intrathecal infusion of NT-3 significantly attenuates neuronal expression of Nav1.8 and Nav1.9 mRNA contralateral and most notably, ipsilateral to CCI, with a similar impact on relative protein expression at the level of the neuron and constricted nerve. We also observe reduced expression of the common neurotrophin receptor p75 in response to CCI that is not reversed by NT-3 in small to medium sized neurons and may confer an enhanced ability of NT-3 to signal via trkA, as has been previously shown in other cell types. These findings are consistent with an analgesic role for NT-3. [Copyright &y& Elsevier]

Abstract: Differential alterations of sodium channels in small nociceptive C-fiber DRG neurons have been implicated in diabetic neuropathy. In this study, we investigated sodium currents and the expression of sodium channels in large A-fiber DRG neurons in diabetic rats. Compared with controls, large neurons from diabetic rats showed significant increases in both total and TTX-S sodium currents and ∼−15mV shifts in their voltage-dependent activation kinetics. TTX-R Nav1.9 sodium current was also significantly increased, whereas no alteration of TTX-R Nav1.8 current was observed in neurons from diabetic rats. Sodium current induced by fast- or slow-voltage ramps increased markedly in the diabetic neurons as well. Immunofluorescence studies showed significant increases in the levels and number of large DRG neurons from diabetic rats expressing Nav1.2, Nav1.3, Nav1.7, and Nav1.9 whereas Nav1.8 decreased. We also observed a decrease in the number of nodes of Ranvier expressing Nav1.8 and in staining intensity of Nav1.6 and Nav1.8 at nodes. Our results suggest that alterations of sodium channels occur in large DRG neurons and A-fibers, and may play an important role in diabetic sensory neuropathy. [Copyright &y& Elsevier]

Changes in the dorsal root ganglion (DRG) expression of the sodium channels Nav1.8 and Nav1.9 may contribute to injury-induced hyperexcitability and pain. Interferon (IFN)-γ receptor −/− mice display a reduced pain-related behavior after nerve injury as compared to wild-type mice (NeuroReport 8 (1997) 1311). To elucidate a possible role for IFN-γ in the regulation of sodium channels, we have studied the DRG mRNA expression of Nav1.8/Nav1.9 in IFN-γ- or IFN-γ receptor-deficient mice. In both types of mice, nerve damage induced a downregulation of Nav1.8 as well as Nav1.9. The magnitude of this reduction was similar to that observed in wild-type animals. These results indicate that the downregulation of Nav1.8/Nav1.9 in damaged DRG neurons is not influenced by IFN-γ. Thus, the reduced pain-related behavior of nerve-injured IFN-γ receptor null mice is not due to differential changes in the regulation of Nav1.8/Nav1.9 mRNA. [Copyright &y& Elsevier]

Previous electrophysiological studies have demonstrated that tetrodotoxin-resistant (TTX-R) sodium channels contribute to action potential electrogenesis and conduction along non-myelinated PNS axons. Moreover, recent work has established that TTX-R sodium channels play a major role in the generation of action potentials in the terminals of non-myelinated nociceptive axons innervating the cornea. We have utilized subtype-specific antibodies to sodium channels Nav1.8 and Nav1.9 to examine the molecular identity of the TTX-R sodium channels that are present in these axons. Both Nav1.8 and Nav1.9 sodium channels are expressed diffusely along the entire lengths of non-myelinated corneal axons, from the nerve plexus at the corneoscleral limbus to the distal corneal leash fibers. Moreover, both Nav1.8 and Nav1.9 are localized at the bulb-like nerve terminals of the leash fibers within the superficial epithelial layers of the cornea. These observations suggest that both TTX-R sodium channels Nav1.8 and Nav1.9 contribute to the electrogenesis of non-myelinated axons of the cornea. [Copyright &y& Elsevier]

A spared nerve injury of the sciatic nerve (SNI) or a segmental lesion of the L5 and L6 spinal nerves (SNL) lead to behavioral signs of neuropathic pain in the territory innervated by adjacent uninjured nerve fibers, while a chronic constriction injury (CCI) results in pain sensitivity in the affected area. While alterations in voltage-gated sodium channels (VGSCs) have been shown to contribute to the generation of ectopic activity in the injured neurons, little is known about changes in VGSCs in the neighboring intact dorsal root ganglion (DRG) neurons, even though these cells begin to fire spontaneously. We have now investigated changes in the expression of the TTX-resistant VGSCs, Nav1.8 (SNS/PN3) and Nav1.9 (SNS2/NaN) by immunohistochemistry in rat models of neuropathic pain both with an intermingling of intact and degenerated axons in the nerve stump (SNL and CCI) and with a co-mingling in the same DRG of neurons with injured and uninjured axons (sciatic axotomy and SNI). The expression of Nav1.8 and Nav1.9 protein was abolished in all injured DRG neurons, in all models. In intact DRGs and in neighboring non-injured neurons, the expression and the distribution among the A- and C-fiber neuronal populations of Nav1.8 and Nav1.9 was, however, unchanged. While it is unlikely, therefore, that a change in the expression of TTX-resistant VGSCs in non-injured neurons contributes to neuropathic pain, it is essential that molecular alterations in both injured and non-injured neurons in neuropathic pain models are investigated. [Copyright &y& Elsevier]

[Display omitted] • Knock-out one allele of miR-96 causes mechanical and thermal allodynia in mice. • Increased spinal Na v 1.7, 1.8, 1.9, 1.2 contributes to allodynia in miR-96 KO mice. • Intrathecal injection of Na v 1.7 or 1.8 blocker relieves allodynia of miR-96 KO mice. • Gad2::CreERT2 conditional miR-96 knockout mice phenocopies global knockout mice. • Nerve injury induces loss of miR-96 and de-repression of Na v s in GABAergic neurons. Voltage-gated sodium channels (Na v s) 1.7, 1.8, and 1.9 are predominately expressed in peripheral sensory neurons and are critical for action potential propagation in nociceptors. Unexpectedly, we found that expression of SCN9A, SCN10A, SCN11A, and SCN2A, the alpha subunit of Na v 1.7, Na v 1.8, Na v 1.9 and Na v 1.2, respectively, are up-regulated in spinal dorsal horn (SDH) neurons of miR-96 knockout mice. These mice also have de-repression of CACNA2D1/2 in DRG and display thermal and mechanical allodynia that could be attenuated by intrathecal or intraperitoneal injection of Na v 1.7 or Na v 1.8 blockers or Gabapentin. Moreover, Gad2::CreERT2 conditional miR-96 knockout mice phenocopied global knockout mice, implicating inhibitory neurons; nerve injury induced significant loss of miR-96 in SDH GABAergic and Glutamatergic neurons in mice which negatively correlated to up-regulation of Na v 1.7, Na v 1.8, Na v 1.9 and Scn2a , this dis-regulation of miR-96 and Na v s in SDH neurons contributed to neuropathic pain which can be alleviated by intrathecal injection of Na v 1.7 or Na v 1.8 blockers. In conclusion, miR-96 is required to avoid allodynia through limiting the expression of VGCCs and Na v s in DRG and Na v s in SDH in naïve and nerve injury-induced neuropathic pain mice. Our findings suggest that central nervous system penetrating Na v 1.7 and Na v 1.8 blockers may be efficacious for pain relief. [ABSTRACT FROM AUTHOR]

The primary studies have shown that scorpion analgesic peptide N58A has a significant effect on voltage-gated sodium channels (VGSCs) and plays an important role in neuropathic pain. The purpose of this study was to investigate the analgesic effect of N58A on trigeminal neuralgia (TN) and its possible mechanism. The results showed that N58A could significantly increase the threshold of mechanical pain and thermal pain and inhibit the spontaneous asymmetric scratching behavior of rats. Western blotting results showed that N58A could significantly reduce the protein phosphorylation level of ERK1/2, P38, JNK, and ERK5/CREB pathways and the expression of Nav1.8 and Nav1.9 proteins in a dose-dependent manner. The changes in current and kinetic characteristics of Nav1.8 and Nav1.9 channels in TG neurons were detected by the whole-cell patch clamp technique. The results showed that N58A significantly decreased the current density of Nav1.8 and Nav1.9 in model rats, and shifted the activation curve to hyperpolarization and the inactivation curve to depolarization. In conclusion, the analgesic effect of N58A on the chronic constriction injury of the infraorbital (IoN-CCI) model rats may be closely related to the regulation of the MAPK pathway and Nav1.8 and Nav1.9 sodium channels. [ABSTRACT FROM AUTHOR]

Summary 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. Highlights • Hibernating DRG neurons have unchanged resting potential, resistance, and rheobase • Hibernating DRG neurons retain electrogenicity, with decreased firing rate • Properties of hibernating neurons are innate and not triggered by cold alone • Properties of hibernating neurons are dictated by altered Na v channel function Hoffstaetter et al. show that somatosensory neurons from hibernating squirrels exist in a semi-functional state, with decreased ability to generate action potential, partially due to altered activity of Na v channels. This state reflects complex changes associated with hibernation and cannot be recapitulated by merely exposing active neurons to cold. [ABSTRACT FROM AUTHOR]