Early painful diabetic neuropathy is associated with differential changes in tetrodotoxin-sensitive and -resistant sodium channels in dorsal root ganglion neurons in the rat

A Novel Antigen Design Strategy to Isolate Single-Domain Antibodies that Target Human Nav1.7 and Reduce Pain in Animal Models

Genetic studies have identified the voltage-gated sodium channel 1.7 (Nav1.7) as pain target. Due to the ineffectiveness of small molecules and monoclonal antibodies as therapeutics for pain, single-domain antibodies (VHHs) are developed against the human Nav1.7 (hNav1.7) using a novel antigen presentation strategy. A 70 amino-acid peptide from the hNav1.7 protein is identified as a target antigen. A recombinant version of this peptide is grafted into the complementarity determining region 3 (CDR3) loop of an inert VHH in order to maintain the native 3D conformation of the peptide. This antigen is used to isolate one VHH able to i) bind hNav1.7, ii) slow the deactivation of hNav1.7, iii) reduce the ability of eliciting action potentials in nociceptors, and iv) reverse hyperalgesia in in vivo rat and mouse models. This VHH exhibits the potential to be developed as a therapeutic capable of suppressing pain. This novel antigen presentation strategy can be applied to develop biologics against other difficult targets such as ion channels, transporters and GPCRs.

Computational modeling to study the impact of changes in Nav1.8 sodium channel on neuropathic pain

Objective

Nav1.8 expression is restricted to sensory neurons; it was hypothesized that aberrant expression and function of this channel at the site of injury contributed to pathological pain. However, the specific contributions of Nav1.8 to neuropathic pain are not as clear as its role in inflammatory pain. The aim of this study is to understand how Nav1.8 present in peripheral sensory neurons regulate neuronal excitability and induce various electrophysiological features on neuropathic pain.

Methods

To study the effect of changes in sodium channel Nav1.8 kinetics, Hodgkin-Huxley type conductance-based models of spiking neurons were constructed using the NEURON v8.2 simulation software. We constructed a single-compartment model of neuronal soma that contained Nav1.8 channels with the ionic mechanisms adapted from some existing small DRG neuron models. We then validated and compared the model with our experimental data from in vivo recordings on soma of small dorsal root ganglion (DRG) sensory neurons in animal models of neuropathic pain (NEP).

Results

We show that Nav1.8 is an important parameter for the generation and maintenance of abnormal neuronal electrogenesis and hyperexcitability. The typical increased excitability seen is dominated by a left shift in the steady state of activation of this channel and is further modulated by this channel's maximum conductance and steady state of inactivation. Therefore, modified action potential shape, decreased threshold, and increased repetitive firing of sensory neurons in our neuropathic animal models may be orchestrated by these modulations on Nav1.8.

Conclusion

Computational modeling is a novel strategy to understand the generation of chronic pain. In this study, we highlight that changes to the channel functions of Nav1.8 within the small DRG neuron may contribute to neuropathic pain.

Progression of axonal excitability abnormalities with increasing clinical severity of diabetic peripheral neuropathy

OBJECTIVE

Diabetic peripheral neuropathy (DPN) is a frequent complication for persons with type 2 diabetes. Previous studies have failed to demonstrate any significant impact of treatment for DPN. The present study assessed the role of axonal ion channel dysfunction in DPN and explored the hypothesis that there may be a progressive change in ion channel abnormalities that varied with disease stage.

METHODS

Neurophysiological studies were conducted using axonal excitability techniques, a clinical method of assessing ion channel dysfunction. Studies were conducted in 178 persons with type 2 diabetes, with participants allocated into four groups according to clinical severity of neuropathy, assessed using the Total Neuropathy Grade.

RESULTS

Analysis of excitability data demonstrated a progressive and stepwise reduction in two parameters that are related to the activity of Kv1.1 channels, namely superexcitability and depolarizing threshold electrotonus at 10-20 ms (p < 0.001), and mathematical modelling of axonal excitability findings supported progressive upregulation of Kv1.1 conductances with increasing greater disease severity.

CONCLUSION

The findings are consistent with a progressive upregulation of juxtaparanodal Kv1.1 conductances with increasing clinical severity of diabetic peripheral neuropathy.

SIGNIFICANCE

From a translational perspective, the study suggests that blockade of Kv1.1 channels using 4-aminopyridine derivatives such as fampridine may be a potential treatment for DPN.

Painful diabetic neuropathy: The role of ion channels

Painful diabetic neuropathy (PDN) is a common chronic complication of diabetes that causes neuropathic pain and negatively affects the quality of life. The management of PDN is far from satisfactory. At present, interventions are primarily focused on symptomatic treatment. Ion channel disorders are a major cause of PDN, and a complete understanding of their roles and mechanisms may provide better options for the clinical treatment of PDN. Therefore, this review summarizes the important role of ion channels in PDN and the current drug development targeting these ion channels.

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.

Structure-function and rational design of a spider toxin Ssp1a at human voltage-gated sodium channel subtypes

The structure-function and optimization studies of NaV-inhibiting spider toxins have focused on developing selective inhibitors for peripheral pain-sensing NaV1.7. With several NaV subtypes emerging as potential therapeutic targets, structure-function analysis of NaV-inhibiting spider toxins at such subtypes is warranted. Using the recently discovered spider toxin Ssp1a, this study extends the structure-function relationships of NaV-inhibiting spider toxins beyond NaV1.7 to include the epilepsy target NaV1.2 and the pain target NaV1.3. Based on these results and docking studies, we designed analogues for improved potency and/or subtype-selectivity, with S7R-E18K-rSsp1a and N14D-P27R-rSsp1a identified as promising leads. S7R-E18K-rSsp1a increased the rSsp1a potency at these three NaV subtypes, especially at NaV1.3 (∼10-fold), while N14D-P27R-rSsp1a enhanced NaV1.2/1.7 selectivity over NaV1.3. This study highlights the challenge of developing subtype-selective spider toxin inhibitors across multiple NaV subtypes that might offer a more effective therapeutic approach. The findings of this study provide a basis for further rational design of Ssp1a and related NaSpTx1 homologs targeting NaV1.2, NaV1.3 and/or NaV1.7 as research tools and therapeutic leads.

Evaluated periodontal tissues and oxidative stress in rats with neuropathic pain-like behavior

BACKGROUND

Oxidative stress has a critical effect on both persistent pain states and periodontal disease. Voltage-gated sodium NaV1.7 (SCN9A), and transient receptor potential ankyrin 1 (TRPA1) are pain genes. The goal of this study was to investigate oxidative stress markers, periodontal status, SCN9A, and TRPA1 channel expression in periodontal tissues of rats with paclitaxel-induced neuropathic pain-like behavior (NPLB).

METHODS AND RESULTS

Totally 16 male Sprague Dawley rats were used: control (n = 8) and paclitaxel-induced pain (PTX) (n = 8). The alveolar bone loss and 8-hydroxy-2-deoxyguanosine (8-OHdG) levels were analyzed histometrically and immunohistochemically. Gingival superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities (spectrophotometric assay) were measured. The relative TRPA1 and SCN9A genes expression levels were evaluated using quantitative real-time PCR (qPCR) in the tissues of gingiva and brain. The PTX group had significantly higher alveolar bone loss and 8-OHdG compared to the control. The PTX group had significantly lower gingival SOD, GPx and CAT activity than the control groups. The PTX group had significantly higher relative gene expression of SCN9A (p = 0.0002) and TRPA1 (p = 0.0002) than the control in gingival tissues. Increased nociceptive susceptibility may affect the increase in oxidative stress and periodontal destruction.

CONCLUSIONS

Chronic pain conditions may increase TRPA1 and SCN9A gene expression in the periodontium. The data of the current study may help develop novel approaches both to maintain periodontal health and alleviate pain in patients suffering from orofacial pain.

Identification and targeting of a unique NaV1.7 domain driving chronic pain

Small molecules directly targeting the voltage-gated sodium channel (VGSC) NaV1.7 have not been clinically successful. We reported that preventing the addition of a small ubiquitin-like modifier onto the NaV1.7-interacting cytosolic collapsin response mediator protein 2 (CRMP2) blocked NaV1.7 function and was antinociceptive in rodent models of neuropathic pain. Here, we discovered a CRMP2 regulatory sequence (CRS) unique to NaV1.7 that is essential for this regulatory coupling. CRMP2 preferentially bound to the NaV1.7 CRS over other NaV isoforms. Substitution of the NaV1.7 CRS with the homologous domains from the other eight VGSC isoforms decreased NaV1.7 currents. A cell-penetrant decoy peptide corresponding to the NaV1.7-CRS reduced NaV1.7 currents and trafficking, decreased presynaptic NaV1.7 expression, reduced spinal CGRP release, and reversed nerve injury-induced mechanical allodynia. Importantly, the NaV1.7-CRS peptide did not produce motor impairment, nor did it alter physiological pain sensation, which is essential for survival. As a proof-of-concept for a NaV1.7 -targeted gene therapy, we packaged a plasmid encoding the NaV1.7-CRS in an AAV virus. Treatment with this virus reduced NaV1.7 function in both rodent and rhesus macaque sensory neurons. This gene therapy reversed and prevented mechanical allodynia in a model of nerve injury and reversed mechanical and cold allodynia in a model of chemotherapy-induced peripheral neuropathy. These findings support the conclusion that the CRS domain is a targetable region for the treatment of chronic neuropathic pain.

Diabetes-induced electrophysiological alterations on neurosomes in ganglia of peripheral nervous system

Diabetes mellitus (DM) leads to medical complications, the epidemiologically most important of which is diabetic peripheral neuropathy (DPN). Electrophysiology is a major component of neural functioning and several studies have been undertaken to elucidate the neural electrophysiological alterations caused by DM and their mechanisms of action. Due to the importance of electrophysiology for neuronal function, the review of the studies dealing predominantly with electrophysiological parameters and mechanisms in the neuronal somata of peripheral neural ganglia of diabetic animals during the last 45 years is here undertaken. These studies, using predominantly techniques of electrophysiology, most frequently patch clamp for voltage clamp studies of transmembrane currents through ionic channels, have investigated the experimental DPN. They also have demonstrated that various cellular and molecular mechanisms of action of diabetic physiopathology at the level of biophysical electrical parameters are affected in DPN. Thus, they have demonstrated that several passive and active transmembrane voltage parameters, related to neuronal excitability and neuronal functions, are altered in diabetes. The majority of the studies agreed that DM produces depolarization of the resting membrane potential; alters excitability, increasing and decreasing it in dorsal root ganglia (DRG) and in nodose ganglion, respectively. They have tried to relate these changes to sensorial alterations of DPN. Concerning ionic currents, predominantly studied in DRG, the most frequent finding was increases in Na+, Ca2+, and TRPV1 cation current, and decreases in K+ current. This review concluded that additional studies are needed before an understanding of the hierarchized, time-dependent, and integrated picture of the contribution of neural electrophysiological alterations to the DPN could be reached. DM-induced electrophysiological neuronal alterations that so far have been demonstrated, most of them likely important, are either consistent with the DPN symptomatology or suggest important directions for improvement of the elucidation of DPN physiopathology, which the continuation seems to us very relevant.

Targeting the vascular endothelial growth factor A/neuropilin 1 axis for relief of neuropathic pain

ABSTRACT

Vascular endothelial growth factor A (VEGF-A) is a pronociceptive factor that causes neuronal sensitization and pain. We reported that blocking the interaction between the membrane receptor neuropilin 1 (NRP1) and VEGF-A-blocked VEGF-A-mediated sensory neuron hyperexcitability and reduced mechanical hypersensitivity in a rodent chronic neuropathic pain model. These findings identified the NRP1-VEGF-A signaling axis for therapeutic targeting of chronic pain. In an in-silico screening of approximately 480 K small molecules binding to the extracellular b1b2 pocket of NRP1, we identified 9 chemical series, with 6 compounds disrupting VEGF-A binding to NRP1. The small molecule with greatest efficacy, 4'-methyl-2'-morpholino-2-(phenylamino)-[4,5'-bipyrimidin]-6(1H)-one, designated NRP1-4, was selected for further evaluation. In cultured primary sensory neurons, VEGF-A enhanced excitability and decreased firing threshold, which was blocked by NRP1-4. In addition, NaV1.7 and CaV2.2 currents and membrane expression were potentiated by treatment with VEGF-A, and this potentiation was blocked by NRP1-4 cotreatment. Neuropilin 1-4 reduced VEGF-A-mediated increases in the frequency and amplitude of spontaneous excitatory postsynaptic currents in dorsal horn of the spinal cord. Neuropilin 1-4 did not bind to more than 300 G-protein-coupled receptors and receptors including human opioids receptors, indicating a favorable safety profile. In rats with spared nerve injury-induced neuropathic pain, intrathecal administration of NRP1-4 significantly attenuated mechanical allodynia. Intravenous treatment with NRP1-4 reversed both mechanical allodynia and thermal hyperalgesia in rats with L5/L6 spinal nerve ligation-induced neuropathic pain. Collectively, our findings show that NRP1-4 is a first-in-class compound targeting the NRP1-VEGF-A signaling axis to control voltage-gated ion channel function, neuronal excitability, and synaptic activity that curb chronic pain.

Diabetes mellitus differently affects electrical membrane properties of vagal afferent neurons of rats

To study whether diabetes mellitus (DM) would cause electrophysiological alterations in nodose ganglion (NG) neurons, we used patch clamp and intracellular recording for voltage and current clamp configuration, respectively, on cell bodies of NG from rats with DM. Intracellular microelectrodes recording, according to the waveform of the first derivative of the action potential, revealed three neuronal groups (A₀ , A_inf , and C_inf ), which were differently affected. Diabetes only depolarized the resting potential of A₀ (from -55 to -44 mV) and C_inf (from -49 to -45 mV) somas. In A_inf neurons, diabetes increased action potential and the after-hyperpolarization durations (from 1.9 and 18 to 2.3 and 32 ms, respectively) and reduced dV/dt_desc (from -63 to -52 V s^-1 ). Diabetes reduced the action potential amplitude while increasing the after-hyperpolarization amplitude of C_inf neurons (from 83 and -14 mV to 75 and -16 mV, respectively). Using whole cell patch clamp recording, we observed that diabetes produced an increase in peak amplitude of sodium current density (from -68 to -176 pA pF^-1 ) and displacement of steady-state inactivation to more negative values of transmembrane potential only in a group of neurons from diabetic animals (DB2). In the other group (DB1), diabetes did not change this parameter (-58 pA pF^-1 ). This change in sodium current did not cause an increase in membrane excitability, probably explainable by the alterations in sodium current kinetics, which are also induced by diabetes. Our data demonstrate that diabetes differently affects membrane properties of different nodose neuron subpopulations, which likely have pathophysiological implications for diabetes mellitus.

Voltage-gated sodium channels in diabetic sensory neuropathy: Function, modulation, and therapeutic potential

Voltage-gated sodium channels (Na V ) are the main contributors to action potential generation and essential players in establishing neuronal excitability. Na V channels have been widely studied in pain pathologies, including those that develop during diabetes. Diabetic sensory neuropathy (DSN) is one of the most common complications of the disease. DSN is the result of sensory nerve damage by the hyperglycemic state, resulting in a number of debilitating symptoms that have a significant negative impact in the quality of life of diabetic patients. Among those symptoms are tingling and numbness of hands and feet, as well as exacerbated pain responses to noxious and non-noxious stimuli. DSN is also a major contributor to the development of diabetic foot, which may lead to lower limb amputations in long-term diabetic patients. Unfortunately, current treatments fail to reverse or successfully manage DSN. In the current review we provide an updated report on Na V channels including structure/function and contribution to DSN. Furthermore, we summarize current research on the therapeutic potential of targeting Na V channels in pain pathologies, including DSN.

An overview of painful diabetic peripheral neuropathy: Diagnosis and treatment advancements

Diabetes mellitus remains a public health problem, affecting 422 million people worldwide. Currently, there is no consensus around treating painful diabetic peripheral neuropathy in a step-wise manner. Among the non-pharmacological interventions, neuromodulation has become a promising alternative. Over the past decade, significant clinical trials have paved the way for prompt inclusion of high-frequency spinal cord stimulation within the painful diabetic peripheral neuropathy treatment algorithm. This article aims to provide an updated evidence-based approach for the management of painful diabetic peripheral neuropathy.

T-type Ca2+ channels play a dual role in modulating the excitability of dorsal root ganglia neurons

A subgroup of low-threshold dorsal root ganglia (DRG) neurons discharge action potentials (APs) with an afterdepolarizing potential (ADP). The ADP is formed by T-type Ca²⁺ currents. It is known that T-type Ca²⁺ currents contribute to neuropathic pain. However, the change in ADP-firing of injured DRG neurons has not been widely studied yet. Here we applied patch clamp to record ADP-firing and T-type Ca²⁺ currents in intact and chronically compressed DRG (CCD) neurons and examined T-type Ca²⁺ channel proteins expression with western blotting. After CCD injury, the incidences of both ADP firing and non-ADP burst firing increased, and T-type Ca²⁺ channels contributed to both of these firing patterns. The neurons discharging large-amplitude-ADP firing were TTX-insensitive, implying that high-density T-type Ca²⁺ channels might cooperate with TTX-insensitive Na⁺ channels to reduce the AP threshold. By contrast, the neurons displaying non-ADP burst firing were TTX-sensitive, implying that low density T-type Ca²⁺ channels may cooperate with TTX-sensitive Na⁺ channels to increase AP number. In DRG neurons, T-type Ca²⁺ currents density varied widely, ranging between 100 pA/pF and 5 pA/pF. After injury, the proportion of DRG neurons with large T-type Ca²⁺ currents increased in parallel with the increase in the incidence of large-amplitude-ADP firing. And in addition to Cav3.2, Cav3.3 channels are also likely to contribute to low-threshold firing. The data revealed that T-type Ca²⁺ channels may play a dual role in modulating the injured neurons' high excitability through a cooperative process with Na⁺ channels, thereby contributing to neuropathic pain.

Novel Nanotechnological Approaches for Targeting Dorsal Root Ganglion (DRG) in Mitigating Diabetic Neuropathic Pain (DNP)

Diabetic neuropathy is the most entrenched complication of diabetes. Usually, it affects the distal foot and toes, which then gradually approaches the lower part of the legs. Diabetic foot ulcer (DFU) could be one of the worst complications of diabetes mellitus. Long-term diabetes leads to hyperglycemia, which is the utmost contributor to neuropathic pain. Hyperglycemia causing an upregulation of voltage-gated sodium channels in the dorsal root ganglion (DRG) was often observed in models of neuropathic pain. DRG opening frequency increases intracellular sodium ion levels, which further causes increased calcium channel opening and stimulates other pathways leading to diabetic peripheral neuropathy (DPN). Currently, pain due to diabetic neuropathy is managed via antidepressants, opioids, gamma-aminobutyric acid (GABA) analogs, and topical agents such as capsaicin. Despite the availability of various treatment strategies, the percentage of patients achieving adequate pain relief remains low. Many factors contribute to this condition, such as lack of specificity and adverse effects such as light-headedness, languidness, and multiple daily doses. Therefore, nanotechnology outperforms in every aspect, providing several benefits compared to traditional therapy such as site-specific and targeted drug delivery. Nanotechnology is the branch of science that deals with the development of nanoscale materials and products, even smaller than 100 nm. Carriers can improve their efficacy with reduced side effects by incorporating drugs into the novel delivery systems. Thus, the utilization of nanotechnological approaches such as nanoparticles, polymeric nanoparticles, inorganic nanoparticles, lipid nanoparticles, gene therapy (siRNA and miRNA), and extracellular vesicles can extensively contribute to relieving neuropathic pain.

Bupivacaine Induces ROS-Dependent Autophagic Damage in DRG Neurons via TUG1/mTOR in a High-Glucose Environment

Bupivacaine (BP) is a commonly clinically used local anesthetic (LA). Current studies suggest that neurological complications are increased in diabetic patients after LA application, but the molecular mechanism is poorly understood. LA-induced autophagy and neuronal injury have been reported. We hypothesized that a high-glucose environment aggravates BP-induced autophagic damage. Mouse dorsal root ganglion (DRG) neurons were treated with BP in a high-glucose environment, and the results showed that reactive oxygen species (ROS) levels increased, autophagy was activated, autophagy flux was blocked, and cell viability decreased. Pretreatment with the ROS scavenger N-acetyl-cysteine (NAC) attenuated ROS-mediated autophagy regulation. Moreover, the expression of the long noncoding RNA (lncRNA) taurine upregulated gene 1 (TUG1) increased, and NAC and TUG1 siRNA inhibited the expression of TUG1/mammalian target of rapamycin (mTOR) in DRGs treated with BP in a high-glucose environment. Intriguingly, contrary to previous reports on a positive effect on neurons, we found that rapamycin, an autophagy activator, and chloroquine, an autophagy and lysosome inhibitor, both exacerbated autophagic damage. These data suggest that a high-glucose environment exacerbated BP induced ROS-dependent autophagic damage in DRG neurons through the TUG1/mTOR signaling pathway, which provides a theoretical basis and target for the clinical prevention and treatment of BP neurotoxicity in diabeties.

Diabetic Peripheral Neuropathy

Diabetic painless and painful peripheral neuropathy remains the most frequent complication of diabetes mellitus, but the pathophysiology remains undescribed, there are no robust clinical endpoints and no efficient treatment exists. This hampers good clinical practice, fruitful clinical research and successful pharmacological trials, necessary for the development of early detection, prevention and treatment. This chapter supplies an update on background and treatment of diabetic peripheral neuropathy. Goals and perspectives for future clinical and scientific approaches are also described.

Voltage-Gated Sodium Channel Modulation by a New Spider Toxin Ssp1a Isolated From an Australian Theraphosid

Given the important role of voltage-gated sodium (NaV) channel-modulating spider toxins in elucidating the function, pharmacology, and mechanism of action of therapeutically relevant NaV channels, we screened the venom from Australian theraphosid species against the human pain target hNaV1.7. Using assay-guided fractionation, we isolated a 33-residue inhibitor cystine knot (ICK) peptide (Ssp1a) belonging to the NaSpTx1 family. Recombinant Ssp1a (rSsp1a) inhibited neuronal hNaV subtypes with a rank order of potency hNaV1.7 > 1.6 > 1.2 > 1.3 > 1.1. rSsp1a inhibited hNaV1.7, hNaV1.2 and hNaV1.3 without significantly altering the voltage-dependence of activation, inactivation, or delay in recovery from inactivation. However, rSsp1a demonstrated voltage-dependent inhibition at hNaV1.7 and rSsp1a-bound hNaV1.7 opened at extreme depolarizations, suggesting rSsp1a likely interacted with voltage-sensing domain II (VSD II) of hNaV1.7 to trap the channel in its resting state. Nuclear magnetic resonance spectroscopy revealed key structural features of Ssp1a, including an amphipathic surface with hydrophobic and charged patches shown by docking studies to comprise the interacting surface. This study provides the basis for future structure-function studies to guide the development of subtype selective inhibitors.

Effects of High-Voltage Pulsed Radiofrequency on the Ultrastructure and Nav1.7 Level of the Dorsal Root Ganglion in Rats With Spared Nerve Injury

OBJECTIVES

To investigate the analgesic effect of high-voltage pulsed radiofrequency (HV-PRF) on the dorsal root ganglion (DRG) for neuropathic pain induced by spared nerve injury (SNI) in rats, especially the influence of this treatment on the DRG ultrastructure and voltage-gated sodium channel 1.7 (Nav1.7) level in the DRG.

MATERIALS AND METHODS

One hundred fifty adult male Sprague-Dawley rats were randomly divided into five groups: Sham, SNI, Free-PRF, standard-voltage PRF (SV-PRF), and HV-PRF. The 45V-PRF and 85V-PRF procedures applied to the left L5 DRG were performed in SV-PRF group and the HV-PRF group respectively on day 7 after SNI, whereas no PRF was concurrently delivered in Free-PRF group. The paw mechanical withdrawal threshold (PMWT) was detected before SNI (baseline) and on days 1, 3, 7, 8, 10, 14, and 21. The changes of left L5 DRG ultrastructure were analyzed with transmission electron microscopy on days 14 and 21. The expression levels of Nav1.7 in left L5 DRG were detected by immunofluorescence and Western blot.

RESULTS

Compared with the Free-PRF group, PMWT in the SV-PRF group and HV-PRF group were both significantly increased after PRF (all p < 0.05). Meanwhile, the PMWT was significantly higher in the HV-PRF group than that in the SV-PRF group on days 14 and 21 all (p < 0.05). There were statistically significant differences between the SV-PRF and Free-PRF groups (p < 0.05). Similarly, statistically significant difference was found between the HV-PRF and Free-PRF groups (p < 0.05). Especially, comparison of the SV-PRF group and the HV-PRF group revealed statistically significant difference (p < 0.05). The Nav1.7 levels were significantly down-regulated in the SV-PRF group and HV-PRF groups compared to that in the Free-PRF group (all p < 0.01). A significantly lower Nav1.7 level was also found in the HV-PRF group compared to that in the SV-PRF group (p < 0.05).

CONCLUSIONS

The HV-PRF produces a better analgesic effect than SV-PRF applied to the DRG in SNI rats. The underlying mechanisms may be associated with improving the histopathological prognosis and the downregulation of Nav1.7 levels in the DRG.

Tetrodotoxin, a Potential Drug for Neuropathic and Cancer Pain Relief?

Tetrodotoxin (TTX) is a potent neurotoxin found mainly in puffer fish and other marine and terrestrial animals. TTX blocks voltage-gated sodium channels (VGSCs) which are typically classified as TTX-sensitive or TTX-resistant channels. VGSCs play a key role in pain signaling and some TTX-sensitive VGSCs are highly expressed by adult primary sensory neurons. During pathological pain conditions, such as neuropathic pain, upregulation of some TTX-sensitive VGSCs, including the massive re-expression of the embryonic VGSC subtype Na_V1.3 in adult primary sensory neurons, contribute to painful hypersensitization. In addition, people with loss-of-function mutations in the VGSC subtype Na_V1.7 present congenital insensitive to pain. TTX displays a prominent analgesic effect in several models of neuropathic pain in rodents. According to this promising preclinical evidence, TTX is currently under clinical development for chemo-therapy-induced neuropathic pain and cancer-related pain. This review focuses primarily on the preclinical and clinical evidence that support a potential analgesic role for TTX in these pain states. In addition, we also analyze the main toxic effects that this neurotoxin produces when it is administered at therapeutic doses, and the therapeutic potential to alleviate neuropathic pain of other natural toxins that selectively block TTX-sensitive VGSCs.

Studying human nociceptors: from fundamentals to clinic

Chronic pain affects one in five of the general population and is the third most important cause of disability-adjusted life-years globally. Unfortunately, treatment remains inadequate due to poor efficacy and tolerability. There has been a failure in translating promising preclinical drug targets into clinic use. This reflects challenges across the whole drug development pathway, from preclinical models to trial design. Nociceptors remain an attractive therapeutic target: their sensitization makes an important contribution to many chronic pain states, they are located outside the blood-brain barrier, and they are relatively specific. The past decade has seen significant advances in the techniques available to study human nociceptors, including: the use of corneal confocal microscopy and biopsy samples to observe nociceptor morphology, the culture of human nociceptors (either from surgical or post-mortem tissue or using human induced pluripotent stem cell derived nociceptors), the application of high throughput technologies such as transcriptomics, the in vitro and in vivo electrophysiological characterization through microneurography, and the correlation with pain percepts provided by quantitative sensory testing. Genome editing in human induced pluripotent stem cell-derived nociceptors enables the interrogation of the causal role of genes in the regulation of nociceptor function. Both human and rodent nociceptors are more heterogeneous at a molecular level than previously appreciated, and while we find that there are broad similarities between human and rodent nociceptors there are also important differences involving ion channel function, expression, and cellular excitability. These technological advances have emphasized the maladaptive plastic changes occurring in human nociceptors following injury that contribute to chronic pain. Studying human nociceptors has revealed new therapeutic targets for the suppression of chronic pain and enhanced repair. Cellular models of human nociceptors have enabled the screening of small molecule and gene therapy approaches on nociceptor function, and in some cases have enabled correlation with clinical outcomes. Undoubtedly, challenges remain. Many of these techniques are difficult to implement at scale, current induced pluripotent stem cell differentiation protocols do not generate the full diversity of nociceptor populations, and we still have a relatively poor understanding of inter-individual variation in nociceptors due to factors such as age, sex, or ethnicity. We hope our ability to directly investigate human nociceptors will not only aid our understanding of the fundamental neurobiology underlying acute and chronic pain but also help bridge the translational gap.

Essential Oil of Croton zehntneri Prevents Conduction Alterations Produced by Diabetes Mellitus on Vagus Nerve

Autonomic diabetic neuropathy (ADN) is a complication of diabetes mellitus (DM), to which there is no specific treatment. In this study, the efficacy of the essential oil of Croton zehntneri (EOCz) in preventing ADN was evaluated in the rat vagus nerve. For the two fastest conducting myelinated types of axons of the vagus nerve, the conduction velocities and rheobase decreased, whilst the duration of the components of the compound action potential of these fibers increased. EOCz completely prevented these DM-induced alterations of the vagus nerve. Unmyelinated fibers were not affected. In conclusion, this investigation demonstrated that EOCz is a potential therapeutic agent for the treatment of ADN.

Impact of Diabetes Mellitus on Knee Osteoarthritis Pain and Physical and Mental Status: Data From the Osteoarthritis Initiative

OBJECTIVE

Diabetes mellitus (DM) appears to increase osteoarthritic knee pain, which may be related to greater adiposity and more advanced disease status often observed in individuals with osteoarthritis (OA) and DM. We aimed to assess whether OA knee pain and health status are worse in individuals with OA and DM, independent of these potential confounders.

METHODS

We included 202 OA participants with DM and 2,279 without DM from the Osteoarthritis Initiative. Knee pain was evaluated using the Knee Injury and Osteoarthritis Outcome Score (KOOS) and a numeric rating scale (NRS). Physical and mental status were assessed by the Medical Outcomes Study Short Form 12 (SF-12) questionnaire, physical component summary (PCS) score and mental component summary (MCS) score, and by the Center for Epidemiologic Studies Depression Scale (CES-D). Linear regression models assessed the influence of DM, adjusted for age, sex, body mass index (BMI), and radiographic severity.

RESULTS

OA participants with DM reported worse knee pain and greater physical and mental issues compared with participants without DM. Individuals with DM had worse KOOS pain (β = -4.72 [95% confidence interval (95% CI) -7.22, -2.23]) and worse NRS pain (β = 0.42 [95% CI 0.04, 0.80]) independent of BMI, OA severity, age, and sex. The negative influence of DM was also apparent for SF-12 PCS (β = -3.49 [95% CI -4.73, -2.25]), SF-12 MCS (β = -1.42 [95% CI -2.57, -0.26]), and CES-D (β = 1.08 [95% CI 0.08, 2.08]).

CONCLUSION

Individuals with knee OA experience on average higher pain intensity and a worse physical and mental health status if they have DM. Linear regression models show that DM is a risk factor for higher pain, in addition to and independent of greater BMI and radiographic OA severity.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

High-Dose Intravenous Immunoglobulin Is Effective in Painful Diabetic Polyneuropathy Resistant to Conventional Treatments. Results of a Double-Blind, Randomized, Placebo-Controlled, Multicenter Trial

OBJECTIVES

The efficacy and safety of high-dose intravenous immunoglobulin (IVIG) in treatment-resistant diabetic painful polyneuropathy (DPN) were assessed.

DESIGN

This was a randomized, double-blind, placebo-controlled, multicenter trial (EudraCT 2010-023883-42).

SETTING

This trial was conducted at eight sites in Italy with a neurology specialist level of care.

SUBJECTS

Twenty-six diabetic patients with DPN who reported baseline severity of pain >60 units (mm) on a VAS scale at enrollment and were resistant to antidepressants and antiepileptic drugs were enrolled; 23 were randomized (11 in the IVIG arm and 12 in the placebo arm). All patients completed the study and were evaluated. All patients were Caucasian, 15 were male, and 21 had a diagnosis of type II diabetes.

METHODS

IVIG (0.4 g/kg/d) or placebo was given for five consecutive days. Pain intensity (visual analog scale, Neuropathic Pain Symptom Inventory) and quality of life (36-Item Short-Form Health Survey, Clinical/Patient Global Impression of Change questionnaires) assessments were performed at visits: baseline, start of therapy (one week later), end of therapy (five days later), and follow-up (four and eight weeks later).

RESULTS

The study achieved its prespecified primary end point of ≥50% pain reduction at four weeks after IVIG, achieved in seven of 11 patients (63.6%) in the IVIG group vs zero of 12 in the placebo group (P = 0.0013). Only two adverse events were reported during the study: one patient in the treatment arm reported a mild "dermatitis psoriasiform," whereas one patient from the placebo group reported a mild "influenza."

CONCLUSIONS

Treatment with IVIG at the dose given was efficacious and safe for patients with DPN resistant to standard therapies.

Peripheral Neuropathic Pain: From Experimental Models to Potential Therapeutic Targets in Dorsal Root Ganglion Neurons

Neuropathic pain exerts a global burden caused by the lesions in the somatosensory nerve system, including the central and peripheral nervous systems. The mechanisms of nerve injury-induced neuropathic pain involve multiple mechanisms, various signaling pathways, and molecules. Currently, poor efficacy is the major limitation of medications for treating neuropathic pain. Thus, understanding the detailed molecular mechanisms should shed light on the development of new therapeutic strategies for neuropathic pain. Several well-established in vivo pain models were used to investigate the detail mechanisms of peripheral neuropathic pain. Molecular mediators of pain are regulated differentially in various forms of neuropathic pain models; these regulators include purinergic receptors, transient receptor potential receptor channels, and voltage-gated sodium and calcium channels. Meanwhile, post-translational modification and transcriptional regulation are also altered in these pain models and have been reported to mediate several pain related molecules. In this review, we focus on molecular mechanisms and mediators of neuropathic pain with their corresponding transcriptional regulation and post-translational modification underlying peripheral sensitization in the dorsal root ganglia. Taken together, these molecular mediators and their modification and regulations provide excellent targets for neuropathic pain treatment.

Tranilast attenuates neuropathic pain during type-2 diabetes by inhibiting hypoxia-induced pro-inflammatory cytokines in Zucker diabetic fatty rat model

BACKGROUND

The modulatory effect of tranilast on neuropathic pain in type-2 diabetes (T2DM) remains unclear.

METHODS

We monitored interleukin (IL)-1β, nuclear factor-κB (NF-κB) and tumour necrosis factor-α (TNF-α) levels during the progression of T2DM induced neuropathic pain in rats, and assessed the impact of tranilast treatment of increasing concentrations (0, 200 and 400 mg/kg/day via oral gavage in 1% NaHCO₃ delivered as 100 mg/kg twice a day) on the levels of cytokine production, as well as on the thermal hyperalgesia and mechanical allodynia.

RESULTS

The rats developed hyperglycaemia accompanied with elevated levels of NF-κB, IL-1β and TNF-α in the rostral ventromedial medulla at the age of 16 weeks. Tranilast administration dose dependently alleviated thermal hyperalgesia as well as mechanical allodynia, which was associated with its ability in inhibiting hypoxia-induced levels of NF-κB, IL-1β and TNF-α.

CONCLUSION

Tranilast plays crucial roles in modulating T2DM-related neuropathic pain, likely through inhibiting hypoxia.

The effect of henna and linseed herbal ointment blend on wound healing in rats with second-degree burns

The treatment of skin burns is one of the most important challenge in medical science. The aim of this study is evaluation of the efficacy of Artaderm herbal ointment containing the Henna (Lawsonia inermis) extract, Linseed (Linum usitatissimum) oil, and Honey Wax on wound healing in the rat with second-degree burn wounds. The Artaderm ointment had an effective role in controlling burn wound infections due to its antimicrobial and anti-inflammatory properties. In this study, 64 male Wistar rats were randomly divided into 8 groups (n = 8). Four groups received Artaderm, 1% Silver Sulfadiazine (SSD 1%), Cod Liver Oil and Fundermol (Alpha) ointments which used in common practices for burn injuries. Another three groups received Henna, Linseed, and Honey Wax alone and a control group that just underwent a second-degree burn injury without any treatments. A second-degree burn was formed on the back of each rat and dressed daily with one of the agents. Burn wounds were macroscopically and microscopically evaluated on the 7th, 14th, and 21st day after burn induction. Rats treated with the Artaderm ointment had significantly faster wound contraction as well as shorter healing time than the rest groups. No scar was observed in rats treated with the Artaderm ointment on the 21st day, while this level of improvement was not observed in other groups at the same time. More than 90% of wounds were healed after on the 14th day in rats treated with Artaderm (94.10 ± 0.18) and Alpha (92.05 ± 0.23) ointments. According to these findings, it can be concluded that Artaderm herbal ointment can be used as a proper alternative for healing of wounds in second-degree burns.

Fecal transplantation and butyrate improve neuropathic pain, modify immune cell profile, and gene expression in the PNS of obese mice

Obesity affects over 2 billion people worldwide and is accompanied by peripheral neuropathy (PN) and an associated poorer quality of life. Despite high prevalence, the molecular mechanisms underlying the painful manifestations of PN are poorly understood, and therapies are restricted to use of painkillers or other drugs that do not address the underlying disease. Studies have demonstrated that the gut microbiome is linked to metabolic health and its alteration is associated with many diseases, including obesity. Pathologic changes to the gut microbiome have recently been linked to somatosensory pain, but any relationships between gut microbiome and PN in obesity have yet to be explored. Our data show that mice fed a Western diet developed indices of PN that were attenuated by concurrent fecal microbiome transplantation (FMT). In addition, we observed changes in expression of genes involved in lipid metabolism and calcium handling in cells of the peripheral nerve system (PNS). FMT also induced changes in the immune cell populations of the PNS. There was a correlation between an increase in the circulating short-chain fatty acid butyrate and pain improvement following FMT. Additionally, butyrate modulated gene expression and immune cells in the PNS. Circulating butyrate was also negatively correlated with distal pain in 29 participants with varied body mass index. Our data suggest that the metabolite butyrate, secreted by the gut microbiome, underlies some of the effects of FMT. Targeting the gut microbiome, butyrate, and its consequences may represent novel viable approaches to prevent or relieve obesity-associated neuropathies.

Neoline, an active ingredient of the processed aconite root in Goshajinkigan formulation, targets Nav1.7 to ameliorate mechanical hyperalgesia in diabetic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Goshajinkigan (GJG), a traditional Japanese Kampo formula, has been shown to exhibit several pharmacological actions, including antinociceptive effects. Processed aconite root (PA), which is considered to be an active ingredient of GJG, has also been demonstrated to have an ameliorative effect on pain, such as diabetic peripheral neuropathic pain. We recently identified neoline as the active ingredient of both GJG and PA that is responsible for its effects against oxaliplatin-induced neuropathic pain in mice.

AIM OF THE STUDY

In the present study, we investigated whether GJG, PA, and neoline could inhibit Nav1.7 voltage-gated sodium channel (VGSC) current and whether neoline could ameliorate mechanical hyperalgesia in diabetic mice.

MATERIALS AND METHODS

To assess the electrophysiological properties of GJG extract formulation, powdered PA, and neoline on Nav1.7 VGSCs, whole-cell patch clamp recording was performed using human HEK293 cells expressing Nav1.7 VGSCs. In addition, the ameliorative effects of neoline on diabetic peripheral neuropathic pain were evaluated using the von Frey test in streptozotocin (STZ)-induced diabetic model mice.

RESULTS

GJG extract formulation significantly inhibited Nav1.7 VGSC peak current. Powdered PA also inhibited Nav1.7 VGSC peak current. Like GJG and PA, neoline could inhibit Nav1.7 VGSC current. When diabetic mice were treated with neoline by intraperitoneal acute administration, the mechanical threshold was increased in diabetic mice, but not in non-diabetic mice, in a behavioral study.

CONCLUSION

These results suggest that neoline might be a novel active ingredient of GJG and PA that is one of responsible ingredients for ameliorating mechanical hyperalgesia in diabetes via the inhibition of Nav1.7 VGSC current at least.

Emerging importance of satellite glia in nervous system function and dysfunction

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC-SGC and neuron-SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

Putative roles of SLC7A5 (LAT1) transporter in pain

Large amino acid transporter 1 (LAT1), also known as SLC7A5, is an essential amino acid transporter that forms a heterodimeric complex with the glycoprotein cell-surface antigen heavy chain (4F2hc (CD98, SLC3A2)). Within nociceptive pathways, LAT1 is expressed in the dorsal root ganglia and spinal cord. Although LAT1 expression is upregulated following spinal cord injury, little is known about LAT1 in neuropathic pain. To date, only circumstantial evidence supports LAT1/4F2hc's role in pain. Notably, LAT1's expression and regulation link it to key cell types and pathways implicated in pain. Transcriptional regulation of LAT1 expression occurs via the Wnt/frizzled/β-catenin signal transduction pathway, which has been shown to be involved in chronic pain. The LAT1/4F2hc complex may also be involved in pain pathways related to T- and B-cells. LAT1's expression induces activation of the mammalian target of rapamycin (mTOR) signaling axis, which is involved in inflammation and neuropathic pain. Similarly, hypoxia and cancer induce activation of hypoxia-inducible factor 2 alpha, promoting not only LAT1's expression but also mTORC1's activation. Perhaps the strongest evidence linking LAT1 to pain is its interactions with key voltage-gated ion channels connected to nociception, namely the voltage-gated potassium channels Kv1.1 and Kv1.2 and the voltage-gated sodium channel Nav1.7. Through functional regulation of these channels, LAT1 may play a role in governing the excitatory to inhibitory ratio which is altered in chronic neuropathic pain states. Remarkably, the most direct role for LAT1 in pain is to mediate the influx of gabapentin and pregabalin, two first-line neuropathic pain drugs, that indirectly inhibit high voltage-activated calcium channel auxiliary subunit α2δ-1. In this review, we discuss the expression, regulation, relevant signaling pathways, and protein interactions of LAT1 that may link it to the development and/or maintenance of pain. We hypothesize that LAT1 expressed in nociceptive pathways may be a viable new target in pain.

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.

Multi-targeting sodium and calcium channels using venom peptides for the treatment of complex ion channels-related diseases

Venom peptides are amongst the most exquisite group of bioactive molecules able to alter the normal physiology of organisms. These bioactive peptides penetrate tissues and blood vessels to encounter a number of receptors and ion channels to which they bind with high affinity and execute modulatory activities. Arachnid is the most diverse class of venomous animals often rich in peptides modulating voltage-gated sodium (Na_V), calcium (Ca_V), and potassium (K_V) channels. Spider venoms, in particular, contain potent and selective peptides targeting these channels, with a few displaying interesting multi-target properties for Na_V and Ca_V channels underlying disease mechanisms such as in neuropathic pain, motor neuron disease and cancer. The elucidation of the pharmacology and structure-function properties of these venom peptides are invaluable for the development of effective drugs targeting Na_V and Ca_V channels. This perspective discusses spider venom peptides displaying multi-target properties to modulate Na_V and Ca_V channels in regard to their pharmacological features, structure-function relationships and potential to become the next generation of effective drugs to treat neurological disorders and other multi-ion channels related diseases.

Cutaneous Aβ-Non-nociceptive, but Not C-Nociceptive, Dorsal Root Ganglion Neurons Exhibit Spontaneous Activity in the Streptozotocin Rat Model of Painful Diabetic Neuropathy in vivo

Diabetic peripheral neuropathic pain (DPNP) is the most devastating complication of diabetes mellitus. Unfortunately, successful therapy for DPNP remains a challenge because its pathogenesis is still elusive. However, DPNP is believed to be due partly to abnormal hyperexcitability of dorsal root ganglion (DRG) neurons, but the relative contributions of specific functional subtypes remain largely unknown. Here, using the strepotozotocin (STZ) rat model of DPNP induced by a STZ injection (60 mg/kg, i.p), and intracellular recordings of action potentials (APs) from DRG neurons in anesthetized rats, we examined electrophysiological changes in C-and Aβ-nociceptive and Aβ-low threshold mechanoreceptive (LTM) neurons that may contribute to DPNP. Compared with control, we found in STZ-rats with established pain hypersensitivity (5 weeks post-STZ) several significant changes including: (a) A 23% increase in the incidence of spontaneous activity (SA) in Aβ-LTMs (but not C-mechanosensitive nociceptors) that may cause dysesthesias/paresthesia suffered by DPNP patients, (b) membrane hyperpolarization and a ∼85% reduction in SA rate in Aβ-LTMs by Kv7 channel activation with retigabine (6 mg/kg, i.v.) suggesting that Kv7/M channels may be involved in mechanisms of SA generation in Aβ-LTMs, (c) decreases in AP duration and in duration and amplitude of afterhyperpolarization (AHP) in C-and/or Aβ-nociceptors. These faster AP and AHP kinetics may lead to repetitive firing and an increase in afferent input to the CNS and thereby contribute to DPNP development, and (d) a decrease in the electrical thresholds of Aβ-nociceptors that may contribute to their sensitization, and thus to the resulting hypersensitivity associated with DPNP.

Diabetes mellitus alters electrophysiological properties in neurons of superior cervical ganglion of rats

Diabetic neuropathy is the most prevalent complication associated with diabetes mellitus (DM). The superior cervical ganglion (SCG) is an important sympathetic component of the autonomic nervous system. We investigated the changes in cellular electrophysiological properties and on Na⁺K⁺-ATPase activity of SCG neurons of rats with DM induced by streptozotocin (STZ). Three types of action potentials (AP) firing pattern were observed in response to a long (1 s) depolarizing pulse. Whilst some neurons fired a single AP (single firing phasic, SFP), others fired few APs (multiple firing phasic, MFP). A third type fired APs during more than 80% of the stimulus duration (tonic-like, TL). The occurrence of SFP, MFP and TL was 84.5, 13.8, and 1.7%, respectively. SFP and MFP differed significantly in their membrane input resistance (R_in). At the end of the 4th week of its time course, DM differently affected most types of neurons: DM induced depolarization of resting membrane potential (RMP), decreased AP amplitude in SFP, and decreased R_in in MFP. DM decreased spike after-hyperpolarization amplitude in MFP and the duration in SFP. Based on the RMP depolarization, we investigated the Na⁺K⁺-ATPase action and observed that DM caused a significant decrease in Na⁺K⁺-ATPase activity of SCG. In conclusion, we have demonstrated that DM affects several parameters of SCG physiology in a manner likely to have pathophysiological relevance.

TNF-α mediated upregulation of NaV1.7 currents in rat dorsal root ganglion neurons is independent of CRMP2 SUMOylation

Clinical and preclinical studies have shown that patients with Diabetic Neuropathy Pain (DNP) present with increased tumor necrosis factor alpha (TNF-α) serum concentration, whereas studies with diabetic animals have shown that TNF-α induces an increase in Na_V1.7 sodium channel expression. This is expected to result in sensitization of nociceptor neuron terminals, and therefore the development of DNP. For further study of this mechanism, dissociated dorsal root ganglion (DRG) neurons were exposed to TNF-α for 6 h, at a concentration equivalent to that measured in STZ-induced diabetic rats that developed hyperalgesia. Tetrodotoxin sensitive (TTXs), resistant (TTXr) and total sodium current was studied in these DRG neurons. Total sodium current was also studied in DRG neurons expressing the collapsin response mediator protein 2 (CRMP2) SUMO-incompetent mutant protein (CRMP2-K374A), which causes a significant reduction in Na_V1.7 membrane cell expression levels. Our results show that TNF-α exposure increased the density of the total, TTXs and TTXr sodium current in DRG neurons. Furthermore, TNF-α shifted the steady state activation and inactivation curves of the total and TTXs sodium current. DRG neurons expressing the CRMP2-K374A mutant also exhibited total sodium current increases after exposure to TNF-α, indicating that these effects were independent of SUMOylation of CRMP2. In conclusion, TNF-α sensitizes DRG neurons via augmentation of whole cell sodium current. This may underlie the pronociceptive effects of TNF-α and suggests a molecular mechanism responsible for pain hypersensitivity in diabetic neuropathy patients.

Spider Knottin Pharmacology at Voltage-Gated Sodium Channels and Their Potential to Modulate Pain Pathways

Voltage-gated sodium channels (Na_Vs) are a key determinant of neuronal signalling. Neurotoxins from diverse taxa that selectively activate or inhibit Na_V channels have helped unravel the role of Na_V channels in diseases, including chronic pain. Spider venoms contain the most diverse array of inhibitor cystine knot (ICK) toxins (knottins). This review provides an overview on how spider knottins modulate Na_V channels and describes the structural features and molecular determinants that influence their affinity and subtype selectivity. Genetic and functional evidence support a major involvement of Na_V subtypes in various chronic pain conditions. The exquisite inhibitory properties of spider knottins over key Na_V subtypes make them the best lead molecules for the development of novel analgesics to treat chronic pain.

Functional Reorganization of Local Circuit Connectivity in Superficial Spinal Dorsal Horn with Neuropathic Pain States

The spinal dorsal horn is the first relay structure coding for pain transmission and modulation. Previous anatomical and electrophysiological studies have examined spinal dorsal horn circuit connections and network activity. Further work is required to understand spinal cord sensory information processing that underlies pathological neuropathic pain states. Our previous studies suggest that peripheral nerve injury enhances presynaptic excitatory input onto spinal superficial dorsal horn neurons, which in turn contributes to pathologic nociception. The potential changes in local postsynaptic circuits in the dorsal horn that lead to pathologically heightened behavioral responses to pain remain largely unexplored. We combined whole-cell electrophysiological recordings with laser-scanning photostimulation to test whether peripheral nerve injury in the spinal nerve ligation (SNL) mouse model of neuropathic pain leads to alterations in the functional connectivity of spinal cord circuits including lamina II excitatory interneurons. Here we show that SNL enhances excitation and decreases inhibition to lamina II excitatory interneurons along with their increased glutamate-evoked excitability. The enhanced excitatory postsynaptic input and connectivity evoked by SNL eventually return to normal levels concurrently with the resolution of the neuropathic pain states. The physiological pattern highly correlates with mouse pain behaviors following SNL, supporting a neurophysiological mechanism of central sensitization and neuropathic pain that is functionally localized to the spinal dorsal horn. Together, these data support that SNL induces functional changes in synaptic input and connectivity to lamina II excitatory interneurons that code for pain perception, and thus provide new insights into the mechanism and locus of pain hypersensitivity.

Capsaicin: Physicochemical properties, cutaneous reactions and potential applications in painful and inflammatory conditions

Capsaicin is a natural protoalkaloid recognized as the main pungent component in hot peppers (Capsicum annuum L.). The capsaicin receptor is highly expressed in the unmyelinated type C nerve fibers originating from small diameter sensory neurons in dorsal root ganglia and cranial nerve ganglia correspondents. Capsaicin and related vanilloids have a variety of effects on primary sensory neurons function, from sensory neuron excitation characterized by local burning sensation and neurogenic inflammation, followed by conduction blockage accompanied by reversible ultrastructural changes of peripheral nociceptive endings (desensitization), going as far as irreversible degenerative changes (neurotoxicity). The main role in capsaicin-induced neurogenic inflammation relies on the capsaicin sensitive, small diameter primary sensory neurons, therefore its evaluation could be used as a diagnostic instrument in functional alterations of cutaneous sensory nerve fibers. Moreover, capsaicin-induced desensitization and neurotoxicity explain the analgesic/anti-nociceptive and anti-inflammatory effects of topical capsaicin and its potential use in the management of painful and inflammatory conditions. In this study, we describe the effects of capsaicin on neurogenic inflammation and nociception, as well as its potential diagnostic value and therapeutic impact in various conditions involving impairment of sensory nerve fibers.

Disruption of peripheral nerve development in a zebrafish model of hyperglycemia

Diabetes mellitus-induced hyperglycemia is associated with a number of pathologies such as retinopathy, nephropathy, delayed wound healing, and diabetic peripheral neuropathy (DPN). Approximately 50% of patients with diabetes mellitus will develop DPN, which is characterized by disrupted sensory and/or motor functioning, with treatment limited to pain management. Zebrafish ( Danio rerio) are an emerging animal model used to study a number of metabolic disorders, including diabetes. Diabetic retinopathy, nephropathy, and delayed wound healing have all been demonstrated in zebrafish. Recently, our laboratory has demonstrated that following the ablation of the insulin-producing β-cells of the pancreas (and subsequent hyperglycemia), the peripheral nerves begin to show signs of dysregulation. In this study, we take a different approach, taking advantage of the transdermal absorption abilities of zebrafish larvae to extend the period of hyperglycemia. Following 5 days of 60 mM d-glucose treatment, we observed motor axon defasciculation, disturbances in perineurial glia sheath formation, decreased myelination of motor axons, and sensory neuron mislocalization. This study extends our understanding of the structural changes of the peripheral nerve following induction of hyperglycemia and does so in an animal model capable of potential DPN drug discovery in the future.

NEW & NOTEWORTHY Zebrafish are emerging as a robust model system for the study of diabetic complications such as retinopathy, nephropathy, and impaired wound healing. We present a novel model of diabetic peripheral neuropathy in zebrafish in which the integrity of the peripheral nerve is dysregulated following the induction of hyperglycemia. By using this model, future studies can focus on elucidating the underlying molecular mechanisms currently unknown.

Acute effects of hyperglycemia on the peripheral nervous system in zebrafish (Danio rerio) following nitroreductase-mediated β-cell ablation

Diabetic peripheral neuropathy (DPN) is estimated to affect 50% of diabetic patients. Although DPN is highly prevalent, molecular mechanisms remain unknown and treatment is limited to pain relief and glycemic control. We provide a novel model of acute DPN in zebrafish ( Danio rerio) larvae. Beginning 5 days postfertilization (dpf), zebrafish expressing nitroreductase in their pancreatic β-cells were treated with metronidazole (MTZ) for 48 h and checked for β-cell ablation 7 dpf. In experimental design, this was meant to serve as proof of concept that β-cell ablation and hyperglycemia are possible at this time point, but we were surprised to find changes in both sensory and motor nerve components. Compared with controls, neurod+ sensory neurons were often observed outside the dorsal root ganglia in MTZ-treated fish. Fewer motor nerves were properly ensheathed by nkx2.2a+ perineurial cells, and tight junctions were disrupted along the motor nerve in MTZ-treated fish compared with controls. Not surprisingly, the motor axons of the MTZ-treated group were defasciculated compared with the control group, myelination was attenuated, and there was a subtle difference in Schwann cell number between the MTZ-treated and control group. All structural changes occurred in the absence of behavioral changes in the larvae at this time point, suggesting that peripheral nerves are influenced by acute hyperglycemia before becoming symptomatic. Moving forward, this novel animal model of DPN will allow us to access the molecular mechanisms associated with the acute changes in the hyperglycemic peripheral nervous system, which may help direct therapeutic approaches.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

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

Neurons and Microglia; A Sickly-Sweet Duo in Diabetic Pain Neuropathy

Diabetes is a common condition characterized by persistent hyperglycemia. High blood sugar primarily affects cells that have a limited capacity to regulate their glucose intake. These cells include capillary endothelial cells in the retina, mesangial cells in the renal glomerulus, Schwann cells, and neurons of the peripheral and central nervous systems. As a result, hyperglycemia leads to largely intractable complications such as retinopathy, nephropathy, hypertension, and neuropathy. Diabetic pain neuropathy is a complex and multifactorial disease that has been associated with poor glycemic control, longer diabetes duration, hypertension, advanced age, smoking status, hypoinsulinemia, and dyslipidemia. While many of the driving factors involved in diabetic pain are still being investigated, they can be broadly classified as either neuron -intrinsic or -extrinsic. In neurons, hyperglycemia impairs the polyol pathway, leading to an overproduction of reactive oxygen species and reactive nitrogen species, an enhanced formation of advanced glycation end products, and a disruption in Na⁺/K⁺ ATPase pump function. In terms of the extrinsic pathway, hyperglycemia leads to the generation of both overactive microglia and microangiopathy. The former incites a feed-forward inflammatory loop that hypersensitizes nociceptor neurons, as observed at the onset of diabetic pain neuropathy. The latter reduces neurons' access to oxygen, glucose and nutrients, prompting reductions in nociceptor terminal expression and losses in sensation, as observed in the later stages of diabetic pain neuropathy. Overall, microglia can be seen as potent and long-lasting amplifiers of nociceptor neuron activity, and may therefore constitute a potential therapeutic target in the treatment of diabetic pain neuropathy.

Dexmedetomidine enhances ropivacaine-induced sciatic nerve injury in diabetic rats

BACKGROUND

Previous studies suggest that dexmedetomidine has a protective effect against local anaesthetic-induced nerve injury in regional nerve blocks. Whether this potentially protective effect exists in the context of diabetes mellitus is unknown.

METHODS

A diabetic state was established in adult male Sprague-Dawley rats with intraperitoneal injection of streptozotocin. Injections of ropivacaine 0.5%, dexmedetomidine 20 μg kg^-1 (alone and in combination), or normal saline (all in 0.2 ml) were made around the sciatic nerve in control and diabetic rats (n=8 per group). The duration of sensory and motor nerve block and the motor nerve conduction velocity (MNCV) were determined. Sciatic nerves were harvested at post-injection day 7 and assessed with light and electron microscopy or used for pro-inflammatory cytokine measurements.

RESULTS

Ropivacaine and dexmedetomidine alone or in combination did not produce nerve fibre damage in control non-diabetic rats. In diabetic rats, ropivacaine induced significant nerve fibre damage, which was enhanced by dexmedetomidine. This manifested with slowed MNCV, decreased axon density, and decreased ratio of inner to outer diameter of the myelin sheath (G ratio). Demyelination, axon disappearance, and empty vacuoles were also found using electron microscopy. An associated increase in nerve interleukin-1β and tumour necrosis factor-α was also seen.

CONCLUSIONS

Ropivacaine 0.5% causes significant sciatic nerve injury in diabetic rats that is greatly potentiated by high-dose dexmedetomidine. Although the dose of dexmedetomidine used in this study is considerably higher than that used in clinical practice, our data suggest that further studies to assess ropivacaine (alone and in combination with dexmedetomidine) use for peripheral nerve blockade in diabetic patients are warranted.

Long noncoding RNA (lncRNA): a target in neuropathic pain

INTRODUCTION

Current treatments for neuropathic pain are limited in part due to the incomplete understanding of its underlying mechanisms. Recent evidence reveals the dysregulated expression of long non-coding RNAs (lncRNAs) in the damaged nerve, dorsal root ganglion (DRG), and spinal cord dorsal horn following peripheral nerve injury. However, the role of the majority of lncRNAs in neuropathic pain genesis is still elusive. Unveiling the mechanisms of how lncRNAs participate in neuropathic pain may develop new strategies to prevent and/or treat this disorder. Areas covered: This review focuses on the dysregulation of lncRNAs in the DRG, dorsal horn, and the injured nerves from preclinical models of neuropathic pain. We provide evidence of how peripheral nerve injury causes the dysregulation of lncRNAs in these pain-related regions. The potential mechanisms of how dysregulated lncRNAs contribute to the pathogenesis of neuropathic pain are discussed. Expert opinion: The investigation on the role of the dysregulated lncRNAs in neuropathic pain might open up a novel avenue for therapeutic treatment of this disorder. However, current investigation is at the infancy stage, which challenges the translation of preclinical findings. More intensive studies on lncRNAs are required before the preclinical findings are translated into therapeutic management for neuropathic pain.