The Roles of Sodium Channels in Nociception: Implications for Mechanisms of Neuropathic Pain

NaV1.8 as Proarrhythmic Target in a Ventricular Cardiac Stem Cell Model

The sodium channel NaV1.8, encoded by the SCN10A gene, has recently emerged as a potential regulator of cardiac electrophysiology. We have previously shown that NaV1.8 contributes to arrhythmogenesis by inducing a persistent Na+ current (late Na+ current, INaL) in human atrial and ventricular cardiomyocytes (CM). We now aim to further investigate the contribution of NaV1.8 to human ventricular arrhythmogenesis at the CM-specific level using pharmacological inhibition as well as a genetic knockout (KO) of SCN10A in induced pluripotent stem cell CM (iPSC-CM). In functional voltage-clamp experiments, we demonstrate that INaL was significantly reduced in ventricular SCN10A-KO iPSC-CM and in control CM after a specific pharmacological inhibition of NaV1.8. In contrast, we did not find any effects on ventricular APD90. The frequency of spontaneous sarcoplasmic reticulum Ca2+ sparks and waves were reduced in SCN10A-KO iPSC-CM and control cells following the pharmacological inhibition of NaV1.8. We further analyzed potential triggers of arrhythmias and found reduced delayed afterdepolarizations (DAD) in SCN10A-KO iPSC-CM and after the specific inhibition of NaV1.8 in control cells. In conclusion, we show that NaV1.8-induced INaL primarily impacts arrhythmogenesis at a subcellular level, with minimal effects on systolic cellular Ca2+ release. The inhibition or knockout of NaV1.8 diminishes proarrhythmic triggers in ventricular CM. In conjunction with our previously published results, this work confirms NaV1.8 as a proarrhythmic target that may be useful in an anti-arrhythmic therapeutic strategy.

Pain symptoms are associated with two-point discrimination threshold in patients with temporomandibular disorders

OBJECTIVE

This study aimed to explore the associations of orofacial two-point discrimination (2-PD) test result with pain symptoms and psychological factors in patients with Temporomandibular Disorders (TMDs).

METHODS

193 patients with TMDs were included in this study. Patients' demographics, pain intensity, and psychological status were recorded. The 2-PDs in the bilateral temporal, zygomatic, mandibular, and temporomandibular joint (TMJ) regions of the patients were measured. Statistical analyses were conducted to observe the associations between variables.

RESULTS

For Pain-related TMDs (PT) patients, Monthly Visual Analogue Scale (VAS-M) and Current Analogue Scale (VAS-C) were correlated with TMJ, zygomatic and temporal 2-PDs. Patients with PT tended to have higher TMJ 2-PDs[Right: β = 1.827 mm, 95%CI(0.107, 3.548), P = 0.038], zygomatic 2-PDs[Right: β = 1.696 mm, 95%CI(0.344, 3.048), P = 0.014], temporal 2-PDs[Left: β = 2.138 mm, 95%CI(0.127, 4.149), P = 0.037; Right: β = 1.893 mm, 95%CI(0.011, 3.775), P = 0.049]. Associations were also observed between VAS-C and TMJ 2-PDs[Left: β = 0.780, 95%CI(0.190, 1.370), P = 0.01; Right: β = 0.885, 95%CI(0.406, 1.364), P = 0.001], Zygomatic 2-PDs[Right: β = 0.555, 95%CI(0.172, 0.938), P = 0.005]; VAS-M and TMJ 2-PDs[Left: β = 0.812, 95%CI(0.313, 1.311), P = 0.002; Right: β = 0.567, 95%CI(0.152, 0.983), P = 0.008], zygomatic 2-PDs[Left: β = 0.405, 95%CI(0.075, 0.735), P = 0.016; Right: β = 0.545, 95%CI(0.221, 0.870), P = 0.001], and temporal 2-PDs [Left: β = 0.741, 95%CI(0.258, 1.224), P = 0.003; Right: β = 0.519, 95%CI(0.063, 0.975), P = 0.026].

CONCLUSION

TMJ, zygomatic, and temporal 2-PDs were significantly associated with PT and pain intensity. Age, gender and psychological factors were not associated with orofacial 2-PDs. PT patients exhibited weaker tactile acuity compared to Non-PT patients. Further discussion on the underlying mechanism is needed.

CLINICAL RELEVANCE

Orofacial tactile acuity of TMDs patients was associated with their pain symptoms, which researchers should take account into when performing 2-PD tests for TMDs patients. The 2-PD test can be considered as a potential tool along with the current procedures for the differentiations of PT and Non-PT.

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.

Experience from a single-center study on multimodal medication therapy for patients with complex regional pain syndrome

BACKGROUND

Despite the application of various therapeutic methods, pain caused by complex regional pain syndrome (CRPS) is not sufficiently managed and often progresses to a chronic stage. For the systematic and effective treatment of CRPS, we developed an algorithm for multimodal medication therapy based on the established pathophysiology of CRPS to control CRPS-related pain.

OBJECTIVE

In this study, we present the outcomes of our novel algorithm for multimodal medication therapy for patients with CRPS, consisting of three major components: multimodal oral medication, intravenous ketamine, and intravenous lidocaine therapy.

METHODS

We retrospectively investigated patients with CRPS who received multimodal therapy. Pain severity scores were evaluated using a numerical rating scale at four time points (P1, pain at initial consultation; P2, pain after oral medication; P3, pain after ketamine treatment; and P4, pain after lidocaine treatment). The effect of the multimodal medication therapy algorithm on pain management was evaluated at each time point.

RESULTS

In patients with CRPS, multimodal oral medication, intravenous ketamine, and intravenous lidocaine therapies led to significantly improved pain control (p< 0.05). Additionally, the combination of these three therapies (through the multimodal medication therapy algorithm) resulted in significant pain relief in patients with CRPS (p< 0.05).

CONCLUSIONS

Our multimodal medication therapy algorithm effectively controlled pain in patients with CRPS. However, further prospective studies with large sample sizes and randomized controlled trials are needed for more accurate generalization.

Scorpion venom peptides: Molecular diversity, structural characteristics, and therapeutic use from channelopathies to viral infections and cancers

Animal venom is an important evolutionary innovation in nature. As one of the most representative animal venoms, scorpion venom contains an extremely diverse set of bioactive peptides. Scorpion venom peptides not only are 'poisons' that immobilize, paralyze, kill, or dissolve preys but also become important candidates for drug development and design. Here, the review focuses on the molecular diversity of scorpion venom peptides, their typical structural characteristics, and their multiple therapeutic or pharmaceutical applications in channelopathies, viral infections and cancers. Especially, the group of scorpion toxin TRPTx targeting transient receptor potential (TRP) channels is systematically summarized and worthy of attention because TRP channels play a crucial role in the regulation of homeostasis and the occurrence of diseases in human. We also further establish the potential relationship between the molecular characteristics and functional applications of scorpion venom peptides to provide a research basis for modern drug development and clinical utilization of scorpion venom resources.

Role of autophagy in the pathogenesis and regulation of pain

Pain is a ubiquitous and highly concerned clinical symptom, usually caused by peripheral or central nervous injury, tissue damage, or other diseases. The long-term existence of pain can seriously affect daily physical function and quality of life and produce great torture on the physiological and psychological levels. However, the complex pathogenesis of pain involving molecular mechanisms and signaling pathways has not been fully elucidated, and managing pain remains highly challenging. As a result, finding new targets to pursue effective and long-term pain treatment strategies is required and urgent. Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, which can be cytoprotective and is vital in maintaining neural plasticity and proper nervous system function. Much evidence has shown that autophagy dysregulation is linked to the emergence of neuropathic pain, such as postherpetic neuralgia and cancer-related pain. Autophagy has also been connected to pain caused by osteoarthritis and lumbar disc degeneration. It is worth noting that in recent years, studies on traditional Chinese medicine have also proved that several traditional Chinese medicine monomers involve autophagy in the mechanism of pain relief. Therefore, autophagy can serve as a potential regulatory target to provide new ideas and inspiration for pain management.

Molecular and Functional Relevance of NaV1.8-Induced Atrial Arrhythmogenic Triggers in a Human SCN10A Knock-Out Stem Cell Model

In heart failure and atrial fibrillation, a persistent Na⁺ current (I_NaL) exerts detrimental effects on cellular electrophysiology and can induce arrhythmias. We have recently shown that Na_V1.8 contributes to arrhythmogenesis by inducing a I_NaL. Genome-wide association studies indicate that mutations in the SCN10A gene (Na_V1.8) are associated with increased risk for arrhythmias, Brugada syndrome, and sudden cardiac death. However, the mediation of these Na_V1.8-related effects, whether through cardiac ganglia or cardiomyocytes, is still a subject of controversial discussion. We used CRISPR/Cas9 technology to generate homozygous atrial SCN10A-KO-iPSC-CMs. Ruptured-patch whole-cell patch-clamp was used to measure the I_NaL and action potential duration. Ca²⁺ measurements (Fluo 4-AM) were performed to analyze proarrhythmogenic diastolic SR Ca²⁺ leak. The I_NaL was significantly reduced in atrial SCN10A KO CMs as well as after specific pharmacological inhibition of Na_V1.8. No effects on atrial APD₉₀ were detected in any groups. Both SCN10A KO and specific blockers of Na_V1.8 led to decreased Ca²⁺ spark frequency and a significant reduction of arrhythmogenic Ca²⁺ waves. Our experiments demonstrate that Na_V1.8 contributes to I_NaL formation in human atrial CMs and that Na_V1.8 inhibition modulates proarrhythmogenic triggers in human atrial CMs and therefore Na_V1.8 could be a new target for antiarrhythmic strategies.

Resistance to local anesthetics: Truth or myth?

The report of anesthetic technique failure is crucial and the etiology of the problem should be determined. We describe a case of locoregional anesthesia failure, in which, after excluding its most common causes, the resistance to local anesthetics was considered as the most probable clinical hypothesis. For this reason, a genetic test was performed, as well as the efficacy of other local anesthetics was evaluated, constituting a different approach in the cases of locoregional anesthesia failure. True resistance to local anesthetics is difficult to diagnose so information about this is scarce in the literature. One of the proposed causes is a mutation of sodium channels where local anesthetics bind. If not recognized, the application of locorregional anesthesia in this patient's condition can lead to unpleasant experiences and unnecessary risks, related to toxic levels of local anesthetics. For this reason, the resistance to local anesthetics should be always precluded in cases of strong clinical suspicion. This approach could be applied in similar cases.

The Role of microRNAs in Regulating Cancer Cell Response to Oxaliplatin-Containing Regimens

Oxaliplatin (cyclohexane-1,2-diamine; oxalate; platinum [2+]) is a third-generation chemotherapeutic drug with anticancer effects. Oxaliplatin has a role in the treatment of several cancers. It is one of the few drugs which can eliminate the neoplastic cells of colorectal cancer. Also, it has an influential role in breast cancer, lung cancer, bladder cancer, prostate cancer, and gastric cancer. Although oxaliplatin has many beneficial effects in cancer treatment, resistance to this drug is in the way to cure neoplastic cells and reduce treatment efficacy. microRNAs are a subtype of small noncoding RNAs with ∼22 nucleotides that exist among species. They have diverse roles in physiological processes, including cellular proliferation and cell death. Moreover, miRNAs have essential roles in resistance to cancer treatment and can strengthen sensitivity to chemotherapeutic drugs and regimens. In colorectal cancer, the co-treatment of oxaliplatin with anti-miR-19a can partially reverse the oxaliplatin resistance through the upregulation of phosphatase and tensin homolog (PTEN). Moreover, by preventing the spread of gastric cancer cells and downregulating glypican-3 (GPC3), MiR-4510 may modify immunosuppressive signals in the tumor microenvironment. Treatment with oxaliplatin may develop into a specialized therapeutic drug for patients with miR-4510 inhibition and glypican-3-expressing gastric cancer. Eventually, miR-122 upregulation or Wnt/β-catenin signaling suppression boosted the death of HCC cells and made them more sensitive to oxaliplatin. Herein, we have reviewed the role of microRNAs in regulating cancer cells' response to oxaliplatin, with particular attention to gastrointestinal cancers. We also discussed the role of these noncoding RNAs in the pathophysiology of oxaliplatin-induced neuropathic pain.

Targeting Chemokines and Chemokine GPCRs to Enhance Strong Opioid Efficacy in Neuropathic Pain

Neuropathic pain (NP) originates from an injury or disease of the somatosensory nervous system. This heterogeneous origin and the possible association with other pathologies make the management of NP a real challenge. To date, there are no satisfactory treatments for this type of chronic pain. Even strong opioids, the gold-standard analgesics for nociceptive and cancer pain, display low efficacy and the paradoxical ability to exacerbate pain sensitivity in NP patients. Mounting evidence suggests that chemokine upregulation may be a common mechanism driving NP pathophysiology and chronic opioid use-related consequences (analgesic tolerance and hyperalgesia). Here, we first review preclinical studies on the role of chemokines and chemokine receptors in the development and maintenance of NP. Second, we examine the change in chemokine expression following chronic opioid use and the crosstalk between chemokine and opioid receptors. Then, we examine the effects of inhibiting specific chemokines or chemokine receptors as a strategy to increase opioid efficacy in NP. We conclude that strong opioids, along with drugs that block specific chemokine/chemokine receptor axis, might be the right compromise for a favorable risk/benefit ratio in NP management.

Computational design of peptides to target NaV1.7 channel with high potency and selectivity for the treatment of pain

The voltage-gated sodium Na_V1.7 channel plays a key role as a mediator of action potential propagation in C-fiber nociceptors and is an established molecular target for pain therapy. ProTx-II is a potent and moderately selective peptide toxin from tarantula venom that inhibits human Na_V1.7 activation. Here we used available structural and experimental data to guide Rosetta design of potent and selective ProTx-II-based peptide inhibitors of human Na_V1.7 channels. Functional testing of designed peptides using electrophysiology identified the PTx2-3127 and PTx2-3258 peptides with IC₅₀s of 7 nM and 4 nM for hNa_V1.7 and more than 1000-fold selectivity over human Na_V1.1, Na_V1.3, Na_V1.4, Na_V1.5, Na_V1.8, and Na_V1.9 channels. PTx2-3127 inhibits Na_V1.7 currents in mouse and human sensory neurons and shows efficacy in rat models of chronic and thermal pain when administered intrathecally. Rationally designed peptide inhibitors of human Na_V1.7 channels have transformative potential to define a new class of biologics to treat pain.

Persistent pain relief following a single injection of a local anesthetic for neuropathic abdominal wall and groin pain

OBJECTIVES

It is our experience that a small portion of patients with neuropathic abdominal wall pain syndromes such as the anterior cutaneous nerve entrapment syndrome (ACNES) have a long term beneficial response following just one single tender point injection (TPI) with a local anesthetic agent. This report focuses on the phenomenon of ongoing pain relief following a single local anesthetic injection in neuropathic abdominal wall and groin pain syndromes.

METHODS

This report is an overview based on earlier studies from a center of expertise for neuropathic abdominal wall and groin pain syndromes. All studies on neuropathic abdominal wall and groin pain syndromes reporting on efficacy of a diagnostic TPI using a local anesthetic agent were included.

RESULTS

A total of 10 studies including 834 patients fulfilled study criteria. Each of these 10 studies found that approximately 10% (range, 4-25%) of the cases experienced persistent pain relief after a single TPI with lidocaine 1%.

CONCLUSIONS

Persistent pain relief after a single TPI using a local anesthetic agent may be observed in approximately one of 10 patients suffering from neuropathic abdominal wall or groin pain syndromes. When a patient is suspected of having a neuropathic abdominal wall or groin pain syndrome, a single TPI using a local anesthetic agent should be administered as long term pain relief may occasionally occur.

Magnolol attenuates inflammatory pain by inhibiting sodium currents in mouse dorsal root ganglion neurons

Voltage-gated sodium channels are currently recognized as one of the targets of analgesics. Magnolol (Mag), an active component isolated from Magnolia officinalis, has been reported to exhibit analgesic effects. The objective of this study was to investigate whether the analgesic effect of Mag was associated with blocking Na⁺ channels. Inflammatory pain was induced by the injection of carrageenan into the hind paw of mice. Mag was administered orally. Mechanical hyperanalgesia was evaluated by using von Frey filaments. Na⁺ currents and neuronal excitability in acutely isolated mouse dorsal root ganglion (DRG) neurons were recorded with the whole-cell patch clamp technique. Results showed that Mag (10 ~ 40 mg/kg) dose-dependently inhibited the paw edema and reduced mechanical pain in the inflammatory animal model. Injection of carrageenan significantly increased the amplitudes of TTX-sensitive and TTX-resistant Na⁺ currents. Compared with the carrageenan group, Mag inhibited the upregulation of two types of Na⁺ currents induced by carrageenan in a dose-dependent manner. Mag 40 mg/kg shifted the inactivation curves of two types of Na⁺ currents to hyperpolarization and returned to normal animal level without changing their activation curves. Mag 40 mg/kg significantly reduced the percentage of cells firing multiple spikes and inhibited the neuronal hyperexcitability induced by carrageenan. Our data suggest that the analgesic effect of Mag may be associated with a decreased neuronal excitability by blocking Na⁺ current.

Treatment Planning Factors Associated with Long-Term Outcomes of Gamma Knife Surgery in Patients with Trigeminal Neuralgia

OBJECTIVE

Gamma Knife surgery (GKS) is an established treatment option for trigeminal neuralgia (TN). However, the long-term efficacy of GKS for patients with TN has not been well studied. The aim of the study is to evaluate the sequential course of pain control after GKS and analyze the factors associated with the long-term analgesic effect, focusing on radiation dosimetry and neurovascular conflict (NVC) factors.

METHODS

We analyzed 83 patients undergoing GKS for TN in our institution between 2005 and 2013 with a follow-up duration >7 years. Tolerable pain with increased medication, persistent-intractable pain, and recurrence were classified as poor outcomes, and any other outcome was classified as a favorable outcome. The dosimetry factors and locational relationship between NVC and the target were analyzed in terms of their correlation with a favorable outcome.

RESULTS

Adequate pain relief was achieved in 93% of patients a month and a half after GKS, but the pain recurred in 41.5% of patients on average 36 months after treatment. A larger V_40Gy (P = 0.002) and higher homogeneity index (P = 0.027) were significantly associated with the long-term favorable outcomes. About 40% of patients had multiple NVC sites, and insufficient inclusion of the NVC in the target was significantly correlated with long-term poor outcomes (P = 0.002).

CONCLUSIONS

Targeting the center of the trigeminal nerve in the area of NVC with GKS is associated with favorable long-term pain control.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

High-Frequency Alternating Current Block Using Macro-Sieve Electrodes: A Pilot Study

Background and objective

High-frequency alternating current (HFAC) can yield a rapid-acting and reversible nerve conduction block. The present study aimed to demonstrate the successful implementation of HFAC block delivery via regenerative macro-sieve electrodes (MSEs).

Methods

Dual-electrode assemblies in two configurations [dual macro-sieve electrode-1 (DMSE-I), DMSE-II] were fabricated from pairs of MSEs and implanted in the transected and subsequently repaired sciatic nerves of two male Lewis rats. After four months of postoperative nerve regeneration through the MSEs' transit zones, the efficacy of acute HFAC block was tested for both configurations. Frequencies ranging from 10 kHz to 42 kHz, and stimulus amplitudes with peak-to-peak voltages ranging from 2 V to 20 V were tested. Evoked muscle force measurement was used to quantify the nerve conduction block.

Results

HFAC stimulation delivered via DMSE assemblies obtained a complete block at frequencies of 14 to 26 kHz and stimulus amplitudes of 12 to 20 V p-p. The threshold voltage for the complete block showed an approximately linear dependence on frequency. The threshold voltage for the partial conduction block was also approximately linear. For those frequencies that displayed both partial and complete block, the partial block thresholds were consistently lower.

Conclusion

This study provides a proof of concept that regenerative MSEs can achieve complete and reversible conduction block via HFAC stimulation of regenerated nerve tissue. A chronically interfaced DMSE assembly may thereby facilitate the inactivation of targeted nerves in cases wherein pathologic neuronal hyperactivity is involved.

L-bupivacaine Inhibition of Nociceptive Transmission in Rat Peripheral and Dorsal Horn Neurons

BACKGROUND

Although the widely used single L-enantiomers of local anesthetics have less toxic effects on the cardiovascular and central nervous systems, the mechanisms mediating their antinociceptive actions are not well understood. The authors hypothesized that significant differences in the ion channel blocking abilities of the enantiomers of bupivacaine would be identified.

METHODS

The authors performed electrophysiologic analysis on rat dorsal root ganglion neurons in vitro and on spinal transmissions in vivo.

RESULTS

In the dorsal root ganglion, these anesthetics decreased the amplitudes of action potentials. The half-maximum inhibitory concentrations of D-enantiomer D-bupivacaine were almost equal for Aβ (29.5 μM), Aδ (29.7μM), and C (29.8 μM) neurons. However, the half-maximum inhibitory concentrations of L-bupivacaine was lower for Aδ (19.35 μM) and C (19.5 μM) neurons than for A β (79.4 μM) neurons. Moreover, D-bupivacaine almost equally inhibited tetrodotoxin-resistant (mean ± SD: 15.8 ± 10.9% of the control, n = 14, P < 0.001) and tetrodotoxin-sensitive (15.4 ± 15.6% of the control, n = 11, P = 0.004) sodium currents. In contrast, L-bupivacaine suppressed tetrodotoxin-resistant sodium currents (26.1 ± 19.5% of the control, n = 18, P < 0.001) but not tetrodotoxin-sensitive sodium currents (74.5 ± 18.2% of the control, n = 11, P = 0.477). In the spinal dorsal horn, L-bupivacaine decreased the area of pinch-evoked excitatory postsynaptic currents (39.4 ± 11.3% of the control, n = 7, P < 0.001) but not touch-evoked responses (84.2 ± 14.5% of the control, n = 6, P = 0.826). In contrast, D-bupivacaine equally decreased pinch- and touch-evoked responses (38.8 ± 9.5% of the control, n = 6, P = 0.001, 42.9 ± 11.8% of the control, n = 6, P = 0.013, respectively).

CONCLUSIONS

These results suggest that the L-enantiomer of bupivacaine (L-bupivacaine) effectively inhibits noxious transmission to the spinal dorsal horn by blocking action potential conduction through C and Aδ afferent fibers.

EDITOR’S PERSPECTIVE

Engineering of highly potent and selective HNTX-III mutant against hNav1.7 sodium channel for treatment of pain

Human voltage-gated sodium channel Na_v1.7 (hNa_v1.7) is involved in the generation and conduction of neuropathic and nociceptive pain signals. Compelling genetic and preclinical studies have validated that hNa_v1.7 is a therapeutic target for the treatment of pain; however, there is a dearth of currently available compounds capable of targeting hNav1.7 with high potency and specificity. Hainantoxin-III (HNTX-III) is a 33-residue polypeptide from the venom of the spider Ornithoctonus hainana. It is a selective antagonist of neuronal tetrodotoxin-sensitive voltage-gated sodium channels. Here, we report the engineering of improved potency and Na_v selectivity of hNa_v1.7 inhibition peptides derived from the HNTX-III scaffold. Alanine scanning mutagenesis showed key residues for HNTX-III interacting with hNa_v1.7. Site-directed mutagenesis analysis indicated key residues on hNa_v1.7 interacting with HNTX-III. Molecular docking was conducted to clarify the binding interface between HNTX-III and Nav1.7 and guide the molecular engineering process. Ultimately, we obtained H4 [K0G1-P18K-A21L-V] based on molecular docking of HNTX-III and hNa_v1.7 with a 30-fold improved potency (IC₅₀ 0.007 ± 0.001 μM) and >1000-fold selectivity against Na_v1.4 and Na_v1.5. H4 also showed robust analgesia in the acute and chronic inflammatory pain model and neuropathic pain model. Thus, our results provide further insight into peptide toxins that may prove useful in guiding the development of inhibitors with improved potency and selectivity for Na_v subtypes with robust analgesia.

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

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

Intercellular communication and ion channels in neuropathic pain chronicization

BACKGROUND

Neuropathic pain is caused by primary lesion or dysfunction of either peripheral or central nervous system. Due to its complex pathogenesis, often related to a number of comorbidities, such as cancer, neurodegenerative and neurovascular diseases, neuropathic pain still represents an unmet clinical need, lacking long-term effective treatment and complex case-by-case approach.

AIM AND METHODS

We analyzed the recent literature on the role of selective voltage-sensitive sodium, calcium and potassium permeable channels and non-selective gap junctions (GJs) and hemichannels (HCs) in establishing and maintaining chronic neuropathic conditions. We finally focussed our review on the role of extracellular microenvironment modifications induced by resident glial cells and on the recent advances in cell-to-cell and cell-to-extracellular environment communication in chronic neuropathies.

CONCLUSION

In this review, we provide an update on the current knowledge of neuropathy chronicization processes with a focus on both neuronal and glial ion channels, as well as on channel-mediated intercellular communication.

Intravenous Lidocaine for Chronic Neuropathic Pain A Systematic Review Addressing Nursing Care

BACKGROUND

The intravenous administration of lidocaine for patients with chronic neuropathic pain is well documented in the literature. However, little is known about the role of the nurse caring for patients receiving the infusion.

AIM

The purpose of this systematic review was to examine and describe common side effects associated with the intravenous administration of lidocaine to patients with chronic neuropathic pain and outline nursing care described in an effort to develop evidence-based protocols for care.

METHOD

A comprehensive search of databases was completed and yielded eleven (n = 11) articles and one care protocol for analysis.

RESULTS

Evidence was appraised and findings suggest intravenous lidocaine has a low risk of causing adverse events, however patients should be monitored closely.

CONCLUSIONS

Nursing care focuses on pain assessment, close observation and intervention if neurological changes occur.

Molecular determinants of afferent sensitization in a rat model of cystitis with urothelial barrier dysfunction

The internal surface of the urinary bladder is covered by the urothelium, a stratified epithelium that forms an impermeable barrier to urinary solutes. Increased urothelial permeability is thought to contribute to symptom generation in several forms of cystitis by sensitizing bladder afferents. In this report we investigate the physiological mechanisms that mediate bladder afferent hyperexcitability in a rat model of cystitis induced by overexpression in the urothelium of claudin-2 (Cldn2), a tight junction-associated protein upregulated in bladder biopsies from patients with interstitial cystitis/bladder pain syndrome. Patch-clamp studies showed that overexpression of Cldn2 in the urothelium sensitizes a population of isolectin GS-IB4-negative [IB4(-)] bladder sensory neurons with tetrodotoxin-sensitive (TTX-S) action potentials. Gene expression analysis revealed a significant increase in mRNA levels of the delayed-rectifier voltage-gated K⁺ channel (K_v)2.2 and the accessory subunit K_v9.1 in this population of bladder sensory neurons. Consistent with this finding, K_v2/K_v9.1 channel activity was greater in IB4(-) bladder sensory neurons from rats overexpressing Cldn2 in the urothelium than in control counterparts. Likewise, current density of TTX-S voltage-gated Na⁺ (Na_v) channels was greater in sensitized neurons than in control counterparts. Significantly, guangxitoxin-1E (GxTX-1E), a selective blocker of K_v2 channels, blunted the repetitive firing of sensitized IB4(-) sensory neurons. In summary, our studies indicate that an increase in the activity of TTX-S Na_v and K_v2/K_v9.1 channels mediates repetitive firing of sensitized bladder sensory neurons in rats with increased urothelial permeability.NEW & NOTEWORTHY Hyperexcitability of sensitized bladder sensory neurons in a rat model of interstitial cystitis/bladder pain syndrome (IC/BPS) results from increased activity of tetrodotoxin-sensitive voltage-gated Na⁺ and delayed-rectifier voltage-gated K⁺ (K_v)2/K_v9.1 channels. Of major significance, our studies indicate that K_v2/K_v9.1 channels play a major role in symptom generation in this model of IC/BPS by maintaining the sustained firing of the sensitized bladder sensory neurons.

Tertiary Care Clinical Experience with Intravenous Lidocaine Infusions for the Treatment of Chronic Pain

Objective

To evaluate the safety of and long-term pain relief due to intravenous lidocaine infusion for the treatment of chronic pain in a tertiary pain management clinic.

Design

Retrospective chart review.

Methods

Medical records were reviewed from 233 adult chronic pain patients who underwent one to three lidocaine infusions. The initial lidocaine challenge consisted of 1,000 mg/h administered intravenously for up to 30 minutes until infusion was complete, full pain resolution, the patient requested to stop, side effects (SEs) became intolerable, and/or if there were any safety concerns. Subsequent infusions were tailored to patient response. Data reviewed included pain diagnosis, lidocaine dose, SEs, and duration of pain relief documented at a follow-up visit.

Results

Patients primarily had neuropathic pain (80%), were 94% white, 58% were female, and there was an average pain duration of 7.9 years. SEs were usually mild and transient, including perioral tingling, dizziness, tinnitus, and nausea/vomiting, and they were uncommon after the initial infusion. Overall, 41% of patients showed long-lasting pain relief, with positive response to the initial infusion associated with receiving and benefitting from subsequent infusions. Benefit by pain diagnoses varied from 32% to 58%.

Conclusions

Our retrospective study in a heterogeneous population with chronic pain suggests that intravenous lidocaine is a safe treatment. Data also suggest long-term pain relief in a significant proportion of patients. Additional study is important in order to delineate patient selection, determine optimal dosing and treatment frequency, assess pain reduction and duration, and treatment cost-effectiveness.

Spider venom peptides as potential drug candidates due to their anticancer and antinociceptive activities

Spider venoms are known to contain proteins and polypeptides that perform various functions including antimicrobial, neurotoxic, analgesic, cytotoxic, necrotic, and hemagglutinic activities. Currently, several classes of natural molecules from spider venoms are potential sources of chemotherapeutics against tumor cells. Some of the spider peptide toxins produce lethal effects on tumor cells by regulating the cell cycle, activating caspase pathway or inactivating mitochondria. Some of them also target the various types of ion channels (including voltage-gated calcium channels, voltage-gated sodium channels, and acid-sensing ion channels) among other pain-related targets. Herein we review the structure and pharmacology of spider-venom peptides that are being used as leads for the development of therapeutics against the pathophysiological conditions including cancer and pain.

Hyperglycemia induces mechanical hyperalgesia and depolarization of the resting membrane potential of primary nociceptive neurons: Role of ATP-sensitive potassium channels

Cumulating data suggests that ion channel alterations in nociceptive neurons might be involved in the development of diabetic painful neuropathy. In the present study we investigated the involvement of ATP-sensitive potassium (K⁺_ATP) channels in the acute effect of high glucose solution in vitro and in vivo. High glucose concentrations depolarized cultured nociceptive neurons and depolarization was blocked by the K⁺_ATP opener, diazoxide or by insulin. Glucose injection at the rat dorsal root ganglia (L5) resulted in acute mechanical hyperalgesia that was blocked by diazoxide. Mannitol injection indicates that osmolarity changes are not responsible for glucose effect. Therefore, this study suggests that K⁺_ATP channels expressed in peripheral sensory neurons might be involved in the development of diabetic painful neuropathy. Since sulfonylureas, that act by blocking K⁺_ATP are used for diabetes treatment, it is important to evaluate the possible side effects of such drugs at primary sensory neurons.

The Role of Toxins in the Pursuit for Novel Analgesics

Chronic pain is a major medical issue which reduces the quality of life of millions and inflicts a significant burden on health authorities worldwide. Currently, management of chronic pain includes first-line pharmacological therapies that are inadequately effective, as in just a portion of patients pain relief is obtained. Furthermore, most analgesics in use produce severe or intolerable adverse effects that impose dose restrictions and reduce compliance. As the majority of analgesic agents act on the central nervous system (CNS), it is possible that blocking pain at its source by targeting nociceptors would prove more efficient with minimal CNS-related side effects. The development of such analgesics requires the identification of appropriate molecular targets and thorough understanding of their structural and functional features. To this end, plant and animal toxins can be employed as they affect ion channels with high potency and selectivity. Moreover, elucidation of the toxin-bound ion channel structure could generate pharmacophores for rational drug design while favorable safety and analgesic profiles could highlight toxins as leads or even as valuable therapeutic compounds themselves. Here, we discuss the use of plant and animal toxins in the characterization of peripherally expressed ion channels which are implicated in pain.

A role for KCC3 in maintaining cell volume of peripheral nerve fibers

The potassium chloride cotransporter, KCC3, is an electroneutral cotransporter expressed in the peripheral and central nervous system. KCC3 is responsible for the efflux of K+ and Cl- in neurons to help maintain cell volume and intracellular chloride levels. A loss-of-function (LOF) of KCC3 causes Hereditary Motor Sensory Neuropathy with Agenesis of the Corpus Callosum (HMSN/ACC) in a population of individuals in the Charlevoix/Lac-Saint-Jean region of Quebec, Canada. A variety of mouse models have been created to understand the physiological and deleterious effects of a KCC3 LOF. Though this KCC3 LOF in mouse models has recapitulated the peripheral neuropathy phenotype of HMSN/ACC, we still know little about the development of the disease pathophysiology. Interestingly, the most recent KCC3 mouse model that we created recapitulated a peripheral neuropathy-like phenotype originating from a KCC3 gain-of-function (GOF). Despite the past two decades of research in attempting to understand the role of KCC3 in disease, we still do not understand how dysfunction of this cotransporter can lead to the pathophysiology of peripheral neuropathy. This review focuses on the function of KCC3 in neurons and its role in human and health and disease.

Long-term Therapeutic Effects of Extracorporeal Shock Wave-Assisted Melatonin Therapy on Mononeuropathic Pain in Rats

We evaluated the ability of extracorporeal shock wave (ECSW)-assisted melatonin (Mel) therapy to offer an additional benefit for alleviating the neuropathic pain (NP) in rats. Left sciatic nerve was subjected to chronic constriction injury (CCI) to induce NP. Animals (n = 30) were randomized into group 1 (sham-operated control), group 2 (CCI only), group 3 (CCI + ECSW), group 4 (CCI + Mel) and group 5 (CCI + ECSW + Mel). By days 15, 22 and 29 after CCI, the thermal paw withdrawal latency (TPWL) and mechanical paw withdrawal threshold (MPWT) were highest in group 1, lowest in group 2, significantly higher in group 5 than in groups 3 and 4, but they showed no difference between the later two groups (all p < 0.0001). The protein expressions of inflammatory (TNF-α, NF-κB, MMP-9, IL-1ß), oxidative-stress (NOXs-1, -2, -4, oxidized protein), apoptotic (cleaved-caspase3, cleaved-PARP), DNA/mitochondrial-damaged (γ-H2AX/cytosolic-cytochrome C), microglia/astrocyte activation (ox42/GFAP), and MAPKs [phosphorylated (p)-p38, p-JNK, p-ERK] biomarkers in dorsal root ganglia neurons (DRGs) and in spinal dorsal horn were exhibited an opposite pattern of TPWL among the five groups (all p < 0.0001). Additionally, protein expressions of Nav.1.3, Nav.1.8 and Nav.1.9 in sciatic nerve exhibited an identical pattern to inflammation among the five groups (all p < 0.0001). The numbers of cellular expressions of MAPKs (p-ERK1/2+/peripherin + cells, p-ERK1/2+/NF200 + cells and p-JNK+/peripherin + cells, p-JNK+/NF200 + cells) and voltage-gated sodium channels (Nav.1.8+/peripherin + cells, Nav.1.8+/NF200 + cells, Nav.1.9+/peripherin + cells, Nav.1.9+/NF200 + cells) in small and large DRGs displayed an identical pattern to inflammation among the five groups (all p < 0.0001). In conclusion, the synergistic effect of combined ECSW-Mel therapy is superior to either one alone for long-term improvement of mononeuropathic pain-induced by CCI in rats.

Analgesic effects of calcitonin on radicular pain in male rats

Purpose

Radicular pain is a frequently observed symptom of lumbar disk herniation or lumbar spinal canal stenosis. Achieving radicular pain relief is difficult. This type of pain may progress to chronic neuropathic pain. Calcitonin (elcatonin [eCT]) has been used mainly for hypercalcemia and pain associated with osteoporosis. The purpose of this study was to investigate analgesic effects of repeated eCT administration on radicular pain in male rats and changes in mRNA-expression levels of voltage-dependent sodium channels in the dorsal root ganglion (DRG).

Methods

Seventy male Sprague-Dawley rats were used. A right L5 hemilaminectomy and an L5-L6 partial facetectomy were performed to expose the right L5 nerve root. Under a microscope, the right L5 spinal nerve root was tightly ligated extradurally with 8-0 nylon suture proximally to the DRG to cause radicular pain in rats. Mechanical hyperalgesia, thermal hyperalgesia, and analgesic effects of eCT were compared among rats with radicular pain that received eCT, those that received the vehicle, and sham rats that received the vehicle. Real-time reverse-transcription PCR was performed to measure mRNA-expression levels of tetrodotoxin-sensitive (Na_V1.3 and Na_V1.6) and tetrodotoxin-resistant (Na_V1.8 and Na_V1.9) sodium channels in the DRG.

Results

Mechanical and thermal hyperalgesic reactions occurring in rats with radicular pain significantly improved on days 5 and 9 of eCT administration, respectively. In rats with radicular pain, mRNA-expression levels of Na_V1.3, Na_V1.8, and Na_V1.9 increased. After repeated eCT administration, mRNA-expression levels of these sodium channels in rats with radicular pain improved to the same levels as in sham rats.

Conclusion

The present study demonstrated that repeated systemic eCT administration was effective for radicular pain. No serious side effects of eCT have been reported thus far. Therefore, calcitonin may be a preferred therapeutic option for patients with radicular pain or for those requiring long-term treatment.

NRSF and Its Epigenetic Effectors: New Treatments for Neurological Disease

The Neuron Restrictive Silencer Factor (NRSF) is the well-known master transcriptional repressor of the neuronal phenotype. Research to date has shown that it is an important player in the growth and development of the nervous system. Its role in the maturation of neural precursor cells to adult neurons has been well characterized in stem cell models. While much has been characterized from a developmental perspective, research is revealing that NRSF plays a role in various neurological diseases, ranging from neurodegenerative, neuropsychiatric, to cancer. Dysregulation of NRSF activity disrupts downstream gene expression that is responsible for neuronal cell homeostasis in several models that contribute to pathologic states. Interestingly, it is now becoming apparent that the dysregulation of NRSF contributes to neurological disease through epigenetic mechanisms. Although NRSF itself is a transcription factor, its major effectors are chromatin modifiers. At the level of epigenetics, changes in NRSF activity have been well characterized in models of neuropathic pain and epilepsy. Better understanding of the epigenetic basis of brain diseases has led to design and use of small molecules that can prevent NRSF from repressing gene expression by neutralizing its interactions with its chromatin remodelers. This review will address the basic function of NRSF and its cofactors, investigate their mechanisms, then explore how their dysfunction can cause disease states. This review will also address research on NRSF as a therapeutic target and delve into new therapeutic strategies that focus on disrupting NRSF’s ability to recruit chromatin remodelers.

Where cone snails and spiders meet: design of small cyclic sodium-channel inhibitors

A 13 aa residue voltage-gated sodium (Na_V) channel inhibitor peptide, Pn, containing 2 disulfide bridges was designed by using a chimeric approach. This approach was based on a common pharmacophore deduced from sequence and secondary structural homology of 2 Na_V inhibitors: Conus kinoshitai toxin IIIA, a 14 residue cone snail peptide with 3 disulfide bonds, and Phoneutria nigriventer toxin 1, a 78 residue spider toxin with 7 disulfide bonds. As with the parent peptides, this novel Na_V channel inhibitor was active on Na_V1.2. Through the generation of 3 series of peptide mutants, we investigated the role of key residues and cyclization and their influence on Na_V inhibition and subtype selectivity. Cyclic PnCS1, a 10 residue peptide cyclized via a disulfide bond, exhibited increased inhibitory activity toward therapeutically relevant Na_V channel subtypes, including Na_V1.7 and Na_V1.9, while displaying remarkable serum stability. These peptides represent the first and the smallest cyclic peptide Na_V modulators to date and are promising templates for the development of toxin-based therapeutic agents.-Peigneur, S., Cheneval, O., Maiti, M., Leipold, E., Heinemann, S. H., Lescrinier, E., Herdewijn, P., De Lima, M. E., Craik, D. J., Schroeder, C. I., Tytgat, J. Where cone snails and spiders meet: design of small cyclic sodium-channel inhibitors.

Single-Blind Placebo-Controlled Response Test with Phenytoin 10% Cream in Neuropathic Pain Patients

Background: Phenytoin cream applied topically has been explored in neuropathic pain conditions. In several case series, phenytoin 5% and 10% cream could reduce pain in a clinically relevant way with a fast onset of action within 30 min, and with positive effects on sleep. Objective: To evaluate a single-blind placebo-controlled response test (SIBRET) for use in clinical practice. Materials and Methods: Patients with localized neuropathic pain, having an equal pain intensity in at least 2 areas (e.g., both feet), and a pain intensity of at least 4 on the 11-point numerical rating scale (NRS), were selected to perform the SIBRET. In one area, placebo cream consisting of the base cream was applied, and on the other area, phenytoin 10% cream was applied with separate hands to avoid contamination. Responders were defined as patients who experienced within 30 min at least 2-points difference as scored on the NRS, between the phenytoin 10% and the placebo cream applied areas, in favor of the former. Responders were subsequently prescribed phenytoin 10% cream. Results: Of the 21 patients, 15 patients (71.45%) were classified as responders. The mean pain reduction after 30 min as measured with the NRS in the phenytoin 10% cream area was 3.3 (SD: 1.3) and in the placebo cream area 1.2 (SD: 1.1). The difference of the mean percentage pain reduction between phenytoin 10% cream and placebo cream was 33.2% (SD: 17.6, p < 0.001). Using a 50% reduction on the NRS as a full response criterion, we could identify 57.1% of responders on phenytoin 10% cream and only 9.5% responders on placebo cream. Conclusions: The SIBRET helps patients and clinicians to quickly identify the appropriate treatment and can thus be seen as an important contributor to the domain of personalized medicine in pain. These results can also be regarded as a proof of principle for the analgesic activity of 10% phenytoin cream.

Discovery of a Novel Nav1.7 Inhibitor From Cyriopagopus albostriatus Venom With Potent Analgesic Efficacy

Spider venoms contain a vast array of bioactive peptides targeting ion channels. A large number of peptides have high potency and selectivity toward sodium channels. Na_v1.7 contributes to action potential generation and propagation and participates in pain signaling pathway. In this study, we describe the identification of μ-TRTX-Ca2a (Ca2a), a novel 35-residue peptide from the venom of Vietnam spider Cyriopagopus albostriatus (C. albostriatus) that potently inhibits Na_v1.7 (IC₅₀ = 98.1 ± 3.3 nM) with high selectivity against skeletal muscle isoform Na_v1.4 (IC₅₀ > 10 μM) and cardiac muscle isoform Na_v1.5 (IC₅₀ > 10 μM). Ca2a did not significantly alter the voltage-dependent activation or fast inactivation of Na_v1.7, but it hyperpolarized the slow inactivation. Site-directed mutagenesis analysis indicated that Ca2a bound with Na_v1.7 at the extracellular S3–S4 linker of domain II. Meanwhile, Ca2a dose-dependently attenuated pain behaviors in rodent models of formalin-induced paw licking, hot plate test, and acetic acid-induced writhing. This study indicates that Ca2a is a potential lead molecule for drug development of novel analgesics.

Models for Nociception Stimulation and Memory Effects in Awake and Aware Healthy Individuals

OBJECTIVE

This paper introduces a primer in the health care practice, namely a mathematical model and methodology for detecting and analysing nociceptor stimulation followed by related tissue memory effects.

METHODS

Noninvasive nociceptor stimulus protocol and prototype device for measuring bioimpedance is provided. Various time instants, sensor location, and stimulus train have been analysed.

RESULTS

The method and model indicate that nociceptor stimulation perceived as pain in awake healthy volunteers is noninvasively detected. The existence of a memory effect is proven from data. Sensor location had minimal effect on detection level, while day-to-day variability was observed without being significant.

CONCLUSION

Following the experimental study, the model enables a comprehensive management of chronic pain patients, and possibly other analgesia, or pain related regulatory loops.

SIGNIFICANCE

A device and methodology for noninvasive for detecting nociception stimulation have been developed. The proposed method and models have been validated on healthy volunteers.

Efficacy of lidocaine 5% medicated plaster (VERSATIS®) in patients with localized neuropathic pain poorly responsive to pharmacological therapy

BACKGROUND

Localized neuropathic pain (LNP) is a subgroup of neuropathic pain characterized by consistent and circumscribed area(s) of maximum pain, associated with negative or positive sensory signs and/or spontaneous symptoms characteristic of NP. Lidocaine medicated plasters (LMP) have shown to be effective in pain relief in selective LNP syndromes.

METHODS

We collected data of 130 patients in our database with LNP syndromes who used LMP.

RESULTS

Forty-one patients out of 130 patients (32%) were treated with antiepileptics, antidepressants and opioids without improvement and/or with intolerable adverse effects and are not assuming systemic therapy anymore. Globally, during the 12 months follow-up, 15% of patients reached a complete pain relief without any systemic therapy, mainly in trigeminal and post-herpetic neuralgia (P=0.009), 38% of patients reduced analgesic drug consumption with the highest reduction in radiculopathy, post-herpetic neuralgia and trigeminal neuralgia. Topical and transient adverse effects, such as itching or local erythema, were seen in 19/130 (14.6%) patients; 7 of these patients (5.4%) needed to discontinue the treatment due to the occurrence of adverse effects. The dropout rate on global population (excluding cured and lost to follow-up) was 45%, and the main cause of dropouts was the inefficacy of treatment in the first 3 months of therapy with LMP.

CONCLUSIONS

LMP treatment is safe and worth consideration also as add-on therapy in order to reduce analgesic drug consumption in selected LNP.

Behavioral, cellular and molecular maladaptations covary with exposure to pyridostigmine bromide in a rat model of gulf war illness pain

Many veterans of Operation Desert Storm (ODS) struggle with the chronic pain of Gulf War Illness (GWI). Exposure to insecticides and pyridostigmine bromide (PB) have been implicated in the etiology of this multisymptom disease. We examined the influence of 3 (DEET (N,N-diethyl-meta-toluamide), permethrin, chlorpyrifos) or 4 GW agents (DEET, permethrin, chlorpyrifos, pyridostigmine bromide (PB)) on the post-exposure ambulatory and resting behaviors of rats. In three independent studies, rats that were exposed to all 4 agents consistently developed both immediate and delayed ambulatory deficits that persisted at least 16 weeks after exposures had ceased. Rats exposed to a 3 agent protocol (PB excluded) did not develop any ambulatory deficits. Cellular and molecular studies on nociceptors harvested from 16WP (weeks post-exposure) rats indicated that vascular nociceptor Na_v1.9 mediated currents were chronically potentiated following the 4 agent protocol but not following the 3 agent protocol. Muscarinic linkages to muscle nociceptor TRPA1 were also potentiated in the 4 agent but not the 3 agent, PB excluded, protocol. Although K_v7 activity changes diverged from the behavioral data, a K_v7 opener, retigabine, transiently reversed ambulation deficits. We concluded that PB played a critical role in the development of pain-like signs in a GWI rat model and that shifts in Na_v1.9 and TRPA1 activity were critical to the expression of these pain behaviors.

Body mass index interaction with SCN10A effects on cardiovascular autonomic neuropathy risk in a Chinese Han population

Background: The aim of this study was to estimate the extent to which the interaction between SCN10A (rs7375096) and body mass index (BMI) affects susceptibility to cardiovascular autonomic neuropathy (AN) in a Chinese Han sample.

Method: We conducted a large-scale, population-based study including 791 overweight patients and 1186 controls to analyze the interaction between SCN10A and BMI with regard to CAN. All participants were genotyped for the presence of SCN10A (rs7375036) polymorphisms. Univariate and multiple linear regression analyses were employed to detect these relationships. The interaction between rs7375036 and BMI with regard to CAN susceptibility on an additive scale was estimated by using the relative excess risk due to interaction, the proportion attributable to interaction, and the synergy index.

Results: Univariate analyses found no significant association between rs7375036 single nucleotide polymorphism and CAN. After controlling for potential confounding factors, the multiple linear regression analysis showed a significant association between BMI and CAN ([Formula: see text], OR = 1.121). In addition, a significant positive interaction effect of BMI and rs7375036 on CAN was detected ([Formula: see text]).

Conclusion: Our findings suggest that BMI and SCN10A (rs7375036) have interaction effects that influence the CAN.

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

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

Neuropathic Pain: Central vs. Peripheral Mechanisms

PURPOSE OF REVIEW

Our goal is to examine the processes-both central and peripheral-that underlie the development of peripherally-induced neuropathic pain (pNP) and to highlight recent evidence for mechanisms contributing to its maintenance. While many pNP conditions are initiated by damage to the peripheral nervous system (PNS), their persistence appears to rely on maladaptive processes within the central nervous system (CNS). The potential existence of an autonomous pain-generating mechanism in the CNS creates significant implications for the development of new neuropathic pain treatments; thus, work towards its resolution is crucial. Here, we seek to identify evidence for PNS and CNS independently generating neuropathic pain signals.

RECENT FINDINGS

Recent preclinical studies in pNP support and provide key details concerning the role of multiple mechanisms leading to fiber hyperexcitability and sustained electrical discharge to the CNS. In studies regarding central mechanisms, new preclinical evidence includes the mapping of novel inhibitory circuitry and identification of the molecular basis of microglia-neuron crosstalk. Recent clinical evidence demonstrates the essential role of peripheral mechanisms, mostly via studies that block the initially damaged peripheral circuitry. Clinical central mechanism studies use imaging to identify potentially self-sustaining infra-slow CNS oscillatory activity that may be unique to pNP patients. While new preclinical evidence supports and expands upon the key role of central mechanisms in neuropathic pain, clinical evidence for an autonomous central mechanism remains relatively limited. Recent findings from both preclinical and clinical studies recapitulate the critical contribution of peripheral input to maintenance of neuropathic pain. Further clinical investigations on the possibility of standalone central contributions to pNP may be assisted by a reconsideration of the agreed terms or criteria for diagnosing the presence of central sensitization in humans.

Human vs. Mouse Nociceptors - Similarities and Differences

The somatosensory system allows us to detect a diverse range of physical and chemical stimuli including noxious ones, which can initiate protective reflexes to prevent tissue damage. However, the sensation of pain can - under pathological circumstances - outlive its usefulness and perpetrate ongoing suffering. Rodent model systems have been tremendously useful to help understand basic mechanisms of pain perception. Unfortunately, the translation of this knowledge into novel therapies has been challenging. We have investigated similarities and differences of human and mouse peptidergic (TRKA expressing) nociceptors using dual-color fluorescence in situ hybridization of dorsal root ganglia. By comparing the transcripts of a selected group of well-established nociceptive markers, we observed significant differences for some of them. We found co-expression of Trpv1, a key player for sensitization and inflammatory pain, with TrkA in a larger population in humans compared to mice. Similar results could be obtained for Na_v1.8 and Na_v1.9, two voltage gated sodium channels implicated in pathological forms of pain. Additionally, co-expression of Ret and TrkA was also found to be more abundant in human neurons. Moreover, the neurofilament heavy polypeptide was detected in all human sensory DRG neurons compared to a more selective expression pattern observed in rodents. To our knowledge, this is the first time that such detailed comparative analysis has been performed and we believe that our findings will direct future experimentation geared to understand the difficulties we face in translating findings from rodent models to humans.

Translocator protein agonist Ro5-4864 alleviates neuropathic pain and promotes remyelination in the sciatic nerve

Our previous study reported the translocator protein to play a critical role in neuropathic pain and the possible mechanisms in the spinal cord. However, its mechanism in the peripheral nervous system is poorly understood. This study was undertaken to explore the distribution of translocator protein in the dorsal root ganglion and the possible mechanisms in peripheral nervous system in a rat model of spared nerve injury. Our results showed that translocator protein was activated in dorsal root ganglion after spared nerve injury. The translocator protein signals were primarily colocalized with neurons in dorsal root ganglion. A single intrathecal (i.t.) injection of translocator protein agonist (7-chloro-5-4-chlorophenyl)-1,3-dihydro-1-methyl-2-H-1,4-benzodiaze-pine-2) (Ro5-4864) exerted remarkable analgesic effect compared with the spared nerve injury group ( P < 0.01). After i.t. administration of 2 µg Ro5-4864 on day 3, the expression of translocator protein in ipsilateral dorsal root ganglion was significantly increased on day 7( P < 0.01) but decreased on day 14 ( P < 0.05) compared with the same point in time in the control group. The duration of translocator protein activation in dorsal root ganglion was remarkably shortened. Ro5-4864 also inhibited the activation of phospho-extracellular signal-regulated kinase 1(p-ERK1) ( P < 0.01), p-ERK2 (D7: P < 0.01, D14: P < 0.05), and brain-derived neurotrophic factor ( P < 0.05) in dorsal root ganglion. Meanwhile, i.t. administration of 2 µg Ro5-4864 on day 3 further accelerated the expression of myelin protein zero(P0) and peripheral myelin protein 22 (PMP22). Our results suggested Ro5-4864 could alleviate neuropathic pain and attenuate p-ERK and brain-derived neurotrophic factor activation in dorsal root ganglion. Furthermore, Ro5-4864 stimulated the expression of myelin regeneration proteins which may also be an important factor against neuropathic pain development. Translocator protein may present a novel target for the treatment of neuropathic pain both in the central and peripheral nervous systems.

Distribution and function of voltage-gated sodium channels in the nervous system

Voltage-gated sodium channels (VGSCs) are the basic ion channels for neuronal excitability, which are crucial for the resting potential and the generation and propagation of action potentials in neurons. To date, at least nine distinct sodium channel isoforms have been detected in the nervous system. Recent studies have identified that voltage-gated sodium channels not only play an essential role in the normal electrophysiological activities of neurons but also have a close relationship with neurological diseases. In this study, the latest research findings regarding the structure, type, distribution, and function of VGSCs in the nervous system and their relationship to neurological diseases, such as epilepsy, neuropathic pain, brain tumors, neural trauma, and multiple sclerosis, are reviewed in detail.

Pharmacological Regulation of Neuropathic Pain Driven by Inflammatory Macrophages

Neuropathic pain can have a major effect on quality of life but current therapies are often inadequate. Growing evidence suggests that neuropathic pain induced by nerve damage is caused by chronic inflammation. Upon nerve injury, damaged cells secrete pro-inflammatory molecules that activate cells in the surrounding tissue and recruit circulating leukocytes to the site of injury. Among these, the most abundant cell type is macrophages, which produce several key molecules involved in pain enhancement, including cytokines and chemokines. Given their central role in the regulation of peripheral sensitization, macrophage-derived cytokines and chemokines could be useful targets for the development of novel therapeutics. Inhibition of key pro-inflammatory cytokines and chemokines prevents neuroinflammation and neuropathic pain; moreover, recent studies have demonstrated the effectiveness of pharmacological inhibition of inflammatory (M1) macrophages. Nicotinic acetylcholine receptor ligands and T helper type 2 cytokines that reduce M1 macrophages are able to relieve neuropathic pain. Future translational studies in non-human primates will be crucial for determining the regulatory mechanisms underlying neuroinflammation-associated neuropathic pain. In turn, this knowledge will assist in the development of novel pharmacotherapies targeting macrophage-driven neuroinflammation for the treatment of intractable neuropathic pain.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Discovery and mode of action of a novel analgesic β-toxin from the African spider Ceratogyrus darlingi

Spider venoms are rich sources of peptidic ion channel modulators with important therapeutical potential. We screened a panel of 60 spider venoms to find modulators of ion channels involved in pain transmission. We isolated, synthesized and pharmacologically characterized Cd1a, a novel peptide from the venom of the spider Ceratogyrus darlingi. Cd1a reversibly paralysed sheep blowflies (PD₅₀ of 1318 pmol/g) and inhibited human Ca_v2.2 (IC₅₀ 2.6 μM) but not Ca_v1.3 or Ca_v3.1 (IC₅₀ > 30 μM) in fluorimetric assays. In patch-clamp electrophysiological assays Cd1a inhibited rat Ca_v2.2 with similar potency (IC₅₀ 3 μM) without influencing the voltage dependence of Ca_v2.2 activation gating, suggesting that Cd1a doesn’t act on Ca_v2.2 as a classical gating modifier toxin. The Cd1a binding site on Ca_v2.2 did not overlap with that of the pore blocker ω-conotoxin GVIA, but its activity at Ca_v2.2-mutant indicated that Cd1a shares some molecular determinants with GVIA and MVIIA, localized near the pore region. Cd1a also inhibited human Na_v1.1–1.2 and Na_v1.7–1.8 (IC₅₀ 0.1–6.9 μM) but not Na_v1.3–1.6 (IC₅₀ > 30 μM) in fluorimetric assays. In patch-clamp assays, Cd1a strongly inhibited human Na_v1.7 (IC₅₀ 16 nM) and produced a 29 mV depolarising shift in Na_v1.7 voltage dependence of activation. Cd1a (400 pmol) fully reversed Na_v1.7-evoked pain behaviours in mice without producing side effects. In conclusion, Cd1a inhibited two anti-nociceptive targets, appearing to interfere with Ca_v2.2 inactivation gating, associated with the Ca_v2.2 α-subunit pore, while altering the activation gating of Na_v1.7. Cd1a was inactive at some of the Na_v and Ca_v channels expressed in skeletal and cardiac muscles and nodes of Ranvier, apparently contributing to the lack of side effects at efficacious doses, and suggesting potential as a lead for development of peripheral pain treatments.

Old Friends With New Faces: Are Sodium Channel Blockers the Future of Adjunct Pain Medication Management?

Providers are being asked to decrease the emphasis and overutilization of long-term opioid therapy, but many are left without proper guidance on appropriate utilization of nonopioid therapies. Furthermore, therapeutic options are quite limited and many providers lack confidence in distinguishing available alternatives. When first-line therapy has failed in a patient, there is an apparent lack of knowledge on how to proceed with choosing subsequent therapy. To choose among alternative agents, an understanding of pharmacology, pharmacokinetics, and efficacy in targeting various pain conditions is necessary. This article focuses on the use of the carboxamide class of sodium channel blockers (carbamazepine, oxcarbazepine, eslicarbazepine) for adjunct pain medication management including research updates in pharmacology, pharmacokinetics, and current evidence for pain along with promising areas of research. It is an evidence update for clinical use of sodium channel blockers, clarifies misconceptions regarding their use, and highlights emerging research for improved pain targets that justifies additional study. We performed a complete review of the literature using the search terms, "oxcarbazepine," "carbamazepine," and "eslicarbazepine" in conjunction with "pharmacokinetics," "adverse effects," "pharmacology," "voltage-gated sodium channel subtype," "neuropathic pain," "inflammatory pain," "metabolism," "epoxide metabolite formation," "drug interactions," "CYP450 interactions," "pain phenotype," and "chronic pain management." Databases searched included PubMed and Google Scholar. Package inserts were used for drug structure illustration, adverse reactions, and bioavailability. Pharmacology and pharmacokinetic data were taken from randomized controlled trials evaluating this area as well as in vitro published results. For validity, only peer-reviewed literature was included. Evidence for sodium channel blockers in chronic pain management was limited. This review focuses on highlighting the data available for the use of sodium channel blockers for certain pain syndromes as well as underutilized potential. Emerging literature on sodium channel subtypes and their connection to neuropathic, inflammatory, and mechanical pain transmission is elucidated. The authors also scrutinize literature surrounding the pharmacokinetics of oxcarbazepine and eslicarbazepine to provide clearer guidance to the significance of any drug interactions and refute assumptions made on the basis of structural similarity to carbamazepine and its known undesirable drug interactions. Side effect profiles are outlined and compared, emphasizing the differences between agents. Sodium channel blocker doses used in certain pain syndromes are outlined with a call for further research to better understand their place in chronic pain management. Identification of sodium channel subtypes with links to specific pain conditions and the ability to target them hints at the potential for truly individualized therapy. Sodium channel inhibitors are underutilized on the basis of available evidence, and emerging research has identified this area as promising for additional clinical trials to better guide clinical practice.

PERSPECTIVE

This article provides a review of the pharmacology, evidence for pain management, and pharmacokinetics of oxcarbazepine, carbamazepine, and eslicarbazepine. There is a disparity in evidence using sodium channel blockers for pain and this article highlights the potential that is currently underutilized. The authors believe this will catalyze interest for further studies.

Ambroxol for the treatment of fibromyalgia: science or fiction?

Fibromyalgia appears to present in subgroups with regard to biological pain induction, with primarily inflammatory, neuropathic/neurodegenerative, sympathetic, oxidative, nitrosative, or muscular factors and/or central sensitization. Recent research has also discussed glial activation or interrupted dopaminergic neurotransmission, as well as increased skin mast cells and mitochondrial dysfunction. Therapy is difficult, and the treatment options used so far mostly just have the potential to address only one of these aspects. As ambroxol addresses all of them in a single substance and furthermore also reduces visceral hypersensitivity, in fibromyalgia existing as irritable bowel syndrome or chronic bladder pain, it should be systematically investigated for this purpose. Encouraged by first clinical observations of two working groups using topical or oral ambroxol for fibromyalgia treatments, the present paper outlines the scientific argument for this approach by looking at each of the aforementioned aspects of this complex disease and summarizes putative modes of action of ambroxol. Nevertheless, at this point the evidence basis for ambroxol is not strong enough for clinical recommendation.

Modulatory features of the novel spider toxin μ-TRTX-Df1a isolated from the venom of the spider Davus fasciatus

BACKGROUND AND PURPOSE

Naturally occurring dysfunction of voltage-gated sodium (Na_V ) channels results in complex disorders such as chronic pain, making these channels an attractive target for new therapies. In the pursuit of novel Na_V modulators, we investigated spider venoms for new inhibitors of Na_V channels.

EXPERIMENTAL APPROACH

We used high-throughput screens to identify a Na_V modulator in venom of the spider Davus fasciatus. Further characterization of this venom peptide was undertaken using fluorescent and electrophysiological assays, molecular modelling and a rodent pain model.

KEY RESULTS

We identified a potent Na_V inhibitor named μ-TRTX-Df1a. This 34-residue peptide fully inhibited responses mediated by Na_V 1.7 endogenously expressed in SH-SY5Y cells. Df1a also inhibited voltage-gated calcium (Ca_V 3) currents but had no activity against the voltage-gated potassium (K_V 2) channel. The modelled structure of Df1a, which contains an inhibitor cystine knot motif, is reminiscent of the Na_V channel toxin ProTx-I. Electrophysiology revealed that Df1a inhibits all Na_V subtypes tested (hNa_V 1.1-1.7). Df1a also slowed fast inactivation of Na_V 1.1, Na_V 1.3 and Na_V 1.5 and modified the voltage-dependence of activation and inactivation of most of the Na_V subtypes. Df1a preferentially binds to the domain II voltage-sensor and has additional interactions with the voltage sensors domains III and IV, which probably explains its modulatory features. Df1a was analgesic in vivo, reversing the spontaneous pain behaviours induced by the Na_V activator OD1.

CONCLUSION AND IMPLICATIONS

μ-TRTX-Df1a shows potential as a new molecule for the development of drugs to treat pain disorders mediated by voltage-gated ion channels.