Potentiation of capsaicin receptor activity by metabotropic ATP receptors as a possible mechanism for ATP-evoked pain and hyperalgesia

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Sustained depolarization-induced propagation of [Ca2+]i oscillations in cultured DRG neurons: the involvement of extracellular ATP and P2Y receptor activation

Recently emerging evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the ganglia. In the present study, we characterized the nonsynaptic chemical coupling mediated by extracellular ATP in dorsal root ganglia (DRG) neuron cultures by using the real time imaging of ATP, whole-cell patch clamping, in conjunction with confocal calcium imaging. Sustained depolarization by electrical stimulation evoked intracellular Ca2+ concentrations ([Ca2+]i) oscillations in individual DRG neurons, and subsequent ATP-dependent propagation [Ca2+]i oscillations to surrounding non-stimulated neighbors. [Ca2+]i oscillations were suppressed by inositol-1,4,5-trisphosphate (IP3) receptor antagonist 2-APB, but not ryanodine. The propagation of [Ca2+]i oscillations was prevented by the presence of the ATP-degrading enzyme, apyrase, and completely abolished by the blockase of G protein-coupled purinergic receptors-PLC-IP3 pathway with suramin, U73122 or 2-APB. In parallel, sustained depolarization elicited robust ATP release and diffusion from the stimulation site. Moreover, exogenous application of ATP to DRG cultures in large concentration elicits the [Ca2+]i oscillations in most neurons. Taken together, this data demonstrates that sustained membrane depolarization elicited ATP release, acting through a highly sensitive P2Y receptors/IP3-mediated signaling pathway to mediate the propagation of intercellular Ca2+ signaling, which suggest a novel signaling pathway for neuronal communication in DRG.

Mechanisms of action of intravesical botulinum treatment in refractory detrusor overactivity

Urinary retention is one of a multitude of autonomic deficits resulting from acute botulism (oral botulinum intoxication). The powerful influence of botulinum-A neurotoxin (BoNT-A) on autonomic function has now been harnessed to the benefit of patients with detrusor overactivity (DO), by injecting the agent intramurally, with consequent improvement in urodynamic and clinical variables. Nonetheless, the complexity of bladder cellular physiology and putative mechanisms underlying the pathophysiological basis of DO even now render the precise mechanisms of clinical response to intravesical BoNT-A uncertain. In this review, the processes by which BoNT-A affects nerve function and the state-of-the-art in the physiological understanding of bladder dysfunction are discussed together, conveying how much must be reckoned when attempting to understand the mechanisms by which this powerful agent can improve refractory and bothersome DO.

Transient receptor potential vanilloid channels in hypertension, inflammation, and end organ damage: an imminent target of therapy for cardiovascular disease?

PURPOSE OF REVIEW

The possible role of several neurohormonal factors in pathogenesis of hypertension has been studied extensively both in humans and in experimental animal models. However, controversial data from some previous studies are indecisive and call for reassessment and development of new targets. This mini-review presents some of the most recent findings about the role of transient receptor potential vanilloid type 1 channels in the development of hypertension and its pathology.

RECENT FINDINGS

The transient receptor potential vanilloid type 1, channel activated by novel endovanilloids or altered pH, temperature, and/or local hemodynamics, may serve as a distinct molecular sensor detecting sodium and water balance and may play a role in preventing salt-induced hypertension and tissue damage. Impairment of the function of the transient receptor potential vanilloid type 1 channels may contribute to increased salt sensitivity, inflammation, and end organ damage.

SUMMARY

Emerging evidence indicates that the transient receptor potential vanilloid type 1 channel plays a key role in cardiovascular health and disease by acting as a sensor and regulator of cardiovascular homeostasis and a protector against cardiovascular injury. Given the huge population who suffers from cardiovascular disease, the study of the transient receptor potential vanilloid channels may improve our understanding of pathogenesis of several common cardiovascular disorders and may lead to the development of therapy for hypertension, inflammation, and organ damage.

Protein kinase A anchoring via AKAP150 is essential for TRPV1 modulation by forskolin and prostaglandin E2 in mouse sensory neurons

Phosphorylation-dependent modulation of the vanilloid receptor TRPV1 is one of the key mechanisms mediating the hyperalgesic effects of inflammatory mediators, such as prostaglandin E(2) (PGE(2)). However, little is known about the molecular organization of the TRPV1 phosphorylation complex and specifically about scaffolding proteins that position the protein kinase A (PKA) holoenzyme proximal to TRPV1 for effective and selective regulation of the receptor. Here, we demonstrate the critical role of the A-kinase anchoring protein AKAP150 in PKA-dependent modulation of TRPV1 function in adult mouse dorsal root ganglion (DRG) neurons. We found that AKAP150 is expressed in approximately 80% of TRPV1-positive DRG neurons and is coimmunoprecipitated with the capsaicin receptor. In functional studies, PKA stimulation with forskolin markedly reduced desensitization of TRPV1. This effect was blocked by the PKA selective inhibitors KT5720 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylicacid hexyl ester] and H89 (N-[2-(p-bromo-cinnamylamino)-ethyl]-5-isoquinoline-sulfon-amide 2HCl), as well as by the AKAP inhibitory peptide Ht31. Similarly, PGE(2) decreased TRPV1 desensitization in a manner sensitive to the PKA inhibitor KT5720. Both the forskolin and PGE(2) effects were strongly impaired in DRG neurons from knock-in mice that express a mutant AKAP150 lacking the PKA-binding domain (Delta36 mice). Protein kinase C-dependent sensitization of TRPV1 remained intact in Delta36 mice. The PGE(2)/PKA signaling defect in DRG neurons from Delta36 mice was rescued by overexpressing the full-length human ortholog of AKAP150 in these cells. In behavioral testing, PGE(2)-induced thermal hyperalgesia was significantly diminished in Delta36 mice. Together, these data suggest that PKA anchoring by AKAP150 is essential for the enhancement of TRPV1 function by activation of the PGE(2)/PKA signaling pathway.

Phospholipase C mediated modulation of TRPV1 channels

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

Roles of peripheral P2X and P2Y receptors in the development of melittin-induced nociception and hypersensitivity

A recent report from our laboratory shows that subcutaneous (s.c.) injection of melittin could induce persistent spontaneous nociception (PSN) and primary thermal or mechanical hyperalgesia. However, the exact peripheral mechanisms underlying melittin-induced multiple pain-related behaviors remain unclear. In this study, behavioral tests combined with pharmacological manipulations were used to explore potential roles of local P2X and P2Y receptors in melittin-induced inflammatory pain and hyperalgesia. Post-treatment of the primary injury site with s.c. injection of A-317491 (a potent P2X(3)/P2X(2/3) receptor antagonist) and Reactive Blue 2 (a potent P2Y receptor antagonist) could significantly suppress the development of melittin-evoked PSN and hypersensitivity (thermal and mechanical). Our control experiments demonstrated that local administration of either antagonist into the contralateral hindpaw produced no significant effect on any kind of pain-associated behaviors. Taken together, these data indicate that activation of P2X and P2Y receptors might be essential to the maintenance of melittin-induced primary thermal and mechanical hyperalgesia as well as on-going pain.

P2Y12 receptor upregulation in activated microglia is a gateway of p38 signaling and neuropathic pain

Microglia in the spinal cord may play an important role in the development and maintenance of neuropathic pain. A metabotropic ATP receptor, P2Y(12), has been shown to be expressed in spinal microglia constitutively and be involved in chemotaxis. Activation of p38 mitogen-activated protein kinase (MAPK) occurs in spinal microglia after nerve injury and may be related to the production of cytokines and other mediators, resulting in neuropathic pain. However, it remains unknown whether any type of P2Y receptor in microglia is involved in the activation of p38 MAPK and the pain behaviors after nerve injury. Using the partial sciatic nerve ligation (PSNL) model in the rat, we found that P2Y(12) mRNA and protein increased in the spinal cord and peaked at 3 d after PSNL. Double labeling studies revealed that cells expressing increased P2Y(12) mRNA and protein after nerve injury were exclusively microglia. Both pharmacological blockades by intrathecal administration of P2Y(12) antagonist and antisense knockdown of P2Y(12) expression suppressed the development of pain behaviors and the phosphorylation of p38 MAPK in spinal microglia after PSNL. The intrathecal infusion of the P2Y(12) agonist 2-(methythio) adenosine 5'-diphosphate trisodium salt into naive rats mimicked the nerve injury-induced activation of p38 in microglia and elevated pain behaviors. These data suggest a new mechanism of neuropathic pain, in which the increased P2Y(12) works as a gateway of the following events in microglia after nerve injury. Activation of this receptor by released ATP or the hydrolyzed products activate p38 MAPK pathway and may play a crucial role in the generation of neuropathic pain.

Influence of TRPV1 on diabetes-induced alterations in thermal pain sensitivity

A common complication associated with diabetes is painful or painless diabetic peripheral neuropathy (DPN). The mechanisms and determinants responsible for these peripheral neuropathies are poorly understood. Using both streptozotocin (STZ)-induced and transgene-mediated murine models of type 1 diabetes (T1D), we demonstrate that Transient Receptor Potential Vanilloid 1 (TRPV1) expression varies with the neuropathic phenotype. We have found that both STZ- and transgene-mediated T1D are associated with two distinct phases of thermal pain sensitivity that parallel changes in TRPV1 as determined by paw withdrawal latency (PWL). An early phase of hyperalgesia and a late phase of hypoalgesia are evident. TRPV1-mediated whole cell currents are larger and smaller in dorsal root ganglion (DRG) neurons collected from hyperalgesic and hypoalgesic mice. Resiniferatoxin (RTX) binding, a measure of TRPV1 expression is increased and decreased in DRG and paw skin of hyperalgesic and hypoalgesic mice, respectively. Immunohistochemical labeling of spinal cord lamina I and II, dorsal root ganglion (DRG), and paw skin from hyperalgesic and hypoalgesic mice reveal increased and decreased TRPV1 expression, respectively. A role for TRPV1 in thermal DPN is further suggested by the failure of STZ treatment to influence thermal nociception in TRPV1 deficient mice. These findings demonstrate that altered TRPV1 expression and function contribute to diabetes-induced changes in thermal perception.

TRPV1 receptor signaling mediates afferent nerve sensitization during colitis-induced motility disorders in rats

Rats with experimental colitis suffer from impaired gastric emptying (GE). We previously showed that this phenomenon involves afferent neurons within the pelvic nerve. In this study, we aimed to identify the mediators involved in this afferent hyperactivation. Colitis was induced by trinitrobenzene sulfate (TNBS) instillation. We determined GE, distal front, and geometric center (GC) of intestinal transit 30 min after intragastric administration of a semiliquid Evans blue solution. We evaluated the effects of the transient receptor potential vanilloid type 1 (TRPV1) antagonists capsazepine (5-10 mg/kg) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropyrazine-1(2H)carboxamide (BCTC; 1-10 mg/kg) and the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP-(8-37) (150 microg/kg). To determine TRPV1 receptor antagonist sensitivity, we examined their effect on capsaicin-induced relaxations of isolated gastric fundus muscle strips. Immunocytochemical staining of TRPV1 and RT-PCR analysis of TRPV1 mRNA were performed in dorsal root ganglion (DRG) L6-S1. TNBS-induced colitis reduced GE but had no effect on intestinal motility. Capsazepine reduced GE in controls but had no effect in rats with colitis. At doses that had no effects in controls, BCTC and CGRP-(8-37) significantly improved colitis-induced gastroparesis. Capsazepine inhibited capsaicin-induced relaxations by 35% whereas BCTC completely abolished them. TNBS-induced colitis increased TRPV1-like immunoreactivity and TRPV1 mRNA content in pelvic afferent neuronal cell bodies in DRG L6-S1. In conclusion, distal colitis in rats impairs GE via sensitized pelvic afferent neurons. We provided pharmacological, immunocytochemical, and molecular biological evidence that this sensitization is mediated by TRPV1 receptors and involves CGRP release.

Painful purinergic receptors

Multiple P2 receptor-mediated mechanisms exist by which ATP can alter nociceptive sensitivity following tissue injury. Evidence from a variety of experimental strategies, including genetic disruption studies and the development of selective antagonists, has indicated that the activation of P2X receptor subtypes, including P2X(3), P2X(2/3), P2X(4) and P2X(7), and P2Y (e.g., P2Y(2)) receptors, can modulate pain. For example, administration of a selective P2X(3) antagonist, A-317491, has been shown to effectively block both hyperalgesia and allodynia in different animal models of pathological pain. Intrathecally delivered antisense oligonucleotides targeting P2X(4) receptors decrease tactile allodynia following nerve injury. Selective antagonists for the P2X(7) receptor also reduce sensitization in animal models of inflammatory and neuropathic pain, providing evidence that purinergic glial-neural interactions are important modulators of noxious sensory neurotransmission. Furthermore, activation of P2Y(2) receptors leads to sensitization of polymodal transient receptor potential-1 receptors. Thus, ATP acting at multiple purinergic receptors, either directly on neurons (e.g., P2X(3), P2X(2/3), and P2Y receptors) or indirectly through neural-glial cell interactions (P2X(4) and P2X(7) receptors), alters nociceptive sensitivity. The development of selective antagonists for some of these P2 receptors has greatly aided investigations into the nociceptive role of ATP. This perspective highlights some of the recent advances to identify selective P2 receptor ligands, which has enhanced the investigation of ATP-related modulation of pain sensitivity.

Endothelin potentiates TRPV1 via ETA receptor-mediated activation of protein kinase C

Background

Endothelin-1 (ET-1) both stimulates nociceptors and sensitizes them to noxious stimuli, an effect probably mediated by the ET_A receptor (ET_AR) expressed in sensory neurons. The cellular mechanisms of this ET-1-mediated effect are only poorly understood. TRPV1, the heat-, pH- and capsaicin-sensitive cation channel already known to be modulated by a number of cellular mediators released in response to noxious stimuli and during inflammation, is a potential target for the action of ET-1.

Results

We studied the effects of ET-1 on TRPV1 in sensory neurons from the dorsal root ganglion (DRG) and in HEK293 cells coexpressing TRPV1 and the ET_AR. Specific ¹²⁵I-ET-1 binding sites (817 ± 92 fmol/mg) were detected in membrane preparations of DRG with an ET_AR/ET_BR ratio of 60:40. In an immunofluorescence analysis, coexpression of TRPV1 and the ET_AR was found in a subpopulation of primary sensory neurons. ET-1 strongly potentiated capsaicin-induced TRPV1 currents in some neurons, and in HEK293 cells co-expressing TRPV1 and the ET_AR. Weaker potentiation was observed in HEK293 cells coexpressing TRPV1 and the ET_BR. ET_AR activation also increased responses to low pH and heat. In HEK293 cells, strong potentiation of TRPV1 like that induced by ET-1 via the ET_AR could be induced by PKC activation, but not with activators of the adenylyl cyclase or the PKA pathway. Furthermore, inhibition of PKC with bisindolylmaleimide X (BIM X) or mutation of the PKC phosphorylation site S800 completely prevented ET_AR-mediated potentiation.

Conclusion

We conclude that ET-1 potentiates TRPV1 by a PKC-dependent mechanism and that this could play a major role in the algogenic and hyperalgesic effects of ET-1 described in previous studies.

The complications of promiscuity: endocannabinoid action and metabolism

In this review, we present our understanding of the action and metabolism of endocannabinoids and related endogenous molecules. It is clear that the interactions between the multiple endocannabinoid-like molecules (ECLs) are highly complex, both at the level of signal transduction and metabolism. Thus, ECLs are a group of ligands active at 7-transmembrane and nuclear receptors, as well as transmitter-gated and ion channels. ECLs and their metabolites can converge on common endpoints (either metabolic or signalling) through contradictory or reinforcing pathways. We highlight the complexity of the endocannabinoid system, based on the promiscuous nature of ECLs and their metabolites, as well as the synthetic modulators of the endocannabinoid system.

TRPV1 antagonists as potential antitussive agents

Cough is an important defensive pulmonary reflex that removes irritants, fluids, or foreign materials from the airways. However, when cough is exceptionally intense or when it is chronic and/or nonproductive it may require pharmacologic suppression. For many patients, antitussive therapies consist of OTC products with inconsequential efficacies. On the other hand, the prescription antitussive market is dominated by older opioid drugs such as codeine. Unfortunately, "codeine-like" drugs suppress cough at equivalent doses that also often produce significant ancillary liabilities such as GI constipation, sedation, and respiratory depression. Thus, the discovery of a novel and effective antitussive drug with an improved side effect profile relative to codeine would fulfill an unmet clinical need in the treatment of cough. Afferent pulmonary nerves are endowed with a multitude of potential receptor targets, including TRPV1, that could act to attenuate cough. The evidence linking TRPV1 to cough is convincing. TRPV1 receptors are found on sensory respiratory nerves that are important in the generation of the cough reflex. Isolated pulmonary vagal afferent nerves are responsive to TRPV1 stimulation. In vivo, TRPV1 agonists such as capsaicin elicit cough when aerosolized and delivered to the lungs. Pertinent to the debate on the potential use of TRPV1 antagonist as antitussive agents are the observations that airway afferent nerves become hypersensitive in diseased and inflamed lungs. For example, the sensitivity of capsaicin-induced cough responses following upper respiratory tract infection and in airway inflammatory diseases such as asthma and COPD is increased relative to that of control responses. Indeed, we have demonstrated that TRPV1 antagonism can attenuate antigen-induced cough in the allergic guinea pig. However, it remains to be determined if the emerging pharmacologic profile of TRPV1 antagonists will translate into a novel human antitussive drug. Current efforts in clinical validation of TRPV1 antagonists revolve around various pain indications; therefore, clinical evaluation of TRPV1 antagonists as antitussive agents will have to await those outcomes.

Facial pain induces the alteration of transient receptor potential vanilloid receptor 1 expression in rat trigeminal ganglion

OBJECTIVE

To investigate the involvement of transient receptor potential vanilloid receptor 1 (TRPV1) in the facial inflammatory pain in relation to thermal hyperalgesia and cold pain sensation.

METHODS

Facial inflammatory pain model was developed by subcutaneous injection of turpentine oil (TO) into rat facial area. Head withdrawal thermal latency (HWTL) and head withdrawal cold latency (HWCL) were measured once a day for 21 d after TO treatment using thermal and cold measurement apparatus. The immunohistochemical staining, cell-size frequency analysis and the survey of average optical density (OD) value were used to observe the changes of TRPV1 expression in the neurons of the trigeminal ganglion (TG), peripheral nerve fibers in the vibrissal pad, and central projection processes in the trigeminal sensory nuclei caudalis (Vc) on day 3, 5, 7, 14, and 21 after TO injection.

RESULTS

HWTL and HWCL decreased significantly from day 1 to day 14 after TO injection with the lowest value on day 5 and day 3, respectively, and both recovered on day 21. The number of TRPV1-labeled neurons increased remarkably from day 1 to day 14 with a peak on day 7, and returned back to the normal level on day 21. In control rats, only small and medium-sized TG neurons were immunoreactive (IR) to TRPV1, and the TRPV1-IR terminals were abundant in both the vibrissal pad and the Vc. Within 2 weeks of inflammation, the expression of TRPV1 in small and medium-sized TG neurons increased obviously. Also the TRPV1 stained terminals and fibers appeared more frequent and denser in both the vibrissal pad skin and throughout laminae I and the outer zone of laminae II (IIo) of Vc.

CONCLUSION

Facial inflammatory pain could induce hyperalgesia to noxious heat and cold stimuli, and result in increase of the numbers of TRPV1 positive TG neurons and the peripheral and central terminals of TG. These results suggest that the phenotypic changes of TRPV1 expression in small and medium-sized TG neurons and terminals might play an important role in the development and maintenance of TO-induced inflammatory thermal hyperalgesia and cold pain sensation.

TRP channels as novel players in the pathogenesis and therapy of itch

Itch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including itch and pain. In particular, we argue that - due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities - certain thermosensitive TRP channels are key players in pruritus pathogenesis. We close by proposing a novel, TRP-centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch.

Dual regulation of TRPV1 by phosphoinositides

The membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2 or PIP2] regulates many ion channels. There are conflicting reports on the effect of PtdIns(4,5)P2 on transient receptor potential vanilloid 1 (TRPV1) channels. We show that in excised patches PtdIns(4,5)P2 and other phosphoinositides activate and the PIP2 scavenger poly-Lys inhibits TRPV1. TRPV1 currents undergo desensitization on exposure to high concentrations of capsaicin in the presence of extracellular Ca2+. We show that in the presence of extracellular Ca2+, capsaicin activates phospholipase C (PLC) in TRPV1-expressing cells, inducing depletion of both PtdIns(4,5)P2 and its precursor PtdIns(4)P (PIP). The PLC inhibitor U73122 and dialysis of PtdIns(4,5)P2 or PtdIns(4)P through the patch pipette inhibited desensitization of TRPV1, indicating that Ca2+-induced activation of PLC contributes to desensitization of TRPV1 by depletion of PtdIns(4,5)P2 and PtdIns(4)P. Selective conversion of PtdIns(4,5)P2 to PtdIns(4)P by a rapamycin-inducible PIP2 5-phosphatase did not inhibit TRPV1 at high capsaicin concentrations, suggesting a significant role for PtdIns(4)P in maintaining channel activity. Currents induced by low concentrations of capsaicin and moderate heat, however, were potentiated by conversion of PtdIns(4,5)P2 to PtdIns(4)P. Increasing PtdIns(4,5)P2 levels by coexpressing phosphatidylinositol-4-phosphate 5-kinase inhibited TRPV1 at low but not at saturating capsaicin concentrations. These data show that at low capsaicin concentrations and other moderate stimuli, PtdIns(4,5)P2 partially inhibits TRPV1 in a cellular context, but this effect is likely to be indirect, because it is not detectable in excised patches. We conclude that phosphoinositides have both inhibitory and activating effects on TRPV1, resulting in complex and distinct regulation at various stimulation levels.

Signaling Pathways in Sensitization: Toward a Nociceptor Cell Biology

The electrophysiological properties of peripheral neurons activated by noxious stimuli, the primary afferent nociceptors, have been investigated intensively, and our knowledge about the molecular basis of transducers for noxious stimuli has increased greatly. In contrast, understanding of the intracellular signaling mechanisms regulating nociceptor sensitization downstream of ligand binding to the receptors is still at a relatively nascent stage. After outlining the initiated signaling cascades, we discuss the emerging plasticity within these cascades and the importance of subcellular compartmentalization. In addition, the recently realized importance of functional interactions with the extracellular matrix, cytoskeleton, intracellular organelles such as mitochondria, and sex hormones will be introduced. This burgeoning literature establishes new cellular features crucial for the function of nociceptive neurons and argues that additional focus should be placed on understanding the complex integration of cellular events that make up the "cell biology of pain."

The uroepithelial-associated sensory web

An important, but not well understood, function of epithelial cells is their ability to sense changes in their extracellular environment and then communicate these changes to the underlying nervous, connective, and muscular tissues. This communication is likely to be important for tube- and sac-shaped organs such as blood vessels, the lungs, the gut, and the bladder, whose normal function can be modulated by stimuli initiated within the epithelium. We propose that the uroepithelium, which lines the renal pelvis, ureters, and inner surface of the bladder, functions as an integral part of a 'sensory web.' Through uroepithelial-associated channels and receptors, the uroepithelium receives sensory 'inputs' such as changes in hydrostatic pressure and binding of mediators including adenosine triphosphate (ATP). These input signals stimulate membrane turnover in the outermost umbrella cell layer and release of sensory 'outputs' from the uroepithelium in the form of neurotransmitters and other mediators that communicate changes in the uroepithelial milieu to the underlying tissues, altering their function. The global consequence of this sensory web is the coordinated function of the bladder during the cycles of filling and voiding, and disruption of this web is likely to lead to bladder dysfunction.

Differential involvement of TRPV1 receptors at the central and peripheral nerves in CFA-induced mechanical and thermal hyperalgesia

Transient receptor potential vanilloid 1 (TRPV1) antagonists are known to attenuate two typical symptoms of inflammatory hyperalgesia: thermal and mechanical. However, it is not clear whether the sites of participation of TRPV1 for each symptom are different. In this study, we clarified the difference between the site of TRPV1 involvement in both symptoms by analysing the anti-hyperalgesic activity of two kinds of TRPV1 antagonists given locally (i.e. intraplantarly and intrathecally) in rats with CFA (complete Freund's adjuvant)-induced inflammation. TRPV1 antagonists BCTC (N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide, 1-300 microg) and SB-366791 (N-(3-methoxyphenyl)-4-chlorocinnamide, 30-300 microg) administered intraplantarly in a dose-dependent manner inhibited CFA-induced thermal hyperalgesia. In addition, CFA-induced thermal hyperalgesia was significantly reversed by intrathecal administration of 1-100 microg of BCTC and SB-366791. While intraplantar BCTC (1-300 microg) and SB-366791 (30-300 microg) did not reverse CFA-induced mechanical hyperalgesia, 1-100 microg of intrathecally administered BCTC and SB-366791 dose-dependently reduced mechanical hyperalgesia. Regression analysis showed that a correlation exists between the inhibitory effects on thermal hyperalgesia and mechanical hyperalgesia after intrathecal administration (correlation factor = 0.6521), but not after intraplantar administration (correlation factor = 0.0215). These data suggest that TRPV1 in the peripheral endings of the primary afferents plays a key role in thermal hyperalgesia, but it makes only a minor contribution in CFA-induced mechanical hyperalgesia. Furthermore, it is suggested that the spinal TRPV1 is critical in the development of both types of hyperalgesia.

Atrial natriuretic peptide reduces ischemia/reperfusion-induced spinal cord injury in rats by enhancing sensory neuron activation

We recently demonstrated that calcitonin gene-related peptide (CGRP) released from sensory neurons reduces spinal cord injury (SCI) by inhibiting neutrophil activation through an increase in the endothelial production of prostacyclin (PGI(2)). Carperitide, a synthetic alpha-human atrial natriuretic peptide (ANP), reduces ischemia/reperfusion (I/R)-induced tissue injury. However, its precise therapeutic mechanism(s) remains to be elucidated. In the present study, we examined whether ANP reduces I/R-induced spinal cord injury by enhancing sensory neuron activation using rats. ANP increased CGRP release and cellular cAMP levels in dorsal root ganglion neurons isolated from rats in vitro. The increase in CGRP release induced by ANP was reversed by pretreatment with capsazepine, an inhibitor of vanilloid receptor-1 activation, or with (9S, 10S, 12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]-benzodiazocine-10-carboxylic acid hexyl ester (KT5720), an inhibitor of protein kinase A (PKA), suggesting that ANP might increase CGRP release from sensory neurons by activating PKA through an increase in the cellular cAMP level. Spinal cord ischemia was induced in rats using a balloon catheter placed in the aorta. ANP reduced mortality and motor disturbances by inhibiting reduction of the number of motor neurons in animals subjected to SCI. ANP significantly enhanced I/R-induced increases in spinal cord tissue levels of CGRP and 6-keto-prostaglandin F(1alpha). a stable metabolite of PGI(2). ANP inhibited I/R-induced increases in spinal cord tissue levels of tumor necrosis factor and myeloperoxidase. Pretreatment with 4'-chloro-3-methoxycinnamanilide (SB366791), a specific vanilloid receptor-1 antagonist, and indomethacin reversed the effects of ANP. These results strongly suggest that ANP might reduce I/R-induced SCI in rats by inhibiting neutrophil activation through enhancement of sensory neuron activation.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Regulation of transient receptor potential (TRP) channels by phosphoinositides

This review summarizes the modulation of transient receptor potential (TRP) channels, by phosphoinositides. TRP channels are characterized by polymodal activation and a surprising complexity of regulation mechanisms. Possibly, most if not all TRP channels are modulated by phosphoinositides. Modulation by phosphatidylinositol 4,5-biphosphate (PIP(2)) has been shown in detail for TRP vanilloid (TRPV) 1, TRPV5, TRP melastatin (TRPM) 4, TRPM5, TRPM7, TRPM8, TRP polycystin 2, and the Drosophila TPR-like (TRPL) channels. This review describes mechanisms of modulation of TRP channels mainly by PIP(2) and discusses some future challenges of this fascinating topic.

The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept

The clinical use of TRPV1 (transient receptor potential vanilloid subfamily, member 1; also known as VR1) antagonists is based on the concept that endogenous agonists acting on TRPV1 might provide a major contribution to certain pain conditions. Indeed, a number of small-molecule TRPV1 antagonists are already undergoing Phase I/II clinical trials for the indications of chronic inflammatory pain and migraine. Moreover, animal models suggest a therapeutic value for TRPV1 antagonists in the treatment of other types of pain, including pain from cancer. We argue that TRPV1 antagonists alone or in conjunction with other analgesics will improve the quality of life of people with migraine, chronic intractable pain secondary to cancer, AIDS or diabetes. Moreover, emerging data indicate that TRPV1 antagonists could also be useful in treating disorders other than pain, such as urinary urge incontinence, chronic cough and irritable bowel syndrome. The lack of effective drugs for treating many of these conditions highlights the need for further investigation into the therapeutic potential of TRPV1 antagonists.

Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states

ATP-sensitive P2X(7) receptors are localized on cells of immunological origin including peripheral macrophages and glial cells in the CNS. Activation of P2X(7) receptors leads to rapid changes in intracellular calcium concentrations, release of the proinflammatory cytokine interleukin-1beta and following prolonged agonist exposure, the formation of cytolytic pores in plasma membranes. Both the localization and functional consequences of P2X(7) receptor activation indicate a role in inflammatory processes. The phenotype of P2X(7) receptor gene-disrupted mice also indicates that P2X(7) receptor activation contributes to ongoing inflammation. More recently, P2X(7) receptor knockout data has also suggested a specific role in inflammatory and neuropathic pain states. The recent discovery of potent and highly selective antagonists for P2X(7) receptors has helped to further clarify P2X receptor pharmacology, expanded understanding of P2X(7) receptor signaling, and offers new evidence that P2X(7) receptors play a specific role in nociceptive signaling in chronic pain states. In this review, we incorporate the recent discoveries of novel P2X(7) receptor-selective antagonists with a brief update on P2X(7) receptor pharmacology and its therapeutic potential.

Protein kinase C in pain: involvement of multiple isoforms

Pain is the primary reason that people seek medical care. At present, chronic unremitting pain is the third greatest health problem after heart disease and cancer. Chronic pain is an economic burden in lost wages, lost productivity, medical expenses, legal fees and compensation. Chronic pain is defined as a pain of greater than 2 months duration. It can be of inflammatory or neuropathic origin that can arise following nerve injury or in the absence of any apparent injury. Chronic pain is characterized by an altered pain perception that includes allodynia (a response to a normally non-noxious stimuli) and hyperalgesia (an exaggerated response to a normally noxious stimuli). This type of pain is often insensitive to the traditional analgesics or surgical intervention. The study of the cellular and molecular mechanisms that contribute to chronic pain are of the up-most importance for the development of a new generation of analgesic agents. Protein kinase C isozymes are under investigation as potential therapeutics for the treatment of chronic pain conditions. The anatomical localization of protein kinase C isozymes in both peripheral and central nervous system sites that process pain have made them the topic of basic science research for close to two decades. This review will outline the research to date on the involvement of protein kinase C in pain and analgesia. In addition, this review will try to synthesize these works to begin to develop a comprehensive mechanistic understanding of how protein kinase C may function as a master regulator of the peripheral and central sensitization that underlies many chronic pain conditions.

Inflammatory mediators modulating the transient receptor potential vanilloid 1 receptor: therapeutic targets to treat inflammatory and neuropathic pain

The transient receptor potential vanilloid 1 receptor (TRPV1) plays an important role in inflammatory heat hyperalgesia. TRPV1 is a non-selective cation channel gated by noxious heat, protons and capsaicin, thus being regarded as a polymodal molecular integrator in nociception. Abundant evidence has demonstrated that TRPV1 is also modulated by numerous inflammatory mediators, including growth factors, neurotransmitters, peptides or small proteins, lipids, chemokines and cytokines. By activating multiple protein kinases to increase the phosphorylation of TRPV1, pronociceptive inflammatory mediators sensitise the TRPV1 response to noxious heat, protons and capsaicin, thus augmenting thermal hyperalgesia. In contrast, by inhibiting protein kinases or other mechanisms, antinociceptive inflammatory mediators suppress the response of TRPV1 to these stimuli, thus damping thermal hyperalgesia. The positive modulation of TRPV1 by inflammatory mediators may constitute a novel mechanism underlying sustained inflammatory or neuropathic pain. Blocking pronociceptive inflammatory mediator-exerted sensitising effects or boosting antinociceptive inflammatory mediator-induced suppressing effects on TRPV1 should be considered as sources of novel potential therapies to more effectively treat chronic pain conditions.

Mechanisms of disease: involvement of the urothelium in bladder dysfunction

Although the urinary bladder urothelium has classically been thought of as a passive barrier to ions and solutes, a number of novel properties have been recently attributed to urothelial cells. Studies have revealed that the urothelium is involved in sensory mechanisms (i.e. the ability to express a number of sensor molecules or respond to thermal, mechanical and chemical stimuli) and can release chemical mediators. Localization of afferent nerves next to the urothelium suggests that urothelial cells could be targets for neurotransmitters released from bladder nerves or that chemicals released by urothelial cells could alter afferent nerve excitability. Taken together, these and other findings highlighted in this article suggest a sensory function for the urothelium. Elucidation of mechanisms that influence urothelial function might provide insights into the pathology of bladder dysfunction.

Potentiation of glutamatergic synaptic transmission by protein kinase C-mediated sensitization of TRPV1 at the first sensory synapse

Sensory input from the periphery to the CNS is critically dependent on the strength of synaptic transmission at the first sensory synapse formed between primary afferent dorsal root ganglion (DRG) and superficial dorsal horn (DH) neurons of the spinal cord. Transient receptor potential vanilloid 1 (TRPV1) expressed on a subset of sensory neurons plays an important role in chronic inflammatory thermal nociception. Activation of protein kinase C (PKC) sensitizes TRPV1, which may contribute to the pathophysiology of chronic pain conditions. In this study, we have examined the modulation of TRPV1-mediated enhancement of excitatory synaptic transmission in response to PKC activation. Miniature excitatory postsynaptic currents (mEPSCs) from embryonic rat DRG-DH neuronal cocultures were recorded by patch clamping DH neurons. Capsaicin potently increased the frequency but not the amplitude of mEPSCs in a calcium-dependent manner, suggesting TRPV1-mediated glutamate release from presynaptic terminals of sensory neurons. Continued or repeated applications of capsaicin reduced the frequency of mEPSCs over time. The PKC activator phorbol 12,13-dibutyrate (PDBu) alone increased mEPSC events to a certain extent in a reversible manner but capsaicin further synergistically enhanced the frequency of mEPSCs. The PKC inhibitor bisindolylmaleimide (BIM) abolished PDBu-mediated potentiation of TRPV1-dependent increases in mEPSC frequency, suggesting modulation of TRPV1 by PKC-induced phosphorylation. In addition, at normal body temperatures ( approximately 37 degrees C) PKC-mediated enhancement of mEPSC frequency is significantly decreased by a specific TRPV1 antagonist, suggesting a physiological role of TRPV1 at the central terminals. Furthermore, bradykinin (BK) significantly potentiated TRPV1-modulated synaptic responses by activating the PLC-PKC pathway. Our results indicate that TRPV1 activation can modulate excitatory synaptic transmission at the first sensory synapse and its effects can further be augmented by activation of PKC. Increased gain of sensory input by TRPV1-induced enhancement of glutamate release and its potentiation by various inflammatory mediators may contribute to persistent pain conditions. Selective targeting of TRPV1 expressed on the central terminals of sensory neurons may serve as a strategy to alleviate chronic intractable pain conditions.

Epidermal keratinocytes as the forefront of the sensory system

Various sensors that respond to physical or chemical environmental factors have been identified in the peripheral nervous system. Some of them, which respond to mechanical stress, osmotic pressure, temperature and chemical stimuli (such as pH), are also expressed in epidermal keratinocytes. Neurotransmitters and their receptors, as well as receptors that regulate the neuroendocrine system of the skin, are also present in keratinocytes. Thus, broadly speaking, epidermal keratinocytes appear to be equipped with sensing systems similar to those of the peripheral and central nervous systems. It had long been considered that only nerve C-terminals in the epidermis play a role in skin surface perception. However, building on earlier work on skin receptors and new findings introduced here, we present in this review a novel hypothesis of skin sensory perception, i.e. first, keratinocytes recognize various environmental factors, and then the information is processed and conveyed to the nervous system.

Vanilloid receptor-1 (TRPV1)-dependent activation of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Inhibitory neurotransmission in spinal cord dorsal horn is mainly mediated by gamma-amino butyric acid (GABA) and glycine. By patch clamp recordings and correlative immunocytochemistry, we studied here the effect of 2 microM capsaicin-induced vanilloid receptor-1 (TRPV1) activation on IPSCs in spinal lamina II neurons from post-natal mice. Specificity was confirmed after pre-incubation with the competitive antagonist SB366791 (10 microM). After a single capsaicin pulse, an intense increase of spontaneous IPSC (sIPSC) frequency was observed in the presence of NBQX 10 microM (62/81 neurons; approximately 76%) or NBQX 10 microM + AP-5 20-100 microM (27/42 neurons; approximately 64%). Only a subpopulation (approximately 40%) of responsive neurons showed a significant amplitude increase. Seventy-two percent of the neurons displayed pure GABA(A) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After two consecutive capsaicin pulses, frequency rises were very similar, and both significantly higher than controls. When the second pulse was given in the presence of 4 microM L732,138, a selective antagonist of the substance P (SP) preferred receptor NK1, we observed a significant loss in frequency increase (63.90% with NBQX and 52.35% with NBQX + AP-5). TTX (1 microM) largely (approximately 81.5%) blocked the effect of capsaicin. These results show that TRPV1 activation on primary afferent fibers releases SP. The peptide then excites inhibitory neurons in laminae I, III and IV, leading to an increased release of GABA/glycine in lamina II via a parallel alternative pathway to glutamate.

Heat sensitization in skin and muscle nociceptors expressing distinct combinations of TRPV1 and TRPV2 protein

Recordings were made from small and medium diameter dorsal root ganglia (DRG) neurons that expressed transient receptor potential (TRP) proteins. Physiologically characterized skin nociceptors expressed either TRPV1 (type 2) or TRPV2 (type 4) in isolation. Other nociceptors co-expressed both TRP proteins and innervated deep tissue sites (gastrocnemius muscle, distal colon; type 5, type 8) and skin (type 8). Subpopulations of myelinated (type 8) and unmyelinated (type 5) nociceptors co-expressed both TRPs. Cells that expressed TRPV1 were excellent transducers of intense heat. Proportional inward currents were obtained from a threshold of approximately 46.5 to approximately 56 degrees C. In contrast, cells expressing TRPV2 alone (52 degrees C threshold) did not reliably transduce the intensity of thermal events. Studies were undertaken to assess the capacity of skin and deep nociceptors to exhibit sensitization to repeated intense thermal stimuli [heat-heat sensitization (HHS)]. Only nociceptors that expressed TRPV2, alone or in combination with TRPV1, exhibited HHS. HHS was shown to be Ca(2+) dependent in either case. Intracellular Ca(2+) dependent pathways to HHS varied with the pattern of TRP protein expression. Cells co-expressing both TRPs modulated heat reactivity through serine/threonine phosphorylation or PLA(2)-dependent pathways. Cells expressing only TRPV2 may have relied on tyrosine kinases for HHS. We conclude that heat sensitization in deep and superficial capsaicin and capsaicin-insensitive C and Adelta nociceptors varies with the distribution of TRPV1 and TRPV2 proteins. The expression pattern of these proteins are specific to subclasses of physiologically identified C and A fiber nociceptors with highly restricted tissue targets.

Biochemistry and pharmacology of endovanilloids

Endovanilloids are defined as endogenous ligands and activators of transient receptor potential (TRP) vanilloid type 1 (TRPV1) channels. The first endovanilloid to be identified was anandamide (AEA), previously discovered as an endogenous agonist of cannabinoid receptors. In fact, there are several similarities, in terms of opposing actions on the same intracellular signals, role in the same pathological conditions, and shared ligands and tissue distribution, between TRPV1 and cannabinoid CB(1) receptors. After AEA and some of its congeners (the unsaturated long chain N-acylethanolamines), at least 2 other families of endogenous lipids have been suggested to act as endovanilloids: (i) unsaturated long chain N-acyldopamines and (ii) some lipoxygenase (LOX) metabolites of arachidonic acid (AA). Here we discuss the mechanisms for the regulation of the levels of the proposed endovanilloids, as well as their TRPV1-mediated pharmacological actions in vitro and in vivo. Furthermore, we outline the possible pathological conditions in which endovanilloids, acting at sometimes aberrantly expressed TRPV1 receptors, might play a role.

Adenosine and ATP receptors

Adenosine and ATP, via P1 and P2 receptors respectively, can modulate pain transmission under physiological, inflammatory, and neuropathic pain conditions. Such influences reflect peripheral and central actions and effects on neurons as well as other cell types. In general, adenosine A1 receptors produce inhibitory effects on pain in a number of preclinical models and are a focus of attention. In humans, i.v. infusions of adenosine reduce some aspects of neuropathic pain and can reduce postoperative pain. For P2X receptors, there is a significant body of information indicating that inhibition of P2X3 receptors may be useful for relieving inflammatory and neuropathic pain. More recently, data have begun to emerge implicating P2X4, P2X7 and P2Y receptors in aspects of pain transmission. Both P1 and P2 receptors may represent novel targets for pain relief.

Mouse colon sensory neurons detect extracellular acidosis via TRPV1

Extracellular acidification contributes to pain by activating or modulating nociceptor activity. To evaluate acidic signaling from the colon, we characterized acid-elicited currents in thoracolumbar (TL) and lumbosacral (LS) dorsal root ganglion (DRG) neurons identified by content of a fluorescent dye (DiI) previously injected into the colon wall. In 13% of unidentified LS DRG neurons (not labeled with DiI) and 69% of LS colon neurons labeled with DiI, protons activated a sustained current that was significantly and reversibly attenuated by the transient receptor potential vanilloid receptor 1 (TRPV1) antagonist capsazepine. In contrast, 63% of unidentified LS DRG neurons and 4% of LS colon neurons exhibited transient amiloride-sensitive acid-sensing ion channel (ASIC) currents. The peak current density of acid-elicited currents was significantly reduced in colon sensory neurons from TRPV1-null mice, supporting predominant expression of TRPV1 in LS colon sensory neurons, which was also confirmed immunohistochemically. Similar to LS colon DRG neurons, acid-elicited currents in TL colon DRG neurons were mediated predominantly by TRPV1. However, the pH producing half-activation of responses significantly differed between TL and LS colon DRG neurons. The properties of acid-elicited currents in colon DRG neurons suggest differential contributions of ASICs and TRPV1 to colon sensation and likely nociception.

P2 receptors and chronic pain

There is abundant evidence that extracellular ATP and other nucleotides have an important role in pain signaling at both the periphery and in the CNS. The focus of attention now is on the possibility that endogenous ATP and its receptor system might be activated in chronic pathological pain states, particularly in neuropathic and inflammatory pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes or infection. This type of pain can be so severe that even light touching can be intensely painful; unfortunately, this state is generally resistant to currently available treatments. In this review, we summarize the role of ATP receptors, particularly the P2X₄, P2X₃ and P2X₇ receptors, in neuropathic and inflammatory pain. The expression of P2X₄ receptors in the spinal cord is enhanced in spinal microglia after peripheral nerve injury, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produce a reduction of the neuropathic pain behaviour. Understanding the key roles of these ATP receptors may lead to new strategies for the management of intractable chronic pain.

Nociception and TRP channels

Pain is initiated when noxious stimuli excite the peripheral terminals of specialized primary afferent neurons called nociceptors. Many molecules are involved in conversion of the noxious stimuli to the electrical signals in the nociceptor endings. Among them, TRP channels play important roles in detecting noxious stimuli.

Insulin and insulin-like growth factor type-I up-regulate the vanilloid receptor-1 (TRPV1) in stably TRPV1-expressing SH-SY5Y neuroblastoma cells

The capsaicin receptor, transient receptor potential, vanilloid type 1 (TRPV1), is a Ca(2+)-permeable ion channel activated by noxious stimuli eliciting pain. Several reports have shown modulation of TRPV1 activity and expression by neuronal growth factors. Here, we study the long-term effects on TRPV1 expression mediated by insulin-like growth factor type-I (IGF-I) and insulin in a stably TRPV1-expressing SH-SY5Y neuroblastoma cell line. We show that, after 72 hr of 10 nM IGF-I or insulin exposure, the TRPV1 protein level was up-regulated 2.5- and 2-fold, respectively. By blocking phosphatidylinositol-3-kinase [PI(3)K] or mitogen-activated protein kinase (MAPK) signaling, we concluded that the increase in total TRPV1 protein content induced by IGF-I was controlled by PI(3)K signaling, whereas insulin seemed to regulate TRPV1 protein expression via both PI(3)K and MAPK pathways. Inhibiting protein kinase C (PKC) blocked the effects of both IGF-I and insulin. Furthermore, the concentrations causing a 50% Ca(2+) increase (EC(50)) after insulin and IGF-I treatments were significantly lowered compared with untreated cells. We conclude that IGF-I and insulin enhance TRPV1 protein expression and activity, and impaired pain sensation might result from distorted TRPV1 regulation in the peripheral nervous system.

TRPV1 antagonists attenuate antigen-provoked cough in ovalbumin sensitized guinea pigs

We examined the molecular pharmacology and in vivo effects of a TRPV1 receptor antagonist, N-(4-Tertiarybutylphenyl)-4(3-cholorphyridin-2-yl)-tetrahydro-pyrazine1(2H) - carboxamide (BCTC) on the guinea pig TRPV1 cation channel. BCTC antagonized capsaicin-induced activation and PMA-mediated activation of guinea pig TRPV1 with IC50 values of 12.2 +/- 5.2 nM, and 0.85 +/- 0.10 nM, respectively. In addition, BCTC (100 nM) completely blocked the ability of heterologously expressed gpTRPV1 to respond to decreases in pH. Thus, BCTC is able to block polymodal activation of gpTRPV1. Furthermore, in nodose ganglia cells, capsaicin induced Ca2+ influx through TRPV1 channel was inhibited via BCTC in a concentration dependent manner. In in vivo studies capsaicin (10 - 300 muM) delivered by aerosol to the pulmonary system of non-sensitized guinea pigs produced an increase in cough frequency. In these studies, the tussigenic effects of capsaicin (300 muM) were blocked in a dose dependent fashion when BCTC (0.01-3.0 mg/kg, i.p.) was administered 30 minutes before challenge. The high dose of BCTC (3.0 mg/kg, i.p) produced a maximum inhibition of capsaicin-induced cough of 65%. We also studied the effects of BCTC (0.03 and 3.0) when administered 60 minutes before capsaicin. Under these conditions, BCTC (3.0 mg/kg, i.p) produced a maximum decrease in capsaicin-induced cough of 31%. In ovalbumin passively sensitized guinea pigs, we found that BCTC (1 and 3 mg/kg, i.p.) attenuated antigen ovalbumin (0.3%) cough responses by 27% and 60%, respectively. We conclude that TRPV1 channel activation may play role in cough mediated by antigen in sensitized guinea pigs. Our results supports increasing evidence that TRPV1 may play a role in the generation of the cough response.

Impaired vasodilation in response to perivascular nerve stimulation in mesenteric arteries of TRPV1-null mutant mice

BACKGROUND

The role of the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in perivascular sensory nerves in the regulation of vascular reactivity is largely unknown. This study was designed to test the hypothesis that vasodilation induced by electrical field stimulation (EFS) of perivascular sensory nerves is mediated by the TRPV1 via release of sensory neurotransmitters in wild-type (WT) mice, and this effect is abolished in gene-targeted TRPV1-null mutant (TRPV1(-/-)) mice.

METHODS

Isolated mesenteric resistance arteries from WT and TRPV1(-/-)) mice were perfused and pretreated with guanethedine and atropine to block sympathetic and parasympathetic nerve activity, respectively. After precontracting with phenylephrine, changes of vascular diameters induced by EFS were monitored in the absence or presence of the TRPV1 receptor antagonist capsazepine; the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP8-37; or the substance P (SP) receptor antagonist, RP67580.

RESULTS

EFS-induced vasodilation was significantly reduced in arteries in TRPV1(-/-)) mice when compared to that of WT mice. Capsazepine and CGRP8-37 attenuated vasodilation induced by EFS in WT but not TRPV1(-/-)) mice. In contrast, RP67580 had no effect on the EFS-induced vasodilation in WT or TRPV1(-/-)) mice. The release of CGRP in the face of EFS challenge was significantly increased in both WT and TRPV1(-/-)) arteries, which was attenuated by capsazepine in WT but not TRPV1(-/-)) arteries. Exogenous CGRP caused dose-dependent vasodilation to a similar degree in WT and TRPV1(-/-)) arteries.

CONCLUSIONS

Our data show that in WT mice transmural stimulation of perivascular sensory nerves activates the TRPV1, leading to CGRP release from sensory nerve endings; and blockade of CGRP, but not SP, receptors abolishes TRPV1-mediated vasodilation during EFS. All these effects are impaired in TRPV1(-/-)) mice, indicating that TRPV1 plays a key role in modulating perivascular sensory nerve-mediated vasodilation.

Protease-activated receptor 2 sensitizes the transient receptor potential vanilloid 4 ion channel to cause mechanical hyperalgesia in mice

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

Impaired pain sensation in mice lacking prokineticin 2

Prokineticins (PKs), consisting of PK1 and PK2, are a pair of newly identified regulatory peptides. Two closely related G-protein coupled receptors, PKR1 and PKR2, mediate the signaling of PKs. PKs/PKRs participate in the regulation of diverse biological processes, ranging from development to adult physiology. A number of studies have indicated the involvement of PKs/PKRs in nociception. Here we show that PK2 is a sensitizer for nociception. Intraplantar injection of recombinant PK2 resulted in a strong and localized hyperalgesia with reduced thresholds to nociceptive stimuli. PK2 mobilizes calcium in dissociated dorsal root ganglion (DRG) neurons. Mice lacking the PK2 gene displayed strong reduction in nociception induced by thermal and chemical stimuli, including capsaicin. However, PK2 mutant mice showed no difference in inflammatory response to capsaicin. As the majority of PK2-responsive DRG neurons also expressed transient receptor potential vanilloid (TRPV1) and exhibited sensitivity to capsaicin, TRPV1 is likely a significant downstream molecule of PK2 signaling. Taken together, these results reveal that PK2 sensitize nociception without affecting inflammation.

How important is protein kinase C in μ-opioid receptor desensitization and morphine tolerance?

The repeated administration of opiate drugs such as morphine results in the development of tolerance to their analgesic, rewarding (euphoric) and respiratory-depressant effects; thus, to obtain the same level of response with subsequent administrations, a greater dose must be used. Tolerance can limit the clinical efficacy of opiate drugs and enhance the social problems that are inherent in recreational opioid abuse. Surprisingly, the mechanism (or mechanisms) underlying the development of morphine tolerance remains controversial. Here, we propose that protein kinase C could have a crucial role in the desensitization of mu-opioid receptors by morphine and that this cellular process could contribute to the development and maintenance of morphine tolerance in vivo.

Phosphorylation of extracellular signal-related protein kinase is required for rapid facilitation of heat-induced currents in rat dorsal root ganglion neurons

A subgroup of dorsal root ganglion (DRG) neurons responds to noxious heat with an influx of cations carried by specific ion channels such as the transient receptor potential channel of the vanilloid receptor type, subtype 1 (TRPV1). Application of capsaicin induces a reversible facilitation of these currents. This facilitation could be an interaction of two agonists at their common receptor or be caused by an influx of calcium ions into the cell. Calcium influx into the cell can activate protein kinases such as the extracellular signal-related protein kinase (ERK) pathway. This study explored the kinetics, calcium-dependency and intracellular signals following application of capsaicin and leading to facilitation of heat-induced currents (Iheat) in rat DRG neurons. Application of 0.5 microM capsaicin caused a 2.65-fold increase of Iheat within 2 s, which was significantly correlated to a small capsaicin-induced current. Intracellular application of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a fast calcium chelator, did not change capsaicin-induced currents or Iheat itself, but inhibited facilitation of Iheat by capsaicin. ERK is activated by calcium influx and membrane depolarization via the mitogen-activated protein kinase/extracellular signal-related protein kinase kinase (MEK). Application of the MEK inhibitor U0126 also inhibited facilitation of Iheat by capsaicin. We conclude that the MEK/ERK cascade is an intracellular signaling pathway playing a vital role in the regulation of nociceptive neurons' sensitivity. The very fast kinetics (less than two seconds) are only explainable with a membrane-attached or at least membrane-near localization of these kinases.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.