Prostaglandin-induced activation of nociceptive neurons via direct interaction with transient receptor potential A1 (TRPA1)

The G protein-coupled receptor-transient receptor potential channel axis: molecular insights for targeting disorders of sensation and inflammation

Sensory nerves are equipped with receptors and ion channels that allow them to detect and respond to diverse chemical, mechanical, and thermal stimuli. These sensory proteins include G protein-coupled receptors (GPCRs) and transient receptor potential (TRP) ion channels. A subclass of peptidergic sensory nerves express GPCRs and TRP channels that detect noxious, irritant, and inflammatory stimuli. Activation of these nerves triggers protective mechanisms that lead to withdrawal from danger (pain), removal of irritants (itch, cough), and resolution of infection (neurogenic inflammation). The GPCR-TRP axis is central to these mechanisms. Signals that emanate from the GPCR superfamily converge on the small TRP family, leading to channel sensitization and activation, which amplify pain, itch, cough, and neurogenic inflammation. Herein we discuss how GPCRs and TRP channels function independently and synergistically to excite sensory nerves that mediate noxious and irritant responses and inflammation in the skin and the gastrointestinal and respiratory systems. We discuss the signaling mechanisms that underlie the GPCR-TRP axis and evaluate how new information about the structure of GPCRs and TRP channels provides insights into their functional interactions. We propose that a deeper understanding of the GPCR-TRP axis may facilitate the development of more selective and effective therapies to treat dysregulated processes that underlie chronic pain, itch, cough, and inflammation.

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.

Resiniferatoxin Mediated Ablation of TRPV1+ Neurons Removes TRPA1 as Well

Objectives:

Resiniferatoxin, the most potent agonist of inflammatory pain/vanilloid receptor/cation channel (TRPV1) can be used for neuron subtype specific ablation of pain generating cells at the level of the peripheral nervous system by Ca2+-excytotoxicity. Molecular neurosurgery is an emerging technology either to alleviate severe pain in cancer or treat/prevent different local neuropathies. Our aim was determining sensory modalities that may be lost after resiniferatoxin treatment.

Methods:

Newborn or adult mice were treated with resiniferatoxin, then changes in chemical and heat sensitivity were correlated with alterations of the cell composition of sensory ganglions.

Results:

Only mice treated at adult age became less sensitive to heat stimuli, while both treatment groups lost sensitivity to specific vanilloid agonists of TRPV1 and, interestingly, to allyl-isothiocyanate, a selective agonist of TRPA1. Our in vivo and post mortem analytical results confirmed that TRPV1 and TRPA1 function together and resiniferatoxin-mediated neurosurgery removes both sensor molecules

Discussion:

In adult mice resiniferatoxin causes: i) desensitization to heat and ii) sensitization to cold. Cold hyperalgesia, an imbalance in thermosensation, might be conferred by a prominent cold receptor that is expressed in surviving resiniferatoxin-resistant sensory neurons and compensates for pain signals lost with TRPA1 and TRPV1 double positive cells in the peripheral nervous system.

Transient receptor potential channels and occupational exposure

PURPOSE OF REVIEW

The discovery that a number of transient receptor potential (TRP) channels are expressed in a subpopulation of primary sensory neurons innervating the upper and lower airways as well as in nonneuronal cells in the airways and lungs has initiated a quest for the understanding of their role in the physiology and pathophysiology of the respiratory tract.

RECENT FINDINGS

Various members of the TRP vanilloid subfamily (TRPV1, TRPV4) and the TRP ankyrin 1 (TRPA1), because of their localization in peptidergic sensory neurons, promote airway neurogenic inflammation. In particular, TRPA1, which is gated by oxidative and nitrative stress byproducts, has been found to mediate inflammatory responses produced by an unprecedented series of toxic and irritant agents produced by air pollution, contained in cigarette smoke, and produced by accidental events at the workplace. The observation that reactive molecules endogenously produced in the airways/lungs of asthma, work-related asthma, and chronic obstructive pulmonary disease target TRPA1 underscores the primary role of the TRPA1 channel in these conditions.

SUMMARY

Identification of TRP channels, and especially TRPA1, as major targets of oxidative/nitrative stress and a variety of irritant environmental agents supports the hypothesis that neurogenic inflammation plays an important role in work-related inflammatory diseases and that antagonists for such channels may be novel therapeutic options for the treatment of these diseases.

Chemical structure and morphology of dorsal root ganglion neurons from naive and inflamed mice

Fourier transform infrared spectromicroscopy provides label-free imaging to detect the spatial distribution of the characteristic functional groups in proteins, lipids, phosphates, and carbohydrates simultaneously in individual DRG neurons. We have identified ring-shaped distributions of lipid and/or carbohydrate enrichment in subpopulations of neurons which has never before been reported. These distributions are ring-shaped within the cytoplasm and are likely representative of the endoplasmic reticulum. The prevalence of chemical ring subtypes differs between large- and small-diameter neurons. Peripheral inflammation increased the relative lipid content specifically in small-diameter neurons, many of which are nociceptive. Because many small-diameter neurons express an ion channel involved in inflammatory pain, transient receptor potential ankyrin 1 (TRPA1), we asked whether this increase in lipid content occurs in TRPA1-deficient (knock-out) neurons. No statistically significant change in lipid content occurred in TRPA1-deficient neurons, indicating that the inflammation-mediated increase in lipid content is largely dependent on TRPA1. Because TRPA1 is known to mediate mechanical and cold sensitization that accompanies peripheral inflammation, our findings may have important implications for a potential role of lipids in inflammatory pain.

TRPV1 Antagonism: From Research to Clinic

The capsaicin receptor, TRPV1, has been one of the most extensively studied molecules in sensory research. Its contribution to the sensation of pain in numerous pre-clinical inflammatory and neuropathic paradigms has been well-established and expression analysis suggests a potential role clinically in pain and bladder conditions. The field has now reached an exciting point in time with the development of a number of high quality TRPV1 antagonist drug candidates and the release of clinical data. What has become apparent from this work is that inhibition of TRPV1 function brings with it the potential liabilities of increased body temperature and altered thermal perception. However, there is cause for optimism because it appears that not all antagonists have the same properties and compounds can be identified that lack significant on-target side-effects whilst retaining efficacy, at least pre-clinically. What is perhaps now more critical to address is the question of how effective the analgesia provided by a TRPV1 antagonist will be. Although tantalizing clinical data showing effects on experimentally-induced pain or pain following molar extraction have been reported, no clear efficacy in a chronic pain condition has yet been demonstrated making it difficult to perform an accurate risk-benefit analysis for TRPV1 antagonists. Here we provide an overview of some of the most advanced clinical candidates and discuss the approaches being taken to avoid the now well established on-target effects of TRPV1 antagonists.

Are sensory TRP channels biological alarms for lipid peroxidation?

Oxidative stress induces numerous biological problems. Lipid oxidation and peroxidation appear to be important steps by which exposure to oxidative stress leads the body to a disease state. For its protection, the body has evolved to respond to and eliminate peroxidation products through the acquisition of binding proteins, reducing and conjugating enzymes, and excretion systems. During the past decade, researchers have identified a group of ion channel molecules that are activated by oxidized lipids: transient receptor potential (TRP) channels expressed in sensory neurons. These ion channels are fundamentally detectors and signal converters for body-damaging environments such as heat and cold temperatures, mechanical attacks, and potentially toxic substances. When messages initiated by TRP activation arrive at the brain, we perceive pain, which results in our preparing defensive responses. Excessive activation of the sensory neuronal TRP channels upon prolonged stimulations sometimes deteriorates the inflammatory state of damaged tissues by promoting neuropeptide release from expresser neurons. These same paradigms may also work for pathologic changes in the internal lipid environment upon exposure to oxidative stress. Here, we provide an overview of the role of TRP channels and oxidized lipid connections during abnormally increased oxidative signaling, and consider the sensory mechanism of TRP detection as an alert system.

Transient receptor potential channel A1 involved in calcitonin gene-related peptide release in neurons

Transient receptor potential channel A1 is one of the important transducers of noxious stimuli in the primary afferents, which may contribute to generation of neurogenic inflammation and hyperalgesia. The present study was designed to investigate if activation of transient receptor potential channel A1 may induce calcitonin gene-related peptide release from the primary afferent neurons. We found that application of allyl isothiocyanate, a transient receptor potential channel A1 activator, caused calcitonin gene-related peptide release from the cultured rat dorsal root ganglion neurons. Knockdown of transient receptor potential channel A1 with an antisense oligodeoxynucleotide prevented calcitonin gene-related peptide release by allyl isothiocyanate application in cultured dorsal root ganglion neurons. Thus, we concluded that transient receptor potential channel A1 activation caused calcitonin gene-related peptide release in sensory neurons.

The blockade of transient receptor potential ankirin 1 (TRPA1) signalling mediates antidepressant- and anxiolytic-like actions in mice

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid 1 (TRPV1) and TRP ankyrin 1 (TRPA1) are involved in many biological processes, including nociception and hyperalgesia. Whereas the involvement of TRPV1 in psychiatric disorders such as anxiety and depression has been reported, little is known regarding the role of TRPA1 in these conditions.

EXPERIMENTAL APPROACH

We investigated the role of TRPA1 in mice models of depression [forced swimming test (FST)] and anxiety [elevated plus maze (EPM) test].

KEY RESULTS

Administration of the TRPA1 antagonist (HC030031, 30 nmol in 2 μL, i.c.v.) reduced immobility time in the FST. Similar results were obtained after oral administration of HC030031 (30-300 mg·kg(-1) ). The reduction in immobility time in FST induced by HC030031 (100 mg·kg(-1) ) was completely prevented by pretreatment with TRPA1 agonist, cinnamaldehyde (50 mg·kg(-1) , p.o.), which per se was inactive. In the EPM test, pretreatment with cinnamaldehyde (50 mg·kg(-1) , p.o.), which per se did not affect behaviour response, prevented the anxiolytic-like effect (increased open arm exploration) evoked by TRPA1 blockade (HC030031, 100 mg·kg(-1) , p.o.). Treatment with either cinnamaldehyde or HC030031 did not affect spontaneous ambulation. Furthermore, TRPA1-deficient mice showed anxiolytic- and antidepressant-like phenotypes in the FST and EPM test respectively.

CONCLUSION AND IMPLICATIONS

The present findings indicate that genetic deletion or pharmacological blockade of TRPA1 produces inhibitory activity in mouse models of anxiety and depression. These results imply that TRPA1 exerts tonic control, promoting anxiety and depression, and that TRPA1 antagonism has potential as an innovative strategy for the treatment of anxiety and mood disorders.

Redox regulation of transient receptor potential channels

SIGNIFICANCE

Environmental and endogenous reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and other electrophiles are not only known to exert toxic effects on organisms, but are also emerging as molecules that mediate cell signaling responses. However, the mechanisms underlying this cellular redox signaling by reactive species remains largely uncharacterized.

RECENT ADVANCES

Ca2+-permeable cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that multiple TRP channels sense reactive species and induce diverse physiological and pathological responses, such as cell death, chemokine production, and pain transduction. TRP channels sense reactive species either indirectly through second messengers or directly via oxidative modification of cysteine residues. In this review, we describe the activation mechanisms and biological roles of redox-sensitive TRP channels, including TRPM2, TRPM7, TRPC5, TRPV1, and TRPA1.

CRITICAL ISSUES

The sensitivity of TRP channels to reactive species in vitro has been well characterized using molecular and pharmacological approaches. However, the precise activation mechanism(s) and in vivo function(s) of ROS/RNS-sensitive TRP channels remain elusive.

FUTURE DIRECTIONS

Redox sensitivity of TRP channels has been shown to mediate previously unexplained biological phenomena and is involved in various pathologies. Understanding the physiological significance and activation mechanisms of TRP channel regulation by reactive species may lead to TRP channels becoming viable pharmacological targets, and modulators of these channels may offer therapeutic options for previously untreatable diseases.

Intraganglionic signaling as a novel nasal-meningeal pathway for TRPA1-dependent trigeminovascular activation by inhaled environmental irritants

Headache is the most common symptom associated with air pollution, but little is understood about the underlying mechanism. Nasal administration of environmental irritants activates the trigeminovascular system by a TRPA1-dependent process. This report addresses questions about the anatomical pathway involved and the function of TRP channels in this pathway. TRPV1 and TRPA1 are frequently co-localized and interact to modulate function in sensory neurons. We demonstrate here that resiniferatoxin ablation of TRPV1 expressing neurons significantly reduces meningeal blood flow responses to nasal administration of both TRPV1 and TRPA1 agonists. Accordingly resiniferatoxin also significantly reduces TRPV1 and CGRP immunostaining and TRPV1 and TRPA1 message levels in trigeminal ganglia. Sensory neurons of the trigeminal ganglia innervate the nasal epithelium and the meninges, but the mechanism and anatomical route by which nasal administration evokes meningeal vasodilatation is unclear. Double retrograde labeling from the nose and meninges reveals no co-localization of fluorescent label, however nasal and meningeal labeled cells are located in close proximity to each other within the trigeminal ganglion. Our data demonstrate that TRPV1 expressing neurons are important for TRPA1 responses in the nasal-meningeal pathway. Our data also suggest that the nasal-meningeal pathway is not primarily by axon reflex, but may instead result from intraganglionic transmission.

A structural view of ligand-dependent activation in thermoTRP channels

Transient Receptor Potential (TRP) proteins are a large family of ion channels, grouped into seven sub-families. Although great advances have been made regarding the activation and modulation of TRP channel activity, detailed molecular mechanisms governing TRP channel gating are still needed. Sensitive to electric, chemical, mechanical, and thermal cues, TRP channels are tightly associated with the detection and integration of sensory input, emerging as a model to study the polymodal activation of ion channel proteins. Among TRP channels, the temperature-activated kind constitute a subgroup by itself, formed by Vanilloid receptors 1–4, Melastatin receptors 2, 4, 5, and 8, TRPC5, and TRPA1. Some of the so-called “thermoTRP” channels participate in the detection of noxious stimuli making them an interesting pharmacological target for the treatment of pain. However, the poor specificity of the compounds available in the market represents an important obstacle to overcome. Understanding the molecular mechanics underlying ligand-dependent modulation of TRP channels may help with the rational design of novel synthetic analgesics. The present review focuses on the structural basis of ligand-dependent activation of TRPV1 and TRPM8 channels. Special attention is drawn to the dissection of ligand-binding sites within TRPV1, PIP₂-dependent modulation of TRP channels, and the structure of natural and synthetic ligands.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

Sensory TRP channel interactions with endogenous lipids and their biological outcomes

Lipids have long been studied as constituents of the cellular architecture and energy stores in the body. Evidence is now rapidly growing that particular lipid species are also important for molecular and cellular signaling. Here we review the current information on interactions between lipids and transient receptor potential (TRP) ion channels in nociceptive sensory afferents that mediate pain signaling. Sensory neuronal TRP channels play a crucial role in the detection of a variety of external and internal changes, particularly with damaging or pain-eliciting potentials that include noxiously high or low temperatures, stretching, and harmful substances. In addition, recent findings suggest that TRPs also contribute to altering synaptic plasticity that deteriorates chronic pain states. In both of these processes, specific lipids are often generated and have been found to strongly modulate TRP activities, resulting primarily in pain exacerbation. This review summarizes three standpoints viewing those lipid functions for TRP modulations as second messengers, intercellular transmitters, or bilayer building blocks. Based on these hypotheses, we discuss perspectives that account for how the TRP-lipid interaction contributes to the peripheral pain mechanism. Still a number of blurred aspects remain to be examined, which will be answered by future efforts and may help to better control pain states.

Identification of a splice variant of mouse TRPA1 that regulates TRPA1 activity

Transient receptor potential ankyrin 1 (TRPA1) protein is a nonselective cation channel. Although many studies suggest that TRPA1 is involved in inflammatory and neuropathic pain, its mechanism remains unclear. Here we identify an alternative splice variant of the mouse Trpa1 gene. TRPA1a (full-length) and TRPA1b (splice variant) physically interact with each other and TRPA1b increases the expression of TRPA1a in the plasma membrane. TRPA1a and TRPA1b co-expression significantly increases current density in response to different agonists without affecting their single-channel conductance. Exogenous overexpression of Trpa1b gene in wild-type and TRPA1KO DRG neurons also increases TRPA1a-mediated AITC responses. Moreover, expression levels of Trpa1a and Trpa1b mRNAs change dynamically in two pain models (complete Freund’s adjuvant-induced inflammatory pain and partial sciatic nerve ligation-induced neuropathic pain models). These results suggest that TRPA1 may be regulated through alternative splicing under these pathological conditions.

TRPA1 is a transient receptor potential channel family member and is involved in the detection of nociceptive stimuli. Zhou et al. identify an alternative splice variant of TRPA1, which increases TRPA1 plasma expression and channel function, and enhances pain-like behaviour in mice.

The contribution of transient receptor potential ankyrin 1 (TRPA1) to the in vivo nociceptive effects of prostaglandin E₂

AIMS

Although evidence suggest that TRPA1 mediates some effects of prostaglandins, it is not known whether TRPA1 contributes to the in vivo nociceptive effects of prostaglandin E2 (PGE2), a key mediator of inflammatory pain.

MAIN METHODS

To address this issue, the effect of the pharmacological blockade of TRPA1 or of its gene silencing on the hyperalgesia induced in the rat paw by PGE2 or its downstream signaling molecules, protein kinase A (PKA) or protein kinase C-epsilon (PKCε), was evaluated. TRPA1 expression on dorsal root ganglia cells was assessed by western blot.

KEY FINDINGS

The pharmacological blockade of local TRPA1 by its selective antagonist, HC 030031 decreased and reversed PGE2-induced hyperalgesia. The TRPA1 gene silencing induced by intrathecal pre-treatment with antisense oligodeoxynucleotide blocked PGE2-induced hyperalgesia and strongly reduced TRPA1 expression in dorsal root ganglia cells (L5 and L6). PGE2 injection into the hind paw did not significantly increase TRPA1 expression in dorsal root ganglia cells. Treatment with either HC 030031 or antisense oligodeoxynucleotide significantly decreased the hyperalgesia induced by PKA or PKCε. Since both kinases are the major components of PGE2-induced intracellular signal transduction, the modulation of TRPA1 function by PGE2 may be downstream PKA and PKC-epsilon.

SIGNIFICANCE

These findings show that TRPA1 is essential to the in vivo nociceptive effects induced by one of the most important mediators of inflammatory pain, PGE2. This is one of the crucial findings necessary to support TRPA1 as a promising target for the development of future drugs to pain treatment and control.

Ligand determinants of fatty acid activation of the pronociceptive ion channel TRPA1

Background and purpose. Arachidonic acid (AA) and its derivatives are important modulators of cellular signalling. The transient receptor potential cation channel subfamily A, member 1 (TRPA1) is a cation channel with important functions in mediating cellular responses to noxious stimuli and inflammation. There is limited information about the interactions between AA itself and TRPA1, so we investigated the effects of AA and key ethanolamide and amino acid/neurotransmitter derivatives of AA on hTRPA1.

Experimental approach. HEK 293 cells expressing hTRPA1 were studied by measuring changes in intracellular calcium ([Ca]_i) with a fluorescent dye and by standard whole cell patch clamp recordings.

Key results. AA (30 μM) increased fluorescence in hTRPA1 expressing cells by 370% (notional EC₅₀ 13 μM). The covalent TRPA1 agonist cinnamaldehyde (300 μM) increased fluorescence by 430% (EC₅₀, 11 μM). Anandamide (230%) and N-arachidonoyl tyrosine (170%) substantially activated hTRPA1 at 30 μM, however, N-arachidonoyl conjugates of glycine and taurine were less effective while N-acyl conjugates of 5-HT did not affect hTRPA1. Changing the acyl chain length or the number and position of double bonds reduced fatty acid efficacy at hTRPA1. Mutant hTRPA1 (Cys621, Cys641 and Cys665 changed to Ser) could be activated by AA (100 μM, 40% of wild type) but not by cinnamaldehyde (300 μM).

Conclusions and implications. AA is a more potent activator of TRPA1 than its ethanolamide or amino acid/neurotransmitter derivatives and acts via a mechanism distinct from that of cinnamaldehyde, further underscoring the likelyhood of multiple pharmacologically exploitable sites on hTRPA1.

Mechanisms underlying the neuronal-based symptoms of allergy

Persons with allergies present with symptoms that often are the result of alterations in the nervous system. Neuronally based symptoms depend on the organ in which the allergic reaction occurs but can include red itchy eyes, sneezing, nasal congestion, rhinorrhea, coughing, bronchoconstriction, airway mucus secretion, dysphagia, altered gastrointestinal motility, and itchy swollen skin. These symptoms occur because mediators released during an allergic reaction can interact with sensory nerves, change processing in the central nervous system, and alter transmission in sympathetic, parasympathetic, and enteric autonomic nerves. In addition, evidence supports the idea that in some subjects this neuromodulation is, for reasons poorly understood, upregulated such that the same degree of nerve stimulus causes a larger effect than seen in healthy subjects. There are distinctions in the mechanisms and nerve types involved in allergen-induced neuromodulation among different organ systems, but general principles have emerged. The products of activated mast cells, other inflammatory cells, and resident cells can overtly stimulate nerve endings, cause long-lasting changes in neuronal excitability, increase synaptic efficacy, and also change gene expression in nerves, resulting in phenotypically altered neurons. A better understanding of these processes might lead to novel therapeutic strategies aimed at limiting the suffering of those with allergies.

Targeting Pain-evoking Transient Receptor Potential Channels for the Treatment of Pain

Chronic pain affects billions of lives globally and is a major public health problem in the United States. However, pain management is still a challenging task due to a lack of understanding of the fundamental mechanisms of pain. In the past decades transient receptor potential (TRP) channels have been identified as molecular sensors of tissue damage and inflammation. Activation/sensitization of TRP channels in peripheral nociceptors produces neurogenic inflammation and contributes to both somatic and visceral pain. Pharmacological and genetic studies have affirmed the role of TRP channels in multiple forms of inflammatory and neuropathic pain. Thus pain-evoking TRP channels emerge as promising therapeutic targets for a wide variety of pain and inflammatory conditions.

TRPA1 channels: chemical and temperature sensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal excitatory ion channel found in sensory neurons of different organisms, ranging from worms to humans. Since its discovery as an uncharacterized transmembrane protein in human fibroblasts, TRPA1 has become one of the most intensively studied ion channels. Its function has been linked to regulation of heat and cold perception, mechanosensitivity, hearing, inflammation, pain, circadian rhythms, chemoreception, and other processes. Some of these proposed functions remain controversial, while others have gathered considerable experimental support. A truly polymodal ion channel, TRPA1 is activated by various stimuli, including electrophilic chemicals, oxygen, temperature, and mechanical force, yet the molecular mechanism of TRPA1 gating remains obscure. In this review, we discuss recent advances in the understanding of TRPA1 physiology, pharmacology, and molecular function.

TRPA1

The transient receptor potential ankyrin subtype 1 protein (TRPA1) is a nonselective cation channel permeable to Ca(2+), Na(+), and K(+). TRPA1 is a promiscuous chemical nocisensor that is also involved in noxious cold and mechanical sensation. It is present in a subpopulation of Aδ- and C-fiber nociceptive sensory neurons as well as in other sensory cells including epithelial cells. In primary sensory neurons, Ca(2+) and Na(+) flowing through TRPA1 into the cell cause membrane depolarization, action potential discharge, and neurotransmitter release both at peripheral and central neural projections. In addition to being activated by cysteine and lysine reactive electrophiles and oxidants, TRPA1 is indirectly activated by pro-inflammatory agents via the phospholipase C signaling pathway, in which cytosolic Ca(2+) is an important regulator of channel gating. The finding that non-electrophilic compounds, including menthol and cannabinoids, activate TRPA1 may provide templates for the design of non-tissue damaging activators to fine-tune the activity of TRPA1 and raises the possibility that endogenous ligands sharing binding sites with such non-electrophiles exist and regulate TRPA1 channel activity. TRPA1 is promising as a drug target for novel treatments of pain, itch, and sensory hyperreactivity in visceral organs including the airways, bladder, and gastrointestinal tract.

Transnitrosylation directs TRPA1 selectivity in N-nitrosamine activators

S-Nitrosylation, the addition of a nitrosyl group to cysteine thiols, regulates various protein functions to mediate nitric oxide (NO) bioactivity. Recent studies have demonstrated that selectivity in protein S-nitrosylation signaling pathways is conferred through transnitrosylation, a transfer of the NO group, between proteins via interaction. We previously demonstrated that sensitivity to activation by synthetic NO-releasing agents via S-nitrosylation is a common feature of members of the transient receptor potential (TRP) family of Ca(2+)-permeable cation channels. However, strategies to confer subtype selectivity to nitrosylating agents targeted to TRP channels are yet to be developed. Here, we show selective activation of TRPA1 channels by novel NO donors derived from the ABBH (7-azabenzobicyclo[2.2.1]heptane) N-nitrosamines, which exhibit transnitrosylation reactivity to thiols without releasing NO. The NNO-ABBH1 (N-nitroso-2-exo,3-exo-ditrifluoromethyl-7-azabenzobicyclo[2.2.1]heptane) elicits S-nitrosylation of TRPA1 proteins, and dose-dependently induces robust Ca(2+) influx via both recombinant and native TRPA1 channels, but not via other NO-activated TRP channels. TRPA1 activation by NNO-ABBH1 is suppressed by specific cysteine mutations but not by NO scavenging, suggesting that cysteine transnitrosylation underlies the activation of TRPA1 by NNO-ABBH1. This is supported by the correlation of N-NO bond reactivity and TRPA1-activating potency in a congeneric series of ABBH N-nitrosamines. Interestingly, nonelectrophilic derivatives of ABBH also activate TRPA1 selectively, but less potently, compared with NNO-ABBH1. Thus, ABBH N-nitrosamines confer subtype selectivity on S-nitrosylation in TRP channels through synergetic effects of two chemical processes: cysteine transnitrosylation and molecular recognition of the nonelectrophilic moiety.

TRPs as chemosensors (ROS, RNS, RCS, gasotransmitters)

The transient receptor potential (trp) gene superfamily encodes TRP proteins that act as multimodal sensor cation channels for a wide variety of stimuli from outside and inside the cell. Upon chemical or physical stimulation of cells, TRP channels transduce electrical and/or Ca(2+) signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive carbonyl species (RCS), and gaseous messenger molecules including molecular oxygen (O2), hydrogen sulfide (H2S), and carbon dioxide (CO2). Hydrogen peroxide (H2O2), an ROS, triggers the production of ADP-ribose, which binds and activates TRPM2. In addition to TRPM2, TRPC5, TRPV1, and TRPA1 are also activated by H2O2 via modification of cysteine (Cys) free sulfhydryl groups. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via Cys S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Anoxia induced by O2-glucose deprivation and severe hypoxia activates TRPM7 and TRPC6, respectively, whereas TRPA1 serves as a sensor of mild hypoxia and hyperoxia in vagal and sensory neurons. TRPA1 also detects other gaseous molecules, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). In this review, we highlight our current knowledge of TRP channels as chemosensors for ROS, RNS, RCS, and gaseous molecules and discuss their functional impacts on physiological and pathological events.

TRP-channels as key integrators of lipid pathways in nociceptive neurons

TRP-channels are the most prominent family of ligand-gated ion channels for pain perception. In sensory neurons, TRPV1-V4, TRPA1 and TRPM8 are expressed and are responsible for the conversion of external stimuli to painful sensations. Under pathophysiological conditions, excessive activity of TRP-channels leads to mechanical allodynia and thermal hyperalgesia. Among the endogenous TRP-channel sensitizers, activators and inhibitors, more than 50 arachidonic acid- and linoleic acid-metabolites from the COX-, LOX- and CYP-pathways, as well as lysophospholipids and isoprenoids can be found. As a consequence, these lipids represent the vast majority of endogenous TRP-channel modulators in sensory neurons. Although the precise mechanisms of TRP-channel modulation by most lipids are still unknown, it became clear that lipids can either bind directly to the target TRP-channel or modulate TRP-channels indirectly by activating G-protein coupled receptors. Thus, TRP-channels seem to be key sensors for lipids, integrating and interpreting incoming signals from the different metabolic lipid pathways. Here, we discuss the specific properties of the currently known endogenous lipid-derived TRP-channel modulators concerning their ability to activate or inhibit TRP-channels, the molecular mechanisms of lipid/TRP-channel interactions and specific TRP-regulatory characteristics of the individual lipid families.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

Changes in sensory activity of ocular surface sensory nerves during allergic keratoconjunctivitis

Peripheral neural mechanisms underlying the sensations of irritation, discomfort, and itch accompanying the eye allergic response have not been hitherto analyzed. We explored this question recording the changes in the electrical activity of corneoconjunctival sensory nerve fibers of the guinea pig after an ocular allergic challenge. Sensitization was produced by i.p. ovalbumin followed by repeated application in the eye of 10% ovalbumin on days 14 to 18. Blinking and tearing rate were measured. Spontaneous and stimulus-evoked (mechanical, thermal, chemical) impulse activity was recorded from mechanonociceptor, polymodal nociceptor and cold corneoscleral sensory afferent fibers. After a single (day 14) or repeated daily exposures to the allergen during the following 3 to 4days, tearing and blinking rate increased significantly. Also, sensitization was observed in mechanonociceptors (transient reduction of mechanical threshold only on day 14) and in polymodal nociceptors (sustained enhancement of the impulse response to acidic stimulation). In contrast, cold thermoreceptors showed a significant decrease in basal ongoing activity and in the response to cooling. Treatment with the TRPV1 and TRPA1 blockers capsazepine and HC-030031 reversed the augmented blinking. Only capsazepine attenuated tearing rate increase and sensitization of the polymodal nociceptors response to CO2. Capsazepine also prevented the decrease in cold thermoreceptor activity caused by the allergic challenge. We conclude that changes in nerve impulse activity accompanying the ocular allergic response, primarily mediated by activation of nociceptor's TRPV1 and to a lesser degree by activation of TRPA1 channels, explain the eye discomfort sensations accompanying allergic episodes.

Methylglyoxal evokes pain by stimulating TRPA1

Diabetic neuropathy is a severe complication of long-standing diabetes and one of the major etiologies of neuropathic pain. Diabetes is associated with an increased formation of reactive oxygen species and the electrophilic dicarbonyl compound methylglyoxal (MG). Here we show that MG stimulates heterologously expressed TRPA1 in CHO cells and natively expressed TRPA1 in MDCK cells and DRG neurons. MG evokes [Ca²⁺]_i-responses in TRPA1 expressing DRG neurons but is without effect in neurons cultured from Trpa1^−/− mice. Consistent with a direct, intracellular action, we show that methylglyoxal is significantly more potent as a TRPA1 agonist when applied to the intracellular face of excised membrane patches than to intact cells. Local intraplantar administration of MG evokes a pain response in Trpa1^+/+ but not in Trpa1^−/− mice. Furthermore, persistently increased MG levels achieved by two weeks pharmacological inhibition of glyoxalase-1 (GLO-1), the rate-limiting enzyme responsible for detoxification of MG, evokes a progressive and marked thermal (cold and heat) and mechanical hypersensitivity in wildtype but not in Trpa1^−/− mice. Our results thus demonstrate that TRPA1 is required both for the acute pain response evoked by topical MG and for the long-lasting pronociceptive effects associated with elevated MG in vivo. In contrast to our observations in DRG neurons, MG evokes indistinguishable [Ca²⁺]_i-responses in pancreatic β-cells cultured from Trpa1^+/+ and Trpa1^−/− mice. In vivo, the TRPA1 antagonist HC030031 impairs glucose clearance in the glucose tolerance test both in Trpa1^+/+ and Trpa1^−/− mice, indicating a non-TRPA1 mediated effect and suggesting that results obtained with this compound should be interpreted with caution. Our results show that TRPA1 is the principal target for MG in sensory neurons but not in pancreatic β-cells and that activation of TRPA1 by MG produces a painful neuropathy with the behavioral hallmarks of diabetic neuropathy.

Mediator mechanisms involved in TRPV1, TRPA1 and P2X receptor-mediated sensory transduction of pulmonary ROS by vagal lung C-fibers in rats

We investigated the mediator mechanisms involved in the sensory transduction of pulmonary reactive oxygen species (ROS) by vagal lung C-fibers in anesthetized rats. Airway challenge of aerosolized H₂O₂ (0.4%) stimulated these afferent fibers. The H₂O₂-induced responses were reduced by a cyclooxygenase inhibitor or ATP scavengers and also attenuated by an antagonist of TRPV1, TRPA1 or P2X receptors. The suppressive effect of the cyclooxygenase inhibitor was not affected by a combined treatment with the TRPV1 or TRPA1 antagonist, but was amplified by a combined treatment with the P2X antagonists. The suppressive effect of ATP scavengers was not affected by a combined treatment with the P2X antagonist, but was amplified by a combined treatment with the TRPV1 or TRPA1 antagonist. Thus, the actions of cyclooxygenase metabolites are mediated through the functioning of the TRPV1 and TRPA1 receptors, whereas the action of ATP is mediated through the functioning of P2X receptors.

Role of prostaglandin D2 in mast cell activation-induced sensitization of esophageal vagal afferents

Sensitization of esophageal afferents plays an important role in esophageal nociception, but the mechanism is less clear. Our previous studies demonstrated that mast cell (MC) activation releases the preformed mediators histamine and tryptase, which play important roles in sensitization of esophageal vagal nociceptive C fibers. PGD2 is a lipid mediator released by activated MCs. Whether PGD2 plays a role in this sensitization process has yet to be determined. Expression of the PGD2 DP1 and DP2 receptors in nodose ganglion neurons was determined by immunofluorescence staining, Western blotting, and RT-PCR. Extracellular recordings were performed in ex vivo esophageal-vagal preparations. Action potentials evoked by esophageal distension were compared before and after perfusion of PGD2, DP1 and DP2 receptor agonists, and MC activation, with or without pretreatment with antagonists. The effect of PGD2 on 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled esophageal nodose neurons was determined by patch-clamp recording. Our results demonstrate that DP1 and DP2 receptor mRNA and protein were expressed mainly in small- and medium-diameter neurons in nodose ganglia. PGD2 significantly increased esophageal distension-evoked action potential discharges in esophageal nodose C fibers. The DP1 receptor agonist BW 245C mimicked this effect. PGD2 directly sensitized DiI-labeled esophageal nodose neurons by decreasing the action potential threshold. Pretreatment with the DP1 receptor antagonist BW A868C significantly inhibited PGD2 perfusion- or MC activation-induced increases in esophageal distension-evoked action potential discharges in esophageal nodose C fibers. In conclusion, PGD2 plays an important role in MC activation-induced sensitization of esophageal nodose C fibers. This adds a novel mechanism of visceral afferent sensitization.

Sensory nerve terminal mitochondrial dysfunction activates airway sensory nerves via transient receptor potential (TRP) channels

Mitochondrial dysfunction and subsequent oxidative stress has been reported for a variety of cell types in inflammatory diseases. Given the abundance of mitochondria at the peripheral terminals of sensory nerves and the sensitivity of transient receptor potential (TRP) ankyrin 1 (A1) and TRP vanilloid 1 (V1) to reactive oxygen species (ROS) and their downstream products of lipid peroxidation, we investigated the effect of nerve terminal mitochondrial dysfunction on airway sensory nerve excitability. Here we show that mitochondrial dysfunction evoked by acute treatment with antimycin A (mitochondrial complex III Qi site inhibitor) preferentially activated TRPA1-expressing "nociceptor-like" mouse bronchopulmonary C-fibers. Action potential discharge was reduced by the TRPA1 antagonist HC-030031. Inhibition of TRPV1 further reduced C-fiber activation. In mouse dissociated vagal neurons, antimycin A induced Ca(2+) influx that was significantly reduced by pharmacological inhibition or genetic knockout of either TRPA1 or TRPV1. Inhibition of both TRPA1 and TRPV1 was required to abolish antimycin A-induced Ca(2+) influx in vagal neurons. Using an HEK293 cell expression system, antimycin A induced concentration-dependent activation of both hTRPA1 and hTRPV1 but failed to activate nontransfected cells. Myxothiazol (complex III Qo site inhibitor) inhibited antimycin A-induced TRPA1 activation, as did the reducing agent dithiothreitol. Scavenging of both superoxide and hydrogen peroxide inhibited TRPA1 activation following mitochondrial modulation. In conclusion, we present evidence that acute mitochondrial dysfunction activates airway sensory nerves preferentially via TRPA1 through the actions of mitochondrially-derived ROS. This represents a novel mechanism by which inflammation may be transduced into nociceptive electrical signaling.

Transient receptor potentials (TRPs) and anaphylaxis

The transient receptor potential (TRP) superfamily consists of 28 members in mammals (27 in human) that act as polymodal sensors and ion channels. They regulate cellular calcium influx, generate depolarization thereby triggering voltage dependent cellular processes, and in turn they are critical in inducing the metabolic activities of cells. It is increasingly apparent that many of the inflammatory mediators released in allergic reactions involve at least two of these ion channels, the 'Vanilloid' TRPV1 and the 'Ankyrin" TRPA1. This review mainly focuses on TRPV1 and TRPA1 and the role they have in the allergic response and how these receptors may be influenced in exercise-induced anaphylaxis. The threshold to react to an allergen for mast cells and lymphocytes can be reduced by activating the melastatin channel TRPM4. This channel is briefly discussed in the context of allergy.

Pre-clinical studies in cough research: role of Transient Receptor Potential (TRP) channels

Cough is a protective reflex and defence mechanism in healthy individuals, which helps clear excessive secretions and foreign material from the lungs. Cough often presents as the first and most persistent symptom of many respiratory diseases and some non-respiratory disorders, but can also be idiopathic, and is a common respiratory complaint for which medical attention is sought. Chronic cough of various aetiologies is a regular presentation to specialist respiratory clinics, and is reported as a troublesome symptom by a significant proportion of the population. Despite this, the treatment options for cough are limited. The lack of effective anti-tussives likely stems from our incomplete understanding of how the tussive reflex is mediated. However, research over the last decade has begun to shed some light on the mechanisms which provoke cough, and may ultimately provide us with better anti-tussive therapies. This review will focus on the in vitro and in vivo models that are currently used to further our understanding of the sensory innervation of the respiratory tract, and how these nerves are involved in controlling the cough response. Central to this are the Transient Receptor Potential (TRP) ion channels, a family of polymodal receptors that can be activated by such diverse stimuli as chemicals, temperature, osmotic stress, and mechanical perturbation. These ion channels are thought to be molecular pain integrators and targets for novel analgesic agents for the treatment of various pain disorders but some are also being developed as anti-tussives.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Prostaglandin metabolite induces inhibition of TRPA1 and channel-dependent nociception

Background

The Transient Receptor Potential (TRP) ion channel TRPA1 is a key player in pain pathways. Irritant chemicals activate ion channel TRPA1 via covalent modification of N-terminal cysteines. We and others have shown that 15-Deoxy-Δ12, 14-prostaglandin J₂ (15d-PGJ₂) similarly activates TRPA1 and causes channel-dependent nociception. Paradoxically, 15d-PGJ₂ can also be anti-nociceptive in several pain models. Here we hypothesized that activation and subsequent desensitization of TRPA1 in dorsal root ganglion (DRG) neurons underlies the anti-nociceptive property of 15d-PGJ₂. To investigate this, we utilized a battery of behavioral assays and intracellular Ca²⁺ imaging in DRG neurons to test if pre-treatment with 15d-PGJ₂ inhibited TRPA1 to subsequent stimulation.

Results

Intraplantar pre-injection of 15d-PGJ₂, in contrast to mustard oil (AITC), attenuated acute nocifensive responses to subsequent injections of 15d-PGJ₂ and AITC, but not capsaicin (CAP). Intraplantar 15d-PGJ₂—administered after the induction of inflammation—reduced mechanical hypersensitivity in the Complete Freund’s Adjuvant (CFA) model for up to 2 h post-injection. The 15d-PGJ₂-mediated reduction in mechanical hypersensitivity is dependent on TRPA1, as this effect was absent in TRPA1 knockout mice. Ca²⁺ imaging studies of DRG neurons demonstrated that 15d-PGJ₂ pre-exposure reduced the magnitude and number of neuronal responses to AITC, but not CAP. AITC responses were not reduced when neurons were pre-exposed to 15d-PGJ₂ combined with HC-030031 (TRPA1 antagonist), demonstrating that inhibitory effects of 15d-PGJ₂ depend on TRPA1 activation. Single daily doses of 15d-PGJ₂, administered during the course of 4 days in the CFA model, effectively reversed mechanical hypersensitivity without apparent tolerance or toxicity.

Conclusions

Taken together, our data support the hypothesis that 15d-PGJ₂ induces activation followed by persistent inhibition of TRPA1 channels in DRG sensory neurons in vitro and in vivo. Moreover, we demonstrate novel evidence that 15d-PGJ₂ is analgesic in mouse models of pain via a TRPA1-dependent mechanism. Collectively, our studies support that TRPA1 agonists may be useful as pain therapeutics.

TRPA1: A gatekeeper for inflammation

Tissue damage evokes an inflammatory response that promotes the removal of harmful stimuli, tissue repair, and protective behaviors to prevent further damage and encourage healing. However, inflammation may outlive its usefulness and become chronic. Chronic inflammation can lead to a host of diseases, including asthma, itch, rheumatoid arthritis, and colitis. Primary afferent sensory neurons that innervate target organs release inflammatory neuropeptides in the local area of tissue damage to promote vascular leakage, the recruitment of immune cells, and hypersensitivity to mechanical and thermal stimuli. TRPA1 channels are required for neuronal excitation, the release of inflammatory neuropeptides, and subsequent pain hypersensitivity. TRPA1 is also activated by the release of inflammatory agents from nonneuronal cells in the area of tissue injury or disease. This dual function of TRPA1 as a detector and instigator of inflammatory agents makes TRPA1 a gatekeeper of chronic inflammatory disorders of the skin, airways, and gastrointestinal tract.

The functions of TRPA1 and TRPV1: moving away from sensory nerves

The transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) channels are members of the TRP superfamily of structurally related, non-selective cation channels. It is rapidly becoming clear that the functions of TRPV1 and TRPA1 interlink with each other to a considerable extent. This is especially clear in relation to pain and neurogenic inflammation where TRPV1 is coexpressed on the vast majority of TRPA1-expressing sensory nerves and both integrate a variety of noxious stimuli. The more recent discovery that both TRPV1 and TRPA1 are expressed on a multitude of non-neuronal sites has led to a plethora of research into possible functions of these receptors. Non-neuronal cells on which TRPV1 and TRPA1 are expressed vary from vascular smooth muscle to keratinocytes and endothelium. This review will discuss the expression, functionality and roles of these non-neuronal TRP channels away from sensory nerves to demonstrate the diverse nature of TRPV1 and TRPA1 in addition to a direct role in pain and neurogenic inflammation.

TRPA1 mediates mechanical sensitization in nociceptors during inflammation

Inflammation is a part of the body's natural response to tissue injury which initiates the healing process. Unfortunately, inflammation is frequently painful and leads to hypersensitivity to mechanical stimuli, which is difficult to treat clinically. While it is well established that altered sensory processing in the spinal cord contributes to mechanical hypersensitivity (central sensitization), it is still debated whether primary afferent neurons become sensitized to mechanical stimuli after tissue inflammation. We induced inflammation in C57BL/6 mice via intraplantar injection of Complete Freund's Adjuvant. Cutaneous C fibers exhibited increased action potential firing to suprathreshold mechanical stimuli. We found that abnormal responses to intense mechanical stimuli were completely suppressed by acute incubation of the receptive terminals with the TRPA1 inhibitor, HC-030031. Further, elevated responses were predominantly exhibited by a specific subgroup of C fibers, which we determined to be C-Mechano Cold sensitive fibers. Thus, in the presence of HC-030031, C fiber mechanical responses in inflamed mice were not different than responses in saline-injected controls. We also demonstrate that injection of the HC-030031 compound into the hind paw of inflamed mice alleviates behavioral mechanical hyperalgesia without affecting heat hyperalgesia. Further, we pharmacologically anesthetized the TRPA1-expressing fibers in vivo by co-injecting the membrane-impermeable sodium channel inhibitor QX-314 and the TRPA1 agonist cinnamaldehyde into the hind paw. This approach also alleviated behavioral mechanical hyperalgesia in inflamed mice but left heat hypersensitivity intact. Our findings indicate that C-Mechano Cold sensitive fibers exhibit enhanced firing to suprathreshold mechanical stimuli in a TRPA1-dependent manner during inflammation, and that input from these fibers drives mechanical hyperalgesia in inflamed mice.

Activation of TRPA1 on dural afferents: a potential mechanism of headache pain

Activation of transient receptor potential ankyrin-1 (TRPA1) on meningeal nerve endings has been suggested to contribute to environmental irritant-induced headache, but this channel may also contribute to other forms of headache, such as migraine. The preclinical studies described here examined functional expression of TRPA1 on dural afferents and investigated whether activation of TRPA1 contributes to headache-like behaviors. Whole-cell patch-clamp recordings were performed in vitro with 2 TRPA1 agonists, mustard oil (MO), and the environmental irritant umbellulone (UMB) on dural-projecting trigeminal ganglion neurons. Application of MO and UMB to dural afferents produced TRPA1-like currents in approximately 42% and 38% of cells, respectively. By means of an established in vivo behavioral model of migraine-related allodynia, dural application of MO and UMB produced robust time-related tactile facial and hind paw allodynia that was attenuated by pretreatment with the TRPA1 antagonist HC-030031. Additionally, MO or UMB were applied to the dura, and exploratory activity was monitored for 30min with an automated open-field activity chamber. Dural MO and UMB decreased the number of vertical rearing episodes and the time spent rearing in comparison to vehicle-treated animals. This change in activity was prevented in rats pretreated with HC-030031 as well as sumatriptan, a clinically effective antimigraine agent. These data indicate that TRPA1 is expressed on a substantial fraction of dural afferents, and activation of meningeal TRPA1 produces behaviors consistent with those observed in patients during migraine attacks. Further, they suggest that activation of meningeal TRPA1 via endogenous or exogenous mechanisms can lead to afferent signaling and headache.

Etodolac blocks the allyl isothiocyanate-induced response in mouse sensory neurons by selective TRPA1 activation

BACKGROUND AND PURPOSE

The excitability of nociceptors is modulated by the transient receptor potential cation channel, ankyrin subfamily, member 1 (TRPA1). We have previously reported that etodolac, a nonsteroidal anti-inflammatory drug, attenuates mechanical allodynia in a mouse model of neuropathic pain by a mechanism that is independent of cyclooxygenase inhibition. Here, we investigate the role of TRPA1 in the mechanism of the antinociceptive action of etodolac in vitro and in vivo.

EXPERIMENTAL APPROACH

Ca(2+) influx was measured in HEK-293 cells expressing mouse TRPA1 and in mouse dorsal root ganglion (DRG) neurons. The effect of etodolac on the nociceptive behavior induced in mice by the TRPA1 agonist allyl isothiocyanate (AITC) was also measured.

RESULTS

Etodolac induced Ca(2+) influx in HEK-293 cells expressing mouse TRPA1 and in mouse DRG neurons. The Ca(2+) influx induced by etodolac was inhibited by pretreatment with the TRPA1-specific antagonist HC-030031. In contrast, etodolac did not induce Ca(2+) influx in cells expressing TRPV1, TRPV2 or TRPM8. In addition, pretreatment with etodolac inhibited the Ca(2+) influx induced by AITC.

CONCLUSION AND IMPLICATION

Etodolac showed a selective TRPA1 agonist action, providing evidence that etodolac desensitizes nociceptors by the selective activation of TRPA1. Etodolac may be clinically useful in the treatment of neuropathic pain.

TRPA1 is a polyunsaturated fatty acid sensor in mammals

Fatty acids can act as important signaling molecules regulating diverse physiological processes. Our understanding, however, of fatty acid signaling mechanisms and receptor targets remains incomplete. Here we show that Transient Receptor Potential Ankyrin 1 (TRPA1), a cation channel expressed in sensory neurons and gut tissues, functions as a sensor of polyunsaturated fatty acids (PUFAs) in vitro and in vivo. PUFAs, containing at least 18 carbon atoms and three unsaturated bonds, activate TRPA1 to excite primary sensory neurons and enteroendocrine cells. Moreover, behavioral aversion to PUFAs is absent in TRPA1-null mice. Further, sustained or repeated agonism with PUFAs leads to TRPA1 desensitization. PUFAs activate TRPA1 non-covalently and independently of known ligand binding domains located in the N-terminus and 5^th transmembrane region. PUFA sensitivity is restricted to mammalian (rodent and human) TRPA1 channels, as the drosophila and zebrafish TRPA1 orthologs do not respond to DHA. We propose that PUFA-sensing by mammalian TRPA1 may regulate pain and gastrointestinal functions.

Oxidative stress--cause or consequence of male genital tract disorders?

BACKGROUND

Inflammatory prostatitis patients are characterized by oxidative stress (OxS) at local and systemic levels. Less is known about the occurrence of OxS in the case of other frequent male genital tract disorders.

METHODS

The study included 196 men: controls (n = 28), asymptomatic inflammatory (NIH category IV) prostatitis patients (n = 21), non-inflammatory (NIH category IIIb) chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) patients (n = 48), inflammatory (NIH category IIIa) CP/CPPS patients (n = 44), benign prostate hyperplasia (BPH) patients (n = 33), and patients with BPH and NIH IV category prostatitis (n = 22). In all subjects, 8-isoprostanes (8-EPI) in urine were determined by competitive enzyme-linked immunoassay.

RESULTS

The levels of 8-EPI were substantially higher in all diseased groups-inflammatory CP/CPPS (P < 0.001), non-inflammatory CP/CPPS (P = 0.03), asymptomatic inflammatory prostatitis (AIP; P = 0.02), BPH (P = 0.007), and BPH + AIP (P = 0.014) in comparison with controls. Importantly, our study showed that OxS is also present in the case of NIH IIIb category prostatitis when the patients have just chronic pelvic pain but no inflammation in prostate-specific materials, as well as in the patients with just lower urinary tract symptoms without pain or overt inflammation.

CONCLUSIONS

This study revealed that several male genital tract disorders-BPH and different forms of prostatitis (NIH categories IIIa, IIIb, and IV)-are tightly interconnected via OxS-mediated pathways. Acknowledging OxS as an important pathogenesis mechanism of these diseases helps to open up new horizons for their treatment.

Comparison of TRPA1-versus TRPV1-mediated cough in guinea pigs

TRPA1 receptor is activated by endogenous inflammatory mediators and exogenous pollutant molecules relevant to respiratory diseases. Previous studies have implicated TRPA1 as a drug target for antitussive therapy. Here we evaluated the relative efficacy of TRPA1 activation to evoke cough. In conscious guinea pigs the TRPA1 agonist allyl-isothiocyanate (AITC) evoked cough with a maximally effective concentration of 10mM that was abolished by the selective TRPA1 antagonist AP-18. AITC (10mM) was approximately 3-times less effective in inducing cough than capsaicin (50 μM). Ex vivo single fiber extracellular recordings revealed that, similarly to capsaicin, AITC evoked activation in airway jugular C-fibers, but not in airway nodose Aδ-fibers. Consistent with the cough studies, AITC was approximately 3-times less effective than capsaicin in evoking sustained activation of the jugular C-fibers. Another TRPA1 agonist, cinnamaldehyde, was approximately twofold more effective than AITC in inducing cough. However, the cinnamaldehyde (10mM)-induced cough was only partially inhibited by the TRPA1 antagonist AP-18, and was abolished by combination of AP-18 and the TRPV1 antagonist I-RTX. We conclude that in naïve guinea pigs, TRPA1 activation initiates cough that is relatively modest compared to the cough initiated by TRPV1, likely due to lower efficacy of TRPA1 stimulation to induce sustained activation of airway C-fibers.

17(R)-resolvin D1 specifically inhibits transient receptor potential ion channel vanilloid 3 leading to peripheral antinociception

BACKGROUND AND PURPOSE

Transient receptor potential ion channel vanilloid 3 (TRPV3) is expressed in skin keratinocytes and plays an important role in thermal and chemical nociceptions in the periphery. The presence of TRPV3 inhibitors would improve our understanding of TRPV3 function and help to develop receptor-specific analgesics. However, little is known about physiological substances that specifically inhibit TRPV3 activity. Here, we investigated whether 17(R)-resolvin D1 (17R-RvD1), a naturally occurring pro-resolving lipid specifically affects TRPV3 activity.

EXPERIMENTAL APPROACH

We examined the effect of 17R-RvD1 on sensory TRP channels using Ca(2+) imaging and whole cell electrophysiology experiments in a HEK cell heterologous expression system, cultured sensory neurons and keratinocytes. We also examined changes in sensory TRP agonist-specific acute licking/flicking or flinching behaviours and mechanical and thermal pain behaviours using Hargreaves, Randall-Selitto and von Frey assay systems in the absence and presence of inflammation.

KEY RESULTS

We showed that 17R-RvD1 specifically suppresses TRPV3-mediated activity at nanomolar and micromolar concentrations. The voltage-dependence of TRPV3 activation by camphor was shifted rightwards by 17R-RvD1, which indicates its inhibitory mechanism is as a result of a shift in voltage-dependence. Consistently, TRPV3-specific acute pain behaviours were attenuated by locally injected 17R-RvD1. Moreover, the administration of 17R-RvD1 significantly reversed the thermal hypersensitivity that occurs during an inflammatory response. Knockdown of epidermal TRPV3 blunted these antinociceptive effects of 17R-RvD1.

CONCLUSIONS AND IMPLICATIONS

17R-RvD1 is a novel natural inhibitory substance specific for TRPV3. The results of our behavioural studies suggest that 17R-RvD1 has acute analgesic potential via TRPV3-specific mechanisms.