Protein kinase C activation potentiates gating of the vanilloid receptor VR1 by capsaicin, protons, heat and anandamide

NGF rapidly increases membrane expression of TRPV1 heat-gated ion channels

Nociceptors, or pain-sensitive receptors, are unique among sensory receptors in that their sensitivity is increased by noxious stimulation. This process, called sensitization or hyperalgesia, is mediated by a variety of proinflammatory factors, including bradykinin, ATP and NGF, which cause sensitization to noxious heat stimuli by enhancing the membrane current carried by the heat- and capsaicin-gated ion channel, TRPV1. Several different mechanisms for sensitization of TRPV1 have been proposed. Here we show that NGF, acting on the TrkA receptor, activates a signalling pathway in which PI3 kinase plays a crucial early role, with Src kinase as the downstream element which binds to and phosphorylates TRPV1. Phosphorylation of TRPV1 at a single tyrosine residue, Y200, followed by insertion of TRPV1 channels into the surface membrane, explains most of the rapid sensitizing actions of NGF.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

The vanilloid receptor as a putative target of diverse chemicals in multiple chemical sensitivity

The vanilloid receptor (TRPV1 or VR1), widely distributed in the central and peripheral nervous system, is activated by a broad range of chemicals similar to those implicated in Multiple Chemical Sensitivity (MCS) Syndrome. The vanilloid receptor is reportedly hyperresponsive in MCS and can increase nitric oxide levels and stimulate N-methyl-D-aspartate (NMDA) receptor activity, both of which are important features in the previously proposed central role of nitric oxide and NMDA receptors in MCS. Vanilloid receptor activity is markedly altered by multiple mechanisms, possibly providing an explanation for the increased activity in MCS and symptom masking by previous chemical exposure. Activation of this receptor by certain mycotoxins may account for some cases of sick building syndrome, a frequent precursor of MCS. Twelve types of evidence implicate the vanilloid receptor as the major target of chemicals, including volatile organic solvents (but not pesticides) in MCS.

Activin induces tactile allodynia and increases calcitonin gene-related peptide after peripheral inflammation

Calcitonin gene-related peptide (CGRP) is a sensory neuropeptide important in inflammatory pain that conveys pain information centrally and dilates blood vessels peripherally. Previous studies indicate that activin A increases CGRP-immunoreactive (IR) sensory neurons in vitro, and following wound, activin A protein increases in the skin and more neurons have detectable CGRP expression in the innervating dorsal root ganglion (DRG). These data suggest some adult sensory neurons respond to activin A or other target-derived factors with increased neuropeptide expression. This study was undertaken to test whether activin contributes to inflammatory pain and increased CGRP and to learn which neurons retained plasticity. After adjuvant-induced inflammation, activin mRNA, but not NGF or glial cell line-derived neurotrophic factor, increased in the skin. To examine which DRG neurons increased CGRP immunoreactivity, retrograde tracer-labeled cutaneous neurons were characterized after inflammation. The proportion and size of tracer-labeled DRG neurons with detectable CGRP increased after inflammation. One-third of CGRP-IR neurons that appear after inflammation also had isolectin B4 binding, suggesting that some mechanoreceptors became CGRP-IR. In contrast, the increased proportion of CGRP-IR neurons did not appear to come from RT97-IR neurons. To learn whether central projections were altered after inflammation, CGRP immunoreactivity in the protein kinase Cgamma-IR lamina IIi was quantified and found to increase. Injection of activin A protein alone caused robust tactile allodynia and increased CGRP in the DRG. Together, these data support the hypothesis that inflammation and skin changes involving activin A cause some sensory neurons to increase CGRP expression and pain responses.

Lipid mediators of sensitivity in sensory neurons

Growing evidence implicates an increasing number of novel lipids, including eicosanoids, diacylglycerols, lysophosphatidic acids and ceramides, in augmenting the sensitivity of sensory neurons and enhancing pain perception. Many of these lipids are second messengers in signaling pathways that are associated with increasing the sensitivity of sensory neurons, whereas others are putative inflammatory mediators that activate either surface receptors or ion channels in these neurons. Based on the studies we review, it is clear that lipid-derived inflammatory mediators are a novel group of targets for therapeutics to treat inflammation and chronic pain states. However, much work remains to define the roles of these lipids in inflammation and the cellular mechanisms by which they alter the sensitivity of sensory neurons.

Co-activation of P2Y2 receptor and TRPV channel by ATP: implications for ATP induced pain

1. Extracellular ATP is recognized as a peripheral modulator of pain. Activation of ionotropic P2X receptors in sensory neurons has been implicated in induction of pain, whereas metabotropic P2Y receptors in potentiation of pain induced by chemical or physical stimuli via capsaicin sensitive TRPV1 channel. Here we report that P2Y2 receptor activation by ATP can activate the TRPV1 channel in absence of any other stimuli. 2. ATP-induced Ca2+ signaling was studied in Neuro2a cells. ATP evoked release of intracellular Ca2+ from ER and Ca2+ influx through a fast inactivating channel. The Ca2+ response was induced by P2Y receptor agonists in the order of potency ATP>or=UTP>or=ATPgammaS>ADP and was inhibited by suramin and PPADS. The P2X receptor agonist alpha beta methyl ATP was ineffective. 3. The Ca2+ influx was blocked by ruthenium red, an inhibitor of TRPV1 channel. Capsaicin, the most potent activator of the TRPV1 channel, evoked a fast inactivating Ca2+ transient suggesting the presence of endogenous TRPV1 channels in Neuro2a cells. NMS and PDBu, repressors of IP3 formation, drastically inhibited both the components of Ca2+ response. 4. Our data show co-activation of the P2Y2 receptor and capsaicin sensitive TRPV1 channel by ATP. Such functional interaction between endogenous P2Y2 receptor and TRPV1 channels could explain the ATP-induced pain.

Investigation of the role of TRPV1 receptors in acute and chronic nociceptive processes using gene-deficient mice

Capsaicin-sensitive, TRPV1 (transient receptor potential vanilloid 1) receptor-expressing primary sensory neurons exert local and systemic efferent effects besides the classical afferent function. The TRPV1 receptor is considered a molecular integrator of various physico-chemical noxious stimuli. In the present study its role was analysed in acute nociceptive tests and chronic neuropathy models by comparison of wild-type (WT) and TRPV1 knockout (KO) mice. The formalin-induced acute nocifensive behaviour, carrageenan-evoked inflammatory mechanical hyperalgesia and partial sciatic nerve lesion-induced neuropathic mechanical hyperalgesia were not different in WT and KO animals. Acute nocifensive behaviour after intraplantar injection of phorbol 12-myristate 13-acetate, an activator of protein kinase C (PKC), was absent in TRPV1 KO animals showing that PKC activation elicits nociception exclusively through TRPV1 receptor sensitization/activation. Thermal hyperalgesia (drop of noxious heat threshold) and mechanical hyperalgesia induced by a mild heat injury (51 degrees C, 15s) was smaller in KO mice suggesting a pronociceptive role for TRPV1 receptor in burn injury. Chronic mechanical hyperalgesia evoked by streptozotocin-induced diabetic and cisplatin-evoked toxic polyneuropathy occurred earlier and were greater in the TRPV1 KO group. In both polyneuropathy models, at time points when maximal difference in mechanical hyperalgesia between the two groups was measured, plasma somatostatin concentrations determined by radioimmunoassay significantly increased in WT but not in TRPV1 KO mice. It is concluded that sensitization/activation of the TRPV1 receptor plays a pronociceptive role in certain models of acute tissue injury but under chronic polyneuropathic conditions it can initiate antinociceptive counter-regulatory mechanisms possibly mediated by somatostatin released from sensory neurons.

Inflammatory mediators convert anandamide into a potent activator of the vanilloid type 1 transient receptor potential receptor in nociceptive primary sensory neurons

The endogenous ligand, anandamide activates at least two receptors on nociceptors; the excitatory vanilloid type 1 transient receptor potential receptor, the activity of which is indispensable for the development and maintenance of inflammatory heat hyperalgesia, and the inhibitory cannabinoid 1 receptor, the activity of which reduces that pathological pain sensation. Recent data are equivocal on whether increasing anandamide levels at the peripheral terminals of nociceptors in pathological conditions increases or decreases inflammatory heat hyperalgesia. Here, by using the cobalt-uptake technique we examined whether vanilloid type 1 transient receptor potential receptor activity evoked by 10 nM-100 microM anandamide is increased or decreased in inflammatory conditions. An inflammatory milieu for cultured rat primary sensory neurons was established by incubating the cells in the presence of the inflammatory mediators, bradykinin and prostaglandin E2. Anandamide, similarly to the archetypical vanilloid type 1 transient receptor potential receptor agonist, capsaicin induced concentration-dependent cobalt-uptake in a proportion of neurons. However, the potency of anandamide was significantly lower than that of capsaicin. While pre-incubation of cultures with bradykinin and prostaglandin E2 alone did not evoke cobalt-entry, the inflammatory mediators potentiated the effect of both capsaicin and anandamide. Application of the competitive vanilloid type 1 transient receptor potential receptor antagonist, capsazepine, or inhibitors of protein kinase A, protein kinase C or phospholipase C inhibited the anandamide-evoked cobalt-uptake both in the presence and absence of bradykinin and prostaglandin E2. These findings show that inflammatory mediators significantly increase the excitatory potency and efficacy of anandamide on vanilloid type 1 transient receptor potential receptor, thus, increasing the anandamide concentration in, or around the peripheral terminals of nociceptors might rather evoke than decrease inflammatory heat hyperalgesia.

Systemic and site-specific effects of A-425619, a selective TRPV1 receptor antagonist, on wide dynamic range neurons in CFA-treated and uninjured rats

Systemic administration of A-425619, a potent and selective TRPV1 receptor antagonist that does not readily enter the CNS, produces antinociception in several rat models of pathological nociception, including complete Freund's adjuvant (CFA)-induced thermal hyperalgesia. To further understand the peripheral mechanisms of TRPV1-related antinociception, we examined the effects of systemic and site-specific injections of A-425619 on evoked and spontaneous firing of spinal wide dynamic range (WDR) neurons in uninjured rats and rats with peripheral inflammation (CFA; 48 h). In uninjured rats, capsaicin-evoked (1 microg) WDR activity was completely blocked by intraplantar administration of A-425619 (3-100 nmol). Systemic injection of A-425619 (3-30 micromol/kg, iv) reduced WDR responses to thermal stimulation in both CFA-inflamed (47 degrees C) and uninjured (52 degrees C) rats. However, the efficacy of A-425619 to attenuate thermal-evoked WDR activity was significantly greater (P < 0.01) in CFA-treated rats. Both intradorsal root ganglion (DRG; L5; 20 nmol) and intraplantar (30-300 nmol) injection of A-425619 reduced WDR responses to thermal stimulation. While the effectiveness of A-425619 was similar between CFA-inflamed and uninjured rats after intraplantar injection, the effects of A-425619 after intra-DRG injection were enhanced in the inflamed rats (compared with the uninjured rats). Spontaneous WDR discharges were unaltered by systemic or site-specific injections of A-425619. Thus noxious thermal stimulation triggers the transmission of TRPV1-related signals to spinal WDR neurons in both inflamed and uninjured animals. The apparent increase in TRPV1 signaling to WDR neurons after injury may be the result of changes to the distribution/sensitization of peripheral TRPV1 receptors.

The temperature-sensitive ion channel TRPV2 is endogenously expressed and functional in the primary sensory cell line F-11

In sensory neurons heat is transduced by a subfamily of TRP channels sharing sequence homology with the capsaicin-sensitive vanilloid receptor subtype 1 (TRPV1), but differing in their thermal response thresholds. To identify a neuronal cell line endogenously expressing noxious heat-transducing ion channels, we examined F-11 cells, a hybridoma derived from rat dorsal root ganglia and mouse neuroblastoma. Using RT-PCR, transcripts homologous to TRPV2 and TRPV4, but not to TRPV1 or TRPV3, were found. We isolated a full-length cDNA of 2.4 kb coding for a 757-amino acid protein corresponding to mouse TRPV2, which was further characterized by immunocytochemistry and electrophysiology. Using the whole-cell patch-clamp technique, we observed a heat-evoked increase in outward and inward currents with a threshold of 51.6 +/- 0.2 degrees C. The current-voltage relationship stimulated by a temperature of 52 degrees C was characterized by an outward rectification with a reversal potential close to -10 mV. Heat-evoked currents could be inhibited by ruthenium red. There was no activation by stimulation with capsaicin or 2-aminoethoxydiphenyl borate. Our results indicate that F-11 cells express functional noxious heat-sensitive TRPV2 channels. Thus, we propose that F-11 cells represent a valuable in vitro model to characterize the properties of TRPV2 in a native neuronal environment.

Endogenous unsaturated C18 N-acylethanolamines are vanilloid receptor (TRPV1) agonists

The endogenous C18 N-acylethanolamines (NAEs) N-linolenoylethanolamine (18:3 NAE), N-linoleoylethanolamine (18:2 NAE), N-oleoylethanolamine (18:1 NAE), and N-stearoylethanolamine (18:0 NAE) are structurally related to the endocannabinoid anandamide (20:4 NAE), but these lipids are poor ligands at cannabinoid CB(1) receptors. Anandamide is also an activator of the transient receptor potential (TRP) vanilloid 1 (TRPV(1)) on primary sensory neurons. Here we show that C18 NAEs are present in rat sensory ganglia and vascular tissue. With the exception of 18:3 NAE in rat sensory ganglia, the levels of C18 NAEs are equal to or substantially exceed those of anandamide. At submicromolar concentrations, 18:3 NAE, 18:2 NAE, and 18:1 NAE, but not 18:0 NAE and oleic acid, activate native rTRPV(1) on perivascular sensory nerves. 18:1 NAE does not activate these nerves in TRPV(1) gene knock-out mice. Only the unsaturated C18 NAEs elicit whole cell currents and fluorometric calcium responses in HEK293 cells expressing hTRPV(1). Molecular modeling revealed a low energy cluster of U-shaped unsaturated NAE conformers, sharing several pharmacophoric elements with capsaicin. Furthermore, one of the two major low energy conformational families of anandamide also overlaps with the cannabinoid CB(1) receptor ligand HU210, which is in line with anandamide being a dual activator of TRPV(1) and the cannabinoid CB(1) receptor. This study shows that several endogenous non-cannabinoid NAEs, many of which are more abundant than anandamide in rat tissues, activate TRPV(1) and thus may play a role as endogenous TRPV(1) modulators.

Lipids as regulators of the activity of transient receptor potential type V1 (TRPV1) channels

After 7 years from its cloning, the transient receptor potential vanilloid type-1 (TRPV1) channel remains the sole membrane receptor mediating the pharmacological effects of the hot chilli pepper pungent component, capsaicin, and of the Euphorbia toxin, resiniferatoxin. Yet, this ion channel represents one of the most complex examples of how the activity of a protein can be regulated. Among the several chemicophysical stimuli that can modulate TRPV1 permeability to cations, endogenous lipids appear to play a major role, either as allosteric effectors or as direct agonists, or both. Furthermore, the capability of some mediators, such as the endocannabinoid anandamide, or the eicosanoid precursors 12- and 5-hydroperoxy-eicosatetraenoic acids, to activate TRPV1 receptors provides a striking example of the "site-dependent" and "metabolic" functional plasticity, respectively, typical of bioactive lipids. In this article, the multi-faceted and most recently discovered aspects of TRPV1 regulation are reviewed, with particular emphasis on the interaction between these membrane channels and some lipid molecules.

Sensitization of vanilloid receptor involves an increase in the phosphorylated form of the channel

A vanilloid receptor (VR1, now known as TRPV1) is an ion channel activated by vanilloids, including capsaicin (CAP) and resiniferatoxin (RTX), which are pungent ingredients of plants. Putative endogenous activators (anandamide and metabolites of arachidonic acid) are weak activators of VR1 compared to capsaicin and RTX, and the concentrations of the physiological condition of those activators are not sufficient to induce significant activation of VR1. One way to overcome the weak activation of endogenous activators would be the sensitization of VR1, with the phosphorylation of the channel being one possibility. The phosphorylation of VR1 by several kinases has been reported, mostly by indirect evidence. Here, using an in vivo phosphorylation method, the VR1 channel was shown to be sensitized by phosphorylation of the channel itself by multiple pathways involving PKA, PKC and acid. Also, in sensitizing VR1, BK appeared to show activation of PKC for the sensitization of VR1 by phosphorylation of the channel.

Sensitization of pulmonary chemosensitive neurons by bombesin-like peptides in rats

Small cell lung cancer (SCLC) patients suffer from pulmonary stresses such as dyspnea and chest pain, and the pathogenic mechanisms are not known. SCLC cells secrete a variety of bioactive neuropeptides, including bombesin-like peptides. We hypothesize that these peptides may enhance the sensitivity of the pulmonary chemosensitive nerve endings, contributing to the development of these pulmonary stresses in SCLC patients. This study was therefore carried out to determine the effects of bombesin and gastrin-releasing peptide (GRP), a major bombesin-like peptide, on the sensitivities of pulmonary chemoreflex and isolated pulmonary vagal chemosensitive neurons. In anesthetized, spontaneously breathing rats, intravenous infusion of bombesin or GRP significantly amplified the pulmonary chemoreflex responses to chemical stimulants such as capsaicin and ATP. The enhanced responses were completely abolished by perineural capsaicin treatment of both cervical vagi, suggesting the involvement of pulmonary C-fiber afferents. In isolated pulmonary vagal chemosensitive neurons, pretreatment with bombesin or GRP potentiated the capsaicin-induced Ca(2+) transient. This sensitizing effect was further demonstrated in patch-clamp recording studies; the sensitivities of these neurons to both chemical (capsaicin and ATP) and electrical stimuli were significantly enhanced by the presence of either bombesin or GRP. In summary, our results have demonstrated that bombesin and GRP upregulate the pulmonary chemoreflex sensitivity in vivo and the excitability of isolated pulmonary chemosensitive neurons in vitro.

Identification of a protein that interacts with the vanilloid receptor

The vanilloid receptor (VR1 or TRPV1) is a capsaicin (CAP)-sensitive non-selective cation channel. Although its channel activity is reportedly modulated through protein-protein interactions, to date very few VR1 interacting proteins have been identified. To address this issue, a yeast two-hybrid screening technique using the C-terminus of rVR1 as bait was employed. Upon interrogation of a mouse brain library, one gene product that interacts with VR1 and is highly homologous to human eferin was found. Its interaction with VR1 was confirmed by GST-pull-down and co-immunoprecipitation. When cotransfected into HEK cells, VR1 and eferin largely colocalize. Furthermore, in rat dorsal root ganglion cells, the rat eferin homologue also colocalizes with rVR1. However, this protein had no significant effect on VR1 channel activity in response to CAP. This was determined by two-electrode recording of oocytes and whole cell recording of HEK cells that were cotransfected with VR1 and human eferin.

The contribution of TRPM8 channels to cold sensing in mammalian neurones

Different classes of ion channels have been implicated in sensing cold temperatures at mammalian thermoreceptor nerve endings. A major candidate is TRPM8, a non-selective cation channel of the transient receptor potential family, activated by menthol and low temperatures. We investigated the role of TRPM8 in cold sensing during transient expression in mouse cultured hippocampal neurones, a tissue that lacks endogenous expression of thermosensitive TRPs. In the absence of synaptic input, control hippocampal neurones were not excited by cooling. In contrast, all TRPM8-transfected hippocampal neurones were excited by cooling and menthol. However, in comparison to cold-sensitive trigeminal sensory neurones, hippocampal neurones exhibited much lower threshold temperatures, requiring temperatures below 27 degrees C to fire action potentials. These results directly demonstrate that expression of TRPM8 in mammalian neurones induces cold sensing, albeit at lower temperatures than native TRPM8-expressing neurones, suggesting the presence of additional modulatory mechanisms in the cold response of sensory neurones.

TRPV channels as thermosensory receptors in epithelial cells

Temperature-sensitive transient receptor potential vanilloid (TRPV) ion channels are critical contributors to normal pain and temperature sensation and therefore represent attractive targets for pain therapy. When these channels were first discovered, most attention was focused on their potential contributions to direct thermal activation of peripheral sensory neurons. However, recent anatomical, physiological, and behavioral studies have provided evidence that TRPV channels expressed in skin epithelial cells may also contribute to thermosensation in vitro and in vivo. Here, we review these studies and speculate on possible communication mechanisms from cutaneous epithelial cells to sensory neurons.

Heat hyperalgesia after incision requires TRPV1 and is distinct from pure inflammatory pain

Postoperative pain significantly impacts patient recovery. However, postoperative pain management remains suboptimal, perhaps because treatment strategies are based mainly on studies using inflammatory pain models. We used a recently developed mouse model of incisional pain to investigate peripheral and spinal mechanisms contributing to heat hyperalgesia after incision. Behavioral experiments involving TRPV1 KO mice demonstrate that, as previously observed in inflammatory models, TRPV1 is necessary for heat (but not mechanical) hyperalgesia after incision. However, in WT mice, neither the proportion of TRPV1 immunoreactive neurons in the DRG nor the intensity of TRPV1 staining in the sciatic nerve was different from that in controls up to 4 days after incision. This result was corroborated by immunoblot analysis of sciatic nerve in rats subjected to an incision, and is distinct from that following inflammation of the rat hind paw, a situation in which TRPV1 expression levels in sciatic nerve increases. In the absence of heat exposure, spinal c-Fos staining was similar between incised TRPV1 KO and WT mice. However, differences in c-Fos staining between heat exposed TRPV1 KO and WT mice after incision suggest that the incision-mediated enhancement of heat-evoked signaling to the spinal cord involves a TRPV1-dependent mechanism. Finally, heat hyperalgesia after incision was reversed by antagonism of spinal non-NMDA receptors, unlike inflammatory hyperalgesia, which is mediated via NMDA receptors . Thus, TRPV1 is important for the generation of thermal hyperalgesia after incision. Our observations suggest that all experimental pain models may not be equally appropriate to guide the development of postoperative pain therapies.

Functional aspects and mechanisms of TRPV1 involvement in neurogenic inflammation that leads to thermal hyperalgesia

Neurogenic inflammation is produced by overstimulation of peripheral nociceptor terminals by injury or inflammation of tissues. Excessive activity of sensory neurons produces vasodilation, plasma extravasation and hypersensitivity. Mechanistically, neurogenic inflammation is due to the release of substances from primary sensory nerve terminals that act directly or indirectly at the peripheral terminals, either activating or sensitizing nociceptors, endothelial cells and immunocytes. Notably, small-diameter sensory neurons that are sensitive to capsaicin play a key role in the generation of neurogenic inflammation. The cloning of the vanilloid receptor 1 (TRPV1) has been a breakthrough that has propelled our understanding of the molecular mechanisms involved in neurogenic inflammation. TRPV1 pivotally contributes to the integration of various stimuli and modulates nociceptor excitability, thus making it a true gateway for pain transduction. In addition, TRPV1 is the endpoint target of intracellular signalling pathways triggered by inflammatory mediators. Phosphorylation-induced potentiation of TRPV1 channel activity, along with an incremented TRPV1 surface expression are major events underlying the nociceptor activation and sensitization that leads to thermal hyperalgesia. The important contribution of TRPV1 receptor to the onset and maintenance of neurogenic inflammation has validated it as a therapeutic target for inflammatory pain management. As a result, the development of specific TRPV1 antagonists is a central focus of current drug discovery programs.

Desensitization of canonical transient receptor potential channel 5 by protein kinase C

The classic type of transient receptor potential channel (TRPC) is a molecular candidate for Ca(2+)-permeable cation channel in mammalian cells. TRPC5 is desensitized rapidly after activation by G protein-coupled receptor. Herein we report our investigation into the desensitization of mTRPC5 and localization of the molecular determinants of this desensitization using mutagenesis. TRPC5 was initially activated by muscarinic stimulation using 100 microM carbachol (CCh) and then decayed rapidly even in the presence of CCh (desensitization). Increased EGTA or omission of MgATP in the pipette solution slowed the rate of this desensitization. The protein kinase C (PKC) inhibitors, 1 microM chelerythrine, 100 nM GF109203X, or PKC peptide inhibitor (19-36), inhibited this desensitization of TRPC5 activated by 100 microM CCh. When TRPC5 current was activated by intracellular GTPgammaS, PKC inhibitors prevented TRPC5 desensitization and the mutation of TRPC5 T972 to alanine slowed the desensitization process dramatically. We conclude that the desensitization of TRPC5 occurs via PKC phosphorylation and suggest that threonine at residue 972 of mouse TRPC5 might be required for its phosphorylation by PKC.

A-425619 [1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea], a novel and selective transient receptor potential type V1 receptor antagonist, blocks channel activation by vanilloids, heat, and acid

The vanilloid receptor transient receptor potential type V1 (TRPV1) integrates responses to multiple stimuli, such as capsaicin, acid, heat, and endovanilloids and plays an important role in the transmission of inflammatory pain. Here, we report the identification and in vitro characterization of A-425619 [1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea], a novel, potent, and selective TRPV1 antagonist. A-425619 was found to potently block capsaicin-evoked increases in intracellular calcium concentrations in HEK293 cells expressing recombinant human TRPV1 receptors (IC50 = 5 nM). A-425619 showed similar potency (IC50 = 3-4 nM) to block TRPV1 receptor activation by anandamide and N-arachidonoyl-dopamine. Electrophysiological experiments showed that A-425619 also potently blocked the activation of native TRPV1 channels in rat dorsal root ganglion neurons (IC50 = 9 nM). When compared with other known TRPV1 antagonists, A-425619 exhibited superior potency in blocking both naive and phorbol ester-sensitized TRPV1 receptors. Like capsazepine, A-425619 demonstrated competitive antagonism (pA2 = 2.5 nM) of capsaicin-evoked calcium flux. Moreover, A-425619 was 25- to 50-fold more potent than capsazepine in blocking TRPV1 activation. A-425619 showed no significant interaction with a wide range of receptors, enzymes, and ion channels, indicating a high degree of selectivity for TRPV1 receptors. These data show that A-425619 is a structurally novel, potent, and selective TRPV1 antagonist.

Block of native and cloned vanilloid receptor 1 (TRPV1) by aminoglycoside antibiotics

Vanilloid receptor 1 (TRPV1) is a Ca2+ permeable non-specific cation channel located at the peripheral nerve terminals and functions as a polymodal nociceptor. Neomycin, an aminoglycoside antibiotic induces analgesia in various animal models. However, the mechanism of action of neomycin has not been fully understood. In this study, we have determined the effect of neomycin on native TRPV1 in cultured embryonic DRG neurons and cloned TRPV1 heterologously expressed in Xenopus oocytes using patch clamp, double electrode voltage clamp, and Ca2+ fluorescence imaging techniques. Here, we show that neomycin potently (IC50 approximately 400 nM) blocks TRPV1-mediated membrane currents in DRG neurons and the block is unrelated to capsaicin concentrations used to evoke currents, suggesting a non-competitive block. Similarly, capsaicin- and proton-induced currents are blocked in oocytes, but to a lesser extent. Increases in capsaicin-induced intracellular Ca2+ levels are also reduced by neomycin. Single-channel current analyses reveal that single-channel conductance is unaffected by neomycin and there is no indication of open channel block. The predominant effect is to lower, the open probability (Po) at both, negative and positive potentials. Kinetic analyses reveal that the number of exponential components required to fit the open time distributions remains the same or reduced, however, the longest open time constant and the area of distribution are shortened at negative and positive potentials, respectively. The area of distribution of longest closed-time constants were significantly prolonged at negative and positive potentials. We conclude that neomycin inhibits TRPV1 channel activity by allosteric binding and altering channel gating.

Fast modulation of heat-activated ionic current by proinflammatory interleukin 6 in rat sensory neurons

The pro-inflammatory cytokine interleukin-6 (IL-6) together with its soluble receptor (sIL-6R) induces and maintains thermal hyperalgesia. It facilitates the heat-induced release of calcitonin gene-related peptide from rat cutaneous nociceptors in vivo and in vitro. Here we report that exposure of nociceptive neurons to the IL-6-sIL-6R complex or the gp130-stimulating designer IL-6-sIL-6R fusion protein Hyper-IL-6 (HIL-6) resulted in a potentiation of heat-activated inward currents (I(heat)) and a shift of activation thresholds towards lower temperatures without affecting intracellular calcium levels. The Janus tyrosine kinase inhibitor AG490, the selective protein kinase C (PKC) inhibitor, bisindolylmaleimide 1 (BIM1), as well as rottlerin, a selective blocker of the PKCdelta isoform, but not the cyclooxygenase inhibitor indomethacin, effectively reduced the effect. Reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization revealed expression of mRNA for the signal-transducing beta subunit of the receptor gp130 in neuronal somata, rather than satellite cells in rat dorsal root ganglia. Together, the results suggest that IL-6-sIL-6R acts directly on sensory neurons. It increases their susceptibility to noxious heat via the gp130/Jak/PKCdelta signalling pathway.

A proinflammatory chemokine, CCL3, sensitizes the heat- and capsaicin-gated ion channel TRPV1

Pain, a critical component of host defense, is one hallmark of the inflammatory response. We therefore hypothesized that pain might be exacerbated by proinflammatory chemokines. To test this hypothesis, CCR1 was cotransfected into human embryonic kidney (HEK)293 cells together with transient receptor potential vanilloid 1 (TRPV1), a cation channel required for certain types of thermal hyperalgesia. In these cells, capsaicin and anandamide induced Ca(2+) influx mediated by TRPV1. When CCR1:TRPV1/HEK293 cells were pretreated with CCL3, the sensitivity of TRPV1, as indicated by the Ca(2+) influx, was increased approximately 3-fold. RT-PCR analysis showed that a spectrum of chemokine and cytokine receptors is expressed in rat dorsal root ganglia (DRG). Immunohistochemical staining of DRG showed that CCR1 is coexpressed with TRPV1 in >85% of small-diameter neurons. CCR1 on DRG neurons was functional, as demonstrated by CCL3-induced Ca(2+) ion influx and PKC activation. Pretreatment with CCL3 enhanced the response of DRG neurons to capsaicin or anandamide. This sensitization was inhibited by pertussis toxin, U73122, or chelerythrine chloride, inhibitors of Gi-protein, phospholipase C, and protein kinase C, respectively. Intraplantar injection of mice with CCL3 decreased their hot-plate response latency. That a proinflammatory chemokine, by interacting with its receptor on small-diameter neurons, sensitizes TRPV1 reveals a previously undescribed mechanism of receptor cross-sensitization that may contribute to hyperalgesia during inflammation.

A proinflammatory chemokine, CCL3, sensitizes the heat- and capsaicin-gated ion channel TRPV1

Pain, a critical component of host defense, is one hallmark of the inflammatory response. We therefore hypothesized that pain might be exacerbated by proinflammatory chemokines. To test this hypothesis, CCR1 was cotransfected into human embryonic kidney (HEK)293 cells together with transient receptor potential vanilloid 1 (TRPV1), a cation channel required for certain types of thermal hyperalgesia. In these cells, capsaicin and anandamide induced Ca(2+) influx mediated by TRPV1. When CCR1:TRPV1/HEK293 cells were pretreated with CCL3, the sensitivity of TRPV1, as indicated by the Ca(2+) influx, was increased approximately 3-fold. RT-PCR analysis showed that a spectrum of chemokine and cytokine receptors is expressed in rat dorsal root ganglia (DRG). Immunohistochemical staining of DRG showed that CCR1 is coexpressed with TRPV1 in >85% of small-diameter neurons. CCR1 on DRG neurons was functional, as demonstrated by CCL3-induced Ca(2+) ion influx and PKC activation. Pretreatment with CCL3 enhanced the response of DRG neurons to capsaicin or anandamide. This sensitization was inhibited by pertussis toxin, U73122, or chelerythrine chloride, inhibitors of Gi-protein, phospholipase C, and protein kinase C, respectively. Intraplantar injection of mice with CCL3 decreased their hot-plate response latency. That a proinflammatory chemokine, by interacting with its receptor on small-diameter neurons, sensitizes TRPV1 reveals a previously undescribed mechanism of receptor cross-sensitization that may contribute to hyperalgesia during inflammation.

NGF and GDNF differentially regulate TRPV1 expression that contributes to development of inflammatory thermal hyperalgesia

The transient receptor potential ion channel, TRPV1 plays an essential role in the development of inflammatory thermal hyperalgesia. We investigated the dependence of inflammatory TRPV1 induction on neurotrophic factor. Rat dorsal root ganglia (DRG) neurons were classified according to immunostaining for trk-A and IB4 and the effects of antibodies against NGF or GDNF on TRPV1 expression within the groups were then analysed by immunohistochemical means. The data were compared with the time course of trophic factor expression and the effects of their antibodies on thermal hyperalgesia against radiant heat after inflammation. Although the levels of both NGF and GDNF were increased by inflammation, NGF rapidly and transiently increased whereas GDNF increased gradually over a period of approximately one week. TRPV1 expression was increased within both trk-A positive and IB4 positive neurons after inflammation. Increased TRPV1 expression within trk-A positive neurons was prevented by anti-NGF but not by anti-GDNF, whereas TRPV1 induction within the IB4 positive group was blocked by anti-GDNF but not by anti-NGF. Both antibodies prevented the short latency of withdrawing an inflamed paw from radiant heat. These results suggest that inflammation differentially increases both NGF and GDNF, which facilitate TRPV1 expression within distinctive neurons to induce thermal hyperalgesia.

Protein kinase C contributes to abnormal capsaicin responses in DRG neurons from cats with feline interstitial cystitis

Interstitial cystitis (IC) is a painful disorder which affects urinary bladder function in cats and humans. We have used patch clamp techniques to examine the possibility that the properties of primary afferent neurons are changed in feline interstitial cystitis (FIC). We measured transient receptor potential vanilloid receptor 1 (TRPV1) responses to capsaicin (CAPS) in dorsal root ganglion (DRG) neurons (L4-S3) from normal cats and cats with FIC. We show that FIC neurons are increased in size and exhibit CAPS responses which are increased in amplitude and desensitize slowly. CAPS responses desensitized seven times slower in FIC neurons. Phorbol 12,13-dibutyrate (PDBu), an activator of PKC, slowed the desensitization of CAPS responses in normal cat bladder and non-bladder neurons, but had no effect in FIC neurons. Bisindolylmaleimide, an inhibitor of PKC, reversed the PDBu effects in normal cat neurons and normalized the desensitization of CAPS responses in FIC neurons. Our data suggest that FIC afferent neurons exhibit abnormal CAPS responses. The latter may be due to enhanced endogenous activities of PKC.

Oleoylethanolamide excites vagal sensory neurones, induces visceral pain and reduces short-term food intake in mice via capsaicin receptor TRPV1

Oleoylethanolamide (OEA) is an endogenous lipid that regulates feeding and body weight. Although the effects of OEA are believed to depend on activation of vagal sensory afferent neurones, the mechanisms involved in exciting these neurones are unclear. Here we show that OEA directly excited nodose ganglion neurones, the cell bodies of vagal afferents. OEA depolarized these neurones and evoked inward currents that were restricted to capsaicin-sensitive cells. These currents were fully blocked by the TRPV1 inhibitor, capsazepine, and no responses to OEA were observed in neurones cultured from TRPV1-null mice. Similarly, OEA induced a rise in Ca(+) concentration in wild-type but not TRPV1-deficient neurones, and responses to OEA were greater at 37 degrees C compared to room temperature. Significantly, OEA administration in mice induced visceral pain-related behaviours that were inhibited by capsazepine and absent in TRPV1-null animals. Further, OEA reduced 30-min food intake in wild-type but not in TRPV1-null mice. Thus, the acute behavioural effects of OEA may result from visceral malaise via the activation of TRPV1.

Regulation of Ca2+-dependent desensitization in the vanilloid receptor TRPV1 by calcineurin and cAMP-dependent protein kinase

The vanilloid receptor TRPV1 is a polymodal nonselective cation channel of nociceptive sensory neurons involved in the perception of inflammatory pain. TRPV1 exhibits desensitization in a Ca2+-dependent manner upon repeated activation by capsaicin or protons. The cAMP-dependent protein kinase (PKA) decreases desensitization of TRPV1 by directly phosphorylating the channel presumably at sites Ser116 and Thr370. In the present study we investigated the influence of protein phosphatase 2B (calcineurin) on Ca2+-dependent desensitization of capsaicin- and proton-activated currents. By using site-directed mutagenesis, we generated point mutations at PKA and protein kinase C consensus sites and studied wild type (WT) and mutant channels transiently expressed in HEK293t or HeLa cells under whole cell voltage clamp. We found that intracellular application of the cyclosporin A.cyclophilin A complex (CsA.CyP), a specific inhibitor of calcineurin, significantly decreased desensitization of capsaicin- or proton-activated TRPV1-WT currents. This effect was similar to that obtained by extracellular application of forskolin (FSK), an indirect activator of PKA. Simultaneous applications of CsA.CyP and FSK in varying concentrations suggested that these substances acted independently from each other. In mutation T370A, application of CsA.CyP did not reduce desensitization of capsaicin-activated currents as compared with WT and to mutant channels S116A and T144A. In a double mutation at candidate protein kinase C phosphorylation sites, application of CsA.CyP or FSK decreased desensitization of capsaicin-activated currents similar to WT channels. We conclude that Ca2+-dependent desensitization of TRPV1 might be in part regulated through channel dephosphorylation by calcineurin and channel phosphorylation by PKA possibly involving Thr370 as a key amino acid residue.

Protease-activated receptors: regulation of neuronal function

Certain serine proteases from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast-cell tryptase, neutrophil proteinase 3), and from many other cell types (e.g., trypsins) can specifically signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. Proteases cleave PARs at specific sites within the extracellular amino-terminus to expose amino-terminal tethered ligand domains that bind to and activate the cleaved receptors. The proteases that activate PARs are often generated and released during injury and inflammation, and activated PARs orchestrate tissue responses to injury, including hemostasis, inflammation, pain, and repair. This review concerns protease and PAR signaling in the nervous system. Neurons of the central and peripheral nervous systems express all four PARs. Proteases that may derive from the circulation, inflammatory cells, or neural tissues can cleave PARs on neurons and thereby activate diverse signaling pathways that control survival, morphology, release of neurotransmitters, and activity of ion channels. In this manner proteases and PARs regulate neurodegeneration, neurogenic inflammation, and pain transmission. Thus, PARs may participate in disease states and PAR antagonists or agonists may be useful therapies for certain disorders.

Posttranslational mechanisms of peripheral sensitization

The sensation of pain can be dramatically altered in response to injury or disease. This sensitization can occur at the level of the primary sensory neuron, and can be mediated by multiple biochemical mechanisms, including, but not limited to, changes in gene transcription, changes in translation, stability, or subcellular localization of translated proteins, and posttranslational modifications. This review focuses on posttranslational modifications to ion channels expressed in primary sensory neurons that form the machinery driving peripheral sensitization and pain hypersensitivity. Studies published to date show strong evidence for modulation of ion channels involved in transduction and transmission of nociceptive inputs coincident with biophysical and behavioral sensitization. The roles of phosphorylation and oxidation/reduction reactions of voltage-dependent sodium, potassium, and calcium channels are discussed, as well as phosphorylation-mediated modulation of sensory transduction channels.

Expression of vanilloid VR1 receptor in PC12 cells

Capsaicin, a pungent ingredient of chili pepper, activates vanilloid receptor subtype 1 (VR1), which is a nonselective cation channel with high Ca(2+) permeability. Although VR1 and its splice variant are highly expressed in sensory neurons, they are expressed in neuronal cells in brain and peripheral non-neuronal cells. In this study, we investigated whether VR1 is expressed in PC12 cells, rat pheochromocytoma. Capsaicin at concentrations above 100 microM induced an increase in intracellular free Ca(2+) concentrations by influx from extracellular spaces, and the effect was blocked by capsazepine, a selective antagonist of VR1. VR1 transcript and protein were detected by reverse transcription-polymerase chain reaction and Western blotting analysis, respectively. Immunocytochemical analysis revealed that VR1 protein was expressed in the cytosol and the plasma membrane of PC12 cells, and treatment with the antisense oligonucleotide for VR1 decreased the expression. VR1 in PC12 cells showed different characters from that in sensory neurons; capsaicin concentration-dependency and heat- and nerve growth factor-sensitivities. These results suggested that VR1 was functionally expressed in PC12 cells. The usefulness of PC12 cell line for studying functions and/or expression of VR1 is discussed.

Activation of protein kinase C reverses capsaicin-induced calcium-dependent desensitization of TRPV1 ion channels

Ca2+ selective ion channels of vanilloid receptor subtype-1 (TRPV1) in capsaicin-sensitive dorsal root ganglion (DRG) neurons and TRPV1 transfected Chinese hamster ovarian (CHO) cells are desensitized following calcium-dependent tachyphylaxis induced by successive applications of 100 nM capsaicin. Tachyphylaxis of TRPV1 to 100 nM capsaicin stimuli was not observed in the absence of extracellular calcium. Capsaicin sensitivity of desensitized TRPV1 ion channels recovered on application of phorbol-12-myristate-13-acetate (PMA). PMA-induced recovery of desensitized TRPV1 was primarily due to influx of extracellular calcium observed during re-application of capsaicin following desensitization. Capsazepine blocked the re-sensitization to capsaicin by PMA. Protein kinase C (PKC) inhibitory peptide PKC fragment 19-36 also inhibited re-sensitization to capsaicin by PMA. Reversal of capsaicin-induced desensitization by PMA was prevented by a mutation of TRPV1 where phosphorylation sites serine502 and serine800 were replaced with alanine. This study provides evidence for a role of PKC in reversing capsaicin-induced calcium-dependent desensitization of TRPV1 ion channels.

Muscle mechanoreflex and metaboreflex responses after myocardial infarction in rats

BACKGROUND

During exercise, the sympathetic nervous system is activated and blood pressure and heart rate increase. In heart failure (HF), the muscle metaboreceptor contribution to sympathetic outflow is attenuated and the mechanoreceptor contribution is accentuated. Previous studies suggest that (1) capsaicin stimulates muscle metabosensitive vanilloid receptor subtype 1 (VR1), inducing a neurally mediated pressor response, and (2) activation of ATP-sensitive P2X receptors enhances the pressor response seen when muscle mechanoreceptors are engaged by muscle stretch. Thus, we hypothesized that the pressor response to VR1 stimulation would be smaller and the sensitizing effects of P2X stimulation greater in rats with HF due to chronic myocardial infarction (MI) than in controls.

METHODS AND RESULTS

Eight to 14 weeks after coronary ligation, rats with infarcts >35% had an increased left ventricular end-diastolic pressure and a marked increase in heart weight. Capsaicin injected into the arterial supply of the hindlimb increased blood pressure by 39% (baseline, 93.9+/-9.5 mm Hg) in control animals but only by 8% (baseline, 94.8+/-10.1 mm Hg) in rats with large MIs (P<0.05). P2X receptor stimulation by alpha,beta-methylene ATP enhanced the pressor response to muscle stretch by 42% in control animals and by 72% in rats with large MIs (P<0.05).

CONCLUSIONS

Compared with control animals, cardiovascular responses to VR1 stimulation are blunted and P2X-mediated responses are augmented in rats with HF owing to large MIs.

Cloning and pharmacological characterization of mouse TRPV1

The Transient Receptor Potential cation channel V1 (TRPV1) is expressed in peripheral nociceptive neurons and is subject to polymodal activation via various agents including capsaicin, noxious heat, low extracellular pH, and direct phosphorylation by protein kinase C (PKC). We have cloned and heterologously expressed mouse TRPV1 (mTRPV1) and characterized its function utilizing FLIPR-based calcium imaging to measure functional responses to various small molecule agonists, low pH and direct phosphorylation via PKC. The various TRPV1 agonists activated mTRPV1 with a rank order of agonist potency of (resiniferatoxin (RTX) = arvanil > capsaicin = olvanil > OLDA > PPAHV) (EC50 values of 0.15+/-0.04 nM, 0.27+/-0.07 nM, 9.1+/-1.2 nM, 3.7+/-0.3 nM, 258+/-105 nM, and 667+/-151 nM, respectively). Additionally, mTRPV1 was activated by either low pH or with addition of the PKC activator phorbol 12-myristate 13-acetate (PMA). The TRPV1 antagonists iodinated-resiniferatoxin (I-RTX) or BCTC were both able to block capsaicin, pH and PKC-induced responses of mTRPV1 (IC50 (I-RTX) = 0.35+/-0.12 nM, 1.9+/-0.7 nM, and 0.80+/-0.68 nM, IC50 (BCTC) = 1.3+/-0.36 nM, 0.59+/-0.16 nM, and 0.37+/-0.15 nM, respectively). However, the antagonist capsazepine was only able to inhibit a capsaicin-evoked response of mTRPV1 with an IC50 of 1426+/-316 nM. Comparable results were achieved with rat TRPV1, while capsazepine blocked all modes of human TRPV1 activation. Thus, the mTRPV1 cation channel has a molecular pharmacological profile more akin to rat TRPV1 than either human or guinea pig TRPV1 and the molecular pharmacology suggests that capsazepine may be an ineffective TRPV1 antagonist for in vivo models of inflammatory pain in the mouse.

Early painful diabetic neuropathy is associated with differential changes in the expression and function of vanilloid receptor 1

Diabetes mellitus is associated with one or more kinds of stimulus-evoked pain including hyperalgesia and allodynia. The mechanisms underlying painful diabetic neuropathy remain poorly understood. Previous studies demonstrate an important role of vanilloid receptor 1 (VR1) in inflammation and injury-induced pain. Here we investigated the function and expression of VR1 in dorsal root ganglion (DRG) neurons isolated from streptozotocin-induced diabetic rats between 4 and 8 weeks after onset of diabetes. DRG neurons from diabetic rats showed significant increases in capsaicin- and proton-activated inward currents. These evoked currents were completely blocked by the capsaicin antagonist capsazepine. Capsaicin-induced desensitization of VR1 was down-regulated, whereas VR1 re-sensitization was up-regulated in DRG neurons from diabetic rats. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate blunted VR1 desensitization, and this effect was reversible in the presence of the PKC inhibitor bisindolylmaleimide I. Compared with the controls, VR1 protein was decreased in DRG whole-cell homogenates from diabetic rats, but increased levels of VR1 protein were observed on plasma membranes. Of interest, the tetrameric form of VR1 increased significantly in DRGs from diabetic rats. Increased phosphorylation levels of VR1 were also observed in DRG neurons from diabetic rats. Colocalization studies demonstrated that VR1 expression was increased in large myelinated A-fiber DRG neurons, whereas it was decreased in small unmyelinated C-fiber neurons as a result of diabetes. These results suggest that painful diabetic neuropathy is associated with altered cell-specific expression of the VR1 receptor that is coupled to increased function through PKC-mediated phosphorylation, oligomerization, and targeted expression on the cell surface membrane.

TRPV1 and the gut: from a tasty receptor for a painful vanilloid to a key player in hyperalgesia

Capsaicin, the pungent ingredient in red pepper, has been used since ancient times as a spice, despite the burning sensation associated with its intake. More than 50 years ago, Nikolaus Jancso discovered that capsaicin can selectively stimulate nociceptive primary afferent neurons. The ensuing research established that the neuropharmacological properties of capsaicin are due to its activation of the transient receptor potential ion channel of the vanilloid type 1 (TRPV1). Expressed by primary afferent neurons innervating the gut and other organs, TRPV1 is gated not only by vanilloids such as capsaicin, but also by noxious heat, acidosis and intracellular lipid mediators such as anandamide and lipoxygenase products. Importantly, TRPV1 can be sensitized by acidosis and activation of various pro-algesic pathways. Upregulation of TRPV1 in inflammatory bowel disease and the beneficial effect of TRPV1 downregulation in functional dyspepsia and irritable bladder make this polymodal nociceptor an attractive target of novel therapies for chronic abdominal pain.

Inhibitory modulation of distal C-terminal on protein kinase C-dependent phospho-regulation of rat TRPV1 receptors

The vanilloid receptor TRPV1, previously known as VR1, has been implicated in pain sensation under both physiological and pathological conditions. The channel is highly expressed in sensory ganglion neurones and is activated by a range of noxious stimuli including irritant chemicals, acids and heat. In order to understand the structural basis underlying this polymodal activation and the regulation by intracellular signalling pathways, we have investigated the functional roles of the cytoplasmic C-terminal of rat TRPV1. A mutant with the maximal truncation of the distal C-terminal encompassing the last 88 residues was constructed. Of interest, this mutant exhibited a Ca(2+)-dependent functional loss; it was irresponsive to capsaicin in the presence of extracellular Ca(2+), but fully functional otherwise. Further studies of this construct revealed that extracellular Ca(2+) alone could activate the channel, and that the activation required protein kinase C (PKC) phosphorylation at S502, an event that was up-regulated by external Ca(2+) entry. We compared the truncation mutant with wild-type TRPV1 and demonstrated that it had a significantly increased sensitivity to PKC phosphorylation. These results suggest the distal C-terminal of TRPV1 can inhibit phosphorylation-induced potentiation of the wild-type channel. They also call into question some established functions of the distal C-terminal of TRPV1, including its roles in agonist binding and functional desensitization. We suggest that the functional loss of the truncation mutant, in the presence of extracellular Ca(2+), was not due to disruption of agonist binding or gating, but rather to desensitization promoted by unstimulated extracellular Ca(2+) entry.

TRPV1 and CB(1) receptor-mediated effects of the endovanilloid/endocannabinoid N-arachidonoyl-dopamine on primary afferent fibre and spinal cord neuronal responses in the rat

N-arachidonoyl-dopamine (NADA) is an endogenous ligand at TRPV1 and CB(1) receptors, which are expressed on primary afferent nociceptors. The aim of this study was to determine contributions of proposed pronociceptive TRPV1 and antinociceptive CB(1) receptors to effects of peripheral NADA on primary afferent fibre function. Effects of NADA on primary afferent nociceptor function, determined by whole cell patch clamp and calcium imaging studies of adult dorsal root ganglion (DRG) neurons, were determined. Application of NADA (1 microm) to DRG neurons depolarized the resting membrane potential (Vm) from -58 +/- 1 to -44 +/- 3 mV (P < 0.00001) and evoked a significant increase (P < 0.0001) in intracellular calcium (74 +/- 11% of response to 60 mm KCl), compared to basal. The TRPV1 receptor antagonist capsazepine abolished NADA-evoked depolarization of Vm (P < 0.0001) and NADA-evoked calcium responses (P < 0.001), which were also blocked by the CB(1) receptor antagonist SR141716A (P < 0.001). Effects of NADA (1.5 microg and 5 microg/50 microL) on mechanically evoked responses of dorsal horn neurons in anaesthetized Sprague-Dawley rats were studied. Intraplantar injection of the higher dose of NADA (5 microg/50 microL) studied significantly inhibited innocuous (8, 10 g) mechanically evoked responses of dorsal horn neurons compared to vehicle, effects blocked by intraplantar injection of SR141716A. Higher weight (26-100 g) noxious-evoked responses of dorsal horn neurons were also significantly inhibited by NADA (5 microg/50 microL), effects blocked by intraplantar injection of the TRPV1 antagonist, iodo-resiniferatoxin. NADA has a complex pattern of effects on DRG neurons and primary afferent fibres, which is likely to reflect its dual site of action at TRPV1 and CB(1) receptors and the differential expression of these receptors by primary afferent fibres.

Protease-activated receptor 2 sensitizes the capsaicin receptor transient receptor potential vanilloid receptor 1 to induce hyperalgesia

Inflammatory proteases (mast cell tryptase and trypsins) cleave protease-activated receptor 2 (PAR2) on spinal afferent neurons and cause persistent inflammation and hyperalgesia by unknown mechanisms. We determined whether transient receptor potential vanilloid receptor 1 (TRPV1), a cation channel activated by capsaicin, protons, and noxious heat, mediates PAR2-induced hyperalgesia. PAR2 was coexpressed with TRPV1 in small- to medium-diameter neurons of the dorsal root ganglia (DRG), as determined by immunofluorescence. PAR2 agonists increased intracellular [Ca2+] ([Ca2+]i) in these neurons in culture, and PAR2-responsive neurons also responded to the TRPV1 agonist capsaicin, confirming coexpression of PAR2 and TRPV1. PAR2 agonists potentiated capsaicin-induced increases in [Ca2+]i in TRPV1-transfected human embryonic kidney (HEK) cells and DRG neurons and potentiated capsaicin-induced currents in DRG neurons. Inhibitors of phospholipase C and protein kinase C (PKC) suppressed PAR2-induced sensitization of TRPV1-mediated changes in [Ca2+]i and TRPV1 currents. Activation of PAR2 or PKC induced phosphorylation of TRPV1 in HEK cells, suggesting a direct regulation of the channel. Intraplantar injection of a PAR2 agonist caused persistent thermal hyperalgesia that was prevented by antagonism or deletion of TRPV1. Coinjection of nonhyperalgesic doses of PAR2 agonist and capsaicin induced hyperalgesia that was inhibited by deletion of TRPV1 or antagonism of PKC. PAR2 activation also potentiated capsaicin-induced release of substance P and calcitonin gene-related peptide from superfused segments of the dorsal horn of the spinal cord, where they mediate hyperalgesia. We have identified a novel mechanism by which proteases that activate PAR2 sensitize TRPV1 through PKC. Antagonism of PAR2, TRPV1, or PKC may abrogate protease-induced thermal hyperalgesia.

Nociceptin inhibits vanilloid TRPV-1-mediated neurosensitization induced by fenoterol in human isolated bronchi

Chronic exposure to beta(2)-adrenoceptor agonists, especially fenoterol, has been shown to increase smooth muscle contraction to endothelin-1 in human bronchi partly through tachykinin-mediated pathways. The purpose of this work was to further investigate the role of sensory nerves in fenoterol-induced sensitization of human airways and the effect of nociceptin, a nociceptin/orphanin FQ (NOP) receptor agonist, on the increase in contraction after fenoterol exposure. Human bronchi from 62 patients were sensitized to endothelin-1 by prolonged incubation with fenoterol (0.1 microM, 15 h). The sensitizing effect of fenoterol was inhibited by high concentration of capsaicin (10 microM, 30 min before fenoterol sensitization), which induces depletion of mediators from sensory nerves, or co-incubation of fenoterol and capsazepine (1 microM), a vanilloid TRPV-1 receptor antagonist. Moreover, short pretreatment of bronchi with capsaicin (10 microM) or capsazepine (1 microM) after sensitization by fenoterol decreased the rise in smooth muscle contraction to endothelin-1. Nociceptin (1 microM) also inhibited the increased contraction in fenoterol-sensitized bronchi. Tertiapin (10 microM), an inhibitor of the inward-rectifier K(+) channels, but not naloxone (0.1 microM), a DOP/KOP/MOP receptor antagonist, prevented the inhibitory effect of nociceptin. In conclusion, fenoterol induces sensitization of human isolated bronchi to endothelin-1 in part through the stimulation of the vanilloid TRPV-1 receptor on tachykininergic sensory nerves. Nociceptin inhibits airway hyperresponsiveness via NOP receptor activation. This effect involves inward-rectifier K(+) channels.

Functional bradykinin B1 receptors are expressed in nociceptive neurones and are upregulated by the neurotrophin GDNF

Bradykinin (BK) has long been recognized as an important mediator of pain and inflammation. In normal tissue bradykinin causes an acute sensation of pain by an action at B2 receptors, but in inflamed tissue the pharmacology of the response changes to that of B1 receptors. Attempts to demonstrate the presence of functional B1 receptors in sensory neurones have failed, however, and the actions of B1 agonists have therefore been presumed to be indirect. Here we show that specific B1 receptor activation causes translocation of the epsilon isoform of protein kinase C (PKC-epsilon) to the membrane of a small fraction of freshly isolated sensory neurones from rats and mice. The proportion of neurones in which PKC-epsilon translocation was observed increased to around 20% of neurones after 3 days in culture with the neurotrophins glial cell line derived neurotrophic factor (GDNF) and neurturin, but not with nerve growth factor (NGF). Using in situ hybridization we found that the proportion of neurones expressing B1 mRNA increased from close to zero to 20.4% after 8 h culture in GDNF. Neurones expressing functional B1 receptors were negative for the neuropeptides CGRP and substance P, but most expressed functional TRPV1 receptors for capsaicin (60%) and bound the lectin IB4 (68%), both markers characteristic of nociceptors. B1 activation enhanced the heat-activated membrane current approximately 3-fold, and the enhancement was much more prolonged than was the case with B2 activation, consistent with a role for B1 receptors in sustained pain. We conclude that GDNF and neurturin potently upregulate functional B1 receptor expression in small non-peptidergic nociceptive neurones.

Modulation of TRPV1 by nonreceptor tyrosine kinase, c-Src kinase

The capsaicin receptor TRPV1 is a nonselective cation channel that is expressed in sensory neurons. In this study, we examined the role of the nonreceptor cellular tyrosine kinase c-Src kinase in the modulation of the rat TRPV1. Capsaicin-induced currents in identified colonic dorsal root ganglion neurons were blocked by the c-Src kinase inhibitor PP2 and enhanced by the tyrosine phosphatase inhibitor sodium orthovandate. PP2 also abolished currents in human embryonic kidney-293 cells transfected with rat TRPV1, whereas cotransfection of TRPV1 with v-Src resulted in fivefold increase in capsaicin-induced currents. In cells transfected with dominant-negative c-Src and TRPV1, capsaicin-induced currents were decreased by approximately fourfold. TRPV1 co-immunoprecipitated with Src kinase and was tyrosine phosphorylated. These studies demonstrate that TRPV1 is a potential target for cellular tyrosine kinase-dependent phosphorylation.

TRPV1 acts as proton channel to induce acidification in nociceptive neurons

The low extracellular pH of inflamed or ischemic tissues enhances painful sensations by sensitizing and activating the vanilloid receptor 1 (TRPV1). We report here that activation of TRPV1 results in a marked intracellular acidification in nociceptive dorsal root ganglion neurons and in a heterologous expression system. A characterization of the underlying mechanisms revealed a Ca(2+)-dependent intracellular acidification operating at neutral pH and an additional as yet unrecognized direct proton conductance through the poorly selective TRPV1 pore operating in acidic extracellular media. Large organic cations permeate through the activated TRPV1 pore even in the presence of physiological concentrations of Na(+), Mg(2+), and Ca(2+). The wide pore and the unexpectedly high proton permeability of TRPV1 point to a proton hopping permeation mechanism along the water-filled channel pore. In acidic media, the high relative proton permeability through TRPV1 defines a novel proton entry mechanism in nociceptive neurons.

Enhancement of potency and efficacy of NADA by PKC-mediated phosphorylation of vanilloid receptor

The search for an endogenous ligand for the vanilloid receptor (VR or TRPV1) has led to the identification of N-arachidonyl dopamine (NADA). This study investigates the role of protein kinase C (PKC)-mediated phosphorylation on NADA-induced membrane currents in Xenopus oocytes heterologously expressing TRPV1 and in dorsal root ganglion (DRG) neurons. In basal state, current induced by 10 microM NADA is 5-10% of the current induced by 1 microM capsaicin or protons at pH 5. However, PKC activator, phorbol 12,13-dibutyrate (PDBu) strongly potentiated ( approximately 15-fold) the NADA-induced current. Repeated application of NADA at short intervals potentiated its own response approximately fivefold in a PKC-dependent manner. PKC inhibitor, bisindolylmaleimide (BIM, 500 nM), a mutant TRPV1 (S800A/S502A), and maximal activation of PKC abolished the potentiation induced by repeated application of NADA. As a further confirmation that NADA could stimulate PKC, pretreatment with NADA potentiated the response of protons at pH 5 (approximately 20 fold), which was dramatically reduced in the mutant TRPV1. In DRG neurons, capsaicin (100 nM) induced a approximately 15 mV depolarization and initiated a train of action potentials compared with 1 microM NADA that produced a approximately 5 mV response. Pretreatment with PDBu induced significantly larger depolarization and potentiated NADA-induced current. Furthermore, exposure of NADA to the intracellular surface of the membrane-induced larger currents suggesting inaccessibility to the intracellular binding site might contribute to its weaker action. These results indicate that NADA is a potent agonist of VR when the receptor is in the PKC-mediated phosphorylation state.

Identification and characterisation of SB-366791, a potent and selective vanilloid receptor (VR1/TRPV1) antagonist

Vanilloid receptor-1 (TRPV1) is a non-selective cation channel, predominantly expressed by peripheral sensory neurones, which is known to play a key role in the detection of noxious painful stimuli, such as capsaicin, acid and heat. To date, a number of antagonists have been used to study the physiological role of TRPV1; however, antagonists such as capsazepine are somewhat compromised by non-selective actions at other receptors and apparent modality-specific properties. SB-366791 is a novel, potent, and selective, cinnamide TRPV1 antagonist isolated via high-throughput screening of a large chemical library. In a FLIPR-based Ca(2+)-assay, SB-366791 produced a concentration-dependent inhibition of the response to capsaicin with an apparent pK(b) of 7.74 +/- 0.08. Schild analysis indicated a competitive mechanism of action with a pA2 of 7.71. In electrophysiological experiments, SB-366791 was demonstrated to be an effective antagonist of hTRPV1 when activated by different modalities, such as capsaicin, acid or noxious heat (50 degrees C). Unlike capsazepine, SB-366791 was also an effective antagonist vs. the acid-mediated activation of rTRPV1. With the aim of defining a useful tool compound, we also profiled SB-366791 in a wide range of selectivity assays. SB-366791 had a good selectivity profile exhibiting little or no effect in a panel of 47 binding assays (containing a wide range of G-protein-coupled receptors and ion channels) and a number of electrophysiological assays including hippocampal synaptic transmission and action potential firing of locus coeruleus or dorsal raphe neurones. Furthermore, unlike capsazepine, SB-366791 had no effect on either the hyperpolarisation-activated current (I(h)) or Voltage-gated Ca(2+)-channels (VGCC) in cultured rodent sensory neurones. In summary, SB-366791 is a new TRPV1 antagonist with high potency and an improved selectivity profile with respect to other commonly used TRPV1 antagonists. SB-366791 may therefore prove to be a useful tool to further study the biology of TRPV1.

Regulated exocytosis contributes to protein kinase C potentiation of vanilloid receptor activity

The vanilloid receptor-1 (TRPV1) plays a key role in the perception of peripheral thermal and inflammatory pain. TRPV1 expression and channel activity are notably up-regulated by proalgesic agents. The transduction pathways involved in TRPV1 sensitization are still elusive. We have used a yeast two-hybrid screen to identify proteins that associate with the N terminus of TRPV1. We report that two vesicular proteins, Snapin and synaptotagmin IX (Syt IX), strongly interact in vitro and in vivo with the TRPV1 N-terminal domain. In primary dorsal root ganglion neurons, TRPV1 co-distributes in vesicles with Syt IX and the vesicular protein synaptobrevin. Neither Snapin nor Syt IX affected channel function, but they notably inhibited protein kinase C (PKC)-induced potentiation of TRPV1 channel activity with a potency that rivaled the blockade evoked by botulinum neurotoxin A, a potent blocker of neuronal exocytosis. Noteworthily, we found that PKC activation induced a rapid delivery of functional TRPV1 channels to the plasma membrane. Botulinum neurotoxin A blocked the TRPV1 membrane translocation induced by PKC that was activated with a phorbol ester or the metabotropic glutamate receptor mGluR5. Therefore, our results indicate that PKC signaling promotes at least in part the SNARE-dependent exocytosis of TRPV1 to the cell surface. Taken together, these findings imply that activity-dependent delivery of channels to the neuronal surface may contribute to the buildup and maintenance of thermal inflammatory hyperalgesia in peripheral nociceptor terminals.

Activation and sensitisation of the vanilloid receptor: role in gastrointestinal inflammation and function

The exquisite specific excitatory and desensitising actions of capsaicin on a subpopulation of primary sensory neurons have been instrumental in identifying the roles of these neurons in nociception, reflex responses and neurogenic inflammation. Structure activity studies with capsaicin-like molecules have suggested that a "receptor" should mediate the effects of capsaicin on sensory neurons. The cloning of the vanilloid receptor-1 (VR1) has confirmed this hypothesis. VR1 (TRPV1) belongs to the transient receptor potential (TRP) family of channels, and its activation by various xenobiotics, noxious temperature, extracellular low pH and high concentration of certain lipid derivatives results in cation influx and sensory nerve terminal excitation. TRPV1 may dimerise or form tetramers or heteromers with PLC-gamma and TrkA or even with other TRPs. TRPV1 is markedly upregulated and/or "sensitised" under inflammatory conditions via protein kinase C-epsilon-, cAMP-dependent PK- and PLC-gamma-dependent pathways or by exposure to dietary agents as ethanol. TRPV1 is expressed on sensory neurons distributed in all the regions of the gastrointestinal tract in myenteric ganglia, muscle layer and mucosa. There is evidence of TRPV1 expression also in epithelial cells of the gastrointestinal tract. High expression of TRPV1 has been detected in several inflammatory diseases of the colon and ileum, whereas neuropeptides released upon sensory nerve stimulation triggered by TRPV1 activation seem to play a role in intestinal motility disorders. TRPV1 antagonists, which will soon be available for clinical testing, may undergo scrutiny for the treatment of inflammatory diseases of the gut.

Sensitisation of gastrointestinal tract afferents

Sensory innervation of the viscera serves a number of important functions, including regulation of visceral motility and secretory activity, and transmission of visceral sensations, including pain. There are many ways in which the sensitivity of visceral sensory neurones might be modulated, and these are discussed. Altered sensory neurone responsiveness may contribute to pathophysiological states such as irritable bowel syndrome, and the mechanisms leading to sensory neurone sensitisation offer novel targets for the treatment of such disorders.