Extracellular cations sensitize and gate capsaicin receptor TRPV1 modulating pain signaling

Solvent-mediated analgesia via the suppression of water permeation through TRPV1 ion channels

Activation of the ion channel transient receptor potential vanilloid 1 (TRPV1), which is integral to pain perception, leads to an expansion of channel width, facilitating the passage of cations and large organic molecules. However, the permeability of TRPV1 channels to water remains uncertain, owing to a lack of suitable tools to study water dynamics. Here, using upconversion nanophosphors to discriminate between H2O and D2O, by monitoring water permeability across activated TRPV1 at the single-cell and single-molecule levels, and by combining single-channel current measurements with molecular dynamics simulations, we show that water molecules flow through TRPV1 and reveal a direct connection between water migration, cation flow and TRPV1 functionality. We also show in mouse models of acute or chronic inflammatory pain that the administration of deuterated water suppresses TRPV1 activity, interrupts the transmission of pain signals and mitigates pain without impacting other neurological responses. Solvent-mediated analgesia may inspire alternative options for pain management.

Role of taste receptors in salty taste perception of minerals and amino acids and developments in salt reduction strategies: A review

Salt (sodium chloride) plays a key role in maintaining the textural, microbiological, and sensorial aspects of the foods. However high dietary salt intake in the population has led to a series of health problems. Currently manufacturers are under pressure to reduce the sodium levels in foods without compromising the consumer experience. Because of the clean salty taste produced by sodium chloride, it has been challenging for the food industry to develop a suitable salt substitute. Studies have shown that different components within a food matrix can influence the perception of saltiness. This review aims to comprehend the potential synergistic effect of compounds such as minerals and amino acids on the perception of saltiness and covers the mechanism of perception where relevant to taste resulting from sodium ions and other metallic ions (such as K, Mg, Ca), as well as various amino acids and their derivatives. Finally, the review summarizes various salt reduction strategies explored by researchers, government organizations and food industry, including the potential use of plant-based extracts.

Temperature-sensing riboceptors

Understanding how cells sense temperature is a fundamental question in biology and is pivotal for the evolution of life. In numerous organisms, temperature is not only sensed but also generated due to cellular processes. Consequently, the mechanisms governing temperature sensation in various organisms have been experimentally elucidated. Extending upon others' proposals and demonstration of protein- and nucleic acid-based thermosensors, and utilizing a colonial India 'punkah-wallahs' analogy, I present my rationale for the necessity of temperature sensing in every organelle in a cell. Finally, I propose temperature-sensing riboceptors (ribonucleic acid receptors) to integrate all the RNA molecules (mRNA, non-coding RNA, and so forth) capable of sensing temperature and triggering a signaling event, which I call as thermocrine signaling. This approach could enable the identification of riboceptors in every cell of almost every organism, not only for temperature but also for other classes of ligands, including gaseous solutes, and water.

Roles of TRPM4 in immune responses in keratinocytes and identification of a novel TRPM4-activating agent

The skin is a protective interface between the internal organs and environment and functions not only as a physical barrier but also as an immune organ. However, the immune system in the skin is not fully understood. A member of the thermo-sensitive transient receptor potential (TRP) channel family, TRPM4, which acts as a regulatory receptor in immune cells, was recently reported to be expressed in human skin and keratinocytes. However, the function of TRPM4 in immune responses in keratinocytes has not been investigated. In this study, we found that treatment with BTP2, a known TRPM4 agonist, reduced cytokine production induced by tumor necrosis factor (TNF) α in normal human epidermal keratinocytes and in immortalized human epidermal keratinocytes (HaCaT cells). This cytokine-reducing effect was not observed in TRPM4-deficient HaCaT cells, indicating that TRPM4 contributed to the control of cytokine production in keratinocytes. Furthermore, we identified aluminum potassium sulfate, as a new TRPM4 activating agent. Aluminum potassium sulfate reduced Ca²⁺ influx by store-operated Ca²⁺ entry in human TRPM4-expressing HEK293T cells. We further confirmed that aluminum potassium sulfate evoked TRPM4-mediated currents, showing direct evidence for TRPM4 activation. Moreover, treatment with aluminum potassium sulfate reduced cytokine expression induced by TNFα in HaCaT cells. Taken together, our data suggested that TRPM4 may serve as a new target for the treatment of skin inflammatory reactions by suppressing the cytokine production in keratinocytes, and aluminum potassium sulfate is a useful ingredient to prevent undesirable skin inflammation through TRPM4 activation.

Diamine Oxidase Activity Deficit and Idiopathic Rhinitis: A New Subgroup of Non-Allergic Rhinitis?

Idiopathic rhinitis represents more than 50% of non-allergic rhinitis, a heterogeneous group that involves the symptomatic inflammation of the nasal mucosa. The TRPV1 receptor of unmyelinated C-type neurons appears to be involved in its pathophysiology. Histamine, whose main catabolic enzyme is DAO, is one of the mediators that can activate this receptor. The failure of DAO causes an increase in the level of histamine in the body and, consequently, the activation of TRPV1. The objective was to investigate the existence of a DAO enzyme activity deficit in idiopathic rhinitis and its correlation with symptoms. A cross-sectional study was conducted in 116 idiopathic rhinitis patients, and DAO activity, nasal peak inspiratory flow, and rhinitis severity were recorded. The prevalence of a DAO activity deficit was 41.38% (95%CI 0.33−0.50; p = 0.05). The DAO activity in patients with mild rhinitis was 52.93 ± 8.72 HDU/mL, in those with moderate rhinitis it was 120.33 ± 71.63 HDU/mL, and in those with severe rhinitis it was 92.58 ± 27.75 HDU/mL (p = 0.006). The NPIF in patients with a DAO activity deficit was 107.92 ± 34.05 L/min, compared to 72.35 ± 27.16 L/min in patients with normal enzymatic activity (p < 0.001), demonstrating a linear correlation between activity levels and nasal obstruction (−0.45; p < 0.001). Therefore, patients with a DAO deficiency and idiopathic rhinitis could present a milder disease course, because the repeated and continuous activation of TRPV1 led to a partial or total decrease in their response (desensitization). This new theory represents a different perspective for the study of idiopathic rhinitis and its relationship with TRPV1, with the regulation or modulation of the desensitization of TRPV1 being an important therapeutic target for patients with idiopathic rhinitis in the future.

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

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

Estrogen metabolites increase nociceptor hyperactivity in a mouse model of uterine pain

Pain emanating from the female reproductive tract is notoriously difficult to be treated and the prevalence of transient pelvic pain has been placed as high as 70-80% in women surveyed. Although sex hormones, especially estrogen, are thought to underlie enhanced pain perception in females, the underlying molecular and cellular mechanisms are not completely understood. Here we show that the pain-initiating TRPA1 channel is required for pain-related behaviors in a mouse model of estrogen-induced uterine pain in ovariectomized female mice. Surprisingly, 2- and 4-hydroxylated estrogen metabolites (HEMs) in the estrogen hydroxylation pathway, but not estrone, estradiol and 16-HEMs, directly increase nociceptor hyperactivity through TRPA1 and TRPV1 channels, and picomolar concentrations of 2- and 4-hydroxylation estrone (OHE1) can sensitize TRPA1 channel function. Moreover, both TRPA1 and TRPV1 are expressed in uterine-innervating primary nociceptors and their expressions are increased in the estrogen-induced uterine pain model. Importantly, pretreatment of 2- or 4-OHE1 recapitulates estrogen-induced uterine pain-like behaviors and intraplantar injections of 2- and 4-OHE1 directly produce a TRPA1-dependent mechanical hypersensitivity. Our findings demonstrate that TRPA1 is critically involved in estrogen-induced uterine pain-like behaviors, which may provide a potential drug target for treating female reproductive tract pain.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

A molecular perspective on identifying TRPV1 thermosensitive regions and disentangling polymodal activation

TRPV1 is a polymodal receptor ion channel that is best known to function as a molecular thermometer. It is activated in diverse ways, including by heat, protons (low pH), and vanilloid compounds, such as capsaicin. In this review, we summarize molecular studies of TRPV1 thermosensing, focusing on the cross-talk between heat and other activation modes. Additional insights from TRPV1 isoforms and non-rodent/non-human TRPV1 ortholog studies are also discussed in this context. While the molecular mechanism of heat activation is still emerging, it is clear that TRPV1 thermosensing is modulated allosterically, i.e., at a distance, with contributions from many distinct regions of the channel. Similarly, current studies identify cross-talk between heat and other TRPV1 activation modes, such as protons and capsaicin, and that these modes can generally be selectively disentangled. In aggregate, this suggests that future TRPV1 molecular studies should define allosteric pathways and provide mechanistic insight, thereby enabling mode-selective manipulation of the polymodal receptor. These advances are anticipated to have significant implications in both basic and applied biomedical sciences.

Structural snapshots of TRPV1 reveal mechanism of polymodal functionality

Many transient receptor potential (TRP) channels respond to diverse stimuli and conditionally conduct small and large cations. Such functional plasticity is presumably enabled by a uniquely dynamic ion selectivity filter that is regulated by physiological agents. What is currently missing is a "photo series" of intermediate structural states that directly address this hypothesis and reveal specific mechanisms behind such dynamic channel regulation. Here, we exploit cryoelectron microscopy (cryo-EM) to visualize conformational transitions of the capsaicin receptor, TRPV1, as a model to understand how dynamic transitions of the selectivity filter in response to algogenic agents, including protons, vanilloid agonists, and peptide toxins, permit permeation by small and large organic cations. These structures also reveal mechanisms governing ligand binding substates, as well as allosteric coupling between key sites that are proximal to the selectivity filter and cytoplasmic gate. These insights suggest a general framework for understanding how TRP channels function as polymodal signal integrators.

Contribution of non-selective membrane channels and receptors in epilepsy

Overcoming refractory epilepsy's resistance to the combination of antiepileptic drugs (AED), mitigating side effects, and preventing sudden unexpected death in epilepsy are critical goals for therapy of this disorder. Current therapeutic strategies are based primarily on neurocentric mechanisms, overlooking the participation of astrocytes and microglia in the pathophysiology of epilepsy. This review is focused on a set of non-selective membrane channels (permeable to ions and small molecules), including channels and ionotropic receptors of neurons, astrocytes, and microglia, such as: the hemichannels formed by Cx43 and Panx1; the purinergic P2X7 receptors; the transient receptor potential vanilloid (TRPV1 and TRPV4) channels; calcium homeostasis modulators (CALHMs); transient receptor potential canonical (TRPC) channels; transient receptor potential melastatin (TRPM) channels; voltage-dependent anion channels (VDACs) and volume-regulated anion channels (VRACs), which all have in common being activated by epileptic activity and the capacity to exacerbate seizure intensity. Specifically, we highlight evidence for the activation of these channels/receptors during epilepsy including neuroinflammation and oxidative stress, discuss signaling pathways and feedback mechanisms, and propose the functions of each of them in acute and chronic epilepsy. Studying the role of these non-selective membrane channels in epilepsy and identifying appropriate blockers for one or more of them could provide complementary therapies to better alleviate the disease.

Airway Exposure to Polyethyleneimine Nanoparticles Induces Type 2 Immunity by a Mechanism Involving Oxidative Stress and ATP Release

Polyethyleneimine (PEI) induced immune responses were investigated in human bronchial epithelial (hBE) cells and mice. PEI rapidly induced ATP release from hBE cells and pretreatment with glutathione (GSH) blocked the response. PEI activated two conductive pathways, VDAC-1 and pannexin 1, which completely accounted for ATP efflux across the plasma membrane. Moreover, PEI increased intracellular Ca²⁺ concentration ([Ca²⁺]_i), which was reduced by the pannexin 1 inhibitor, ¹⁰Panx (50 μM), the VDAC-1 inhibitor, DIDS (100 μM), and was nearly abolished by pretreatment with GSH (5 mM). The increase in [Ca²⁺]_i involved Ca²⁺ uptake through two pathways, one blocked by oxidized ATP (oATP, 300 μM) and another that was blocked by the TRPV-1 antagonist A784168 (100 nM). PEI stimulation also increased IL-33 mRNA expression and protein secretion. In vivo experiments showed that acute (4.5 h) PEI exposure stimulated secretion of Th2 cytokines (IL-5 and IL-13) into bronchoalveolar lavage (BAL) fluid. Conjugation of PEI with ovalbumin also induced eosinophil recruitment and secretion of IL-5 and IL-13 into BAL fluid, which was inhibited in IL-33 receptor (ST2) deficient mice. In conclusion, PEI-induced oxidative stress stimulated type 2 immune responses by activating ATP-dependent Ca²⁺ uptake leading to IL-33 secretion, similar to allergens derived from Alternaria.

Molecular mechanism underlying modulation of TRPV1 heat activation by polyols

Sensing noxiously high temperatures is crucial for living organisms to avoid heat-induced injury. The TRPV1 channel has long been known as a sensor for noxious heat. However, the mechanism of how this channel is activated by heat remains elusive. Here we found that a series of polyols including sucrose, sorbitol, and hyaluronan significantly elevate the heat activation threshold temperature of TRPV1. The modulatory effects of these polyols were only observed when they were perfused extracellularly. Interestingly, mutation of residues E601 and E649 in the outer pore region of TRPV1 largely abolished the effects of these polyols. We further observed that intraplantar injection of polyols into the hind paws of rats reduced their heat-induced pain response. Our observations not only suggest that the extracellular regions of TRPV1 are critical for the modulation of heat activation by polyols, but also indicate a potential role of polyols in reducing heat-induced pain sensation.

Capsaicin and Proton Differently Modulate Activation Kinetics of Mouse Transient Receptor Potential Vanilloid-1 Channel Induced by Depolarization

The transient receptor potential vanilloid type 1 (TRPV1) channel is a non-selective cation channel expressed with transient receptor potential ankyrin type 1 (TRPA1) in small and medial size neurons of the dorsal root ganglions and trigeminal ganglions. TRPV1 is activated by capsaicin, thermal stimuli higher than 43°C, mechanical stress, and protons (H⁺). Although the TRPV1 channel does not have positively charged residues at regular intervals on its transmembrane segments, alterations in membrane potential also affect the state of TRPV1 channel. In the presence of capsaicin, voltage-dependent probability of opening of the TRPV1 channel and its kinetics have been examined, but the characteristics in the low pH remain unclear. To understand the voltage-dependency of the TRPV1 channel activation, we recorded capsaicin- and proton-induced mouse TRPV1 channel currents in a heterologous expression system. Outward current evoked by depolarizing square pulses in the presence of capsaicin or protons was fitted to a two-exponential function with a time-independent component. The voltage-dependent changes in amplitude of the three components displayed shallow curves and the changes in their ratio to the total current display similar tendencies in the presence of capsaicin and under the low pH. However, the fast and slow time constants in the presence of capsaicin were respectively 5- and 8-fold lower than those obtained under low pH conditions. These results suggest that the TRPV1 channel slowly drives the feed-forward cycle of pain sensation, and capsaicin and protons differently modulate the voltage-dependent TRPV1 channel gating.

Chemical Mediators' Expression Associated with the Modulation of Pain in Rheumatoid Arthritis

BACKGROUND

The management of pain in patients with rheumatoid arthritis (RA) is a complex subject due to the autoimmune nature of the pathology. Studies have shown that chemical mediators play a fundamental role in the determination, susceptibility and modulation of pain at different levels of the central and peripheral nervous system, resulting in interesting novel molecular targets to mitigate pain in patients with RA. However, due to the complexity of pain physiology in RA cand the many chemical mediators, the results of several studies are controversial.

OBJECTIVE

The aim of this study was to identify the chemical mediators that are able to modulate pain in RA.

METHOD

In this review, a search was conducted on PubMed, ProQuest, EBSCO, and the Science Citation index for studies that evaluated the expression of chemical mediators on the modulation of pain in RA.

RESULTS

Few studies have highlighted the importance of the expression of some chemical mediators that modulate pain in patients with rheumatoid arthritis. The expression of TRPV1, ASIC-3, and TDV8 encode ionic channels in RA and modulates pain, likewise, the transcription factors in RA, such as TNFα, TGF-β1, IL-6, IL-10, IFN-γ, IL-1b, mTOR, p21, caspase 3, EDNRB, CGRPCALCB, CGRP-CALCA, and TAC1 are also directly involved in pain perception.

CONCLUSION

The expression of all chemical mediators is directly related to RA and the modulation of pain by a complex intra and extracellular signaling pathway, however, transcription factors are involved in modulating acute pain, while the ionic channels are involved in chronic pain in RA.

A review of the flavor profile of metal salts: understanding the complexity of metallic sensation

The oral sensation of metallic is a complex experience. Much of our current understanding of metallic sensation is from the investigation of metal salts, which elicit diverse sensations, including taste, smell, and chemesthetic sensations, and therefore meet the definition of a flavor rather than a taste. Due to the involvement of multiple chemosensory systems, it can be challenging to define and characterize metallic sensation. Here, we provide a comprehensive review of the psychophysical studies quantifying and characterizing metallic sensation, focusing on metal salts. We examine the factors that impact perception, including anion complex, concentration, nasal occlusion, and pH. In addition, we summarize the receptors thought to be involved in the perception of metallic sensation (i.e., TRPV1, T1R3, TRPA1, and T2R7) either as a result of in vitro assays or from studies in knock-out mice. By enhancing our scientific understanding of metallic sensation and its transduction pathways, it has the potential to improve food and pharmaceuticals, help identify suppression or masking strategies, and improve the ability to characterize individual differences in metallic sensation. It also has the potential to translate to clinical populations by addressing the disparities in knowledge and treatment options for individuals suffering from metallic taste disorder (i.e., phantom taste or "metal mouth"). Future psychophysical studies investigating the sensory perception of metal salts should include a range of compounds and diverse food matrices, coupled with modern sensory methods, which will help to provide a more comprehensive understanding of metallic sensation.

Reduction of extracellular sodium evokes nociceptive behaviors in the chicken via activation of TRPV1

Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel, is mainly expressed in nociceptive primary sensory neurons. Sensitivity of TRPV1 to several stimuli is known to vary among species, specifically, the avian orthologue is nearly insensitive to capsaicin. Extracellular sodium ions ([Na⁺]_o) regulate TRPV1 activity in mammals, but their regulatory role on chicken TRPV1 (cTRPV1) is unknown. Here, we focused on the actions of capsaicin and low [Na⁺]_o on cTRPV1 activity. In chicken dorsal root ganglion (cDRG) neurons, capsaicin elicited [Ca²⁺]_i increases, but its effective concentration was much higher than those in mammals. Low [Na⁺]_o evoked [Ca²⁺]_i increases in cDRG neurons in a decreasing [Na⁺]_o-dependent manner and the complete removal of [Na⁺]_o (0Na) produced maximal effects. The population of 0Na-sensitive neurons was mostly overlapped with those of proton- and capsaicin-sensitive ones. Low [Na⁺]_o synergistically potentiated the capsaicin- and proton-induced TRPV1 activation in cDRG neurons. In HEK293 cells expressing cTRPV1 (cTRPV1-HEK), capsaicin elicited [Ca²⁺]_i increases with an EC₅₀ of 11.8 µM, and low [Na⁺]_o also did. Well-defined mammalian TRPV1 antagonists hardly suppressed cTRPV1 activation by low [Na⁺]_o. 0Na evoked outwardly rectified currents in cTRPV1-HEK. Mutagenesis analyses revealed a possible interaction of [Na⁺]_o with the proton-binding sites of cTRPV1. The administration of capsaicin and 0Na to chick eyes elicited pain-related behaviors. These results suggest that low [Na⁺]_o is capable of activating cTRPV1 in vitro, resulting in pain in vivo. Our data demonstrate that characterization of the cTRPV1 function is important to understand activation mechanisms of TRPV1.

Centipede Venom Peptides Acting on Ion Channels

Centipedes are among the oldest venomous arthropods that use their venom to subdue the prey. The major components of centipede venom are a variety of low-molecular-weight peptide toxins that have evolved to target voltage-gated ion channels to interfere with the central system of prey and produce pain or paralysis for efficient hunting. Peptide toxins usually contain several intramolecular disulfide bonds, which confer chemical, thermal and biological stability. In addition, centipede peptides generally have novel structures and high potency and specificity and therefore hold great promise both as diagnostic tools and in the treatment of human disease. Here, we review the centipede peptide toxins with reported effects on ion channels, including Nav, Kv, Cav and the nonselective cation channel polymodal transient receptor potential vanilloid 1 (TRPV1).

Zerumbone Modulates α2A-Adrenergic, TRPV1, and NMDA NR2B Receptors Plasticity in CCI-Induced Neuropathic Pain In Vivo and LPS-Induced SH-SY5Y Neuroblastoma In Vitro Models

Zerumbone has shown great potential in various pathophysiological models of diseases, particularly in neuropathic pain conditions. Further understanding the mechanisms of action is important to develop zerumbone as a potential anti-nociceptive agent. Numerous receptors and pathways function to inhibit and modulate transmission of pain signals. Previously, we demonstrated involvement of the serotonergic system in zerumbone's anti-neuropathic effects. The present study was conducted to determine zerumbone's modulatory potential involving noradrenergic, transient receptor potential vanilloid type 1 (TRPV1) and N-methyl-D-aspartate (NMDA) receptors in chronic constriction injury (CCI)-induced in vitro and lipopolysaccharide (LPS)-induced SH-SY5Y in vitro neuroinflammatory models. von Frey filament and Hargreaves plantar tests were used to assess allodynia and hyperalgesia in the chronic constriction injury-induced neuropathic pain mouse model. Involvement of specific adrenoceptors were investigated using antagonists- prazosin (α₁-adrenoceptor antagonist), idazoxan (α₂-adrenoceptor antagonist), metoprolol (β₁-adrenoceptor antagonist), ICI 118,551 (β₂-adrenoceptor antagonist), and SR 59230 A (β₃-adrenoceptor antagonist), co-administered with zerumbone (10 mg/kg). Involvement of excitatory receptors; TRPV and NMDA were conducted using antagonists capsazepine (TRPV1 antagonist) and memantine (NMDA antagonist). Western blot was conducted to investigate the effect of zerumbone on the expression of α_2A-adrenoceptor, TRPV1 and NMDA NR2B receptors in CCI-induced whole brain samples of mice as well as in LPS-induced SH-SY5Y neuroblastoma cells. Pre-treatment with α₁- and α₂-adrenoceptor antagonists significantly attenuated both anti-allodynic and anti-hyperalgesic effects of zerumbone. For β-adrenoceptors, only β₂-adrenoceptor antagonist significantly reversed the anti-allodynic and anti-hyperalgesic effects of zerumbone. β₁-adrenoceptor antagonist only reversed the anti-allodynic effect of zerumbone. The anti-allodynic and anti-hyperalgesic effects of zerumbone were both absent when TRPV1 and NMDA receptors were antagonized in both nociceptive assays. Zerumbone treatment markedly decreased the expression of α_2A-adrenoceptor, while an up-regulation was observed of NMDA NR2B receptors. Expression of TRPV1 receptors however did not significantly change. The in vitro study, representing a peripheral model, demonstrated the reduction of both NMDA NR2B and TRPV1 receptors while significantly increasing α_2A-adrenoceptor expression in contrast to the brain samples. Our current findings suggest that the α₁-, α₂-, β₁- and β₂-adrenoceptors, TRPV1 and NMDA NR2B are essential for the anti-allodynic and antihyperalgesic effects of zerumbone. Alternatively, we demonstrated the plasticity of these receptors through their response to zerumbone's administration.

Transient receptor potential vanilloid 1 mediates cocaine reinstatement via the D1 dopamine receptor in the nucleus accumbens

PURPOSE

The transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that mediates synaptic modification in the nucleus accumbens (NAc). However, no study has yet examined the mechanism of TRPV1 in the NAc on cocaine reinstatement. We investigated the mechanism of TRPV1 in NAc on cocaine reinstatement using the conditioned place preference (CPP) test in mice.

METHODS

We examined the effect of capsazepine (5 mg/kg, a TRPV1 antagonist, administered intraperitoneally (i.p.)), capsaicin (0.3 mg/kg, a TRPV1 agonist, administered i.p.), and genetic deletion of TRPV1 on the reinstatement of cocaine-induced CPP (15 mg/kg, administered i.p.). The expression of TRPV1 and Ca2⁺/calmodulin-mediated kinase II (CaMKII) in the NAc were determined after cocaine reinstatement. Microinjection of SB366791 (0.2 ng, a selective TRPV1 antagonist) in the NAc was assessed on SKF-81297 (1 µg, D1-like dopamine (DA) receptor agonist) primed cocaine reinstatement.

RESULTS

Capsazepine suppressed and capsaicin potentiated cocaine CPP in the reinstatement phase. In addition, genetic deletion of TRPV1 inhibited cocaine-priming reinstatement. Cocaine reinstatement was mediated by increased TRPV1 expression in the NAc, which involves CaMKII. Microinjection of SB366791 in the NAc prevented the cocaine reinstatement evoked by microinjection of SKF-81297 in the NAc.

CONCLUSIONS

These findings suggest that activation of TRPV1 mediates the stimulation of D1-like DA receptors and CaMKII in the NAc, resulting in the facilitation of cocaine reinstatement behaviors. Thus, our findings reveal a previously unknown TRPV1 mechanism in the reinstatement to drugs of abuse.

Selective Effects of Temperature on the Sensory Irritation but not Taste of NaCl and Citric Acid

This study investigated the effect of temperature on taste and chemesthetic sensations produced by the prototypical salty and sour stimuli NaCl and citric acid. Experiment 1 measured the perceived intensity of irritation (burning, stinging) and taste (saltiness, sourness) produced on the tongue tip by brief (3 s) exposures to suprathreshold concentrations of NaCl and citric acid at 3 different temperatures (12, 34, and 42 °C). No significant effects of temperature were found on the taste or sensory irritation of either stimulus. Experiment 2 investigated the potential effects of temperature on sensory irritation at peri-threshold concentrations and its sensitization over time. Measurements were again made on the tongue tip at the same 3 temperatures. Heating was found to enhance the perception of irritation at peri-threshold concentrations for both stimuli, whereas cooling suppressed sensitization of irritation for NaCl but not for citric acid. These results (i) confirm prior evidence that perception of suprathreshold salty and sour tastes are independent of temperature; (ii) demonstrate that heat has only weak effects on sensory irritation produced by brief exposures to NaCl and citric acid; and (iii) suggest that sensitization of the irritation produced by NaCl and citric acid occur via different peripheral mechanisms that have different thermal sensitivities. Overall, the results are consistent with involvement of the heat-sensitive channel TRPV1 in the sensory irritation of both stimuli together with one or more additional channels (e.g., acid-sensing channel, epithelial sodium channel, TRPA1) that are insensitive to heat and may possibly be sensitive to cooling.

Gastric vagal afferent neuropathy following experimental spinal cord injury

Dramatic impairment of gastrointestinal (GI) function accompanies high-thoracic spinal cord injury (T3-SCI). The vagus nerve contains mechano- and chemosensory fibers as well as the motor fibers necessary for the central nervous system (CNS) control of GI reflexes. Cell bodies for the vagal afferent fibers are located within the nodose gangla (NG) and the majority of vagal afferent axons are unmyelinated C fibers that are sensitive to capsaicin through activation of transient receptor potential vanilloid-1 (TRPV1) channels. Vagal afferent fibers also express receptors for GI hormones, including cholecystokinin (CCK). Previously, T3-SCI provokes a transient GI inflammatory response as well as a reduction of both gastric emptying and centrally-mediated vagal responses to GI peptides, including CCK. TRPV1 channels and CCK-A receptors (CCKar) expressed in vagal afferents are upregulated in models of visceral inflammation. The present study investigated whether T3-SCI attenuates peripheral vagal afferent sensitivity through plasticity of TRPV1 and CCK receptors. Vagal afferent response to graded mechanical stimulation of the stomach was significantly attenuated by T3-SCI at 3-day and 3-week recovery. Immunocytochemical labeling for CCKar and TRPV1 demonstrated expression on dissociated gastric-projecting NG neurons. Quantitative assessment of mRNA expression by qRT-PCR revealed significant elevation of CCKar and TRPV1 in the whole NG following T3-SCI in 3-day recovery, but levels returned to normal after 3-weeks. Three days after injury, systemic administration of CCK-8 s showed a significantly diminished gastric vagal afferent response in T3-SCI rats compared to control rats while systemic capsaicin infusion revealed a significant elevation of vagal response in T3-SCI vs control rats. These findings demonstrate that T3-SCI provokes peripheral remodeling and prolonged alterations in the response of vagal afferent fibers to the physiological signals associated with digestion.

Different agonists induce distinct single-channel conductance states in TRPV1 channels

TRPV1 is a polymodal ion channel that can be activated by lysophosphatidic acid (LPA), resulting in pain. Here we show that TRPV1 activation by LPA promotes a distinct open state with a different single-channel conductance from that induced by capsaicin.

The TRPV1 ion channel is a membrane protein that is expressed in primary afferent nociceptors, where it is activated by a diverse array of stimuli. Our prior work has shown that this channel is activated by lysophosphatidic acid (LPA), an unsaturated lysophospholipid that is produced endogenously and released under certain pathophysiological conditions, resulting in the sensation of pain. Macroscopic currents activated by saturating concentrations of LPA applied to excised membrane patches are larger in magnitude than those activated by saturating concentrations of capsaicin, which causes near-maximal TRPV1 open probability. Here we show that activation of TRPV1 by LPA is associated with a higher single-channel conductance than activation by capsaicin. We also observe that the effects of LPA on TRPV1 are not caused by an increase in the surface charge nor are they mimicked by a structurally similar lipid, ruling out the contribution of change in membrane properties. Finally, we demonstrate that the effects of LPA on the unitary conductance of TRPV1 depend upon the presence of a positively charged residue in the C terminus of the channel, suggesting that LPA induces a distinct conformational change.

Ultraviolet light-induced gasdermin C expression is mediated via TRPV1/calcium/calcineurin/NFATc1 signaling

Gasdermin (GSDM)‑C is a member of the GSDM gene family and is expressed in the epithelial cells of various tissue types, including skin. GSDMC expression is induced by ultraviolet (UV) irradiation and contributes to UV‑induced matrix metalloproteinase 1 expression in human skin keratinocytes. However, how UV irradiation induces GSDMC expression remains unclear. The present study aimed to investigate the role of transient receptor potential cation channel subfamily V member 1 (TRPV1) and a calcium/calcineurin‑signaling pathway in UV‑induced GSDMC expression in human skin keratinocytes. Suppression of TRPV1 activity by treatment with the TRPV1 antagonists capsazepine and ruthenium red significantly reduced UV‑induced GSDMC expression, whereas direct activation of TRPV1 by capsaicin, a TRPV1 agonist, increased GSDMC expression. The results demonstrated that extracellular calcium and calcineurin activity may be necessary for UV‑induced GSDMC expression in HaCaT cells. In addition, UV‑induced GSDMC expression was either decreased or increased following knockdown or overexpression of nuclear factor of activated T‑cells, cytoplasmic 1 (NFATc1), respectively. These data suggested that TRPV1 may serve an important role in the induction of GSDMC expression by UV and that UV‑induced GSDMC expression may be mediated via a calcium/calcineurin/NFATc1 pathway.

Irreversible temperature gating in trpv1 sheds light on channel activation

Temperature-activated TRP channels or thermoTRPs are among the only proteins that can directly convert temperature changes into changes in channel open probability. In spite of a wealth of functional and structural information, the mechanism of temperature activation remains unknown. We have carefully characterized the repeated activation of TRPV1 by thermal stimuli and discovered a previously unknown inactivation process, which is irreversible. We propose that this form of gating in TRPV1 channels is a consequence of the heat absorption process that leads to channel opening.

Tumour-Derived Glutamate: Linking Aberrant Cancer Cell Metabolism to Peripheral Sensory Pain Pathways

Background

Chronic pain is a major symptom that develops in cancer patients, most commonly emerging during advanced stages of the disease. The nature of cancer-induced pain is complex, and the efficacy of current therapeutic interventions is restricted by the dose-limiting side-effects that accompany common centrally targeted analgesics.

Methods

This review focuses on how up-regulated glutamate production and export by the tumour converge at peripheral afferent nerve terminals to transmit nociceptive signals through the transient receptor cation channel, TRPV1, thereby initiating central sensitization in response to peripheral disease-mediated stimuli.

Results

Cancer cells undergo numerous metabolic changes that include increased glutamine catabolism and over-expression of enzymes involved in glutaminolysis, including glutaminase. This mitochondrial enzyme mediates glutaminolysis, producing large pools of intracellular glutamate. Up-regulation of the plasma membrane cystine/glutamate antiporter, system xc-, promotes aberrant glutamate release from cancer cells. Increased levels of extracellular glutamate have been associated with the progression of cancer-induced pain and we discuss how this can be mediated by activation of TRPV1.

Conclusion

With a growing population of patients receiving inadequate treatment for intractable pain, new targets need to be considered to better address this largely unmet clinical need for improving their quality of life. A better understanding of the mechanisms that underlie the unique qualities of cancer pain will help to identify novel targets that are able to limit the initiation of pain from a peripheral source–the tumour.

Presynaptic inhibition of transient receptor potential vanilloid type 1 (TRPV1) receptors by noradrenaline in nociceptive neurons

KEY POINTS

The transient receptor potential vanilloid type 1 (TRPV1) receptor is a polymodal molecular integrator in the pain pathway expressed in Aδ- and C-fibre nociceptors and is responsible for the thermal hyperalgesia associated with inflammatory pain. Noradrenaline strongly inhibited the activity of TRPV1 channels in dorsal root ganglia neurons. The effect of noradrenaline was reproduced by clonidine and antagonized by yohimbine, consistent with contribution of α2 adrenergic receptors. The inhibitory effect of noradrenaline on TRPV1 channels was dependent on calcium influx and linked to calcium/calmodulin-dependent protein kinase II. In spinal cord slices, clonidine reduced the frequency of capsaicin-induced miniature EPSCs in the presence of tetrodotoxin and ω-conotoxin-MVIIC, consistent with inhibition of presynaptic TRPV1 channels by α2 adrenergic receptors. We suggest that modulation of presynaptic TRPV1 channels in nociceptive neurons by descending noradrenergic inputs may constitute a mechanism for noradrenaline to modulate incoming noxious stimuli in the dorsal horn of the spinal cord.

ABSTRACT

The transient receptor potential vanilloid type 1 (TRPV1) receptor is a well-known contributor to nociceptor excitability. To address whether noradrenaline can down-regulate TRPV1 channel activity in nociceptors and reduce their synaptic transmission, the effects of noradrenaline and clonidine were tested on the capsaicin-activated current recorded from acutely dissociated small diameter (<27 μm) dorsal root ganglia (DRG) neurons and on miniature (m)EPSCs recorded from large lamina I neurons in horizontal spinal cord slices. Noradrenaline or clonidine inhibited the capsaicin-activated current by ∼60%, and the effect was reversed by yohimbine, confirming that it was mediated by activation of α2 adrenergic receptors. Similarly, clonidine reduced the frequency of capsaicin-induced mEPSCs by ∼60%. Inhibition of capsaicin-activated current by noradrenaline was mediated by GTP binding proteins, and was highly dependent on calcium influx. The inhibitory effect of noradrenaline on the capsaicin-activated current was not affected either by blocking the activity of protein kinase A with H89, or by blocking the activity of protein kinase C with bisindolylmaleimide II. In contrast, when the calcium/calmodulin-dependent protein kinase II (CaMKII) was blocked with KN-93, the inhibitory effect of noradrenaline on the capsaicin-activated current was greatly reduced, suggesting that activation of adrenergic receptors in DRG neurons is preferentially linked to CaMKII activity. We suggest that modulation of TRPV1 channels by noradrenaline in nociceptive neurons is a mechanism whereby noradrenaline may suppress incoming noxious stimuli at the primary synaptic afferents in the dorsal horn of the spinal cord.

Role of transient receptor potential vanilloid 1 in regulating erythropoietin-induced activation of endothelial nitric oxide synthase

AIMS

Erythropoietin (EPO), the key hormone involved in erythropoiesis, beneficially affects endothelial cells (ECs), but the detailed mechanisms are yet to be completely understood. In this study, we investigated the role of transient receptor potential vanilloid type 1 (TRPV1), a ligand-gated non-selective calcium (Ca²⁺ ) channel, in EPO-mediated endothelial nitric oxide synthase (eNOS) activation and angiogenesis.

METHODS AND RESULTS

In ECs, EPO time dependently increased intracellular levels of calcium; this increase was abrogated by the Ca²⁺ chelators and pharmacological inhibitors of TRPV1 in bovine aortic ECs (BAECs) and TRPV1-transfected HEK293 cells. In addition, EPO-induced nitrite oxide (NO) production, phosphorylation of eNOS, Akt and AMP-activated protein kinase (AMPK) and the formation of TRPV1-Akt-AMPK-eNOS complex as well as tube formation were diminished by the pharmacological inhibition of TRPV1 in BAECs. Moreover, EPO time dependently induced the phosphorylation of phospholipase C-γ1 (PLC-γ1). Inhibition of PLC-γ1 activity blunted the EPO-induced Ca²⁺ influx, eNOS phosphorylation, TRPV1-eNOS complex formation and NO production. The phosphorylated level of eNOS increased in the aortas of EPO-treated wild-type (WT) mice or EPO-transgenic (Tg) mice but not in those of EPO-treated TRPV1-deficient (TRPV1^-/- ) mice or EPO-Tg/TRPV1^-/- mice. Matrigel plug assay showed that EPO-induced angiogenesis was abrogated in TRPV1 antagonist capsazepine-treated WT mice and TRPV1^-/- mice.

CONCLUSION

These findings indicate the EPO-induced Ca²⁺ influx via the activation of the PLC-γ1 signalling pathway, which leads to TRPV1 activation and consequently increases the association of the TRPV1-Akt-AMPK-eNOS complex, eNOS activation, NO production and angiogenesis.

Understand spiciness: mechanism of TRPV1 channel activation by capsaicin

Capsaicin in chili peppers bestows the sensation of spiciness. Since the discovery of its receptor, transient receptor potential vanilloid 1 (TRPV1) ion channel, how capsaicin activates this channel has been under extensive investigation using a variety of experimental techniques including mutagenesis, patch-clamp recording, crystallography, cryo-electron microscopy, computational docking and molecular dynamic simulation. A framework of how capsaicin binds and activates TRPV1 has started to merge: capsaicin binds to a pocket formed by the channel’s transmembrane segments, where it takes a “tail-up, head-down” configuration. Binding is mediated by both hydrogen bonds and van der Waals interactions. Upon binding, capsaicin stabilizes the open state of TRPV1 by “pull-and-contact” with the S4-S5 linker. Understanding the ligand-host interaction will greatly facilitate pharmaceutical efforts to develop novel analgesics targeting TRPV1.

Participation of peripheral TRPV1, TRPV4, TRPA1 and ASIC in a magnesium sulfate-induced local pain model in rat

We previously showed that magnesium sulfate (MS) has systemic antinociceptive and local peripheral pronociceptive effects. The role of transient receptor potential (TRP) channels and acid-sensing ion channels (ASICs) in the mechanism of action of MS has not been investigated in detail. The aim of this study was to explore the participation of TRP channels in the pronociceptive action of MS in rats after its intraplantar injection. The paw withdrawal threshold (PWT) to mechanical stimuli was measured by the electronic von Frey test. Drugs that were tested were either co-administered with an isotonic pH-unadjusted or pH-adjusted solution of MS intraplantarily, or to the contralateral paw to exclude systemic effects. We found that the subcutaneous administration of both pH-adjusted (7.4) and pH-unadjusted (about 6.0) isotonic (6.2% w/v in water) solutions of MS induce the pain at the injection site. The pH-unadjusted MS solution-induced mechanical hyperalgesia decreased in a dose-dependent manner as a consequence of co-injection of capsazepine, a selective TRPV1 antagonist (20, 100 and 500pmol/paw), RN-1734, a selective TRPV4 antagonist (1.55, 3.1 and 6.2μmol/paw), HC-030031, a selective TRPA1 antagonist (5.6, 28.1 and 140nmol/paw), and amiloride hydrochloride, a non-selective ASIC inhibitor (0.83, 2.5 and 7.55μmol/paw). In pH-adjusted MS-induced hyperalgesia, the highest doses of TRPV1, TRPV4 and TRPA1 antagonists displayed effects that were, respectively, either similar, less pronounced or delayed in comparison to the effect induced by administration of the pH-unadjusted MS solution; the ASIC antagonist did not have any effect. These results suggest that the MS-induced local peripheral mechanical hyperalgesia is mediated via modulation of the activity of peripheral TRPV1, TRPV4, TRPA1 and ASICs. Specific local inhibition of TRP channels represents a novel approach to treating local injection-related pain.

Exploring functional roles of TRPV1 intracellular domains with unstructured peptide-insertion screening

TRPV1 is a polymodal nociceptor for diverse physical and chemical stimuli that interact with different parts of the channel protein. Recent cryo-EM studies revealed detailed channel structures, opening the door for mapping structural elements mediating activation by each stimulus. Towards this goal, here we have combined unstructured peptide-insertion screening (UPS) with electrophysiological and fluorescence recordings to explore structural and functional roles of the intracellular regions of TRPV1 in mediating various activation stimuli. We found that most of the tightly packed protein regions did not tolerate structural perturbation by UPS when tested, indicating that structural integrity of the intracellular region is critical. In agreement with previous reports, Ca²⁺-dependent desensitization is strongly dependent on both intracellular N- and C-terminal domains; insertions of an unstructured peptide between these domains and the transmembrane core domain nearly eliminated Ca²⁺-dependent desensitization. In contrast, channel activations by capsaicin, low pH, divalent cations, and even heat are mostly intact in mutant channels containing the same insertions. These observations suggest that the transmembrane core domain of TRPV1, but not the intracellular domains, is responsible for sensing these stimuli.

TRPV currents and their role in the nociception and neuroplasticity

Transient receptor potential channels sensitive to vanilloids (TRPVs) are group of ion channels which are sensitive to various tissue damaging signals and their activation is generally perceived as pain. Therefore, they are generally named as nociceptors. Understanding their activation and function as well as their interaction with intracellular pathways is crucial for the development of pharmacological interference in order to reduce pain perception. The current review summarizes basic facts in regard to TRPV and discusses their relevance in the sensing of (pain-) signals and their intracellular processing, focussing on their modulation of the intracellular calcium ([Ca(2+)]i) signal. Furthermore we discuss the basic mechanisms how the modification of [Ca(2+)]i through TRPV might induce long-term-potentiation (LTP) or long-term- depression (LTD) and from "memories" of pain. Understanding of these mechanisms is needed to localize the best point of interference for pharmacological treatment. Therefore, high attention is given to highlight physiological and pathological processes and their interaction with significant modulators and their roles in neuroplasticity and pain modulation.

Dopamine modulation of transient receptor potential vanilloid type 1 (TRPV1) receptor in dorsal root ganglia neurons

The transient receptor potential vanilloid type 1 (TRPV1) receptor plays a key role in the modulation of nociceptor excitability. To address whether dopamine can modulate the activity of TRPV1 channels in nociceptive neurons, the effects of dopamine and dopamine receptor agonists were tested on the capsaicin-activated current recorded from acutely dissociated small diameter (<27 μm) dorsal root ganglia (DRG) neurons. Dopamine or SKF 81297 (an agonist at D1/D5 receptors), caused inhibition of both inward and outward currents by ∼60% and ∼48%, respectively. The effect of SKF 81297 was reversed by SCH 23390 (an antagonist at D1/D5 receptors), confirming that it was mediated by activation of D1/D5 dopamine receptors. In contrast, quinpirole (an agonist at D2 receptors) had no significant effect on the capsaicin-activated current. Inhibition of the capsaicin-activated current by SKF 81297 was mediated by G protein coupled receptors (GPCRs), and highly dependent on external calcium. The inhibitory effect of SKF 81297 on the capsaicin-activated current was not affected when the protein kinase A (PKA) activity was blocked with H89, or when the protein kinase C (PKC) activity was blocked with bisindolylmaleimide II (BIM). In contrast, when the calcium-calmodulin-dependent protein kinase II (CaMKII) was blocked with KN-93, the inhibitory effect of SKF 81297 on the capsaicin-activated current was greatly reduced, suggesting that activation of D1/D5 dopamine receptors may be preferentially linked to CaMKII activity. We suggest that modulation of TRPV1 channels by dopamine in nociceptive neurons may represent a way for dopamine to modulate incoming noxious stimuli.

The effects of intra-articular resiniferatoxin on monosodium iodoacetate-induced osteoarthritic pain in rats

This study was performed to investigate whether an intra-articular injection of transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX) would alleviate behavioral signs of arthritic pain in a rat model of osteoarthritis (OA). We also sought to determine the effect of RTX treatment on calcitonin gene-related peptide (CGRP) expression in the spinal cord. Knee joint inflammation was induced by intra-articular injection of monosodium iodoacetate (MIA, 8 mg/50 µl) and weight bearing percentage on right and left hindpaws during walking, paw withdrawal threshold to mechanical stimulation, and paw withdrawal latency to heat were measured to evaluate pain behavior. Intra-articular administration of RTX (0.03, 0.003 and 0.0003%) at 2 weeks after the induction of knee joint inflammation significantly improved reduction of weight bearing on the ipsilateral hindlimb and increased paw withdrawal sensitivity to mechanical and heat stimuli. The reduction of pain behavior persisted for 3~10 days according to each behavioral test. The MIA-induced increase in CGRP immunoreactivity in the spinal cord was decreased by RTX treatment in a dose-dependent manner. The present study demonstrated that a single intra-articular administration of RTX reduced pain behaviors for a relatively long time in an experimental model of OA and could normalize OA-associated changes in peptide expression in the spinal cord.

TRPV1 and PLC Participate in Histamine H4 Receptor-Induced Itch

Histamine H4 receptor has been confirmed to play a role in evoking peripheral pruritus. However, the ionic and intracellular signaling mechanism of activation of H4 receptor on the dorsal root ganglion (DRG) neurons is still unknown. By using cell culture and calcium imaging, we studied the underlying mechanism of activation of H4 receptor on the DRG neuron. Immepip dihydrobromide (immepip)—a histamine H4 receptor special agonist under cutaneous injection—obviously induced itch behavior of mice. Immepip-induced scratching behavior could be blocked by TRPV1 antagonist AMG9810 and PLC pathway inhibitor U73122. Application of immepip (8.3–50 μM) could also induce a dose-dependent increase in intracellular Ca²⁺ ([Ca²⁺]_i) of DRG neurons. We found that 77.8% of the immepip-sensitized DRG neurons respond to the TRPV1 selective agonist capsaicin. U73122 could inhibit immepip-induced Ca²⁺ responses. In addition, immepip-induced [Ca²⁺]_i increase could be blocked by ruthenium red, capsazepine, and AMG9810; however it could not be blocked by TRPA1 antagonist HC-030031. These results indicate that TRPV1 but not TRPA1 is the important ion channel to induce the DRG neurons' responses in the downstream signaling pathway of histamine H4 receptor and suggest that TRPV1 may be involved in the mechanism of histamine-induced itch response by H4 receptor activation.

Measurement of relative Ca²⁺ permeability during sustained activation of TRPV1 receptors

Some cation permeable ligand-gated ion channels, including the capsaicin-sensitive TRPV1, have been reported to exhibit a time-dependent increase in permeability to large inorganic cations during sustained activation, a phenomenon termed "pore dilation." TRPV1 conducts substantial Ca(2+) entry, and it has been suggested that this channel undergoes a time-dependent change in Ca(2+) permeability relative to Na(+) (P Ca/P Na) that parallels pore dilation. However, our experiments employing whole cell patch clamp photometry and single channel recordings to directly measure relative Ca(2+) current in TRPV1 expressing HEK293 cells show that relative Ca(2+) influx remains constant for the duration of capsaicin-evoked channel activation. Further, we present evidence from patch clamp photometry experiments suggesting that sustained activation of Ca(2+) permeable ion channels in the voltage-clamp configuration leads to rapid saturation of the pipette Ca(2+) chelator, and that subsequent observed shifts in the current reversal potentials in the presence of extracellular Ca(2+) are likely due to intracellular accumulation of this ion and a movement of the Ca(2+) equilibrium potential (E Ca) towards zero. Finally, using an adapted reversal potential-based protocol in which cells are only exposed to Ca(2+) after sustained capsaicin exposure in the absence of added extracellular Ca(2+), we demonstrate that the calculated P Ca/P Na is unaffected by duration of TRPV1 activation. In conclusion, we find no evidence in support of a time-dependent change in P Ca/P Na for TRPV1. Our data further urges caution in estimating relative Ca(2+) permeability using reversal potentials, as there is a limited time window in which the cytosolic Ca(2+) chelator included in the patch pipette can prevent localised elevations in cytosolic free Ca(2+) and thus allow for an accurate estimate of this important channel permeability parameter.

A pain-inducing centipede toxin targets the heat activation machinery of nociceptor TRPV1

The capsaicin receptor TRPV1 ion channel is a polymodal nociceptor that responds to heat with exquisite sensitivity through an unknown mechanism. Here we report the identification of a novel toxin, RhTx, from the venom of the Chinese red-headed centipede that potently activates TRPV1 to produce excruciating pain. RhTx is a 27-amino-acid small peptide that forms a compact polarized molecule with very rapid binding kinetics and high affinity for TRPV1. We show that RhTx targets the channel's heat activation machinery to cause powerful heat activation at body temperature. The RhTx-TRPV1 interaction is mediated by the toxin's highly charged C terminus, which associates tightly to the charge-rich outer pore region of the channel where it can directly interact with the pore helix and turret. These findings demonstrate that RhTx binding to the outer pore can induce TRPV1 heat activation, therefore providing crucial new structural information on the heat activation machinery.

The pain receptor TRPV1 displays agonist-dependent activation stoichiometry

The receptor channel TRPV1 (Transient Receptor Potential Vanilloid 1) is expressed by primary afferent sensory neurons of the pain pathway, where it functions as a sensor of noxious heat and various chemicals, including eicosanoids, capsaicin, protons and peptide toxins. Comprised of four identical subunits that organize into a non-selective cationic permeable channel, this receptor has a variety of binding sites responsible for detecting their respective agonists. Although its physiological role as a chemosensor has been described in detail, the stoichiometry of TRPV1 activation by its different ligands remains unknown. Here, we combined the use of concatemeric constructs harboring mutated binding sites with patch-clamp recordings in order to determine the stoichiometry for TRPV1 activation through the vanilloid binding site and the outer-pore domain by capsaicin and protons, respectively. We show that, while a single capsaicin-bound subunit was sufficient to achieve a maximal open-channel lifetime, all four proton-binding sites were required. Thus, our results demonstrate a distinct stoichiometry of TRPV1 activation through two of its different agonist-binding domains.

Proton block of proton-activated TRPV1 current

H⁺ not only activates the TRPV1 cation channel but inhibits ion permeation, yielding a prominent OFF current response.

The TRPV1 cation channel is a polymodal nociceptor that is activated by heat and ligands such as capsaicin and is highly sensitive to changes in extracellular pH. In the body core, where temperature is usually stable and capsaicin is normally absent, H⁺ released in response to ischemia, tissue injury, or inflammation is the best-known endogenous TRPV1 agonist, activating the channel to mediate pain and vasodilation. Paradoxically, removal of H⁺ elicits a transient increase in TRPV1 current that is much larger than the initial H⁺-activated current. We found that this prominent OFF response is caused by rapid recovery from H⁺ inhibition of the excitatory current carried by H⁺-activated TRPV1 channels. H⁺ inhibited current by interfering with ion permeation. The degree of inhibition is voltage and permeant ion dependent, and it can be affected but not eliminated by mutations to acidic residues within or near the ion selectivity filter. The opposing H⁺-mediated gating and permeation effects produce complex current responses under different cellular conditions that are expected to greatly affect the response of nociceptive neurons and other TRPV1-expressing cells.

BOLD fMRI of C-Fiber Mediated Nociceptive Processing in Mouse Brain in Response to Thermal Stimulation of the Forepaws

Functional magnetic resonance imaging (fMRI) in rodents enables non-invasive studies of brain function in response to peripheral input or at rest. In this study we describe a thermal stimulation paradigm using infrared laser diodes to apply noxious heat to the forepaw of mice in order to study nociceptive processing. Stimulation at 45 and 46°C led to robust BOLD signal changes in various brain structures including the somatosensory cortices and the thalamus. The BOLD signal amplitude scaled with the temperature applied but not with the area irradiated by the laser beam. To demonstrate the specificity of the paradigm for assessing nociceptive signaling we administered the quaternary lidocaine derivative QX-314 to the forepaws, which due to its positive charge cannot readily cross biological membranes. However, upon activation of TRPV1 channels following the administration of capsaicin the BOLD signal was largely abolished, indicative of a selective block of the C-fiber nociceptors due to QX-314 having entered the cells via the now open TRPV1 channels. This demonstrates that the cerebral BOLD response to thermal noxious paw stimulation is specifically mediated by C-fibers.

Role of calcium ions in the positive interaction between TRPA1 and TRPV1 channels in bronchopulmonary sensory neurons

Both transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) receptors are abundantly expressed in bronchopulmonary C-fiber sensory nerves and can be activated by a number of endogenous inflammatory mediators. A recent study has reported a synergistic effect of simultaneous TRPA1 and TRPV1 activations in vagal pulmonary C-fiber afferents in anesthetized rats, but its underlying mechanism was not known. This study aimed to characterize a possible interaction between these two TRP channels and to investigate the potential role of Ca(2+) as a mediator of this interaction in isolated rat vagal pulmonary sensory neurons. Using the perforated patch-clamp recording technique, our study demonstrated a distinct positive interaction occurring abruptly between TRPA1 and TRPV1 when they were activated simultaneously by their respective agonists, capsaicin (Cap) and allyl isothiocyanate (AITC), at near-threshold concentrations in these neurons. AITC at this low concentration evoked only minimal or undetectable responses, but it markedly amplified the Cap-evoked current in the same neurons. This potentiating effect was eliminated when either AITC or Cap was replaced by non-TRPA1 and non-TRPV1 chemical activators of these neurons, demonstrating the selectivity of the interaction between these two TRP channels. Furthermore, when Ca(2+) was removed from the extracellular solution, the synergistic effect of Cap and AITC on pulmonary sensory neurons was completely abrogated, clearly indicating a critical role of Ca(2+) in mediating the action. These results suggest that this TRPA1-TRPV1 interaction may play a part in regulating the sensitivity of pulmonary sensory neurons during airway inflammatory reaction.

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.

Calcium ions facilitate body heat emission response to warming

Involvement of various areas of the body surface in heat emission response to warming is characterized by a certain succession. The first response preceding the deep body temperature rise is dilation of ear skin vessels. Then, an increase in deep body temperature is counterbalanced by vascular reaction in the tail region, which plays the leading role in up-regulation of heat emission. Calcium ions accelerate the vascular response to warming in both regions, although they produce no effect on the maximum level of heat emission. Our findings confirm the involvement of Ca(2+)-dependent mechanisms in activation of the processes aimed at stabilization of body temperature in warm-blooded animals. The role of heat-sensitive TRPV1 ion channels determining modality of the temperature signal and direction of effector reactions is discussed.

Role of the outer pore domain in transient receptor potential vanilloid 1 dynamic permeability to large cations

Transient receptor potential vanilloid 1 (TRPV1) has been shown to alter its ionic selectivity profile in a time- and agonist-dependent manner. One hallmark of this dynamic process is an increased permeability to large cations such as N-methyl-D-glucamine (NMDG). In this study, we mutated residues throughout the TRPV1 pore domain to identify loci that contribute to dynamic large cation permeability. Using resiniferatoxin (RTX) as the agonist, we identified multiple gain-of-function substitutions within the TRPV1 pore turret (N628P and S629A), pore helix (F638A), and selectivity filter (M644A) domains. In all of these mutants, maximum NMDG permeability was substantially greater than that recorded in wild type TRPV1, despite similar or even reduced sodium current density. Two additional mutants, located in the pore turret (G618W) and selectivity filter (M644I), resulted in significantly reduced maximum NMDG permeability. M644A and M644I also showed increased and decreased minimum NMDG permeability, respectively. The phenotypes of this panel of mutants were confirmed by imaging the RTX-evoked uptake of the large cationic fluorescent dye YO-PRO1. Whereas none of the mutations selectively altered capsaicin-induced changes in NMDG permeability, the loss-of-function phenotypes seen with RTX stimulation of G618W and M644I were recapitulated in the capsaicin-evoked YO-PRO1 uptake assay. Curiously, the M644A substitution resulted in a loss, rather than a gain, in capsaicin-evoked YO-PRO1 uptake. Modeling of our mutations onto the recently determined TRPV1 structure revealed several plausible mechanisms for the phenotypes observed. We conclude that side chain interactions at a few specific loci within the TRPV1 pore contribute to the dynamic process of ionic selectivity.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.