Upregulation of the sodium channel NaVβ4 subunit and its contributions to mechanical hypersensitivity and neuronal hyperexcitability in a rat model of radicular pain induced by local dorsal root ganglion inflammation

Molecular determinants of resurgent sodium currents mediated by Navβ4 peptide and A-type FHFs

Introduction

Resurgent current (INaR ) generated by voltage-gated sodium channels (VGSCs) plays an essential role in maintaining high-frequency firing of many neurons and contributes to disease pathophysiology such as epilepsy and painful disorders. Targeting INaR may present a highly promising strategy in the treatment of these diseases. Navβ4 and A-type fibroblast growth factor homologous factors (FHFs) have been identified as two classes of important INaR mediators; however, their receptor sites in VGSCs remain unknown, which hinders the development of novel agents to effectively target INaR .

Methods

Navβ4 and FHF4A can mediate INaR generation through the amino acid segment located in their C-terminus and N-terminus, respectively. We mainly employed site-directed mutagenesis, chimera construction and whole-cell patch-clamp recording to explore the receptor sites of Navβ4 peptide and FHF4A in Nav1.7 and Nav1.8.

Results

We show that the receptor of Navβ4-peptide involves four residues, N395, N945, F1737 and Y1744, in Nav1.7 DI-S6, DII-S6, and DIV-S6. We show that A-type FHFs generating INaR depends on the segment located at the very beginning, not at the distal end, of the FHF4 N-terminus domain. We show that the receptor site of A-type FHFs also resides in VGSC inner pore region. We further show that an asparagine at DIIS6, N891 in Nav1.8, is a major determinant of INaR generated by A-type FHFs in VGSCs.

Discussion

Cryo-EM structures reveal that the side chains of the critical residues project into the VGSC channel pore. Our findings provide additional evidence that Navβ4 peptide and A-type FHFs function as open-channel pore blockers and highlight channel inner pore region as a hotspot for development of novel agents targeting INaR .

Fabry disease Schwann cells release p11 to induce sensory neuron hyperactivity

Patients with Fabry disease suffer from chronic debilitating pain and peripheral sensory neuropathy with minimal treatment options, but the cellular drivers of this pain are unknown. Here, we propose a mechanism we believe to be novel in which altered signaling between Schwann cells and sensory neurons underlies the peripheral sensory nerve dysfunction we observed in a genetic rat model of Fabry disease. Using in vivo and in vitro electrophysiological recordings, we demonstrated that Fabry rat sensory neurons exhibited pronounced hyperexcitability. Schwann cells probably contributed to this finding because application of mediators released from cultured Fabry Schwann cells induced spontaneous activity and hyperexcitability in naive sensory neurons. We examined putative algogenic mediators using proteomic analysis and found that Fabry Schwann cells released elevated levels of the protein p11 (S100A10), which induced sensory neuron hyperexcitability. Removal of p11 from Fabry Schwann cell media caused hyperpolarization of neuronal resting membrane potentials, indicating that p11 may contribute to the excessive neuronal excitability caused by Fabry Schwann cells. These findings demonstrate that sensory neurons from rats with Fabry disease exhibit hyperactivity caused in part by Schwann cell release of the protein p11.

Differential expression of slow and fast-repriming tetrodotoxin-sensitive sodium currents in dorsal root ganglion neurons

Sodium channel Nav1.7 triggers the generation of nociceptive action potentials and is important in sending pain signals under physiological and pathological conditions. However, studying endogenous Nav1.7 currents has been confounded by co-expression of multiple sodium channel isoforms in dorsal root ganglion (DRG) neurons. In the current study, slow-repriming (SR) and fast-repriming (FR) tetrodotoxin-sensitive (TTX-S) currents were dissected electrophysiologically in small DRG neurons of both rats and mice. Three subgroups of small DRG neurons were identified based on the expression pattern of SR and FR TTX-S currents. A majority of rat neurons only expressed SR TTX-S currents, while a majority of mouse neurons expressed additional FR TTX-S currents. ProTx-II inhibited SR TTX-S currents with variable efficacy among DRG neurons. The expression of both types of TTX-S currents was higher in Isolectin B4-negative (IB4-) compared to Isolectin B4-positive (IB4+) neurons. Paclitaxel selectively increased SR TTX-S currents in IB4- neurons. In simulation experiments, the Nav1.7-expressing small DRG neuron displayed lower rheobase and higher frequency of action potentials upon threshold current injections compared to Nav1.6. The results suggested a successful dissection of endogenous Nav1.7 currents through electrophysiological manipulation that may provide a useful way to study the functional expression and pharmacology of endogenous Nav1.7 channels in DRG neurons.

Targeting dorsal root ganglia and primary sensory neurons for the treatment of chronic pain: an update

INTRODUCTION

Current treatments for chronic pain are inadequate. Here, we provide an update on the new therapeutic strategies that target dorsal root ganglia (DRGs) in the peripheral nervous system for a better and safer treatment of chronic pain.

AREAS COVERED

Despite the complex nature of chronic pain and its underlying mechanisms, we do know that changes in the plasticity and modality of neurons in DRGs play a pivotal role. DRG neurons are heterogenous and offer potential pain targets for different therapeutic interventions. We discuss the last advancements of these interventions, which include the use of systemic and local administrations, selective nerve drug delivery, and gene therapy. In particular, we provide updates and further details on the molecular characterization of primary sensory neurons, new analgesics entering the market, and future gene therapy approaches.

EXPERT OPINION

DRGs and primary sensory neurons are promising targets for chronic pain treatment due to their key role in pain signaling, unique anatomical location, and the potential for different targeted therapeutic interventions.

Schwann cell release of p11 induces sensory neuron hyperactivity in Fabry disease

Patients with Fabry disease suffer from chronic debilitating pain and peripheral sensory neuropathy with minimal treatment options, but the cellular drivers of this pain are unknown. Here, we propose a novel mechanism by which altered signaling between Schwann cells and sensory neurons underlies the peripheral sensory nerve dysfunction we observe in a genetic rat model of Fabry disease. Using in vivo and in vitro electrophysiological recordings, we demonstrate that Fabry rat sensory neurons exhibit pronounced hyperexcitability. Schwann cells likely contribute to this finding as application of mediators released from cultured Fabry Schwann cells induces spontaneous activity and hyperexcitability in naïve sensory neurons. We examined putative algogenic mediators using proteomic analysis and found that Fabry Schwann cells release elevated levels of the protein p11 (S100-A10) which induces sensory neuron hyperexcitability. Removal of p11 from Fabry Schwann cell media causes hyperpolarization of neuronal resting membrane potential, indicating that p11 contributes to the excessive neuronal excitability caused by Fabry Schwann cells. These findings demonstrate that rats with Fabry disease exhibit sensory neuron hyperexcitability caused in part by Schwann cell release of the protein p11.

SARS-CoV-2 airway infection results in the development of somatosensory abnormalities in a hamster model

Although largely confined to the airways, SARS-CoV-2 infection has been associated with sensory abnormalities that manifest in both acute and chronic phenotypes. To gain insight on the molecular basis of these sensory abnormalities, we used the golden hamster model to characterize and compare the effects of infection with SARS-CoV-2 and influenza A virus (IAV) on the sensory nervous system. We detected SARS-CoV-2 transcripts but no infectious material in the cervical and thoracic spinal cord and dorsal root ganglia (DRGs) within the first 24 hours of intranasal virus infection. SARS-CoV-2-infected hamsters exhibited mechanical hypersensitivity that was milder but prolonged compared with that observed in IAV-infected hamsters. RNA sequencing analysis of thoracic DRGs 1 to 4 days after infection suggested perturbations in predominantly neuronal signaling in SARS-CoV-2-infected animals as opposed to type I interferon signaling in IAV-infected animals. Later, 31 days after infection, a neuropathic transcriptome emerged in thoracic DRGs from SARS-CoV-2-infected animals, which coincided with SARS-CoV-2-specific mechanical hypersensitivity. These data revealed potential targets for pain management, including the RNA binding protein ILF3, which was validated in murine pain models. This work elucidates transcriptomic signatures in the DRGs triggered by SARS-CoV-2 that may underlie both short- and long-term sensory abnormalities.

SARS-CoV-2 Airway Infection Results in Time-dependent Sensory Abnormalities in a Hamster Model

Despite being largely confined to the airways, SARS-CoV-2 infection has been associated with sensory abnormalities that manifest in both acute and long-lasting phenotypes. To gain insight on the molecular basis of these sensory abnormalities, we used the golden hamster infection model to characterize the effects of SARS-CoV-2 versus Influenza A virus (IAV) infection on the sensory nervous system. Efforts to detect the presence of virus in the cervical/thoracic spinal cord and dorsal root ganglia (DRGs) demonstrated detectable levels of SARS-CoV-2 by quantitative PCR and RNAscope uniquely within the first 24 hours of infection. SARS-CoV-2-infected hamsters demonstrated mechanical hypersensitivity during acute infection; intriguingly, this hypersensitivity was milder, but prolonged when compared to IAV-infected hamsters. RNA sequencing (RNA-seq) of thoracic DRGs from acute infection revealed predominantly neuron-biased signaling perturbations in SARS-CoV-2-infected animals as opposed to type I interferon signaling in tissue derived from IAV-infected animals. RNA-seq of 31dpi thoracic DRGs from SARS-CoV-2-infected animals highlighted a uniquely neuropathic transcriptomic landscape, which was consistent with substantial SARS-CoV-2-specific mechanical hypersensitivity at 28dpi. Ontology analysis of 1, 4, and 30dpi RNA-seq revealed novel targets for pain management, such as ILF3. Meta-analysis of all SARS-CoV-2 RNA-seq timepoints against preclinical pain model datasets highlighted both conserved and unique pro-nociceptive gene expression changes following infection. Overall, this work elucidates novel transcriptomic signatures triggered by SARS-CoV-2 that may underlie both short- and long-term sensory abnormalities while also highlighting several therapeutic targets for alleviation of infection-induced hypersensitivity.

One Sentence Summary

SARS-CoV-2 infection results in an interferon-associated transcriptional response in sensory tissues underlying time-dependent hypersensitivity.

Anti-inflammatory effects of cannabidiol against lipopolysaccharides in cardiac sodium channels

BACKGROUND

Sepsis, caused by a dysregulated host response to infections, can lead to cardiac arrhythmias. However, the mechanisms underlying sepsis-induced inflammation, and how inflammation provokes cardiac arrhythmias, are not well understood. We hypothesized that CBD may ameliorate lipopolysaccharides (LPS)-induced cardiotoxicity via Toll-like receptor 4 (TLR-4) and cardiac sodium channels (Nav1.5).

METHODS AND RESULTS

We incubated human immune cells (THP-1 macrophages) with LPS for 24 hours, then extracted the THP-1 incubation media. ELISA assay showed that LPS (1 or 5 μg/ml), in a concentration-dependent manner, or MPLA (TLR-4 agonist, 5 μg/ml) stimulated the THP-1 cells to release inflammatory cytokines (TNF-α and IL-6). Prior incubation (4 hours) with cannabidiol (CBD: 5 μM) or C34 (TLR-4 antagonist: 5 μg/ml) inhibited LPS and MPLA-induced release of both IL-6 and TNF-α. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) were subsequently incubated for 24 hours in the media extracted from THP-1 cells incubated with LPS, MPLA alone, or in combination with CBD or C34. Voltage-clamp experiments showed a right shift in the voltage dependence of Nav1.5 activation, steady state fast inactivation (SSFI), increased persistent current and prolonged in silico action potential duration in hiSPC-CM incubated in the LPS or MPLA-THP-1 media. Co-incubation with CBD or C34 rescued the biophysical dysfunction caused by LPS and MPLA.

CONCLUSION

Our results suggest that CBD may protect against sepsis-induced inflammation and subsequent arrhythmias through (i) inhibition of the release of inflammatory cytokines, antioxidant and anti-apoptotic effects and/or (ii) direct effect on Nav1.5.

IL-10 alleviates radicular pain by inhibiting TNF-α/p65 dependent Nav1.7 up-regulation in DRG neurons of rats

BACKGROUND

Lumbar disc herniation (LDH) may induce radicular pain, the upregulation of voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) contributes to radicular pain by generating ectopic discharge of neurons, but the mechanism is unclear. Previously, we reported pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) up-regulated VGSCs in diabetic neuropathy. In this study, we explored the effect of anti-inflammatory cytokine interleukin-10 (IL-10) on radicular pain and the possible mechanisms.

METHODS

Rat model of LDH was induced by implanting autologous nucleus pulposus (NP). Mechanical and thermal pain thresholds were assessed by von Frey filaments and hotplate test respectively. IL-10 and TNF-α level in DRG and cerebrospinal fluid (CSF) were assessed by Enzyme-linked immunosorbent assay (ELISA). IL-10 was intrathecally delivered for 12 days. The expression of IL-10R1 and sodium channel Nav1.7 was displayed by immunofluorescence staining. The protein level of TNF-α and p-p65 was measured by western blotting.

RESULTS

NP implantation increased Nav1.7 expression in DRG neurons, decreased IL-10 level and increased TNF-α level in DRG and CSF. IL-10 significantly alleviated pain behaviors of rats with NP. IL-10R1 was co-localized with neurons but not with satellite cells in DRG. IL-10 decreased Nav1.7 and TNF-α/p-p65 expression in DRG of rats with NP. Co-administration of TNF-α with IL-10 counteracted the effect of IL-10 on pain behaviors, Nav1.7 and TNF-α/p-p65 expression of rats with NP.

CONCLUSIONS

The study revealed that IL-10 alleviated radicular pain by inhibiting TNF-α/p-p65 dependent Nav1.7 up-regulation in DRG neurons.

A-type FHFs mediate resurgent currents through TTX-resistant voltage-gated sodium channels

Resurgent currents (I_NaR) produced by voltage-gated sodium channels are required for many neurons to maintain high-frequency firing, and contribute to neuronal hyperexcitability and disease pathophysiology. Here we show, for the first time, that I_NaR can be reconstituted in a heterologous system by co-expression of sodium channel α-subunits and A-type fibroblast growth factor homologous factors (FHFs). Specifically, A-type FHFs induces I_NaR from Nav1.8, Nav1.9 tetrodotoxin-resistant neuronal channels and, to a lesser extent, neuronal Nav1.7 and cardiac Nav1.5 channels. Moreover, we identified the N-terminus of FHF as the critical molecule responsible for A-type FHFs-mediated I_NaR. Among the FHFs, FHF4A is the most important isoform for mediating Nav1.8 and Nav1.9 I_NaR. In nociceptive sensory neurons, FHF4A knockdown significantly reduces I_NaR amplitude and the percentage of neurons that generate I_NaR, substantially suppressing excitability. Thus, our work reveals a novel molecular mechanism underlying TTX-resistant I_NaR generation and provides important potential targets for pain treatment.

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.

Pain-related toxins in scorpion and spider venoms: a face to face with ion channels

Pain is a common symptom induced during envenomation by spiders and scorpions. Toxins isolated from their venom have become essential tools for studying the functioning and physiopathological role of ion channels, as they modulate their activity. In particular, toxins that induce pain relief effects can serve as a molecular basis for the development of future analgesics in humans. This review provides a summary of the different scorpion and spider toxins that directly interact with pain-related ion channels, with inhibitory or stimulatory effects. Some of these toxins were shown to affect pain modalities in different animal models providing information on the role played by these channels in the pain process. The close interaction of certain gating-modifier toxins with membrane phospholipids close to ion channels is examined along with molecular approaches to improve selectivity, affinity or bioavailability in vivo for therapeutic purposes.

Inhibitory Effects of Columbianadin on Nociceptive Behaviors in a Neuropathic Pain Model, and on Voltage-Gated Calcium Currents in Dorsal Root Ganglion Neurons in Mice

Radix angelicae pubescentis (RAP) has been used in Chinese traditional medicine to treat painful diseases such as rheumatism and headache. A previous study has reported that columbianadin (CBN), a major coumarin in RAP inhibits acute and inflammatory pain behaviors. However, the effects of CBN on neuropathic pain behaviors, and the potential underlying mechanism have not been reported. In the present study, the effects of CBN, compared to another major coumarin of RAP osthole (OST), on oxaliplatin-induced neuropathic pain behaviors and on the voltage-gated calcium currents in small dorsal root ganglion (DRG) neurons were studied in mice. It was found that CBN and OST inhibited both mechanical and cold hypersensitivity induced by oxaliplatin. Moreover, CBN and OST might preferentially inhibit T- and L-type calcium currents (Ica). The inhibitory effects of CBN and OST on the oxaliplatin-induced mechanical allodynia were prevented by gabapentin. These results suggest that CBN, as well as OST might inhibit neuropathic pain behaviors through an inhibition of T- and L-type calcium currents in nociceptive DRG neurons.

Distinct functional alterations in SCN8A epilepsy mutant channels

KEY POINTS

Mutations in the SCN8A gene cause early infantile epileptic encephalopathy. We characterize a new epilepsy-related SCN8A mutation, R850Q, in the human SCN8A channel and present gain-of-function properties of the mutant channel. Systematic comparison of R850Q with three other SCN8A epilepsy mutations, T761I, R1617Q and R1872Q, identifies one common dysfunction in resurgent current, although these mutations alter distinct properties of the channel. Computational simulations in two different neuron models predict an increased excitability of neurons carrying these mutations, which explains the over-excitation that underlies seizure activities in patients. These data provide further insight into the mechanism of SCN8A-related epilepsy and reveal subtle but potentially important distinction of functional characterization performed in the human vs. rodent channels.

ABSTRACT

SCN8A is a novel causal gene for early infantile epileptic encephalopathy. It is well accepted that gain-of-function mutations in SCN8A underlie the disorder, although the remarkable heterogeneity of its clinical presentation and poor treatment response demand a better understanding of the disease mechanisms. Here, we characterize a new epilepsy-related SCN8A mutation, R850Q, in human Nav1.6. We show that it is a gain-of-function mutation, with a hyperpolarizing shift in voltage dependence of activation, a two-fold increase of persistent current and a slowed decay of resurgent current. We systematically compare its biophysics with three other SCN8A epilepsy mutations, T767I, R1617Q and R1872Q, in the human Nav1.6 channel. Although all of these mutations are gain-of-function, the mutations affect different aspects of channel properties. One commonality that we discovered is an alteration of resurgent current kinetics, although the mechanisms by which resurgent currents are augmented remain unclear for all of the mutations. Computational simulations predict an increased excitability of neurons carrying these mutations with differential enhancement by open channel blockade.

Injury-Induced Effectors of Neuropathic Pain

Injuries typically result in the development of neuropathic pain, which decreases in parallel with wound healing. However, the pain may remain after the injury appears to have healed, which is generally associated with an ongoing underlying pro-inflammatory state. Injury induces many cells to release factors that contribute to the development of a pro-inflammatory state, which is considered an essential first step towards wound healing. However, pain elimination requires a transition of the injury site from pro- to anti-inflammatory. Therefore, developing techniques that eliminate chronic pain require an understanding of the cells resident at and recruited to injury sites, the factors they release, that promote a pro-inflammatory state, and promote the subsequent transition of that site to be anti-inflammatory. Although a relatively large number of cells, factors, and gene expression changes are involved in these processes, it may be possible to control a relatively small number of them leading to the reduction and elimination of chronic neuropathic pain. This first of two papers examines the roles of the most salient cells and mediators associated with the development and maintenance of chronic neuropathic pain. The following paper examines the cells and mediators involved in reducing and eliminating chronic neuropathic pain.

Effects of FGF14 and NaVβ4 deletion on transient and resurgent Na current in cerebellar Purkinje neurons

Voltage-gated Na channels of Purkinje cells are specialized to maintain high availability during high-frequency repetitive firing. They enter fast-inactivated states relatively slowly and undergo a voltage-dependent open-channel block by an intracellular protein (or proteins) that prevents stable fast inactivation and generates resurgent Na current. These properties depend on the pore-forming α subunits, as well as modulatory subunits within the Na channel complex. The identity of the factors responsible for open-channel block remains a question. Here we investigate the effects of genetic mutation of two Na channel auxiliary subunits highly expressed in Purkinje cells, Na_Vβ4 and FGF14, on modulating Na channel blocked as well as inactivated states. We find that although both Na_Vβ4 and the FGF14 splice variant FGF14-1a contain sequences that can generate resurgent-like currents when applied to Na channels in peptide form, deletion of either protein, or both proteins simultaneously, does not eliminate resurgent current in acutely dissociated Purkinje cell bodies. Loss of FGF14 expression does, however, reduce resurgent current amplitude and leads to an acceleration and stabilization of inactivation that is not reversed by application of the site-3 toxin, anemone toxin II (ATX). Tetrodotoxin (TTX) sensitivity is higher for resurgent than transient components of Na current, and loss of FGF14 preferentially affects a highly TTX-sensitive subset of Purkinje α subunits. The data suggest that Na_V1.6 channels, which are known to generate the majority of Purkinje cell resurgent current, bind TTX with high affinity and are modulated by FGF14 to facilitate open-channel block.

The role of NaV channels in synaptic transmission after axotomy in a microfluidic culture platform

Voltage gated sodium channels are key players in aberrant pain signaling and sensitization of nociceptors after peripheral nerve injury. The extent to which sodium channel activity after injury contributes to synaptic transmission at the first pain synapse however remains unclear. To investigate the effect of axotomy on synaptic transmission between dorsal root ganglia neurons and dorsal horn neurons, we reconstructed the first pain synapse in a novel microfluidic based compartmentalized cell culture system, which recapitulates the connectivity of peripheral pain signaling. We show that following axotomy of the distal axons, inhibition of Na_V1.7 and Na_V1.8 sodium channels in incoming presynaptic DRG axons is no longer sufficient to block activation of these axons and the resulting synaptic transmission to dorsal horn neurons. We found that blockade of Na_V1.6 activity is highly effective in reducing activation of incoming axons contributing to synaptic transmission after axotomy of DRG neurons. The microfluidic culture system described here offers an in vitro platform to recapitulate and study the first pain synapse.

Extracellular signal-regulated kinases mediate the enhancing effects of inflammatory mediators on resurgent currents in dorsal root ganglion neurons

Previously we reported that a group of inflammatory mediators significantly enhanced resurgent currents in dorsal root ganglion neurons. To understand the underlying intracellular signaling mechanism, we investigated the effects of inhibition of extracellular signal-regulated kinases and protein kinase C on the enhancing effects of inflammatory mediators on resurgent currents in rat dorsal root ganglion neurons. We found that the extracellular signal-regulated kinases inhibitor U0126 completely prevented the enhancing effects of the inflammatory mediators on both Tetrodotoxin-sensitive and Tetrodotoxin-resistant resurgent currents in both small and medium dorsal root ganglion neurons. U0126 substantially reduced repetitive firing in small dorsal root ganglion neurons exposed to inflammatory mediators, consistent with prevention of resurgent current amplitude increases. The protein kinase C inhibitor Bisindolylmaleimide I also showed attenuating effects on resurgent currents, although to a lesser extent compared to extracellular signal-regulated kinases inhibition. These results indicate a critical role of extracellular signal-regulated kinases signaling in modulating resurgent currents and membrane excitability in dorsal root ganglion neurons treated with inflammatory mediators. It is also suggested that targeting extracellular signal-regulated kinases-resurgent currents might be a useful strategy to reduce inflammatory pain.

Brucine alleviates neuropathic pain in mice via reducing the current of the sodium channel

ETHNOPHARMACOLOGICAL RELEVANCE

Strychnos nux-vomica L. (Loganiaceae) is grown extensively in South Asian. The dried seed of this plant, nux vomica, has been clinically used in Chinese medicine for relieving rheumatic pain, reducing swelling and treating cancer. Brucine, the second abundant alkaloid constituent of nux vomica, shows excellent clinical therapeutic effect, especially in relieving pain, but mechanism of brucine in relieving pain is still unclear.

AIM OF THE STUDY

Explore the analgesic effect of brucine, reveal the molecular mechanism of brucine analgesia.

MATERIALS AND METHODS

Antinociceptive effects of brucine were assessed in acute and chronic pain mice model. Electrophysiological experiments were used to evaluate the effects of brucine on neuronal activity and sodium channel function.

RESULTS

In acute pain models, brucine significantly inhibits response induced by nociceptive heat and mechanical stimulation. Furthermore, thermal hypersensitivity and mechanical allodynia were also alleviated by brucine treatment in a chronic constriction injury (CCI) mouse model. Sodium channel plays a crucial role in neuropathic pain. Electrophysiological results show that brucine inhibits the excitability of DRG neurons directly, the number of action potential (AP) was significantly reduced after brucine treatment, and this kind of inhibition is due to brucine inhibits both tetrodotoxin-sensitive (TTXs) and tetrodotoxin-resistant (TTXr) sodium channel.

CONCLUSIONS

Taken together, brucine is a novel drug candidate in treating acute and chronic pain diseases, which might be attributed to inhibition the excitability of sodium channel directly.

Sensory Neurons of the Dorsal Root Ganglia Become Hyperexcitable in a T-Cell-Mediated MOG-EAE Model of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune, demyelinating disease of the central nervous system. Patients with MS typically present with visual, motor, and sensory deficits. However, an additional complication of MS in large subset of patients is neuropathic pain. To study the underlying immune-mediated pathophysiology of pain in MS we employed the myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalitis (EAE) model in mice. Since sensory neurons are crucial for nociceptive transduction, we investigated the effect of this disease on sensory neurons of the lumbar dorsal root ganglia (DRG). Here, we report the disease was associated with activation of the complement system and the NLRP3 inflammasome in the DRG. We further observe a transient increase in the number of complement component 5a receptor 1-positive (C5aR1+) immune cells, CD4+ T-cells, and Iba1+ macrophages in the DRG. The absence of any significant change in the levels of mRNA for myelin proteins in the DRG and the sciatic nerve suggests that demyelination in the PNS is not a trigger for the immune response in the DRG. However, we did observe an induction of activating transcription factor 3 (ATF3) at disease onset and chronic disruption of cytoskeletal proteins in the DRG demonstrating neuronal injury in the PNS in response to the disease. Electrophysiological analysis revealed the emergence of hyperexcitability in medium-to-large (≥26 µm) diameter neurons, especially at the onset of MOG-EAE signs. These results provide conclusive evidence of immune activation, neuronal injury, and peripheral sensitization in MOG-EAE, a model classically considered to be centrally mediated.

Role of NaV1.6 and NaVβ4 Sodium Channel Subunits in a Rat Model of Low Back Pain Induced by Compression of the Dorsal Root Ganglia

Low back pain is a common cause of chronic pain and disability. It is modeled in rodents by chronically compressing the lumbar dorsal root ganglia (DRG) with small metal rods, resulting in ipsilateral mechanical and cold hypersensitivity, and hyperexcitability of sensory neurons. Sodium channels are implicated in this hyperexcitability, but the responsible isoforms are unknown. In this study, we used siRNA-mediated knockdown of the pore-forming Na_V1.6 and regulatory Na_Vβ4 sodium channel isoforms that have been previously implicated in a different model of low back pain caused by locally inflaming the L5 DRG. Knockdown of either subunit markedly reduced spontaneous pain and mechanical and cold hypersensitivity induced by DRG compression, and reduced spontaneous activity and hyperexcitability of sensory neurons with action potentials <1.5 msec (predominately cells with myelinated axons, based on conduction velocities measured in a subset of cells) 4 days after DRG compression. These results were similar to those previously obtained in the DRG inflammation model and some neuropathic pain models, in which sensory neurons other than nociceptors seem to play key roles. The cytokine profiles induced by DRG compression and DRG inflammation were also very similar, with upregulation of several type 1 pro-inflammatory cytokines and downregulation of type 2 anti-inflammatory cytokines. Surprisingly, the cytokine profile was largely unaffected by Na_Vβ4 knockdown in either model. The Na_V1.6 channel, and the Na_Vβ4 subunit that can regulate Na_V1.6 to enhance repetitive firing, play key roles in both models of low back pain; targeting the abnormal spontaneous activity they generate may have therapeutic value.

Increased Resurgent Sodium Currents in Nav1.8 Contribute to Nociceptive Sensory Neuron Hyperexcitability Associated with Peripheral Neuropathies

Neuropathic pain is a significant public health challenge, yet the underlying mechanisms remain poorly understood. Painful small fiber neuropathy (SFN) may be caused by gain-of-function mutations in Nav1.8, a sodium channel subtype predominantly expressed in peripheral nociceptive neurons. However, it is not clear how Nav1.8 disease mutations induce sensory neuron hyperexcitability. Here we studied two mutations in Nav1.8 associated with hypersensitive sensory neurons: G1662S reported in painful SFN; and T790A, which underlies increased pain behaviors in the Possum transgenic mouse strain. We show that, in male DRG neurons, these mutations, which impair inactivation, significantly increase TTX-resistant resurgent sodium currents mediated by Nav1.8. The G1662S mutation doubled resurgent currents, and the T790A mutation increased them fourfold. These unusual currents are typically evoked during the repolarization phase of action potentials. We show that the T790A mutation greatly enhances DRG neuron excitability by reducing current threshold and increasing firing frequency. Interestingly, the mutation endows DRG neurons with multiple early afterdepolarizations and leads to substantial prolongation of action potential duration. In DRG neurons, siRNA knockdown of sodium channel β4 subunits fails to significantly alter T790A current density but reduces TTX-resistant resurgent currents by 56%. Furthermore, DRG neurons expressing T790A channels exhibited reduced excitability with fewer early afterdepolarizations and narrower action potentials after β4 knockdown. Together, our data demonstrate that open-channel block of TTX-resistant currents, enhanced by gain-of-function mutations in Nav1.8, can make major contributions to the hyperexcitability of nociceptive neurons, likely leading to altered sensory phenotypes including neuropathic pain in SFN.SIGNIFICANCE STATEMENT This work demonstrates that two disease mutations in the voltage-gated sodium channel Na_v1.8 that induce nociceptor hyperexcitability increase resurgent currents. Nav1.8 is crucial for pain sensations. Because resurgent currents are evoked during action potential repolarization, they can be crucial regulators of action potential activity. Our data indicate that increased Nav1.8 resurgent currents in DRG neurons greatly prolong action potential duration and enhance repetitive firing. We propose that Nav1.8 open-channel block is a major factor in Nav1.8-associated pain mechanisms and that targeting the molecular mechanism underlying these unique resurgent currents represents a novel therapeutic target for the treatment of aberrant pain sensations.

GpTx-1 and [Ala5 , Phe6 , Leu26 , Arg28 ]GpTx-1, two peptide NaV 1.7 inhibitors: analgesic and tolerance properties at the spinal level

BACKGROUND AND PURPOSE

The voltage-gated sodium channel Na_V 1.7 is considered a therapeutic target for pain treatment based on human genetic evidence. GpTx-1 and its potent analogue [Ala⁵ , Phe⁶ , Leu²⁶ , Arg²⁸ ]GpTx-1 (GpTx-1-71) were recently characterized as Na_V 1.7 inhibitors in vitro. Furthermore, the present work was conducted to investigate the analgesic properties of these two peptides in different pain models after spinal administration.

EXPERIMENTAL APPROACH

The antinociceptive activities of both GpTx-1 and GpTx-1-71 were investigated in mouse models of acute, visceral, inflammatory and neuropathic pain. Furthermore, the side effects of GpTx-1 and GpTx-1-71 were evaluated in rotarod, antinociceptive tolerance, acute hyperlocomotion and gastrointestinal transit tests.

KEY RESULTS

The i.t. administration of both GpTx-1 and GpTx-1-71 dose-dependently produced powerful antinociception in the different pain models. This effect was attenuated by the opioid receptor antagonist naloxone, suggesting the involvement of the opioid system. In this study, repeated administration of these two_peptides produced spinal analgesia without a loss of potency over 8 days in mouse models of acute, inflammatory and neuropathic pain. Moreover, spinal administration of GpTx-1 and GpTx-1-71 did not induce significant effects on motor coordination, evoke acute hyperlocomotion or increase gastrointestinal transit time.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that the Na_V 1.7 peptide inhibitors GpTx-1 and GpTx-1-71 produce powerful, nontolerance-forming analgesia in preclinical pain models, which might be dependent on the endogenous opioid system. In addition, at the spinal level, the limited side effects imply that these Na_V 1.7 peptide inhibitors could be potentially developed as GpTx-1-based drugs for pain relief.

The NaV1.7 Channel Subtype as an Antinociceptive Target for Spider Toxins in Adult Dorsal Root Ganglia Neurons

Although necessary for human survival, pain may sometimes become pathologic if long-lasting and associated with alterations in its signaling pathway. Opioid painkillers are officially used to treat moderate to severe, and even mild, pain. However, the consequent strong and not so rare complications that occur, including addiction and overdose, combined with pain management costs, remain an important societal and economic concern. In this context, animal venom toxins represent an original source of antinociceptive peptides that mainly target ion channels (such as ASICs as well as TRP, Ca_V, K_V and Na_V channels) involved in pain transmission. The present review aims to highlight the Na_V1.7 channel subtype as an antinociceptive target for spider toxins in adult dorsal root ganglia neurons. It will detail (i) the characteristics of these primary sensory neurons, the first ones in contact with pain stimulus and conveying the nociceptive message, (ii) the electrophysiological properties of the different Na_V channel subtypes expressed in these neurons, with a particular attention on the Na_V1.7 subtype, an antinociceptive target of choice that has been validated by human genetic evidence, and (iii) the features of spider venom toxins, shaped of inhibitory cysteine knot motif, that present high affinity for the Na_V1.7 subtype associated with evidenced analgesic efficacy in animal models.

Magnesium sulfate attenuates brain edema by lowering AQP4 expression and inhibits glia-mediated neuroinflammation in a rodent model of eclampsia

Eclampsia is characterized by high morbidity and mortality wordwide. Magnesium sulfate (MgSO₄) is used frequently as a prophylaxis for eclamptic seizure in clinical settings. However, the underlying mechanism is less studied, we have previously demonstrated that MgSO₄ pretreatment decreases eclampsia-like seizure threshold. Here, we further evaluated the hypothesis that MgSO₄ exert neuroprotective actions in eclampsia-like rats model by ameliorating neuroinflammation and brain edema. In this study, the eclampsia-like model was established by administering lipopolysaccharide plus pentylenetetrazol in pregnant Sprague-Dawley rats. Rats were given MgSO₄ from gestation day14-19. Then, Iba-1 (a marker for microglia) and S100-B (a marker for astrocytes) expression levels in the hippocampus CA3 region were detected by Enzyme-linked immunosorbent assay. Cerebrospinal fluid (CSF) levels of inflammatory cytokines were measured by Luminex assays. Aquaporin-4 (a transmembrane water channel protein) expression levels in cortex were analyzed using immunohistochemistry. Astrocyte and microglia expressions were detected by immunofluorescence, neuronal damage were evaluated by Nissl staining, and changes in neuronal number in the hippocampal CA3 region (CA3) among different groups were detected by neuronal nuclei staining. Our results demonstrated that MgSO₄ effectively attenuated astrocyte and microglia activation and promoted the neuronal survival in the CA3. Additionally, MgSO₄ significantly reduced inflammatory cytokines response in the CSF, and decreased the expression of AQP-4 protein in the cortex. Collectively, the findings of this study indicated that MgSO₄ has a neuroprotective role in eclampsia-like seizure rats through its anti-neuroninflammatory and brain edema-attenuating properties.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

Localized Sympathectomy Reduces Mechanical Hypersensitivity by Restoring Normal Immune Homeostasis in Rat Models of Inflammatory Pain

Some forms of chronic pain are maintained or enhanced by activity in the sympathetic nervous system (SNS), but attempts to model this have yielded conflicting findings. The SNS has both pro- and anti-inflammatory effects on immunity, confounding the interpretation of experiments using global sympathectomy methods. We performed a "microsympathectomy" by cutting the ipsilateral gray rami where they entered the spinal nerves near the L4 and L5 DRG. This led to profound sustained reductions in pain behaviors induced by local DRG inflammation (a rat model of low back pain) and by a peripheral paw inflammation model. Effects of microsympathectomy were evident within one day, making it unlikely that blocking sympathetic sprouting in the local DRGs or hindpaw was the sole mechanism. Prior microsympathectomy greatly reduced hyperexcitability of sensory neurons induced by local DRG inflammation observed 4 d later. Microsympathectomy reduced local inflammation and macrophage density in the affected tissues (as indicated by paw swelling and histochemical staining). Cytokine profiling in locally inflamed DRG showed increases in pro-inflammatory Type 1 cytokines and decreases in the Type 2 cytokines present at baseline, changes that were mitigated by microsympathectomy. Microsympathectomy was also effective in reducing established pain behaviors in the local DRG inflammation model. We conclude that the effect of sympathetic fibers in the L4/L5 gray rami in these models is pro-inflammatory. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some chronic inflammatory pain conditions.

SIGNIFICANCE STATEMENT

Sympathetic blockade is used for many pain conditions, but preclinical studies show both pro- and anti-nociceptive effects. The sympathetic nervous system also has both pro- and anti-inflammatory effects on immune tissues and cells. We examined effects of a very localized sympathectomy. By cutting the gray rami to the spinal nerves near the lumbar sensory ganglia, we avoided widespread sympathetic denervation. This procedure profoundly reduced mechanical pain behaviors induced by a back pain model and a model of peripheral inflammatory pain. One possible mechanism was reduction of inflammation in the sympathetically denervated regions. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some inflammatory conditions.

Inflammatory Changes in Paravertebral Sympathetic Ganglia in Two Rat Pain Models

Injury to peripheral nerves can lead to neuropathic pain, along with well-studied effects on sensory neurons, including hyperexcitability, abnormal spontaneous activity, and neuroinflammation in the sensory ganglia. Neuropathic pain can be enhanced by sympathetic activity. Peripheral nerve injury may also damage sympathetic axons or expose them to an inflammatory environment. In this study, we examined the lumbar sympathetic ganglion responses to two rat pain models: ligation of the L5 spinal nerve, and local inflammation of the L5 dorsal root ganglion (DRG), which does not involve axotomy. Both models resulted in neuroinflammatory changes in the sympathetic ganglia, as indicated by macrophage responses, satellite glia activation, and increased numbers of T cells, along with very modest increases in sympathetic neuron excitability (but not spontaneous activity) measured in ex vivo recordings. The spinal nerve ligation model generally caused larger responses than DRG inflammation. Plasticity of the sympathetic system should be recognized in studies of sympathetic effects on pain.

Selective phosphodiesterase-2A inhibitor alleviates radicular inflammation and mechanical allodynia in non-compressive lumbar disc herniation rats

PURPOSE

Phosphodiesterase inhibitors possess anti-inflammatory properties. In addition, some studies report that phosphodiesterase 2A (PDE2A) are highly expressed in the dorsal horn of the spinal cord. The present study aimed to investigate whether intrathecal administration of Bay 60-7550, a specific PDE2A inhibitor, could alleviate mechanical allodynia in non-compressive lumbar disc herniation (NCLDH) rats.

METHODS

Rat NCLDH models by autologous nucleus pulposus implantation to dorsal root ganglion were established. Vehicle or Bay 60-7550 (0.1, 1.0 mg/kg) was injected by intrathecal catheter at day 1 post-operation. The ipsilateral mechanical withdrawal thresholds were analyzed from the day before surgery to day 7 after surgery. At day 7 post-operation, the ipsilateral lumbar (L4-L6) segments of the spinal dorsal horns were removed, and tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), cyclic adenosine monophosphate (cAMP), and cyclic guanosine monophosphate (cGMP) expressions were measured by ELISA. Furthermore, PDE2A mRNA and protein expressions in spinal cord were measured by Real-Time PCR and Western blot.

RESULTS

Intrathecal administration of the PDE2A inhibitor Bay 60-7550, significantly attenuated mechanical allodynia, down-regulated spinal TNF-α, IL-1β and IL-6 over-expressions, increased the expression of spinal cAMP, as well as cGMP in a more remarkable manner, and decreased the spinal PDE2A expression in NCLDH rats in a dose-dependent manner.

CONCLUSIONS

Bay 60-7550 alleviated mechanical allodynia and inflammation in NCLDH rats, which might be associated with increased cAMP and especially cGMP increase. Thus, spinal PDE2A inhibition might represent a potential analgesic strategy for radiculopathy treatment in non-compressive lumbar disc herniation.

FHF2 isoforms differentially regulate Nav1.6-mediated resurgent sodium currents in dorsal root ganglion neurons

Nav1.6 and Nav1.6-mediated resurgent currents have been implicated in several pain pathologies. However, our knowledge of how fast resurgent currents are modulated in neurons is limited. Our study explored the potential regulation of Nav1.6-mediated resurgent currents by isoforms of fibroblast growth factor homologous factor 2 (FHF2) in an effort to address the gap in our knowledge. FHF2 isoforms colocalize with Nav1.6 in peripheral sensory neurons. Cell line studies suggest that these proteins differentially regulate inactivation. In particular, FHF2A mediates long-term inactivation, a mechanism proposed to compete with the open-channel blocker mechanism that mediates resurgent currents. On the other hand, FHF2B lacks the ability to mediate long-term inactivation and may delay inactivation favoring open-channel block. Based on these observations, we hypothesized that FHF2A limits resurgent currents, whereas FHF2B enhances resurgent currents. Overall, our results suggest that FHF2A negatively regulates fast resurgent current by enhancing long-term inactivation and delaying recovery. In contrast, FHF2B positively regulated resurgent current and did not alter long-term inactivation. Chimeric constructs of FHF2A and Navβ4 (likely the endogenous open channel blocker in sensory neurons) exhibited differential effects on resurgent currents, suggesting that specific regions within FHF2A and Navβ4 have important regulatory functions. Our data also indicate that FHFAs and FHF2B isoform expression are differentially regulated in a radicular pain model and that associated neuronal hyperexcitability is substantially attenuated by a FHFA peptide. As such, these findings suggest that FHF2A and FHF2B regulate resurgent current in sensory neurons and may contribute to hyperexcitability associated with some pain pathologies.

A gain-of-function mutation in Nav1.6 in a case of trigeminal neuralgia

Idiopathic trigeminal neuralgia (TN) is a debilitating pain disorder characterized by episodic unilateral facial pain along the territory of branches of the trigeminal nerve. Human painful disorders, but not TN, have been linked to gain-of-function mutations in peripheral voltage-gated sodium channels (Na_V1.7, Na_V1.8 and Na_V1.9). Gain-of-function mutations in Na_V1.6, which is expressed in myelinated and unmyelinated CNS and peripheral nervous system neurons and supports neuronal high-frequency firing, have been linked to epilepsy but not to pain. Here, we describe an individual who presented with evoked and spontaneous paroxysmal unilateral facial pain, and carried a diagnosis of TN. Magnetic resonance imaging showed unilateral neurovascular compression, consistent with pain in areas innervated by the second branch of the trigeminal nerve. Genetic analysis as part of a phase 2 clinical study in patients with TN conducted by Convergence Pharmaceuticals Ltd revealed a previously undescribed de novo missense mutation in Na_V1.6 (c.A406G; p.Met136Val). Whole-cell voltage-clamp recordings show that the Met136Val mutation significantly increases peak current density (1.5-fold) and resurgent current (1.6-fold) without altering gating properties. Current-clamp studies in trigeminal ganglion (TRG) neurons showed that Met136Val increased the fraction of high-firing neurons, lowered the current threshold and increased the frequency of evoked action potentials in response to graded stimuli. Our results demonstrate a novel Na_V1.6 mutation in TN, and show that this mutation potentiates transient and resurgent sodium currents and leads to increased excitability in TRG neurons. We suggest that this gain-of-function Na_V1.6 mutation may exacerbate the pathophysiology of vascular compression and contribute to TN.