Inhibitory effect of amiloride and gadolinium on fine afferent nerves in the rat knee: evidence of mechanogated ion channels in joints

The contribution of clock genes BMAL1 and PER2 in osteoarthritis-associated pain

Joint pain is the primary symptom of osteoarthritis (OA) and the main motivator for patients to seek medical care. OA-related pain significantly restricts joint function and diminishes quality of life. Despite the availability of various pain-relieving medications for OA, current treatment strategies often fall short in delivering adequate pain relief. Furthermore, long-term use of pain medications for OA management is frequently linked with notable side effects and toxicities, suggesting the need to explore new potential targets to treat pain in OA patients. In this context, clock genes, particularly brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (BMAL1) and period circadian protein homolog 2 (PER2), known for their role in circadian rhythms, represent promising opportunities for pharmacological interventions due to their involvement in both the development and maintenance of OA pain. While BMAL1 and PER2 have been extensively studied in neuropathic and inflammatory pain, their specific contributions to OA pain remain less clear, demanding further investigation. This narrative review aims to synthesize the relationship between OA pain and the BMAL1 and PER2 signaling pathways, ultimately exploring the potential therapeutic role of clock genes in addressing this challenging condition.

Comparison of nociceptor properties using electrophysiology in preclinical models of osteoarthritis

Osteoarthritis (OA) pain originates in the joint by sensitization of articular nociceptors. While behavioural assessments provide valuable information regarding pain symptoms, the techniques are subjective and open to interpretation by the experimenter. This study used in vivo electrophysiological approaches to measure objectively joint nociceptor properties in three rodent models of OA. Single unit extracellular recordings of joint mechanosensitive afferents were carried out in male and female rats following either (1) transection of the medial meniscus (MMT: post-traumatic OA), (2) intra-articular injection of sodium monoiodoacetate (MIA: chemically-induced OA), or (3) intra-articular injection of lysophosphatidic acid (LPA: neuropathic OA). In naïve male control rats, the mechanical threshold of joint mechanonociceptors (23.5 ± 1.8 mNm) was significantly reduced with MMT (9.4 ± 1.1 mNm) and MIA (15.1 ± 1.6 mNm). In females, the mechanical threshold of naïve rats (23.2 ± 3.1 mNm) was reduced following induction of MMT (8.3 ± 1.0 mNm) and LPA (10.6 ± 2.2 mNm). Afferent firing frequency increased in male MMT (∼275 %), LPA (∼175 %), MIA (225 %), and female MMT (∼146 %), LPA (∼200 %), and MIA (∼192 %). Mechanical threshold and evoked firing were negatively correlated in all models for both sexes except LPA rats (male + female) and female MMT. These data indicate that MMT, MIA, and LPA induce peripheral sensitization of joint afferents thereby validating their use in OA pain studies.

Innate Immunity at the Core of Sex Differences in Osteoarthritic Pain?

Osteoarthritis (OA) is a progressive whole-joint disease; no disease-modifying drugs are currently available to stop or slow its process. Symptoms alleviation is the only treatment option. OA is the major cause of chronic pain in adults, with pain being the main symptom driving patients to seek medical help. OA pathophysiology is closely associated with the innate immune system, which is also closely linked to pain mediators leading to joint pain. Pain research has shown sex differences in the biology of pain, including sexually dimorphic responses from key cell types in the innate immune system. Not only is OA more prevalent in women than in men, but women patients also show worse OA outcomes, partially due to experiencing more pain symptoms despite having similar levels of structural damage. The cause of sex differences in OA and OA pain is poorly understood. This review provides an overview of the involvement of innate immunity in OA pain in joints and in the dorsal root ganglion. We summarize the emerging evidence of sex differences regarding innate immunity in OA pain. Our main goal with this review was to provide a scientific foundation for future research leading to alternative pain relief therapies targeting innate immunity that consider sex differences. This will ultimately lead to a more effective treatment of pain in both women and men.

Update on pain in arthritis

PURPOSE OF REVIEW

Osteoarthritis is a degenerative joint disease that features pain as a hallmark symptom. This review summarises progress and obstacles in our understanding of pain mechanisms in arthritis.

RECENT FINDINGS

Pain phenotypes in osteoarthritis are poorly characterized in clinical studies and animal studies are largely carti-centric. Different animal models incur variable disease progression patterns and activation of distinct pain pathways, but studies reporting both structural and pain outcomes permit better translational insights. In patients, classification of osteoarthritis disease severity is only based on structural integrity of the joint, but pain outcomes do not consistently correlate with joint damage. The complexity of this relationship underlines the need for pain detection in criteria for osteoarthritis classification and patient-reported outcome measures.

SUMMARY

Variable inflammatory and neuropathic components and spatiotemporal evolution underlie the heterogeneity of osteoarthritis pain phenotypes, which must be considered to adequately stratify patients. Revised classification of osteoarthritis at different stages encompassing both structural and pain outcomes would significantly improve detection and diagnosis at both early and late stages of disease. These are necessary advancements in the field that would also improve trial design and provide better understanding of basic mechanisms of disease progression and pain in osteoarthritis.

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

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

Synovial nerve fiber density decreases with naturally-occurring osteoarthritis in horses

OBJECTIVE

To measure the nerve fiber density in synovial membranes from healthy and OA equine joints and to investigate the relationship between synovial innervation and OA severity, synovial vascularity and synovitis.

DESIGN

Twenty-five equine metacarpophalangeal joints were collected post-mortem. The joints were dissected and the macroscopic lesions of the articular cartilage were scored. Synovial membrane specimens (n = 50) were harvested, fixed, sectioned and scored histologically. Immunohistochemical staining and immunofluorescence with S-100 protein, that identifies nerve fibers, and ⍺-actin, that stains vascular smooth muscle, were also performed on site-matched specimens and the relationships between these tissues was interrogated.

RESULTS

The nerve fiber density was higher in the superficial layer (≤200 μm) of the synovium when compared to the deeper layer in control equine joints (mean difference (95% C.I.): 0.054% (0.018%, 0.11%)). In osteoarthritic joints, synovial innervation decreased in the superficial layer with increasing macroscopic OA score (β (SEM), 95% C.I.: -0.0061 (0.00021), -0.0011, -0.00017). The blood vessel density was also higher in the superficial layer of the synovium compared to the deep layer in the control (mean difference (95% C.I.): 1.1% (0.36%, 2.3%)) and OA (mean difference (95% C.I.): 0.60% (0.22%, 1.2%)) equine joints. Moreover, considering all synovial specimens, higher nerve fiber density in the deep layer positively correlated with blood vessel density (β (SEM), 95% C.I.: 0.11 (0.036), 0.035, 0.18).

CONCLUSION

The reduction in nerve fiber density with advanced cartilage degeneration suggests that peripheral neuropathy is associated with equine OA. Whether this link is associated with neuropathic pain, requires further investigation.

Nutraceuticals and osteoarthritis pain

Arthritis is a chronic disease of joints. It is highly prevalent, particularly in the elderly, and is commonly associated with pain that interferes with quality of life. Because of its chronic nature, pharmacological approaches to pain relief and joint repair must be safe for long term use, a quality many current therapies lack. Nutraceuticals refer to compounds or materials that can function as nutrition and exert a potential therapeutic effect, including the relief of pain, such as pain related to arthritis, of which osteoarthritis (OA) is the most common form. Of interest, nutraceuticals have recently been shown to have potential in relieving OA pain in human clinical trials. Emerging evidence indicates nutraceuticals may represent promising alternatives for the relief of OA pain. In this paper, we will overview OA pain and the use of nutraceuticals in OA pain management, focusing on those that have been evaluated by clinical trials. Furthermore, we discuss the biologic and pharmacologic actions underlying the nutraceutical effects on pain relief based on the potential active ingredients identified from traditional nutraceuticals in OA pain management and their potential for drug development. The review concludes by sharing our viewpoints that future studies should prioritize elucidating the mechanisms of action of nutraceuticals in OA and developing nutraceuticals that not only relieve OA pain, but also mitigate OA pathology.

Mechanosensitive ion channels in articular nociceptors drive mechanical allodynia in osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is a disabling and highly prevalent condition affecting millions worldwide. Pain is the major complaint of OA patients and is presently inadequately managed. It manifests as mechanical allodynia, a painful response to innocuous stimuli such as joint movement. Allodynia is due in part to the sensitization of articular nociceptors to mechanical stimuli. These nociceptors respond to noxious mechanical stimuli applied to their terminals via the expression of depolarizing high-threshold mechanosensitive ion channels (MSICs) that convert painful mechanical forces into electrical signals. In this study, we examined the contribution of MSICs to mechanical allodynia in a mouse model of OA.

METHOD

Sodium mono-iodoacetate (MIA) was injected in the left knee of adult male Trpv1:Cre; GFP mice. Primary mechanical allodynia was monitored using the knee-bend test. Single-channel patch clamp electrophysiology was performed on visually-identified knee-innervating nociceptors. Dorsal horn neuronal activation was assessed by Fos immunoreactivity.

RESULTS

In examining the gating properties of MSICs of naïve and OA mice, we discovered that their activation threshold is greatly reduced, causing their opening at significantly lower stimuli intensities. Consequently, nociceptors are activated by mild mechanical stimuli. These channels are reversibly inhibited by the selective MSIC inhibitor GsMTx4, and the intra-articular injection of this peptide significantly reduced the activation of dorsal horn nociceptive circuits and primary mechanical allodynia in OA mice.

CONCLUSIONS

These results suggest that MSICs are sensitized during OA and directly contribute to mechanical allodynia. They therefore represent potential therapeutic targets in the treatment of OA pain.

Mechanisms and Mediators That Drive Arthritis Pain

There are over 100 different types of arthritis and each can differ greatly in their aetiology and pathophysiology; however, one characteristic that is common to all arthritic conditions is joint pain. Musculoskeletal pain is the leading cause of disability in the world, and the number one reason arthritis patients visit their primary care physician. Despite the prevalence and burden of arthritis pain, current analgesics lack sufficient efficacy and are plagued by multiple adverse side effects. In this review, we outline the current landscape of research concerning joint pain, drawing from both preclinical and clinical studies. Specifically, this review is a discussion of the different neurophysiological processes that occur during joint disease and how inflammatory and neuropathic aspects contribute to the development of arthritis pain.

Osteoarthritis pain

Osteoarthritis (OA) represents one of the most frequently occurring painful conditions. Pain is the major OA symptom, involving both peripheral and central neurological mechanisms. OA pain is initiated from free axonal endings located in the synovium, periosteum bone, and tendons, but not in the cartilage. The nociceptive message involves not only neuromediators and regulating factors such as neuronal growth factor (NGF) but also central modifications of pain pathways. OA pain is a mixed phenomenon where nociceptive and neuropathic mechanisms are involved in both the local and central levels. OA pain perception is influenced by multiple environmental, psychological, or constitutional factors, and OA pain intensity is not correlated with joint degradation. OA pain may present with different clinical features: constant and intermittent pain, with or without a neuropathic component, and with or without central sensitization. Finally, OA pain should be considered as a complex and not unique pain condition, where precise clinical assessment may drive specific therapeutic approaches.

Towards a mechanism-based approach to pain management in osteoarthritis

Pain is the defining symptom of osteoarthritis (OA), yet available treatment options, of which NSAIDs are the most common, provide inadequate pain relief and are associated with serious health risks when used long term. Chronic pain pathways are subject to complex levels of control and modulation, both in the periphery and in the central nervous system. Ongoing clinical and basic research is uncovering how these pathways operate in OA. Indeed, clinical investigation into the types of pain associated with progressive OA, the presence of central sensitization, the correlation with structural changes in the joint, and the efficacy of novel analgesics affords new insights into the pathophysiology of OA pain. Moreover, studies in disease-specific animal models enable the unravelling of the cellular and molecular pathways involved. We expect that increased understanding of the mechanisms by which chronic OA-associated pain is generated and maintained will offer opportunities for targeting and improving the safety of analgesia. In addition, using clinical and genetic approaches, it might become possible to identify subsets of patients with pain of different pathophysiology, thus enabling a tailored approach to pain management.

A commentary on modelling osteoarthritis pain in small animals

OBJECTIVE

To describe the currently used animal models for the study of osteoarthritis (OA) pain, with an emphasis on small animals (predominantly mice and rats).

OUTLINE

Narrative review summarizing the opportunities and limitations of the most commonly used small animal models for the study of pain and pain pathways associated with OA, and discussing currently used methods for pain assessment. Involvement of neural degeneration in OA is briefly discussed. A list of considerations when studying pain-related behaviours and pathways in animal models of OA is proposed.

CONCLUSIONS

Animal models offer great potential to unravel the complex pathophysiology of OA pain, its molecular and temporal regulation. They constitute a critical pathway for developing and testing disease-specific symptom-modifying therapeutic interventions. However, a number of issues remain to be resolved in order to standardize pre-clinical OA pain research and to optimize translation to clinical trials and patient therapies.

Neurosensory mechanotransduction through acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-insensitive cation channels responding to extracellular acidification. ASIC proteins have two transmembrane domains and a large extracellular domain. The molecular topology of ASICs is similar to that of the mechanosensory abnormality 4- or 10-proteins expressed in touch receptor neurons and involved in neurosensory mechanotransduction in nematodes. The ASIC proteins are involved in neurosensory mechanotransduction in mammals. The ASIC isoforms are expressed in Merkel cell-neurite complexes, periodontal Ruffini endings and specialized nerve terminals of skin and muscle spindles, so they might participate in mechanosensation. In knockout mouse models, lacking an ASIC isoform produces defects in neurosensory mechanotransduction of tissue such as skin, stomach, colon, aortic arch, venoatrial junction and cochlea. The ASICs are thus implicated in touch, pain, digestive function, baroreception, blood volume control and hearing. However, the role of ASICs in mechanotransduction is still controversial, because we lack evidence that the channels are mechanically sensitive when expressed in heterologous cells. Thus, ASIC channels alone are not sufficient to reconstruct the path of transducing molecules of mechanically activated channels. The mechanotransducers associated with ASICs need further elucidation. In this review, we discuss the expression of ASICs in sensory afferents of mechanoreceptors, findings of knockout studies, technical issues concerning studies of neurosensory mechanotransduction and possible missing links. Also we propose a molecular model and a new approach to disclose the molecular mechanism underlying the neurosensory mechanotransduction.

Involvement of Nav 1.8 sodium ion channels in the transduction of mechanical pain in a rodent model of osteoarthritis

INTRODUCTION

A subgroup of voltage gated sodium channels including Nav1.8 are exclusively expressed on small diameter primary afferent neurons and are therefore believed to be integral to the neurotransmission of nociceptive pain. The present study examined whether local application of A-803467, a selective blocker of the Nav 1.8 sodium channel, can reduce nociceptive transmission from the joint in a rodent model of osteoarthritis (OA).

METHODS

OA-like changes were induced in male Wistar rats by an intra-articular injection of 3 mg sodium monoiodoacetate (MIA). Joint nociception was measured at day 14 by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious rotation of the joint both before and following close intra-arterial injection of A-803467. The effect of Nav1.8 blockade on joint pain perception and secondary allodynia were determined in MIA treated animals by hindlimb incapacitance and von Frey hair algesiometry respectively.

RESULTS

A-803467 significantly reduced the firing rate of joint afferents during noxious rotation of the joint but had no effect during non-noxious rotation. In the pain studies, peripheral injection of A-803467 into OA knees attenuated hindlimb incapacitance and secondary allodynia.

CONCLUSIONS

These studies show for the first time that the Nav1.8 sodium channel is part of the molecular machinery involved in mechanotransduction of joint pain. Targeting the Nav1.8 sodium channel on joint nociceptors could therefore be useful for the treatment of OA pain, avoiding the unwanted side effects of non-selective nerve blocks.

Understanding sensory nerve mechanotransduction through localized elastomeric matrix control

Background

While neural systems are known to respond to chemical and electrical stimulation, the effect of mechanics on these highly sensitive cells is still not well understood. The ability to examine the effects of mechanics on these cells is limited by existing approaches, although their overall response is intimately tied to cell-matrix interactions. Here, we offer a novel method, which we used to investigate stretch-activated mechanotransduction on nerve terminals of sensory neurons through an elastomeric interface.

Methodology/Principal Findings

To apply mechanical force on neurites, we cultured dorsal root ganglion neurons on an elastic substrate, polydimethylsiloxane (PDMS), coated with extracellular matrices (ECM). We then implemented a controlled indentation scheme using a glass pipette to mechanically stimulate individual neurites that were adjacent to the pipette. We used whole-cell patch clamping to record the stretch-activated action potentials on the soma of the single neurites to determine the mechanotransduction-based response. When we imposed specific mechanical force through the ECM, we noted a significant neuronal action potential response. Furthermore, because the mechanotransduction cascade is known to be directly affected by the cytoskeleton, we investigated the cell structure and its effects. When we disrupted microtubules and actin filaments with nocodozale or cytochalasin-D, respectively, the mechanically induced action potential was abrogated. In contrast, when using blockers of channels such as TRP, ASIC, and stretch-activated channels while mechanically stimulating the cells, we observed almost no change in action potential signalling when compared with mechanical activation of unmodified cells.

Conclusions/Significance

These results suggest that sensory nerve terminals have a specific mechanosensitive response that is related to cell architecture.

Unravelling the relationship between age, nociception and joint destruction in naturally occurring osteoarthritis of Dunkin Hartley guinea pigs

Osteoarthritis (OA) is a debilitating and painful disease, the incidence of which increases with advancing age. One of the confounding aspects of OA is that there is a disconnect between the severity of joint degeneration and the intensity of pain reported. This study examined the relationship between age, joint nociception, and joint pathology in an animal model of naturally occurring OA. Dunkin Hartley guinea pigs were grouped according to age: young (2-5 months) and senescent (17-37 months). Joint nociception was objectively measured in these animals by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious movements of the knee. Joint pathology in the same knees was then determined by histomorphology and micro-computerized tomography (micro-CT). A principal components analysis was carried out on the data to determine if any correlation exists between each of the measured variables. In aged guinea pigs, 33% of joint mechanosensory nerves were spontaneously active, whereas young animals showed no such neural activity at rest. The frequency of afferent firing evoked by noxious movements was greater in old guinea pigs. Micro-CT and histopathological determination of OA positively correlated with age; however, there was no significant correlation between the severity of joint degeneration and nociception. In the Dunkin Hartley model of inveterate OA, the level of joint pathology correlates well with increasing age. This study also provides the first objective evidence that there is no correlation between joint nociception and articular damage, thereby corroborating the clinical observation that pain is a poor predictor of OA severity.

Antinociceptive effects of tumor necrosis factor alpha neutralization in a rat model of antigen-induced arthritis: evidence of a neuronal target

OBJECTIVE

The reduction of pain in the course of antiinflammatory therapy can result from an attenuation of the inflammatory process and/or from the neutralization of endogenous mediators of inflammation that act directly on nociceptive neurons. The purpose of this study was to investigate whether analgesic effects of the neutralization of tumor necrosis factor alpha (TNFalpha) are due to an attenuation of inflammation or whether direct neuronal effects significantly contribute to pain relief in the course of therapy.

METHODS

Locomotor and pain-related behavior and histology were assessed in rats with chronic antigen-induced arthritis (AIA) in the knee joint, and the rats were treated with systemic saline, etanercept, or infliximab. The expression of TNF receptors (TNFRs) in dorsal root ganglia was measured using immunohistochemical analysis and polymerase chain reaction. Action potentials were recorded from afferent Adelta fibers and C fibers of the medial knee joint nerve, and etanercept and infliximab were injected intraarticularly into normal or inflamed knee joints (AIA or kaolin/carrageenan-induced inflammation).

RESULTS

In rats with AIA, both etanercept and infliximab significantly decreased inflammation-induced locomotor and pain-related behavior, while joint swelling was only weakly attenuated and histomorphology still revealed pronounced inflammation. A large proportion of dorsal root ganglion neurons showed TNFRI- and TNFRII-like immunoreactivity. Intraarticular injection of etanercept reduced the responses of joint afferents to mechanical stimulation of the inflamed joint starting 30 minutes after injection, but had no effect on responses to mechanical stimulation of the uninflamed joint.

CONCLUSION

Overall, these data show the pronounced antinociceptive effects of TNFalpha neutralization, thus suggesting that reduction of the effects of TNFalpha on pain fibers themselves significantly contributes to pain relief.

Age alters the ability of substance P to sensitize joint nociceptors in guinea pigs

Pain perception is altered during senescence and it is thought that this could in part be due to changes in peripheral pain sensing processes. The present study examined the effect of substance P (SP) (10(-10)-10(-8) mol; 0.1 mL bolus close intraarterial) on knee joint afferent mechanosensitivity in young (2- to 5-mo-old) and aged (17- to 36-mo-old) Dunkin-Hartley guinea pigs. Single unit electrophysiological recordings were made from knee joint primary afferent nerves in response to normal (nonnoxious) and painful (noxious) rotation of the joint. In young and old animals, local application of SP had a sensitizing effect on joint afferents in response to movements made in the normal working range of the knee. With noxious hyper-rotation of the joint, SP was able to increase afferent firing rate in young but not in old animals. These data demonstrate a lack of SP-mediated sensitization of joint nociceptors during senescence and suggest a peripheral deficiency in joint nociception with respect to age.

Arthritis and pain. Neurogenic origin of joint pain

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

Sensitization of unmyelinated sensory fibers of the joint nerve to mechanical stimuli by interleukin-6 in the rat: an inflammatory mechanism of joint pain

OBJECTIVE

Pain during mechanical stimulation of the joint and spontaneous pain are major symptoms of arthritis. An important neuronal process of mechanical hypersensitivity of the joint is the sensitization of thin myelinated Adelta fibers and unmyelinated C fibers innervating the joint. Because interleukin-6 (IL-6) is a major inflammatory mediator, we investigated whether this cytokine has the potential to sensitize joint afferents to mechanical stimuli.

METHODS

In electrophysiologic experiments conducted on anesthetized rats, action potentials were recorded from afferent fibers supplying the knee joint. Responses to innocuous and noxious rotation of the tibia against the femur in the knee joint were monitored before and 1-2 hours after injection of test compounds into the joint cavity.

RESULTS

Injection of IL-6 and coinjection of IL-6 plus soluble IL-6 receptor (sIL-6R) caused a gradual increase in the responses of C fibers to innocuous and noxious rotation within 1 hour. The increase in responses to IL-6 and IL-6 plus sIL-6R was prevented by coadministration of soluble glycoprotein 130 (sgp130), but sgp130 did not reverse established mechanical hyperexcitability. Responses of Adelta fibers were not altered by the compounds. While injection of sIL-6R alone into the normal knee joint did not influence responses to mechanical stimulation, injection of sIL-6R into the acutely inflamed knee joint caused an increase in responses.

CONCLUSION

IL-6 has the potential to sensitize C fibers in the joint to mechanical stimulation. Thus, IL-6 contributes to mechanical hypersensitivity, most likely due to an action of IL-6 on nerve fibers themselves.

Acid-sensing ion channels: advances, questions and therapeutic opportunities

Extracellular acid can have important effects on neuron function. In central and peripheral neurons, acid-sensing ion channels (ASICs) have emerged as key receptors for extracellular protons, and recent studies suggest diverse roles for these channels in the pathophysiology of pain, ischemic stroke and psychiatric disease. ASICs have also been implicated in mechanosensation in the peripheral nervous system and in neurotransmission in the central nervous system. Here, we briefly review advances in our understanding of ASICs, their potential contributions to disease, and the possibility for their therapeutic modification.