Reducing and oxidizing agents sensitize heat-activated vanilloid receptor (TRPV1) current

TRP Channels in Respiratory Pathophysiology: the Role of Oxidative, Chemical Irritant and Temperature Stimuli

There is rapidly growing evidence indicating multiple and important roles of Ca(2+)- permeable cation TRP channels in the airways, both under normal and disease conditions. The aim of this review was to summarize the current knowledge of TRP channels in sensing oxidative, chemical irritant and temperature stimuli by discussing expression and function of several TRP channels in relevant cell types within the respiratory tract, ranging from sensory neurons to airway smooth muscle and epithelial cells. Several of these channels, such as TRPM2, TRPM8, TRPA1 and TRPV1, are discussed in much detail to show that they perform diverse, and often overlapping or contributory, roles in airway hyperreactivity, inflammation, asthma, chronic obstructive pulmonary disease and other respiratory disorders. These include TRPM2 involvement in the disruption of the bronchial epithelial tight junctions during oxidative stress, important roles of TRPA1 and TRPV1 channels in airway inflammation, hyperresponsiveness, chronic cough, and hyperplasia of airway smooth muscles, as well as TRPM8 role in COPD and mucus hypersecretion. Thus, there is increasing evidence that TRP channels not only function as an integral part of the important endogenous protective mechanisms of the respiratory tract capable of detecting and ensuring proper physiological responses to various oxidative, chemical irritant and temperature stimuli, but that altered expression, activation and regulation of these channels may also contribute to the pathogenesis of respiratory diseases.

Oxidant-induced activation of cGMP-dependent protein kinase Iα mediates neuropathic pain after peripheral nerve injury

AIMS

Emerging lines of evidence indicate that oxidants such as hydrogen peroxide exert specific signaling functions during the processing of chronic pain. However, the mechanisms by which oxidants regulate pain processing in vivo remain poorly understood. Here, we investigated whether cyclic guanosine monophosphate (cGMP)-dependent protein kinase Iα (cGKIα), which can be activated by oxidants independently of cGMP, serves as a primary redox target during pain processing.

RESULTS

After peripheral nerve injury, oxidant-induced cGKIα activation is increased in dorsal root ganglia of mice. Knock-in (KI) mice in which cGKIα cannot transduce oxidant signals demonstrated reduced neuropathic pain behaviors after peripheral nerve injury, and reduced pain behaviors after intrathecal delivery of oxidants. In contrast, acute nociceptive, inflammatory, and cGMP-induced pain behaviors were not impaired in these mice.

INNOVATION

Studying cGKIα KI mice, we provide the first evidence that oxidants activate cGKIα in sensory neurons after peripheral nerve injury in vivo.

CONCLUSION

Our results suggest that oxidant-induced activation of cGKIα specifically contributes to neuropathic pain processing, and that prevention of cGKIα redox activation could be a potential novel strategy to manage neuropathic pain.

Electromagnetic radiation (Wi-Fi) and epilepsy induce calcium entry and apoptosis through activation of TRPV1 channel in hippocampus and dorsal root ganglion of rats

Incidence rates of epilepsy and use of Wi-Fi worldwide have been increasing. TRPV1 is a Ca(2+) permeable and non-selective channel, gated by noxious heat, oxidative stress and capsaicin (CAP). The hyperthermia and oxidant effects of Wi-Fi may induce apoptosis and Ca(2+) entry through activation of TRPV1 channel in epilepsy. Therefore, we tested the effects of Wi-Fi (2.45 GHz) exposure on Ca(2+) influx, oxidative stress and apoptosis through TRPV1 channel in the murine dorsal root ganglion (DRG) and hippocampus of pentylentetrazol (PTZ)-induced epileptic rats. Rats in the present study were divided into two groups as controls and PTZ. The PTZ groups were divided into two subgroups namely PTZ + Wi-Fi and PTZ + Wi-Fi + capsazepine (CPZ). The hippocampal and DRG neurons were freshly isolated from the rats. The DRG and hippocampus in PTZ + Wi-Fi and PTZ + Wi-Fi + CPZ groups were exposed to Wi-Fi for 1 hour before CAP stimulation. The cytosolic free Ca(2+), reactive oxygen species production, apoptosis, mitochondrial membrane depolarization, caspase-3 and -9 values in hippocampus were higher in the PTZ group than in the control although cell viability values decreased. The Wi-Fi exposure induced additional effects on the cytosolic Ca(2+) increase. However, pretreatment of the neurons with CPZ, results in a protection against epilepsy-induced Ca(2+) influx, apoptosis and oxidative damages. In results of whole cell patch-clamp experiments, treatment of DRG with Ca(2+) channel antagonists [thapsigargin, verapamil + diltiazem, 2-APB, MK-801] indicated that Wi-Fi exposure induced Ca(2+) influx via the TRPV1 channels. In conclusion, epilepsy and Wi-Fi in our experimental model is involved in Ca(2+) influx and oxidative stress-induced hippocampal and DRG death through activation of TRPV1 channels, and negative modulation of this channel activity by CPZ pretreatment may account for the neuroprotective activity against oxidative stress.

TRPV1 antagonism by capsazepine modulates innate immune response in mice infected with Plasmodium berghei ANKA

Thousands of people suffer from severe malaria every year. The innate immune response plays a determinant role in host's defence to malaria. Transient receptor potential vanilloid 1 (TRPV1) modulates macrophage-mediated responses in sepsis, but its role in other pathogenic diseases has never been addressed. We investigated the effects of capsazepine, a TRPV1 antagonist, in malaria. C57BL/6 mice received 10(5) red blood cells infected with Plasmodium berghei ANKA intraperitoneally. Noninfected mice were used as controls. Capsazepine or vehicle was given intraperitoneally for 6 days. Mice were culled on day 7 after infection and blood and spleen cell phenotype and activation were evaluated. Capsazepine decreased circulating but not spleen F4/80(+)Ly6G(+) cell numbers as well as activation of both F4/80(+)and F4/80(+)Ly6G(+) cells in infected animals. In addition, capsazepine increased circulating but not spleen GR1(+) and natural killer (NK) population, without interfering with natural killer T (NKT) cell numbers and blood NK and NKT activation. However, capsazepine diminished CD69 expression in spleen NKT but not NK cells. Infection increased lipid peroxidation and the release of TNFα and IFNγ, although capsazepine-treated group exhibited lower levels of lipid peroxidation and TNFα. Capsazepine treatment did not affect parasitaemia. Overall, TRPV1 antagonism modulates the innate immune response to malaria.

Redox regulation of transient receptor potential channels

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

Sensitization by pulmonary reactive oxygen species of rat vagal lung C-fibers: the roles of the TRPV1, TRPA1, and P2X receptors

Sensitization of vagal lung C-fibers (VLCFs) induced by mediators contributes to the pathogenesis of airway hypersensitivity, which is characterized by exaggerated sensory and reflex responses to stimulants. Reactive oxygen species (ROS) are mediators produced during airway inflammation. However, the role of ROS in VLCF-mediated airway hypersensitivity has remained elusive. Here, we report that inhalation of aerosolized 0.05% H2O2 for 90 s potentiated apneic responses to intravenous capsaicin (a TRPV1 receptor agonist), α,β-methylene-ATP (a P2X receptor agonist), and phenylbiguanide (a 5-HT3 receptor agonist) in anesthetized rats. The apneic responses to these three stimulants were abolished by vagatomy or by perivagal capsaicin treatment, a procedure that blocks the neural conduction of VLCFs. The potentiating effect of H2O2 on the apneic responses to these VLCF stimulants was prevented by catalase (an enzyme that degrades H2O2) and by dimethylthiourea (a hydroxyl radical scavenger). The potentiating effect of H2O2 on the apneic responses to capsaicin was attenuated by HC-030031 (a TRPA1 receptor antagonist) and by iso-pyridoxalphosphate-6-azophenyl-2',5'-disulphonate (a P2X receptor antagonist). The potentiating effect of H2O2 on the apneic responses to α,β-methylene-ATP was reduced by capsazepine (a TRPV1 receptor antagonist), and by HC-030031. The potentiating effect of H2O2 on the apneic responses to phenylbiguanide was totally abolished when all three antagonists were combined. Consistently, our electrophysiological studies revealed that airway delivery of aerosolized 0.05% H2O2 for 90 s potentiated the VLCF responses to intravenous capsaicin, α,β-methylene-ATP, and phenylbiguanide. The potentiating effect of H2O2 on the VLCF responses to phenylbiguanide was totally prevented when all antagonists were combined. Inhalation of 0.05% H2O2 indeed increased the level of ROS in the lungs. These results suggest that 1) increased lung ROS sensitizes VLCFs, which leads to exaggerated reflex responses in rats and 2) the TRPV1, TRPA1, and P2X receptors are all involved in the development of this airway hypersensitivity.

GCH1 variants, tetrahydrobiopterin and their effects on pain sensitivity

Background A great proportion of the variation in pain experience and chronicity is caused by heritable factors. Within the last decades several candidate genes have been discovered either increasing or decreasing pain sensitivity or the risk of chronic pain in humans. One of the most studied genes is the GCH1 gene coding for the enzyme GTP cyclohydrolase 1 (GCH1). GCH1 catalyses the initial and rate-limiting step in the biosynthesis of tetrahydrobiopterin (BH4). The main function of BH4 is regulation of monoamine and nitric oxide biosynthesis, all involved in nociceptive signalling. Methods In this topical review we focus on the implication of the GCH1 gene and BH4 in painful conditions. We discuss experimental evidence from our group in relation to relevant research publications evaluating the BH4 pathway in pain. Studies assessing the role of GCH1 and BH4 in pain consist of human and animal studies, including DOPA-responsive dystonia (DRD) patients and hph-1 mice (a genetic mouse model of DRD) having mutations in the GCH1 gene as well as preclinical studies with the GCH1 inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP). The hypothesis is that genetic and pharmacological reduction of GCH1 would result in lower pain sensitivity. Results Previous studies have demonstrated that a particular "pain protective" GCH1 haplotype, found in 15% of the general human population, is linked to decreased pain sensitivity. We further support these findings in DRD patients, showing normal thresholds to mechanical and thermal stimuli, whereas a trend towards lower pain sensitivity is seen following chemical pain sensitisation. Consistent with these observations, non-injured hph-1 mice displayed normal mechano- and thermosensation compared to wild-type mice. After peripheral inflammation with Complete Freund' Adjuvant or sensitisation with capsaicin the mutant mice exhibited lower sensitivity to mechanical and heat stimuli. Moreover, hph-1 mice showed decreased nociception in the first phase of the formalin test. Several studies report analgesic effects of GCH1 inhibition with 90-270 mg/kg DAHP in rat models of inflammatory and neuropathic pain. However, we could not completely replicate these findings in mice. Fairly higher doses of DAHP (≥270 mg/kg) were needed to reduce inflammatory pain in mice, but the window between antinociception and toxic effects was small, since 400 mg/kg DAHP affected motor performance and general appearance. Also, the analgesic effects were marginal in mice compared to that observed in rats. Conclusions Variations in the GCH1 gene in both humans and mice appear to regulate pain sensitivity and pain behaviours, particularly after pain sensitisation, whereas pain sensitivity to phasic mechanical and thermal stimuli is normal. Moreover, pharmacological inhibition of GCH1 shows antinociceptive effects in preclinical pain studies, though our studies imply that GCH1 inhibition may have a small therapeutic index. Implications The implication of the GCH1 gene in pain may increase our understanding of the risk factors of chronic pain development and improve current pain therapy by personalised medicine. In addition, inhibition of GCH1 provides a potential target for analgesic drug development, though GCH1 inhibitors should possess local or partial effects to avoid serious side-effects to the central nervous system and cardiovascular system.

Identification of voltage-gated K(+) channel beta 2 (Kvβ2) subunit as a novel interaction partner of the pain transducer Transient Receptor Potential Vanilloid 1 channel (TRPV1)

The Transient Receptor Potential Vanilloid 1 (TRPV1, vanilloid receptor 1) ion channel plays a key role in the perception of thermal and inflammatory pain, however, its molecular environment in dorsal root ganglia (DRG) is largely unexplored. Utilizing a panel of sequence-directed antibodies against TRPV1 protein and mouse DRG membranes, the channel complex from mouse DRG was detergent-solubilized, isolated by immunoprecipitation and subsequently analyzed by mass spectrometry. A number of potential TRPV1 interaction partners were identified, among them cytoskeletal proteins, signal transduction molecules, and established ion channel subunits. Based on stringent specificity criteria, the voltage-gated K(+) channel beta 2 subunit (Kvβ2), an accessory subunit of voltage-gated K(+) channels, was identified of being associated with native TRPV1 channels. Reverse co-immunoprecipitation and antibody co-staining experiments confirmed TRPV1/Kvβ2 association. Biotinylation assays in the presence of Kvβ2 demonstrated increased cell surface expression levels of TRPV1, while patch-clamp experiments resulted in a significant increase of TRPV1 sensitivity to capsaicin. Our work shows, for the first time, the association of a Kvβ subunit with TRPV1 channels, and suggests that such interaction may play a role in TRPV1 channel trafficking to the plasma membrane.

Functionally important amino acid residues in the transient receptor potential vanilloid 1 (TRPV1) ion channel--an overview of the current mutational data

This review aims to create an overview of the currently available results of site-directed mutagenesis studies on transient receptor potential vanilloid type 1 (TRPV1) receptor. Systematization of the vast number of data on the functionally important amino acid mutations of TRPV1 may provide a clearer picture of this field, and may promote a better understanding of the relationship between the structure and function of TRPV1. The review summarizes information on 112 unique mutated sites along the TRPV1, exchanged to multiple different residues in many cases. These mutations influence the effect or binding of different agonists, antagonists, and channel blockers, alter the responsiveness to heat, acid, and voltage dependence, affect the channel pore characteristics, and influence the regulation of the receptor function by phosphorylation, glycosylation, calmodulin, PIP2, ATP, and lipid binding. The main goal of this paper is to publish the above mentioned data in a form that facilitates in silico molecular modelling of the receptor by promoting easier establishment of boundary conditions. The better understanding of the structure-function relationship of TRPV1 may promote discovery of new, promising, more effective and safe drugs for treatment of neurogenic inflammation and pain-related diseases and may offer new opportunities for therapeutic interventions.

Reactive oxygen species enhance excitatory synaptic transmission in rat spinal dorsal horn neurons by activating TRPA1 and TRPV1 channels

Central neuropathic pain (CNP) in the spinal cord, such as chronic pain after spinal cord injury (SCI), is an incurable ailment. However, little is known about the spinal cord mechanisms underlying CNP. Recently, reactive oxygen species (ROS) have been recognized to play an important role in CNP of the spinal cord. However, it is unclear how ROS affect synaptic transmission in the dorsal horn of the spinal cord. To clarify how ROS impact on synaptic transmission, we investigated the effects of ROS on synaptic transmission in rat spinal cord substantia gelatinosa (SG) neurons using whole-cell patch-clamp recordings. Administration of tert-butyl hydroperoxide (t-BOOH), an ROS donor, into the spinal cord markedly increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in SG neurons. This t-BOOH-induced enhancement was not suppressed by the Na(+) channel blocker tetrodotoxin. However, in the presence of a non-N-methyl-D-aspartate glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, t-BOOH did not generate any sEPSCs. Furthermore, in the presence of a transient receptor potential ankyrin 1 (TRPA1) channel antagonist (HC-030031) or a transient receptor potential vanilloid 1 (TRPV1) channel antagonist (capsazepine or AMG9810), the t-BOOH-induced increase in the frequency of sEPSCs was inhibited. These results indicate that ROS enhance the spontaneous release of glutamate from presynaptic terminals onto SG neurons through TRPA1 and TRPV1 channel activation. Excessive activation of these ion channels by ROS may induce central sensitization in the spinal cord and result in chronic pain such as that following SCI.

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

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

Neuroprotection induced by N-acetylcysteine against cytosolic glutathione depletion-induced Ca2+ influx in dorsal root ganglion neurons of mice: role of TRPV1 channels

Glutathione (GSH) and N-acetylcysteine (NAC) are thiol-containing antioxidants, and also act through a direct reaction with free radicals. Transient receptor potential vanilloid 1 (TRPV1) is the principal transduction channel serving as a polymodal detector. Despite the importance of oxidative stress in pain sensitivity, its role in TRPV1 modulation is poorly understood. NAC may also have a regulator role on TRPV1 channel activity in the dorsal root ganglion (DRG) neuron. Therefore, we tested the effects of GSH and NAC on TRPV1 channel current, Ca(2+) influx, oxidative stress and caspase activity in the DRG of mice. DRG neurons were freshly isolated from mice and the neurons were incubated for 6 and 24h with buthionine sulfoximine (BSO). Pretreatment of cultured DRG neurons with NAC, results in a protection against oxidative damages. This neuroprotection is associated with the attenuation of a Ca(2+) influx triggered by oxidative agents such as H2O2, 5,5'-dithiobis-(2-nitrobenzoic acid) and GSH depletion via BSO. Here, we demonstrate the contribution of cytosolic factors (related to thiol group depletion) on the activation of TRPV1 channels in this mechanism. TRPV1 channels are activated by various agents including capsaicin (CAP), the pungent component of hot chili peppers, and are blocked by capsazepine. An oxidative environment also increased CAP-evoked TRPV1 currents in the neurons. When NAC and GSH were included in the patch pipette as well as extracellularly in the chamber, TRPV1 channels were not activated by CAP and H2O2. TRPV1 inhibitors, 2-aminoethyl diphenylborinate and N-(p-amylcinnamoyl)anthranilic acid strongly reduced BSO-induced oxidative toxicity and Ca(2+) influx, in a manner similar to pretreatment with NAC and GSH. Caspase-3 and -9 activities of all groups were not changed by the agonists or antagonists. In conclusion, in our experimental model, TRPV1 channels are involved in the oxidative stress-induced neuronal death, and negative modulation of this channel activity by GSH and NAC pretreatment may account for their neuroprotective activity against oxidative stress.

NOXious signaling in pain processing

Chronic pain affects millions of people and often causes major health problems. Accumulating evidence indicates that the production of reactive oxygen species (ROS), such as superoxide anion or hydrogen peroxide, is increased in the nociceptive system during chronic inflammatory and neuropathic pain, and that ROS can act as specific signaling molecules in pain processing. Reduction of ROS levels by administration of scavengers or antioxidant compounds attenuated the nociceptive behavior in various animal models of chronic pain. However, the sources of increased ROS production during chronic pain and the role of ROS in pain processing are poorly understood. Current work revealed pain-relevant functions of the Nox family of NADPH oxidases, a group of electron-transporting transmembrane enzymes whose sole function seems to be the generation of ROS. In particular, significant expression of the Nox family members Nox1, Nox2, and Nox4 in various cells of the nociceptive system has been discovered. Studies using knockout mice suggest that these Nox enzymes specifically contribute to distinct signaling pathways in chronic inflammatory and/or neuropathic pain states. Accordingly, targeting Nox1, Nox2, and Nox4 could be a novel strategy for the treatment of chronic pain. Currently selective inhibitors of Nox enzymes are being developed. Here, we introduce the distinct roles of Nox enzymes in pain processing, we summarize recent findings in the understanding of ROS-dependent signaling pathways in the nociceptive system, and we discuss potential analgesic properties of currently available Nox inhibitors.

Differential effects of acute hypoxia on the activation of TRPV1 by capsaicin and acidic pH

Transient receptor potential vanilloid 1 (TRPV1) is a Ca(2+)-permeable cation channel activated by a variety of physicochemical stimuli. The effect of hypoxia (P(O(2)), 3%) on rat TRPV1 overexpressed in HEK293T has been studied. The basal TRPV1 current (I (TRPV1)) was partly activated by hypoxia, whereas capsaicin-induced TRPV1 (I (TRPV1,Cap)) was attenuated. Such changes were also suggested from hypoxia- and capsaicin-induced Ca(2+) signals in TRPV1-expressing cells. Regarding plausible changes of reactive oxygen species (ROS) under hypoxia, the effects of antioxidants, vitamin C and tiron, as membrane-impermeable and -permeable, respectively, were tested. Both I (TRPV1) and I (TRPV1,Cap) were increased by vitamin C, while only I (TRPV1) was slightly increased by tiron. The hypoxic inhibition of I (TRPV1,Cap) was still persistent under hypoxia/vitamin C. Interestingly, hypoxia/tiron strongly inhibited both I (TRPV1) and I (TRPV1,Cap). Also, with vitamin C applied through a pipette solution, hypoxia inhibited I (TRPV1) and I (TRPV1,Cap). In contrast, hypoxia and hypoxia/tiron had no effect on the I (TRPV1) induced by acid (pH 6.2, I (TRPV1,Acid)). Taken together, hypoxia partly activated TRPV1 while it decreased their sensitivity to capsaicin. Putative changes of ROS under hypoxia might underlie the side-specific effects of ROS on TRPV1: inhibitory at the extracellular and stimulatory at the intracellular side, respectively. The differential effects of hypoxia on I (TRPV1,Cap) and I (TRPV1,Acid) suggested that the intracellular ROS increase might attenuate the pharmacological potency of capsaicin.

TRP Channels as Sensors and Signal Integrators of Redox Status Changes

Proteins are capable of sensing the redox status of cells. Cysteine residues, which react with oxidants, reductants, and electrophiles, have been increasingly recognized as the mediators of this redox sensitivity. Cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that a class of TRP channels is sensitive to changes in redox status and is notably susceptible to modifications of cysteine residues, such as oxidation, electrophilic reaction, and S-nitrosylation of sulfhydryls. In this review, we focus on TRP channels, which directly sense redox status, and discuss the biological significance of cysteine modifications and the consequences of this chemical reaction for physiological responses.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Nitro-oleic acid targets transient receptor potential (TRP) channels in capsaicin sensitive afferent nerves of rat urinary bladder

Nitro-oleic acid (9- and 10-nitro-octadeca-9-enoic acid, OA-NO(2)) is an electrophilic fatty acid nitroalkene derivative that modulates gene transcription and protein function via post-translational protein modification. Nitro-fatty acids are generated from unsaturated fatty acids by oxidative inflammatory reactions and acidic conditions in the presence of nitric oxide or nitrite. Nitroalkenes react with nucleophiles such as cysteine and histidine in a variety of susceptible proteins including transient receptor potential (TRP) channels in sensory neurons of the dorsal root and nodose ganglia. The present study revealed that OA-NO(2) activates TRP channels on afferent nerve terminals in the urinary bladder and thereby increases bladder activity. The TRPV1 agonist capsaicin (CAPS, 1 μM) and the TRPA1 agonist allyl isothiocyanate (AITC, 30 μM), elicited excitatory effects in bladder strips, increasing basal tone and amplitude of phasic bladder contractions (PBC). OA-NO(2) mimicked these effects in a concentration-dependent manner (1 μM-33 μM). The TRPA1 antagonist HC3-030031 (HC3, 30 μM) and the TRPV1 antagonist diaryl piperazine analog (DPA, 1 μM), reduced the effect of OA-NO(2) on phasic contraction amplitude and baseline tone. However, the non-selective TRP channel blocker, ruthenium red (30 μM) was a more effective inhibitor, reducing the effects of OA-NO(2) on basal tone by 75% and the effects on phasic amplitude by 85%. In bladder strips from CAPS-treated rats, the effect of OA-NO(2) on phasic contraction amplitude was reduced by 65% and the effect on basal tone was reduced by 60%. Pretreatment of bladder strips with a combination of neurokinin receptor antagonists (NK1 selective antagonist, CP 96345; NK2 selective antagonist, MEN 10,376; NK3 selective antagonist, SB 234,375, 1 μM each) reduced the effect of OA-NO(2) on basal tone, but not phasic contraction amplitude. These results indicate that nitroalkene fatty acid derivatives can activate TRP channels on CAPS-sensitive afferent nerve terminals, leading to increased bladder contractile activity. Nitrated fatty acids produced endogenously by the combination of fatty acids and oxides of nitrogen released from the urothelium and/or afferent nerves may play a role in modulating bladder activity.

Oxidative stress-induced posttranslational modification of TRPV1 expressed in esophageal epithelial cells

Human esophageal epithelium is continuously exposed to physical stimuli or to gastric acid that sometimes causes inflammation of the mucosa. Transient receptor potential vanilloid 1 (TRPV1) is a nociceptive, Ca(2+)-selective ion channel activated by capsaicin, heat, and protons. It has been reported that activation of TRPV1 expressed in esophageal mucosa is involved in gastroesophageal reflux disease (GERD) or in nonerosive GERD symptoms. In this study, we examined the expression and function of TRPV1 in the human esophageal epithelial cell line Het1A, focusing in particular on the role of oxidative stress. Interleukin-8 (IL-8) secreted by Het1A cells upon stimulation by capsaicin or acid with/without 4-hydroxy-2-nonenal (HNE) was measured by ELISA. Following capsaicin stimulation, the intracellular production of reactive oxygen species (ROS) was determined using a redox-sensitive fluorogenic probe, and ROS- and HNE-modified proteins were determined by Western blotting using biotinylated cysteine and anti-HNE antibody, respectively. HNE modification of TRPV1 proteins was further investigated by immunoprecipitation after treatment with synthetic HNE. Capsaicin and acid induced IL-8 production in Het1A cells, and this production was diminished by antagonists of TRPV1. Capsaicin also significantly increased the production of intracellular ROS and ROS- or HNE-modified proteins in Het1A cells. Moreover, IL-8 production in capsaicin-stimulated Het1A cells was enhanced by synthetic HNE treatment. Immunoprecipitation studies revealed that TRPV1 was modified by HNE in synthetic HNE-stimulated Het1A cells. We concluded that TRPV1 functions in chemokine production in esophageal epithelial cells, and this function may be regulated by ROS via posttranslational modification of TRPV1.

Nobilamides A-H, long-acting transient receptor potential vanilloid-1 (TRPV1) antagonists from mollusk-associated bacteria

New compounds nobilamides A-H and related known compounds A-3302-A and A-3302-B were isolated based upon their suppression of capsaicin-induced calcium uptake in a mouse dorsal root ganglion primary cell culture assay. Two of these compounds, nobilamide B and A-3302-A, were shown to be long-acting antagonists of mouse and human TRPV1 channels, abolishing activity for >1 h after removal of drug presumably via a covalent attachment. Other derivatives also inhibited the TRPV1 channel, albeit with low potency, affording a structure-activity profile to support the proposed mechanism of action. While the activities were modest, we propose a new mechanism of action and a new site of binding for these inhibitors that may spur development of related analogues for treatment of pain.

Sensing pulmonary oxidative stress by lung vagal afferents

Oxidative stress in the bronchopulmonary airways can occur through a variety of inflammatory mechanisms and also following the inhalation of environmental pollutants. Oxidative stress causes cellular dysfunction and thus mammals (including humans) have developed mechanisms for detecting oxidative stress, such that defensive behavior and defensive biological mechanisms can be induced to lessen its potential damage. Vagal sensory nerves innervating the airways play a critical role in the detection of the microenvironment in the airways. Oxidative stress and associated compounds activate unmyelinated bronchopulmonary C-fibers, initiating action potentials in these nerves that conduct centrally to evoke unpleasant sensations (e.g. urge to cough, dyspnea, chest-tightness) and to stimulate/modulate reflexes (e.g. cough, bronchoconstriction, respiratory rate, inspiratory drive). This review will summarize the published evidence regarding the mechanisms by which oxidative stress, reactive oxygen species, environmental pollutants and lipid products of peroxidation activate bronchopulmonary C-fibers. Evidence suggests a key role for transient receptor potential ankyrin 1 (TRPA1), although transient receptor potential vanilloid 1 (TRPV1) and purinergic P2X channels may also play a role. Knowledge of these pathways greatly aids our understanding of the role of oxidative stress in health and disease and represents novel therapeutic targets for diseases of the airways.

Intracellular redox state alters NMDA receptor response during aging through Ca2+/calmodulin-dependent protein kinase II

The contribution of the NMDA receptors (NMDARs) to synaptic plasticity declines during aging, and the decline is thought to contribute to memory deficits. Here, we demonstrate that an age-related shift in intracellular redox state contributes to the decline in NMDAR responses through Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). The oxidizing agent xanthine/xanthine oxidase (X/XO) decreased the NMDAR-mediated synaptic responses at hippocampal CA3-CA1 synapses in slices from young (3-8 months) but not aged (20-25 months) rats. Conversely, the reducing agent dithiothreitol (DTT) selectively enhanced NMDAR response to a greater extent in aged hippocampal slices. The enhancement of NMDAR responses facilitated induction of long-term potentiation in aged but not young animals. The DTT-mediated growth in the NMDAR response was not observed for the AMPA receptor-mediated synaptic responses. A similar increase was observed by intracellular application of the membrane-impermeable reducing agent, L-glutathione (L-GSH), through the intracellular recording pipette, indicating that the increased NMDAR response was dependent on intracellular redox state. DTT enhancement of the NMDAR response was dependent on CaMKII activity and was blocked by the CaMKII inhibitor--myristoylated autocamtide-2-related inhibitory peptide (myr-AIP)--but not by inhibition of the activity of protein phosphatases--PP1 and calcineurin (CaN/PP2B) or protein kinase C. CaMKII activity assays established that DTT increased CaMKII activity in CA1 cytosolic extracts in aged but not in young animals. These findings indicate a link between oxidation of CaMKII during aging, a decline in NMDAR responses, and altered synaptic plasticity.

TRPV1: on the road to pain relief

Historically, drug research targeted to pain treatment has focused on trying to prevent the propagation of action potentials in the periphery from reaching the brain rather than pinpointing the molecular basis underlying the initial detection of the nociceptive stimulus: the receptor itself. This has now changed, given that many receptors of nociceptive stimuli have been identified and/or cloned. Transient Receptor Potential (TRP) channels have been implicated in several physiological processes such as mechanical, chemical and thermal stimuli detection. Ten years after the cloning of TRPV1, compelling data has been gathered on the role of this channel in inflammatory and neuropathic states. TRPV1 activation in nociceptive neurons, where it is normally expressed, triggers the release of neuropeptides and transmitters resulting in the generation of action potentials that will be sent to higher CNS areas where they will often be perceived as pain. Its activation also will evoke the peripheral release of pro-inflammatory compounds that may sensitize other neurons to physical, thermal or chemical stimuli. For these reasons as well as because its continuous activation causes analgesia, TRPV1 has become a viable drug target for clinical use in the management of pain. This review will provide a general picture of the physiological and pathophysiological roles of the TRPV1 channel and of its structural, pharmacological and biophysical properties. Finally, it will provide the reader with an overall view of the status of the discovery of potential therapeutic agents for the management of chronic and neuropathic pain.

Oxidative challenges sensitize the capsaicin receptor by covalent cysteine modification

The capsaicin receptor TRPV1, one of the major transduction channels in the pain pathway, integrates information from extracellular milieu to control excitability of primary nociceptive neurons. Sensitization of TRPV1 heightens pain sensation to moderately noxious or even innocuous stimuli. We report here that oxidative stress markedly sensitizes TRPV1 in multiple species' orthologs. The sensitization can be recapitulated in excised inside-out membrane patches, reversed by strong reducing agents, and blocked by pretreatment with maleimide that alkylates cysteines. We identify multiple cysteines required for full modulation of TRPV1 by oxidative challenges. Robust oxidative modulation recovers the agonist sensitivity of receptors desensitized by prolonged exposure to capsaicin. Moreover, oxidative modulation operates synergistically with kinase or proton modulations. Thus, oxidative modulation is a robust mechanism tuning TRPV1 activity via covalent modification of evolutionarily conserved cysteines and may play a role in pain sensing processes during inflammation, infection, or tissue injury.

Effect of acute copper sulfate exposure on olfactory responses to amino acids and pheromones in goldfish (Carassius auratus)

Exposure of olfactory epithelium to environmentally relevant concentrations of copper disrupts olfaction in fish. To examine the dynamics of recovery at both functional and morphological levels after acute copper exposure, unilateral exposure of goldfish olfactory epithelia to 100 microM CuSO(4) (10 min) was followed by electro-olfactogram (EOG) recording and scanning electron microscopy. Sensitivity to amino acids (l-arginine and l-serine), generally considered food-related odorants, recovered most rapidly (three days), followed by that to catecholamines (3-O-methoxytyramine), bile acids (taurolithocholic acid) and the steroid pheromone, 17,20beta-dihydroxy-4-pregnen-3-one 20-sulfate, which took 28 days to reach full recovery. Sensitivity to the postovulatory pheromone prostaglandin F(2alpha) had not fully recovered even at 28 days. These changes in sensitivity were correlated with changes in the recovery of ciliated and microvillous receptor cell types. Microvillous cells appeared largely unaffected by CuSO(4) treatment. Cilia in ciliated receptor neurones, however, appeared damaged one day post-treatment and were virtually absent after three days but had begun to recover after 14 days. Together, these results support the hypothesis that microvillous receptor neurones detect amino acids whereas ciliated receptor neurones were not functional and are responsible for detection of social stimuli (bile acids and pheromones). Furthermore, differences in sensitivity to copper may be due to different transduction pathways in the different cell types.

Pharmacology of vanilloid transient receptor potential cation channels

Depending on their primary structure, the 28 mammalian transient receptor potential (TRP) cation channels identified so far can be sorted into 6 subfamilies: TRPC ("Canonical"), TRPV ("Vanilloid"), TRPM ("Melastatin"), TRPP ("Polycystin"), TRPML ("Mucolipin"), and TRPA ("Ankyrin"). The TRPV subfamily (vanilloid receptors) comprises channels critically involved in nociception and thermosensing (TRPV1, TRPV2, TRPV3, and TRPV4), whereas TRPV5 and TRPV6 are involved in renal Ca(2+) absorption/reabsorption. Apart from TRPV1, the pharmacology of these channels is still insufficiently known. Furthermore, only few small-molecule ligands for non-TRPV1 vanilloid receptors have been identified, and little is known of their endogenous ligands, resulting in a substantial "orphan" state for these channels. In this review, we summarize the pharmacological properties of members of the TRPV subfamily, highlighting the critical issues and challenges facing their "deorphanization" and clinical exploitation.

Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia, acting via transient receptor potential vanilloid 1-dependent and independent mechanisms

Inflammatory diseases associated with pain are often difficult to treat in the clinic due to insufficient understanding of the nociceptive pathways involved. Recently, there has been considerable interest in the role of reactive oxygen species (ROS) in inflammatory disease, but little is known of the role of hydrogen peroxide (H(2)O(2)) in hyperalgesia. In the present study, intraplantar injection of H(2)O(2)-induced a significant dose- and time-dependent mechanical and thermal hyperalgesia in the mouse hind paw, with increased c-fos activity observed in the dorsal horn of the spinal cord. H(2)O(2) also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H(2)O(2) levels were significantly raised in the carrageenan-induced hind paw inflammation model, showing that this ROS is produced endogenously in a model of inflammation. Moreover, superoxide dismutase and catalase significantly reduced carrageenan-induced mechanical and thermal hyperalgesia, providing evidence of a functionally significant endogenous role. Thermal, but not mechanical, hyperalgesia in response to H(2)O(2) (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H(2)O(2). In conclusion, we demonstrate a notable effect of H(2)O(2) in mediating inflammatory hyperalgesia, thus highlighting H(2)O(2) removal as a novel therapeutic target for anti-hyperalgesic drugs in the clinic.

Reduced hyperalgesia in homozygous carriers of a GTP cyclohydrolase 1 haplotype

BACKGROUND

Carriers of a particular haplotype of the GTP cyclohydrolase gene (GCH1) had less pain after surgery for chronic lumbar radiculopathy and a decreased sensitivity to some experimental mechanical pain stimuli. Ex-vivo, GCH1 upregulation and BH4 production after forskolin stimulation were reduced, while baseline BH4 concentrations were not affected. This suggested that the haplotype may mainly exert its modulating function when the GCH1 system is provoked. The present study aimed at (i) testing this hypothesis and (ii) independently reproducing the pain-decreasing effects of a particular GCH1 haplotype having been previously associated with pain protection.

METHODS

Experimental pain models with sensitization (local skin inflammation, dermal capsaicin application) and without sensitization (punctate pressure, blunt pressure, thermal and electrical pain) were assessed in 10 homozygous and 22 non-carriers of the particular GCH1 haplotype reportedly associated with pain protection. GCH1, iNOS upregulation and BH4 production were assessed ex-vivo in white blood cells after lipopolysaccharide stimulation for 24 h.

RESULTS

Carriers of the particular GCH1 haplotype addressed in this study had higher thresholds to punctate mechanical pain (von Frey hairs) following local skin inflammation (18.1+/-11.3 vs. 9+/-2.8 g; p=0.005) and, to a lesser degree, to heat pain following capsaicin sensitization (35.2+/-0.9 vs. 36.6+/-2.4 degrees C; p=0.026). In contrast, heat and pressure thresholds and tolerance to electrical stimulation in pain models without sensitization did not differ among the genotypes. GCH1, BH4 and iNOS upregulation in white blood cells after lipopolysaccharide stimulation were decreased in carriers of the GCH1 haplotype, which verified that the genotype groups differed with respect to regulation of the biopterin pathway.

CONCLUSIONS

This study verifies previous results that decreased GCH1 function or inducibility as a result of genetic polymorphisms protects against pain. This study extents previous results by showing that this pain protection is mainly conferred under conditions of hyperalgesia resulting from sensitization, supporting specific functions of BH4 in relation to particular aspects of pain.

Redox modulation of long-term potentiation in the hippocampus via regulation of the glycogen synthase kinase-3beta pathway

Alzheimer disease (AD) is an age-related neurodegenerative disorder. Many observations indicate that impaired redox regulation is implicated in AD with synaptic failure. The aim of the current investigation was to characterize the role of redox-active agents on long-term potentiation (LTP) in the CA1 region of rat hippocampal slices and to elucidate the molecular sequence of events leading to these changes. The results presented here indicate that the membrane-permeable oxidizing agent chloramine-T (CH-T) inhibits the induction of LTP, whereas the membrane-permeable reducing agent dithiothreitol (DTT) enhances the induction of LTP. In contrast, neither the membrane-impermeable oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) nor the membrane-impermeable reducing agent tris-(2-carboxyethyl) phosphine (TCEP) can affect the induction of LTP. The inhibition of LTP by CH-T can be restored by pretreatment with DTT but not with TCEP, whereas the enhancement of LTP by DTT can be reversed by pretreatment with CH-T but not with DTNB. We also provide evidence that the CH-T-evoked inhibition of LTP is mediated via activation of glycogen synthase kinase-3beta (GSK-3beta), whereas the DTT-evoked enhancement of LTP is mediated via inactivation of GSK-3beta. These findings will benefit the understanding of the redox contribution to the mechanisms underlying synaptic plasticity and AD pathogenesis.

Activation of transient receptor potential ankyrin 1 by hydrogen peroxide

Hydrogen peroxide (H(2)O(2)), which is contained in industrial products, is also generated within cells. H(2)O(2) causes pain but it has not been elucidated how it activates sensory neurons in the pain pathway. Here we show that transient receptor potential ankyrin 1 (TRPA1), expressed by sensory neurons in the pain pathway, is a receptor for H(2)O(2). H(2)O(2) activated mouse TRPA1 to induce Ca(2+) influx and elicit non-selective cation currents. These effects of H(2)O(2) were mimicked by both reactive oxygen species and reactive nitrogen species. Cysteine-reducing agents suppressed H(2)O(2)-induced TRPA1 activation, whereas cysteine-oxidizing agents activated TRPA1. H(2)O(2) caused Ca(2+) influx in a subset of dorsal root ganglia neurons, which responded to allyl isothiocyanate, a TRPA1 ligand. These results suggest that TRPA1 might be involved in the sensation of pain caused by H(2)O(2).

A single N-terminal cysteine in TRPV1 determines activation by pungent compounds from onion and garlic

Some members of the transient receptor potential (TRP) family of cation channels mediate sensory responses to irritant substances. Although it is well known that TRPA1 channels are activated by pungent compounds found in garlic, onion, mustard and cinnamon extracts, activation of TRPV1 by these extracts remains controversial. Here we establish that TRPV1 is activated by pungent extracts from onion and garlic, as well as by allicin, the active compound in these preparations, and participates together with TRPA1 in the pain-related behavior induced by this compound. We found that in TRPV1 these agents act by covalent modification of cysteine residues. In contrast to TRPA1 channels, modification of a single cysteine located in the N-terminal region of TRPV1 was necessary and sufficient for all the effects we observed. Our findings point to a conserved mechanism of activation in TRP channels, which provides new insights into the molecular basis of noxious stimuli detection.

Channel regulation by extracellular redox protein

The most widely studied stimuli for ion channel activation are changes in membrane voltage and binding of a chemical ligand in a pocket of the channel protein. While modulation by redox potential has also been appreciated our study shows previously unrecognised channel activation via electron donation from the extracellular redox protein thioredoxin (TRX).(1) The ion channel type involved is a member of the Transient Receptor Potential (TRP) family. Activation by TRX led us to consider the relevance of TRP channels to the inflammatory condition of rheumatoid arthritis, where functions of ion channels are relatively unknown and TRX concentrations are high. TRP channel activation was found to be inhibitory for secretion of matrix metalloproteinases, suggesting activation by TRX may have a protective role against disease. Here we expand on our original article and discuss the potential wider implications of the findings in terms of concepts for channel activation and relevance to other ion channel types and systems.

Reducing agents sensitize C-type nociceptors by relieving high-affinity zinc inhibition of T-type calcium channels

Recent studies have demonstrated an important role for T-type Ca2+ channels (T-channels) in controlling the excitability of peripheral pain-sensing neurons (nociceptors). However, the molecular mechanisms underlying the functions of T-channels in nociceptors are poorly understood. Here, we demonstrate that reducing agents as well as endogenous metal chelators sensitize C-type dorsal root ganglion nociceptors by chelating Zn2+ ions off specific extracellular histidine residues on Ca(v)3.2 T-channels, thus relieving tonic channel inhibition, enhancing Ca(v)3.2 currents, and lowering the threshold for nociceptor excitability in vitro and in vivo. Collectively, these findings describe a novel mechanism of nociceptor sensitization and firmly establish reducing agents, as well as Zn2+, Zn2+-chelating amino acids, and Zn2+-chelating proteins as endogenous modulators of Ca(v)3.2 and nociceptor excitability.