Rapid sodium channel augmentation in response to inflammation induced by complete Freund's adjuvant

Cancer as a Channelopathy—Appreciation of Complimentary Pathways Provides a Different Perspective for Developing Treatments

Simple Summary

While improvements in technology have improved our ability to treat many forms of cancer when diagnosed at an early stage of the disease, the ability to improve survival and quality of life for patients with late stage disease has been limited, largely due to the ability of cancer cells to evade destruction when treatments block preferred paths for survival. Here, we review the role that ions and ion channels play in normal cell function, the development of disease and their role in the life and death of a cell. It is hoped that viewing cancer from the perspective of altered ion channel expression and ion balance may provide a novel approach for developing more effective treatments for this devastating disease.

Abstract

Life depends upon the ability of cells to evaluate and adapt to a constantly changing environment and to maintain internal stability to allow essential biochemical reactions to occur. Ions and ion channels play a crucial role in this process and are essential for survival. Alterations in the expression of the transmembrane proteins responsible for maintaining ion balance that occur as a result of mutations in the genetic code or in response to iatrogenically induced changes in the extracellular environment is a characteristic feature of oncogenesis and identifies cancer as one of a constellation of diseases known as channelopathies. The classification of cancer as a channelopathy provides a different perspective for viewing the disease. Potentially, it may expand opportunities for developing novel ways to affect or reverse the deleterious changes that underlie establishing and sustaining disease and developing tolerance to therapeutic attempts at treatment. The role of ions and ion channels and their interactions in the cell’s ability to maintain ionic balance, homeostasis, and survival are reviewed and possible approaches that mitigate gain or loss of ion channel function to contribute to new or enhance existing cancer therapies are discussed.

Targeted Osmotic Lysis: A Novel Approach to Targeted Cancer Therapies

The conventional treatment of cancer has been based on the delivery of non-selective toxins and/or ionizing energy that affect both the cancer and normal tissues in the hope of destroying the offending disease before killing the patient. Unfortunately, resistance often develops to these treatments and patients experience severe, dose-limiting adverse effects that reduce treatment efficacy and compromise quality of life. Recent advances in our knowledge of the biology of tumor cells and their microenvironment, the recognition of surface proteins that are unique to specific cancers and essential to cell growth and survival and signaling pathways associate with invasion and metastasis have led to the development of targeted therapies that are able to identify specific cellular markers and more selectively deliver lethal treatment to the invading cancer thus improving efficacy and limiting adverse effects. In the context of targeted approaches to cancer therapy, we present targeted osmotic lysis as a novel and fundamentally different approach for treating advanced-stage carcinoma that exploits the conserved relationship between voltage-gated sodium channels and Na⁺, K⁺-ATPase and has the potential to increase survival without compromising quality of life in a broad spectrum of highly malignant forms of cancer.

Peripheral Inflammation Results in Increased Excitability of Capsaicin-Insensitive Nociceptive DRG Neurons Mediated by Upregulation of ASICs and Voltage-Gated Ion Channels

Previously, we have characterized the capsaicin-insensitive low pH-sensitive (caps⁻lpH⁺) subtype of small-sized nociceptive dorsal root ganglion (DRG) neurons that express acid-sensing ion channels, T-type Ca²⁺ channels, and have isolectin B4-negative phenotype. These neurons demonstrated increased excitability in a model of long-term diabetes, contributing to chronic pain sensation. Here we studied changes in the excitability of the caps⁻lpH⁺ neurons and underlying changes in the functional expression and gating properties of ion channels under complete Freund's adjuvant (CFA)-induced peripheral inflammation. We have found that, under these pathological conditions, the functional expression of the acid-sensing ion channels (ASICs) and voltage-gated Na⁺ channels, was increased. In addition, T-type Ca²⁺ current was significantly increased in the neurons at the membrane potentials close to its resting value. Altogether, the observed changes in the channel functioning shifted a pH level evoking an action potential (AP) toward its physiological value and led to an increase of evoked and spontaneous excitability of the caps⁻lpH⁺ neurons that may contribute to hyperalgesia and chronic inflammatory pain.

Antinociception produced by nonsteroidal anti-inflammatory drugs in female vs male rats

The primary aim of this study was to examine sex differences in acute antinociceptive and anti-inflammatory effects of nonsteroidal anti-inflammatory drugs (NSAIDs) in rats. Complete Freund's adjuvant (CFA) was administered to adult Sprague-Dawley rats to induce pain and inflammation in one hindpaw; 2.5 h later, vehicle or a single dose of the NSAIDs ibuprofen (1.0-32 mg/kg) or ketoprofen (0.1-10 mg/kg), or the COX-2-preferring inhibitor celecoxib (1.0-10 mg/kg) was injected i.p. Mechanical allodynia, heat hyperalgesia, biased weight-bearing, and hindpaw thickness were assessed 0.5-24 h after drug injection. Ibuprofen and ketoprofen were more potent or efficacious in females than males in reducing mechanical allodynia and increasing weight-bearing on the CFA-injected paw, and celecoxib was longer-acting in females than males on these endpoints. In contrast, ketoprofen and celecoxib were more potent or efficacious in males than females in reducing hindpaw edema. When administered 3 days rather than 2.5 h after CFA, ketoprofen (3.2-32 mg/kg) was minimally effective in attenuating mechanical allodynia and heat hyperalgesia, and did not restore weight-bearing or significantly decrease hindpaw edema, with no sex differences in any effect. Neither celecoxib nor ketoprofen effects were significantly attenuated by cannabinoid receptor 1 or 2 (CB1 or CB2) antagonists in either sex. These results suggest that common NSAIDs administered shortly after induction of inflammation are more effective in females than males in regard to their antinociceptive effects, whereas their anti-inflammatory effects tend to favor males; effect sizes indicate that sex differences in NSAID effect may be functionally important in some cases.

Conserved Expression of Nav1.7 and Nav1.8 Contribute to the Spontaneous and Thermally Evoked Excitability in IL-6 and NGF-Sensitized Adult Dorsal Root Ganglion Neurons In Vitro

Sensory neurons respond to noxious stimuli by relaying information from the periphery to the central nervous system via action potentials driven by voltage-gated sodium channels, specifically Nav1.7 and Nav1.8. These channels play a key role in the manifestation of inflammatory pain. The ability to screen compounds that modulate voltage-gated sodium channels using cell-based assays assumes that key channels present in vivo is maintained in vitro. Prior electrophysiological work in vitro utilized acutely dissociated tissues, however, maintaining this preparation for long periods is difficult. A potential alternative involves multi-electrode arrays which permit long-term measurements of neural spike activity and are well suited for assessing persistent sensitization consistent with chronic pain. Here, we demonstrate that the addition of two inflammatory mediators associated with chronic inflammatory pain, nerve growth factor (NGF) and interleukin-6 (IL-6), to adult DRG neurons increases their firing rates on multi-electrode arrays in vitro. Nav1.7 and Nav1.8 proteins are readily detected in cultured neurons and contribute to evoked activity. The blockade of both Nav1.7 and Nav1.8, has a profound impact on thermally evoked firing after treatment with IL-6 and NGF. This work underscores the utility of multi-electrode arrays for pharmacological studies of sensory neurons and may facilitate the discovery and mechanistic analyses of anti-nociceptive compounds.

Pomegranate as a Potential Alternative of Pain Management: A Review

The use of complementary medicine has recently increased in an attempt to find effective alternative therapies that reduce the adverse effects of drugs. Punica granatum L. (pomegranate) has been used in traditional medicine for different kinds of pain. This review aims to explore the scientific evidence about the antinociceptive effect of pomegranate. A selection of original scientific articles that accomplished the inclusion criteria was carried out. It was found that different parts of pomegranate showed an antinociceptive effect; this effect can be due mainly by the presence of polyphenols, flavonoids, or fatty acids. It is suggested in the literature that the mechanisms of action may be related to the activation of the L-arginine / NO pathway, members of the TRP superfamily (TRPA1 or TRPV1) and the opioid system. The implications for the field are to know the mechanisms of action by which this effect is generated and thus be able to create alternative treatments for specific types of pain, which help alleviate it and reduce the adverse effects produced by drugs. The results propose that pomegranate and secondary metabolites could be considered in the treatment of inflammatory, nociceptive, and neuropathic pain.

Neurological structures and mediators of pain sensation in anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) tears is a devastating injury and one of the most common knee injuries experienced by athletes in the United States. Although patients reach maximal subjective improvement by one-year following ACL reconstruction, many patients often experience moderate to severe post-operative pain. Opioids, intra-articular injections, and regional anesthesia have been previously implemented to mediate post-operative pain. However, chronic opioid usage has become an epidemic in the United States. Alternative analgesic modalities, such as nerve blocks, have been implemented in clinical practice to provide adequate pain relief and minimize opioid usage. Periarticular injections targeted towards local neurological structures performed concomitantly with nerve blocks provides superior pain relief and satisfaction than isolated nerve blocks. Therefore, it is imperative for physicians to understand local neurological anatomy around the knee joint in order to provide adequate analgesia while minimizing opioid consumption. This purpose of this investigation is to summarize (1) neurogenic origins of pain generators and mediators in sites affected by ACL reconstruction and autograft harvest sites and (2) analgesia utilized in ACL reconstruction.

The ion channel TRPM4 in murine experimental autoimmune encephalomyelitis and in a model of glutamate-induced neuronal degeneration

Transient receptor potential melastatin member 4 (TRPM4), a Ca²⁺-activated nonselective cation channel, has been found to mediate cell membrane depolarization in immune response, insulin secretion, cardiovascular diseases, and cancer. In murine experimental autoimmune encephalomyelitis (EAE), TRPM4 deletion and administration of glibenclamide were found to ameliorate clinical symptoms and attenuate disease progression. However, the exact role of TRPM4 in EAE, as well as the molecular mechanisms underlining TRPM4 contribution in EAE, remain largely unclear. In the present study, EAE was induced in WT C57BL/6 N mice using myelin oligodendrocyte glycoprotein 35–55 (MOG_35–55) and TRPM4 protein and mRNA expression were examined in spinal cord membrane extracts. Our results showed that TRPM4 protein and mRNA are upregulated in EAE, and that their upregulation correlated with disease progression. Moreover, newly-developed TRPM4 inhibitors, named compound 5 and compound 6, were shown to exert a better neuroprotection compared to currently used TRPM4 inhibitors in an in vitro model of glutamate-induced neurodegeneration. These results support the hypothesis that TRPM4 is crucial from early stages of EAE, and suggest that these more potent TRPM4 inhibitors could be used as novel protective therapeutic tools in glutamate-induced neurodegeneration.

Selective lysis of breast carcinomas by simultaneous stimulation of sodium channels and blockade of sodium pumps

Sodium influx through voltage-gated sodium channels (VGSCs) coupled with balanced removal of sodium ions via Na⁺, K⁺-ATPase is a major determinant of cellular homeostasis and intracellular ionic concentration. Interestingly, many metastatic carcinomas express high levels of these channels. We hypothesized that if excess VGSCs are activated and Na⁺, K⁺-ATPase is simultaneously blocked, the intracellular Na⁺ concentration should increase, resulting in water movement into the cell, causing swelling and lytic cell death. MDA-MB-231 breast cancer cells over-express VGSCs by 7-fold. To test our hypothesis, we treated these cells in vitro with the Na⁺, K⁺-ATPase blocker, ouabain, and then stimulated with a sublethal electric current. For in vivo histologic and survival studies, MDA-MB-231 xenografts were established in Nu/J mice. Mice injected with saline or ouabain were electrically stimulated with trains of 10 msec 10V DC pulses. Within seconds to minutes, the cells swelled and lysed. MCF-10a cells, which express normal VGSCs levels, were unaffected by this treatment. Cells from the weakly-malignant cell line, MCF-7, which express 3-fold greater VGSCs than MCF-10a cells, displayed an intermediate time-to-lysis. The rate of lysis correlated directly with the degree of sodium channel expression and malignancy. We also demonstrated efficacy in cell lines from prostate, colon and lung carcinomas. Treated MDA-MB-231 xenografts showed 60-80% cell death. In survival studies, TOL-treated mice showed significantly slower tumor growth vs. controls. These results are evidence that this "targeted osmotic lysis" represents a novel method for selectively killing cancer cells and warrants further investigation as a potential treatment for advanced and end-stage breast cancer.

The NAv1.7 blocker protoxin II reduces burn injury-induced spinal nociceptive processing

Controlling pain in burn-injured patients poses a major clinical challenge. Recent findings suggest that reducing the activity of the voltage-gated sodium channel Na_v1.7 in primary sensory neurons could provide improved pain control in burn-injured patients. Here, we report that partial thickness scalding-type burn injury on the rat paw upregulates Na_v1.7 expression in primary sensory neurons 3 h following injury. The injury also induces upregulation in phosphorylated cyclic adenosine monophosphate response element-binding protein (p-CREB), a marker for nociceptive activation in primary sensory neurons. The upregulation in p-CREB occurs mainly in Na_v1.7-immunopositive neurons and exhibits a peak at 5 min and, following a decline at 30 min, a gradual increase from 1 h post-injury. The Na_v1.7 blocker protoxin II (ProTxII) or morphine injected intraperitoneally 15 min before or after the injury significantly reduces burn injury-induced spinal upregulation in phosphorylated serine 10 in histone H3 and phosphorylated extracellular signal-regulated kinase 1/2, which are both markers for spinal nociceptive processing. Further, ProTxII significantly reduces the frequency of spontaneous excitatory post-synaptic currents in spinal dorsal horn neurons following burn injury. Together, these findings indicate that using Na_v1.7 blockers should be considered to control pain in burn injury.

KEY MESSAGES

• Burn injury upregulates Na_v1.7 expression in primary sensory neurons. • Burn injury results in increased activity of Na_v1.7-expressing primary sensory neurons. • Inhibiting Na_v1.7 by protoxin II reduces spinal nociceptive processing. • Na_v1.7 represents a potential target to reduce pain in burn injury.

Gi-DREADD Expression in Peripheral Nerves Produces Ligand-Dependent Analgesia, as well as Ligand-Independent Functional Changes in Sensory Neurons

Designer receptors exclusively activated by designer drugs (DREADDs) are an advanced experimental tool that could potentially provide a novel approach to pain management. In particular, expression of an inhibitory (Gi-coupled) DREADD in nociceptors might enable ligand-dependent analgesia. To test this possibility, TRPV1-cre mice were used to restrict expression of Gi-DREADDs to predominantly C-fibers. Whereas baseline heat thresholds in both male and female mice expressing Gi-DREADD were normal, 1 mg/kg clozapine-N-oxide (CNO) produced a significant 3 h increase in heat threshold that returned to baseline by 5 h after injection. Consistent with these behavioral results, CNO decreased action potential firing in isolated sensory neurons from Gi-DREADD mice. Unexpectedly, however, the expression of Gi-DREADD in sensory neurons caused significant changes in voltage-gated Ca²⁺ and Na⁺ currents in the absence of CNO, as well as an increase in Na⁺ channel (NaV1.7) expression. Furthermore, CNO-independent excitatory and inhibitory second-messenger signaling was also altered in these mice, which was associated with a decrease in the analgesic effect of endogenous inhibitory G-protein-coupled receptor activation. These results highlight the potential of this exciting technology, but also its limitations, and that it is essential to identify the underlying mechanisms for any observed behavioral phenotypes.

SIGNIFICANCE STATEMENT

DREADD technology is a powerful tool enabling manipulation of activity and/or transmitter release from targeted cell populations. The purpose of this study was to determine whether inhibitory DREADDs in nociceptive afferents could be used to produce analgesia, and if so, how. DREADD activation produced a ligand-dependent analgesia to heat in vivo and a decrease in neuronal firing at the single-cell level. However, we observed that expression of Gi-DREADD also causes ligand-independent changes in ion channel activity and second-messenger signaling. These findings highlight both the potential and the limitations of this exciting technology as well as the necessity to identify the mechanisms underlying any observed phenotype.

Voltage-gated Na+ currents in human dorsal root ganglion neurons

Available evidence indicates voltage-gated Na⁺ channels (VGSCs) in peripheral sensory neurons are essential for the pain and hypersensitivity associated with tissue injury. However, our understanding of the biophysical and pharmacological properties of the channels in sensory neurons is largely based on the study of heterologous systems or rodent tissue, despite evidence that both expression systems and species differences influence these properties. Therefore, we sought to determine the extent to which the biophysical and pharmacological properties of VGSCs were comparable in rat and human sensory neurons. Whole cell patch clamp techniques were used to study Na⁺ currents in acutely dissociated neurons from human and rat. Our results indicate that while the two major current types, generally referred to as tetrodotoxin (TTX)-sensitive and TTX-resistant were qualitatively similar in neurons from rats and humans, there were several differences that have important implications for drug development as well as our understanding of pain mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.23235.001

Functional up-regulation of Nav1.8 sodium channel on dorsal root ganglia neurons contributes to the induction of scorpion sting pain

BmK I, purified from the venom of scorpion Buthus martensi Karsch (BmK), is a receptor site-3-specific modulator of voltage-gated sodium channels (VGSCs) and can induce pain-related behaviors in rats. The tetrodotoxin-resistant (TTX-R) sodium channel Nav1.8 contributes to most of the sodium current underlying the action potential upstroke in dorsal root ganglia (DRG) neurons and may serve as a critical ion channel targeted by BmK I. Herein, using electrophysiological, molecular, and behavioral approaches, we investigated whether the aberrant expression of Nav1.8 in DRG contributes to generation of pain induced by BmK I. The expression of Nav1.8 was found to be significantly increased at both mRNA and protein levels following intraplantar injection of BmK I in rats. In addition, the current density of TTX-R Nav1.8 sodium channel is significantly increased and the gating kinetics of Nav1.8 is also altered in DRG neurons from BmK I-treated rats. Furthermore, spontaneous pain and mechanical allodynia, but not thermal hyperalgesia induced by BmK I, are significantly alleviated through either blockade of the Nav1.8 sodium channel by its selective blocker A-803467 or knockdown of the Nav1.8 expression in DRG by antisense oligodeoxynucleotide (AS-ODN) targeting Nav1.8 in rats. Finally, BmK I was shown to induce enhanced pain behaviors in complete freund's adjuvant (CFA)-inflamed rats, which was partly due to the over-expression of Nav1.8 in DRG. Our results suggest that functional up-regulation of Nav1.8 channel on DRG neurons contributes to the development of BmK I-induced pain in rats.

The in vitro mechanisms and in vivo efficacy of intravenous lidocaine on the neuroinflammatory response in acute and chronic pain

INTRODUCTION

The neuroinflammatory response plays a key role in several pain syndromes. Intravenous (iv) lidocaine is beneficial in acute and chronic pain. This review delineates the current literature concerning in vitro mechanisms and in vivo efficacy of iv lidocaine on the neuroinflammatory response in acute and chronic pain.

DATABASES AND DATA TREATMENT

We searched PUBMED and the Cochrane Library for in vitro and in vivo studies from July 1975 to August 2014. In vitro articles providing an explanation for the mechanisms of action of lidocaine on the neuroinflammatory response in pain were included. Animal or clinical studies were included concerning iv lidocaine for acute or chronic pain or during inflammation.

RESULTS

Eighty-eight articles regarding iv lidocaine were included: 36 in vitro studies evaluating the effect on ion channels and receptors; 31 animal studies concerning acute and chronic pain and inflammatory models; 21 clinical studies concerning acute and chronic pain. Low-dose lidocaine inhibits in vitro voltage-gated sodium channels, the glycinergic system, some potassium channels and Gαq-coupled protein receptors. Higher lidocaine concentrations block potassium and calcium channels, and NMDA receptors. Animal studies demonstrate lidocaine to have analgesic effects in acute and neuropathic pain syndromes and anti-inflammatory effects early in the inflammatory response. Clinical studies demonstrate lidocaine to have advantage in abdominal surgery and in some neuropathic pain syndromes.

CONCLUSIONS

Intravenous lidocaine has analgesic, anti-inflammatory and antihyperalgesic properties mediated by an inhibitory effect on ion channels and receptors. It attenuates the neuroinflammatory response in perioperative pain and chronic neuropathic pain.

Changing channels in pain and epilepsy: Exploiting ion channel gene therapy for disorders of neuronal hyperexcitability

Chronic pain and epilepsy together affect hundreds of millions of people worldwide. While traditional pharmacotherapy provides essential relief to the majority of patients, a large proportion remains resistant, and surgical intervention is only possible for a select few. As both disorders are characterised by neuronal hyperexcitability, manipulating the expression of the most direct modulators of excitability - ion channels - represents an attractive common treatment strategy. A number of viral gene therapy approaches have been explored to achieve this. These range from the up- or down-regulation of channels that control excitability endogenously, to the delivery of exogenous channels that permit manipulation of excitability via optical or chemical means. In this review we highlight the key experimental successes of each approach and discuss the challenges facing their clinical translation.

Dexmedetomidine inhibits Tetrodotoxin-resistant Nav1.8 sodium channel activity through Gi/o-dependent pathway in rat dorsal root ganglion neurons

Background

Systemically administered dexmedetomidine (DEX), a selective α2 adrenergic receptor (α2-AR) agonists, produces analgesia and sedation. Peripherally restricted α2-AR antagonist could block the analgesic effect of systemic DEX on neuropathic pain, with no effect on sedation, indicating peripheral analgesic effect of DEX. Tetrodotoxin-resistant (TTX-R) sodium channel Na_v1.8 play important roles in the conduction of nociceptive sensation. Both α2-AR and Nav1.8 are found in small nociceptive DRG neurons. We, therefore, investigated the effects of DEX on the Na_v1.8 currents in acutely dissociated small-diameter DRG neurons.

Results

Whole-cell patch-clamp recordings demonstrated that DEX concentration-dependently suppressed TTX-R Na_v1.8 currents in small-diameter lumbar DRG neurons. DEX also shifted the steady-state inactivation curves of Na_v1.8 in a hyperpolarizing direction and increased the threshold of action potential and decrease electrical and chemical stimuli-evoked firings in small-diameter DRG neurons. The α2-AR antagonist yohimbine or α2_A-AR antagonist BRL44408 but not α2_B-AR antagonist imiloxan blocked the inhibition of Na_v1.8 currents by DEX. Immunohistochemistry results showed that Na_v1.8 was predominantly expressed in peripherin-positive small-diameter DRG neurons, and some of them were α2_A-AR-positive ones. Our electrophysiological recordings also demonstrated that DEX-induced inhibition of Na_v1.8 currents was prevented by intracellular application of G-protein inhibitor GDPβ-s or G_i/o proteins inhibitor pertussis toxin (PTX), and bath application of adenylate cyclase (AC) activator forskolin or membrane-permeable cAMP analogue 8-Bromo-cAMP (8-Br-cAMP). PKA inhibitor Rp-cAMP could mimic DEX-induced inhibition of Na_v1.8 currents.

Conclusions

We established a functional link between α2-AR and Na_v1.8 in primary sensory neurons utilizing the G_i/o/AC/cAMP/PKA pathway, which probably mediating peripheral analgesia of DEX.

Regulation and pharmacological blockade of sodium-potassium ATPase: a novel pathway to neuropathy

Inflammation causes upregulation of NaV1.7 sodium channels in the associated dorsal root ganglia (DRG). The resultant increase in sodium influx must be countered to maintain osmotic homeostasis. The primary mechanism to pump sodium out of neurons is Na(+), K(+)-ATPase. To test whether there is a compensatory upregulation of Na(+), K(+)-ATPase after inflammation, rats received an injection of complete Freund's adjuvant (CFA) into one hindpaw and saline into the contralateral hindpaw. Three days later, L4-L6 DRGs were extracted and analyzed using gel electrophoresis and immunohistochemistry. Immunoreactivity for both the α-1 and α-3 subunits were increased in DRG associated with CFA-treatment, compared to saline-treatment. To test whether dysregulation of Na(+), K(+)-ATPase may cause cell death after inflammation, we produced a pharmacological blockade with ouabain (10mg/kg, s.c.) three days after CFA injection and paws were stimulated or not. Twenty-four hours later, DRG were removed and stained with cresyl violet. Greater cell death was seen in DRG from ouabain-treated animals on the CFA treated side than the saline-treated side. Paw stimulation doubled this difference. Control DRG showed little neuronal death. These results are evidence that regulation of Na(+), K(+)-ATPase during major inflammatory disease states is critical for homeostatic protection of primary afferent neurons.

Functional up-regulation of Nav1.8 sodium channel in Aβ afferent fibers subjected to chronic peripheral inflammation

Background

Functional alterations in the properties of Aβ afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Na_v1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na_v1.8 in controlling Aβ-fiber excitability following persistent inflammation.

Methods

Distribution and expression of Na_v1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund’s adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total I_Na and TTX-R Na_v1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na_v1.8-preferring blocker on the electrophysiological properties of Na_v1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.

Results

Our findings revealed that Na_v1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total I_Na and Na_v1.8 peak current densities are enhanced in inflamed large myelinated Aβ-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aβ-fiber neuron excitability by shifting the voltage-dependent activation of Na_v1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na_v1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na_v1.8 regulation in Aβ-fibers contributes to inflammatory pain.

Conclusions

Collectively, these findings support a key role for Na_v1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

The role of sodium channels in chronic pain

Here we review recent research into the mechanisms of chronic pain that has focused on neuronal sodium channels, a target of classic analgesic agents. We first discuss evidence that specific sodium channel isoforms are essential for the detection and conduction of normal acutely painful stimuli from nociceptors. We then review findings that show changes in sodium channel expression and localization in chronic inflammation and nerve injury in animal and human tissues. We conclude by discussing the role that myelination plays in organizing and maintaining sodium channel clusters at nodes of Ranvier in normal development and how inflammatory processes or nerve injury alter the characteristics of such clusters. Based on these findings, we suggest that chronic pain may in part result from partial demyelination of axons during chronic injury, which creates aberrant sodium channel clusters that serve as sites of ectopic sensitivity or spontaneous activity.

Reduction of voltage gated sodium channel protein in DRG by vector mediated miRNA reduces pain in rats with painful diabetic neuropathy

Background

Painful neuropathy is a common complication of diabetes. Previous studies have identified significant increases in the amount of voltage gated sodium channel isoforms Na_V1.7 and Na_V1.3 protein in the dorsal root ganglia (DRG) of rats with streptozotocin (STZ)-induced diabetes. We found that gene transfer-mediated release of the inhibitory neurotransmitters enkephalin or gamma amino butyric acid (GABA) from DRG neurons in diabetic animals reduced pain-related behaviors coincident with a reduction in Na_V1.7 protein levels in DRG in vivo. To further evaluate the role of Na_Vα subunit levels in DRG in the pathogenesis of pain in diabetic neuropathy, we constructed a non-replicating herpes simplex virus (HSV)-based vector expressing a microRNA (miRNA) against Na_Vα subunits.

Results

Subcutaneous inoculation of the miRNA-expressing HSV vector into the feet of diabetic rats to transduce DRG resulted in a reduction in Na_Vα subunit levels in DRG neurons, coincident with a reduction in cold allodynia, thermal hyperalgesia and mechanical hyperalgesia.

Conclusions

These data support the role of increased Na_Vα protein in DRG in the pathogenesis of pain in diabetic neuropathy, and provide a proof-of-principle demonstration for the development of a novel therapy that could be used to treat intractable pain in patients with diabetic neuropathy.

A-kinase anchoring protein 150 expression in a specific subset of TRPV1- and CaV 1.2-positive nociceptive rat dorsal root ganglion neurons

Modulation of phosphorylation states of ion channels is a critical step in the development of hyperalgesia during inflammation. Modulatory enhancement of channel activity may increase neuronal excitability and affect downstream targets such as gene transcription. The specificity required for such regulation of ion channels quickly occurs via targeting of protein kinases and phosphatases by the scaffolding A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 has been implicated in inflammatory pain by targeting protein kinase A (PKA) and protein kinase C (PKC) to the transient receptor potential vanilloid 1 (TRPV1) channel in peripheral sensory neurons, thus lowering threshold for activation of the channel by multiple inflammatory reagents. However, the expression pattern of AKAP150 in peripheral sensory neurons is unknown. Here we identify the peripheral neuron subtypes that express AKAP150, the subcellular distribution of AKAP150, and the potential target ion channels in rat dorsal root ganglion (DRG) slices. We found that AKAP150 is expressed predominantly in a subset of small DRG sensory neurons, where it is localized at the plasma membrane of the soma, axon initial segment, and small fibers. Most of these neurons are peripherin positive and produce C fibers, although a small portion produce Aδ fibers. Furthermore, we demonstrate that AKAP79/150 colocalizes with TRPV1 and Ca(V) 1.2 in the soma and axon initial segment. Thus AKAP150 is expressed in small, nociceptive DRG neurons, where it is targeted to membrane regions and where it may play a role in the modulation of ion channel phosphorylation states required for hyperalgesia.

Chronic compression of mouse dorsal root ganglion alters voltage-gated sodium and potassium currents in medium-sized dorsal root ganglion neurons

Chronic compression (CCD) of the dorsal root ganglion (DRG) is a model of human radicular pain produced by intraforaminal stenosis and other disorders affecting the DRG, spinal nerve, or root. Previously, we examined electrophysiological changes in small-diameter lumbar level 3 (L3) and L4 DRG neurons treated with CCD; the present study extends these observations to medium-sized DRG neurons, which mediate additional sensory modalities, both nociceptive and non-nociceptive. Whole-cell patch-clamp recordings were obtained from medium-sized somata in the intact DRG in vitro. Compared with neurons from unoperated control animals, CCD neurons exhibited a decrease in the current threshold for action potential generation. In the CCD group, current densities of TTX-resistant and TTX-sensitive Na(+) current were increased, whereas the density of delayed rectifier voltage-dependent K(+) current was decreased. No change was observed in the transient or "A" current after CCD. We conclude that CCD in the mouse produces hyperexcitability in medium-sized DRG neurons, and the hyperexcitability is associated with an increased density of Na(+) current and a decreased density of delayed rectifier voltage-dependent K(+) current.

Antisense-mediated knockdown of Na(V)1.8, but not Na(V)1.9, generates inhibitory effects on complete Freund's adjuvant-induced inflammatory pain in rat

Tetrodotoxin-resistant (TTX-R) sodium channels Na_V1.8 and Na_V1.9 in sensory neurons were known as key pain modulators. Comparing with the widely reported Na_V1.8, roles of Na_V1.9 on inflammatory pain are poorly studied by antisense-induced specific gene knockdown. Here, we used molecular, electrophysiological and behavioral methods to examine the effects of antisense oligodeoxynucleotide (AS ODN) targeting Na_V1.8 and Na_V1.9 on inflammatory pain. Following complete Freund's adjuvant (CFA) inflammation treatment, Na_V1.8 and Na_V1.9 in rat dorsal root ganglion (DRG) up-regulated mRNA and protein expressions and increased sodium current densities. Immunohistochemical data demonstrated that Na_V1.8 mainly localized in medium and small-sized DRG neurons, whereas Na_V1.9 only expressed in small-sized DRG neurons. Intrathecal (i.t.) delivery of AS ODN was used to down-regulate Na_V1.8 or Na_V1.9 expressions confirmed by immunohistochemistry and western blot. Unexpectedly, behavioral tests showed that only Na_V1.8 AS ODN, but not Na_V1.9 AS ODN could reverse CFA-induced heat and mechanical hypersensitivity. Our data indicated that TTX-R sodium channels Na_V1.8 and Na_V1.9 in primary sensory neurons played distinct roles in CFA-induced inflammatory pain and suggested that antisense oligodeoxynucleotide-mediated blocking of key pain modulator might point toward a potential treatment strategy against certain types of inflammatory pain.

Increased Na+ and K+ currents in small mouse dorsal root ganglion neurons after ganglion compression

We investigated the effects of chronic compression (CCD) of the L3 and L4 dorsal root ganglion (DRG) on pain behavior in the mouse and on the electrophysiological properties of the small-diameter neuronal cell bodies in the intact ganglion. CCD is a model of human radicular pain produced by intraforaminal stenosis and other disorders affecting the DRG, spinal nerve, or root. On days 1, 3, 5, and 7 after the onset of compression, there was a significant decrease from preoperative values in the threshold mechanical force required to elicit a withdrawal of the foot ipsilateral to the CCD (tactile allodynia). Whole cell patch-clamp recordings were obtained, in vitro, from small-sized somata and, for the first time, in the intact DRG. Under current clamp, CCD neurons exhibited a significantly lower rheobase compared with controls. A few CCD but no control neurons exhibited spontaneous action potentials. CCD neurons showed an increase in the density of TTX-resistant and TTX-sensitive Na(+) current. CCD neurons also exhibited an enhanced density of voltage-dependent K(+) current, due to an increase in delayed rectifier K(+) current, without a change in the transient or "A" current. We conclude that CCD in the mouse produces a model of radicular pain, as we have previously demonstrated in the rat. While the role of enhanced K(+) current remains to be elucidated, we speculate that it represents a compensatory neuronal response to reduce ectopic or aberrant levels of neuronal activity produced by the injury.

Pharmacological management of neuropathic pain in older adults: an update on peripherally and centrally acting agents

The burden of neuropathic pain in older adults is great and the practitioner is challenged to reduce symptoms and improve quality of life. Many common neuropathic pain syndromes are more prevalent in the older population, and older adults also carry greater sensitivity to certain side effects. The health care professional should have a thorough familiarity with all medications available to treat this difficult group of disorders.

Role of acid-sensing ion channel 3 in sub-acute-phase inflammation

BACKGROUND

Inflammation-mediated hyperalgesia involves tissue acidosis and sensitization of nociceptors. Many studies have reported increased expression of acid-sensing ion channel 3 (ASIC3) in inflammation and enhanced ASIC3 channel activity with pro-inflammatory mediators. However, the role of ASIC3 in inflammation remains inconclusive because of conflicting results generated from studies of ASIC3 knockout (ASIC3-/-) or dominant-negative mutant mice, which have shown normal, decreased or increased hyperalgesia during inflammation.

RESULTS

Here, we tested whether ASIC3 plays an important role in inflammation of subcutaneous tissue of paw and muscle in ASIC3-/- mice induced by complete Freund's adjuvant (CFA) or carrageenan by investigating behavioral and pathological responses, as well as the expression profile of ion channels. Compared with the ASIC3+/+ controls, ASIC3-/- mice showed normal thermal and mechanical hyperalgesia with acute (4-h) intraplantar CFA- or carrageenan-induced inflammation, but the hyperalgesic effects in the sub-acute phase (1-2 days) were milder in all paradigms except for thermal hyperalgesia with CFA-induced inflammation. Interestingly, carrageenan-induced primary hyperalgesia was accompanied by an ASIC3-dependent Nav1.9 up-regulation and increase of tetrodotoxin (TTX)-resistant sodium currents. CFA-inflamed muscle did not evoke hyperalgesia in ASIC3-/- or ASIC3+/+ mice, whereas carrageenan-induced inflammation in muscle abolished mechanical hyperalgesia in ASIC3-/- mice, as previously described. However, ASIC3-/- mice showed attenuated pathological features such as less CFA-induced granulomas and milder carrageenan-evoked vasculitis as compared with ASIC3+/+ mice.

CONCLUSION

We provide a novel finding that ASIC3 participates in the maintenance of sub-acute-phase primary hyperalgesia in subcutaneous inflammation and mediates the process of granuloma formation and vasculitis in intramuscular inflammation.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Changes in the expression of NaV1.7, NaV1.8 and NaV1.9 in a distinct population of dorsal root ganglia innervating the rat knee joint in a model of chronic inflammatory joint pain

Voltage-gated sodium channels play an essential role in regulating the excitability of nociceptive primary afferent neurones. In particular the tetrodotoxin-sensitive (TTX-S) Na(V)1.7 and the tetrodotoxin-resistant (TTX-R) Na(V)1.8 and Na(V)1.9 channels have been suggested to play a role in inflammatory pain. Previous work has revealed acute administration of inflammatory mediators, such as Freund's complete adjuvant (FCA) or carrageenan caused an upregulation in the levels of Na(V)1.7 and Na(V)1.8 protein in DRG (dorsal root ganglia) tissue up to 4 days post-insult. In the present study, the expression of Na(V)1.7, Na(V)1.8 and Na(V)1.9 was examined over a 28 day timecourse during a rat model of FCA-induced chronic inflammatory joint pain. Using the retrograde tracer Fast Blue (FB) and specific Na(V)1.7, Na(V)1.8 and Na(V)1.9 sodium channel antibodies, immunohistochemical staining techniques were used to study sodium channel expression in a distinct population of L3-L5 knee joint afferent DRGs. In the ganglia, counts were made of positively labelled cells in the FB population. The results demonstrate that, following FCA injection, Na(V)1.9 expression is upregulated at days 14, 21 and 28 post-FCA, with Na(V)1.7 and Na(V)1.8 showing increased channel expression at days 14 and 28. These observations are accompanied by a unilateral joint hypersensitivity in the FCA-injected knee indicated by a behavioural shift in weight distribution measured using an incapacitance tester. The increased presence of these channels suggests that Na(V)1.7, Na(V)1.8 and Na(V)1.9 play a role, at least in part, in the maintenance of chronic inflammatory pain several weeks after the initial insult.

Topical agents for the management of musculoskeletal pain

The recent recognition of the magnitude of cardiovascular risk of both nonselective nonsteroidal anti-inflammatory drugs and COX-2 selective inhibitors, in addition to the persistent concerns about the use of opioids, has brought increased attention to nonsystemic, topical analgesics. These agents have a favorable safety profile and there is increasing evidence indicating their efficacy for a variety of pain disorders. The use of topical analgesics in the treatment of the most prevalent musculoskeletal pain syndromes is described, with a focus on mechanisms for drug delivery and clinical trials data.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Prostaglandin E2 has no effect on two components of tetrodotoxin-resistant Na+ current in mouse dorsal root ganglion

One possible mechanism underlying inflammation-induced sensitization of the primary afferent neuron is the upregulation of tetrodotoxin-resistant (TTX-R) Na(+) current by inflammatory mediators such as prostaglandins. This notion is based on reports that showed an augmentation of TTX-R Na(+) current following an application of prostaglandin E(2) (PGE(2)) in dorsal root ganglion (DRG) neurons. However, no information was available on the properties of the novel type of TTX-R Na(+) channel, Na(V)1.9, at times when these reports were published. Hence, the contribution of Na(V)1.9 to the PGE(2)-induced upregulation of TTX-R Na(+) current remains to be elucidated. To further examine the modulation of TTX-R Na(+) current by PGE(2), we recorded two components of TTX-R Na(+) current in isolation from small (<25 microm in diameter) DRG neurons using wild-type and Na(V)1.8 knock-out mice. Unexpectedly, neither the component mediated by Na(V)1.8 nor the persistent component mediated by Na(V)1.9 was affected by PGE(2) (1 and 10 microM). Our results raise a question regarding the well-known modulatory role of PGE(2) on TTX-R Na(+) current in inflammatory hyperalgesia.

Glycosylation-induced depolarization facilitates subthreshold membrane oscillation in injured primary sensory neurons

Subthreshold membrane potential oscillations (SMPO) in the injured dorsal root ganglion (DRG) neurons are involved in the generation of spontaneous activity, which can directly evoke neuropathic pain. Nerve injury usually triggers the synthesis of large quantities of membrane protein in nerve injured DRG neurons. Membrane proteins are glycosylated by addition of sugars, especially negatively charged sialic acid residues, which may depolarize the resting membrane potential (Vm), open voltage-gated channels in injured neurons, and cause spontaneous activity. In the present study, we aimed to determine if increased negative charge on the cell surface, carried by the sialic acid residues, could contribute to the generation of SMPO in injured DRG neurons. Intracellular recording was performed in DRG neurons following chronic constrictive injury (CCI) of the sciatic nerve. Results indicated that both A- and C-type injured DRG neurons exhibited a higher incidence of SMPO and more depolarized Vm than those of the control neurons. Ca(2+), Mg(2+), Mn(2+), or poly-lysine, a positively charged organic compound, when topically applied to the DRG, not only reduced SMPO but also caused a rapid hyperpolarizing shift in Vm. Topical application of neuraminidase to selectively remove sialic acid residues on the extracellular membrane normalized the depolarized Vm and inhibited both spontaneous and evoked SMPO. However, application of Ca(2+), Mg(2+), Mn(2+) or neuraminidase had no effect on excitability and Vm in normal neurons. The results demonstrated that the increase in negatively charged sialic acid residues on the extracellular membrane of neuronal somata is a critical factor in the generation of SMPO and hyperexcitability in injured sensory neurons.

Up-regulation of acid-sensing ion channel 3 in dorsal root ganglion neurons following application of nucleus pulposus on nerve root in rats

STUDY DESIGN

Immunocytochemistry for acid-sensing ion channel 3 (ASIC3) in neurons of rat dorsal root ganglions (DRGs) from animals exposed to a model of lumbar disc herniation.

OBJECTIVE

To examine expression of ASIC3 in DRGs and the effect of a sodium channel blocker applied to the nerve root in a rat model of lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

Radicular pain is a common symptom of lumbar disc herniation in human beings. A depolarizing sodium channel gated by protons during tissue acidosis, ASIC3, is specifically expressed in sensory neurons. It has been associated with cardiac ischemic and inflammatory pain. We often perform spinal nerve root block for radicular pain using a sodium channel blocker, such as lidocaine; however, it has been unclear whether the effective period of this treatment is usually longer than the expected duration of efficacy.

METHODS

For the lumbar disc herniation model, nucleus pulposus was harvested from the tail and applied to the L5 nerve root, and the nerve roots were pinched. We evaluated mechanical allodynia in sham-operated animals and a disc herniation model. Immunohistochemistry was used to examine ASIC3 expression in L5 DRGs. Finally, the effect of lidocaine on pain and ASIC3 expression in the disc herniation model was examined.

RESULTS

Animals exposed to the lumbar disc herniation model showed allodynia for 8 days, and ASIC3 immunoreactivity was up-regulated in DRG neurons. After administration of lidocaine to spinal nerve roots affected by disc herniation, ASIC3 immunoreactivity was down-regulated in DRG neurons, and the level of mechanical allodynia was significantly decreased for 8 days.

CONCLUSIONS

Our results suggest that ASIC3 in DRG neurons may play an important role in nerve root pain caused by lumbar disc herniation. Lidocaine decreased ASIC3 expression in DRG neurons and pain associated with the disc herniation model.

Antinociceptive effects of tetrodotoxin (TTX) in rodents

BACKGROUND

Tetrodotoxin (TTX) is a powerful sodium channel blocker extracted from the puffer fish. The analgesic effects of TTX were investigated in different animal pain models.

METHODS

Wistar rats were submitted to the formalin test and to partial ligation of the sciatic nerve (Seltzer's model). Swiss Webster mice were used in the writhing test. Rodents were divided into six groups receiving a s.c. injection of either 0.9% NaCl, TTX 0.3, 1, 3, or 6 microg kg(-1), or morphine (5 mg kg(-1)). Substances were injected 30 min before 2.5% formalin injection into the hind paw, acetic acid administration intraperitoneally or neuropathic pain testing consisting of mechanical allodynia (von Frey filament) and thermal hyperalgesia (Plantar test).

RESULTS

TTX decreased pain behaviour in the formalin test at the highest dose and in the writhing test at 3 and 6 microg kg(-1). It also diminished mechanical allodynia and thermal hyperalgesia with an ED(50) of 1.08 (0.89) and 0.62 (0.33) microg kg(-1), respectively. Observation of the rats after TTX injection did not show any motor deficit, respiratory distress or sedation. Morphine was also effective in relieving pain in all three tests but with signs of considerable sedation.

CONCLUSION

Systemic injections of TTX diminished pain behaviour in a dose-dependent manner in models of inflammatory, visceral and neuropathic pain without causing adverse events, whereas morphine analgesia was associated with heavy sedation. TTX is a very promising substance for the treatment of various types of pain but needs further evaluation.

Inflammatory hyperalgesia: a role for the C-fiber sensory neuron cell body?

Peripheral nerve injury increases the chemosensitivity and excitability of injured afferents, resulting in ectopic activity arising from within dorsal root ganglia. Studies of dissociated sensory ganglion neurons in vitro suggest afferent somata might be sensitized by persistent inflammation. However, it is unknown whether this inflammation-induced sensitization is manifest in somata within the intact ganglia. To explore this possibility, intracellular electrophysiologic recording was used with a sciatic nerve-L4-dorsal root ganglia preparation to compare excitability and chemosensitivity of cutaneous C-fiber somata from control and inflamed rats. Cutaneous afferents were identified with the retrograde dye DiI. Excitability was assessed before and after the application of inflammatory soup (IS) containing bradykinin, serotonin, and prostaglandin E2 all at a pH of 7.0. Persistent inflammation decreased the excitability of cutaneous afferents in intact ganglia and had no significant influence on the magnitude of IS-induced increase in excitability. Opposite to the effects observed in intact ganglia, excitability was greater in dissociated cutaneous nociceptors obtained from inflamed rats, although the magnitude of the IS-induced increase in excitability was not significantly affected by inflammation. These results suggest that the cell bodies of putative cutaneous nociceptors in the intact ganglia contribute minimally to pain and hyperalgesia associated with persistent inflammation.

PERSPECTIVE

Results of the present study suggest that inflammation-induced changes in afferent somata are minimal. However, they also suggest that inflammatory mediator-induced increase in the excitability of sensory neuron somata might contribute to global changes in nociception observed under high systemic inflammatory mediator loads.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

The role of Na(V)1.8 sodium channel in the maintenance of chronic inflammatory hypernociception

We previously described an animal model of persistent inflammatory sensitization of nociceptors. In this model the hypernociception persists for more than 30 days after the cessation of 2 weeks of daily intraplantar treatment with prostaglandin E(2) (PGE(2)). The tetrodotoxin-resistant (TTX-R) voltage-gated sodium channel Na(V)1.8 is considered a characteristic of primary afferent nociceptive C fibers and plays an important role in acute hypernociception. In the present study, the relevance of the Na(V)1.8 channel was investigated in this model of persistent mechanical hypernociception in rats. In the PGE(2)-induced persistent hypernociception, but not in the single injection-induced acute hypernociception, the mRNA expression (RT-PCR) of Na(V)1.8 in dorsal root ganglia (DRG) was up-regulated. A similar increase of Na(V)1.8 mRNA was observed when DbcAMP was used to induce persistent hypernociception. Four daily intrathecal administrations of oligodeoxynucleotides (ODN) antisense against Na(V)1.8 decreased the mRNA encoding Na(V)1.8 in DRG. The intrathecal administration of ODN antisense prevented the PGE(2)-induced acute hypernociception and significantly reduced ongoing PGE(2)-induced persistent hypernociception. A parallel restoration of the persistent hypernociception and up-regulation of Na(V)1.8 mRNA was observed after the cessation of ODN antisense treatment. These results suggest the participation of Na(V)1.8 channels in the development and maintenance of chronic inflammatory hyperalgesia, and confirm their involvement in the acute inflammatory hypernociception.

Effects of ethanol and water extracts of propolis (bee glue) on acute inflammatory animal models

The anti-inflammatory effects of ethanol (EEP) and water (WSD) extracts in ICR mice and Wistar rats were analyzed. Both WSD and EEP exhibited significant anti-inflammatory effects in animal models with respect to thoracic capillary vessel leakage in mice, carrageenan-induced oedema, carrageenan-induced pleurisy, acute lung damage in rats. The mechanisms for the anti-inflammatory effects probably involve decreasing prostaglandin-E(2) (PGE(2)) and nitric oxide (NO) levels. In rats with Freund's complete adjuvant (FCA) induced arthritis, propolis extracts significantly inhibited the increase of interleukin-6 (IL-6) in inflamed tissues, but had no significant effect on levels of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma). The results are consistent with the interpretation that EEP and WSD may exert these effects by inhibiting the activation and differentiation of mononuclear macrophages.

Inflammation alters sodium currents and excitability of temporomandibular joint afferents

Inflammation-induced changes in voltage-gated sodium currents (I(Na)) in primary afferent neurons may contribute to hyperexcitability and pain. The present study was designed to test the hypothesis that persistent inflammation of the temporomandibular joint (TMJ) increases I(Na) in TMJ afferents. Acutely dissociated retrogradely labeled TMJ afferents were studied using whole-cell patch clamp techniques three days following Complete Freund's Adjuvant-induced inflammation of the TMJ. Inflammation was associated with a decrease in tetrodotoxin (TTX)-sensitive Na+ conductance and no significant change in slowly inactivating TTX-resistant Na+ conductance. However, inflammation increased the excitability of TMJ afferents. These results suggest that changes in ion channels other than those underlying TTX-sensitive and the slowly inactivating TTX-resistant Na+ conductance are likely to account for the inflammation-induced increase in the excitability of TMJ afferents.

Inflammation-induced hyperexcitability of nociceptive gastrointestinal DRG neurones: the role of voltage-gated ion channels

Gastrointestinal (GI) inflammation modulates the intrinsic properties of nociceptive dorsal root ganglia neurones, which innervate the GI tract and these changes are important in the genesis of abdominal pain and visceral hyperalgesia neurones exhibit hyperexcitability characterized by a decreased threshold for activation and increased firing rate, and changes in voltages-gated Na(+) and K(+) channels play a major role in this plasticity. This review highlights emerging evidence that specific subsets of channels and signalling pathways are involved and their potential to provide novel selective therapeutics targets for the treatment of abdominal pain.

Functional assay of voltage-gated sodium channels using membrane potential-sensitive dyes

The discovery of novel therapeutic agents that act on voltage-gated sodium channels requires the establishment of high-capacity screening assays that can reliably measure the activity of these proteins. Fluorescence resonance energy transfer (FRET) technology using membrane potential-sensitive dyes has been shown to provide a readout of voltage-gated sodium channel activity in stably transfected cell lines. Due to the inherent rapid inactivation of sodium channels, these assays require the presence of a channel activator to prolong channel opening. Because sodium channel activators and test compounds may share related binding sites on the protein, the assay protocol is critical for the proper identification of channel inhibitors. In this study, high throughput, functional assays for the voltage-gated sodium channels, hNa(V)1.5 and hNa(V)1.7, are described. In these assays, channels stably expressed in HEK cells are preincubated with test compound in physiological medium and then exposed to a sodium channel activator that slows channel inactivation. Sodium ion movement through open channels causes membrane depolarization that can be measured with a FRET dye membrane potential-sensing system, providing a large and reproducible signal. Unlike previous assays, the signal obtained in the agonist initiation assay is sensitive to all sodium channel modulators that were tested and can be used in high throughput mode, as well as in support of Medicinal Chemistry efforts for lead optimization.

Sialic acid contributes to hyperexcitability of dorsal root ganglion neurons in rats with peripheral nerve injury

Axonal injury of the dorsal root ganglion (DRG) neurons may alter the synthesis of certain membrane proteins that are responsible for the development of abnormal hyperexcitability. The external domains of most of these membrane proteins are sialylated. Because sialic acid carries heavy negative charges, the increase of sialylated proteins may increase neurons' negative surface charges, which will have predictable effects on the voltage-gated channels, and affect the excitability of injured neurons. Using intracellular electrophysiological recording, we demonstrated that following chronic constriction injury (CCI) of the sciatic nerve, Aalpha/beta DRG neurons become hyperexcitable, as indicated by a more depolarized resting membrane potential (Vm) and a lowered threshold current (TIC). More interestingly, the excitability of injured DRG neurons was reduced substantially when the extracellular sialic acid was removed by pretreatment with neuraminidase. The Vm was less depolarized and the TIC increased robustly as compared to the CCI neurons without neuraminidase treatment. However, desialylation of normal, intact neurons had no significant effect on the Vm and less effect on the TIC. Our results suggest that the hyperexcitability of injured sensory neurons may be associated with increased negatively charged sialic acid residues on the surface of the neuronal somata.

Partial sciatic nerve transection causes redistribution of pain-related peptides and lowers withdrawal threshold

Complete nerve transection results in loss of sensation and paralysis of the involved extremity. Such injury drastically reduces content of the nociceptive peptides, substance P, and somatostatin in the dorsal horn of the spinal cord and dorsal root ganglia innervating the limb. Partial nerve injuries occur more commonly in clinical practice, however, and frequently result in the development of chronic neuropathic pain. To investigate mechanisms underlying this pathologic pain syndrome, rats were subjected to partial sciatic nerve transection. Withdrawal thresholds determined with Von Frey hairs dropped dramatically in the operated limb. On postoperative Day 4, thresholds had decreased from 15 g to less than 5 g on the operated side, whereas those on the contralateral (unoperated) side or those from sham-operated rats did not change. Sciatic hemisection had no effect on total content of either substance P or somatostatin in the dorsal spinal cord and lumbar dorsal root ganglia as measured by radioimmunoassay on postoperative Days 4, 7, or 14. However, when examined immunohistochemically, there was a marked redistribution of both peptides associated with partial transection. On the contralateral side or in sham-operated rats, both substance P and somatostatin were confined to the superficial laminae of the dorsal horn. By contrast, on the operated side, content of both peptides was reduced by more than half in the superficial laminae. There was a compensatory increase in content in the deeper laminae where nociceptive peptides are not usually found. Redistribution of substance P and somatostatin may be due to axonal sprouting, increased peptide expression by interneurons, or aberrant expression of nociceptive peptides by neurons normally mediating mechanical sensation. The presence of increased levels of nociceptive peptides in regions of the spinal cord that mediate innocuous sensation may underlie development of allodynia.

Ibuprofen blocks changes in nav 1.7 and 1.8 sodium channels associated with complete freund's adjuvant–induced inflammation in rat

Although nerve growth factor plays a role in augmenting sodium channel expression in small dorsal root ganglion (DRG) cells, the cytochemical mediators responsible for enhanced expression in large DRG neurons are unknown. To narrow the search for mediators involved in the increased production of sodium channels in large DRG neurons, we examined the effect of cyclooxygenase inhibition on sodium channel production during inflammation. Thirty minutes before the subcutaneous injection of complete Freund's adjuvant (CFA), rats received ibuprofen (nonselective, cyclooxygenase inhibitor), NS-398 (selective, cyclooxygenase inhibitor), or vehicle. Withdrawal thresholds from thermal and mechanical stimulation were measured before and immediately after CFA injection and at selected hourly intervals after injection for the next 24 hours. Sodium channel up-regulation was then examined in DRG by using site-specific, anti-sodium channel antibodies, Na(v) 1.7 and 1.8. Both ibuprofen and NS-398 provided analgesia during the second phase of inflammatory hyperalgesia that begins 3 hours after CFA injection. The up-regulation, predominantly of Na(v) 1.7 and minimally of Na(v) 1.8 channels, seen in vehicle-treated rats was suppressed by both drugs at 24 hours after injection. By 72 hours after injection, no difference in labeling between the drug- and vehicle-treated animals was observed. Sodium channel labeling in large DRG neurons returned to baseline between 1 and 2 weeks after CFA injection, whereas small cell labeling persisted. The cytochemical signal for sodium channel up-regulation in the large DRG cells that most closely correlates with inflammatory hyperalgesia is mediated at least in part through products of the cyclooxygenase pathway.

PERSPECTIVE

Expression of sodium channels in dorsal root ganglia increases dramatically during inflammation. The increase in sodium channels is thought to enhance neuronal excitability and to play a role in hyperalgesia and wound vigilance during healing. We provide evidence that prostaglandins play a role in signaling channel augmentation.