Pain as a channelopathy

Physiological implications of the Slack channel in the central and peripheral nervous systems

The Slack channels, encoded by KCNT1, are found in neurons across the central nervous system (CNS) and peripheral nervous system (PNS), generating a delayed outward current known as sodium-activated potassium current (IKNa). IKNa plays a crucial role in shaping neuronal excitability and facilitating adaptation in response to sustained stimulation. Slack channels are crucial for neural plasticity and cognitive function; however, mutations in these channels cause profound learning and developmental abnormalities in humans. The physiological significance of Slack conductance in both systems is still unclear. This review provides a comprehensive overview of the role of Slack channels in both CNS and PNS, along with their associated modulators. It offers a summary of diseases that can result from abnormal expression of the Slack channel in both systems. Furthermore, identifying potent blockers and activators of Slack channels could greatly improve our understanding of Slack functions.

Decoding the neuroimmune axis in the atopic march: mechanisms and implications

The immune and nervous systems have co-evolved complex mechanisms to sense environmental dangers and orchestrate a concerted response to safeguard tissue and mobilize host defenses. This sophisticated interplay, marked by a shared repertoire of receptors and ligands, influences disease pathogenesis. Neuro-immune interactions in allergic diseases are pivotal for symptom development, from anaphylaxis to chronic conditions like asthma and atopic dermatitis. This review explores the neuro-immune interplay within the atopic march, emphasizing its role in host defense, inflammation resolution, and tissue repair. We delve into the multifaceted functions of nociceptors in orchestrating type 2 immune responses and the progression of allergic disorders, focusing on key regulators such as CGRP-RAMP1 and SP-MRGPRB2/A2. Additionally, we discuss the potential of nociceptor neuron-blocking drugs to target neuro-immunity, offering the possibility of reversing the progression of the atopic march. Altogether, we underscore the need for targeted interventions to disrupt the pathological processes and enhance therapeutic outcomes at various stages of the atopic march.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Small Fiber Neuropathy in Burning Mouth Syndrome: A Systematic Review

Burning mouth syndrome (BMS) is a chronic idiopathic orofacial pain disorder, characterized by persistent burning sensations and pain without clear pathological causes. Recent research suggests that small fiber neuropathy (SFN) may play a significant role in the neuropathic pain and sensory disturbances associated with BMS. Following PRISMA guidelines, this systematic review aims to evaluate and synthesize current evidence supporting SFN's involvement in BMS. The protocol is registered in PROSPERO (CRD42024555839). The results show eight studies reported reductions in nerve fiber density in tongue biopsies (ranging from 30% to 60%), along with morphological changes indicative of small fiber damage. Additionally, an increase in TRPV1-positive, NGF-positive, and P2X3-positive fibers, overexpression of Nav1.7, and slight underexpression of Nav1.9 mRNA were observed in BMS patients. Quantitative Sensory Testing in seven studies revealed sensory abnormalities such as reduced cool detection and cold pain thresholds. Blink reflex and corneal confocal microscopy also indicated peripheral and central small fiber damage, along with increased artemin mRNA expression. The evidence strongly supports SFN as a key factor in the pathophysiology of BMS, particularly due to reductions in nerve fiber density and altered sensory thresholds. However, variability across studies highlights the need for larger, standardized research to establish causal relationships and guide therapeutic strategies.

Effects of Wrist Stability Training Combined with Grip Strength Exercise on Pain and Function in Patients with Nonspecific Chronic Wrist Pain

Background and Objectives: Non-specific chronic wrist pain is wrist pain that occurs without a specific cause, such as trauma, and may limit the range of motion of the joints of the wrist and hand, affecting muscle strength, grip strength, and function. This study aimed to determine the effects of grip-strengthening exercises combined with wrist stability training on pain and function in patients with non-specific chronic wrist pain. Materials and Methods: The subjects of the study were 31 patients with wrist pain. To determine the effect of grip-strengthening exercises combined with wrist stability training, 15 participants participated in grip-strengthening exercises combined with wrist stability training and 16 control subjects participated. The experimental group participated in wrist-stability training. Grip-strengthening exercises combined with wrist stability training were performed for 20 min/day twice a week for 4 weeks, and relaxation massage and conservative physical therapy were performed for 20 min/day twice a week for 4 weeks. The control group received relaxation massage and conservative physical therapy for 40 min/day twice a week for 4 weeks. A visual pain scale was used to evaluate the degree of pain before and after treatment, and a patient-rated wrist evaluation was used to evaluate wrist function. Results: The results showed that the visual score significantly decreased in the time effect before and after the intervention in both groups (p < 0.001), patient-rated wrist evaluation significantly decreased (p < 0.001), and grip strength and muscle strength significantly increased (p < 0.001). The results of this study showed that grip-strengthening exercises combined with wrist stability training were effective in improving pain, function, grip strength, and muscle strength in patients with non-specific chronic wrist pain. Conclusions: Grip-strengthening exercises combined with wrist stability training can be used as an effective intervention method to improve pain, function, grip strength, and muscle strength, emphasizing the need for wrist exercise interventions in patients with non-specific chronic wrist pain in the future.

Molecular characterization of positively selected genes contributing aquatic adaptation in marine mammals

BACKGROUND

Marine mammals, which have evolved independently into three distinct lineages, share common physiological features that contribute to their adaptation to the marine environment.

OBJECTIVE

To identify positively selected genes (PSGs) for adaptation to the marine environment using available genomic data from three taxonomic orders: cetaceans, pinnipeds, and sirenians.

METHODS

Based on the genomes within each group of Artiodactyla, Carnivora and Afrotheria, we performed selection analysis using the branch-site model in CODEML.

RESULTS

Based on the branch-site model, 460, 614, and 359 PSGs were predicted for the cetaceans, pinnipeds, and sirenians, respectively. Functional enrichment analysis indicated that genes associated with hemostasis were positively selected across all lineages of marine mammals. We observed positive selection signals for the hemostasis and coagulation-related genes plasminogen activator, urokinase (PLAU), multimerin 1 (MMRN1), gamma-glutamyl carboxylase (GGCX), and platelet endothelial aggregation receptor 1 (PEAR1). Additionally, we found out that the sodium voltage-gated channel alpha subunit 9 (SCN9A), serine/arginine repetitive matrix 4 (SRRM4), and Ki-ras-induced actin-interacting protein (KRAP) are under positive selection pressure and are associated with cognition, neurite outgrowth, and IP3-mediated Ca2 + release, respectively.

CONCLUSION

This study will contribute to our understanding of the adaptive evolution of marine mammals by providing information on a group of candidate genes that are predicted to influence adaptation to aquatic environments, as well as their functional characteristics.

Genetic Analysis of SCN11A, SCN10A, and SCN9A in Familial Episodic Pain Syndrome (FEPS) in Japan and Proposal of Clinical Diagnostic Criteria

Familial episodic pain syndrome (FEPS) is an early childhood onset disorder of severe episodic limb pain caused mainly by pathogenic variants of SCN11A, SCN10A, and SCN9A, which encode three voltage-gated sodium channels (VGSCs) expressed as key determinants of nociceptor excitability in primary sensory neurons. There may still be many undiagnosed patients with FEPS. A better understanding of the associated pathogenesis, epidemiology, and clinical characteristics is needed to provide appropriate diagnosis and care. For this study, nationwide recruitment of Japanese patients was conducted using provisional clinical diagnostic criteria, followed by genetic testing for SCN11A, SCN10A, and SCN9A. In the cohort of 212 recruited patients, genetic testing revealed that 64 patients (30.2%) harbored pathogenic or likely pathogenic variants of these genes, consisting of 42 (19.8%), 14 (6.60%), and 8 (3.77%) patients with variants of SCN11A, SCN10A, and SCN9A, respectively. Meanwhile, the proportions of patients meeting the tentative clinical criteria were 89.1%, 52.0%, and 54.5% among patients with pathogenic or likely pathogenic variants of each of the three genes, suggesting the validity of these clinical criteria, especially for patients with SCN11A variants. These clinical diagnostic criteria of FEPS will accelerate the recruitment of patients with underlying pathogenic variants who are unexpectedly prevalent in Japan.

Syntaxin1A overexpression and pain insensitivity in individuals with 7q11.23 duplication syndrome

Genetic modifications leading to pain insensitivity phenotypes, while rare, provide invaluable insights into the molecular biology of pain and reveal targets for analgesic drugs. Pain insensitivity typically results from Mendelian loss-of-function mutations in genes expressed in nociceptive (pain-sensing) dorsal root ganglion (DRG) neurons that connect the body to the spinal cord. We document a pain insensitivity mechanism arising from gene overexpression in individuals with the rare 7q11.23 duplication syndrome (Dup7), who have 3 copies of the approximately 1.5-megabase Williams syndrome (WS) critical region. Based on parental accounts and pain ratings, people with Dup7, mainly children in this study, are pain insensitive following serious injury to skin, bones, teeth, or viscera. In contrast, diploid siblings (2 copies of the WS critical region) and individuals with WS (1 copy) show standard reactions to painful events. A converging series of human assessments and cross-species cell biological and transcriptomic studies identified 1 likely candidate in the WS critical region, STX1A, as underlying the pain insensitivity phenotype. STX1A codes for the synaptic vesicle fusion protein syntaxin1A. Excess syntaxin1A was demonstrated to compromise neuropeptide exocytosis from nociceptive DRG neurons. Taken together, these data indicate a mechanism for producing "genetic analgesia" in Dup7 and offer previously untargeted routes to pain control.

A microfluidic model of the first sensory synapse for analgesic target discovery

The synaptic connections between dorsal root ganglia (DRG) and dorsal horn (DH) neurons are a crucial relay point for the transmission of painful stimuli. To delineate how synaptic plasticity may modulate the excitability of DH neurons, we have devised a microfluidic co-culture model that recapitulates the first sensory synapse using postnatal mouse sensory neurons. We show that DRG-DH co-cultures characterize salient features of the in vivo physiology of sensory neurons. Immunocytcochemical experiments of the cultured DH neurons show a co-localization of Map2 with VGlut2 and of Map2 with Synapsin 1, corroborating the glutamatergic identity of the DH neurons and further suggesting the potential formation of active synapses in this neuronal set. Fluorometric imaging experiments demonstrate the elicitation of calcium responses in DH neurons following the stimulation of DRG cell bodies or axons. Selective NMDA and AMPA receptor blockade appreciably silences DH neuron responses, suggesting that glutamatergic signaling is maintained in vitro. Last, a surrogate model of peripheral nerve injury is introduced in the form of an axotomy, which results in elevated and prolonged calcium responses of DH neurons. Overall, the microfluidic mouse co-cultures provide a method advancement in the study of periphery-to-center pain signaling, where the potential of utilizing the platform for drug target identification is underscored.

Ion channels in cancer-induced bone pain: from molecular mechanisms to clinical applications

Cancer-induced bone pain (CIBP) caused by bone metastasis is one of the most prevalent diseases, and current treatments rely primarily on opioids, which have significant side effects. However, recent developments in pharmaceutical science have identified several new mechanisms for CIBP, including the targeted modification of certain ion channels and receptors. Ion channels are transmembrane proteins, which are situated on biological cell membranes, which facilitate passive transport of inorganic ions across membranes. They are involved in various physiological processes, including transmission of pain signals in the nervous system. In recent years, there has been an increasing interest in the role of ion channels in chronic pain, including CIBP. Therefore, in this review, we summarize the current literature on ion channels, related receptors, and drugs and explore the mechanism of CIBP. Targeting ion channels and regulating their activity might be key to treating pain associated with bone cancer and offer new treatment avenues.

The Biology of Chronic Pain and Its Implications for Pain Neuroscience Education: State of the Art

Pain is an individualized experience for the person suffering from chronic pain. Significant strides have been made in the last few decades in understanding various biological changes that coincide with chronic pain. This state-of-the-art overview looks at the current evidence related to the biology of chronic pain and the implications these findings have on the delivery of pain neuroscience education (PNE). The paper summarizes the various (epi)genetic, neural, endocrine, and immune factors discovered and explored in the scientific literature concerning chronic pain. Each of these biological factors has various implications for the content and delivery of PNE. We discuss the future directions these biological factors have for the clinical implementation of PNE by linking the importance of behavior change, optimizing the learning environment, and using an individualized multimodal treatment approach with PNE. In addition, future directions for research of PNE based on these biological factors are provided with importance placed on individualized patient-centered care and how PNE can be used with traditional modes of care and growing trends with other care methods. PNE was originally and continues to be rooted in understanding chronic pain biology and how that understanding can improve patient care and outcomes.

Rubbing Salt in the Wound: Molecular Evolutionary Analysis of Pain-Related Genes Reveals the Pain Adaptation of Cetaceans in Seawater

Pain, usually caused by a strong or disruptive stimulus, is an unpleasant sensation that serves as a warning to organisms. To adapt to extreme environments, some terrestrial animals have evolved to be inherently insensitive to pain. Cetaceans are known as supposedly indifferent to pain from soft tissue injury representatives of marine mammals. However, the molecular mechanisms that explain how cetaceans are adapted to pain in response to seawater environment remain unclear. Here, we performed a molecular evolutionary analysis of pain-related genes in selected representatives of cetaceans. ASIC4 gene was identified to be pseudogenized in all odontocetes (toothed whales) except from Physeter macrocephalus (sperm whales), and relaxed selection of this gene was detected in toothed whales with pseudogenized ASIC4. In addition, positive selection was detected in pain perception (i.e., ASIC3, ANO1, CCK, and SCN9A) and analgesia (i.e., ASIC3, ANO1, CCK, and SCN9A) genes among the examined cetaceans. In this study, potential convergent amino acid substitutions within predicted proteins were found among the examined cetaceans and other terrestrial mammals, inhabiting extreme environments (e.g., V441I of TRPV1 in cetaceans and naked mole rats). Moreover, specific amino acid substitutions within predicted sequences of several proteins were found in the studied representatives of cetaceans (e.g., F56L and D163A of ASIC3, E88G of GRK2, and F159L of OPRD1). Most of the substitutions were located within important functional domains of proteins, affecting their protein functions. The above evidence suggests that cetaceans might have undergone adaptive molecular evolution in pain-related genes through different evolutionary patterns to adapt to pain, resulting in greater sensitivity to pain and more effective analgesia. This study could have implications for diagnosis and treatment of human pain.

Familial Episodic Pain Syndromes

Over the past decades, advances in genetic sequencing have opened a new world of discovery of causative genes associated with numerous pain-related syndromes. Familial episodic pain syndromes (FEPS) are one of the distinctive syndromes characterized by early-childhood onset of severe episodic pain mainly affecting the distal extremities and tend to attenuate or diminish with age. According to the phenotypic and genetic properties, FEPS at least includes four subtypes of FEPS1, FEPS2, FEPS3, and FEPS4, which are caused by mutations in the TRPA1, SCN10A, SCN11A, and SCN9A genes, respectively. Functional studies have revealed that all missense mutations in these genes are closely associated with the gain-of-function of cation channels. Because some FEPS patients may show a relative treatability and favorable prognosis, it is worth paying attention to the diagnosis and management of FEPS as early as possible. In this review, we state the common clinical manifestations, pathogenic mechanisms, and potential therapies of the disease, and provide preliminary opinions about future research for FEPS.

Suppression of ASIC activity by the activation of A1 adenosine receptors in rat primary sensory neurons

Peripheral A1 adenosine receptor signaling has been shown to have analgesic effects in a variety of pain conditions. However, it is not yet fully elucidated for the precise molecular mechanisms. Acid sensing ion channels (ASICs) are expressed predominantly in nociceptive sensory neurons responding to protons. Given that both A1 adenosine receptors and ASICs are present in dorsal root ganglia (DRG) neurons, we therefore investigated whether there was a cross-talk between the two types of receptors. Herein, electrophysiological recordings showed that the A1 adenosine receptor agonist N⁶-cyclopentyladenosine (CPA) suppressed acid-induced currents and action potentials, which were mediated by ASICs, in rat DRG neurons. CPA inhibited the maximum response to protons, as shown a downward shift of concentration-response curve for protons. The CPA-induced suppression of ASIC currents was blocked by the A1 adenosine receptor antagonist KW-3902 and also prevented by intracellular application of the G_i/o-protein inhibitor pertussis toxin, the adenylate cyclase activator forskolin, and the cAMP analog 8-Br-cAMP. Finally, intraplantar pretreatment of CPA dose-dependently relieved acid-induced nociceptive responses in rats through peripheral A1 adenosine receptors. These results suggested that CPA suppressed ASICs via A1 adenosine receptors and intracellular G_i/o-proteins and cAMP signaling cascades in rat DRG neurons, which was a novel potential mechanism underlying analgesia of peripheral A1 adenosine receptors.

The widely used antihistamine mepyramine causes topical pain relief through direct blockade of nociceptor sodium channels

Mepyramine, a first-generation antihistamine targeting the histamine H(1) receptor, was extensively prescribed to patients suffering from allergic reactions and urticaria. Serious adverse effects, especially in case of overdose, were frequently reported, including drowsiness, impaired thinking, convulsion, and coma. Many of these side effects were associated with the blockade of histaminergic or cholinergic receptors. Here we show that mepyramine directly inhibits a variety of voltage-gated sodium channels, including the Tetrodotoxin-sensitive isoforms and the main isoforms (Nav1.7, Nav1.8, and Nav1.9) of nociceptors. Estimated IC₅₀ were within the range of drug concentrations detected in poisoned patients. Mepyramine inhibited sodium channels through fast- or slow-inactivated state preference depending on the isoform. Moreover, mepyramine inhibited the firing responses of C- and Aβ-type nerve fibers in ex vivo skin-nerve preparations. Locally applied mepyramine had analgesic effects on the scorpion toxin-induced excruciating pain and produced pain relief in acute, inflammatory, and chronic pain models. Collectively, these data provide evidence that mepyramine has the potential to be developed as a topical analgesic agent.

Selective Expression of a SNARE-Cleaving Protease in Peripheral Sensory Neurons Attenuates Pain-Related Gene Transcription and Neuropeptide Release

Chronic pain is a leading health and socioeconomic problem and an unmet need exists for long-lasting analgesics. SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are required for neuropeptide release and noxious signal transducer surface trafficking, thus, selective expression of the SNARE-cleaving light-chain protease of botulinum neurotoxin A (LCA) in peripheral sensory neurons could alleviate chronic pain. However, a safety concern to this approach is the lack of a sensory neuronal promoter to prevent the expression of LCA in the central nervous system. Towards this, we exploit the unique characteristics of Pirt (phosphoinositide-interacting regulator of TRP), which is expressed in peripheral nociceptive neurons. For the first time, we identified a Pirt promoter element and cloned it into a lentiviral vector driving transgene expression selectively in peripheral sensory neurons. Pirt promoter driven-LCA expression yielded rapid and concentration-dependent cleavage of SNAP-25 in cultured sensory neurons. Moreover, the transcripts of pain-related genes (TAC1, tachykinin precursor 1; CALCB, calcitonin gene-related peptide 2; HTR3A, 5-hydroxytryptamine receptor 3A; NPY2R, neuropeptide Y receptor Y2; GPR52, G protein-coupled receptor 52; SCN9A, sodium voltage-gated channel alpha subunit 9; TRPV1 and TRPA1, transient receptor potential cation channel subfamily V member 1 and subfamily A member 1) in pro-inflammatory cytokines stimulated sensory neurons were downregulated by viral mediated expression of LCA. Furthermore, viral expression of LCA yielded long-lasting inhibition of pain mediator release. Thus, we show that the engineered Pirt-LCA virus may provide a novel means for long lasting pain relief.

An overview on transient receptor potential channels superfamily

The transient receptor potential (TRP) channel superfamily is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling and are involved in plethora of animal behaviors. These channels are activated through a wide variety of mechanisms and participate in virtually every sensory modality. Modulating TRP channel activity provides an important way to regulate membrane excitability and intracellular calcium levels. This is reflected by the fact that small molecule compounds modulating different TRPs have all entered clinical trials for a variety of diseases. The role of TRPs will be further elucidated in complex diseases of the nervous, intestinal, renal, urogenital, respiratory, and cardiovascular systems in diverse therapeutic areas including pain and itch, headache, pulmonary function, oncology, neurology, visceral organs, and genetic diseases. This review focuses on recent developments in the TRP ion channel-related area and highlights evidence supporting TRP channels as promising targets for new analgesic drugs for therapeutic intervention. This review presents a variety of: (1) phylogeny aspects of TRP channels; (2) some structural and functional characteristics of TRPs; (3) a general view and short characteristics of main seven subfamilies of TRP channels; (4) the evidence for consider TRP channels as therapeutic and analgesic targets; and finally (5) further perspectives of TRP channels research.

Ion channels and pain in Fabry disease

Fabry disease (FD) is a progressive, X-linked inherited disorder of glycosphingolipid metabolism due to deficient or absent lysosomal α-galactosidase A (α-Gal A) activity which results in progressive accumulation of globotriaosylceramide (Gb3) and related metabolites. One prominent feature of Fabry disease is neuropathic pain. Accumulation of Gb3 has been documented in dorsal root ganglia (DRG) as well as other neurons, and has lately been associated with the mechanism of pain though the pathophysiology is still unclear. Small fiber (SF) neuropathy in FD differs from other entities in several aspects related to the perception of pain, alteration of fibers as well as drug therapies used in the practice with patients, with therapies far from satisfying. In order to develop better treatments, more information on the underlying mechanisms of pain is needed. Research in neuropathy has gained momentum from the development of preclinical models where different aspects of pain can be modelled and further analyzed. This review aims at describing the different in vitro and FD animal models that have been used so far, as well as some of the insights gained from their use. We focus especially in recent findings associated with ion channel alterations -that apart from the vascular alterations-, could provide targets for improved therapies in pain.

Review/overview of pain in sickle cell disease

Sickle cell disease (SCD) is a highly complex inherited disorder of hemoglobin structure. Although the molecular lesion is a single-point mutation, the sickle gene is pleiotropic in nature causing multiple phenotypic expressions that constitute the various complications of the disease. Its manifestations could be acute, chronic, nociceptive, neuropathic that could occur singly or in various combinations. Pain continues to be the major factor of SCD phenotypic complications and the most common cause of admissions to the Emergency Department and/or the hospital. Although progress has been made in understanding the pathophysiology of SCD as well as in developing curative therapies such as hematopoietic stem cell transplantation and gene therapy, effective pain management continues to lag behind. Palliative therapies continue to be the major approach to the management of SCD and its complications. The advent of hydroxyurea made partial success in preventing the frequency of vaso-occlusive crises and l-glutamine awaits post-trial confirmation of benefits. The search for additional pharmacotherapeutic agents that could be used singly or in combination with hydroxyurea and/or l-glutamine awaits their dawn hopefully in the near future. The purpose of this review is to describe the various manifestations of SCD, their pathophysiology and their current management. Recent impressive advances in understanding the pathophysiology of pain promise the determination of agents that could replace or minimize the use of opioids.

Receptors for pro-resolving mediators are increased in Alzheimer's disease brain

Neuroinflammation is a key element of AD pathology and conceivably a result of a disturbed resolution. Resolution of inflammation is an active process which is strictly orchestrated following the acute inflammatory response after removal of the inflammatory stimuli. Acute inflammation is actively terminated by specialized pro‐resolving mediators (SPMs) thereby promoting healing and return to homeostasis. Failed resolution may contribute to persistent neuroinflammation and aggravate AD pathology. BLT1 (leukotriene B₄ receptor) and ChemR23 (chemerin receptor 23) are receptors for the SPM resolvin (Rv) E1 and are important clinical targets for ending inflammation. In AD, the levels of SPMs are decreased, and pro‐inflammatory mediators are increased. In the current study, the distribution of BLT1 and ChemR23 receptors in control brains and in AD as well as correlations with AD pathology was examined for the first time. BLT1 and ChemR23 were analyzed in different regions of post‐mortem human brain from cases with AD, early‐onset AD and mild cognitive impairment (MCI) and healthy controls, using western blotting and immunohistochemistry. BLT1 and ChemR23 were detected in neurons and glial cells in all examined regions of the human brain, with markedly higher levels in AD than in controls. The receptor levels correlated with the density of staining for the inflammation markers HLA‐DR and YKL‐40 for microglia and astrocytes, respectively, and elevated staining coincided with high Braak stages in AD. The relative staining densities of these receptors were higher in the basal forebrain, cingulate gyrus and hippocampal regions compared to the cerebellum and frontal cortex (BA46).

In conclusion, alterations in the expression of the resolution receptor BLT1 in AD have not been reported previously and the changes in both BLT1 and ChemR23 suggest a disturbed resolution pathway in several regions of the AD brain that may play a role in disease pathology.

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

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

Treatment of an Upper Extremity Chronic Repetitive Strain Injury of 28 Years Duration in a Professional Jazz Saxophonist Using 5% Dextrose

The treatment of chronic repetitive strain injury is a frustrating discourse of potential pathoanatomical causes and their treatments. This case describes an overlooked pathoanatomical cause and successful treatment for a chronic and debilitating repetitive strain injury of the upper extremities that lasted for 28 years and was resistant to a variety of conventional and alternative treatments in a professional jazz saxophone player. A series of fascial tissue infiltrations using 5% dextrose was used to successfully downregulate c-fiber activity within the upper extremities. This treatment resulted in the complete resolution of upper extremity pain and dysfunction with a full return to normal instrument practice and performance that has been sustained without recurrence for four years following treatment.

Kv7 channels a potential therapeutic target in fibromyalgia: A hypothesis

Fibromyalgia is characterized by the primary symptoms of persistent diffuse pain, fatigue, sleep disturbance and cognitive dysfunction. Persistent pain conditions, such as fibromyalgia, are often refractory to current available therapies. An involvement of K⁺ channels in the pathophysiology of fibromyalgia is emerging and supported by drug treatments for this condition exhibiting action at these molecular processes. K⁺ channels constitute potential novel target candidates for pain therapy offering peripheral and/or central actions. The Kv7 channel activators, flupirtine and retigabine, have exhibited pharmacological profiles compatible to the requirements needed for use as a therapeutic approach to fibromyalgia. Clinical trials to address the multidimensional challenges of fibromyalgia with flupirtine and retigabine will provide important insight to the role of K⁺ channels in this condition.

Region-Resolved Quantitative Proteome Profiling Reveals Molecular Dynamics Associated With Chronic Pain in the PNS and Spinal Cord

To obtain a thorough understanding of chronic pain, large-scale molecular mapping of the pain axis at the protein level is necessary, but has not yet been achieved. We applied quantitative proteome profiling to build a comprehensive protein compendium of three regions of the pain neuraxis in mice: the sciatic nerve (SN), the dorsal root ganglia (DRG), and the spinal cord (SC). Furthermore, extensive bioinformatics analysis enabled us to reveal unique protein subsets which are specifically enriched in the peripheral nervous system (PNS) and SC. The immense value of these datasets for the scientific community is highlighted by validation experiments, where we monitored protein network dynamics during neuropathic pain. Here, we resolved profound region-specific differences and distinct changes of PNS-enriched proteins under pathological conditions. Overall, we provide a unique and validated systems biology proteome resource (summarized in our online database painproteome.em.mpg.de), which facilitates mechanistic insights into somatosensory biology and chronic pain—a prerequisite for the identification of novel therapeutic targets.

Targeting the TREK-1 potassium channel via riluzole to eliminate the neuropathic and depressive-like effects of oxaliplatin

Neurotoxicity remains the most common adverse effect of oxaliplatin, limiting its clinical use. In the present study, we developed a mouse model of chronic oxaliplatin-induced neuropathy, which mimics both sensory and motor deficits observed in patients, in a clinically relevant time course. Repeated oxaliplatin administration in mice induced both cephalic and extracephalic long lasting mechanical and cold hypersensitivity after the first injection as well as delayed sensorimotor deficits and a depression-like phenotype. Using this model, we report that riluzole prevents both sensory and motor deficits induced by oxaliplatin as well as the depression-like phenotype induced by cumulative chemotherapeutic drug doses. All the beneficial effects are due to riluzole action on the TREK-1 potassium channel, which plays a central role in its therapeutic action. Riluzole has no negative effect on oxaliplatin antiproliferative capacity in human colorectal cancer cells and on its anticancer effect in a mouse model of colorectal cancer. Moreover, riluzole decreases human colorectal cancer cell line viability in vitro and inhibits polyp development in vivo. The present data in mice may support the need to clinically test riluzole in oxaliplatin-treated cancer patients and state for the important role of the TREK-1 channel in pain perception.

Pain in Urology: Pathophysiological Aspects of Pain and Chronicity

Chronic pain has been traditionally defined by pain duration, but this approach has limited empirical support and does not account for chronic pain multidimensionality. Defining chronic pain solely by duration is based on the view that acute pain signals potential tissue damage, whereas chronic pain results from central sensitization in which pain is sustained after nociceptive inputs have diminished. Chronic urological pain is a prevalent condition, which can represent a major challenge to health care providers due to its complex aetiology and poor response to therapy. In most cases, clear signs of on-going tissue trauma, inflammation or infection are not present. Despite this, more underhanded pathophysiological mechanisms, affecting the urinary system or other pelvic organ systems (musculoskeletal, neurologic, urologic, gynaecologic) and some psychological aspects may be present. In this article, some pathophysiological aspects of visceral pain are discussed; the definition of ‘chronic pain’, the mechanism of action of drugs used in the treatment of pain and the rationale for association therapy are also reviewed.

Role of Asic3, Nav1.7 and Nav1.8 in Electroacupuncture-Induced Analgesia in a Mouse Model of Fibromyalgia Pain

Background

The mechanisms underlying fibromyalgia (FM) pain are not understood. The US Food and Drug Administration has recommended three drugs for treating FM—namely, pregabalin, duloxetine and milnacipran; however, these medications are associated with severe side effects.

Objective

To create a mouse model of FM pain using dual injections of acidic saline to cause mechanical hyperalgesia and test whether ASIC3, Nav1.7 and Nav1.8 are involved in this process and whether electroacupuncture (EA) can reverse these phenomena.

Methods

The FM model was established by injecting acidic saline twice into 40 ICR mice. The mice were assigned to subgroups (n=8 each) treated with different EA frequencies (2, 15 and 50 Hz). ASIC3, Nav1.7 and Nav1.8 expression levels were measured by Western blotting and immunohistochemistry.

Results

Significant mechanical hyperalgesia was induced on day 8 in FM mice, which was reversed by 2, 15 and 50 Hz EA. ASIC3, Nav1.7 and Nav1.8 protein levels increased significantly in both the dorsal root ganglion and in the spinal cord of FM model mice. These changes were further attenuated by 2, 15 and 50 Hz EA.

Conclusion

Reduced nociceptive ASIC3, Nav1.7 and Nav1.8 proteins are involved in the preventive effects of EA against FM, and this series of molecules may represent targets for FM treatment.

SHANK3 Deficiency Impairs Heat Hyperalgesia and TRPV1 Signaling in Primary Sensory Neurons

Abnormal pain sensitivity is commonly associated with autism spectrum disorders (ASDs) and affects the life quality of ASD individuals. SHANK3 deficiency was implicated in ASD and pain dysregulation. Here, we report functional expression of SHANK3 in mouse dorsal root ganglion (DRG) sensory neurons and spinal cord presynaptic terminals. Homozygous and heterozygous Shank3 complete knockout (Δe4-22) results in impaired heat hyperalgesia in inflammatory and neuropathic pain. Specific deletion of Shank3 in Nav1.8-expressing sensory neurons also impairs heat hyperalgesia in homozygous and heterozygous mice. SHANK3 interacts with transient receptor potential subtype V1 (TRPV1) via Proline-rich region and regulates TRPV1 surface expression. Furthermore, capsaicin-induced spontaneous pain, inward currents in DRG neurons, and synaptic currents in spinal cord neurons are all reduced after Shank3 haploinsufficiency. Finally, partial knockdown of SHANK3 expression in human DRG neurons abrogates TRPV1 function. Our findings reveal a peripheral mechanism of SHANK3, which may underlie pain deficits in SHANK3-related ASDs.

Ion channelopathies in functional GI disorders

In the gastrointestinal (GI) tract, abnormalities in secretion, absorption, motility, and sensation have been implicated in functional gastrointestinal disorders (FGIDs). Ion channels play important roles in all these GI functions. Disruptions of ion channels' ability to conduct ions can lead to diseases called ion channelopathies. Channelopathies can result from changes in ion channel biophysical function or expression due to mutations, posttranslational modification, and accessory protein malfunction. Channelopathies are strongly established in the fields of cardiology and neurology, but ion channelopathies are only beginning to be recognized in gastroenterology. In this review, we describe the state of the emerging field of GI ion channelopathies. Several recent discoveries show that channelopathies result in alterations in GI motility, secretion, and sensation. In the epithelium, mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) or CFTR-associating proteins result in channelopathies with constipation or diarrhea as phenotypes. In the muscle, mutations in the SCN5A-encoded voltage-gated sodium channel Na_V1.5 are associated with irritable bowel syndrome. In the sensory nerves, channelopathies of voltage-gated sodium channels Na_V1.7 and Na_V1.9 (encoded by SCN9A, SCN11A, respectively) manifest by either GI hyper- or hyposensation. Recent advances in structural biology and ion channel biophysics, coupled with personalized medicine, have fueled rapid discoveries of novel channelopathies and direct drug targeting of specific channelopathies. In summary, the emerging field of GI ion channelopathies has significant implications for functional GI disease stratification, diagnosis, and treatment.

Standardized Profiling of The Membrane-Enriched Proteome of Mouse Dorsal Root Ganglia (DRG) Provides Novel Insights Into Chronic Pain

Chronic pain is a complex disease with limited treatment options. Several profiling efforts have been employed with the aim to dissect its molecular underpinnings. However, generated results are often inconsistent and nonoverlapping, which is largely because of inherent technical constraints. Emerging data-independent acquisition (DIA)-mass spectrometry (MS) has the potential to provide unbiased, reproducible and quantitative proteome maps - a prerequisite for standardization among experiments. Here, we designed a DIA-based proteomics workflow to profile changes in the abundance of dorsal root ganglia (DRG) proteins in two mouse models of chronic pain, inflammatory and neuropathic. We generated a DRG-specific spectral library containing 3067 DRG proteins, which enables their standardized quantification by means of DIA-MS in any laboratory. Using this resource, we profiled 2526 DRG proteins in each biological replicate of both chronic pain models and respective controls with unprecedented reproducibility. We detected numerous differentially regulated proteins, the majority of which exhibited pain model-specificity. Our approach recapitulates known biology and discovers dozens of proteins that have not been characterized in the somatosensory system before. Functional validation experiments and analysis of mouse pain behaviors demonstrate that indeed meaningful protein alterations were discovered. These results illustrate how the application of DIA-MS can open new avenues to achieve the long-awaited standardization in the molecular dissection of pathologies of the somatosensory system. Therefore, our findings provide a valuable framework to qualitatively extend our understanding of chronic pain and somatosensation.

Future Directions in Pain Management: Integrating Anatomically Selective Delivery Techniques With Novel Molecularly Selective Agents

Treatment for chronic, locoregional pain ranks among the most prevalent unmet medical needs. The failure of systemic analgesic drugs, such as opioids, is often due to their off-target toxicity, development of tolerance, and abuse potential. Interventional pain procedures provide target specificity but lack pharmacologically selective agents with long-term efficacy. Gene therapy vectors are a new tool for the development of molecularly selective pain therapies, which have already been proved to provide durable analgesia in preclinical models. Taken together, advances in image-guided delivery and gene therapy may lead to a new class of dual selective analgesic treatments integrating the molecular selectivity of analgesic genes with the anatomic selectivity of interventional delivery techniques.

Neuroimmune interactions in itch: Do chronic itch, chronic pain, and chronic cough share similar mechanisms?

Itch and pain are closely related but also clearly distinct sensations. Pain is known to suppress itch, while analgesics such as morphine can provoke itch. However, in pathological and chronic conditions, pain and itch also have similarities. Dysfunction of the nervous system, as manifested by neural plastic changes in primary sensory neurons of the peripheral nervous system (peripheral sensitization) and spinal cord and brain stem neurons in the central nervous system (central sensitization) will result in chronic pain and itch. Importantly, these diseases also result from immune dysfunction, since inflammatory mediators can directly activate or sensitize nociceptive and pruriceptive neurons in the peripheral and central nervous system, leading to pain and itch hypersensitivity. In this mini-review, I discuss the roles of Toll-like receptors (TLRs), transient receptor potential ankyrin 1 (TRPA1) ion channel, and Nav1.7 sodium channel in regulating itch and inflammation, with special emphasis of neuronal TLR signaling and the interaction of TLR7 and TRPA1. Chronic pain and chronic itch are debilitating diseases and dramatically impact the life quality of patients. Targeting TLRs for the control of inflammation, neuroinflammation (inflammation restricted in the nervous system), and hyperexcitability of nociceptors and pruriceptors will lead to new therapeutics for the relief of chronic pain and chronic itch. Finally, given the shared mechanisms among chronic cough, chronic pain, and chronic itch and the demonstrated efficacy of the neuropathic pain drug gabapentin in treating chronic cough, novel therapeutics targeting TRPA1, Nav1.7, and TLRs may also help to alleviate refractory cough via modulating neuron-immune interaction.

Hyperexcitability and sensitization of sodium channels of dorsal root ganglion neurons in a rat model of lumber disc herniation

PURPOSE

Low back pain and sciatica are the most common symptoms of patients with lumbar disc herniation (LDH). The pathophysiology of lumbocrural pain and sciatica is not fully understood. The aim of the present study was to define the membrane properties and activities of voltage-gated sodium channels of dorsal root ganglion (DRG) neurons in a rat model of LDH.

METHODS

LDH was established by transplantation of autologous nucleus pulposus (NP) to lumbar 5 and 6 spinal nerves (L5-L6 DRG) of adult male rats. Mechanical paw withdrawal threshold (PWT) and thermal paw withdrawal latency (PWL) were measured 1 day before and through 35 days after transplantation of NP. Changes in expression of VGSCs were determined by western blotting. L5-L6 DRGs neurons innervating the hindpaw were labeled with DiI and acutely dissociated for measuring excitability and sodium channel currents under whole-cell patch clamp configurations.

RESULTS

NP transplantation significantly reduced the PWT and PWL in association with a significant reduction in rheobase and an increase in numbers of action potentials evoked by 2X and 3X rheobase current stimulation. Voltage-gated sodium current density was significantly enhanced in L5-L6 DRG neurons from LDH rats. The inactivation curve showed a leftward shift in LDH rats while activation curve did not significantly alter. However, NP transplantation remarkably enhanced expression of NaV1.7 and NaV1.8 in L5-L6 DRGs but not in T10-12 DRGs.

CONCLUSION

These data suggest that NP application produces pain-related behavior and potentiates sodium current density of DRG neurons, which is most likely mediated by enhanced expression of NaV1.7 and NaV1.8.

G protein-gated inwardly rectifying potassium channel subunits 1 and 2 are down-regulated in rat dorsal root ganglion neurons and spinal cord after peripheral axotomy

Background

Increased nociceptive neuronal excitability underlies chronic pain conditions. Various ion channels, including sodium, calcium and potassium channels have pivotal roles in the control of neuronal excitability. The members of the family of G protein-gated inwardly rectifying potassium (GIRK) channels, GIRK1–4, have been implicated in modulating excitability. Here, we investigated the expression and distribution of GIRK1 and GIRK2 in normal and injured dorsal root ganglia (DRGs) and spinal cord of rats.

Results

We found that ~70% of the DRG neurons expressed GIRK1, while only <10% expressed GIRK2. The neurochemical profiles of GIRK1- and GIRK2-immunoreactive neurons were characterized using the neuronal markers calcitonin gene-related peptide, isolectin-B4 and neurofilament-200, and the calcium-binding proteins calbindin D28k, calretinin, parvalbumin and secretagogin. Both GIRK subunits were expressed in DRG neurons with nociceptive characteristics. However, while GIRK1 was widely expressed in several sensory neuronal subtypes, GIRK2 was detected mainly in a group of small C-fiber neurons. In the spinal dorsal horn, GIRK1- and -2-positive cell bodies and processes were mainly observed in lamina II, but also in superficial and deeper layers. Abundant GIRK1-, but not GIRK2-like immunoreactivity, was found in the ventral horn (laminae VI–X). Fourteen days after axotomy, GIRK1 and GIRK2 were down-regulated in DRG neurons at the mRNA and protein levels. Both after axotomy and rhizotomy there was a reduction of GIRK1- and -2-positive processes in the dorsal horn, suggesting a presynaptic localization of these potassium channels. Furthermore, nerve ligation caused accumulation of both subunits on both sides of the lesion, providing evidence for anterograde and retrograde fast axonal transport.

Conclusions

Our data support the hypothesis that reduced GIRK function is associated with increased neuronal excitability and causes sensory disturbances in post-injury conditions, including neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0044-z) contains supplementary material, which is available to authorized users.

Modulation of nociceptive ion channels and receptors via protein-protein interactions: implications for pain relief

In the last 2 decades biomedical research has provided great insights into the molecular signatures underlying painful conditions. However, chronic pain still imposes substantial challenges to researchers, clinicians and patients alike. Under pathological conditions, pain therapeutics often lack efficacy and exhibit only minimal safety profiles, which can be largely attributed to the targeting of molecules with key physiological functions throughout the body. In light of these difficulties, the identification of molecules and associated protein complexes specifically involved in chronic pain states is of paramount importance for designing selective interventions. Ion channels and receptors represent primary targets, as they critically shape nociceptive signaling from the periphery to the brain. Moreover, their function requires tight control, which is usually implemented by protein-protein interactions (PPIs). Indeed, manipulation of such PPIs entails the modulation of ion channel activity with widespread implications for influencing nociceptive signaling in a more specific way. In this review, we highlight recent advances in modulating ion channels and receptors via their PPI networks in the pursuit of relieving chronic pain. Moreover, we critically discuss the potential of targeting PPIs for developing novel pain therapies exhibiting higher efficacy and improved safety profiles.

Slack channels expressed in sensory neurons control neuropathic pain in mice

Slack (Slo2.2) is a sodium-activated potassium channel that regulates neuronal firing activities and patterns. Previous studies identified Slack in sensory neurons, but its contribution to acute and chronic pain in vivo remains elusive. Here we generated global and sensory neuron-specific Slack mutant mice and analyzed their behavior in various animal models of pain. Global ablation of Slack led to increased hypersensitivity in models of neuropathic pain, whereas the behavior in models of inflammatory and acute nociceptive pain was normal. Neuropathic pain behaviors were also exaggerated after ablation of Slack selectively in sensory neurons. Notably, the Slack opener loxapine ameliorated persisting neuropathic pain behaviors. In conclusion, Slack selectively controls the sensory input in neuropathic pain states, suggesting that modulating its activity might represent a novel strategy for management of neuropathic pain.

Redox and nitric oxide-mediated regulation of sensory neuron ion channel function

SIGNIFICANCE

Reactive oxygen and nitrogen species (ROS and RNS, respectively) can intimately control neuronal excitability and synaptic strength by regulating the function of many ion channels. In peripheral sensory neurons, such regulation contributes towards the control of somatosensory processing; therefore, understanding the mechanisms of such regulation is necessary for the development of new therapeutic strategies and for the treatment of sensory dysfunctions, such as chronic pain.

RECENT ADVANCES

Tremendous progress in deciphering nitric oxide (NO) and ROS signaling in the nervous system has been made in recent decades. This includes the recognition of these molecules as important second messengers and the elucidation of their metabolic pathways and cellular targets. Mounting evidence suggests that these targets include many ion channels which can be directly or indirectly modulated by ROS and NO. However, the mechanisms specific to sensory neurons are still poorly understood. This review will therefore summarize recent findings that highlight the complex nature of the signaling pathways involved in redox/NO regulation of sensory neuron ion channels and excitability; references to redox mechanisms described in other neuron types will be made where necessary.

CRITICAL ISSUES

The complexity and interplay within the redox, NO, and other gasotransmitter modulation of protein function are still largely unresolved. Issues of specificity and intracellular localization of these signaling cascades will also be addressed.

FUTURE DIRECTIONS

Since our understanding of ROS and RNS signaling in sensory neurons is limited, there is a multitude of future directions; one of the most important issues for further study is the establishment of the exact roles that these signaling pathways play in pain processing and the translation of this understanding into new therapeutics.

Nav1.7 protein and mRNA expression in the dorsal root ganglia of rats with chronic neuropathic pain

Neuropathic pain was produced by chronic constriction injury of the sciatic nerve in rats. Behavioral tests showed that the thresholds for thermal and mechanical hyperalgesia were significantly reduced in neuropathic pain rats 3-28 days following model induction. The results of immunohistochemistry, western blot assays and reverse transcription-PCR showed that Nav1.7 protein and mRNA expression was significantly increased in the injured dorsal root ganglia. These findings indicated that Nav1.7 might play an important role in the model of chronic neuropathic pain.

Ion channels in somatosensory transmission: an introduction to the collection

Excitation of peripheral endings of sensory nerves is a primary event in most types of somatosensation, including pain. This excitation and transmission of action potentials within somatosensory pathways is brought about by the concerted action of the wide array of plasmalemmal ion channels, some of which are specific to somatosensory nerves. Accordingly, ion channel deficiencies or ‘channelopathies’ often underlie sensory disorders and pathological pain states and many current and prospective analgesics target ion channels. This F1000Research article collection is focused on the current advances in understanding function and regulation of ion channels controlling excitability and synaptic transmission within somatosensory pathways. The focus is on the peripheral neurons but studies of central mechanisms that integrate peripheral inputs are also welcome. We also welcome discussions of emerging approaches, methods and techniques in somatosensory physiology.

Gain-of-function mutations in the transient receptor potential channels TRPV1 and TRPA1: how painful?

Gain-of-function (GOF) mutations in ion channels are rare events, which lead to increased agonist sensitivity or altered gating properties, and may render the channel constitutively active. Uncovering and following characterization of such mutants contribute substantially to the understanding of the molecular basis of ion channel functioning. Here we give an overview of some GOF mutants in polymodal ion channels specifically involved in transduction of painful stimuli--TRPV1 and TRPA1, which are scrutinized by scientists due to their important role in development of some pathological pain states. Remarkably, a substitution of single amino acid in the S4-S5 region of TRPA1 (N855S) has been recently associated with familial episodic pain syndrome. This mutation increases chemical sensitivity of TRPA1, but leaves the voltage sensitivity unchanged. On the other hand, mutations in the analogous region of TRPV1 (R557K and G563S) severely affect all aspects of channel activation and lead to spontaneous activity. Comparison of the effects induced by mutations in homologous positions in different TRP receptors (or more generally in other distantly related ion channels) may elucidate the gating mechanisms conserved during evolution.

Endogenous regulation of inflammatory pain by T-cell-derived opioids: when friend turns to foe

Painful sensation is a hallmark of microbe-induced inflammation. This inflammatory pain is downregulated a few days after infection by opioids locally released by effector T lymphocytes generated in response to microbe-derived antigens. This review focuses on the endogenous regulation of inflammatory pain associated with adaptive T-cell response and puts in perspective the clinical consequences of the opioid-mediated analgesic activity of colitogenic T lymphocytes in inflammatory bowel disease.

Sustained neuronal hyperexcitability is evident in the thalamus after a transient cervical radicular injury

STUDY DESIGN

This study used extracellular electrophysiology to examine neuronal hyperexcitability in the ventroposterolateral nucleus (VPL) of the thalamus in a rat model of painful radiculopathy.

OBJECTIVE

The goal of this study was to quantify evoked neuronal excitability in the VPL at day 14 after a cervical nerve root compression to determine thalamic processing of persistent radicular pain.

SUMMARY OF BACKGROUND DATA

Nerve root compression often leads to radicular pain. Chronic pain is thought to induce structural and biochemical changes in the brain affecting supraspinal signaling. In particular, the VPL of the thalamus has been implicated in chronic pain states.

METHODS

Rats underwent a painful transient C7 nerve root compression or sham procedure. Ipsilateral forepaw mechanical allodynia was assessed on days 1, 3, 5, 7, 10, and 14 and evoked thalamic neuronal recordings were collected at day 14 from the contralateral VPL, whereas the injured forepaw was stimulated using a range of non-noxious and noxious mechanical stimuli. Neurons were classified on the basis of their response to stimulation.

RESULTS

Behavioral sensitivity was elevated after nerve root compression starting at day 3 and persisted until day 14 (P < 0.049). Thalamic recordings at day 14 demonstrated increased neuronal hyperexcitability after injury for all mechanical stimuli (P < 0.024). In particular, wide dynamic range neurons demonstrated significantly more firing after injury compared with sham in response to von Frey stimulation (P < 0.0001). Firing in low threshold mechanoreceptive neurons was not different between groups.

CONCLUSION

These data demonstrate that persistent radicular pain is associated with sustained neuronal hyperexcitability in the contralateral VPL of the thalamus. These findings suggest that thalamic processing is altered during radiculopathy and these changes in neuronal firing are associated with behavioral sensitivity.

LEVEL OF EVIDENCE

N/A.

Fruit flies as a powerful model to drive or validate pain genomics efforts

Chronic pain is a disabling condition that persists even after normal healing processes are complete and presents considerable physical, psychological and financial burdens for patients globally. However, current analgesic treatments do not meet clinical needs. Here, we review genomic and pharmacogenomic studies of pain in humans and nociception in the fruit fly Drosophila melanogaster, and provide evidence supporting the use of fly genetics to compliment genome-wide and pharmacogenomic studies of human conditions, such as pain. Combining genomic and pharmacogenomic techniques to study chronic pain in humans with functional genomic assessment in model organisms may provide molecular rationale for developing more personalized or improving generalized chronic pain therapies.

The renal channelopathies

Specific channels permit movement of selected ions through cellular membranes, and are of vital importance in a number of physiological processes, particularly in excitable tissues such as nerve and muscle, but also in endocrine organs and in epithelial biology. Disorders of channel proteins are termed channelopathies, and their importance is increasingly recognised within medicine. In the kidney, ion channels have critical roles enabling sodium and potassium reuptake or excretion along the nephron, in magnesium homeostasis, in the control of water reabsorption in the collecting duct, and in determining glomerular permeability. In this review, we assess the channelopathies encountered in each nephron segment, and see how their molecular and genetic characterisation in the past 20–30 years has furthered our understanding of normal kidney physiology and disease processes, aids correct diagnosis and promises future therapeutic opportunities.

HCN2 channels account for mechanical (but not heat) hyperalgesia during long-standing inflammation

There is emerging evidence that hyperpolarization-activated cation (HCN) channels are involved in the development of pathological pain, including allodynia and hyperalgesia. Mice lacking the HCN isoform 2 display reduced heat but unchanged mechanical pain behavior, as recently shown in preclinical models of acute inflammatory pain. However, the impact of HCN2 to chronic pain conditions is less clear and has not been examined so far. In this report, we study the role of HCN2 in the complete Freund's adjuvant inflammation model reflecting chronic pain conditions. We used sensory neuron-specific as well as inducible global HCN2 mutants analyzing pain behavior in persistent inflammation and complemented this by region-specific administration of an HCN channel blocker. Our results demonstrate that the absence of HCN2 in primary sensory neurons reduces tactile hypersensitivity in chronic inflammatory conditions but leaves heat hypersensitivity unaffected. This result is in remarkable contrast to the recently described role of HCN2 in acute inflammatory conditions. We show that chronic inflammation results in an increased expression of HCN2 and causes sensitization in peripheral and spinal terminals of the pain transduction pathway. The contribution of HCN2 to peripheral sensitization mechanisms was further supported by single-fiber recordings from isolated skin-nerve preparations and by conduction velocity measurements of saphenous nerve preparations. Global HCN2 mutants revealed that heat hypersensitivity-unaffected in peripheral HCN2 mutants-was diminished by the additional disruption of central HCN2 channels, suggesting that thermal hyperalgesia under chronic inflammatory conditions is mediated by HCN2 channels beyond primary sensory afferents.