Systemic lidocaine for neuropathic pain relief

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Pharmacological characterisation of the rat brachial plexus avulsion model of neuropathic pain

Recently, our laboratory has proposed the avulsion of rat brachial plexus as a new and reliable model for the study of neuropathic pain. In this model, the neuropathy can be detected even at distant sites from the injury, both in ipsilateral and contralateral hindpaws. The purpose of this study was to pharmacologically characterise this behavioural model of persistent peripheral neuropathic pain by assessing the effects of several analgesic drugs currently used in clinical practice. For this purpose, the effects of these drugs on the mechanical and cold allodynia were analysed 20-40 days after rat brachial plexus avulsion. Injection of saline, administered by the same route as the other drugs, did not significantly affect the nociceptive threshold either in sham-operated or in neuropathic rats. However, administration of the opioid analgesic morphine (5 mg/kg, s.c.), the alpha2 adrenoceptor agonist clonidine (300 microg/kg, i.p.), the NMDA receptor antagonist ketamine (25 mg/kg, i.p.) or the anticonvulsant drug gabapentin (70 mg/kg, p.o.) consistently reduced both mechanical and cold allodynia following avulsion of rat brachial plexus. The administration of the selective COX-2 inhibitor celecoxib (10 mg/kg, p.o.) blocked mechanical allodynia, but not cold allodynia, whereas the sodium channel blocker lidocaine (40 mg/kg, i.p.) attenuated only cold allodynia. The non-steroidal anti-inflammatory drug diclofenac (100 mg/kg, i.p.), the steroidal anti-inflammatory dexamethasone (1.5 mg/kg, i.p.) and the antidepressant imipramine (10 mg/kg, i.p.) all failed to significantly attenuate both mechanical and cold allodynia in the rats following avulsion of brachial plexus. These findings suggest that avulsion-associated mechanical and cold allodynia, two classic signs of persistent neuropathic pain, were consistently prevented by several analgesics currently available in clinical practice, namely morphine, clonidine, ketamine and gabapentin, and to a lesser extent by celecoxib and lidocaine. Therefore, this new proposed model of persistent nociception seems to be suitable for the study of the underlying mechanisms involved in neuropathic pain and for the identification of potential clinically relevant drugs to treat this aspect of peripheral neuropathy.

The Effects of Class Ic Antiarrhythmics on Tetrodotoxin-Resistant Na+ Currents in Rat Sensory Neurons

IV or oral administration of antiarrhythmics has been reported to be effective for relieving neuropathic pain. Recent reports have indicated that tetrodotoxin-resistant (TTX-R) Na(+) channels play important roles in the nerve conduction of nociceptive sensation. In the present study, we investigated the effects of flecainide, pilsicainide (class Ic antiarrhythmics), and lidocaine (a class Ib drug) on TTX-R Na(+) currents in rat dorsal root ganglion neurons using the whole-cell patch-clamp method. Flecainide, pilsicainide, and lidocaine reversibly blocked the peak amplitude of TTX-R Na(+) currents in a concentration-dependent manner with half-maximum inhibitory concentration values of 8.5 +/- 6.6 microM (n = 7), 78 +/- 6.9 microM (n = 7), and 73 +/- 6.8 microM (n = 7), respectively. Each drug shifted the inactivation curve for the TTX-R Na(+) currents in the hyperpolarizing direction and caused a use-dependent block. We also studied an interaction between these antiarrhythmics on TTX-R Na(+) channels. Additional application of flecainide or pilsicainide to lidocaine resulted in an additive increase of tonic and use-dependent block. These results suggest that the inhibition of TTX-R Na(+) currents of dorsal root ganglion neurons by such antiarrhythmics is attributable, at least partly, to their antinociceptive effects.

Differential blockade of nerve injury–induced thermal and tactile hypersensitivity by systemically administered brain-penetrating and peripherally restricted local anesthetics

Systemic administration of local anesthetics has been shown to transiently reverse thermal and tactile hypersensitivity induced by peripheral nerve injury, effects that have been taken as suggesting direct actions on the peripheral nerves. The present study sought to determine whether a central site of action could contribute to, or account for, the effects of lidocaine on nerve injury-induced thermal and tactile hypersensitivity. Systemic lidocaine and its peripherally restricted analogues, QX-314 or QX-222, effectively reversed thermal hypersensitivity after spinal nerve ligation injury. Nerve injury-induced tactile hypersensitivity, however, was reversed by systemic lidocaine but not QX-314 or QX-222. Microinjection of either lidocaine or QX-314 into the rostral ventromedial medulla fully reversed spinal nerve ligation-induced thermal and tactile hypersensitivity. The data strongly suggest that nerve injury-induced thermal and tactile hypersensitivity are mediated through different mechanisms. In addition, the present study supports a prominent contribution of the central nervous system in the activity of systemically given lidocaine against nerve injury-induced tactile and thermal hypersensitivity. Thus, lidocaine might reverse tactile hypersensitivity mainly through its actions within the central nervous system, whereas its reversal of thermal hypersensitivity might occur through either central or peripheral sites.

PERSPECTIVE

Nerve injury-induced neuropathic pain has proved remarkably difficult to treat. Systemic administration of ion channel blockers such as lidocaine has been explored for the management of chronic pain. This work indicates that systemic rather than local administration of lidocaine would be more effective in treating tactile allodynia.

The medical management of chronic pain

Pain is defined by the International Association for Study of Pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage." This article reviews the medical management of chronic pain.

Voltage-gated sodium channels and hyperalgesia

Physiological and pharmacological evidence both have demonstrated a critical role for voltage-gated sodium channels (VGSCs) in many types of chronic pain syndromes because these channels play a fundamental role in the excitability of neurons in the central and peripheral nervous systems. Alterations in function of these channels appear to be intimately linked to hyperexcitability of neurons. Many types of pain appear to reflect neuronal hyperexcitability, and importantly, use-dependent sodium channel blockers are effective in the treatment of many types of chronic pain. This review focuses on the role of VGSCs in the hyperexcitability of sensory primary afferent neurons and their contribution to the inflammatory or neuropathic pain states. The discrete localization of the tetrodotoxin (TTX)-resistant channels, in particular NaV1.8, in the peripheral nerves may provide a novel opportunity for the development of a drug targeted at these channels to achieve efficacious pain relief with an acceptable safety profile.

Upregulation of spinal cannabinoid-1-receptors following nerve injury enhances the effects of Win 55,212-2 on neuropathic pain behaviors in rats

Exogenous cannabinoids are effective in attenuating neuropathic pain behaviors induced by peripheral nerve injury, but the mechanisms of their effectiveness remain unclear. Here we examined the expression of spinal cannabinoid-1-receptors (CB1Rs) following chronic constriction sciatic nerve injury (CCI) and its relation to the effects of a CBR agonist (Win 55,212-2) on neuropathic pain in rats. CCI induced a time-dependent upregulation of spinal CB1Rs primarily within the ipsilateral superficial spinal cord dorsal horn as revealed by both Western blot and immunohistochemistry. This CCI-induced CB1R upregulation was at least in part mediated through tyrosine kinase receptors (Trk), because intrathecal treatment with the Trk inhibitor K252a (1 microg) for postoperative days 1-6 significantly reduced the CB1R upregulation in CCI rats. At the intracellular level, the mitogen-activated protein kinase (ERK-MAPK) inhibitor PD98059 (1 microg) prevented, while the protein kinase C inhibitor chelerythrine (10 microg) partially reduced, the CCI-induced CB1R upregulation when each agent was administered intrathecally for postoperative days 1-6. Importantly, the CCI-induced upregulation of spinal CB1Rs enhanced the effects of Win 55,212-2 on both thermal hyperalgesia and mechanical allodynia, since inhibition of the CB1R upregulation by PD98059 resulted in a significant reduction of the effects of Win 55,212-2 in CCI rats. These results indicate that upregulation of spinal CB1Rs following peripheral nerve injury may contribute to the therapeutic effects of exogenous cannabinoids on neuropathic pain.

Advances in pain therapeutics

Recent work in defining molecular targets for neuropathic pain has been plentiful and varied. Three novel targets have received much attention recently: N-methyl-D-aspartate receptor subtypes such as the glycine and NR2B sites, and the tetrodotoxin-resistant voltage-gated sodium channel (Na(v) 1.8; SNS/PN3). Preclinical data have been encouraging as a number of selective NR2B and glycine site antagonists have shown efficacy in animal models. Selective Na(v) 1.8 channel blockers have yet to emerge; however, strong genetic evidence and data from non-selective Na channel blockers indicate that this target too may hold much promise.

Management strategies for the treatment of neuropathic pain in the elderly

Pain caused by dysfunction or damage to the peripheral or central nervous system is typified by the symptoms described by patients with painful diabetic neuropathy, post-herpetic neuralgia and central poststroke pain. All these conditions are more common in the elderly. Neuropathic pain has long been recognised as one of the more difficult types of pain to treat; however, with the current emphasis on providing a multidisciplinary assessment and approach to management, more patients will be offered relief of their symptoms and an improved quality of life. Despite the use of combination drug therapy, adequate pain relief in the elderly is difficult to achieve without adverse effects. In an attempt to minimise these it is important to include non-drug treatment options in the management plan. Lifestyle changes and environmental modification, together with encouragement to adopt an appropriate exercise programme and an emphasis on maintaining mobility and independence should always be considered. Interventional therapy ranging from simple nerve blocks to intrathecal drug delivery can be of value. Drug treatment remains the mainstay of therapy. Tricyclic antidepressants such as amitriptyline, while having significant adverse effects in the elderly, have a number needed to treat (NNT) of 3.5 for 50% pain relief in diabetic neuropathy and 2.1 for 50% pain relief in postherpetic neuralgia. The newer antiepileptic drugs, such as gabapentin, appear to have a better adverse effect profile and provide similar efficacy with the NNT for treating painful diabetic neuropathy being 3.7 and 3.2 for treating pain in postherpetic neuralgia. As our understanding of the complexities of the pain processes increases, we are becoming more able to appropriately combine treatments to achieve not only improved pain relief but also improved function.

Complex regional pain syndrome: a review of evidence-supported treatment options

Complex regional pain syndrome consists of pain and other symptoms that are unexpectedly severe or protracted after an injury. In type II complex regional pain syndrome, major nerve injury, often with motor involvement, is the cause; in complex regional pain syndrome I, the culprit is a more occult lesion, often a lesser injury that predominantly affects unmyelinated axons. In florid form, disturbances of vasoregulation (eg, edema) and abnormalities of other innervated tissues (skin, muscle, bone) can appear. Because of these various symptoms and the difficulty in identifying causative lesions, complex regional pain syndrome is difficult to treat or cure. Complex regional pain syndrome has not been systematically investigated; there are few controlled treatment trials for established complex regional pain syndrome. This article reviews the existing studies (even if preliminary) to direct clinicians toward the best options. Treatments for other neuropathic pain syndromes that may be efficacious for complex regional pain syndrome also are discussed. Some common treatments (eg, local anesthetic blockade of sympathetic ganglia) are not supported by the aggregate of published studies and should be used less frequently. Other treatments with encouraging published results (eg, neural stimulators) are not used often enough. We hope to encourage clinicians to rely more on evidence-supported treatments for complex regional pain syndrome.

Multiple phases of relief from experimental mechanical allodynia by systemic lidocaine: responses to early and late infusions

Systemic lidocaine can relieve various forms of neuropathic pain that develop after nerve injury. Mechanical allodynia, defined by a significant drop in paw withdrawal threshold force following spinal nerve ligation (L5-L6) in rats, can be reversed by one 30min lidocaine infusion at a constant plasma concentration as low as 1-2 microg/ml, an effect that is still present when the rats are tested days and weeks afterwards. In this study, we resolved the detailed time course of reversal of ipsilateral and contralateral allodynia in rats with spinal nerve ligation by a single systemic infusion of lidocaine, to 4 microg/ml, given either 2 days after ligation (POD2) or 7 days after ligation (POD7). Male Sprague-Dawley rats were examined for 21 days after undergoing sham operation or spinal nerve ligation to produce allodynia, which was quantified by a lower force of von Frey hairs at the plantar hind paw just required to produce paw withdrawal (paw withdrawal threshold, PWT). Six experimental protocols were followed: rats were infused with lidocaine on POD2 (L2) or on POD7 (L7), or with saline on POD2 (S2) or on POD7 (S7), and sham operated rats were infused with lidocaine on POD2 or on POD7. PWTs were measured during the last 5min of a single 30min lidocaine infusion; at 30, 60, 90, 120, 240 and 360min, and 24, 48 and 72h after beginning infusion, and then every 1-3 days up to 21 days. Three distinct sequential phases of ipsilateral relief were apparent in both L2 and L7 groups: (1) an acute elevation of PWT during the infusion, returning to the pre-infusion allodynic level within 30-60min after infusion; (2) a second, transient elevation of PWT within the next 360min; (3) a sustained elevation of PWT developing slowly over 24h after infusion and maintained over the next 21 days. A significant, although weaker contralateral allodynia developed more slowly (>POD8) than the ipsilateral condition, and could be delayed for more than 2 weeks by lidocaine infusion on POD2 but for only 1 week by the same treatment on POD7. None of the sham operated animals had any allodynic signs and no saline infusions elevated PWT in ligated, allodynic rats. These results of separate phases imply that there are mechanistic differences between the acute relief and the sustained relief of allodynia after a single infusion of lidocaine, and may present an experimental paradigm for investigating the advantages of earlier rather than late therapeutic intervention.

Lidocaine block of neonatal Nav1.3 is differentially modulated by co-expression of beta1 and beta3 subunits

The effects of lidocaine on neonatal Na(v)1.3 (Na(v)1.3n) expressed alone and in combination with beta1 and beta3 subunits in Xenopus oocytes were examined. Lidocaine reversibly inhibited the peak Na(v)1.3n current, shifted the steady-state inactivation curve to hyperpolarized potentials and delayed recovery from inactivation. These effects were attenuated by the co-expression of the beta subunits, with greater attenuating effects being observed in oocytes co-expressing beta1 compared to those co-expressing beta3. Use-dependent block by lidocaine was assessed at 1 Hz train frequency for 60 pulses. Lidocaine caused similar use-dependent block of current amplitude at pulse 60 for Na(v)1.3n and Na(v)1.3n+beta3. In oocytes co-expressing beta1, these use-dependent actions were reduced. In conclusion, the effects of lidocaine on Na(v)1.3n are differentially modulated by beta1 and beta3 subunits. Since these subunits exhibit a complementary distribution, this finding may have importance in our understanding of lidocaine action.

A comparison of the antinociceptive effects of voltage-activated Na+ channel blockers in two rat models of neuropathic pain

The pain-relieving effects of various voltage-activated Na(+) channel blockers have been evaluated in two rat models of neuropathic pain; the photochemically induced nerve injury model (Gazelius) and spared nerve injury model. Lidocaine (up to 40 mg/kg, i.p.) and lamotrigine (up to 60 mg/kg, i.p.) had no effect on mechanical or cold allodynia in either model. However, lamotrigine (10, 30 and 60 mg/kg) significantly attenuated mechanical hyperalgesia in the spared nerve injury model, while mexiletine (25 and 37.5 mg/kg, i.p.) attenuated mechanical allodynia in the Gazelius model. Tocainide (50, 75 and 100 mg/kg, i.p.) significantly reduced all types of pain behaviour measured. The present results show that these voltage-activated Na(+) channel blockers have broadly similar antinociceptive effects in these two models of neuropathic pain. They also show that these drugs can have markedly different effects on distinct neuropathic pain-related behaviours within models.

Micromolar lidocaine selectively blocks propagating ectopic impulses at a distance from their site of origin

Abnormal impulses caused by very slowly inactivating Na channels of peripheral nerve have been proposed to play a critical role in neuropathic pain. Low concentrations of local anesthetics, often effective in treating experimental and clinical neuropathic pain, are also known to potently suppress the long after-depolarizations induced by these persistently open Na channels. However, these drug actions on impulses that have propagated away from such sites are undetermined. In the present study, the focal application of anemone toxin II (ATX, 300 nM), which slows Na-channel inactivation, produced prolonged depolarizing after-potentials and, coincidentally, induced spontaneous bursting impulse activity that propagated away from the site of ATX application in the frog sciatic nerve in vitro. The application of low concentrations of lidocaine (1-10 microM), both at the site of ATX exposure and at a distant site, selectively and reversibly inhibited the spontaneous bursting, while having no effect on the electrically stimulated initial spike of the compound action potential. Inhibition occurred as a shortening of burst episodes rather than a reduction in frequency of impulses within a burst or a reduction of intraburst impulse amplitude. Tetrodotoxin also inhibited the induced spontaneous activity, but only at concentrations that also depressed the compound action potential spike. These findings show that low concentrations of lidocaine can restore normal firing patterns in nerve where hyperexcitability has been caused by delayed Na-channel inactivation, without acting directly at the site where ectopic impulses are generated. Thus, it appears that the pattern of abnormal activity rather than an abnormally gating Na channel per se can be a target for lidocaine's therapeutic action.

Caudal ventrolateral medulla mediates the depressor response elicited by the greater splanchnic nerve afferent stimulation in rats

Caudal ventrolateral medulla (CVLM) plays an important role in the regulation of reflex cardiovascular activity. In the present study, the possible involvement of the CVLM in mediating the depressor response elicited by the greater splanchnic nerve (GSPL) afferent stimulation was explored in rats anesthetized with urethane and alpha-chloralose. Microinjection of lidocaine, and the glutamate receptor antagonists, kynurenic acid and 2-amino-7-phosphonolieptanoic acid, into the CVLM significantly blocked the depressor response induced by the GSPL afferent stimulation. Electrical stimulation of the GSPL inputs excited 48 of 75 CVLM neurons tested (64%). Sixteen out of 21 excited CVLM neurons tested received baroreceptor inputs. Coherence analysis revealed a strong cardiac-related rhythm in the discharges of 11 out of these 21 excited neurons. These results suggest the involvement of CVLM neurons and activation of glutamate receptors in the CVLM in mediating the depressor response induced by the GSPL afferent stimulation in rats.

Continual systemic infusion of lidocaine provides analgesia in an animal model of neuropathic pain

We examined whether continual constant-rate infusion of lidocaine would provide analgesia during the initial post-injury phase in the chronic constriction injury model of neuropathic pain. Male Sprague-Dawley rats were divided into control and ligated groups and infused with saline or lidocaine (0.15, 0.33, 0.67, and 1.3mg/kg/h) via subcutaneously implanted Alzet((R)) osmotic minipumps. Thermal withdrawal latencies were obtained prior (Day 0) and 3 days after loose sciatic ligation and pump implantation surgery. Ligated animals receiving lidocaine at 0.67 or 1.3mg/kg/h exhibited no change in withdrawal latency on Day 3 after surgery, indicating that lidocaine at these doses prevented the development of thermal hyperalgesia as a sign of neuropathic pain. In contrast, ligated animals treated with saline or lidocaine at 0.15 or 0.33mg/kg/h exhibited hyperalgesia on Day 3 after surgery, indicating that these lower doses of lidocaine failed to provide analgesia. Control animals treated with saline or any of the lidocaine doses exhibited no change in withdrawal latencies between Day 0 and Day 3. In a separate group of ligated animals, lidocaine infusion (0.67mg/kg/h) that was started 24h after sciatic ligation surgery reversed the already present thermal hyperalgesia. Average plasma lidocaine concentrations were 0.11, 0.36, and 0.45microg/ml for animals receiving 0.33, 0.67 and 1.3mg/kg/h of lidocaine, respectively. These results suggest that continual systemic infusion of lidocaine prevents or reverses the development of neuropathic pain following chronic constriction injury. These results add to the increasing body of evidence supporting the therapeutic value of preemptive and post-operative lidocaine administration for the relief of neuropathic pain.

Reflex sympathetic dystrophy

Reflex sympathetic dystrophy (RSD) is composed of five major features: pain, swelling, autonomic dysregulation, movement disorders, and atrophy and dystrophy. RSD is caused by an injury to a specific nerve or the C- and A-delta fibers that innervate the involved tissue. It is a progressive illness that spreads with time and may encompass the entire body. There is no psychological disposition to the problem, but all patients are severely depressed because of the constant pain, lack of sleep, and complete disruption of their lifestyle. The continuing pain is usually secondary to the process of central sensitization. The autonomic dysregulation has a major central nervous system component. Atrophy and dystrophy are partly due to loss of nutritive blood supply to the affected tissues. The movement disorder is partly due to deficiency of GABAergic mechanisms; the tremor is an exaggeration of the normal physiologic tremor. Treatment consists of decreasing the afferent pain, maintaining barrage from the underlying defect, and blocking the sympathetic component of the process. New developments include the use of neurotrophic factors to reverse the phenotypic changes that occur in the dorsal horn and the use of pharmacologic agents to block the activity-dependent NMDA channels that appear to be instrumental in maintaining central sensitization.

Flecainide reverses neuropathic pain and suppresses ectopic nerve discharge in rats

We investigated effects of flecainide, a Class IC sodium channel blocker, in the rat chronic constrictive injury (CCI) and ectopic nerve discharge models. In the behavioral evaluation, 2, 6, and 12 mg/kg flecainide were intravenously given to the CCI model, and a dose-dependent analgesic effect was shown on both thermal hyperalgesia and tactile allodynia. In the electrophysiological evaluation using the ectopic nerve discharge model produced by saphenous neurectomy, i.v. administration of 2, 6, and 12 mg/kg flecainide suppressed spontaneous discharge at the peripheral nerve level in a dose-dependent fashion as with the behavioral evaluation, but flecainide did not affect nerve conduction at the dose of 12 mg/kg.

Pain Processing: Paradoxes and Predictions

During the last 25 years, there have been substantial advances in our understanding of the physiology and pathophysiology of pain. The development of animal models that more closely mimic clinical pain in humans has helped elucidate the putative mechanisms by which chronic pain develops and is maintained. However, our increased understanding of the neurobiology of pain has not translated into breakthrough treatments for pain management. As such, chronic pain is still primarily managed by drugs whose primary indication does not include pain (eg, antidepressants, anticonvulsants, antiarrhythmics, local anesthetics). These adjuvant analgesics have come into favor despite the fact that the mechanisms through which these drugs provide pain relief remain either largely unknown or are not selective for a single target. Moreover, the efficacy of adjuvant analgesics in animal models of pain is often validated only after case studies or clinical trials have been reported. This retrospective validation of "novel" analgesics in animal models of pain raises a question of the predictive validity of these models. This article reviews the use of several adjuvant and standard analgesics currently used to treat difficult-to-manage pain. What can these drugs teach us about the development of novel pain medicines? Within this context, the use of animal models of pain to predict analgesic efficacy in clinical pain conditions is considered.

Sodium channels and pain therapy

Researchers have characterized changes in the nervous system that occur in response to tissue injury in order to identify possible targets for novel therapeutic interventions for the treatment of pain. That blockers of voltage-gated sodium channels (VGSCs) are clinically effective for the treatment of pain associated with certain types of tissue injury suggests that these channels constitute such a target. Furthermore, there are changes in biophysical properties, expression, and/or distribution of VGSCs in subpopulations of primary afferent and central nervous system neurons in response to injury that are consistent with a role for VGSCs in the generation and maintenance of pain. Injury-induced changes in four unique VGSCs have been described. However, each of these channels appears to contribute to pain associated with different forms of injury in different ways.