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

Preferential activation of the vagal nodose nociceptive subtype by TRPA1 agonists in the guinea pig esophagus

BACKGROUND

The TRPA1 receptor is directly activated by a wide range of chemicals including many endogenous molecules relevant for esophageal pathophysiology. We addressed the hypothesis that the TRPA1 agonists differentially activate esophageal nociceptive subtypes depending on their embryological source (neural crest or epibranchial placodes).

METHODS

Single cell RT-PCR and whole cell patch clamp recordings were performed on the vagal neurons retrogradely labeled from the guinea pig esophagus. Extracellular recordings were made in the isolated innervated esophagus preparation ex vivo.

KEY RESULTS

Single cell RT-PCR revealed that the majority of the nodose (placodes-derived) and jugular (neural crest-derived) TRPV1-positive esophageal nociceptors express TRPA1. Single fiber recording showed that the TRPA1 agonists allyl-isothiocyanate (AITC) and cinnamaldehyde were effective in inducing robust action potential discharge in the nerve terminals of nodose nociceptors, but had far less effect in jugular nociceptors (approximately fivefold less). Higher efficacy of the TRPA1 agonists to activate nodose nociceptors was confirmed in the isolated esophagus-labeled vagal neurons in the whole cell patch clamp studies. Similarly to neural crest-derived vagal jugular nociceptors, the spinal DRG nociceptors that are also neural crest-derived were only modestly activated by allyl-isothiocyanate.

CONCLUSIONS & INFERENCES

We conclude that the TRPA1 agonists are substantially more effective activators of the placodes-derived than the neural crest-derived esophageal nociceptors. Our data predict that in esophageal diseases the presence of endogenous TRPA1 activators will be preferentially signaled by the vagal nodose nociceptors.

Emerging roles of resolvins in the resolution of inflammation and pain

Resolvins, including D and E series resolvins, are endogenous lipid mediators generated during the resolution phase of acute inflammation from the omega-3 polyunsaturated fatty acids docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Resolvins have potent anti-inflammatory and pro-resolution actions in several animal models of inflammation. Recent findings also demonstrate that resolvin E1 and resolvin D1 can each potently dampen inflammatory and postoperative pain. This review focuses on the mechanisms by which resolvins act on their receptors in immune cells and neurons to normalize exaggerated pain via regulation of inflammatory mediators, transient receptor potential (TRP) ion channels, and spinal cord synaptic transmission. Resolvins may offer novel therapeutic approaches for preventing and treating pain conditions associated with inflammation.

Curcumin ((E,E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) activates and desensitizes the nociceptor ion channel TRPA1

The ion channel TRPA1 is activated by a wide variety of noxious stimuli, such as pollutants, products of oxidative tissue damage, and pungent natural products. Many TRPA1 activators are reactive electrophiles that form Michael adducts with cysteine and lysine residues of TRPA1's intracellular N-terminus. Curcumin, the active principle of turmeric root (Curcuma longa), can also form Michael adducts. In order to test the hypothesis that the electrophilic curcumin activates TRPA1, we have performed whole-cell, voltage-clamp analysis on both HEK293 cells expressing human TRPA1 (hTRPA1-HEK) and native mouse vagal neurons. In nominally calcium-free extracellular and intracellular solutions which minimized the chances of calcium-dependent activation of TRPA1, curcumin increased TRPA1 currents in hTRPA1-HEK cells in a concentration-dependent manner (1-30μM) but did not cause block or activation of recombinant TRPM8 and TRPV1. In addition, 7 out of 11 vagal sensory neurons from wild type mice responded to curcumin (30μM) with inward currents (11.6±5.4pA/pF) that were largely reversed by TRPA1 blockers. In marked contrast, neurons from TRPA1-deficient mice did not respond to curcumin (30μM). With physiological levels of calcium added to the external solution to facilitate channel desensitization, curcumin-dependent currents in hTRPA1-HEK cells were completely desensitized and exhibited marked tachyphylaxis upon subsequent application of curcumin. Taken together, these results demonstrate that curcumin causes activation and subsequent desensitization of native and recombinant TRPA1 ion channels of multiple mammalian species.

Transient receptor potential ankyrin 1 is expressed by inhibitory motoneurons of the mouse intestine

BACKGROUND & AIMS

Transient receptor potential ankyrin (TRPA) 1, an excitatory ion channel expressed by sensory neurons, mediates somatic and visceral pain in response to direct activation or noxious mechanical stimulation. Although the intestine is routinely exposed to irritant alimentary compounds and inflammatory mediators that activate TRPA1, there is no direct evidence for functional TRPA1 receptors on enteric neurons, and the effects of TRPA1 activation on intestinal function have not been determined. We characterized expression of TRPA1 by enteric neurons and determined its involvement in the control of intestinal contractility and transit.

METHODS

TRPA1 expression was characterized by reverse-transcription polymerase chain reaction and immunofluorescence analyses. TRPA1 function was examined by Ca(2+) imaging and by assays of contractile activity and transit.

RESULTS

We detected TRPA1 messenger RNA in the mouse intestine and TRPA1 immunoreactivity in enteric neurons. The cecum and colon had immunoreactivity for neuronal TRPA1, but the duodenum did not. TRPA1 immunoreactivity was also detected in inhibitory motoneurons and descending interneurons, cholinergic neurons, and intrinsic primary afferent neurons. TRPA1 activators, including cinnamaldehyde, allyl isothiocyanate (AITC), and 4-hydroxynonenal, increased [Ca(2+)](i) in myenteric neurons. These were reduced by a TRPA1 antagonist (HC-030031) or deletion of Trpa1. TRPA1 activation inhibited contractility of the segments of colon but not stomach or small intestine of Trpa1(+/+) but not Trpa1(-/-) mice; this effect was reduced by tetrodotoxin or N(G)-nitro-l-arginine methyl ester. Administration of AITC by gavage did not alter gastric emptying or small intestinal transit, but luminal AITC inhibited colonic transit via TRPA1.

CONCLUSIONS

Functional TRPA1 is expressed by enteric neurons, and activation of neuronal TRPA1 inhibits spontaneous neurogenic contractions and transit of the colon.

Tooth injury increases expression of the cold sensitive TRP channel TRPA1 in trigeminal neurons

OBJECTIVE

Transient receptor potential (TRP) channels, a family of structurally related proteins have been implicated in the sensation of pain and hyperalgesia caused by exogenous and endogenous agonists, as well as touch, pH, and temperature. The objective of this study was to determine the effects of tooth injury on the expression of the cold sensitive channel TRPA1, in the trigeminal ganglion, the primary source of sensory and nociceptive innervation of teeth.

DESIGN

We analyzed TRPA1 expression in a rodent model of tooth injury, by Western blot analyses of proteins extracted from trigeminal ganglia.

RESULTS

We found that TRPA1 was selectively increased in trigeminal ganglia innervating injured teeth when compared to TRPA1 expression in trigeminal ganglia innervating healthy teeth.

CONCLUSIONS

Our results provide the first evidence of increased expression of a cold-sensitive TRP channel in trigeminal ganglia after pulp exposure, and are consistent with the possibility that increased expression and function of TRPA1 in trigeminal neurons contributes to hyperalgesia and allodynia following tooth injury.

Recent advances in the biology and medicinal chemistry of TRPA1

The transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is a nonselective cation channel that is highly expressed in small-diameter sensory neurons, where it functions as a polymodal receptor, responsible for detecting potentially harmful chemicals, mechanical forces and temperatures. TRPA1 is also activated and/or sensitized by multiple endogenous inflammatory mediators. As such, TRPA1 likely mediates the pain and neurogenic inflammation caused by exposure to reactive chemicals. In addition, it is also possible that this channel may mediate some of the symptoms of chronic inflammatory conditions such as asthma. We review recent advances in the biology of TRPA1 and summarize the evidence for TRPA1 as a therapeutic drug target. In addition, we provide an update on TRPA1 medicinal chemistry and the progress in the search for novel TRPA1 antagonists.

Proteinase-activated receptor 2 sensitizes transient receptor potential vanilloid 1, transient receptor potential vanilloid 4, and transient receptor potential ankyrin 1 in paclitaxel-induced neuropathic pain

Paclitaxel chemotherapy is limited by a long-lasting painful neuropathy that lacks an effective therapy. In this study, we tested the hypothesis that paclitaxel may release mast cell tryptase, which activates protease-activated receptor 2 (PAR2) and, subsequently, protein kinases A and C, resulting in mechanical and thermal (both heat and cold) hypersensitivity. Correlating with the development of neuropathy after repeated administration of paclitaxel, mast cell tryptase activity was found to be increased in the spinal cord, dorsal root ganglia, and peripheral tissues in mice. FSLLRY-amide, a selective PAR2 antagonist, blocked paclitaxel-induced neuropathic pain behaviors in a dose- and time-dependent manner. In addition, blocking downstream signaling pathways of PAR2, including phospholipase C (PLC), protein kinase A (PKA), and protein kinase Cε (PKC), effectively attenuated paclitaxel-induced mechanical, heat, or cold hypersensitivity. Furthermore, sensitized pain response was selectively inhibited by antagonists of transient receptor potential (TRP) V1, TRPV4, or TRPA1. These results revealed specific cellular signaling pathways leading to paclitaxel-induced neuropathy, including the activation of PAR2 and downstream enzymes PLC, PKCε, and PKA and resultant sensitization of TRPV1, TRPV4, and TRPA1. Targeting one or more of these signaling molecules may present new opportunities for the treatment of paclitaxel-induced neuropathy.

Chronic alteration in phosphatidylinositol 4,5-biphosphate levels regulates capsaicin and mustard oil responses

There is an agreement that acute (in minutes) hydrolysis and accumulation of phosphatidylinositol 4,5-bisphosphate (PIP(2) ) modulate TRPV1 and TRPA1 activities. Because inflammation results in PIP(2) depletion, persisting for long periods (hours to days) in pain models and in the clinic, we examined whether chronic depletion and accumulation of PIP(2) affect capsaicin (CAP) and mustard oil (MO) responses. In addition, we wanted to evaluate whether the effects of PIP(2) depend on TRPV1 and TRPA1 coexpression and whether the PIP(2) actions vary in expression cells vs. sensory neurons. Chronic PIP(2) production was stimulated by overexpression of phosphatidylinositol-4-phosphate-5-kinase, and PIP(2) -specific phospholipid 5'-phosphatase was selected to reduce plasma membrane levels of PIP(2) . Our results demonstrate that CAP (100 nM) responses and receptor tachyphylaxis are not significantly influenced by chronic changes in PIP(2) levels in wild-type (WT) or TRPA1 null-mutant sensory neurons as well as CHO cells expressing TRPV1 alone or with TRPA1. However, low concentrations of CAP (20 nM) produced a higher response after PIP(2) depletion in cells containing TRPV1 alone but not TRPV1 together with TRPA1. MO (25 μM) responses were also not affected by PIP(2) in WT sensory neurons and cells coexpressing TRPA1 and TRPV1. In contrast, PIP(2) reduction leads to pronounced tachyphylaxis to MO in cells with both channels. Chronic effect of PIP(2) on TRPA1 activity depends on presence of the TRPV1 channel and cell type (CHO vs. sensory neurons). In summary, chronic alterations in PIP(2) levels regulate magnitude of CAP and MO responses as well as MO tachyphylaxis. This regulation depends on coexpression profile of TRPA1 and TRPV1 and cell type.

Inhibition of TRPA1 channel activity in sensory neurons by the glial cell line-derived neurotrophic factor family member, artemin

Background

The transient receptor potential (TRP) channel subtype A1 (TRPA1) is known to be expressed on sensory neurons and respond to changes in temperature, pH and local application of certain noxious chemicals such as allyl isothiocyanate (AITC). Artemin is a neuronal survival and differentiation factor and belongs to the glial cell line-derived neurotrophic factor (GDNF) family. Both TRPA1 and artemin have been reported to be involved in pathological pain initiation and maintenance. In the present study, using whole-cell patch clamp recording technique, in situ hybridization and behavioral analyses, we examined the functional interaction between TRPA1 and artemin.

Results

We found that 85.8 ± 1.9% of TRPA1-expressing neurons also expressed GDNF family receptor alpha 3 (GFR α3), and 87.5 ± 4.1% of GFRα3-expressing neurons were TRPA1-positive. In whole-cell patch clamp analysis, a short-term treatment of 100 ng/ml artemin significantly suppressed the AITC-induced TRPA1 currents. A concentration-response curve of AITC resulting from the effect of artemin showed that this inhibition did not change EC₅₀ but did lower the AITC-induced maximum response. In addition, pre-treatment of artemin significantly suppressed the number of paw lifts induced by intraplantar injection of AITC, as well as the formalin-induced pain behaviors.

Conclusions

These findings that a short-term application of artemin inhibits the TRPA1 channel's activity and the sequential pain behaviors suggest a role of artemin in regulation of sensory neurons.

The protease-activated receptor-2-specific agonists 2-aminothiazol-4-yl-LIGRL-NH2 and 6-aminonicotinyl-LIGRL-NH2 stimulate multiple signaling pathways to induce physiological responses in vitro and in vivo

Protease-activated receptor-2 (PAR(2)) is one of four protease-activated G-protein-coupled receptors. PAR(2) is expressed on multiple cell types where it contributes to cellular responses to endogenous and exogenous proteases. Proteolytic cleavage of PAR(2) reveals a tethered ligand that activates PAR(2) and two major downstream signaling pathways: mitogen-activated protein kinase (MAPK) and intracellular Ca(2+) signaling. Peptides or peptidomimetics can mimic binding of the tethered ligand to stimulate signaling without the nonspecific effects of proteases. The most commonly used peptide activators of PAR(2) (e.g. SLIGRL-NH(2) and SLIGKV-NH(2)) lack potency at the receptor. However, although the potency of 2-furoyl-LIGRLO-NH(2) (2-f-LIGRLO-NH(2)) underscores the use of peptidomimetic PAR(2) ligands as a mechanism to enhance pharmacological action at PAR(2), 2-f-LIGRLO-NH(2) has not been thoroughly evaluated. We evaluated the known agonist 2-f-LIGRLO-NH(2) and two recently described pentapeptidomimetic PAR(2)-specific agonists, 2-aminothiazol-4-yl-LIGRL-NH(2) (2-at-LIGRL-NH(2)) and 6-aminonicotinyl-LIGRL-NH(2) (6-an-LIGRL-NH(2)). All peptidomimetic agonists stimulated PAR(2)-dependent in vitro physiological responses, MAPK signaling, and Ca(2+) signaling with an overall rank order of potency of 2-f-LIGRLO-NH(2) ≈ 2-at-LIGRL-NH(2) > 6-an-LIGRL-NH(2) ≫ SLIGRL-NH(2). Because PAR(2) plays a major role in pathological pain conditions and to test potency of the peptidomimetic agonists in vivo, we evaluated these agonists in models relevant to nociception. All three agonists activated Ca(2+) signaling in nociceptors in vitro, and both 2-at-LIGRL-NH(2) and 2-f-LIGRLO-NH(2) stimulated PAR(2)-dependent thermal hyperalgesia in vivo. We have characterized three high potency ligands that can be used to explore the physiological role of PAR(2) in a variety of systems and pathologies.

Relevance of mast cell-nerve interactions in intestinal nociception

Cross-talk between the immune- and nervous-system is considered an important biological process in health and disease. Because mast cells are often strategically placed between nerves and surrounding (immune)-cells they may function as important intermediate cells. This review summarizes the current knowledge on bidirectional interaction between mast cells and nerves and its possible relevance in (inflammation-induced) increased nociception. Our main focus is on mast cell mediators involved in sensitization of TRP channels, thereby contributing to nociception, as well as neuron-released neuropeptides and their effects on mast cell activation. Furthermore we discuss mechanisms involved in physical mast cell-nerve interactions. This article is part of a Special Issue entitled: Mast cells in inflammation.

TRP channels: emerging targets for respiratory disease

The mammalian transient receptor potential (TRP) superfamily of cation channels is divided into six subfamilies based on sequence homology TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPP (polycystin) and TRPML (mucolipin). The expression of these channels is especially abundant in sensory nerves, and there is increasing evidence demonstrating their existence in a broad range of cell types which are thought to play a key role in respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD). These ion channels can be activated by a diverse range of chemical and physical stimuli. Physical stimuli include temperature, membrane potential changes and osmotic stress, and some of the more well known chemical stimuli include capsaicin (TRPV1), menthol (TRPM8) and acrolein (TRPA1). There is increasing evidence in this rapidly moving field to suggest that selective blockers of these channels may represent attractive novel strategies to treat characteristic features of respiratory diseases such as asthma and COPD. This review focuses on summarising the evidence that modulation of selected TRP channels may have beneficial effects at targeting key features of these respiratory diseases including airways inflammation, airways hyper-reactivity, mucus secretion and cough.

Proteinase-activated receptor 2 mediates thermal hyperalgesia and is upregulated in a rat model of chronic pancreatitis

OBJECTIVES

The mechanism of pain in chronic pancreatitis (CP) has yet to be explored. Proteinase-activated receptor 2 (PAR2) plays a pronociceptive role in visceral pain. The study aimed to assess the expression of PAR2 in dorsal root ganglia (DRGs) and validate its role of thermal hyperalgesia in CP.

METHODS

Chronic pancreatitis model was induced by trinitrobenzene sulfonic acid infusion into rat pancreatic ducts. Abdominal hyperalgesia was measured by thermal withdrawal latencies. The expression of PAR2 and transient receptor potential vanilloid 1 (TRPV1) were analyzed by immunofluorescence and Western blot. The messenger RNA encoding PAR2 was quantitated by real-time polymerase chain reaction. The effects of short-term and long-term ulinastatin treatment on abdominal thermal hyperalgesia of rats with CP were measured.

RESULTS

Rats with CP showed a decreased thermal withdrawal latency. Proteinase-activated receptor 2 and TRPV1 were significantly upregulated in DRGs. The increased PAR2 protein expression was tightly correlated with thermal withdrawal latencies and TRPV1 expression. Short-term ulinastatin treatment inhibited the development of thermal hyperalgesia of rats with CP in a dose-dependent manner.

CONCLUSIONS

The thermal hyperalgesia in CP is associated with an up-regulation of the PAR2 in DRGs. Proteinase-activated receptor 2 was involved in the pain generation in rats with CP.

TRP channels in neurogastroenterology: opportunities for therapeutic intervention

The members of the superfamily of transient receptor potential (TRP) cation channels are involved in a plethora of cellular functions. During the last decade, a vast amount of evidence is accumulating that attributes an important role to these cation channels in different regulatory aspects of the alimentary tract. In this review we discuss the expression patterns and roles of TRP channels in the regulation of gastrointestinal motility, enteric nervous system signalling and visceral sensation, and provide our perspectives on pharmacological targeting of TRPs as a strategy to treat various gastrointestinal disorders. We found that the current knowledge about the role of some members of the TRP superfamily in neurogastroenterology is rather limited, whereas the function of other TRP channels, especially of those implicated in smooth muscle cell contractility (TRPC4, TRPC6), visceral sensitivity and hypersensitivity (TRPV1, TRPV4, TRPA1), tends to be well established. Compared with expression data, mechanistic information about TRP channels in intestinal pacemaking (TRPC4, TRPC6, TRPM7), enteric nervous system signalling (TRPCs) and enteroendocrine cells (TRPM5) is lacking. It is clear that several different TRP channels play important roles in the cellular apparatus that controls gastrointestinal function. They are involved in the regulation of gastrointestinal motility and absorption, visceral sensation and visceral hypersensitivity. TRP channels can be considered as interesting targets to tackle digestive diseases, motility disorders and visceral pain. At present, TRPV1 antagonists are under development for the treatment of heartburn and visceral hypersensitivity, but interference with other TRP channels is also tempting. However, their role in gastrointestinal pathophysiology first needs to be further elucidated.

Role of TRP channels in pain sensation

It is crucial for a living organism to recognize and discern potentially harmful noxious stimuli from innocuous stimuli to avoid hazards in the environment. However, unnecessary or exaggerated nociception is at best unpleasant and often compromises the quality of life. In order to lessen the intensity of nociception or eliminate the pathological pain, it is important to understand the nature of nociception and the mechanisms of hyperalgesia or allodynia. Transient receptor potential (TRP) channels play central roles in nociception under physiological and pathological conditions including inflammation and neuropathy. In this chapter, we will highlight the enormous progress in understanding the role of TRP channels in nociception. We will mainly focus on two TRP channels (TRPV1 and TRPA1) that have been particularly implicated in transducing signals associated with pain sensation, and briefly discuss the role of TRPM8, TRPV3 and TRPV4. We will stress debatable issues that needed to be resolved and provide perspectives for the future studies.

Transient receptor potential A1 channel contributes to activation of the muscle reflex

This study was undertaken to elucidate the role played by transient receptor potential A1 channels (TRPA1) in activating the muscle reflex, a sympathoexcitatory drive originating in contracting muscle. First, we tested the hypothesis that stimulation of the TRPA1 located on muscle afferents reflexly increases sympathetic nerve activity. In decerebrate rats, allyl isothiocyanate, a TRPA1 agonist, was injected intra-arterially into the hindlimb muscle circulation. This led to a 33% increase in renal sympathetic nerve activity (RSNA). The effect of allyl isothiocyanate was a reflex because the response was prevented by sectioning the sciatic nerve. Second, we tested the hypothesis that blockade of TRPA1 reduces RSNA response to contraction. Thirty-second continuous static contraction of the hindlimb muscles, induced by electrical stimulation of the peripheral cut ends of L(4) and L(5) ventral roots, increased RSNA and blood pressure. The integrated RSNA during contraction was reduced by HC-030031, a TRPA1 antagonist, injected intra-arterially (163 ± 24 vs. 95 ± 21 arbitrary units, before vs. after HC-030031, P < 0.05). Third, we attempted to identify potential endogenous stimulants of TRPA1, responsible for activating the muscle reflex. Increases in RSNA in response to injection into the muscle circulation of arachidonic acid, bradykinin, and diprotonated phosphate, which are metabolic by-products of contraction and stimulants of muscle afferents during contraction, were reduced by HC-030031. These observations suggest that the TRPA1 located on muscle afferents is part of the muscle reflex and further support the notion that arachidonic acid metabolites, bradykinin, and diprotonated phosphate are candidates for endogenous agonists of TRPA1.

The multiple pathways for itch and their interactions with pain

Multiple neural pathways and molecular mechanisms responsible for producing the sensation of itch have recently been identified, including histamine-independent pathways. Physiological, molecular, behavioral and brain imaging studies are converging on a description of these pathways and their close association with pain processing. Some conflicting results have arisen and the precise relationship between itch and pain remains controversial. A better understanding of the generation of itch and of the intrinsic mechanisms that inhibit itch after scratching should facilitate the search for new methods to alleviate clinical pruritus (itch). In this review we describe the current understanding of the production and inhibition of itch. A model of itch processing within the CNS is proposed.

Nociceptors: the sensors of the pain pathway

Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin by detecting extremes in temperature and pressure and injury-related chemicals, and transducing these stimuli into long-ranging electrical signals that are relayed to higher brain centers. The activation of functionally distinct cutaneous nociceptor populations and the processing of information they convey provide a rich diversity of pain qualities. Current work in this field is providing researchers with a more thorough understanding of nociceptor cell biology at molecular and systems levels and insight that will allow the targeted design of novel pain therapeutics.

Sensory detection and responses to toxic gases: mechanisms, health effects, and countermeasures

The inhalation of reactive gases and vapors can lead to severe damage of the airways and lung, compromising the function of the respiratory system. Exposures to oxidizing, electrophilic, acidic, or basic gases frequently occur in occupational and ambient environments. Corrosive gases and vapors such as chlorine, phosgene, and chloropicrin were used as warfare agents and in terrorist acts. Chemical airway exposures are detected by the olfactory, gustatory, and nociceptive sensory systems that initiate protective physiological and behavioral responses. This review focuses on the role of airway nociceptive sensory neurons in chemical sensing and discusses the recent discovery of neuronal receptors for reactive chemicals. Using physiological, imaging, and genetic approaches, Transient Receptor Potential (TRP) ion channels in sensory neurons were shown to respond to a wide range of noxious chemical stimuli, initiating pain, respiratory depression, cough, glandular secretions, and other protective responses. TRPA1, a TRP ion channel expressed in chemosensory C-fibers, is activated by almost all oxidizing and electrophilic chemicals, including chlorine, acrolein, tear gas agents, and methyl isocyanate, the highly noxious chemical released in the Bhopal disaster. Chemicals likely activate TRPA1 through covalent protein modification. Animal studies using TRPA1 antagonists or TRPA1-deficient mice confirmed the role of TRPA1 in chemically induced respiratory reflexes, pain, and inflammation in vivo. New research shows that sensory neurons are not merely passive sensors of chemical exposures. Sensory channels such as TRPA1 are essential for maintenance of airway inflammation in asthma and may contribute to the progression of airway injury following high-level chemical exposures.

The proteinase/proteinase-activated receptor-2/transient receptor potential vanilloid-1 cascade impacts pancreatic pain in mice

AIMS

Proteinase-activated receptor-2 (PAR2) and transient receptor potential vanilloid-1 (TRPV1) are co-localized in the primary afferents, and the trans-activation of TRPV1 by PAR2 activation is involved in processing of somatic pain. Given evidence for contribution of PAR2 to pancreatic pain, the present study aimed at clarifying the involvement of TRPV1 in processing of pancreatic pain by the proteinase/PAR2 pathway in mice.

MAIN METHODS

Acute pancreatitis was created by repeated administration of cerulein in conscious mice, and the referred allodynia/hyperalgesia was assessed using von Frey filaments. Injection of PAR2 agonists into the pancreatic duct was achieved in anesthetized mice, and expression of Fos in the spinal cord was determined by immunohistochemistry.

KEY FINDINGS

The established referred allodynia/hyperalgesia following cerulein treatment was abolished by post-treatment with nafamostat mesilate, a proteinase inhibitor, and with capsazepine, a TRPV1 antagonist, in mice. Injection of trypsin, an endogenous PAR2 agonist, or SLIGRL-NH(2), a PAR2-activating peptide, into the pancreatic duct caused expression of Fos protein in the spinal superficial layers at T8-T10 levels in the mice. The spinal Fos expression caused by trypsin and by SLIGRL-NH(2) was partially blocked by capsazepine, the former effect abolished by nafamostat mesilate.

SIGNIFICANCE

Our data thus suggest that the proteinase/PAR2/TRPV1 cascade might impact pancreatic pain, in addition to somatic pain, and play a role in the maintenance of pancreatitis-related pain in mice.

Development of an in vitro coculture of primary sensitive pig neurons and keratinocytes for the study of cutaneous neurogenic inflammation

Cutaneous neurogenic inflammation (CNI) is often associated with skin disorders. Activated sensory neurons secrete neuropeptides, such as substance P (SP), which initiate or aggravate inflammation in the skin. The discovery of new molecules acting on these neurons is hampered by the difficulty of reproducing the interactions between nerve endings and skin in vitro. We developed an in vitro model based on the coculture of porcine primary keratinocytes and sensory neurons, which mimics skin innervation. To test the relevance of this model, we compared the effects of different substances on CNI by measuring SP secretion in vitro using a sensitive enzyme immunoassay. Collectively, our results indicate that the use of porcine cells could be very useful to perform an in vitro model of CNI. By adding capsaicin, which induces the secretion of SP by neurons, to the culture, we show that our model mimics CNI in vitro, allowing us to screen for molecules that inhibit this inflammatory response. Such a model can be used to test the effects of different substances on CNI and may be useful for dermatological or cosmetic applications. Based on our screen, we found that extracts of Laminaria digitata and Vernonia sublutea inhibit CNI.

Transient receptor potential ion channels V4 and A1 contribute to pancreatitis pain in mice

The mechanisms of pancreatic pain, a cardinal symptom of pancreatitis, are unknown. Proinflammatory agents that activate transient receptor potential (TRP) channels in nociceptive neurons can cause neurogenic inflammation and pain. We report a major role for TRPV4, which detects osmotic pressure and arachidonic acid metabolites, and TRPA1, which responds to 4-hydroxynonenal and cyclopentenone prostaglandins, in pancreatic inflammation and pain in mice. Immunoreactive TRPV4 and TRPA1 were detected in pancreatic nerve fibers and in dorsal root ganglia neurons innervating the pancreas, which were identified by retrograde tracing. Agonists of TRPV4 and TRPA1 increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in these neurons in culture, and neurons also responded to the TRPV1 agonist capsaicin and are thus nociceptors. Intraductal injection of TRPV4 and TRPA1 agonists increased c-Fos expression in spinal neurons, indicative of nociceptor activation, and intraductal TRPA1 agonists also caused pancreatic inflammation. The effects of TRPV4 and TRPA1 agonists on [Ca(2+)](i), pain and inflammation were markedly diminished or abolished in trpv4 and trpa1 knockout mice. The secretagogue cerulein induced pancreatitis, c-Fos expression in spinal neurons, and pain behavior in wild-type mice. Deletion of trpv4 or trpa1 suppressed c-Fos expression and pain behavior, and deletion of trpa1 attenuated pancreatitis. Thus TRPV4 and TRPA1 contribute to pancreatic pain, and TRPA1 also mediates pancreatic inflammation. Our results provide new information about the contributions of TRPV4 and TRPA1 to inflammatory pain and suggest that channel antagonists are an effective therapy for pancreatitis, when multiple proinflammatory agents are generated that can activate and sensitize these channels.

Enhanced scratching evoked by PAR-2 agonist and 5-HT but not histamine in a mouse model of chronic dry skin itch

Chronic itch is a symptom of many skin conditions and systemic disease, and it has been hypothesized that the chronic itch may result from sensitization of itch-signaling pathways. We induced experimental chronic dry skin on the rostral back of mice, and observed a significant increase in spontaneous hindlimb scratches directed to the dry skin. Spontaneous scratching was significantly attenuated by a PAR-2 antibody and 5-HT2A receptor antagonist, indicating activation of these receptors by endogenous mediators released under dry skin conditions. We also observed a significant increase in the number of scratch bouts evoked by acute intradermal injections of a protease-activated receptor (PAR)-2 agonist and serotonin (5-HT), but not histamine. We additionally investigated if pruritogen-evoked activity of dorsal root ganglion (DRG) neurons is enhanced in this model. DRG cells from dry skin mice exhibited significantly larger responses to the PAR-2 agonist and 5-HT, but not histamine. Spontaneous scratching may reflect ongoing itch, and enhanced pruritogen-evoked scratching may represent hyperknesis (enhanced itch), both potentially due to sensitization of itch-signaling neurons. The correspondence between enhanced behavioral scratching and DRG cell responses suggest that peripheral pruriceptors that respond to proteases and 5-HT, but not histamine, may be sensitized in dry skin itch.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

[Activation and regulation of nociceptive transient receptor potential (TRP) channels, TRPV1 and TRPA1]

TRP channels are well recognized for their contributions to sensory transduction, responding to a wide variety of stimuli including temperature, nociceptive stimuli, touch, osmolarity and pheromones. In particular, the involvement of TRP channels in nociception has been extensively studied following the cloning of the capsaicin receptor, TRPV1. Painful diabetic peripheral neuropathy is described as a superficial burning pain, and it is one of the most commonly encountered neuropathic pain syndromes in clinical practice. We found that hypoxic and high glucose conditions commonly observed in diabetes potentiate TRPV1 activity without affecting TRPV1 expression both in native rat sensory neurons and HEK293 cells expressing rat TRPV1. The potentiation seems to be caused by phosphorylation of the serine residues of TRPV1 by PKC. These data indicate that PKC-dependent potentiation of TRPV1 activities under hypoxia and hyperglycemia might be involved in early diabetic neuropathy. Mechanisms for the detection of alkaline pH by sensory neurons are not well understood, although it is well accepted that acidic pH monitoring can be attributed to several ion channels, including TRPV1 and ASICs. We found that alkaline pH activates TRPA1 and that the TRPA1 activation is involved in nociception, using Ca(2+)-imaging and patch-clamp methods. In addition, intracellular alkalization activated TRPA1 at the whole-cell level, and single-channel openings were observed in the inside-out configuration. Furthermore, intraplantar injection of ammonium chloride into the mouse hind paw caused pain-related behaviors, which were not observed in TRPA1-deficient mice. These results suggest that alkaline pH causes pain sensation through activation of TRPA1.

Acute PAR2 activation reduces alpha, beta-MeATP sensitive currents in rat dorsal root ganglion neurons

It has been reported that proteinase-activated receptor 2 (PAR2) receptor activation enhances the animal's pain response and PAR2 coexpresses with P2X3 in dorsal root ganglion neurons. However, whether PAR2 activation has a direct impact on P2X3 currents is still not clear. In this study, we performed the patch-clamp experiments in cultured dorsal root ganglion neurons and found that when incubated with trypsin or the PAR2 agonist SL-NH2 for a short time (3 min), instead of increasing, P2X3 currents amplitude decreased significantly. Meanwhile, the opening of P2X3 ion channel accelerated. Protein kinase A inhibitor H89 could not reverse above phenomenon, but played a synergistic effect on the contrary. These results suggest that the enhanced pain response caused by PAR2 activation is not through direct increase of the P2X3 current amplitude, and the acceleration of P2X3 opening may participate in the enhanced pain response in a long-time view. Moreover, protein kinase A does not participate in the inhibition of P2X3 currents caused by PAR2 activation.

Expression of transient receptor potential ankyrin 1 (TRPA1) in the rat trigeminal sensory afferents and spinal dorsal horn

Transient receptor potential ankyrin 1 (TRPA1), responding to noxious cold and pungent compounds, is implicated in the mediation of nociception, but little is known about the processing of the TRPA1-mediated nociceptive information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized the TRPA1-positive (+) neurons in the trigeminal ganglion (TG) and investigated the distribution of TRPA1(+) afferent fibers and their synaptic connectivity within the rat TSN and DH by using light and electron microscopic immunohistochemistry. In the TG, TRPA1 was expressed in unmyelinated and small myelinated axons and also occasionally in large myelinated axons. Many TRPA1(+) neurons costained for the marker for peptidergic neurons substance P (26.8%) or the marker for nonpeptidergic neurons IB4 (44.5%). In the CNS, small numbers of axons and terminals were immunopositive for TRPA1 throughout the rostral TSN, in contrast to the dense network of positive fibers and terminals in the superficial laminae of the trigeminal caudal nucleus (Vc) and DH. The TRPA1(+) terminals contained clear round vesicles, were presynaptic to one or two dendrites, and rarely participated in axoaxonic contacts, suggesting involvement in relatively simple synaptic circuitry with a small degree of synaptic divergence and little presynaptic modulation. Immunoreactivity for TRPA1 was also occasionally observed in postsynaptic dendrites. These results suggest that TRPA1-dependent orofacial and spinal nociceptive input is processed mainly in the superficial laminae of the Vc and DH in a specific manner and may be processed differently between the rostral TSN and Vc.

TRPA1 modulation of spontaneous and mechanically evoked firing of spinal neurons in uninjured, osteoarthritic, and inflamed rats

Background

There is growing evidence supporting a role for TRPA1 receptors in the neurotransmission of peripheral mechanical stimulation. In order to enhance understanding of TRPA1 contributions to mechanotransmission, we examined the effects a selective TRPA1 receptor antagonist, A-967079, on spinal neuronal activity following peripheral mechanical stimulation in uninjured, CFA-inflamed, and osteoarthritc (OA) rats.

Results

Systemic injection of A-967079 (30 μmol/kg, i.v.) decreased the responses of wide dynamic range (WDR), and nociceptive specific (NS) neurons following noxious pinch stimulation of the ipsilateral hind paw in uninjured and CFA-inflamed rats. Similarly, A-967079 reduced the responses of WDR neurons to high-intensity mechanical stimulation (300 g von Frey hair) of the knee joint in both OA and OA-sham rats. WDR neuronal responses to low-intensity mechanical stimulation (10 g von Frey hair) were also reduced by A-967079 administration to CFA-inflamed rats, but no effect was observed in uninjured rats. Additionally, the spontaneous activity of WDR neurons was decreased after A-967079 injection in CFA-inflamed rats but was unaltered in uninjured, OA, and OA-sham animals.

Conclusions

Blockade of TRPA1 receptors disrupts transmission of high-intensity mechanical stimulation to the spinal cord in both uninjured and injured rats indicating that TRPA1 receptors have an important role in noxious mechanosensation in both normal and pathological conditions. TRPA1 receptors also contribute to the transmission of low-intensity mechanical stimulation, and to the modulation of spontaneous WDR firing, but only after an inflammatory injury.

Proteinase-Activated Receptor-4 (PAR4) Activation Leads to Sensitization of Rat Joint Primary Afferents Via a Bradykinin B2 Receptor-Dependent Mechanism

The G-protein-linked receptor, proteinase-activated receptor-4 (PAR4) is activated by proteinases released into the joint during inflammation. It is unclear whether PAR4 has a pro- or anti-nociceptive effect and whether it directly affects nerve activity. In this study, we examined the expression of PAR4 in joints and dorsal root ganglion (DRG) neurons and whether activation of PAR4 has an effect on nociception in normal rat knee joints. Electrophysiological recordings were made from joint primary afferents in male Wistar rats during both nonnoxious and noxious rotations of the knee. Afferent firing rate was recorded for 15 min post close intra-arterial injection of 10−9–10−5 mol of the PAR4 activating peptide, AYPGKF-NH2, or the inactive peptide, YAPGKF-NH2 (100 μl bolus). Rats were either naive or pretreated with the selective PAR4 antagonist, pepducin P4pal-10, the transient receptor potential vanilloid-1 (TRPV1) antagonist, SB366791, or the bradykinin B2 receptor antagonist, HOE140. Immunofluorescence experiments showed extensive PAR4 expression in the knee joint and in sensory neurons projecting from the joint. AYPGKF-NH2 significantly increased joint afferent firing during nonnoxious and noxious rotation of the knee. The inactive control peptide, YAPGKF-NH2 was without effect. Systemic pretreatment with the PAR4 antagonist, pepducin P4pal-10, inhibited the AYPGKF-NH2-induced increase in firing rate. Pretreatment with HOE140, but not SB366791, also blocked this increase in firing rate. These data reveal that in normal rat knee joints, PAR4 activation increases joint primary afferent activity in response to mechanical stimuli. This PAR4-induced sensitization is TRPV1-independent but involves B2 receptor activation, suggesting a role for kinins in this process.

Transient receptor potential ankyrin-1 has a major role in mediating visceral pain in mice

The excitatory ion channel transient receptor potential ankyrin-1 (TRPA1) is prominently expressed by primary afferent neurons and is a mediator of inflammatory pain. Inflammatory agents can directly activate [e.g., hydroxynonenal (HNE), prostaglandin metabolites] or indirectly sensitize [e.g., agonists of protease-activated receptor (PAR(2))] TRPA1 to induce somatic pain and hyperalgesia. However, the contribution of TRPA1 to visceral pain is unknown. We investigated the role of TRPA1 in visceral hyperalgesia by measuring abdominal visceromotor responses (VMR) to colorectal distention (CRD) after intracolonic administration of TRPA1 agonists [mustard oil (MO), HNE], sensitizing agents [PAR(2) activating peptide (PAR(2)-AP)], and the inflammatory agent trinitrobenzene sulfonic acid (TNBS) in trpa1(+/+) and trpa1(-/-) mice. Sensory neurons innervating the colon, identified by retrograde tracing, coexpressed immunoreactive TRPA1, calcitonin gene-related peptide, and substance P, expressed TRPA1 mRNA and responded to MO with depolarizing currents. Intracolonic MO and HNE increased VMR to CRD and induced immunoreactive c-fos in spinal neurons in trpa1+/+ but not in trpa1(-/-) mice. Intracolonic PAR(2)-AP induced mechanical hyperalgesia in trpa1+/+ but not in trpa1(-/-) mice. TNBS-induced colitis increased in VMR to CRD and induced c-fos in spinal neurons in trpa1(+/+) but not in trpa1(-/-) mice. Thus TRPA1 is expressed by colonic primary afferent neurons. Direct activation of TRPA1 causes visceral hyperalgesia, and TRPA1 mediates PAR(2)-induced hyperalgesia. TRPA1 deletion markedly reduces colitis-induced mechanical hyperalgesia in the colon. Our results suggest that TRPA1 has a major role in visceral nociception and may be a therapeutic target for colonic inflammatory pain.

Nociceptive signals induce trafficking of TRPA1 to the plasma membrane

Transient receptor potential A1 (TRPA1) ion channel senses a variety of noxious stimuli and is involved in nociception. Many TRPA1 agonists covalently modify the channel, which can lead to desensitization. The fate of modified TRPA1 and the mechanism of preserving its response to subsequent stimuli are not understood. Moreover, inflammatory signals sensitize TRPA1 by involving protein kinase A (PKA) and phospholipase C (PLC) through unknown means. We show that TRPA1-mediated nocifensive behavior can be sensitized in vivo via PKA/PLC signaling and by activating TRPA1 with the ligand mustard oil (MO). Interestingly, both stimuli increased TRPA1 membrane levels in vitro. Tetanus toxin attenuated the response to the second of two pulses of MO in neurons, suggesting that vesicle fusion increases functional surface TRPA1. Capacitance recordings suggest that MO can induce exocytosis. We propose that TRPA1 translocation to the membrane might represent one of the mechanisms controlling TRPA1 functionality upon acute activation or inflammatory signals.

The ion channel TRPA1 is required for normal mechanosensation and is modulated by algesic stimuli

BACKGROUND & AIMS

The transient receptor potential (TRP) channel family includes transducers of mechanical and chemical stimuli for visceral sensory neurons. TRP ankyrin 1 (TRPA1) is implicated in inflammatory pain; it interacts with G-protein-coupled receptors, but little is known about its role in the gastrointestinal (GI) tract. Sensory information from the GI tract is conducted via 5 afferent subtypes along 3 pathways.

METHODS

Nodose and dorsal root ganglia whose neurons innnervate 3 different regions of the GI tract were analyzed from wild-type and TRPA1(-/-) mice using quantitative reverse-transcription polymerase chain reaction, retrograde labeling, and in situ hybridization. Distal colon sections were analyzed by immunohistochemistry. In vitro electrophysiology and pharmacology studies were performed, and colorectal distension and visceromotor responses were measured. Colitis was induced by administration of trinitrobenzene sulphonic acid.

RESULTS

TRPA1 is required for normal mechano- and chemosensory function in specific subsets of vagal, splanchnic, and pelvic afferents. The behavioral responses to noxious colonic distension were substantially reduced in TRPA1(-/-) mice. TRPA1 agonists caused mechanical hypersensitivity, which increased in mice with colitis. Colonic afferents were activated by bradykinin and capsaicin, which mimic effects of tissue damage; wild-type and TRPA1(-/-) mice had similar direct responses to these 2 stimuli. After activation by bradykinin, wild-type afferents had increased mechanosensitivity, whereas, after capsaicin exposure, mechanosensitivity was reduced: these changes were absent in TRPA1(-/-) mice. No interaction between protease-activated receptor-2 and TRPA1 was evident.

CONCLUSIONS

These findings demonstrate a previously unrecognized role for TRPA1 in normal and inflamed mechanosensory function and nociception within the viscera.

Ion channels involved in cold detection in mammals: TRP and non-TRP mechanisms

Substantial progress in understanding thermal transduction in peripheral sensory nerve endings was achieved with the recent cloning of six thermally gated ion channels from the TRP (transient receptor potential) super-family. Two of these channels, TRP melastatin 8 (TRPM8) and TRP ankyrin 1 (TRPA1), are expressed in dorsal root ganglion (DRG) and trigeminal ganglion (TG) neurons, are activated by various degrees of cooling, and are candidates for mediating gentle cooling and noxious cold, respectively. However, accumulating evidence suggests that more than just these two channels are involved in cold sensing in mammals. A recent report described a critical role of the voltage-gated tetrodotoxin-resistant sodium channel Na_v1.8 in perceiving intense cold and noxious stimuli at cold temperatures. Other ion channels, such as two-pore domain background potassium channels (K2P), are known to be expressed in peripheral nerves, have pronounced temperature dependence, and may contribute to cold sensing and/or cold hypersensitivity in pain states. This article reviews the evidence supporting a role for each of these channels in cold transduction, focusing on their biophysical properties, expression pattern, and modulation by pro-inflammatory mediators.

Protein kinase D isoforms are expressed in rat and mouse primary sensory neurons and are activated by agonists of protease-activated receptor 2

Serine proteases generated during injury and inflammation cleave protease-activated receptor 2 (PAR(2)) on primary sensory neurons to induce neurogenic inflammation and hyperalgesia. Hyperalgesia requires sensitization of transient receptor potential vanilloid (TRPV) ion channels by mechanisms involving phospholipase C and protein kinase C (PKC). The protein kinase D (PKD) serine/threonine kinases are activated by diacylglycerol and PKCs and can phosphorylate TRPV1. Thus, PKDs may participate in novel signal transduction pathways triggered by serine proteases during inflammation and pain. However, it is not known whether PAR(2) activates PKD, and the expression of PKD isoforms by nociceptive neurons is poorly characterized. By using HEK293 cells transfected with PKDs, we found that PAR(2) stimulation promoted plasma membrane translocation and phosphorylation of PKD1, PKD2, and PKD3, indicating activation. This effect was partially dependent on PKCepsilon. By immunofluorescence and confocal microscopy, with antibodies against PKD1/PKD2 and PKD3 and neuronal markers, we found that PKDs were expressed in rat and mouse dorsal root ganglia (DRG) neurons, including nociceptive neurons that expressed TRPV1, PAR(2), and neuropeptides. PAR(2) agonist induced phosphorylation of PKD in cultured DRG neurons, indicating PKD activation. Intraplantar injection of PAR(2) agonist also caused phosphorylation of PKD in neurons of lumbar DRG, confirming activation in vivo. Thus, PKD1, PKD2, and PKD3 are expressed in primary sensory neurons that mediate neurogenic inflammation and pain transmission, and PAR(2) agonists activate PKDs in HEK293 cells and DRG neurons in culture and in intact animals. PKD may be a novel component of a signal transduction pathway for protease-induced activation of nociceptive neurons and an important new target for antiinflammatory and analgesic therapies.

TRPA1 in mast cell activation-induced long-lasting mechanical hypersensitivity of vagal afferent C-fibers in guinea pig esophagus

Sensitization of esophageal sensory afferents by inflammatory mediators plays an important role in esophageal nociception. We have shown esophageal mast cell activation induces long-lasting mechanical hypersensitivity in vagal nodose C-fibers. However, the roles of mast cell mediators and downstream ion channels in this process are unclear. Mast cell tryptase via protease-activated receptor 2 (PAR2)-mediated pathways sensitizes sensory nerves and induces hyperalgesia. Transient receptor potential A1 (TRPA1) plays an important role in mechanosensory transduction and nociception. Here we tested the hypothesis that mast cell activation via a PAR2-dependent mechanism sensitizes TRPA1 to induce mechanical hypersensitivity in esophageal vagal C-fibers. The expression profiles of PAR2 and TRPA1 in vagal nodose ganglia were determined by immunostaining, Western blot, and RT-PCR. Extracellular recordings from esophageal nodose neurons were performed in ex vivo guinea pig esophageal-vagal preparations. Action potentials evoked by esophageal distention and chemical perfusion were compared. Both PAR2 and TRPA1 expressions were identified in vagal nodose neurons by immunostaining, Western blot, and RT-PCR. Ninety-one percent of TRPA1-positive neurons were of small and medium diameters, and 80% coexpressed PAR2. Esophageal mast cell activation significantly enhanced the response of nodose C-fibers to esophageal distension (mechanical hypersensitivity). This was mimicked by PAR2-activating peptide, which sustained for 90 min after wash, but not by PAR2 reverse peptide. TRPA1 inhibitor HC-030031 pretreatment significantly inhibited mechanical hypersensitivity induced by either mast cell activation or PAR2 agonist. Collectively, our data provide new evidence that sensitizing TRPA1 via a PAR2-dependent mechanism plays an important role in mast cell activation-induced mechanical hypersensitivity of vagal nodose C-fibers in guinea pig esophagus.

Phosphoinositide regulation of non-canonical transient receptor potential channels

Transient receptor potential (TRP) channels are involved in a wide range of physiological processes, and characterized by diverse activation mechanisms. Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PIP(2), or PtdIns(4,5)P(2)] recently emerged as regulators of many TRP channels. Several TRP channels require PIP(2) for activity, and depletion of the lipid inhibits them. For some TRP channels, however, phosphoinositide regulation seems more complex, both activating and inhibitory effects have been reported. This review will discuss phosphoinositide regulation of members of the TRPM (Melastatin), TRPV (Vanilloid), TRPA (Ankyrin) and TRPP (Polycystin) families. Lipid regulation of TRPC (Canonical) channels is discussed elsewhere in this volume.

Activation of TRPV1 and TRPA1 leads to muscle nociception and mechanical hyperalgesia

The involvement of TRPV1 and TRPA1 in mediating craniofacial muscle nociception and mechanical hyperalgesia was investigated in male Sprague-Dawley rats. First, we confirmed the expression of TRPV1 in masseter afferents in rat trigeminal ganglia (TG), and provided new data that TRPA1 is also expressed in primary afferents innervating masticatory muscles in double-labeling immunohistochemistry experiments. We then examined whether the activation of each TRP channel in the masseter muscle evokes acute nocifensive responses and leads to the development of masseter hypersensitivity to mechanical stimulation using the behavioral models that have been specifically designed and validated for the craniofacial system. Intramuscular injections with specific agonists for TRPV1 and TRPA1, capsaicin and mustard oil (MO), respectively, produced immediate nocifensive hindpaw responses followed by prolonged mechanical hyperalgesia in a concentration-dependent manner. Pretreatment of the muscle with a TRPV1 antagonist, capsazepine, effectively attenuated the capsaicin-induced muscle nociception and mechanical hyperalgesia. Similarly, pretreatment of the muscle with a selective TRPA1 antagonist, AP18, significantly blocked the MO-induced muscle nociception and mechanical hyperalgesia. We confirmed these data with another set of selective antagonist for TRPV1 and TRPA1, AMG9810 and HC030031, respectively. Collectively, these results provide compelling evidence that TRPV1 and TRPA1 can functionally contribute to muscle nociception and hyperalgesia, and suggest that TRP channels expressed in muscle afferents can engage in the development of pathologic muscle pain conditions.

A sensory neuronal ion channel essential for airway inflammation and hyperreactivity in asthma

Asthma is an inflammatory disorder caused by airway exposures to allergens and chemical irritants. Studies focusing on immune, smooth muscle, and airway epithelial function revealed many aspects of the disease mechanism of asthma. However, the limited efficacies of immune-directed therapies suggest the involvement of additional mechanisms in asthmatic airway inflammation. TRPA1 is an irritant-sensing ion channel expressed in airway chemosensory nerves. TRPA1-activating stimuli such as cigarette smoke, chlorine, aldehydes, and scents are among the most prevalent triggers of asthma. Endogenous TRPA1 agonists, including reactive oxygen species and lipid peroxidation products, are potent drivers of allergen-induced airway inflammation in asthma. Here, we examined the role of TRPA1 in allergic asthma in the murine ovalbumin model. Strikingly, genetic ablation of TRPA1 inhibited allergen-induced leukocyte infiltration in the airways, reduced cytokine and mucus production, and almost completely abolished airway hyperreactivity to contractile stimuli. This phenotype is recapitulated by treatment of wild-type mice with HC-030031, a TRPA1 antagonist. HC-030031, when administered during airway allergen challenge, inhibited eosinophil infiltration and prevented the development of airway hyperreactivity. Trpa1(-/-) mice displayed deficiencies in chemically and allergen-induced neuropeptide release in the airways, providing a potential explanation for the impaired inflammatory response. Our data suggest that TRPA1 is a key integrator of interactions between the immune and nervous systems in the airways, driving asthmatic airway inflammation following inhaled allergen challenge. TRPA1 may represent a promising pharmacological target for the treatment of asthma and other allergic inflammatory conditions.

Effect of protease-activated receptor 2 activation on single TRPV1 channel activities in rat vagal pulmonary sensory neurons

Protease-activated receptor 2 (PAR(2)) is involved in airway inflammation and airway hyperresponsiveness; both are the prominent features of asthma. Transient receptor potential vanilloid receptor 1 (TRPV1) is expressed in pulmonary sensory nerves, functions as a thermal and chemical transducer and contributes to neurogenic inflammation. Using cell-attached single-channel recordings we investigated the effect of PAR(2) activation on single TRPV1channel activities in isolated pulmonary sensory neurons. Our immunohistochemical study demonstrated the expression of PAR(2) in rat vagal pulmonary sensory neurons. Our patch clamp study further showed that intracellular application of capsaicin (0.75 microM) induced single channel current that exhibited outward rectification in these neurons. The probability of the channel being open (Po) was significantly increased after the cells were pretreated with PAR2-activating peptide (100 microM, 2 min). Pretreatment with trypsin (0.1 microM, 2 min) also increased the single-channel Po, and the effect was completely inhibited by soybean trypsin inhibitor (0.5 microM, 3 min). In addition, the effect of PAR2 activation was abolished by either U73122 (1 microM, 4 min),a phospholipase C inhibitor, or chelerythrine (10 microM, 4 min), a protein kinase C inhibitor. In conclusion, our data demonstrated that activation of PAR2 upregulated single-channel activitiesofTRPV1and that the effect was mediated through the protein kinase C-dependent transduction pathway.

TPRV1 expression defines functionally distinct pelvic colon afferents

Changes in primary sensory neurons are likely to contribute to the emergence of chronic visceral pain. An important step in understanding visceral pain is the development of comprehensive phenotypes that combines functional and anatomical properties for these neurons. We developed a novel ex vivo physiology preparation in mice that allows intracellular recording from colon sensory neurons during colon distension, in the presence and absence of pharmacologic agents. This preparation also allows recovery of functionally characterized afferents for histochemical analysis. Recordings obtained from L6 dorsal root ganglion cells in C57BL/6 mice identified two distinct populations of distension-responsive colon afferents: high-firing frequency (HF) and low-firing frequency (LF) cells. Fluid distension of the colon elicited rapid firing (>20 Hz) in HF cells, whereas LF cells seldom fired >5 Hz. Distension response thresholds were significantly lower in HF cells (LF, 17.5 +/- 1.1 cmH(2)O; HF, 2.6 +/- 1.0 cmH(2)O). Responses of most LF afferents to colon distension were sensitized by luminal application of capsaicin (1 microm; 8 of 9 LF cells), mustard oil (100 microm; 10 of 12 LF cells), and low pH (pH 4.0; 5 of 6 LF cells). In contrast, few HF afferents were sensitized by capsaicin (3 of 9), mustard oil (2 of 7), or low pH (1 of 6) application. Few HF afferents (4 of 23) expressed the capsaicin receptor, TRPV1. In contrast, 87% (25 of 29) of LF afferents expressed TRPV1. TRPV1 has been shown to be required for development of inflammatory hyperalgesia. These results suggest a unique functional role of TRPV1-positive colon afferents that could be exploited to design specific therapies for visceral hypersensitivity.

Gi-dependent cell signaling responses of the human P2Y14 receptor in model cell systems

Eight G protein-coupled receptors comprise the P2Y receptor family of cell signaling proteins. The goal of the current study was to define native cell signaling pathways regulated by the uridine nucleotide sugar-activated P2Y(14) receptor (P2Y(14)-R). The P2Y(14)-R was stably expressed in human embryonic kidney (HEK) 293 and C6 rat glioma cells by retroviral infection. Nucleotide sugar-dependent P2Y(14)-R activation was examined by measuring inhibition of forskolin-stimulated cAMP accumulation. The effect of P2Y(14)-R activation on mitogen-activated protein kinase signaling also was studied in P2Y(14)-HEK293 cells and in differentiated HL-60 human myeloid leukemia cells. UDP-Glc, UDP-galactose, UDP-glucuronic acid, and UDP-N-acetylglucosamine promoted inhibition of forskolin-stimulated cAMP accumulation in P2Y(14)-HEK293 and P2Y(14)-C6 cells, and this signaling effect was abolished by pretreatment of cells with pertussis toxin. Inhibition of cAMP formation by nucleotide sugars also was observed in direct assays of adenylyl cyclase activity in membranes prepared from P2Y(14)-C6 cells. UDP-Glc promoted concentration-dependent and pertussis toxin-sensitive extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in P2Y(14)-HEK293 cells. P2Y(14)-R mRNA was not observed in wild-type HL-60 cells but was readily detected in dimethyl sulfoxide-differentiated cells. Consistent with this observation, no effect of UDP-Glc was observed in wild-type HL-60 cells, but UDP-Glc-promoted pertussis toxin-sensitive activation of ERK1/2 occurred after differentiation. These results illustrate that the human P2Y(14)-R signals through G(i) to inhibit adenylyl cyclase, and P2Y(14)-R activation also leads to ERK1/2 activation. This work also identifies two stable P2Y(14)-R-expressing cell lines and differentiated HL-60 cells as model systems for the study of P2Y(14)-R-dependent signal transduction.