Modulation of sensory neuron mechanotransduction by PKC- and nerve growth factor-dependent pathways

Transcriptional profiling of dental sensory and proprioceptive trigeminal neurons using single-cell RNA sequencing

Dental primary afferent (DPA) neurons and proprioceptive mesencephalic trigeminal nucleus (MTN) neurons, located in the trigeminal ganglion and the brainstem, respectively, are essential for controlling masticatory functions. Despite extensive transcriptomic studies on various somatosensory neurons, there is still a lack of knowledge about the molecular identities of these populations due to technical challenges in their circuit-validated isolation. Here, we employed high-depth single-cell RNA sequencing (scRNA-seq) in combination with retrograde tracing in mice to identify intrinsic transcriptional features of DPA and MTN neurons. Our transcriptome analysis revealed five major types of DPA neurons with cell type-specific gene enrichment, some of which exhibit unique mechano-nociceptive properties capable of transmitting nociception in response to innocuous mechanical stimuli in the teeth. Furthermore, we discovered cellular heterogeneity within MTN neurons that potentially contribute to their responsiveness to mechanical stretch in the masseter muscle spindles. Additionally, DPA and MTN neurons represented sensory compartments with distinct molecular profiles characterized by various ion channels, receptors, neuropeptides, and mechanoreceptors. Together, our study provides new biological insights regarding the highly specialized mechanosensory functions of DPA and MTN neurons in pain and proprioception.

Role of TMEM100 in mechanically insensitive nociceptor un-silencing

Mechanically silent nociceptors are sensory afferents that are insensitive to noxious mechanical stimuli under normal conditions but become sensitized to such stimuli during inflammation. Using RNA-sequencing and quantitative RT-PCR we demonstrate that inflammation upregulates the expression of the transmembrane protein TMEM100 in silent nociceptors and electrophysiology revealed that over-expression of TMEM100 is required and sufficient to un-silence silent nociceptors in mice. Moreover, we show that mice lacking TMEM100 do not develop secondary mechanical hypersensitivity-i.e., pain hypersensitivity that spreads beyond the site of inflammation-during knee joint inflammation and that AAV-mediated overexpression of TMEM100 in articular afferents in the absence of inflammation is sufficient to induce mechanical hypersensitivity in remote skin regions without causing knee joint pain. Thus, our work identifies TMEM100 as a key regulator of silent nociceptor un-silencing and reveals a physiological role for this hitherto enigmatic afferent subclass in triggering spatially remote secondary mechanical hypersensitivity during inflammation.

Kinin B2 and B1 Receptors Activation Sensitize the TRPA1 Channel Contributing to Anastrozole-Induced Pain Symptoms

Aromatase inhibitors (AIs) cause symptoms of musculoskeletal pain, and some mechanisms have been proposed to explain them. However, signaling pathways downstream from kinin B2 (B2R) and B1 (B1R) receptor activation and their possible sensitizing of the Transient Receptor Potential Ankyrin 1 (TRPA1) remain unknown. The interaction between the kinin receptor and the TRPA1 channel in male C57BL/6 mice treated with anastrozole (an AI) was evaluated. PLC/PKC and PKA inhibitors were used to evaluate the signaling pathways downstream from B2R and B1R activation and their effect on TRPA1 sensitization. Anastrozole caused mechanical allodynia and muscle strength loss in mice. B2R (Bradykinin), B1R (DABk), or TRPA1 (AITC) agonists induced overt nociceptive behavior and enhanced and prolonged the painful parameters in anastrozole-treated mice. All painful symptoms were reduced by B2R (Icatibant), B1R (DALBk), or TRPA1 (A967079) antagonists. We observed the interaction between B2R, B1R, and the TRPA1 channel in anastrozole-induced musculoskeletal pain, which was dependent on the activation of the PLC/PKC and PKA signaling pathways. TRPA1 seems to be sensitized by mechanisms dependent on the activation of PLC/PKC, and PKA due to kinin receptors stimulation in anastrozole-treated animals. Thus, regulating this signaling pathway could contribute to alleviating AIs-related pain symptoms, patients’ adherence to therapy, and disease control.

Mechanotransduction: Exploring New Therapeutic Avenues in Central Nervous System Pathology

Cells are continuously exposed to physical forces and the central nervous system (CNS) is no exception. Cells dynamically adapt their behavior and remodel the surrounding environment in response to forces. The importance of mechanotransduction in the CNS is illustrated by exploring its role in CNS pathology development and progression. The crosstalk between the biochemical and biophysical components of the extracellular matrix (ECM) are here described, considering the recent explosion of literature demonstrating the powerful influence of biophysical stimuli like density, rigidity and geometry of the ECM on cell behavior. This review aims at integrating mechanical properties into our understanding of the molecular basis of CNS disease. The mechanisms that mediate mechanotransduction events, like integrin, Rho/ROCK and matrix metalloproteinases signaling pathways are revised. Analysis of CNS pathologies in this context has revealed that a wide range of neurological diseases share as hallmarks alterations of the tissue mechanical properties. Therefore, it is our belief that the understanding of CNS mechanotransduction pathways may lead to the development of improved medical devices and diagnostic methods as well as new therapeutic targets and strategies for CNS repair.

Structural analysis of TrkA mutations in patients with congenital insensitivity to pain reveals PLCγ as an analgesic drug target

Chronic pain is a major health issue, and the search for new analgesics has become increasingly important because of the addictive properties and unwanted side effects of opioids. To explore potentially new drug targets, we investigated mutations in the NTRK1 gene found in individuals with congenital insensitivity to pain with anhidrosis (CIPA). NTRK1 encodes tropomyosin receptor kinase A (TrkA), the receptor for nerve growth factor (NGF) and that contributes to nociception. Molecular modeling and biochemical analysis identified mutations that decreased the interaction between TrkA and one of its substrates and signaling effectors, phospholipase Cγ (PLCγ). We developed a cell-permeable phosphopeptide derived from TrkA (TAT-pQYP) that bound the Src homology domain 2 (SH2) of PLCγ. In HEK-293T cells, TAT-pQYP inhibited the binding of heterologously expressed TrkA to PLCγ and decreased NGF-induced, TrkA-mediated PLCγ activation and signaling. In mice, intraplantar administration of TAT-pQYP decreased mechanical sensitivity in an inflammatory pain model, suggesting that targeting this interaction may be analgesic. The findings demonstrate a strategy to identify new targets for pain relief by analyzing the signaling pathways that are perturbed in CIPA.

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

The cutaneous mechanisms that trigger spontaneous neuropathic pain in diabetic peripheral neuropathy (PDPN) are far from clear. Two types of nociceptors are found within the epidermal and dermal skin layers. Small-diameter lightly myelinated Aδ and unmyelinated C cutaneous mechano and heat-sensitive (AMH and CMH) and C mechanoinsensitive (CMi) nociceptors transmit pain from the periphery to central nervous system. AMH and CMH fibers are mainly located in the epidermis, and CMi fibers are distributed in the dermis. In DPN, dying back intra-epidermal AMH and CMH fibers leads to reduced pain sensitivity, and the patients exhibit significantly increased pain thresholds to acute pain when tested using traditional methods. The role of CMi fibers in painful neuropathies has not been fully explored. Microneurography has been the only tool to access CMi fibers and differentiate AMH, CMH, and CMi fiber types. Due to the complexity, its use is impractical in clinical settings. In contrast, a newly developed diode laser fiber selective stimulation (DLss) technique allows to safely and selectively stimulate Aδ and C fibers in the superficial and deep skin layers. DLss data demonstrate that patients with painful DPN have increased Aδ fiber pain thresholds, while C-fiber thresholds are intact because, in these patients, CMi fibers are abnormally spontaneously active. It is also possible to determine the involvement of CMi fibers by measuring the area of DLss-induced neurogenic axon reflex flare. The differences in AMH, CMH, and CMi fibers identify patients with painful and painless neuropathy. In this review, we will discuss the role of CMi fibers in PDPN.

Understanding pain perception through genetic painlessness diseases: The role of NGF and proNGF

Nerve growth factor (NGF), by binding to TrkA and p75^NTR receptors, regulates the survival and differentiation of sensory neurons during development and mediates pain transmission and perception during adulthood, by acting at different levels of the nervous system. Key to understanding the role of NGF as a pain mediator is the finding that mutations (namely, R121W, V232fs and R221W) in the NGF gene cause painlessness disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). Here we shall review the consequences of these NGF mutations, each of which results in specific clinical signs: R221W determines congenital pain insensitivity with no overt cognitive disabilities, whereas V232fs and R121W also result in intellectual disability, thus showing similarities to HSAN IV, which is caused by mutations in TrkA, rather than to HSAN V. Comparing the cellular, biochemical and clinical findings of these mutations could help in better understanding not only the possible mechanisms underlying HSAN V, but also mechanisms of NGF signalling and roles. These mutations alter the balance between NGF and proNGF in favour of an accumulation of the latter, suggesting a possible role of proNGF as a molecule with an analgesic role. Furthermore, the neurotrophic and pronociceptive functions of NGF are split by the R221W mutation, making NGF variants based on this mutation interesting for designing therapeutic applications for many diseases. This review emphasizes the possibility of using the mutations involved in "painlessness" clinical disorders as an innovative approach to identify new proteins and pathways involved in pain transmission and perception. OUTSTANDING QUESTIONS: Why do homozygous HSAN V die postnatally? What is the cause of this early postnatal lethality? Is the development of a mouse or a human feeling less pain affecting higher cognitive and perceptual functions? What is the consequence of the HSAN V mutation on the development of joints and bones? Are the multiple fractures observed in HSAN V patients due exclusively to the carelessness consequent to not feeling pain, or also to an intrinsic frailty of their bones? Are heterodimers of NGF^WT and NGF^R221W in the heterozygote state formed? And if so, what are the properties of these heterodimeric proteins? How is the processing of proNGF^R221W to NGF^R221W affected by the mutation?

c-Jun/p38MAPK/ASIC3 pathways specifically activated by nerve growth factor through TrkA are crucial for mechanical allodynia development

Mechanical allodynia is a cardinal sign of several inflammatory pain disorders where nerve growth factor, a prototypic neurotrophin, plays a crucial role by binding to TrkA receptors. Here, we took the advantage of our generated knock-in mouse model expressing a chimeric TrkA/TrkC receptor that seems to not specifically develop mechanical allodynia after inflammation, to identify the TrkA downstream pathways involved in this phenomenon. We confirmed and extended that disrupting TrkA-specific pathways leads to a specific deficit in mechanical hypersensitivity development after somatic (systemic nerve growth factor administration and paw incision) and, to a lesser extent, visceral injuries. Despite a deficit in thin, mainly peptidergic, fibre innervation in TrkAC mice, thermal hyperalgesia development was not different from WT mice. Inflammatory reaction (oedema, IL-6 content), pain behaviours after intraplantar capsaicin, as well as TRPV1 calcium imaging response of dorsal root ganglion neurons were similar between TrkAC and WT mice. This deficiency in mechanical allodynia development in TrkAC mice is likely due to the alteration of the expression of different TrkA transduction pathways (ie, Akt, p38 MAPK, and c-Jun) especially p38 MAPK, in the dorsal root ganglion cell bodies, ultimately leading to an alteration of at least, ASIC3 channel overexpression, known to participate in nociceptor mechanosensory function.

Role of mechanosensitive ion channels in the sensation of pain

Our ability to sense mechanical cues from our environment depend on the capacity of molecular sensor capable of converting mechanical energy into biochemical or electrical signals. This process, termed mechanotransduction, relies on the activity of mechanosensitive ion channels (MSCs) that are expressed in most tissues, including cells of the inner and outer ear, sensory and sympathetic neurons, and vascular cells. However, the precise role these channels play in the physiology of the cells and organs, where they are expressed is not completely understood. In this review, we will explore some of the recent findings on the role of MSCs to our sense of mechanical pain.

Characterizing the Mechanical Properties of Ectopic Axonal Receptive Fields in Inflamed Nerves and Following Axonal Transport Disruption

Radiating pain is a significant feature of chronic musculoskeletal pain conditions such as radiculopathies, repetitive motion disorders and whiplash associated disorders. It is reported to be caused by the development of mechanically-sensitive ectopic receptive fields along intact nociceptor axons at sites of peripheral neuroinflammation (neuritis). Since inflammation disrupts axonal transport, we have hypothesised that anterogradely-transported mechanically sensitive ion channels accumulate at the site of disruption, which leads to axonal mechanical sensitivity (AMS). In this study, we have characterised the mechanical properties of the ectopic axonal receptive fields in the rat and have examined the contribution of mechanically sensitive ion channels to the development of AMS following neuritis and vinblastine-induced axonal transport disruption. In both models, there was a positive force-discharge relationship and mechanical thresholds were low (∼9 mN/mm²). All responses were attenuated by Ruthenium Red and FM1-43, which block mechanically sensitive ion channels. In both models, the transport of TRPV1 and TRPA1 was disrupted, and intraneural injection of agonists of these channels caused responses in neurons with AMS following neuritis but not vinblastine treatment. In summary, these data support a role for mechanically sensitive ion channels in the development of AMS.

Skeletal Muscle Reflex-Induced Sympathetic Dysregulation and Sensitization of Muscle Afferents in Type 1 Diabetic Rats

The blood pressure response to exercise is exaggerated in the type 1 diabetes mellitus (T1DM). An overactive exercise pressor reflex (EPR) contributes to the potentiated pressor response. However, the mechanism(s) underlying this abnormal EPR activity remains unclear. This study tested the hypothesis that the heightened blood pressure response to exercise in T1DM is mediated by EPR-induced sympathetic overactivity. Additionally, the study examined whether the single muscle afferents are sensitized by PKC (protein kinase C) activation in this disease. Sprague-Dawley rats were intraperitoneally administered either 50 mg/kg streptozotocin (T1DM) or saline (control). At 1 to 3 weeks after administration, renal sympathetic nerve activity and mean arterial pressure responses to activation of the EPR, mechanoreflex, and metaboreflex were measured in decerebrate animals. Action potential responses to mechanical and chemical stimulation were determined in group IV afferents with pPKCα (phosphorylated-PKCα) levels assessed in dorsal root ganglia. Compared with control, EPR (58±18 versus 96±33%; P<0.05), mechanoreflex (21±13 versus 51±20%; P<0.05), and metaboreflex (40±20 versus 88±39%; P<0.01) activation in T1DM rats evoked significant increases in renal sympathetic nerve activity as well as mean arterial pressure. The response of group IV afferents to mechanical (18±24 versus 61±45 spikes; P<0.01) and chemical (0.3±0.4 versus 1.6±0.8 Hz; P<0.01) stimuli were significantly greater in T1DM than control. T1DM rats showed markedly increased pPKCα levels in dorsal root ganglia compared with control. These data suggest that in T1DM, abnormally muscle reflex-evoked increases in sympathetic activity mediate exaggerations in blood pressure. Further, sensitization of muscle afferents, potentially via PKC activation, may contribute to this abnormal circulatory responsiveness.

Differential modulation of voltage-gated sodium channels by nerve growth factor in three major subsets of TrkA-expressing nociceptors

Nerve growth factor is an inflammatory mediator that induces long-lasting hyperalgesia, which can partially be attributed to nerve growth factor-induced sensitization of primary afferent nociceptors. It was shown that nerve growth factor increases the excitability of polymodal C-fibre nociceptors by modulating tetrodotoxin-sensitive and tetrodotoxin-resistant voltage-gated sodium channels, but hitherto only little is known about the effects of nerve growth factor on sodium currents in other nociceptor subtypes that express the nerve growth factor receptor TrkA. We previously characterized two reporter mouse lines that allow the unequivocal identification of two important subclasses of TrkA-expressing nociceptors – i.e. neuropeptide Y receptor type 2 (NPY2R⁺ ) Aδ-fibre nociceptors that mediate pinprick pain and nicotinic acetylcholine receptor alpha-3 subunit (CHRNA3⁺ ) silent nociceptors, which are the most abundant TrkA⁺ nociceptors in visceral organs and deep somatic tissues. Here, we utilized these mouse lines to investigate the expression patterns and the possible nerve growth factor-dependent modulation of sodium channels in these neurons using whole-cell patch-clamp recordings and quantitative real-time polymerase chain reaction. We demonstrate that NPY2R⁺ nociceptors, CHRNA3⁺ ‘silent’ nociceptors and polymodal C-fibre nociceptors express different combinations of sodium channel α- and β-subunits and accordingly exhibit functionally different sodium currents. Moreover, we demonstrate that nerve growth factor produces robust hyperpolarizing shifts in the half-activation voltage of tetrodotoxin-resistant currents in NPY2R⁺ nociceptors and polymodal C-fibre nociceptors and also shifts the half-activation of tetrodotoxin-sensitive currents in polymodal C-fibre nociceptors. In silent nociceptors, however, nerve growth factor solely increases the current density of the tetrodotoxin-resistant current but does not alter other sodium channel properties. Considering the different peripheral target tissues and the previously reported roles in different forms of pain of the nociceptor subpopulations that were examined here, our results suggest that nerve growth factor differentially contributes to the development visceral and cutaneous pain hypersensitivity and highlights the importance of developing different therapeutic strategies for different forms of pain.

Functional and Molecular Characterization of Mechanoinsensitive "Silent" Nociceptors

Mechanical and thermal hyperalgesia (pain hypersensitivity) are cardinal signs of inflammation. Although the mechanism underlying thermal hyperalgesia is well understood, the cellular and molecular basis of mechanical hyperalgesia is poorly described. Here, we have identified a subset of peptidergic C-fiber nociceptors that are insensitive to noxious mechanical stimuli under normal conditions but become sensitized to such stimuli when exposed to the inflammatory mediator nerve growth factor (NGF). Strikingly, NGF did not affect mechanosensitivity of other nociceptors. We show that these mechanoinsensitive "silent" nociceptors are characterized by the expression of the nicotinic acetylcholine receptor subunit alpha-3 (CHRNA3) and that the mechanically gated ion channel PIEZO2 mediates NGF-induced mechanosensitivity in these neurons. Retrograde tracing revealed that CHRNA3⁺ nociceptors account for ∼50% of all peptidergic nociceptive afferents innervating visceral organs and deep somatic tissues. Hence, our data suggest that NGF-induced "un-silencing" of CHRNA3⁺ nociceptors significantly contributes to the development of mechanical hyperalgesia during inflammation.

Inhibition of somatosensory mechanotransduction by annexin A6

Mechanically activated, slowly adapting currents in sensory neurons have been linked to noxious mechanosensation. The conotoxin NMB-1 (noxious mechanosensation blocker-1) blocks such currents and inhibits mechanical pain. Using a biotinylated form of NMB-1 in mass spectrometry analysis, we identified 67 binding proteins in sensory neurons and a sensory neuron-derived cell line, of which the top candidate was annexin A6, a membrane-associated calcium-binding protein. Annexin A6-deficient mice showed increased sensitivity to mechanical stimuli. Sensory neurons from these mice showed increased activity of the cation channel Piezo2, which mediates a rapidly adapting mechano-gated current linked to proprioception and touch, and a decrease in mechanically activated, slowly adapting currents. Conversely, overexpression of annexin A6 in sensory neurons inhibited rapidly adapting currents that were partially mediated by Piezo2. Furthermore, overexpression of annexin A6 in sensory neurons attenuated mechanical pain in a mouse model of osteoarthritis, a disease in which mechanically evoked pain is particularly problematic. These data suggest that annexin A6 can be exploited to inhibit chronic mechanical pain.

Peripheral 5-HT3 mediates mirror-image pain by a cross-talk with acid-sensing ion channel 3

Mirror-image pain (MIP), which occurs along with complex regional pain syndrome, rheumatoid arthritis and chronic migraine, is characterized by increased pain sensitivity of healthy body regions other than the actual injured or inflamed sites. A high level of peripheral inflammation may activate central or peripheral glia, triggering mirror-image pain. However, which receptors mediate inflammatory signals to contribute glial activation remains unclear. Intraplantarly injecting mice with 5-hydroxytryptamine (5-HT) or acidic buffer (proton) caused only unilateral hyperalgesia, but co-injection of 5-HT/acid induced bilateral hyperalgesia (MIP). Blocking 5-HT₃ or acid-sensing ion channel 3 (ASIC3) abolished satellite glial activation, inhibiting MIP. Interestingly, intraplantar administration of a 5-HT₃ agonist induced MIP, and 5-HT₃-mediated MIP can be reversed by a 5-HT₃ antagonist or an ASIC3 blocker. Similar results were found using a ASIC3 agonist. Furthermore, 5-HT₃ was observed to co-localize with ASIC3 in DRG neurons; 5-HT₃ activation-induced an increase in intracellular calcium that was inhibited by an ASIC3 blocker and vice versa. A cross-talk between 5-HT₃ and ASIC3 mediates satellite glial activation, thereby triggering mirror-image pain.

Regulation of Piezo Channels by Cellular Signaling Pathways

The recently identified mechanically activated Piezo1 and Piezo2 channels play major roles in various aspects of mechanosensation in mammals, and their mutations are associated with human diseases. Recent reports show that activation of cell surface receptors coupled to heterotrimeric Gq proteins increase the sensitivity of Piezo2 channels to mechanical stimuli. Activation of the cyclic adenosine monophosphate pathway was also shown to potentiate Piezo2 channel activity. This phenomenon may play a role in mechanical allodynia or hyperalgesia during inflammation. Both Piezo1 and Piezo2 channels are inhibited upon depletion of plasma membrane phosphoinositides, in response to phospholipase C activation by Ca²⁺ influx via the transient receptor potential vanilloid 1 channels. This review will discuss current knowledge on regulation of Piezo channels by these intracellular signaling pathways.

Mechanical sensitivity and electrophysiological properties of acutely dissociated dorsal root ganglion neurons of rats

Primary afferent fibers use mechanically activated (MA) currents to transduce innocuous and noxious mechanical stimuli. However, it is largely unknown about the differences in MA currents between the afferents for sensing innocuous and noxious stimuli. In the present study, we used dorsal root ganglion (DRG) neurons acutely dissociated from rats and studied their MA currents and also their intrinsic membrane properties. Recorded from small-sized DRG neurons, we found that most of these neurons were mechanically sensitive (MS) showing MA currents. The MS neurons could be classified into nociceptive-like mechanically sensitive (Noci-MS) and non-nociceptive-like mechanically sensitive (nonNoci-MS) neurons based on their action potential shapes. Noci-MS neurons responded to mechanical stimulation with three types of MA currents, rapidly adapting (RA), intermediately adapting (IA), and slowly adapting (SA) currents. In contrast, almost all nonNoci-MS neurons showed RA current type in response to mechanical stimulation. Mechanical thresholds had a broad range for both nonNoci-MS and Noci-MS neurons, and the thresholds were not significantly different between them. However, MA current densities were significantly smaller in Noci-MS than in nonNoci-MS neurons. Noci-MS and nonNoci-MS neurons also showed significant differences in their electrophysiological properties including action potential (AP) thresholds and AP firing patterns. These differences may contribute to the differential sensory encoding for innocuous and noxious mechanical stimuli.

Regulation of Piezo2 Mechanotransduction by Static Plasma Membrane Tension in Primary Afferent Neurons

The Piezo2 channel is a newly identified mammalian mechanical transducer that confers rapidly adapting mechanically activated (RA-MA) currents in primary afferent neurons. The Piezo2 channels sense rapid membrane displacement, but it is not clear whether they are sensitive to osmotic swelling, which slowly increases static plasma membrane tension (SPMT). Here, we show that SPMT exerts a profound impact on the mechanical sensitivity of RA-MA channels in primary afferent neurons. RA-MA currents are greatly enhanced, and the mechanical threshold was reduced in both primary afferent neurons of rat dorsal root ganglia (DRG) and HEK293 cells heterologously expressing Piezo2 when these cells undergo osmotic swelling to increase SPMT. Osmotic swelling switches the kinetics of RA-MA currents to the slowly adapting type in both cultured DRG neurons and HEK293 cells heterologously expressing Piezo2. The potentiation of RA-MA currents is abolished when cultured DRG neurons are treated with cytochalasin D, an actin filament disruptor that prevents SPMT of cultured DRG neurons from an increase by osmotic swelling. Osmotic swelling significantly increases DRG neuron mechano-excitability such that a subthreshold mechanical stimulus can result in action potential firing. Behaviorally, the mechanical hind paw withdrawal threshold in rats is reduced following the injection of a hypotonic solution, but this osmotic effect is abolished when cytochalasin D or Gd(3+) is co-administered with the hypo-osmotic solution. Taken together, our findings suggest that Piezo2-mediated mechanotransduction is regulated by SPMT in primary afferent neurons. Because SPMT can be changed by multiple biological factors, our findings may have broad implications in mechanical sensitivity under physiological and pathological conditions.

Delayed onset muscle soreness: Involvement of neurotrophic factors

Delayed-onset muscle soreness (DOMS) is quite a common consequence of unaccustomed strenuous exercise, especially exercise containing eccentric contraction (lengthening contraction, LC). Its typical sign is mechanical hyperalgesia (tenderness and movement related pain). Its cause has been commonly believed to be micro-damage of the muscle and subsequent inflammation. Here we present a brief historical overview of the damage-inflammation theory followed by a discussion of our new findings. Different from previous observations, we have observed mechanical hyperalgesia in rats 1-3 days after LC without any apparent microscopic damage of the muscle or signs of inflammation. With our model we have found that two pathways are involved in inducing mechanical hyperalgesia after LC: activation of the B2 bradykinin receptor-nerve growth factor (NGF) pathway and activation of the COX-2-glial cell line-derived neurotrophic factor (GDNF) pathway. These neurotrophic factors were produced by muscle fibers and/or satellite cells. This means that muscle fiber damage is not essential, although it is sufficient, for induction of DOMS, instead, NGF and GDNF produced by muscle fibers/satellite cells play crucial roles in DOMS.

Long-Term Effects of Botulinum Toxin Complex Type A Injection on Mechano- and Metabo-Sensitive Afferent Fibers Originating from Gastrocnemius Muscle

The aim of the present study was to investigate long term effects of motor denervation by botulinum toxin complex type A (BoNT/A) from Clostridium Botulinum, on the afferent fibers originating from the gastrocnemius muscle of rats. Animals were divided in 2 experimental groups: 1) untreated animals acting as control and 2) treated animals in which the toxin was injected in the left muscle, the latter being itself divided into 3 subgroups according to their locomotor recovery with the help of a test based on footprint measurements of walking rats: i) no recovery (B0), ii) 50% recovery (B50) and iii) full recovery (B100). Then, muscle properties, metabosensitive afferent fiber responses to potassium chloride (KCl) and lactic acid injections and Electrically-Induced Fatigue (EIF), and mechanosensitive responses to tendon vibrations were measured. At the end of the experiment, rats were killed and the toxin injected muscles were weighted. After toxin injection, we observed a complete paralysis associated to a loss of force to muscle stimulation and a significant muscle atrophy, and a return to baseline when the animals recover. The response to fatigue was only decreased in the B0 group. The responses to KCl injections were only altered in the B100 groups while responses to lactic acid were altered in the 3 injected groups. Finally, our results indicated that neurotoxin altered the biphasic pattern of response of the mechanosensitive fiber to tendon vibrations in the B0 and B50 groups. These results indicated that neurotoxin injection induces muscle afferent activity alterations that persist and even worsen when the muscle has recovered his motor activity.

Amplified Mechanically Gated Currents in Distinct Subsets of Myelinated Sensory Neurons following In Vivo Inflammation of Skin and Muscle

Primary afferents are sensitized to mechanical stimuli following in vivo inflammation, but whether sensitization of mechanically gated ion channels contributes to this phenomenon is unknown. Here we identified two populations of murine A fiber-type sensory neurons that display markedly different responses to focal mechanical stimuli of the membrane based on their expression of calcitonin gene-related peptide (CGRP). Following inflammation of the hindpaw, myelinated, CGRP-positive neurons projecting to the paw skin displayed elevated mechanical currents in response to mechanical stimuli. Conversely, muscle inflammation markedly amplified mechanical currents in myelinated, CGRP-negative neurons projecting to muscle. These data show, for the first time, that mechanically gated currents are amplified following in vivo tissue inflammation, and also suggest that mechanical sensitization can occur in myelinated neurons after inflammation.

Botulinum Toxin for Neuropathic Pain: A Review of the Literature

Botulinum neurotoxin (BoNT), derived from Clostridium botulinum, has been used therapeutically for focal dystonia, spasticity, and chronic migraine. Its spectrum as a potential treatment for neuropathic pain has grown. Recent opinions on the mechanism behind the antinociceptive effects of BoNT suggest that it inhibits the release of peripheral neurotransmitters and inflammatory mediators from sensory nerves. There is some evidence showing the axonal transport of BoNT, but it remains controversial. The aim of this review is to summarize the experimental and clinical evidence of the antinociceptive effects, mechanisms, and therapeutic applications of BoNT for neuropathic pain conditions, including postherpetic neuralgia, complex regional pain syndrome, and trigeminal neuralgia. The PubMed and OvidSP databases were searched from 1966 to May 2015. We assessed levels of evidence according to the American Academy of Neurology guidelines. Recent studies have suggested that BoNT injection is an effective treatment for postherpetic neuralgia and is likely efficient for trigeminal neuralgia and post-traumatic neuralgia. BoNT could also be effective as a treatment for diabetic neuropathy. It has not been proven to be an effective treatment for occipital neuralgia or complex regional pain syndrome.

Acidosis Mediates the Switching of Gs-PKA and Gi-PKCε Dependence in Prolonged Hyperalgesia Induced by Inflammation

Chronic inflammatory pain, when not effectively treated, is a costly health problem and has a harmful effect on all aspects of health-related quality of life. Previous studies suggested that in male Sprague Dawley rats, prostaglandin E2 (PGE2)-induced short-term hyperalgesia depends on protein kinase A (PKA) activity, whereas long-lasting hyperalgesia induced by PGE2 with carrageenan pre-injection, requires protein kinase Cε (PKCε). However, the mechanism underlying the kinase switch with short- to long-term hyperalgesia remains unclear. In this study, we used the inflammatory agents carrageenan or complete Freund's adjuvant (CFA) to induce long-term hyperalgesia, and examined PKA and PKCε dependence and switching time. Hyperalgesia induced by both agents depended on PKA/PKCε and Gs/Gi-proteins, and the switching time from PKA to PKCε and from Gs to Gi was about 3 to 4 h after inflammation induction. Among the single inflammatory mediators tested, PGE2 and 5-HT induced transient hyperalgesia, which depended on PKA and PKCε, respectively. Only acidic solution-induced hyperalgesia required Gs-PKA and Gi-PKCε, and the switch time for kinase dependency matched inflammatory hyperalgesia, in approximately 2 to 4 h. Thus, acidosis in inflamed tissues may be a decisive factor to regulate switching of PKA and PKCε dependence via proton-sensing G-protein-coupled receptors.

Protein kinase C modulates frequency of micturition and non-voiding contractions in the urinary bladder via neuronal and myogenic mechanisms

Background

Protein Kinase C (PKC) dysfunction is implicated in a variety of smooth muscle disorders including detrusor overactivity associated with frequency and urgency of micturition. In this study, we aimed to evaluate the modulatory effects of endogenous PKC-dependent pathways on bladder storage and emptying function.

Methods

We utilized in vivo cystometry and in vitro organ bath studies using isolated bladder muscle strips (BMS) from rats to measure contractility, intravesical pressure, and voided volume. Both in vitro and in vivo results were statistically analyzed using one-way repeated measures ANOVA between the groups followed by Bonferroni’s post-test, as appropriate (Systat Software Inc., San Jose, CA).

Results

Effects of PKC activators, phorbol-12,13-dibutyrate (PDBu), and phorbol-12,13-myristate (PMA), were concentration-dependent, with high concentrations increasing frequency of micturition, and sensitivity of intramural nerves to electrical field stimulation (EFS), in vitro, while lower concentrations had no effect on BMS sensitivity to EFS. The PKC inhibitors, bisindolylmaleimide1 (Bim-1), (28 nM), and Ro318220 (50 μM) triggered an increase in the number of non-voiding contractions (NVC), and a decrease in the voided volume associated with reduced ability to maintain contractile force upon EFS, but did not affect peak force in vitro. Both low (50 nM) and high PDBu 1 micromolar (1uM) decreased the sensitivity of BMS to carbachol. Application of a low concentration of PDBu inhibited spontaneous contractions, in vitro, and Bim-1-induced NVC, and restored normal voiding frequency during urodynamic recordings in vivo.

Conclusions

In summary, the effects of low PKC stimulation include inhibition of smooth muscle contractile responses, whereas high levels of PKC stimulation increased nerve-mediated contractions in vitro, and micturition contractions in vivo. These results indicate that endogenous PKC signaling displays a concentration-dependent contraction profile in the urinary bladder via both smooth muscle and nerve-mediated pathways.

Contribution of mechanosensitive ion channels to somatosensation

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

Role of nerve growth factor in pain

Nerve growth factor (NGF) was first identified as a substance that is essential for the development of nociceptive primary neurons and later found to have a role in inflammatory hyperalgesia in adults. Involvement of NGF in conditions with no apparent inflammatory signs has also been demonstrated. In this review we look at the hyperalgesic effects of exogenously injected NGF into different tissues, both human and animal, with special emphasis on the time course of these effects. The roles of NGF in inflammatory and neuropathic conditions as well as cancer pain are then reviewed. The role of NGF in delayed onset muscle soreness is described in more detail than its other roles based on the authors' recent observations. Acute effects are considered to be peripherally mediated, and accordingly, sensitization of nociceptors by NGF to heat and mechanical stimulation has been reported. Changes in the conductive properties of axons have also been reported. The intracellular mechanisms so far proposed for heat sensitization are direct phosphorylation and membrane trafficking of TRPV1 by TrkA. Little investigation has been done on the mechanism of mechanical sensitization, and it is still unclear whether mechanisms similar to those for heat sensitization work in mechanical sensitization. Long-lasting sensitizing effects are mediated both by changed expression of neuropeptides and ion channels (Na channels, ASIC, TRPV1) in primary afferents and by spinal NMDA receptors. Therapeutic perspectives are briefly discussed at the end of the chapter.

Sensory mechanotransduction at membrane-matrix interfaces

Sensory cells specialized to detect extremely small mechanical changes are common to the auditory and somatosensory systems. It is widely accepted that mechanosensitive channels form the core of the mechanoelectrical transduction in hair cells as well as the somatic sensory neurons that underlie the sense of touch and mechanical pain. Here, we will review how the activation of such channels can be measured in a meaningful physiological context. In particular, we will discuss the idea that mechanosensitive channels normally occur in transmembrane complexes that are anchored to extracellular matrix components (ECM) both in vitro and in vivo. One component of such complexes in sensory neurons is the integral membrane scaffold protein STOML3 which is a robust physiological regulator of native mechanosensitive currents. In order to better characterize such channels in transmembrane complexes, we developed a new electrophysiological method that enables the quantification of mechanosensitive current amplitude and kinetics when activated by a defined matrix movement in cultured cells. The results of such studies strongly support the idea that ion channels in transmembrane complexes are highly tuned to detect movement of the cell membrane in relation to the ECM.

Mechanical allodynia

Mechanical allodynia (other pain) is a painful sensation caused by innocuous stimuli like light touch. Unlike inflammatory hyperalgesia that has a protective role, allodynia has no obvious biological utility. Allodynia is associated with nerve damage in conditions such as diabetes, and is likely to become an increasing clinical problem. Unfortunately, the mechanistic basis of this enhanced sensitivity is incompletely understood. In this review, we describe evidence for the involvement of candidate mechanosensitive channels such as Piezo2 and their role in allodynia, as well as the peripheral and central nervous system mechanisms that have also been implicated in this form of pain. Specific treatments that block allodynia could be very useful if the cell and molecular basis of the condition could be determined. There are many potential mechanisms underlying this condition ranging from alterations in mechanotransduction and sensory neuron excitability to the actions of inflammatory mediators and wiring changes in the CNS. As with other pain conditions, it is likely that the range of redundant mechanisms that cause allodynia will make therapeutic intervention problematic.

Botulinum toxin-A treatment reduces human mechanical pain sensitivity and mechanotransduction

The mechanisms underlying the analgesic effects of botulinum toxin serotype A (BoNT-A) are not well understood. We have tested the hypothesis that BoNT-A can block nociceptor transduction. Intradermal administration of BoNT-A to healthy volunteers produced a marked and specific decrease in noxious mechanical pain sensitivity, whereas sensitivity to low-threshold mechanical and thermal stimuli was unchanged. BoNT-A did not affect cutaneous innervation. In cultured rodent primary sensory neurons, BoNT-A decreased the proportion of neurons expressing slowly adapting mechanically gated currents linked to mechanical pain transduction. Inhibition of mechanotransduction provides a novel locus of action of BoNT-A, further understanding of which may extend its use as an analgesic agent.

Nerve growth factor and nociception: from experimental embryology to new analgesic therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.

GTP-dependent run-up of Piezo2-type mechanically activated currents in rat dorsal root ganglion neurons

Rapidly adapting mechanically activated channels (RA) are expressed in primary afferent neurons and identified as Piezo2 ion channels. We made whole-cell voltage-clamp recordings from cultured dorsal root ganglion (DRG) neurons to study RA channel regulation. RA currents showed gradual increases in current amplitude (current "run-up") after establishing whole-cell mode when 0.33 mM GTP or 0.33 mM GTPγS was included in the patch pipette internal solution. RA current run-up was also observed in HEK293 cells that heterologously expressed Piezo2 ion channels. No significant RA current run-up was observed in DRG neurons when GTP was omitted from the patch pipette internal solution, when GTP was replaced with 0.33 mM GDP, or when recordings were made under the perforated patch-clamp recording configuration. Our findings revealed a GTP-dependent up-regulation of the function of piezo2 ion channels in DRG neurons.

Glial cell line-derived neurotrophic factor sensitized the mechanical response of muscular thin-fibre afferents in rats

BACKGROUND

The role of glial cell line-derived neurotrophic factor (GDNF) in pain and muscular nociceptor activities is not well understood. We examined pain-related behaviour and mechanical response of muscular thin-fibre afferents after intramuscular injection of GDNF in rats.

METHODS

GDNF and antagonist to transient receptor potential V1 or acid-sensing ion channels were injected into rat gastrocnemius muscle and muscular mechanical hyperalgesia was assessed with a Randall-Selitto analgesiometer. Activities of single C- (conduction velocity < 2.0 m/s) and Aδ-fibres (conduction velocity 2.0-12.0 m/s) were recorded from extensor digitorum longus muscle-nerve preparations in vitro. The changes in the responses to mechanical stimuli before and after GDNF injection were recorded.

RESULTS

Mechanical hyperalgesia was observed from 1 h to 1 day after GDNF (0.03 μM, 20 μL) injection. The decreased withdrawal threshold was temporarily reversed after intramuscular injection of amiloride (50 mM, 20 μL), but not capsazepine (50 μM, 20 μL). In single-fibre recordings, both phosphate buffered saline (PBS) and GDNF failed to induce any significant discharges. GDNF significantly enhanced the mechanical response when compared with the PBS group, but only in Aδ-fibre afferents. C-fibres were not affected. Significantly lowered threshold and increased response magnitude to mechanical stimuli were observed 30 or 60-120 min after injection. These times are compatible with the timing of the onset of the hyperalgesic effect of GDNF.

CONCLUSIONS

These results suggest that GDNF increased the response of muscular Aδ-fibre afferents to mechanical stimuli, resulting in muscular mechanical hyperalgesia.

A role for Piezo2 in EPAC1-dependent mechanical allodynia

Aberrant mechanosensation has an important role in different pain states. Here we show that Epac1 (cyclic AMP sensor) potentiation of Piezo2-mediated mechanotransduction contributes to mechanical allodynia. Dorsal root ganglia Epac1 mRNA levels increase during neuropathic pain, and nerve damage-induced allodynia is reduced in Epac1-/- mice. The Epac-selective cAMP analogue 8-pCPT sensitizes mechanically evoked currents in sensory neurons. Human Piezo2 produces large mechanically gated currents that are enhanced by the activation of the cAMP-sensor Epac1 or cytosolic calcium but are unaffected by protein kinase C or protein kinase A and depend on the integrity of the cytoskeleton. In vivo, 8-pCPT induces long-lasting allodynia that is prevented by the knockdown of Epac1 and attenuated by mouse Piezo2 knockdown. Piezo2 knockdown also enhanced thresholds for light touch. Finally, 8-pCPT sensitizes responses to innocuous mechanical stimuli without changing the electrical excitability of sensory fibres. These data indicate that the Epac1-Piezo2 axis has a role in the development of mechanical allodynia during neuropathic pain.

Generalization of the dynamic clamp concept in neurophysiology and behavior

The idea of closed-loop interaction in in vitro and in vivo electrophysiology has been successfully implemented in the dynamic clamp concept strongly impacting the research of membrane and synaptic properties of neurons. In this paper we show that this concept can be easily generalized to build other kinds of closed-loop protocols beyond (or in addition to) electrical stimulation and recording in neurophysiology and behavioral studies for neuroethology. In particular, we illustrate three different examples of goal-driven real-time closed-loop interactions with drug microinjectors, mechanical devices and video event driven stimulation. Modern activity-dependent stimulation protocols can be used to reveal dynamics (otherwise hidden under traditional stimulation techniques), achieve control of natural and pathological states, induce learning, bridge between disparate levels of analysis and for a further automation of experiments. We argue that closed-loop interaction calls for novel real time analysis, prediction and control tools and a new perspective for designing stimulus-response experiments, which can have a large impact in neuroscience research.

Extracellular matrix proteoglycan plays a pivotal role in sensitization by low pH of mechanosensitive currents in nociceptive sensory neurones

Ischaemia, inflammation, and exercise lead to tissue acidosis, which induces pain and mechanical hyperalgesia. Corresponding to this, enhanced thin-fibre afferent responses to mechanical stimulation have been recorded in vitro at low pH. However, knowledge about how this sensitization by low pH occurs is lacking. In this study, we found that all three types (rapidly adapting (RA), intermediately adapting and slowly adapting) of mechanically activated currents recorded with the whole cell patch-clamp method were sensitized by low pH in rat cultured dorsal root ganglion neurones. This sensitization was mainly observed in neurones positively labelled with isolectin B4 (IB4), which binds to versican, a chondroitin sulfate proteoglycan. Inhibitors of acid-sensitive channels (amiloride and capsazepine) did not block sensitization by low pH except in RA neurones, and extracellular calcium was not involved even in the sensitization of this type of neurone. A broad spectrum kinase inhibitor and a phospholipase C inhibitor (staurosporine and U73122) failed to block pH-induced sensitization in IB4-positive neurones, suggesting that these intracellular signalling pathways are not involved. Notably, both excess chondroitin sulfate in the extracellular solution and pretreatment of the neurone culture with chondroitinase ABC attenuated this low pH-induced sensitization in IB4-positive neurones. These findings suggest that a change in interaction between mechanosensitive channels and/or their auxiliary molecules and the side chain of versican on the cell surface causes this sensitization, at least in IB4-positive neurones. This report proposes a novel mechanism for sensitization that involves extracellular proteoglycans (versican).

Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

The cervical facet joint and its capsule are a common source of neck pain from whiplash. Mechanical hyperalgesia elicited by painful facet joint distraction is associated with spinal neuronal hyperexcitability that can be induced by transmitter/receptor systems that potentiate the synaptic activation of neurons. This study investigated the temporal response of a glutamate receptor and transporters in the dorsal root ganglia (DRG) and spinal cord. Bilateral C6/C7 facet joint distractions were imposed in the rat either to produce behavioral sensitivity or without inducing any sensitivity. Neuronal metabotropic glutamate receptor-5 (mGluR5) and protein kinase C-epsilon (PKCε) expression in the DRG and spinal cord were evaluated on days 1 and 7. Spinal expression of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1), was also quantified at both time points. Painful distraction produced immediate behavioral hypersensitivity that was sustained for 7 days. Increased expression of mGluR5 and PKCε in the DRG was not evident until day 7 and only following painful distraction; this increase was observed in small-diameter neurons. Only painful facet joint distraction produced a significant increase (p<0.001) in neuronal mGluR5 over time, and this increase also was significantly elevated (p≤0.05) over responses in the other groups at day 7. However, there were no differences in spinal PKCε expression on either day or between groups. Spinal EAAC1 expression was significantly increased (p<0.03) only in the nonpainful groups on day 7. Results from this study suggest that spinal glutamatergic plasticity is selectively modulated in association with facet-mediated pain.

Generation of a pain memory in the primary afferent nociceptor triggered by PKCε activation of CPEB

Isolectin B(4)-positive [IB(4)(+)] primary afferent nociceptors challenged with an inflammatory or neuropathic insult develop a PKCε-dependent long-lasting hyperalgesic response to a subsequent challenge by the proinflammatory cytokine prostaglandin E(2) (PGE(2)), a phenomenon known as hyperalgesic priming. Here we demonstrate that the neuroplasticity underlying nociceptor priming requires 72 h to be established; rats that have been challenged with the inflammatory mediator TNFα 24 or 48 h ahead of PGE(2) do not show the enhanced and prolonged hyperalgesic response by which primed IB(4)(+)-nociceptors are being characterized. Moreover, as the underlying plasticity can be interrupted by the peripheral administration of the protein translation inhibitor anisomycin it is reflected by changes in the peripheral protein expression pattern. Finally, the induction of priming by the selective PKCε agonist, psi ε receptor for activated c kinase (ψεRACK) can be prevented, but not reversed by intrathecal injections of antisense oligodeoxynucleotides for the cytoplasmic polyadenylation element binding protein (CPEB) mRNA, a master regulator of protein translation that coimmunoprecipitated with PKCε and is almost exclusively expressed by IB(4)(+)-nociceptors. Our results suggest that CPEB is downstream of PKCε in the cellular signaling cascade responsible for the induction of priming, raising the intriguing possiblity that prion-like misfolding could be a responsible mechanism for the chronification of pain.

Sodium channels and mammalian sensory mechanotransduction

BACKGROUND

Members of the degenerin/epithelial (DEG/ENaC) sodium channel family are mechanosensors in C elegans, and Nav1.7 and Nav1.8 voltage-gated sodium channel knockout mice have major deficits in mechanosensation. β and γENaC sodium channel subunits are present with acid sensing ion channels (ASICs) in mammalian sensory neurons of the dorsal root ganglia (DRG). The extent to which epithelial or voltage-gated sodium channels are involved in transduction of mechanical stimuli is unclear.

RESULTS

Here we show that deleting β and γENaC sodium channels in sensory neurons does not result in mechanosensory behavioural deficits. We had shown previously that Nav1.7/Nav1.8 double knockout mice have major deficits in behavioural responses to noxious mechanical pressure. However, all classes of mechanically activated currents in DRG neurons are unaffected by deletion of the two sodium channels. In contrast, the ability of Nav1.7/Nav1.8 knockout DRG neurons to generate action potentials is compromised with 50% of the small diameter sensory neurons unable to respond to electrical stimulation in vitro.

CONCLUSION

Behavioural deficits in Nav1.7/Nav1.8 knockout mice reflects a failure of action potential propagation in a mechanosensitive set of sensory neurons rather than a loss of primary transduction currents. DEG/ENaC sodium channels are not mechanosensors in mouse sensory neurons.

Noxious mechanosensation - molecules and circuits

Drugs that block mechanically-evoked pain would be useful for many common pain conditions, but appropriate drug development targets have yet to be defined. There is increasing evidence that both peripheral sensory neuron wiring patterns as well as the expression of transducing molecules are important for modality specific pain sensations. Progress in identifying the cell types, candidate transducing molecules and wiring patterns involved in mechanosensation has been dramatic over the past few years. Here we focus on potential mechano-transducing channels, and the relevant cell types and wiring patterns that provide clues for new analgesic drug development strategies.

Recording of mechanosensitive currents using piezoelectrically driven mechanostimulator

Mechanotransduction constitutes the basis of a variety of physiological processes, such as the senses of touch, balance, proprioception and hearing. In vertebrates, mechanosensation is mediated by mechanosensory receptors. The aptitude of these mechanoreceptors for detecting mechanical information relies on the presence of mechanosensitive channels that transform mechanical forces into electrical signals. However, advances in understanding mechanical transduction processes have proven difficult because sensory nerve endings have historically been inaccessible to patch-clamp recording. We report here an in vitro model of mechanotransduction that allows the application of focal force on sensory neuron membrane during whole-cell patch clamping. This technique, called mechano-clamp, provides an opportunity to explore the properties and identities of mechanotransducer channels in mammalian sensory neurons. The protocol-from tissue dissociation to patch-clamp recording-can be completed in 7 h.

Mitochondrial dependence of nerve growth factor-induced mechanical hyperalgesia

Mitochondria are present at high concentration at the site of sensory transduction in the peripheral terminals of nociceptors. Because nerve growth factor (NGF), which induces nociceptor sensitization by acting on the high-affinity tropomyosin receptor kinase A (TrkA) receptor, also produces local recruitment of mitochondria in DRG neurons, we evaluated the role of mitochondria in NGF-induced mechanical hyperalgesia. Inhibition of 3 major mitochondrial functions-oxidation of nutrients, adenosine triphosphate (ATP) production, and generation of reactive oxygen species--markedly attenuated NGF-induced mechanical hyperalgesia in the rat. Disruption of microtubules, which are required for the trafficking and subcellular localization of mitochondria, also attenuated NGF-induced hyperalgesia. Our results suggest a contribution of mitochondrial localization and function to NGF-dependent pain syndromes.

TRPA1 contributes to specific mechanically activated currents and sensory neuron mechanical hypersensitivity

The mechanosensory role of TRPA1 and its contribution to mechanical hypersensitivity in sensory neurons remains enigmatic. We elucidated this role by recording mechanically activated currents in conjunction with TRPA1 over- and under-expression and selective pharmacology. First, we established that TRPA1 transcript, protein and functional expression are more abundant in smaller-diameter neurons than larger-diameter neurons, allowing comparison of two different neuronal populations. Utilising whole cell patch clamping, we applied calibrated displacements to neurites of dorsal root ganglion (DRG) neurons in short-term culture and recorded mechanically activated currents termed intermediately (IAMCs), rapidly (RAMCs) or slowly adapting (SAMCs). Trpa1 deletion (–/–) significantly reduced maximum IAMC amplitude by 43% in small-diameter neurons compared with wild-type (+/+) neurons. All other mechanically activated currents in small- and large-diameter Trpa1−/− neurons were unaltered. Seventy-three per cent of Trpa1+/+ small-diameter neurons responding to the TRPA1 agonist allyl-isothiocyanate (AITC) displayed IAMCs to neurite displacement, which were significantly enhanced after AITC addition. The TRPA1 antagonist HC-030031 significantly decreased Trpa1+/+ IAMC amplitudes, but only in AITC responsive neurons. Using a transfection system we also showed TRPA1 over-expression in Trpa1+/+ small-diameter neurons increases IAMC amplitude, an effect reversed by HC-030031. Furthermore, TRPA1 introduction into Trpa1−/− small-diameter neurons restored IAMC amplitudes to Trpa1+/+ levels, which was subsequently reversed by HC-030031. In summary our data demonstrate TRPA1 makes a contribution to normal mechanosensation in a specific subset of DRG neurons. Furthermore, they also provide new evidence illustrating mechanisms by which sensitisation or over-expression of TRPA1 enhances nociceptor mechanosensitivity. Overall, these findings suggest TRPA1 has the capacity to tune neuronal mechanosensitivity depending on its degree of activation or expression.

The majority of myelinated and unmyelinated sensory nerve fibers that innervate bone express the tropomyosin receptor kinase A

Although skeletal pain is a leading cause of chronic pain and disability, relatively little is known about the specific populations of nerve fibers that innervate the skeleton. Recent studies have reported that therapies blocking nerve growth factor (NGF) or its cognate receptor, tropomyosin receptor kinase A (TrkA) are efficacious in attenuating skeletal pain. A potential factor to consider when assessing the analgesic efficacy of targeting NGF-TrkA signaling in a pain state is the fraction of NGF-responsive TrkA+ nociceptors that innervate the tissue from which the pain is arising, as this innervation and the analgesic efficacy of targeting NGF-TrkA signaling may vary considerably from tissue to tissue. To explore this in the skeleton, tissue slices and whole mount preparations of the normal, adult mouse femur were analyzed using immunohistochemistry and confocal microscopy. Analysis of these preparations revealed that 80% of the unmyelinated/thinly myelinated sensory nerve fibers that express calcitonin gene-related peptide (CGRP) and innervate the periosteum, mineralized bone and bone marrow also express TrkA. Similarly, the majority of myelinated sensory nerve fibers that express neurofilament 200 kDa (NF200) which innervate the periosteum, mineralized bone and bone marrow also co-express TrkA. In the normal femur, the relative density of CGRP+, NF200+ and TrkA+ sensory nerve fibers per unit volume is: periosteum>bone marrow>mineralized bone>cartilage with the respective relative densities being 100:2:0.1:0. The observation that the majority of sensory nerve fibers innervating the skeleton express TrkA+, may in part explain why therapies that block NGF/TrkA pathway are highly efficacious in attenuating skeletal pain.

Avian magnetite-based magnetoreception: a physiologist's perspective

It is now well established that animals use the Earth's magnetic field to perform long-distance migration and other navigational tasks. However, the transduction mechanisms that allow the conversion of magnetic field variations into an electric signal by specialized sensory cells remain largely unknown. Among the species that have been shown to sense Earth-strength magnetic fields, birds have been a model of choice since behavioural tests show that their direction-finding abilities are strongly influenced by magnetic fields. Magnetite, a ferromagnetic mineral, has been found in a wide range of organisms, from bacteria to vertebrates. In birds, both superparamagnetic (SPM) and single-domain magnetite have been found to be associated with the trigeminal nerve. Electrophysiological recordings from cells in the trigeminal ganglion have shown an increase in action potential firing in response to magnetic field changes. More recently, histological evidence has demonstrated the presence of SPM magnetite in the subcutis of the pigeon's upper beak. The aims of the present review are to review the evidence for a magnetite-based mechanism in birds and to introduce physiological concepts in order to refine the proposed models.

Evidence for a protein tether involved in somatic touch

The gating of ion channels by mechanical force underlies the sense of touch and pain. The mode of gating of mechanosensitive ion channels in vertebrate touch receptors is unknown. Here we show that the presence of a protein link is necessary for the gating of mechanosensitive currents in all low-threshold mechanoreceptors and some nociceptors of the dorsal root ganglia (DRG). Using TEM, we demonstrate that a protein filament with of length approximately 100 nm is synthesized by sensory neurons and may link mechanosensitive ion channels in sensory neurons to the extracellular matrix. Brief treatment of sensory neurons with non-specific and site-specific endopeptidases destroys the protein tether and abolishes mechanosensitive currents in sensory neurons without affecting electrical excitability. Protease-sensitive tethers are also required for touch-receptor function in vivo. Thus, unlike the majority of nociceptors, cutaneous mechanoreceptors require a distinct protein tether to transduce mechanical stimuli.

NGF induces non-inflammatory localized and lasting mechanical and thermal hypersensitivity in human skin

Nerve growth factor (NGF) modulates sensitivity and sprouting of nociceptors. We explored the spatial and temporal sensitization induced by NGF injection (1 microg) in human skin. Hyperalgesia was investigated in 16 volunteers (36+/-9 years) at day 1, 3, 7, 21, and 49. Areas of mechanical (brush, pin-prick) and heat (43 degrees C) sensitization were mapped and thermal (heat and cold) pain thresholds, mechanical (impact stimulation) and electrically evoked pain, and axon reflex flare were assessed. No spontaneous pain or local inflammation was recorded upon NGF injection and during 49 days. Sensitization to heat was maximum at day 3 and lasted 21 days. Hyperalgesia to cold was recorded at day 7 and 21. Hypersensitivity to mechanical impact stimuli developed delayed, reached maximum at day 21, and persisted throughout 49 days. Fifty percent of all volunteers reported a static allodynia to tonic pressure until day 21. Electrical stimulation at 7.5 mA was more painful at the NGF site at day 21, which correlated significantly to maximum impact pain. Axon reflex flare was unaffected by NGF. Sensitization was limited to the NGF injection site, no touch- or pin-prick evoked secondary hyperalgesia was observed. Spatially restricted hyperalgesia indicates a peripheral rather than central mechanism. The temporal profile of lasting nociceptor sensitization suggests an altered peripheral axonal expression of sensory proteins specifically leading to mechanical and thermal sensitization. Intradermal NGF administration provokes a pattern of sensitization that can be used as experimental model for neuropathic pain.