Sensitization of TRPA1 by PAR2 contributes to the sensation of inflammatory pain

Role of TRPV1 in inflammation-induced airway hypersensitivity

Airway hypersensitivity is a common pathophysiological feature in various airway inflammatory diseases. Increasing evidence suggests that activation of the transient receptor potential vanilloid type 1 receptor (TRPV1) plays an important part in the manifestation of various symptoms of airway hypersensitivity. This mini-review focuses on recent studies that have revealed several potential contributing factors to the increase in TRPV1 sensitivity in pulmonary sensory neurons during airway inflammatory reaction. In addition, chronic allergic airway inflammation induces a pronounced overexpression of TRPV1 in neurofilament-positive pulmonary sensory neurons in nodose ganglia. A better understanding of the mechanisms underlying the increase in sensitivity and/or expression of TRPV1 during acute and chronic airway inflammation should generate the necessary information for developing effective therapeutic interventions to alleviate airway hypersensitivity.

Breathtaking TRP channels: TRPA1 and TRPV1 in airway chemosensation and reflex control

New studies have revealed an essential role for TRPA1, a sensory neuronal TRP ion channel, in airway chemosensation and inflammation. TRPA1 is activated by chlorine, reactive oxygen species, and noxious constituents of smoke and smog, initiating irritation and airway reflex responses. Together with TRPV1, the capsaicin receptor, TRPA1 may contribute to chemical hypersensitivity, chronic cough, and airway inflammation in asthma, COPD, and reactive airway dysfunction syndrome.

TRPA1 in bradykinin-induced mechanical hypersensitivity of vagal C fibers in guinea pig esophagus

Bradykinin (BK) activates sensory nerves and causes hyperalgesia. Transient receptor potential A1 (TRPA1) is expressed in sensory nerves and mediates cold, mechanical, and chemical nociception. TRPA1 can be activated by BK. TRPA1 knockout mice show impaired responses to BK and mechanical nociception. However, direct evidence from sensory nerve terminals is lacking. This study aims to determine the role of TRPA1 in BK-induced visceral mechanical hypersensitivity. Extracellular recordings of action potentials from vagal nodose and jugular neurons are performed in an ex vivo guinea pig esophageal-vagal preparation. Peak frequencies of action potentials of afferent nerves evoked by esophageal distension and chemical perfusion are recorded and compared. BK activates most nodose and all jugular C fibers. This activation is repeatable and associated with a significant increase in response to esophageal distension, which can be prevented by the B2 receptor antagonist WIN64338. TRPA1 agonist allyl isothiocyanate (AITC) activates most BK-positive nodose and jugular C fibers. This is associated with a transient loss of response to mechanical distensions and desensitization to a second AITC perfusion. Desensitization with AITC and pretreatment with TRPA1 inhibitor HC-030031 both inhibit BK-induced mechanical hypersensitivity but do not affect BK-evoked activation in nodose and jugular C fibers. In contrast, esophageal vagal afferent Adelta fibers do not respond to BK or AITC and fail to show mechanical hypersensitivity after BK perfusion. This provides the first evidence directly from visceral sensory afferent nerve terminals that TRPA1 mediates BK-induced mechanical hypersensitivity. This reveals a novel mechanism of visceral peripheral sensitization.

Transient receptor potential channels: targeting pain at the source

Pain results from the complex processing of neural signals at different levels of the central nervous system, with each signal potentially offering multiple opportunities for pharmacological intervention. A logical strategy for developing novel analgesics is to target the beginning of the pain pathway, and aim potential treatments directly at the nociceptors--the high-threshold primary sensory neurons that detect noxious stimuli. The largest group of receptors that function as noxious stimuli detectors in nociceptors is the transient receptor potential (TRP) channel family. This Review highlights evidence supporting particular TRP channels as targets for analgesics, indicates the likely efficacy profiles of TRP-channel-acting drugs, and discusses the development pathways needed to test candidates as analgesics in humans.

Transient receptor potential channels on sensory nerves

The somatosensory effects of natural products such as capsaicin, mustard oil, and menthol have been long recognized. Over the last decade, the identification of transient receptor potential (TRP) channels in primary sensory neurons as the targets for these agents has led to an explosion of research into the roles of "thermoTRPs" TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8 in nociception. In concert, through the efforts of many industrial and academic teams, a number of agonists and antagonists of these channels have been discovered, paving the way for a better understanding of sensory biology and, potentially, for novel treatments for diseases.

Cough sensors. II. Transient receptor potential membrane receptors on cough sensors

The transient receptor potential (TRP) family of channels is represented by at least six members in primary sensory neurons. These include the TRP vanilloid subtypes 1 (TRPV1), 2, 3, and 4, the cold and menthol receptor TRPM8, and TRPA1. Much interest has been directed to the study of the TRPV1, because capsaicin has been instrumental in discovering the unique role of a subset of primary sensory neurons in causing nociceptive responses, in activating reflex pathways including cough, and in producing neurogenic inflammation. TRPV1 is now regarded as an integrator of diverse sensory modalities because it undergoes marked plasticity and sensitization through a variety of mechanisms, including activation of G-protein-coupled or tyrosine kinase receptors. Evidence in experimental animals and in patients with airway diseases indicates a marked hypersensitivity to cough induced by TRPV1 agonists. Recent studies with newly developed high-affinity and selective TRPV1 antagonists have revealed that TRPV1 inhibition reduces cough induced by citric acid or antigen challenge.

Roles of transient receptor potential channels in pain

Pain perception begins with the activation of primary sensory nociceptors. Over the past decade, flourishing research has revealed that members of the Transient Receptor Potential (TRP) ion channel family are fundamental molecules that detect noxious stimuli and transduce a diverse range of physical and chemical energy into action potentials in somatosensory nociceptors. Here we highlight the roles of TRP vanilloid 1 (TRPV1), TRP melastatin 8 (TRPM8) and TRP ankyrin 1 (TRPA1) in the activation of nociceptors by heat and cold environmental stimuli, mechanical force, and by chemicals including exogenous plant and environmental compounds as well as endogenous inflammatory molecules. The contribution of these channels to pain and somatosensation is discussed at levels ranging from whole animal behavior to molecular modulation by intracellular signaling proteins. An emerging theme is that TRP channels are not simple ion channel transducers of one or two stimuli, but instead serve multidimensional roles in signaling sensory stimuli that are exceptionally diverse in modality and in their environmental milieu.

Transient receptor potential ankyrin 1 mediates toluene diisocyanate-evoked respiratory irritation

Toluene diisocyanate (TDI), a reactive, hazardous irritant, causes respiratory symptoms such as cough, rhinitis, dyspnea, and chest tightness in exposed workers. Although previous animal studies have shown that TDI causes respiratory reflexes that are abolished by desensitization of capsaicin-sensitive sensory nerves, the specific molecular identity of the transducer(s) responsible for sensing this noxious stimulus has, to date, remained elusive. Recent studies have demonstrated that transient receptor potential ankyrin 1 (TRPA1), an ion channel largely restricted to a subset of capsaicin-sensitive sensory nerves, functions as a transducer capable of initiating reflex responses to many reactive chemical stimuli. We therefore hypothesized that TRPA1 is the primary molecular transducer through which TDI causes sensory nerve activation and respiratory reflexes. Consistent with this hypothesis, TDI activated TRPA1, but not the capsaicin-sensitive transient receptor potential vanilloid 1 channel, in heterologous expression systems. TDI also activated a subset of dissociated trigeminal sensory neurons from wild-type but not TRPA1-deficient mice. In vivo, TDI mimicked known TRPA1 agonists by causing a pronounced decrease in breathing rate, indicative of respiratory sensory irritation, and this reflex was abolished in TRPA1-deficient mice. Together, our data suggest that TDI causes sensory nerve activation and airway sensory irritation via the activation of the ion channel, TRPA1.

Transient receptor potential ankyrin 1 antagonists block the noxious effects of toxic industrial isocyanates and tear gases

The release of methyl isocyanate in Bhopal, India, caused the worst industrial accident in history. Exposures to industrial isocyanates induce lacrimation, pain, airway irritation, and edema. Similar responses are elicited by chemicals used as tear gases. Despite frequent exposures, the biological targets of isocyanates and tear gases in vivo have not been identified, precluding the development of effective countermeasures. We use Ca(2+) imaging and electrophysiology to show that the noxious effects of isocyanates and those of all major tear gas agents are caused by activation of Ca(2+) influx and membrane currents in mustard oil-sensitive sensory neurons. These responses are mediated by transient receptor potential ankyrin 1 (TRPA1), an ion channel serving as a detector for reactive chemicals. In mice, genetic ablation or pharmacological inhibition of TRPA1 dramatically reduces isocyanate- and tear gas-induced nocifensive behavior after both ocular and cutaneous exposures. We conclude that isocyanates and tear gas agents target the same neuronal receptor, TRPA1. Treatment with TRPA1 antagonists may prevent and alleviate chemical irritation of the eyes, skin, and airways and reduce the adverse health effects of exposures to a wide range of toxic noxious chemicals.

Inhibition of M current in sensory neurons by exogenous proteases: a signaling pathway mediating inflammatory nociception

Inflammatory pain is thought to be mediated in part through the action of inflammatory mediators on membrane receptors of peripheral nerve terminals, however, the downstream signaling events which lead to pain are poorly understood. In this study we investigated the nociceptive pathways induced by activation of protease-activated receptor 2 (PAR-2) in damage-sensing (nociceptive) neurons from rat dorsal root ganglion (DRG). We found that activation of PAR-2 in these cells strongly inhibited M-type potassium currents (conducted by Kv7 potassium channels). Such inhibition caused depolarization of the neuronal resting membrane potential leading, ultimately, to nociception. Consistent with this mechanism, injection of the specific M channel blocker XE991 into rat paw induced nociception in a concentration-dependent manner. Injection of a PAR-2 agonist peptide also induced nociception but coinjection of XE991 and the PAR-2 agonist did not result in summation of nociception, suggesting that the action of both agents may share a similar mechanism. We also studied the signaling pathway of M current inhibition by PAR-2 using patch-clamp and fluorescence imaging of DRG neurons. These experiments revealed that the PAR-2 effect was mediated by phospholipase C (PLC). Further experiments demonstrated that M current inhibition required concurrent rises in cytosolic Ca(2+) concentration and depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)). We propose that PLC- and Ca(2+)/PIP(2)-mediated inhibition of M current in sensory neurons may represent one of the general mechanisms underlying pain produced by inflammatory mediators, and may therefore open up a new therapeutic window for treatment of this major clinical problem.

Peripheral mechanisms I: plasticity of peripheral pathways

Cough plays a vital role in protecting the lower airways from inhaled irritants, pollutants, and infectious agents. The cough reflex exhibits remarkable plasticity, such that in the context of infectious or inflammatory respiratory diseases such as asthma, chronic bronchitis, and idiopathic pulmonary fibrosis the cough reflex can become dysregulated, leading to a chronic cough. A chronic, nonproductive (dry) cough can rob sufferers of quality of life. Plasticity of the cough reflex likely involves multiple intersecting pathways within the airways, the peripheral nerves that supply them, and the central nervous system. While further studies are needed to determine the presence and relevance of many of these specific pathways in cough associated with chronic respiratory disease, the last decade has yielded unprecedented insight into the molecular identity of the ion channels and associated proteins that initiate and conduct action potentials in the primary sensory nerves involved in reflexes such as cough. We now know, for instance, that members of the transient receptor potential superfamily of nonselective cation channels function as transducers that convert specific external stimuli into neuronal activation. We also know that certain Na+ and K+ channels play specialized roles in regulating action potential discharge in irritant-sensing afferent nerves. In this chapter, we summarize the available information regarding factors that may modulate afferent neuron function acutely, via posttranslational modifications and over the longer term through neurotrophin-dependent alterations of the transcriptional programs of adult sensory neurons.

Caffeine activates mouse TRPA1 channels but suppresses human TRPA1 channels

Caffeine has various well-characterized pharmacological effects, but in mammals there are no known plasma membrane receptors or ion channels activated by caffeine. We observed that caffeine activates mouse transient receptor potential A1 (TRPA1) in heterologous expression systems by Ca(2+)(i) imaging and electrophysiological analyses. These responses to caffeine were confirmed in acutely dissociated dorsal root ganglion sensory neurons from WT mice, which are known to express TRPA1, but were not seen in neurons from TRPA1 KO mice. Expression of TRPA1 was detected immunohistochemically in nerve fibers and bundles in the mouse tongue. Moreover, WT mice, but not KO mice, showed a remarkable aversion to caffeine-containing water. These results demonstrate that mouse TRPA1 channels expressed in sensory neurons cause an aversion to drinking caffeine-containing water, suggesting they mediate the perception of caffeine. Finally, we observed that caffeine does not activate human TRPA1; instead, it suppresses its activity.

HC-030031, a TRPA1 selective antagonist, attenuates inflammatory- and neuropathy-induced mechanical hypersensitivity

Background

Safe and effective treatment for chronic inflammatory and neuropathic pain remains a key unmet medical need for many patients. The recent discovery and description of the transient receptor potential family of receptors including TRPV1 and TRPA1 has provided a number of potential new therapeutic targets for treating chronic pain. Recent reports have suggested that TRPA1 may play an important role in acute formalin and CFA induced pain. The current study was designed to further explore the therapeutic potential of pharmacological TRPA1 antagonism to treat inflammatory and neuropathic pain.

Results

The in vitro potencies of HC-030031 versus cinnamaldehyde or allyl isothiocyanate (AITC or Mustard oil)-induced TRPA1 activation were 4.9 ± 0.1 and 7.5 ± 0.2 μM respectively (IC₅₀). These findings were similar to the previously reported IC₅₀ of 6.2 μM against AITC activation of TRPA1 [1]. In the rat, oral administration of HC-030031 reduced AITC-induced nocifensive behaviors at a dose of 100 mg/kg. Moreover, oral HC-030031 (100 mg/kg) significantly reversed mechanical hypersensitivity in the more chronic models of Complete Freunds Adjuvant (CFA)-induced inflammatory pain and the spinal nerve ligation model of neuropathic pain.

Conclusion

Using oral administration of the selective TRPA1 antagonist HC-030031, our results demonstrated that TRPA1 plays an important role in the mechanisms responsible for mechanical hypersensitivity observed in inflammatory and neuropathic pain models. These findings suggested that TRPA1 antagonism may be a suitable new approach for the development of a potent and selective therapeutic agent to treat both inflammatory and neuropathic pain.

Inflammation reduces mechanical thresholds in a population of transient receptor potential channel A1-expressing nociceptors in the rat

Inflammatory hypersensitivity is characterized by behavioural reductions in withdrawal thresholds to noxious stimuli. Although cutaneous primary afferent neurones are known to have lowered thermal thresholds in inflammation, whether their mechanical thresholds are altered remains controversial. The transient receptor potential channel A1 (TRPA1) is a receptor localized to putative nociceptive neurones and is implicated in mechanical and thermal nociception. Herein, we examined changes in the properties of single primary afferents in normal and acutely inflamed rats and determined whether specific nociceptive properties, particularly mechanical thresholds, are altered in the subpopulation of afferents that responded to the TRPA1 agonist cinnamaldehyde (TRPA1-positive afferents). TRPA1-positive afferents in normal animals belonged to the mechanonociceptive populations, many of which also responded to heat or capsaicin but only a few of which responded to cold. In acute inflammation, a greater proportion of afferents responded to cinnamaldehyde and an increased proportion of dorsal root ganglion neurones expressed TRPA1 protein. Functionally, in inflammation, TRPA1-positive afferents showed significantly reduced mechanical thresholds and enhanced activity to agonist stimulation. Inflammation altered thermal thresholds in both TRPA1-positive and TRPA1-negative afferents. Our data show that a subset of afferents is sensitized to mechanical stimulation by inflammation and that these afferents are defined by expression of TRPA1.

The nociceptor ion channel TRPA1 is potentiated and inactivated by permeating calcium ions

The transient receptor potential A1 (TRPA1) channel is the molecular target for environmental irritants and pungent chemicals, such as cinnamaldehyde and mustard oil. Extracellular Ca(2+) is a key regulator of TRPA1 activity, both potentiating and subsequently inactivating it. In this report, we provide evidence that the effect of extracellular Ca(2+) on these processes is indirect and can be entirely attributed to entry through TRPA1 and subsequent elevation of intracellular calcium. Specifically, we found that in a pore mutant of TRPA1, D918A, in which Ca(2+) permeability was greatly reduced, extracellular Ca(2+) produced neither potentiation nor inactivation. Both processes were restored by reducing intracellular Ca(2+) buffering, which allowed intracellular Ca(2+) levels to become elevated upon entry through D918A channels. Application of Ca(2+) to the cytosolic face of excised patches was sufficient to produce both potentiation and inactivation of TRPA1 channels. Moreover, in whole cell recordings, elevation of intracellular Ca(2+) by UV uncaging of 1-(4,5-dimethoxy-2-nitrophenyl)-EDTA-potentiated TRPA1 currents. In addition, our data show that potentiation and inactivation are independent processes. TRPA1 currents could be inactivated by Mg(2+), Ba(2+), and Ca(2+) but potentiated only by Ba(2+) and Ca(2+). Saturating activation by cinnamaldehyde or mustard oil occluded potentiation but did not interfere with inactivation. Last, neither process was affected by mutation of a putative intracellular Ca(2+)-binding EF-hand motif. In conclusion, we have further clarified the mechanisms of potentiation and inactivation of TRPA1 using the D918A pore mutant, an important tool for investigating the contribution of Ca(2+) influx through TRPA1 to nociceptive signaling.

TRPA1 is a major oxidant sensor in murine airway sensory neurons

Sensory neurons in the airways are finely tuned to respond to reactive chemicals threatening airway function and integrity. Nasal trigeminal nerve endings are particularly sensitive to oxidants formed in polluted air and during oxidative stress as well as to chlorine, which is frequently released in industrial and domestic accidents. Oxidant activation of airway neurons induces respiratory depression, nasal obstruction, sneezing, cough, and pain. While normally protective, chemosensory airway reflexes can provoke severe complications in patients affected by inflammatory airway conditions like rhinitis and asthma. Here, we showed that both hypochlorite, the oxidizing mediator of chlorine, and hydrogen peroxide, a reactive oxygen species, activated Ca(2+) influx and membrane currents in an oxidant-sensitive subpopulation of chemosensory neurons. These responses were absent in neurons from mice lacking TRPA1, an ion channel of the transient receptor potential (TRP) gene family. TRPA1 channels were strongly activated by hypochlorite and hydrogen peroxide in primary sensory neurons and heterologous cells. In tests of respiratory function, Trpa1(-/-) mice displayed profound deficiencies in hypochlorite- and hydrogen peroxide-induced respiratory depression as well as decreased oxidant-induced pain behavior. Our results indicate that TRPA1 is an oxidant sensor in sensory neurons, initiating neuronal excitation and subsequent physiological responses in vitro and in vivo.

How irritating: the role of TRPA1 in sensing cigarette smoke and aerogenic oxidants in the airways

Airway irritants cause a variety of lung pathologies. Two separate studies, the first recently reported in the JCI by Bessac et al. and the second reported by Andrè et al. in the current issue of the JCI (see the related article beginning on page 2574), have identified irritants that activate transient receptor potential cation channel, subfamily A, member 1 (TRPA1) receptors in airway sensory neurons, resulting in neurogenic inflammation and respiratory hypersensitivity. The identification of TRPA1 activation by toxicants from cigarette smoke and polluted air, such as crotonaldehyde, acrolein, and oxidizing agents such as hydrogen peroxide, is an important finding. These two studies enhance our understanding of how pollution and cigarette smoke can damage airway function and will hopefully pave the way for the development of rational alternative therapeutics for such airway injury.

Phospholipase C mediated modulation of TRPV1 channels

The transient receptor potential vanilloid type 1 (TRPV1) channels are involved in both thermosensation and nociception. They are activated by heat, protons, and capsaicin and modulated by a plethora of other agents. This review will focus on the consequences of phospholipase C (PLC) activation, with special emphasis on the effects of phosphatidylinositol 4,5-bisphosphate (PIP2) on these channels. Two opposing effects of PIP2 have been reported on TRPV1. PIP2 has been proposed to inhibit TRPV1, and relief from this inhibition was suggested to be involved in sensitization of these channels by pro-inflammatory agents. In excised patches, however, PIP2 was shown to activate TRPV1. Calcium flowing through TRPV1 activates PLC and the resulting depletion of PIP2 was proposed to play a role in capsaicin-induced desensitization of these channels. We will describe the data indicating involvement of PLC and PIP2 in sensitization and desensitization of TRPV1 and will also discuss other pathways potentially contributing to these two phenomena. We attempt to resolve the seemingly contradictory data by proposing that PIP2 can both activate and inhibit TRPV1 depending on the experimental conditions, more specifically on the level of stimulation of these channels. Finally, we also discuss data in the literature indicating that other TRP channels, TRPA1 and some members of the TRPC subfamily, may also be under a similar dual control by PIP2.

Inhibition of transient receptor potential A1 channel by phosphatidylinositol-4,5-bisphosphate

Membrane phosphatidylinositol-4,5-bisphosphate (PIP2) is critical for the function of many transient receptor potential (TRP) ion channels. The role of PIP2 in TRPA1 function is not well known. The effect of PIP2 on TRPA1 was investigated by direct application of PIP2 and by using polylysine and PIP2 antibody that sequester PIP2. In inside-out patches from HeLa cells expressing mouse TRPA1, polytriphosphate (PPPi) was added to the bath solution to keep TRPA1 sensitive to allyl isothiocyanate (AITC; mustard oil). Direct application of PIP2 (10 microM) to inside-out patches did not activate TRPA1, but AITC and Delta(9)-tetrahydrocannabinol (THC) produced strong activation. In inside-out patches in which TRPA1 was first activated with AITC (in the presence of PPPi), further addition of PIP2 produced a concentration-dependent inhibition of TRPA1 [agonist concentration producing half-maximal activity (K(1/2)), 2.8 microM]. Consistent with the inhibition of TRPA1 by PIP2, AITC activated a large whole cell current when polylysine or PIP2 antibody was added to the pipette but a markedly diminished current when PIP2 was added to the pipette. In inside-out patches with PPPi in the bath solution, application of PIP2 antibody or polylysine caused activation of TRPA1, and this was blocked by PIP2. However, TRPA1 was not activated by polylysine and PIP2 antibody under whole cell conditions, suggesting a more complex regulation of TRPA1 by PIP2 in intact cells. These results show that PIP2 inhibits TRPA1 and reduces the sensitivity of TRPA1 to AITC.

Activation of transient receptor potential A1 channels by mustard oil, tetrahydrocannabinol and Ca2+ reveals different functional channel states

Our recent study has shown that activation of transient receptor potential A1 channel (TRPA1) by pungent chemicals such as allyl-isothiocyanate (AITC) requires an unidentified cytosolic factor whose action can be mimicked by inorganic polyphosphates. Thus, AITC and other pungent chemicals fail to activate TRPA1 in excised patches. It is unclear whether TRPA1 switches to a conformation that is insensitive to the pungent chemicals, or whether TRPA1 simply becomes completely non-functional and insensitive to all activators when the cytosolic factor is absent. To help distinguish between these possibilities, the effects of Delta(9)-tetrahydrocannabinol (THC) and Ca(2+) that are structurally different from pungent chemicals were tested on AITC-sensitive and AITC-insensitive states of TRPA1. In HeLa cells transiently expressing mouse TRPA1, activation of TRPA1 by THC was slow and weak from the extracellular side (cell-attached; K(1/2) >20 microM), but was faster and more potent from the intracellular side (inside-out; K(1/2), approximately 0.7 microM), and this did not require the presence of a polyphosphate. Similar results were observed in rat trigeminal neurons. Increasing the extracellular [Ca(2+)] from approximately 0-1-3 mM activated TRPA1 in cell-attached patches. Elevation of cytosolic [Ca(2+)] using thapsigargin (inhibitor of Ca(2+)-ATPAse) and histamine (that elevates IP(3)) also activated TRPA1 in cell-attached patches. Similar to pungent chemicals, Ca(2+) (1-5 microM) failed to activate TRPA1 in inside-out patches, unless polyphosphates were present. These results show that TRPA1 can exist in different functional states: a native state (cell-attached patch) and a non-native state (excised patch). THC can activate TRPA1 even in the absence of polyphosphates, whereas pungent chemicals and Ca(2+) require it for activation.

Modulation of meningeal nociceptors mechanosensitivity by peripheral proteinase-activated receptor-2: the role of mast cells

Inflammatory-related activation and sensitization of meningeal nociceptors is believed to play a key role in promoting the intracranial throbbing pain of migraine. We have shown recently that mast cell activation and various mast cell-derived inflammatory mediators can promote activation and sensitization of meningeal nociceptors. Mast cell tryptase has also been proposed to promote pain hypersensitivity by activating the proteinase-activated receptor 2 (PAR2) that is expressed on nociceptive neurons. In this study using in vivo single-unit recording in the trigeminal ganglion of anaesthetized rats, we found that local meningeal activation of PAR2 using the specific agonist SLIGRL-NH2 promoted sensitization of the threshold response while provoking desensitization of the suprathreshold responses. SLIGRL-NH2 also excited a subpopulation of meningeal nociceptors. Chronic mast cell depletion enhanced the sensitizing effects of PAR2 activation while curbing its desensitizing effects. Mast cell depletion did not change the PAR2-mediated excitatory effect. We propose that by enhancing the mechanical sensitivity of meningeal nociceptors local PAR2 activation could play a role in promoting the throbbing pain of migraine and that local mast cell degranulation may modulate such an effect.

The vanilloid receptor TRPV1 is activated and sensitized by local anesthetics in rodent sensory neurons

Local anesthetics (LAs) block the generation and propagation of action potentials by interacting with specific sites of voltage-gated Na(+) channels. LAs can also excite sensory neurons and be neurotoxic through mechanisms that are as yet undefined. Nonspecific cation channels of the transient receptor potential (TRP) channel family that are predominantly expressed by nociceptive sensory neurons render these neurons sensitive to a variety of insults. Here we demonstrated that the LA lidocaine activated TRP channel family receptors TRPV1 and, to a lesser extent, TRPA1 in rodent dorsal root ganglion sensory neurons as well as in HEK293t cells expressing TRPV1 or TRPA1. Lidocaine also induced a TRPV1-dependent release of calcitonin gene-related peptide (CGRP) from isolated skin and peripheral nerve. Lidocaine sensitivity of TRPV1 required segments of the putative vanilloid-binding domain within and adjacent to transmembrane domain 3, was diminished under phosphatidylinositol 4,5-bisphosphate depletion, and was abrogated by a point mutation at residue R701 in the proximal C-terminal TRP domain. These data identify TRPV1 and TRPA1 as putative key elements of LA-induced nociceptor excitation. This effect is sufficient to release CGRP, a key component of neurogenic inflammation, and warrants investigation into the role of TRPV1 and TRPA1 in LA-induced neurotoxicity.

Phospholipase C and protein kinase A mediate bradykinin sensitization of TRPA1: a molecular mechanism of inflammatory pain

Bradykinin is an inflammatory mediator that plays a pivotal role in pain and hyperalgesia in inflamed tissues by exciting and/or sensitizing nociceptors. TRPA1 is an important component of the transduction machinery through which environmental irritants and endogenous proalgesic agents depolarize nociceptors to elicit inflammatory pain. Here, using electrophysiological, immunocytochemical and behavioural analyses, we showed a functional interaction of these two inflammation-related molecules in both heterologous expressing systems and primary sensory neurons. We found that bradykinin increased the TRPA1 currents evoked by allyl isothiocyanate (AITC) or cinnamaldehyde in HEK293 cells expressing TRPA1 and bradykinin receptor 2 (B2R). This potentiation was inhibited by phospholipase C (PLC) inhibitor or protein kinase A (PKA) inhibitor, and mimicked by PLC or PKA activator. The functional interaction between B2R and TRPA1, as well as the modulation mechanism, was also observed in rat dorsal root ganglia neurons. In an occlusion experiment, the PLC activator could enhance AITC-induced TRPA1 current further even in saturated PKA-mediated potentiation, indicating the additive potentiating effects of the PLC and PKA pathways. These data for the first time indicate that a cAMP-PKA signalling is involved in the downstream from B2R in dorsal root ganglia neurons in addition to PLC. Finally, subcutaneous pre-injection of a sub-inflammatory dose of bradykinin into rat hind paw enhanced AITC-induced pain behaviours, which was consistent with the observations in vitro. Collectively, these results represent a novel mechanism through which bradykinin released in response to tissue inflammation might trigger the sensation of pain by TRPA1 activation.

ThermoTRP channels in nociceptors: taking a lead from capsaicin receptor TRPV1

Nociceptors with peripheral and central projections express temperature sensitive transient receptor potential (TRP) ion channels, also called thermoTRP's. Chemosensitivity of thermoTRP's to certain natural compounds eliciting pain or exhibiting thermal properties has proven to be a good tool in characterizing these receptors. Capsaicin, a pungent chemical in hot peppers, has assisted in the cloning of the first thermoTRP, TRPV1. This discovery initiated the search for other receptors encoding the response to a wide range of temperatures encountered by the body. Of these, TRPV1 and TRPV2 encode unique modalities of thermal pain when exposed to noxious heat. The ability of TRPA1 to encode noxious cold is presently being debated. The role of TRPV1 in peripheral inflammatory pain and central sensitization during chronic pain is well known. In addition to endogenous agonists, a wide variety of chemical agonists and antagonists have been discovered to activate and inhibit TRPV1. Efforts are underway to determine conditions under which agonist-mediated desensitization of TRPV1 or inhibition by antagonists can produce analgesia. Also, identification of specific second messenger molecules that regulate phosphorylation of TRPV1 has been the focus of intense research, to exploit a broader approach to pain treatment. The search for a role of TRPV2 in pain remains dormant due to the lack of suitable experimental models. However, progress into TRPA1's role in pain has received much attention recently. Another thermoTRP, TRPM8, encoding for the cool sensation and also expressed in nociceptors, has recently been shown to reduce pain via a central mechanism, thus opening a novel strategy for achieving analgesia. The role of other thermoTRP's (TRPV3 and TRPV4) encoding for detection of warm temperatures and expressed in nociceptors cannot be excluded. This review will discuss current knowledge on the role of nociceptor thermoTRPs in pain and therapy and describes the activator and inhibitor molecules known to interact with them and modulate their activity.

Expression and function of the ion channel TRPA1 in vagal afferent nerves innervating mouse lungs

Transient receptor potential (TRP) A1 and TRPM8 are ion channels that have been localized to afferent nociceptive nerves. These TRP channels may be of particular relevance to respiratory nociceptors in that they can be activated by various inhaled irritants and/or cold air. We addressed the hypothesis that mouse vagal sensory nerves projecting to the airways express TRPA1 and TRPM8 and that they can be activated via these receptors. Single cell RT-PCR analysis revealed that TRPA1 mRNA, but not TRPM8, is uniformly expressed in lung-labelled TRPV1-expressing vagal sensory neurons. Neither TRPA1 nor TRPM8 mRNA was expressed in TRPV1-negative neurons. Capsaicin-sensitive, but not capsaicin-insensitive, lung-specific neurons responded to cinnamaldehyde, a TRPA1 agonist, with increases in intracellular calcium. Menthol, a TRPM8 agonist, was ineffective at increasing cellular calcium in lung-specific vagal sensory neurons. Cinnamaldehyde also induced TRPA1-like inward currents (as measured by means of whole cell patch clamp recordings) in capsaicin-sensitive neurons. In an ex vivo vagal innervated mouse lung preparation, cinnamaldehyde evoked action potential discharge in mouse vagal C-fibres with a peak frequency similar to that observed with capsaicin. Cinnamaldehyde inhalation in vivo mimicked capsaicin in eliciting strong central-reflex changes in breathing pattern. Taken together, our results support the hypothesis that TRPA1, but not TRPM8, is expressed in vagal sensory nerves innervating the airways. TRPA1 activation provides a mechanism by which certain environmental stimuli may elicit action potential discharge in airway afferent C-fibres and the consequent nocifensor reflexes.

A role of TRPA1 in mechanical hyperalgesia is revealed by pharmacological inhibition

Mechanical hyperalgesia is a clinically-relevant form of pain sensitization that develops through largely unknown mechanisms. TRPA1, a Transient Receptor Potential ion channel, is a sensor of pungent chemicals that may play a role in acute noxious mechanosensation and cold thermosensation. We have developed a specific small molecule TRPA1 inhibitor (AP18) that can reduce cinnameldehyde-induced nociception in vivo. Interestingly, AP18 is capable of reversing CFA-induced mechanical hyperalgesia in mice. Although TRPA1-deficient mice develop normal CFA-induced hyperalgeisa, AP18 is ineffective in the knockout mice, consistent with an on-target mechanism. Therefore, TRPA1 plays a role in sensitization of nociception, and that compensation in TRPA1-deficient mice masks this requirement.

Nociceptors--noxious stimulus detectors

In order to deal effectively with danger, it is imperative to know about it. This is what nociceptors do--these primary sensory neurons are specialized to detect intense stimuli and represent, therefore, the first line of defense against any potentially threatening or damaging environmental inputs. By sensing noxious stimuli and contributing to the necessary reactions to avoid them--rapid withdrawal and the experience of an intensely unpleasant or painful sensation, nociceptors are essential for the maintenance of the body's integrity. Although nociceptive pain is clearly an adaptive alarm system, persistent pain is maladaptive, essentially an ongoing false alarm. Here, we highlight the genesis of nociceptors during development and the intrinsic properties of nociceptors that enable them to transduce, conduct, and transmit nociceptive information and also discuss how their phenotypic plasticity contributes to clinical pain.