The molecular pathophysiology of pain: abnormal expression of sodium channel genes and its contributions to hyperexcitability of primary sensory neurons

Anoctamin 1, a multi-modal player in pain and itch

Anoctamin 1 (ANO1/TMEM16A) encodes a Ca2+-activated Cl- channel. Among ANO1's many physiological functions, it plays a significant role in mediating nociception and itch. ANO1 is activated by intracellular Ca2+ and depolarization. Additionally, ANO1 is activated by heat above 44 °C, suggesting heat as another activation stimulus. ANO1 is highly expressed in nociceptors, indicating a role in nociception. Conditional Ano1 ablation in dorsal root ganglion (DRG) neurons results in a reduction in acute thermal pain, as well as thermal and mechanical allodynia or hyperalgesia evoked by inflammation or nerve injury. Pharmacological interventions also lead to a reduction in nocifensive behaviors. ANO1 is functionally linked to the bradykinin receptor and TRPV1. Bradykinin stimulates ANO1 via IP3-mediated Ca2+ release from intracellular stores, whereas TRPV1 stimulates ANO1 via a combination of Ca2+ influx and release. Nerve injury causes upregulation of ANO1 expression in DRG neurons, which is blocked by ANO1 antagonists. Due to its role in nociception, strong and specific ANO1 antagonists have been developed. ANO1 is also expressed in pruritoceptors, mediating Mas-related G protein-coupled receptors (Mrgprs)-dependent itch. The activation of ANO1 leads to chloride efflux and depolarization due to high intracellular chloride concentrations, causing pain and itch. Thus, ANO1 could be a potential target for the development of new drugs treating pain and itch.

Schisandrin B from Schisandra chinensis alleviated pain via glycine receptors, Nav1.7 channels and Cav2.2 channels

ETHNOPHARMACOLOGICAL RELEVANCE

Schisandra chinensis, the dried and ripe fruit of the magnolia family plant Schisandra chinensis (Turcz.) Baill, was commonly used in traditional analgesic prescription. Studies have shown that the extract of Schisandra chinensis (SC) displayed analgesic activity. However, the analgesic active component and the exact mechanisms have yet to be revealed.

AIM OF THE STUDY

The present study was to investigate the anti-nociceptive constituent of Schisandra chinensis, assess its analgesic effect, and explore the potential molecular mechanisms.

MATERIALS AND METHODS

The effects of a series of well-recognized compounds from SC on glycine receptors were investigated. The analgesic effect of the identified compound was evaluated in three pain models. Mechanistic studies were performed using patch clamp technique on various targets expressed in recombinant cells. These targets included glycine receptors, Nav1.7 sodium channels, Cav2.2 calcium channels et al. Meanwhile, primary cultured spinal dorsal horn (SDH) neurons and dorsal root ganglion (DRG) neurons were also utilized.

RESULTS

Schisandrin B (SchB) was a positive allosteric modulator of glycine receptors in spinal dorsal horn neurons. The EC50 of SchB on glycine receptors in spinal dorsal horn neurons was 2.94 ± 0.28 μM. In three pain models, the analgesic effect of SchB was comparable to that of indomethacin at the same dose. Besides, SchB rescued PGE2-induced suppression of α3 GlyR activity and alleviated persistent pain. Notably, SchB could also potently decrease the frequency of action potentials and inhibit sodium and calcium channels in DRG neurons. Consistent with the data from DRG neurons, SchB was also found to significantly block Nav1.7 sodium channels and Cav2.2 channels in recombinant cells.

CONCLUSION

Our results demonstrated that, Schisandrin B, the primary lignan component of Schisandra chinensis, may exert its analgesic effect by acting on multiple ion channels, including glycine receptors, Nav1.7 channels, and Cav2.2 channels.

A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons

Hyperexcitability in sensory neurons is known to underlie many of the maladaptive changes associated with persistent pain. Chemogenetics has shown promise as a means to suppress such excitability, yet chemogenetic approaches suitable for human applications are needed. PSAM4-GlyR is a modular system based on the human α7 nicotinic acetylcholine and glycine receptors, which responds to inert chemical ligands and the clinically approved drug varenicline. Here, we demonstrated the efficacy of this channel in silencing both mouse and human sensory neurons by the activation of large shunting conductances after agonist administration. Virally mediated expression of PSAM4-GlyR in mouse sensory neurons produced behavioral hyposensitivity upon agonist administration, which was recovered upon agonist washout. Stable expression of the channel led to similar reversible suppression of pain-related behavior even after 10 months of viral delivery. Mechanical and spontaneous pain readouts were also ameliorated by PSAM4-GlyR activation in acute and joint pain inflammation mouse models. Furthermore, suppression of mechanical hypersensitivity generated by a spared nerve injury model of neuropathic pain was also observed upon activation of the channel. Effective silencing of behavioral hypersensitivity was reproduced in a human model of hyperexcitability and clinical pain: PSAM4-GlyR activation decreased the excitability of human-induced pluripotent stem cell-derived sensory neurons and spontaneous activity due to a gain-of-function NaV1.7 mutation causing inherited erythromelalgia. Our results demonstrate the contribution of sensory neuron hyperexcitability to neuropathic pain and the translational potential of an effective, stable, and reversible humanized chemogenetic system for the treatment of pain.

Synchronized activity of sensory neurons initiates cortical synchrony in a model of neuropathic pain

Increased low frequency cortical oscillations are observed in people with neuropathic pain, but the cause of such elevated cortical oscillations and their impact on pain development remain unclear. By imaging neuronal activity in a spared nerve injury (SNI) mouse model of neuropathic pain, we show that neurons in dorsal root ganglia (DRG) and somatosensory cortex (S1) exhibit synchronized activity after peripheral nerve injury. Notably, synchronized activity of DRG neurons occurs within hours after injury and 1-2 days before increased cortical oscillations. This DRG synchrony is initiated by axotomized neurons and mediated by local purinergic signaling at the site of nerve injury. We further show that synchronized DRG activity after SNI is responsible for increasing low frequency cortical oscillations and synaptic remodeling in S1, as well as for inducing animals' pain-like behaviors. In naive mice, enhancing the synchrony, not the level, of DRG neuronal activity causes synaptic changes in S1 and pain-like behaviors similar to SNI mice. Taken together, these results reveal the critical role of synchronized DRG neuronal activity in increasing cortical plasticity and oscillations in a neuropathic pain model. These findings also suggest the potential importance of detection and suppression of elevated cortical oscillations in neuropathic pain states.

Inflammation and histone modification in chronic pain

Increasing evidence suggests that epigenetic mechanisms have great potential in the field of pain. The changes and roles of epigenetics of the spinal cord and dorsal root ganglia in the chronic pain process may provide broad insights for future pain management. Pro-inflammatory cytokines and chemokines released by microglia and astrocytes, as well as blood-derived macrophages, play critical roles in inducing and maintaining chronic pain, while histone modifications may play an important role in inflammatory metabolism. This review provides an overview of neuroinflammation and chronic pain, and we systematically discuss the regulation of neuroinflammation and histone modifications in the context of chronic pain. Specifically, we analyzed the role of epigenetics in alleviating or exacerbating chronic pain by modulating microglia, astrocytes, and the proinflammatory mediators they release. This review aimed to contribute to the discovery of new therapeutic targets for chronic pain.

Neuropathy, its Profile and Experimental Nerve Injury Neuropathic Pain Models: A Review

Neuropathy is a terrible disorder that has a wide range of etiologies. Drug-induced neuropathy, which happens whenever a chemical agent damages the peripheral nerve system, has been linked here to the iatrogenic creation of some drugs. It is potentially permanent and causes sensory impairments and paresthesia that typically affects the hands, feet, and stockings; motor participation is uncommon. It might appear suddenly or over time, and the long-term outlook varies. The wide range of chronic pain conditions experienced by people has been one of the main obstacles to developing new, more effective medications for the treatment of neuropathic pain. Animal models can be used to examine various neuropathic pain etiologies and symptoms. Several models investigate the peripheral processes of neuropathic pain, whereas some even investigate the central mechanisms, such as drug induce models like vincristine, cisplatin, bortezomib, or thalidomide, etc., and surgical models like sciatic nerve chronic constriction injury (CCI), sciatic nerve ligation through spinal nerve ligation (SNL), sciatic nerve damage caused by a laser, SNI (spared nerve injury), etc. The more popular animal models relying on peripheral nerve ligatures are explained. In contrast to chronic sciatic nerve contraction, which results in behavioral symptoms of less reliable stressful neuropathies, (SNI) spared nerve injury generates behavioral irregularities that are more feasible over a longer period. This review summarizes the latest methods models as well as clinical ideas concerning this mechanism. Every strongest current information on neuropathy is discussed, along with several popular laboratory models for causing neuropathy.

Jmjd3 Mediates Neuropathic Pain by Inducing Macrophage Infiltration and Activation in Lumbar Spinal Stenosis Animal Model

Lumbar spinal stenosis (LSS) is a major cause of chronic neuropathic back and/or leg pain. Recently, we demonstrated that a significant number of macrophages infiltrated into the cauda equina after compression injury, causing neuroinflammation, and consequently mediating neuropathic pain development and/or maintenance. However, the molecular mechanisms underlying macrophage infiltration and activation have not been elucidated. Here, we demonstrated the critical role of histone H3K27 demethylase Jmjd3 in blood-nerve barrier dysfunction following macrophage infiltration and activation in LSS rats. The LSS rat model was induced by cauda equina compression using a silicone block within the epidural spaces of the L5-L6 vertebrae with neuropathic pain developing 4 weeks after compression. We found that Jmjd3 was induced in the blood vessels and infiltrated macrophages in a rat model of neuropathic pain. The blood-nerve barrier permeability in the cauda equina was increased after compression and significantly attenuated by the Jmjd3 demethylase inhibitor, GSK-J4. GSK-J4 also inhibited the expression and activation of MMP-2 and MMP-9 and significantly alleviated the loss of tight junction proteins and macrophage infiltration. Furthermore, the activation of a macrophage cell line, RAW 264.7, by LPS was significantly alleviated by GSK-J4. Finally, GSK-J4 and a potential Jmjd3 inhibitor, gallic acid, significantly inhibited mechanical allodynia in LSS rats. Thus, our findings suggest that Jmjd3 mediates neuropathic pain development and maintenance by inducing macrophage infiltration and activation after cauda equina compression and thus may serve as a potential therapeutic target for LSS-induced neuropathic pain.

Morphological and Functional Changes of Corneal Nerves and Their Contribution to Peripheral and Central Sensory Abnormalities

The cornea is the most densely innervated and sensitive tissue in the body. The cornea is exclusively innervated by C- and A-delta fibers, including mechano-nociceptors that are triggered by noxious mechanical stimulation, polymodal nociceptors that are excited by mechanical, chemical, and thermal stimuli, and cold thermoreceptors that are activated by cooling. Noxious stimulations activate corneal nociceptors whose cell bodies are located in the trigeminal ganglion (TG) and project central axons to the trigeminal brainstem sensory complex. Ocular pain, in particular, that driven by corneal nerves, is considered to be a core symptom of inflammatory and traumatic disorders of the ocular surface. Ocular surface injury affecting corneal nerves and leading to inflammatory responses can occur under multiple pathological conditions, such as chemical burn, persistent dry eye, and corneal neuropathic pain as well as after some ophthalmological surgical interventions such as photorefractive surgery. This review depicts the morphological and functional changes of corneal nerve terminals following corneal damage and dry eye disease (DED), both ocular surface conditions leading to sensory abnormalities. In addition, the recent fundamental and clinical findings of the importance of peripheral and central neuroimmune interactions in the development of corneal hypersensitivity are discussed. Next, the cellular and molecular changes of corneal neurons in the TG and central structures that are driven by corneal nerve abnormalities are presented. A better understanding of the corneal nerve abnormalities as well as neuroimmune interactions may contribute to the identification of a novel therapeutic targets for alleviating corneal pain.

Genomic analysis of 21 patients with corneal neuralgia after refractive surgery

Background

Refractive surgery, specifically laser-assisted in situ keratomileusis and photorefractive keratectomy, are widely applied procedures to treat myopia, hyperopia, and astigmatism. After surgery, a subgroup of cases suffers from persistent and intractable pain of obscure etiology, thought to be neuropathic. We aimed to investigate the contribution of genomic factors in the pathogenesis of these patients with corneal neuralgia.

Methods

We enrolled 21 cases (6 males and 15 females) from 20 unrelated families, who reported persistent pain (>3 months), after refractive surgery (20 laser-assisted in situ keratomileusis and 1 photorefractive keratectomy patients). Whole-exome sequencing and gene-based association test were performed.

Results

Whole-exome sequencing demonstrated low-frequency variants (allele frequency < 0.05) in electrogenisome-related ion channels and cornea-expressed collagens, most frequently in SCN10A (5 cases), SCN9A (4 cases), TRPV1 (4 cases), CACNA1H and CACNA2D2 (5 cases each), COL5A1 (6 cases), COL6A3 (5 cases), and COL4A2 (4 cases). Two variants, p.K655R of SCN9A and p.Q85R of TRPV1, were previously characterized as gain-of-function. Gene-based association test assessing "damaging" missense variants against gnomAD exome database (non-Finnish European or global), identified a gene, SLC9A3R1, with statistically significant effect (odds ratio = 17.09 or 17.04; Bonferroni-corrected P-value < 0.05).

Conclusion

These findings in a small patient cohort did not identify a common gene/variant among most of these cases, as found in other disorders, for example small-fiber neuropathy. Further studies of these candidate genes/variants might enhance understanding of the role of genetic factors in the pathogenesis of corneal neuralgia.

Dorsal root ganglion stimulation for chronic pain modulates Aβ-fiber activity but not C-fiber activity: A computational modeling study

OBJECTIVE

The goal of this project was to use computational models to investigate which types of primary sensory neurons are modulated by dorsal root ganglion stimulation (DRGS) to provide pain relief.

METHODS

We modeled DRGS by coupling an anatomical finite element model of a human L5 dorsal root ganglion to biophysical models of primary sensory neurons. We calculated the stimulation amplitude needed to elicit an action potential in each neuron, and examined how DRGS affected sensory neuron activity.

RESULTS

We showed that within clinical ranges of stimulation parameters, DRGS drives the activity of large myelinated Aβ-fibers but does not directly activate small nonmyelinated C-fibers. We also showed that the position of the active and return electrodes and the polarity of the stimulus pulse influence neural activation.

CONCLUSIONS

Our results indicate that DRGS may provide pain relief by activating pain-gating mechanisms in the dorsal horn via repeated activation of large myelinated afferents.

SIGNIFICANCE

Understanding the mechanisms of action of DRGS-induced pain relief may lead to innovations in stimulation technologies that improve patient outcomes.

Cyclic nucleotide signaling in sensory neuron hyperexcitability and chronic pain after nerve injury

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Activation of cAMP-PKA and cGMP-PKG pathways contributes to injury-induced sensory neuron hyperexcitability.

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Activation of cAMP and cGMP contributes to the development of bone cancer pain.

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PAR2 activation mediates injury-induced cAMP-dependent sensory neuron hyperexcitability.

The cyclic nucleotide signaling, including cAMP-PKA and cGMP-PKG pathways, has been well known to play critical roles in regulating cellular growth, metabolism and many other intracellular processes. In recent years, more and more studies have uncovered the roles of cAMP and cGMP in the nervous system. The cAMP and cGMP signaling mediates chronic pain induced by different forms of injury and stress. Here we summarize the roles of cAMP-PKA and cGMP-PKG signaling pathways in the pathogenesis of chronic pain after nerve injury. In addition, acute dissociation and chronic compression of the dorsal root ganglion (DRG) neurons, respectively, leads to neural hyperexcitability possibly through PAR2 activation-dependent activation of cAMP-PKA pathway. Clinically, radiotherapy can effectively alleviate bone cancer pain at least partly through inhibiting the cancer cell-induced activation of cAMP-PKA pathway. Roles of cyclic nucleotide signaling in neuropathic and inflammatory pain are also seen in many other animal models and are involved in many pro-nociceptive mechanisms including the activation of hyperpolarization-activated cyclic nucleotide (HCN)-modulated ion channels and the exchange proteins directly activated by cAMP (EPAC). Further understanding the roles of cAMP and cGMP signaling in the pathogenesis of chronic pain is theoretically significant and clinically valuable for treatment of chronic pain.

General Pathways of Pain Sensation and the Major Neurotransmitters Involved in Pain Regulation

Pain has been considered as a concept of sensation that we feel as a reaction to the stimulus of our surrounding, putting us in harm's way and acting as a form of defense mechanism that our body has permanently installed into its system. However, pain leads to a huge chunk of finances within the healthcare system with continuous rehabilitation of patients with adverse pain sensations, which might reduce not only their quality of life but also their productivity at work setting back the pace of our economy. It may not look like a huge deal but factor in pain as an issue for majority of us, it becomes an economical burden. Although pain has been researched into and understood by numerous researches, from its definition, mechanism of action to its inhibition in hopes of finding an absolute solution for victims of pain, the pathways of pain sensation, neurotransmitters involved in producing such a sensation are not comprehensively reviewed. Therefore, this review article aims to put in place a thorough understanding of major pain conditions that we experience-nociceptive, inflammatory and physiologically dysfunction, such as neuropathic pain and its modulation and feedback systems. Moreover, the complete mechanism of conduction is compiled within this article, elucidating understandings from various researches and breakthroughs.

Targeting ASIC3 for Relieving Mice Fibromyalgia Pain: Roles of Electroacupuncture, Opioid, and Adenosine

Many scientists are seeking better therapies for treating fibromyalgia (FM) pain. We used a mouse model of FM to determine if ASIC3 and its relevant signaling pathway participated in FM pain. We demonstrated that FM-induced mechanical hyperalgesia was attenuated by electroacupuncture (EA). The decrease in fatigue-induced lower motor function in FM mice was also reversed by EA. These EA-based effects were abolished by the opioid receptor antagonist naloxone and the adenosine A1 receptor antagonist rolofylline. Administration of opioid receptor agonist endomorphin (EM) or adenosine A1 receptor agonist N⁶-cyclopentyladenosine (CPA) has similar results to EA. Similar results were also observed in ASIC3^−/− or ASIC3 antagonist (APETx2) injected mice. Using western blotting, we determined that pPKA, pPI3K, and pERK were increased during a dual acidic injection priming period. Nociceptive receptors, such as ASIC3, Nav1.7, and Nav1.8, were upregulated in the dorsal root ganglion (DRG) and spinal cord (SC) of FM mice. Furthermore, pPKA, pPI3K, and pERK were increased in the central thalamus. These aforementioned mechanisms were completely abolished in ASIC3 knockout mice. Electrophysiological results also indicated that acid potentiated Nav currents through ASIC3 and ERK pathway. Our results highlight the crucial role of ASIC3-mediated mechanisms in the treatment of FM-induced mechanical hyperalgesia.

Veratridine produces distinct calcium response profiles in mouse Dorsal Root Ganglia neurons

Nociceptors are a subpopulation of dorsal root ganglia (DRG) neurons that detect noxious stimuli and signal pain. Veratridine (VTD) is a voltage-gated sodium channel (VGSC) modifier that is used as an "agonist" in functional screens for VGSC blockers. However, there is very little information on VTD response profiles in DRG neurons and how they relate to neuronal subtypes. Here we characterised VTD-induced calcium responses in cultured mouse DRG neurons. Our data shows that the heterogeneity of VTD responses reflects distinct subpopulations of sensory neurons. About 70% of DRG neurons respond to 30-100 μM VTD. We classified VTD responses into four profiles based upon their response shape. VTD response profiles differed in their frequency of occurrence and correlated with neuronal size. Furthermore, VTD response profiles correlated with responses to the algesic markers capsaicin, AITC and α, β-methylene ATP. Since VTD response profiles integrate the action of several classes of ion channels and exchangers, they could act as functional "reporters" for the constellation of ion channels/exchangers expressed in each sensory neuron. Therefore our findings are relevant to studies and screens using VTD to activate DRG neurons.

HYP-17, a novel voltage-gated sodium channel blocker, relieves inflammatory and neuropathic pain in rats

Clinical and experimental studies suggest that voltage-gated sodium channels (VGSCs) play a key role in the pathogenesis of neuropathic pain and that blocking agents against these channels can be potentially therapeutic. In the current study, we investigated whether a novel compound, (-)-2-Amino-1-(4-((4-chlorophenyl)(phenyl)methyl)piperazin-1-yl)-propan-1-one(HYP-17), binds to VGSCs and evaluated its inhibitory effect on Na⁺ currents of the rat dorsal root ganglia (DRG) sensory neurons and its analgesic effect on inflammatory and neuropathic pain. HYP-17 (10μM) reduced both the tetrodotoxin-sensitive (TTX-S) and the TTX-resistant (TTX-R) currents in DRG sensory neurons. However, neither the voltage-dependent activation curves nor the steady-state inactivation curves for TTX-S and TTX-R currents were changed by HYP-17. In rats injected with 5% formalin under the plantar surface of the hind paw, HYP-17 (10μg) significantly reduced both the early and late phase spontaneous pain behaviors. Systemic injection with HYP-17 (60mg/kg, i.p.) also significantly relieved the mechanical, cold, and warm allodynia induced by rat tail nerve injury. Furthermore, HYP-17 (60mg/kg, i.p.) significantly relieved the central neuropathic pain induced by spinal cord injury (SCI), and inhibited c-Fos expression in lumbar (L) 4-L5 spinal segments. Electrophysiological study showed that HYP-17 significantly attenuated the hyper-responsiveness of lumbar dorsal horn neurons. In addition, HYP-17 significantly reduced the levels of pp38MAPK and p-JNK in microglia and astrocytes, respectively, in the L4-L5 spinal dorsal horn. Therefore, our results indicate that HYP-17 has potential analgesic activities against nociceptive, inflammatory and neuropathic pain.

Pulsed Magnetic Field Therapy in Refractory Neuropathic Pain Secondary to Peripheral Neuropathy: Electrodiagnostic Parameters—Pilot Study

Context. Neuropathic pain (NP) from peripheral neuropathy (PN) arises from ectopic firing of unmyelinated C-fibers with accumulation of sodium and calcium channels. Because pulsed electromagnetic fields (PEMF) safely induce extremely low frequency (ELF) quasirectangular currents that can depolarize, repolarize, and hyperpolarize neurons, it was hypothesized that directing this energy into the sole of one foot could potentially modulate neuropathic pain. Objective. To determine if 9 consecutive 1-h treatments in physician’s office (excluding weekends) of a pulsed signal therapy can reduce NP scores in refractory feet with PN. Design/setting/patients. 24 consecutive patients with refractory and symptomatic PN from diabetes, chronic inflammatory demyelinating polyneuropathy (CIDP), pernicious anemia, mercury poisoning, paraneoplastic syndrome, tarsal tunnel, and idiopathic sensory neuropathy were enrolled in this nonplacebo pilot study. The most symptomatic foot received therapy. Primary endpoints were comparison of VAS scores at the end of 9 days and the end of 30 days follow-up compared to baseline pain scores. Additionally, Patients’ Global Impression of Change (PGIC) questionnaire was tabulated describing response to treatment. Subgroup analysis of nerve conduction scores, quantified sensory testing (QST), and serial examination changes were also tabulated. Subgroup classification of pain (Serlin) was utilized to determine if there were disproportionate responses. Intervention. Noninvasive pulsed signal therapy generates a unidirectional quasirectangular waveform with strength about 20 gauss and a frequency about 30 Hz into the soles of the feet for 9 consecutive 1-h treatments (excluding weekends). The most symptomatic foot of each patient was treated. Results. All 24 feet completed 9 days of treatment. 15/24 completed follow-up (62%) with mean pain scores decreasing 21% from baseline to end of treatment (P = 0.19) but with 49% reduction of pain scores from baseline to end of follow-up (P < 0.01). Of this group, self-reported PGIC was improved 67% (n = 10) and no change was 33% (n = 5). An intent-to-treat analysis based on all 24 feet demonstrated a 19% reduction in pain scores from baseline to end of treatment (P = 0.10) and a 37% decrease from baseline to end of follow-up ( P < 0.01). Subgroup analysis revealed 5 patients with mild pain with nonsignificant reduction at end of follow-up. Of the 19 feet with moderate to severe pain, there was a 28% reduction from baseline to end of treatment (P < 0.05) and a 39% decrease from baseline to end of follow-up (P < 0.01). Benefit was better in those patients with axonal changes and advanced CPT baseline scores. The clinical examination did not change. There were no adverse events or safety issues. Conclusions. These pilot data demonstrate that directing PEMF to refractory feet can provide unexpected shortterm analgesic effects in more than 50% of individuals. The role of placebo is not known and was not tested. The precise mechanism is unclear yet suggests that severe and advanced cases are more magnetically sensitive. Future studies are needed with randomized placebo-controlled design and longer treatment periods.

Decreased excitability and voltage-gated sodium currents in aortic baroreceptor neurons contribute to the impairment of arterial baroreflex in cirrhotic rats

Cardiovascular autonomic dysfunction, which is manifested by an impairment of the arterial baroreflex, is prevalent irrespective of etiology and contributes to the increased morbidity and mortality in cirrhotic patients. However, the cellular mechanisms that underlie the cirrhosis-impaired arterial baroreflex remain unknown. In the present study, we examined whether the cirrhosis-impaired arterial baroreflex is attributable to the dysfunction of aortic baroreceptor (AB) neurons. Biliary and nonbiliary cirrhotic rats were generated via common bile duct ligation (CBDL) and intraperitoneal injections of thioacetamide (TAA), respectively. Histological and molecular biological examinations confirmed the development of fibrosis in the livers of both cirrhotic rat models. The heart rate changes during phenylephrine-induced baroreceptor activation indicated that baroreflex sensitivity was blunted in the CBDL and TAA rats. Under the current-clamp mode of the patch-clamp technique, cell excitability was recorded in DiI-labeled AB neurons. The number of action potential discharges in the A- and C-type AB neurons was significantly decreased because of the increased rheobase and threshold potential in the CBDL and TAA rats compared with sham-operated rats. Real-time PCR and Western blotting indicated that the NaV1.7, NaV1.8, and NaV1.9 transcripts and proteins were significantly downregulated in the nodose ganglion neurons from the CBDL and TAA rats compared with the sham-operated rats. Consistent with these molecular data, the tetrodotoxin-sensitive NaV currents and the tetrodotoxin-resistant NaV currents were significantly decreased in A- and C-type AB neurons, respectively, from the CBDL and TAA rats compared with the sham-operated rats. Taken together, these findings implicate a key cellular mechanism in the cirrhosis-impaired arterial baroreflex.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Neurotrophins and Neuropathic Pain: Role in Pathobiology

Neurotrophins (NTs) belong to a family of trophic factors that regulate the survival, growth and programmed cell death of neurons. In mammals, there are four structurally and functionally related NT proteins, viz. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 and neurotrophin 4. Most research on NTs to date has focussed on the effects of NGF and BDNF signalling via their respective cognate high affinity neurotrophic tyrosine kinase viz TrkA and TrkB receptors. Apart from the key physiologic roles of NGF and BDNF in peripheral and central nervous system function, NGF and BDNF signalling via TrkA and TrkB receptors respectively have been implicated in mechanisms underpinning neuropathic pain. Additionally, NGF and BDNF signalling via the low-affinity pan neurotrophin receptor at 75 kDa (p75NTR) may also contribute to the pathobiology of neuropathic pain. In this review, we critically assess the role of neurotrophins signalling via their cognate high affinity receptors as well as the low affinity p75NTR in the pathophysiology of peripheral neuropathic and central neuropathic pain. We also identify knowledge gaps to guide future research aimed at generating novel insight on how to optimally modulate NT signalling for discovery of novel therapeutics to improve neuropathic pain relief.

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.

Contribution of mechanosensitive ion channels to somatosensation

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal, is a central mechanism to several physiological functions in mammals. It relies on the function of mechanosensitive ion channels (MSCs). Although the first single-channel recording from MSCs dates back to 30 years ago, the identity of the genes encoding MSCs has remained largely elusive. Because these channels have an important role in the development of mechanical hypersensitivity, a better understanding of their function may lead to the identification of selective inhibitors and generate novel therapeutic pathways in the treatment of chronic pain. Here, I will describe our current understanding of the role MSCs may play in somatosensation and the potential candidate genes proposed to encode them.

Neuromodulatory properties of inflammatory cytokines and their impact on neuronal excitability

Increasing evidence underlines that prototypical inflammatory cytokines (IL-1β, TNF-α and IL-6) either synthesized in the central (CNS) or peripheral nervous system (PNS) by resident cells, or imported by immune blood cells, are involved in several pathophysiological functions, including an unexpected impact on synaptic transmission and neuronal excitability. This review describes these unconventional neuromodulatory properties of cytokines, that are distinct from their classical action as effector molecules of the immune system. In addition to the role of cytokines in brain physiology, we report evidence that dysregulation of their biosynthesis and cellular release, or alterations in receptor-mediated intracellular pathways in target cells, leads to neuronal cell dysfunction and modifications in neuronal network excitability. As a consequence, targeting of these cytokines, and related signalling molecules, is considered a novel option for the development of therapies in various CNS or PNS disorders associated with an inflammatory component. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Expression of background potassium channels in rat DRG is cell-specific and down-regulated in a neuropathic pain model

Neuropathic pain is associated with hyperexcitability of DRG neurons. Despite the importance of leakage potassium channels for neuronal excitability, little is known about their cell-specific expression in DRGs and possible modulation in neuropathic pain. Multiple leakage channels are expressed in DRG neurons, including TASK1, TASK3, TRESK, TRAAK, TWIK1, TREK1 and TREK2 but little is known about their distribution among different cell types. Our immunohistochemical studies show robust TWIK1 expression in large and medium size neurons, without overlap with TRPV1 or IB4 staining. TASK1 and TASK3, on the contrary, are selectively expressed in small cells; TASK1 expression closely overlaps TRPV1-positive cells, while TASK3 is expressed in TRPV1- and IB4-negative cells. We also studied mRNA expression of these channels in L4-L5 DRGs in control conditions and up to 4 weeks after spared nerve injury lesion. We found that TWIK1 expression is much higher than TASK1 and TASK3 and is strongly decreased 1, 2 and 4 weeks after neuropathic injury. TASK3 expression, on the other hand, decreases 1 week after surgery but reverts to baseline by 2weeks; TASK1 shows no significant change at any time point. These data suggest an involvement of TWIK1 in the maintenance of the pain condition.

A systematic review: current and future directions of dorsal root ganglion therapeutics to treat chronic pain

OBJECTIVE

The purpose of the study was to systematically review the historical therapeutics for chronic pain care directed at the dorsal root ganglion (DRG) and to identify future trends and upcoming treatment strategies.

METHODS

A literature search on bibliographic resources, including EMBASE, PubMed Cochrane Database of Systemic Reviews from literature published from 1966 to December 1, 2012 to identify studies and treatments directed at the DRG to treat chronic pain, and was limited to the English language. Case series, case reports, and preclinical work were excluded. Information on emerging technologies and pharmacologics were captured separately, as they did not meet the inclusion criteria.

RESULTS

The literature review yielded three current clinical treatment strategies: ganglionectomy, conventional radiofrequency treatment of the dorsal root ganglion, and pulsed radiofrequency treatment of the DRG. Seven studies were identified utilizing ganglionectomy, 14 for conventional radiofrequency, and 16 for pulsed radiofrequency. Electrical stimulation and novel therapeutic delivery strategies have been proposed and are in development.

CONCLUSIONS

Despite a robust understanding of the DRG and its importance in acute nociception, as well as the development and maintenance of chronic pain, relatively poor evidence exists regarding current therapeutic strategies. Novel therapies like electrical and pharmacologic strategies are on the horizon, and more prospective study is required to better qualify the role of the DRG in chronic pain care.

Neuroplastic alteration of TTX-resistant sodium channel with visceral pain and morphine-induced hyperalgesia

The discovery of the tetrodotoxin-resistant (TTX-R) Na⁺ channel in nociceptive neurons has provided a special target for analgesic intervention. In a previous study we found that both morphine tolerance and persistent visceral inflammation resulted in visceral hyperalgesia. It has also been suggested that hyperexcitability of sensory neurons due to altered TTX-R Na⁺ channel properties and expression contributes to hyperalgesia; however, we do not know if some TTX-R Na⁺ channel property changes can be triggered by visceral hyperalgesia and morphine tolerance, or whether there are similar molecular or channel mechanisms in both situations. To evaluate the effects of morphine tolerance and visceral inflammation on the channel, we investigated the dorsal root ganglia (DRG) neuronal change following these chronic treatments. Using whole-cell patch clamp recording, we recorded TTX-R Na⁺ currents in isolated adult rat lumbar and sacral (L6−S2) DRG neurons from normal and pathologic rats with colon inflammatory pain or chronic morphine treatment. We found that the amplitudes of TTX-R Na⁺ currents were significantly increased in small-diameter DRG neurons with either morphine tolerance or visceral inflammatory pain. Meanwhile, the result also showed that those treatments altered the kinetics properties of the electrical current (ie, the activating and inactivating speed of the channel was accelerated). Our current results suggested that in both models, visceral chronic inflammatory pain and morphine tolerance causes electrophysiological changes in voltage-gated Na channels due to the chronic administration of these medications. For the first time, the present investigation explored the adaptations of this channel, which may contribute to the hyperexcitability of primary afferent nerves and hyperalgesia during these pathologic conditions. The results also suggest that neurophysiologic mechanisms of morphine tolerance and visceral hyperalgesia are related at the TTX-R Na⁺ channel.

Loureirin B: An Effective Component in Dragon's Blood Modulating Sodium Currents in TG Neurons

To test, analyze and express the relationship between the pharmacological effect of Traditional Chinese Medicine (TCM) dragon's blood and that of its component loureirin B, specify an operational definition for effective component from raw drug of TCM. Using the whole-cell patch-clamp technique, the effects of dragon's blood and its component loureirin B on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in trigeminal ganglion (TG) neurons were observed. The results show that both dragon's blood and loureirin B suppressed two types of peak sodium currents in a dose-dependent way. 0.1% dragon's blood and 0.2mmol/L loureirin B affected the activation and inactivation of sodium channels. The results further prove the analgetic mechanism of dragon's blood interfering with the nociceptive transmission. According to the above definition, loureirin B is the effective component in dragon's blood modulating sodium currents in TG neurons.

N-methyl-D-aspartate receptor- and calpain-mediated proteolytic cleavage of K+-Cl- cotransporter-2 impairs spinal chloride homeostasis in neuropathic pain

Loss of synaptic inhibition by γ-aminobutyric acid and glycine due to potassium chloride cotransporter-2 (KCC2) down-regulation in the spinal cord is a critical mechanism of synaptic plasticity in neuropathic pain. Here we present novel evidence that peripheral nerve injury diminishes glycine-mediated inhibition and induces a depolarizing shift in the reversal potential of glycine-mediated currents (E(glycine)) in spinal dorsal horn neurons. Blocking glutamate N-methyl-D-aspartate (NMDA) receptors normalizes synaptic inhibition, E(glycine), and KCC2 by nerve injury. Strikingly, nerve injury increases calcium-dependent calpain activity in the spinal cord that in turn causes KCC2 cleavage at the C terminus. Inhibiting calpain blocks KCC2 cleavage induced by nerve injury and NMDA, thereby normalizing E(glycine). Furthermore, calpain inhibition or silencing of μ-calpain at the spinal level reduces neuropathic pain. Thus, nerve injury promotes proteolytic cleavage of KCC2 through NMDA receptor-calpain activation, resulting in disruption of chloride homeostasis and diminished synaptic inhibition in the spinal cord. Targeting calpain may represent a new strategy for restoring KCC2 levels and tonic synaptic inhibition and for treating chronic neuropathic pain.

Upregulation of LENK and VIP in paracervical ganglion neurons supplying the urinary bladder of tetrodotoxin- and resiniferatoxin-treated female pigs

Both resiniferatoxin (RTX) and tetrodotoxin (TTX) have been reported to be effective in several clinical trials aiming to cure urinary bladder dysfunction. The goal of this experiment was to study the effect of intravesical administration of RTX and TTX on the chemical coding of paracervical ganglion (PCG) neurons that supply the urinary bladder in pigs. The vasoactive intestinal peptide (VIP) and the opioid family member Leu5-enkephalin (LENK) are both known for their regulatory effects in the function of the porcine genitourinary tract. The PCG neurons innervating the urinary bladder were identified by application of the retrograde tracer Fast Blue (FB), injected into the bladder wall prior to intravesical RTX or TTX administration. Immunocytochemical detection of LENK and VIP expression in the FB-labelled perikarya revealed that in the control group 25.15% of the FB-positive PCG neurons contained LENK, and 9.22% of them expressed VIP. Intravesical infusion of RTX resulted in an increase in the number of LENKIR neurons to 48.19% and VIP-IR perikarya to 11.25%. Optional treatment with TTX induced increase of LENK-IR neurons up to 81.67% and VIP-IR population to 16.46% of the FB-positive PCG cells. The present results show that both neurotoxins affect the chemical coding of PCG nervous cells supplying the porcine urinary bladder and that they stimulate both LENK and VIP expression. Furthermore, the results indicate a possible involvement of LENK and VIP neurons in the mechanisms of action of RTX and TTX in the therapy of overactive bladder disorder.

Ethanolic extract of Aconiti Brachypodi Radix attenuates nociceptive pain probably via inhibition of voltage-dependent Na⁺ channel

Aconiti Brachypodi Radix, belonging to the genus of Aconitum (Family Ranunculaceae), are used clinically as anti-rheumatic, anti-inflammatory and anti-nociceptive in traditional medicine of China. However, its mechanism and influence on nociceptive threshold are unknown and need further investigation. The analgesic effects of ethanolic extract of Aconiti Brachypodi Radix (EABR) were thus studied in vivo and in vitro. Three pain models in mice were used to assess the effect of EABR on nociceptive threshold. In vitro study was conducted to clarify the modulation of the extract on the tetrodotoxin-sensitive (TTX-S) sodium currents in rat's dorsal root ganglion (DRG) neurons using whole-cell patch clamp technique. The results showed that EABR (5-20 mg/kg, i.g.) could produce dose-dependent analgesic effect on hot-plate tests as well as writhing response induced by acetic acid. In addition, administration of 2.5-10 mg/kg EABR (i.g.) caused significant decrease in pain responses in the first and second phases of formalin test without altering the PGE₂ production in the hind paw of the mice. Moreover, EABR (10 µg/ml -1 mg/ml) could suppress TTX-S voltage-gated sodium currents in a dose-dependent way, indicating the underlying electrophysiological mechanism of the analgesic effect of the folk plant medicine. Collectively, our results indicated that EABR has analgesic property in three pain models and useful influence on TTX-S sodium currents in DRG neurons, suggesting that the interference with pain messages caused by the modulation of EABR on TTX-S sodium currents in DRG neurones may explain some of its analgesic effect.

Neuromedin U type 1 receptor stimulation of A-type K+ current requires the βγ subunits of Go protein, protein kinase A, and extracellular signal-regulated kinase 1/2 (ERK1/2) in sensory neurons

Although neuromedin U (NMU) has been implicated in analgesia, the detailed mechanisms still remain unclear. In this study, we identify a novel functional role of NMU type 1 receptor (NMUR1) in regulating the transient outward K(+) currents (I(A)) in small dorsal root ganglion (DRG) neurons. We found that NMU reversibly increased I(A) in a dose-dependent manner, instead the sustained delayed rectifier K(+) current (I(DR)) was not affected. This NMU-induced I(A) increase was pertussis toxin-sensitive and was totally reversed by NMUR1 knockdown. Intracellular application of GDPβS (guanosine 5'-O-(2-thiodiphosphate)), QEHA peptide, or a selective antibody raised against the Gα(o) or Gβ blocked the stimulatory effects of NMU. Pretreatment of the cells with the protein kinase A (PKA) inhibitor or ERK inhibitor abolished the NMU-induced I(A) response, whereas inhibition of phosphatidylinositol 3-kinase or PKC had no such effects. Exposure of DRG neurons to NMU markedly induced the phosphorylation of ERK (p-ERK), whereas p-JNK or p-p38 was not affected. Moreover, the NMU-induced p-ERK increase was attenuated by PKA inhibition and activation of PKA by foskolin would mimic the NMU-induced I(A) increase. Functionally, we observed a significant decrease of the firing rate of neuronal action potential induced by NMU and pretreatment of DRG neurons with 4-AP could abolish this effect. In summary, these results suggested that NMU increases I(A) via activation of NMUR1 that couples sequentially to the downstream activities of Gβγ of the G(o) protein, PKA, and ERK, which could contribute to its physiological functions including neuronal hypoexcitability in DRG neurons.

Increased vanilloid receptor VR1 innervation in vulvodynia

Vulvodynia is characterised by painful burning sensation, allodynia and hyperalgesia in the region of the vulval vestibulus. While in many patients the cause of vulvodynia remains uncertain, we and others have previously shown increased intraepithelial and papillary innervation in vulvodynia. The vanilloid receptor VR1 (TRPV1) is expressed by nociceptors, and is triggered by capsaicin, noxious heat, protons, and chemicals produced during inflammation. In the present study we show increased papillary VR1 fibres by immunostaining and image analysis in vulvodynia tissues compared to controls (p<0.002). VR1 expression was found to be significantly increased when the percentage area immunostained was expressed as a ratio of VR1 to PGP 9.5, a pan-neuronal marker (P=0.01). VR1-positive fine epidermal fibres also appeared to be increased in vulvodynia tissues, by inspection. Fibres immunoreactive to the voltage-gated sodium channel SNS1/PN3 (Nav1.8), also expressed by nociceptors, were relatively scarce in both vulvodynia and control tissues. We hypothesize that increased expression of VR1 by nociceptors could mediate some of the symptoms in vulvodynia, for which systemic or topical specific VR1 antagonists may provide novel treatment.

Pulsed radiofrequency of lumbar dorsal root ganglion for chronic postamputation phantom pain

Chronic pain following lower-limb amputation is now a well-known neuropathic, chronic-pain syndrome that usually presents as a combination of phantom and stump pain. Controlling these types of neuropathic pain is always complicated and challenging. If pharmacotherapy does not control the patient’s pain, interventional procedures have to be taken.

The aim of this study was to evaluate the efficacy of using pulsed radiofrequency (PRF) on the dorsal root ganglia at the L4 and L5 nerve roots to improve phantom pain.

Two patients with phantom pain were selected for the study. After a positive response to segmental nerve blockade at the L4 and L5 nerve roots, PRF was performed on the L4 and L5 dorsal root ganglia.

Global clinical improvement was good in one patient, with a 40% decrease in pain on the visual analogue scale (VAS) in 6 months, and moderate in the second patient, with a 30% decrease in pain scores in 4 months. PRF of the dorsal root ganglia at the L4 and L5 nerve roots may be an effective therapeutic option for patients with refractory phantom pain.

Relationship of axonal voltage-gated sodium channel 1.8 (NaV1.8) mRNA accumulation to sciatic nerve injury-induced painful neuropathy in rats

Painful peripheral neuropathy is a significant clinical problem; however, its pathological mechanism and effective treatments remain elusive. Increased peripheral expression of tetrodotoxin-resistant voltage-gated sodium channel 1.8 (NaV1.8) has been shown to associate with chronic pain symptoms in humans and experimental animals. Sciatic nerve entrapment (SNE) injury was used to develop neuropathic pain symptoms in rats, resulting in increased NaV1.8 mRNA in the injured nerve but not in dorsal root ganglia (DRG). To study the role of NaV1.8 mRNA in the pathogenesis of SNE-induced painful neuropathy, NaV1.8 shRNA vector was delivered by subcutaneous injection of cationized gelatin/plasmid DNA polyplex into the rat hindpaw and its subsequent retrograde transport via sciatic nerve to DRG. This in vivo NaV1.8 shRNA treatment reversibly and repeatedly attenuated the SNE-induced pain symptoms, an effect that became apparent following a distinct lag period of 3-4 days and lasted for 4-6 days before returning to pretreatment levels. Surprisingly, apparent knockdown of NaV1.8 mRNA occurred only in the injured nerve, not in the DRG, during the pain alleviation period. Levels of heteronuclear NaV1.8 RNA were unaffected by SNE or shRNA treatments, suggesting that transcription of the Scn10a gene encoding NaV1.8 was unchanged. Based on these data, we postulate that increased axonal mRNA transport results in accumulation of functional NaV1.8 protein in the injured nerve and the development of painful neuropathy symptoms. Thus, targeted delivery of agents that interfere with axonal NaV1.8 mRNA may represent effective neuropathic pain treatments.

Enhanced artemin/GFRα3 levels regulate mechanically insensitive, heat-sensitive C-fiber recruitment after axotomy and regeneration

We have shown recently that following saphenous nerve transection and successful regeneration, cutaneous polymodal nociceptors (CPMs) lacking transient receptor potential vanilloid 1 (TRPV1) are sensitized to heat stimuli and that mechanically insensitive, heat-sensitive C-fibers (CHs) that contain TRPV1 increase in prevalence. Target-derived neurotrophic factor levels were also enhanced after axotomy and regeneration. In particular, the glial-cell line-derived neurotrophic factor (GDNF) family member artemin was found to be significantly enhanced in the hairy hindpaw skin and its receptor GDNF family receptor α3 (GFRα3) was increased in the L2/L3 dorsal root ganglia (DRGs) following nerve injury. In this study, we assessed the role of enhanced artemin/GFRα3 levels on the changes in mouse cutaneous CH neurons following saphenous nerve regeneration. We used a newly developed siRNA-mediated in vivo knockdown strategy to specifically inhibit the injury-induced expression of GFRα3 and coupled this with an ex vivo recording preparation to examine response characteristics and neurochemical phenotype of different types of functionally defined neurons after injury. We found that inhibition of GFRα3 did not affect the axotomy-induced decrease in CPM threshold, but transiently prevented the recruitment of CH neurons. Western blot and real-time PCR analysis of hairy hindpaw skin and L2/L3 DRGs after saphenous nerve regeneration suggested that inhibition of the potential initial injury-induced increase in enhanced target-derived artemin signaling resulted in dynamic changes in TRPV1 expression after regeneration. These changes in TRPV1 expression may underlie the functional alterations observed in CH neurons after nerve regeneration.

L-acetylcarnitine: a proposed therapeutic agent for painful peripheral neuropathies

During the past two decades, many pharmacological strategies have been investigated for the management of painful neuropathies. However, neuropathic pain still remains a clinical challenge. A combination of therapies is often required, but unfortunately in most cases adequate pain relief is not achieved. Recently, attention has been focused on the physiological and pharmacological effects of L-acetylcarnitine in neurological disorders. There are a number of reports indicating that L-acetylcarnitine can be considered as a therapeutic agent in neuropathic disorders including painful peripheral neuropathies. In this review article, we will examine the antinociceptive and the neuroprotective effects of Lacetylcarnitine as tested in clinical studies and in animal models of nerve injury.

Use of Temporary Implantable Biomaterials to Reduce Leg Pain and Back Pain in Patients with Sciatica and Lumbar Disc Herniation

The principle etiology of leg pain (sciatica) from lumbar disc herniation is mechanical compression of the nerve root. Sciatica is reduced by decompression of the herniated disc, i.e., removing mechanical compression of the nerve root. Decompression surgery typically reduces sciatica more than lumbar back pain (LBP). Decompression surgery reduces mechanical compression of the nerve root. However, decompression surgery does not directly reduce sensitization of the sensory nerves in the epidural space and disc. In addition, sensory nerves in the annulus fibrosus and epidural space are not protected from topical interaction with pain mediators induced by decompression surgery. The secondary etiology of sciatica from lumbar disc herniation is sensitization of the nerve root. Sensitization of the nerve root results from a) mechanical compression, b) exposure to cellular pain mediators, and/or c) exposure to biochemical pain mediators. Although decompression surgery reduces nerve root compression, sensory nerve sensitization often persists. These observations are consistent with continued exposure of tissue in the epidural space, including the nerve root, to increased cellular and biochemical pain mediators following surgery. A potential contributor to lumbar back pain (LBP) is stimulation of sensory nerves in the annulus fibrosus by a) cellular pain mediators and/or b) biochemical pain mediators that accompany annular tears or disruption. Sensory fibers located in the outer one-third of the annulus fibrosus increase in number and depth as a result of disc herniation. The nucleus pulposus is comprised of material that can produce an autoimmune stimulation of the sensory nerves located in the annulus and epidural space leading to LBP. The sensory nerves of the annulus fibrosus and epidural space may be sensitized by topical exposure to cellular and biochemical pain mediators induced by lumbar surgery. Annulotomy or annular rupture allows the nucleus pulposus topical access to sensory nerve fibers, thereby leading to LBP. Coverage of the annulus and adjacent structures in the epidural space by absorbable viscoelastic gels appears to reduce LBP following surgery by protecting sensory fibers from cellular and biochemical pain mediators.

Postherpetic neuralgia: from preclinical models to the clinic

Postherpetic neuralgia (PHN), a common complication of herpes zoster, which results from reactivation of varicella zoster virus, is a challenging neuropathic pain syndrome. The incidence and severity of herpes zoster and PHN increases with immune impairment or age and may become a greater burden both in terms of health economics and individual suffering. A clearer understanding of the underlying mechanisms of this disease and translation of preclinical outcomes to the clinic may lead to more efficacious treatment options. Here we give an overview of recent findings from preclinical models and clinical research on PHN.

Sensitization of cutaneous nociceptors after nerve transection and regeneration: possible role of target-derived neurotrophic factor signaling

Damage to peripheral nerves is known to contribute to chronic pain states, including mechanical and thermal hyperalgesia and allodynia. It is unknown whether the establishment of these states is attributable to peripheral changes, central modifications, or both. In this study, we used several different approaches to assess the changes in myelinated (A) and unmyelinated (C) cutaneous nociceptors after transection and regeneration of the saphenous nerve. An ex vivo recording preparation was used to examine response characteristics and neurochemical phenotype of different types of functionally defined neurons. We found that myelinated nociceptors had significantly lower mechanical and thermal thresholds after regeneration, whereas C-polymodal nociceptors (CPMs) had lower heat thresholds. There was a significant increase in the percentage of mechanically insensitive C-fibers that responded to heat (CHs) after regeneration. Immunocytochemical analysis of identified afferents revealed that most CPMs were isolectin B4 (IB4) positive and transient receptor potential vanilloid 1 (TRPV1) negative, whereas CHs were always TRPV1 positive and IB4 negative in naive animals (Lawson et al., 2008). However, after regeneration, some identified CPMs and CHs stained positively for both markers, which was apparently attributable to an increase in the total number of IB4-positive neurons. Real-time PCR analysis of L2/L3 DRGs and hairy hindpaw skin at various times after saphenous nerve axotomy suggested multiple changes in neurotrophic factor signaling that correlated with either denervation or reinnervation of the cutaneous target. These changes may underlie the functional alterations observed after nerve regeneration and may explain how nerve damage leads to chronic pain conditions.

Understanding the nature and mechanism of foot pain

Approximately one-quarter of the population are affected by foot pain at any given time. It is often disabling and can impair mood, behaviour, self-care ability and overall quality of life. Currently, the nature and mechanism underlying many types of foot pain is not clearly understood. Here we comprehensively review the literature on foot pain, with specific reference to its definition, prevalence, aetiology and predictors, classification, measurement and impact. We also discuss the complexities of foot pain as a sensory, emotional and psychosocial experience in the context of clinical practice, therapeutic trials and the placebo effect. A deeper understanding of foot pain is needed to identify causal pathways, classify diagnoses, quantify severity, evaluate long term implications and better target clinical intervention.