Nociception and TRP channels

Nociceptive transient receptor potential ankyrin 1 (TRPA1) in sensory neurons are targets of the antifungal drug econazole

BACKGROUND

Econazole is a widely used imidazole derivative antifungal for treating skin infections. The molecular targets for its frequent adverse effects of skin irritation symptoms, such as pruritus, burning sensation, and pain, have not been clarified. Transient receptor potential (TRP) channels, non-selective cation channels, are mainly expressed in peripheral sensory neurons and serve as sensors for various irritants.

METHODS

We investigated the effect of econazole on TRP channel activation by measuring intracellular calcium concentration ([Ca2+]i) through fluorescent ratio imaging in mouse dorsal root ganglion (DRG) neurons isolated from wild-type, TRPA1(-/-) and TRPV1(-/-) mice, as well as in heterologously TRP channel-expressed cells. A cheek injection model was employed to assess econazole-induced itch and pain in vivo.

RESULTS

Econazole evoked an increase in [Ca2+]i, which was abolished by the removal of extracellular Ca2+ in mouse DRG neurons. The [Ca2+]i responses to econazole were suppressed by a TRPA1 blocker but not by a TRPV1 blocker. Attenuation of the econazole-induced [Ca2+]i responses was observed in the TRPA1(-/-) mouse DRG neurons but was not significant in the TRPV1(-/-) neurons. Econazole increased the [Ca2+]i in HEK293 cells expressing TRPA1 (TRPA1-HEK) but not in those expressing TRPV1, although at higher concentrations, it induced Ca2+ mobilization from intracellular stores in untransfected naïve HEK293 cells. Miconazole, which is a structural analog of econazole, also increased the [Ca2+]i in mouse DRG neurons and TRPA1-HEK, and its nonspecific action was larger than econazole. Fluconazole, a triazole drug failed to activate TRPA1 and TRPV1 in mouse DRG neurons and TRPA1-HEK. Econazole induced itch and pain in wild-type mice, with reduced responses in TRPA1(-/-) mice.

CONCLUSIONS

These findings suggested that the imidazole derivatives econazole and miconazole may induce skin irritation by activating nociceptive TRPA1 in the sensory neurons. Suppression of TRPA1 activation may mitigate the adverse effects of econazole.

Inhibitory effects of linalool, an essential oil component of lavender, on nociceptive TRPA1 and voltage-gated Ca2+ channels in mouse sensory neurons

Linalool, an essential oil component of lavender is commonly used in fragrances. It is known that linalool has anxiolytic, sedative, and analgesic actions. However, the mechanism of its analgesic action has not yet been fully clarified. Pain signals elicited by the activation of nociceptors on peripheral neurons are transmitted to the central nervous system. In the present study, we investigated the effects of linalool on transient receptor potential (TRP) channels and voltage-gated channels, both of which are important for pain signaling via nociceptors in somatosensory neurons. For detection of channel activity, the intracellular Ca2+ concentration ([Ca2+]i) was measured using a Ca2+-imaging system, and membrane currents were recorded using the whole-cell patch-clamp technique. Analgesic actions were also examined in vivo. In mouse sensory neurons linalool at concentrations that did not induce [Ca2+]i increases did not affect [Ca2+]i responses to capsaicin and acids, TRPV1 agonists, but suppressed those induced by allyl isothiocyanate (AITC) and carvacrol, TRPA1 agonists. Similar inhibitory effects of linalool were observed in cells that heterologously expressed TRPA1. Linalool attenuated the [Ca2+]i increases induced by KCl and voltage-gated Ca2+ currents but only slightly suppressed voltage-gated Na＋currents in mouse sensory neurons. Linalool diminished TRPA1-mediated nociceptive behaviors. The present data suggest that linalool exerts an analgesic action via the suppression of nociceptive TRPA1 and voltage-gated Ca2+ channels.

Lysionotin exerts antinociceptive effects in various models of nociception induction

Background

Lysionotin, a natural flavonoid extracted from Lysionotus pauciflorus Maxim (Gesneriaceae), has several pharmacological effects including anti-bacterial, anti-hypertensive and anti-inflammatory effects. However, its analgesic effect has not been investigated. This study aimed to assess the antinociceptive activity of lysionotin using chemically and thermally induced nociception in a mouse model.

Methods

The antinociceptive effects of various lysionotin doses (50, 100, 150, and 200 μg/kg) were assessed in mice using the acetic acid-induced writhing test, hot plate test, and formalin-induced paw licking assay. The effects were compared to those of mice treated with acetylsalicylic acid or morphine in the presence or absence of naloxone (an opioid receptor antagonist). Capsaicin- and glutamate-induced paw licking tests were also used to evaluate the involvement of the vanilloid and glutamatergic systems, respectively.

Results

Lysionotin produced significant dose-dependent inhibition of nociceptive behavior in the acetic acid-induced writhing test, showing 60% inhibition at a dose of 200 μg/kg. Lysionotin also caused a significant increase in the latency period in response to the hot plate test (76.4% at 200 μg/kg), and significantly inhibited both the neurogenic and inflammatory phases in the formalin-induced paw licking test. Naloxone significantly reverses the effect of lysionotin in both hot plate test and formalin-induced paw licking test. Moreover, lysionotin significantly inhibited the neurogenic nociception induced by intraplantar injections of glutamate and capsaicin (57% and 67.2%, respectively at a dose of 200 μg/kg). Thus, lysionotin exhibited peripheral and central antinociception through the modulation of vanilloid receptors, opioid receptors, and the glutamatergic system.

Conclusion

Lysionotin possesses antinociceptive activity on adult mice that is mediated through both central and peripheral pathways.

The effects of vanilloid analogues structurally related to capsaicin on the transient receptor potential vanilloid 1 channel

Transient receptor potential vanilloid 1 (TRPV1) is known as a receptor of capsaicin, a spicy ingredient of chili peppers. It is also sensitive to a variety of pungent compounds and is involved in nociception. Here, we focused on the structural characteristics of capsaicin, and investigated whether vanillylmanderic acid (VMA), vanillic acid (VAcid), vanillyl alcohol (VAlc), vanillyl butyl ether (VBE), and vanillin, containing a vanillyl skeleton similar to capsaicin, affected the TRPV1 activities. For detection of TRPV1 activity, intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured in HEK 293 cells heterologously expressing mouse TRPV1 (mTRPV1-HEK) and in mouse sensory neurons. Except for vanillin, four vanilloid analogues dose-dependently increased [Ca²⁺]_i in mTRPV1-HEK. The solutions that dissolved VMA, VAcid and vanillin at high concentrations were acidic, whereas those of VAlc and VBE were neutral. Neutralized VAcid evoked [Ca²⁺]_i increases but neutralized VMA did not. Mutation of capsaicin-sensing sites diminished [Ca²⁺]_i responses to VAcid, VAlc and VBE. VAcid, VMA, and vanillin suppressed the activation of TRPV1 induced by capsaicin. VAcid and VMA also inhibited the acid-induced TRPV1 activation. In sensory neurons, VMA diminished TRPV1 activation by capsaicin or acids. The present data indicate that these structural characteristics of chemical compounds on TRPV1 may provide strategies for the development of novel analgesic drugs targeting nociceptive TRPV1.

Chia oil prevents chemical and immune-mediated inflammatory responses in mice: Evidence for the underlying mechanisms

Chia (Salvia hispanica L.) is an herbaceous plant used as omega-3 polyunsaturated fatty acid (ω-3 PUFA) source that presents a range of beneficial effects on human health. Herein, it was used a chia oil containing over than 62% of α-linolenic acid (ALA), a compound widely related to anti-inflammatory actions. Chia oil effect was tested using paw edema and mechanical hyperalgesia induced by carrageenan, and ear edema induced by croton oil, histamine, and capsaicin. Croton oil was used in both preventive and therapeutic treatment schedules of chia oil while histamine and capsaicin were used only in preventive treatment schedule. Chia oil mechanism of action was investigated using nociception and paw edema response induced by intraplantar injection of acidified saline (ASIC activator), PGE₂ (prostaglandin pathway), cinnamaldehyde (TRPA1 activator), bradykinin (BK pathway), menthol (TRPM8 activator), and capsaicin (TRPV1 activator). Further, RT-PCR for inflammatory mediators (TRPA1, NF-κB, PPAR-γ, COX-2, IL-6, TNF, FPR2, FAAH, MAGL, and IL-12A) induced by carrageenan, NLRP3 inflammasome activation, and the cell viability were then accessed. Later, chia oil actions were evaluated in the experimental autoimmune encephalomyelitis (EAE), a multiple sclerosis (MS) model. Chia oil showed anti-edematogenic and anti-hyperalgesic effects when administered 1 h before pro-inflammatory stimulus - particularly carrageenan and croton oil. Moreover, chia oil upregulated the mRNA levels of COX-2 and formyl peptide receptor 2 (FPR2) while reduced IL-6 expression in the spinal cord of mice submitted to i.pl. injection of carrageenan. Interestingly, chia oil mediates antinociceptive effects in mice decreasing the nociceptive response induced by acidified saline, PGE₂, and cinnamaldehyde, but not by bradykinin, menthol, and capsaicin. On the EAE model, chia oil preventively administered attenuated EAE-induced motor deficits and mechanical hyperalgesia in mice, suggesting a valuable effect of chia oil supplementation in regulating inflammatory responses and some immune functions during immune-mediated inflammatory disorders (IMID). Nonetheless, additional reports will need to assess the effect of chia oil in well-controlled clinical trials performed in MS patients.

Selenium and Neurological Diseases: Focus on Peripheral Pain and TRP Channels

Pain is a complex physiological process that includes many components. Growing evidence supports the idea that oxidative stress and Ca2+ signaling pathways participate in pain detection by neurons. The main source of endogenous reactive oxygen species (ROS) is mitochondrial dysfunction induced by membrane depolarization, which is in turn caused by Ca2+ influx into the cytosol of neurons. ROS are controlled by antioxidants, including selenium. Selenium plays an important role in the nervous system, including the brain, where it acts as a cofactor for glutathione peroxidase and is incorporated into selenoproteins involved in antioxidant defenses. It has neuroprotective effects through modulation of excessive ROS production, inflammation, and Ca2+ overload in several diseases, including inflammatory pain, hypersensitivity, allodynia, diabetic neuropathic pain, and nociceptive pain. Ca2+ entry across membranes is mediated by different channels, including transient receptor potential (TRP) channels, some of which (e.g., TRPA1, TRPM2, TRPV1, and TRPV4) can be activated by oxidative stress and have a role in the induction of peripheral pain. The results of recent studies indicate the modulator roles of selenium in peripheral pain through inhibition of TRP channels in the dorsal root ganglia of experimental animals. This review summarizes the protective role of selenium in TRP channel regulation, Ca2+ signaling, apoptosis, and mitochondrial oxidative stress in peripheral pain induction.

Brown Adipose Tissue, Diet-Induced Thermogenesis, and Thermogenic Food Ingredients: From Mice to Men

Since the recent rediscovery of brown adipose tissue (BAT) in adult humans, this thermogenic tissue has been attracting increasing interest. The inverse relationship between BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. Cold exposure activates and recruits BAT, resulting in increased energy expenditure and decreased body fatness. The stimulatory effects of cold exposure are mediated through transient receptor potential (TRP) channels and the sympathetic nervous system (SNS). Most TRP members also function as chemesthetic receptors for various food ingredients, and indeed, agonists of TRP vanilloid 1 such as capsaicin and its analog capsinoids mimic the effects of cold exposure to decrease body fatness through the activation and recruitment of BAT. The antiobesity effect of other food ingredients including tea catechins may be attributable, at least in part, to the activation of the TRP-SNS-BAT axis. BAT is also involved in the facultative thermogenesis induced by meal intake, referred to as diet-induced thermogenesis (DIT), which is a significant component of the total energy expenditure in our daily lives. Emerging evidence suggests a crucial role for the SNS in BAT-associated DIT, particularly during the early phase, but several gut-derived humoral factors may also participate in meal-induced BAT activation. One intriguing factor is bile acids, which activate BAT directly through Takeda G-protein receptor 5 (TGR5) in brown adipocytes. Given the apparent beneficial effects of some TRP agonists and bile acids on whole-body substrate and energy metabolism, the TRP/TGR5-BAT axis represents a promising target for combating obesity and related metabolic disorders in humans.

A critical evaluation of TRPA1-mediated locomotor behavior in zebrafish as a screening tool for novel anti-nociceptive drug discovery

Current medications inadequately treat the symptoms of chronic pain experienced by over 50 million people in the United States, and may come with substantial adverse effects signifying the need to find novel treatments. One novel therapeutic target is the Transient Receptor Potential A1 channel (TRPA1), an ion channel that mediates nociception through calcium influx of sensory neurons. Drug discovery still relies heavily on animal models, including zebrafish, a species in which TRPA1 activation produces hyperlocomotion. Here, we investigated if this hyperlocomotion follows zebrafish TRPA1 pharmacology and evaluated the strengths and limitations of using TRPA1-mediated hyperlocomotion as potential preclinical screening tool for drug discovery. To support face validity of the model, we pharmacologically characterized mouse and zebrafish TRPA1 in transfected HEK293 cells using calcium assays as well as in vivo. TRPA1 agonists and antagonists respectively activated or blocked TRPA1 activity in HEK293 cells, mice, and zebrafish in a dose-dependent manner. However, our results revealed complexities including partial agonist activity of TRPA1 antagonists, bidirectional locomotor activity, receptor desensitization, and off-target effects. We propose that TRPA1-mediated hyperlocomotion in zebrafish larvae has the potential to be used as in vivo screening tool for novel anti-nociceptive drugs but requires careful evaluation of the TRPA1 pharmacology.

Sida tuberculata extract reduces the nociceptive response by chemical noxious stimuli in mice: Implications for mechanism of action, relation to chemical composition and molecular docking

Sida tuberculata R.E.Fr. (Malvaceae) is a medicinal plant widely found in Southern Brazil, and popularly used for inflammatory disorders and to pain relief. A phytochemical analysis followed by an investigation about antinociceptive potential and mechanism of action were performed with leaves and roots extracts. Methanolic extracts, designated as S. tuberculata leaves extract (STLE) and S. tuberculata roots extract, were analyzed both by UHPLC–MS. The in vivo antinociceptive potential of STLE (10–300 mg kg−1) was assessed in mice subjected to the acetic acid‐induced abdominal writhes and formalin model. Agonist/antagonist tests and computational docking suggest the involvement of opioid and adenosinergic systems. The main chemical class detected on extracts was the ecdysteroids, and 20‐hydoxyecdysone (20HE) was confirmed as the major phytoconstituent. The pretreatment with STLE (100 mg kg−1) reduced more than 70% abdominal contortions induced by acetic acid model and produced significant inhibition on formalin‐induced licking response. The mechanism of action study revealed STLE might act through opioid and adenosine systems. Molecular docking suggested kaempferol derivative and 20HE might interacting with μ‐opioid receptor. Thus, the results suggest the existence of antinociceptive potential from S. tuberculata extracts being in accordance to the traditional use.

Antinociceptive activity of Riparin II from Aniba riparia: Further elucidation of the possible mechanisms

Riparin II (RipII) has an anti-inflammatory activity potentially due its ability to decrease TNF-α and IL-1β production and its histamine antagonism. The objective of this study was to evaluate the role of RipII in the pain process and the possible antinociceptive mechanisms involved, using classic models of nociception. Male Swiss mice were used in the assays. Determinate the acute toxicity according to the OECD 425 test guideline. The models used were the acetic acid-, formalin-, hot plate and glutamate-induced nociception. For evaluation of antinociceptive effect, the involvement of TRPV1, TRPA1, TRPM8, ASICS, Bradykinin, PKC and PKA were performed using the paw licking using agonists. The acute toxicity study did not detect any clinical signs or changes in behavior or mortality. RipII, administered orally (25 and 50 mg/kg) caused a reduction of nociception induced by acetic acid, formalin (on the second phase) and glutamate. In the investigation of antinociceptive mechanism, we used capsaicin (2.2 μg/paw), cinnamaldehyde (10 nmol/paw), menthol (1.2 μmol/paw), ASICS (2% acetic acid, pH 1.98) and bradykinin (10 μg/paw). The results showed that TRPV1, TRPA1, TRPM8, ASICS and bradykinin play a role in the antinociceptive effect of RipII. The results also showed that PKA is involved too. These data demonstrate that RipII has a low or not toxicity and produced an important antinociceptive effect through mechanisms that probably involve an interaction, at least in part, TRPV1, TRPA1, TRPM8, ASICS, bradykinin and PKA participate in the RipII's antinociceptive effect.

TRPA1 polymorphisms in chronic and complete spinal cord injury patients with neuropathic pain: a pilot study

Study design

Pilot study.

Objectives

Single-nucleotide polymorphisms (SNPs) in TRPA1 gene are related to the etiology of chronic pain. The study is a pilot study with the primary objective of analyzing these SNPs in Spanish patients with chronic and complete spinal cord injury (SCI) and neuropathic pain (NPP).

Setting

Asepeyo Hospital Department of Chronic and Complete SCI.

Methods

Twelve patients with chronic and complete SCI and NPP, and 12 patients with chronic and complete SCI with no pain were reviewed. International Spinal Cord Injury Pain Classification (LANSS) and visual analog score (VAS) were chosen to classify pain syndrome. SNPs were identified by melting analysis after DNA amplification with real-time fluorescence PCR.

Results

There were differences in rs11988795 variant: GG homozygous (p = 0.01) and G allele (p = 0.001) were more frequent in SCI patients with no pain. There were differences in rs13255063 variant: TT homozygous were prevalent (p = 0.03) in patients with NPP.

Conclusions

Until now this is the first study to show a description of TRPA1 SNPs in Spanish patients with chronic and complete SCI and NPP. These results suggest that GG genotype in rs11988795 variant and G allele could be protective factors against NPP. TT genotype in rs13255063 variant could be a risk factor for NPP. Neuropathic pain after spinal cord injuries may have genetic contributions.

Long Non-coding RNA BC168687 is Involved in TRPV1-mediated Diabetic Neuropathic Pain in Rats

Long noncoding RNAs (lncRNAs) participate in a diverse range of molecular and biological processes, and dysregulation of lncRNAs has been observed in the pathogenesis of various human diseases. We observed alterations in mechanical withdrawal thresholds (MWT) and thermal withdrawal latencies (TWL) in streptozotocin (STZ)-induced diabetic rats treated with small interfering RNA (siRNA) of lncRNA BC168687. We detected expression of transient receptor potential vanilloid type 1 (TRPV1) in rat dorsal root ganglia (DRG) by a series of molecular experiments. We determined relative levels of tumor necrosis factor (TNF)-α and interleukin (IL)-1β in rat serum by enzyme-linked immunosorbent assay (ELISA). In addition, we examined extracellular regulated protein kinases (ERK) and p38 mitogen-activated protein kinase (MAPK) signaling pathways by Western blot (WB). We showed that the MWT and TWL of diabetic rats increased significantly compared with control. Expression of TRPV1 receptors in DRG substantially decreased. Relative levels of TNF-α and IL-1β in the serum of lncRNA BC168687 siRNA-treated rats were reduced. Phosphorylation (p)-ERK and p-p38 signaling pathways in DRG were also decreased. Taken together, we concluded lncRNA BC168687 siRNA may alleviate TRPV1-mediated diabetic neuropathic pain.

Translational Aspects of Brown Fat Activation by Food-Derived Stimulants

Since the rediscovery of brown adipose tissue (BAT) in humans, its energy-dissipating ability has been well-recognized. The negative correlations of BAT activity with adiposity and insulin sensitivity provided an obvious rationale for discerning reliable and practical strategies for stimulating BAT. Though cold exposure or use of pharmacological adrenomimetics can activate BAT, they may have adverse effects. Therefore, determining alternative stimulants of BAT with lower risks such as commonly used food ingredients is highly desirable. Recent observations revealed that chemical activation of temperature-sensitive transient receptor potential (TRP) channels by food ingredients can recruit BAT in humans. Furthermore, animal studies have identified several food-derived stimulants of BAT acting through multiple mechanisms distinct from a TRP-mediated process. Dietary compounds acting as an activator of Sirtuin 1, a critical regulator of mitochondrial biogenesis and brown adipocyte differentiation, are one such class of promising food-derived BAT activators in humans. While the individual effects of various dietary factors are increasingly established in a laboratory setting, the potential synergistic effects of multiple stimulants on BAT remain to be tested in a clinical environment. These investigations may support the development of efficient, flexible dietary regimens capable of boosting BAT thermogenesis.

Effects of Simvastatin Beyond Dyslipidemia: Exploring Its Antinociceptive Action in an Animal Model of Complex Regional Pain Syndrome-Type I

Simvastatin is a lipid-lowering agent that blocks the production of cholesterol through inhibition of 3-hydroxy-methyl-glutaryl coenzyme A (HMG-CoA) reductase. In addition, recent evidence has suggested its anti-inflammatory and antinociceptive actions during inflammatory and pain disorders. Herein, we investigated the effects of simvastatin in an animal model of complex regional pain syndrome-type I, and its underlying mechanisms. Chronic post-ischemia pain (CPIP) was induced by ischemia and reperfusion (IR) injury of the left hind paw. Our findings showed that simvastatin inhibited mechanical hyperalgesia induced by CPIP model in single and repeated treatment schedules, respectively; however simvastatin did not alter inflammatory signs during CPIP model. The mechanisms underlying those actions are related to modulation of transient receptor potential (TRP) channels, especially TRMP8. Moreover, simvastatin oral treatment was able to reduce the nociception induced by acidified saline [an acid-sensing ion channels (ASICs) activator] and bradykinin (BK) stimulus, but not by TRPA1, TRPV1 or prostaglandin-E2 (PGE2). Relevantly, the antinociceptive effects of simvastatin did not seem to be associated with modulation of the descending pain circuits, especially noradrenergic, serotoninergic and dopaminergic systems. These results indicate that simvastatin consistently inhibits mechanical hyperalgesia during neuropathic and inflammatory disorders, possibly by modulating the ascending pain signaling (TRPM8/ASIC/BK pathways expressed in the primary sensory neuron). Thus, simvastatin open-up new standpoint in the development of innovative analgesic drugs for treatment of persistent pain, including CRPS-I.

Human psychophysics and rodent spinal neurones exhibit peripheral and central mechanisms of inflammatory pain in the UVB and UVB heat rekindling models

Translational research is key to bridging the gaps between preclinical findings and the patients, and a translational model of inflammatory pain will ideally induce both peripheral and central sensitisation, more effectively mimicking clinical pathophysiology in some chronic inflammatory conditions. We conducted a parallel investigation of two models of inflammatory pain, using ultraviolet B (UVB) irradiation alone and UVB irradiation with heat rekindling. We used rodent electrophysiology and human quantitative sensory testing to characterise nociceptive processing in the peripheral and central nervous systems in both models. In both species, UVB irradiation produces peripheral sensitisation measured as augmented evoked activity of rat dorsal horn neurones and increased perceptual responses of human subjects to mechanical and thermal stimuli. In both species, UVB with heat rekindling produces central sensitisation. UVB irradiation alone and UVB with heat rekindling are translational models of inflammation that produce peripheral and central sensitisation, respectively. The predictive value of laboratory models for human pain processing is crucial for improving translational research. The discrepancy between peripheral and central mechanisms of pain is an important consideration for drug targets, and here we describe two models of inflammatory pain that involve ultraviolet B (UVB) irradiation, which can employ peripheral and central sensitisation to produce mechanical and thermal hyperalgesia in rats and humans. We use electrophysiology in rats to measure the mechanically- and thermally-evoked activity of rat spinal neurones and quantitative sensory testing to assess human psychophysical responses to mechanical and thermal stimulation in a model of UVB irradiation and in a model of UVB irradiation with heat rekindling. Our results demonstrate peripheral sensitisation in both species driven by UVB irradiation, with a clear mechanical and thermal hypersensitivity of rat dorsal horn neurones and enhanced perceptual responses of human subjects to both mechanical and thermal stimulation. Additional heat rekindling produces markers of central sensitisation in both species, including enhanced receptive field sizes. Importantly, we also showed a correlation in the evoked activity of rat spinal neurones to human thermal pain thresholds. The parallel results in rats and humans validate the translational use of both models and the potential for such models for preclinical assessment of prospective analgesics in inflammatory pain states.

Meningeal afferent signaling and the pathophysiology of migraine

Migraine is the most common neurological disorder. Attacks are complex and consist of multiple phases but are most commonly characterized by intense, unilateral, throbbing headache. The pathophysiology contributing to migraine is poorly understood and the disorder is not well managed with currently available therapeutics, often rendering patients disabled during attacks. The mechanisms most likely to contribute to the pain phase of migraine require activation of trigeminal afferent signaling from the cranial meninges and subsequent relay of nociceptive information into the central nervous system in a region of the dorsal brainstem known as the trigeminal nucleus caudalis. Events leading to activation of meningeal afferents are unclear, but nerve endings within this tissue are mechanosensitive and also express a variety of ion channels including acid-sensing ion channels and transient receptor-potential channels. These properties may provide clues into the pathophysiology of migraine by suggesting that decreased extracellular pH and environmental irritant exposure in the meninges contributes to headache. Neuroplasticity is also likely to play a role in migraine given that attacks are triggered by routine events that are typically nonnoxious in healthy patients and clear evidence of sensitization occurs during an attack. Where and how plasticity develops is also not clear but may include events directly on the afferents and/or within the TNC. Among the mediators potentially contributing to plasticity, calcitonin gene-related peptide has received the most attention within the migraine field but other mechanisms may also contribute. Ultimately, greater understanding of the molecules and mechanisms contributing to migraine will undoubtedly lead to better therapeutics and relief for the large number of patients across the globe who suffer from this highly disabling neurological disorder.

Are sensory TRP channels biological alarms for lipid peroxidation?

Oxidative stress induces numerous biological problems. Lipid oxidation and peroxidation appear to be important steps by which exposure to oxidative stress leads the body to a disease state. For its protection, the body has evolved to respond to and eliminate peroxidation products through the acquisition of binding proteins, reducing and conjugating enzymes, and excretion systems. During the past decade, researchers have identified a group of ion channel molecules that are activated by oxidized lipids: transient receptor potential (TRP) channels expressed in sensory neurons. These ion channels are fundamentally detectors and signal converters for body-damaging environments such as heat and cold temperatures, mechanical attacks, and potentially toxic substances. When messages initiated by TRP activation arrive at the brain, we perceive pain, which results in our preparing defensive responses. Excessive activation of the sensory neuronal TRP channels upon prolonged stimulations sometimes deteriorates the inflammatory state of damaged tissues by promoting neuropeptide release from expresser neurons. These same paradigms may also work for pathologic changes in the internal lipid environment upon exposure to oxidative stress. Here, we provide an overview of the role of TRP channels and oxidized lipid connections during abnormally increased oxidative signaling, and consider the sensory mechanism of TRP detection as an alert system.

NGX-4010, a capsaicin 8% dermal patch, for the treatment of painful HIV-associated distal sensory polyneuropathy: results of a 52-week open-label study

OBJECTIVES

To evaluate the efficacy, safety, and tolerability of repeated NGX-4010 treatments in the open-label extension phase of a 52-week study in patients with neuropathic pain due to HIV-associated distal sensory polyneuropathy (HIV-DSP).

METHODS

Patients completing the 12-week, randomized, double-blind phase of the study could enter a 40-week, open-label phase, and receive up to 3, 60-minute NGX-4010 treatments. Patients recorded their "average pain for the past 24 hours" daily using the Numeric Pain Rating Scale (NPRS). Efficacy assessment included the percentage NPRS score reduction from baseline to weeks 2 to 12 after the final treatment, and Patient Global Impression of Change (PGIC) and Clinician Global Impression of Change (CGIC) questionnaires at study termination.

RESULTS

Of 307 patients randomized, 272 entered the open-label phase; 81, 90, 55, and 46 received 0, 1, 2, and 3 retreatments, respectively. The mean percentage decrease in NPRS score from baseline to weeks 2 to 12 after the final treatment was similar in patients receiving single or multiple NGX-4010 treatments (-25.8%, -27.1%, -24.6%, and -22.7% for 1, 2, 3, and 4 NGX-4010 treatments, respectively). PGIC and CGIC results demonstrated a benefit of NGX-4010 treatment through to the end of the study regardless of the number of treatments received. Transient local application site reactions were the most frequently reported adverse events, and were mainly mild to moderate, nonserious, and did not increase with repeated treatment.

DISCUSSION

Repeated NGX-4010 treatments were generally well tolerated and resulted in consistent reductions in HIV-DSP-associated pain and improvement in patient-reported outcomes.

High-Dose Capsaicin for the Treatment of Neuropathic Pain: What We Know and What We Need to Know

Neuropathic pain is a frequent and disabling condition with diverse underlying etiologies and is often difficult to treat. Systemic drug treatment is often limited in efficacy. Furthermore, adverse effects may be a limiting factor when trying to reach the necessary dose. Analgesics that can be applied topically have the potential to largely overcome this problem. They may be of particular advantage in localized neuropathic pain syndromes such as postherpetic neuralgia or small fiber neuropathy. Capsaicin, the pungent component of chili peppers, is a natural ligand of the transient receptor potential vanilloid 1 channel and has long been used as topically applicable cream with concentrations of 0.025 to 0.075%. In 2009, a high-concentration transdermal capsaicin 8% patch (Qutenza^®; Acorda Therapeutics, Inc., Ardsley, NY, USA; Astellas Pharma Europe Ltd., Chertsey, Surrey, UK) was introduced for the treatment of peripheral neuropathic pain syndromes other than of diabetic origin in adults. It has since been widely used in diverse neuropathic pain disorders. In this review article, we summarize current knowledge on Qutenza, its advantages and problems, and expose unmet needs.

Selective activation of hTRPV1 by N-geranyl cyclopropylcarboxamide, an amiloride-insensitive salt taste enhancer

TRPV1t, a variant of the transient receptor potential vanilloid-1 (TRPV1) has been proposed as a constitutively active, non-selective cation channel as a putative amiloride-insensitive salt taste receptor and shares many properties with TRPV1. Based on our previous chorda tympani taste nerve recordings in rodents and human sensory evaluations, we proposed that N-geranylcyclopropylcarboxamide (NGCC), a novel synthetic compound, acts as a salt taste enhancer by modulating the amiloride/benzamil-insensitive Na⁺ entry pathways. As an extension of this work, we investigated NGCC-induced human TRPV1 (hTRPV1) activation using a Ca²⁺-flux signaling assay in cultured cells. NGCC enhanced Ca²⁺ influx in hTRPV1-expressing cells in a dose-dependent manner (EC₅₀ = 115 µM). NGCC-induced Ca²⁺ influx was significantly attenuated by ruthenium red (RR; 30 µM), a non-specific blocker of TRP channels and capsazepine (CZP; 5 µM), a specific antagonist of TRPV1, implying that NGCC directly activates hTRPV1. TRPA1 is often co-expressed with TRPV1 in sensory neurons. Therefore, we also investigated the effects of NGCC on hTRPA1-expressing cells. Similar to hTRPV1, NGCC enhanced Ca²⁺ influx in hTRPA1-expressing cells (EC₅₀ = 83.65 µM). The NGCC-induced Ca²⁺ influx in hTRPA1-expressing cells was blocked by RR (30 µM) and HC-030031 (100 µM), a specific antagonist of TRPA1. These results suggested that NGCC selectively activates TRPV1 and TRPA1 in cultured cells. These data may provide additional support for our previous hypothesis that NGCC interacts with TRPV1 variant cation channel, a putative amiloride/benzamil-insensitive salt taste pathway in the anterior taste receptive field.

Modulation of transient receptor vanilloid 1 activity by transient receptor potential ankyrin 1

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective ligand-gated cation channel responding to noxious heat, protons, and chemicals such as capsaicin. TRPV1 is expressed in sensory neurons and plays a critical role in pain associated with tissue injury, inflammation, and nerve lesions. Transient receptor potential ankyrin 1 (TRPA1) is coexpressed with TRPV1. It is activated by compounds that cause a burning sensation (e.g., mustard oil) and, indirectly, by components of the inflammatory milieu eliciting nociceptor excitation and pain hypersensitivity. Previous studies indicate an interaction of TRPV1 and TRPA1 signaling pathways. Here we sought to examine the molecular mechanisms underlying such interactions in nociceptive neurons. We first excluded physical interactions of both channels using radioligand binding studies. By microfluorimetry, electrophysiological experiments, cAMP measurements, and site-directed mutagenesis we found a sensitization of TRPV1 after TRPA1 stimulation with mustard oil in a calcium and cAMP/protein kinase A (PKA)-dependent manner. TRPA1 stimulation enhanced TRPV1 phosphorylation via the putative PKA phosphorylation site serine 116. We also detected calcium-sensitive increased TRPV1 activity after TRPA1 activation in dorsal root ganglion neurons. The inhibition of TRPA1 by HC-030031 (1,2,3,6-tetrahydro-1,3-dimethyl-N-[4-(1-methylethyl)phenyl]-2,6-dioxo-7H-purine-7-acetamide, 2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopropylphenyl)acetamide) after its initial stimulation (and the calcium-insensitive TRPA1 mutant D477A) still showed increased capsaicin-induced TRPV1 activity. This excludes a calcium-induced additive TRPA1 current after TRPV1 stimulation. Our study shows sensitization of TRPV1 via activation of TRPA1, which involves adenylyl cyclase, increased cAMP, subsequent translocation and activation of PKA, and phosphorylation of TRPV1 at PKA phosphorylation residues. This suggests that cross-sensitization of TRP channels contributes to enhanced pain sensitivity in inflamed tissues.

Novel anti-nociceptive effects of cardamonin via blocking expression of cyclooxygenase-2 and transglutaminase-2

Recently, we reported that Alpinia katsumadai (AK) has anti-nociceptive activity in vivo and that cardamonin (CDN) from AK suppresses the activity and expression of transglutaminase-2 (Tgase-2). However, it remains unknown whether CDN contributes to the anti-nociceptive activities of AK in vivo. We examined the anti-inflammatory effects of CDN in MG63 osteoblast-like cells and Raw264.7 macrophage-like cells treated with interleukin-1β treatment. CDN suppressed the expression of Tgase-2, cyclooxygenase-2 (COX-2), and p65 (nuclear factor-κB) in a concentration-dependent manner, and restored the expression of IκB in MG63 and Raw264.7 cells. However, CDN did not inhibit the activity of COX-2. Gene silencing of Tgase-2 reduced the COX-2 expression in MG63 cells. Phenylbenzoquinone (PBQ)-induced writhing, carrageenan-induced hyperalgesia, and rota-rod test were used to evaluate the anti-nociceptive activity in vivo. CDN (3-30 mg/kg, orally administered) significantly inhibited PBQ-induced writhing. CDN also produced a significant, dose-dependent increase in the withdrawal response latencies in carrageenan-induced hyperalgesia. The effects of CDN on PBQ-induced writhing were not caused by impaired motor functions. These results suggest that CDN might be helpful in controlling the pain from inflammatory diseases.

Activation of the Chemosensory Ion Channels TRPA1 and TRPV1 by Hydroalcohol Extract of Kalopanax pictus Leaves

TRPA1 and TRPV1 are members of the TRP superfamily of structurally related, nonselective cation channels. TRPA1 and TRPV1 are often co-expressed in sensory neurons and play an important role in somatosense such as cold, pain, and irritants. The first leaves of Kalopanax pictus Nakai (Araliaceae) have long been used as a culinary ingredient in Korea because of their unique chemesthetic flavor. In this study, we observed the intracellular Ca²⁺ response to cultured cells expressing human TRPA1 (hTRPA1) and human TRPV1 (hTRPV1) by Ca²⁺ imaging analysis to investigate the ability of the first leaves of K. pictus to activate the hTRPA1 and hTRPV1. An 80% ethanol extract of K. pictus (KPEx) increased intracellular Ca²⁺ influx in a response time- and concentration-dependent manner via either hTRPA1 or hTRPV1. KPEx-induced response to hTRPA1 was markedly attenuated by ruthenium red, a general blocker of TRP channels, and HC-030031, a specific antagonist of TRPA1. In addition, the intracellular Ca²⁺ influx attained with KPEx to hTRPV1 was mostly blocked by ruthenium red, and capsazepine, a specific antagonist of TRPV1. These results indicate that KPEx selectively activates both hTRPA1 and hTRPV1, which may provide evidence that the first leaves of K. pictus primarily activate TRPA1 and TRPV1 to induce their unique chemesthetic sense.

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.

Zingerone enhances glutamatergic spontaneous excitatory transmission by activating TRPA1 but not TRPV1 channels in the adult rat substantia gelatinosa

Transient receptor potential (TRP) channels are thought to play a role in regulating nociceptive transmission to spinal substantia gelatinosa (SG) neurons. It remains to be unveiled whether the TRP channels in the central nervous system are different in property from those involved in receiving nociceptive stimuli in the peripheral nervous system. We examined the effect of the vanilloid compound zingerone, which activates TRPV1 channels in the cell body of a primary afferent neuron, on glutamatergic excitatory transmission in the SG neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. Bath-applied zingerone reversibly and concentration-dependently increased spontaneous excitatory postsynaptic current (EPSC) frequency. This effect was accompanied by an inward current at -70 mV that was resistant to glutamate receptor antagonists. These zingerone effects were repeated and persisted in Na(+)-channel blocker tetrodotoxin-, La(3+)-, or IP3-induced Ca(2+)-release inhibitor 2-aminoethoxydiphenyl borate-containing or Ca(2+)-free Krebs solution. Zingerone activity was resistant to the selective TRPV1 antagonist capsazepine but sensitive to the nonselective TRP antagonist ruthenium red, the TRPA1 antagonist HC-030031, and the Ca(2+)-induced Ca(2+)-release inhibitor dantrolene. TRPA1 agonist allyl isothiocyanate but not capsaicin inhibited the facilitatory effect of zingerone. On the other hand, zingerone reduced monosynaptically evoked EPSC amplitudes, as did TRPA1 agonists. Like allyl isothiocyanate, zingerone enhanced GABAergic spontaneous inhibitory transmission in a manner sensitive to tetrodotoxin. We conclude that zingerone presynaptically facilitates spontaneous excitatory transmission, probably through Ca(2+)-induced Ca(2+)-release mechanisms, and produces a membrane depolarization in SG neurons by activating TRPA1 but not TRPV1 channels.

Modality-specific nociceptor sensitization following UV-B irradiation of human skin

Ultraviolet-B (UV-B) irradiation is a well-established inflammatory pain model inducing mechanical and thermal hyperalgesia, presumably mediated by released mediators that sensitize sensory nerve endings. Here, we used additional electrical stimulation to investigate axonal hyperexcitability. The lower leg of 13 volunteers was irradiated with 3-fold the minimum erythema UV-B dose and sensitization was recorded at days 1, 3, 7, and 14. Maximum heat pain (47°C, 5 seconds) developed at day 1 (visual analog scale [VAS: 0-100]; 59), was reduced at day 3 (VAS 43, P < .002), and was back to normal at day 7 (VAS 18). Mechanical impact pain (8 m/s), pinprick (150 mN), and pressure (100 kPa) hyperalgesia were maximum throughout days 1 to 3 (VAS 16, 8, and 12, respectively, P < .001) and back to normal at day 7. Suprathreshold transcutaneous electrical stimuli (1.5-fold pain threshold) were delivered in trains of 10 pulses at frequencies of 5 to 100 Hz. Electrical pain thresholds (determined at 2 Hz) decreased significantly (P < .002) and suprathreshold electrical pain increased by about 70% at days 1 to 3 after irradiation (VAS 36, P < .002). Electrical hyperalgesia did not correlate with mechanical sensitization but with reduced heat pain threshold and increased tonic heat pain (r = -.46 and .53; P < .05 and < .01), indicating that axonal hyperexcitability might contribute to heat hyperalgesia. Released inflammatory mediators (eg, prostaglandins) might sensitize both heat transducer molecules and axonal ion channels and receptors, which would explain the simultaneous development and close correlation between heat hyperalgesia and axonal hyperexcitability.

PERSPECTIVE

Local inflammation by UV-B irradiation sensitizes not only sensory endings, but also axons. Increased axonal excitability could contribute to inflammatory hyperalgesia by facilitating spike generation and increasing peak discharge frequencies of nociceptors. Thus, axonal channels and receptors crucial for this sensitization need to be identified to provide new therapeutic targets.

Methyl syringate, a low-molecular-weight phenolic ester, as an activator of the chemosensory ion channel TRPA1

Transient receptor potential channel ankryn 1 (TRPA1) and transient receptor potential channel vanilloid 1 (TRPV1) are members of the TRP superfamily of structurally related, nonselective cation channels and are often coexpressed in sensory neurons. Extracts of the first leaves of Kalopanax pictus Nakai (Araliaceae) have been shown to activate hTRPA1 and hTRPV1. Therefore, the effects of six commercially available chemicals (methyl syringate, coniferyl alcohol, protocatechuic acid, hederacoside C, α-hederin, and eleutheroside B) found in K. pictus were investigated on cultured cells expressing hTRPA1 and hTRPV1. Of the six compounds, methyl syringate selectively activated hTRPA1 (EC(50) = 507.4 μM), but not hTRPV1. Although methyl syringate had a higher EC(50) compared with allyl isothiocyanate (EC(50) = 7.4 μM) and cinnamaldehyde (EC(50) = 22.2 μM), the present study provides evidence that methyl syringate from K. pictus is a specific and selective activator of hTRPA1.

Antinociceptive and anti-inflammatory potential of extract and isolated compounds from the leaves of Salvia officinalis in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Salvia officinalis L. has been used as a traditional herbal medicine for gastric disturbances and inflammatory processes. This study investigated the toxicological, antinociceptive and anti-inflammatory effects of the hydroalcoholic extract (HE) from leaves of Salvia officinalis and its isolated compounds in mice.

MATERIALS AND METHODS

Mice were treated with HE before the induction of nociceptive response by chemical agents (acetic-acid, formalin, glutamate, capsaicin and cinnamaldehyde). Total leukocytes and plasma extravasation induced by acetic acid and paw oedema induced by glutamate, capsaicin and cinnamaldehyde were also measured. The antinociceptive effect of carnosol and ursolic acid/oleanolic acid were evaluated on formalin and cinnamaldehyde models.

RESULTS

In the acute toxicity test the value of estimated LD50 for HE was 44.7579 g/kg. Oral administration of HE (10, 30 and 100 mg/kg) inhibited the number of writhings, total leukocytes and plasma extravasation induced by acetic acid. In the formalin test, HE reduced both neurogenic and inflammatory phases, effect that was affected by naloxone. The glutamate-, capsaicin- and cinnamaldehyde-induced nociception and paw oedema were reduced by HE at doses that did not affect the locomotor activity of mice in the open field test. Carnosol (10mg/kg) and ursolic acid/oleanolic acid (30 mg/kg) inhibited the inflammatory phase of formalin and the nociception and mechanical allodynia induced by cinnamaldehyde.

CONCLUSIONS

These results demonstrate that HE presents significant anti-inflammatory and also antinociceptive effects on chemical behavioral models of nociception that involves an opioid mechanism. In addition, carnosol and ursolic acid/oleanolic acid contained in this plant appears to contribute for the antinociceptive property of the extract, possibly through a modulatory influence on TRPA1-receptors. However, further studies regarding the precise site and the mechanism of action of HE and carnosol and ursolic acid/oleanolic acid merited exploring further.

Antinociception of β-D-glucan from Pleurotus pulmonarius is possibly related to protein kinase C inhibition

β-D-Glucan, a polysaccharide isolated from an edible mushroom Pleurotus pulmonarius (Fr.) Quel., presented antinociceptive activity in mice. This study evaluated the involvement of transient receptor potential (TRP) channels and protein kinase C (PKC) on antinociceptive effect of a (1→3),(1→6)-linked β-D-glucan (GL) in mice. Intraperitoneal administration of GL potently inhibited nociceptive responses induced by intraplantar injections of capsaicin, cinnamaldehyde, menthol, acidified saline and phorbol myristate acetate (PMA). Moreover, Western blot analysis revealed that GL treatment also prevented PMA-induced PKCɛ activation. Collectively, present results demonstrate that GL could constitute an attractive molecule of interest for the development of new analgesic drugs.

Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch

Topical capsaicin formulations are used for pain management. Safety and modest efficacy of low-concentration capsaicin formulations, which require repeated daily self-administration, are supported by meta-analyses of numerous studies. A high-concentration capsaicin 8% patch (Qutenza™) was recently approved in the EU and USA. A single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks. Advantages of the high-concentration capsaicin patch include longer duration of effect, patient compliance, and low risk for systemic effects or drug-drug interactions. The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as 'defunctionalization' of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals. Peripheral neuropathic hypersensitivity is mediated by diverse mechanisms, including altered expression of the capsaicin receptor TRPV1 or other key ion channels in affected or intact adjacent peripheral nociceptive nerve fibres, aberrant re-innervation, and collateral sprouting, all of which are defunctionalized by topical capsaicin. Evidence suggests that the utility of topical capsaicin may extend beyond painful peripheral neuropathies.

Transient receptor potential channel polymorphisms are associated with the somatosensory function in neuropathic pain patients

Transient receptor potential channels are important mediators of thermal and mechanical stimuli and play an important role in neuropathic pain. The contribution of hereditary variants in the genes of transient receptor potential channels to neuropathic pain is unknown. We investigated the frequency of transient receptor potential ankyrin 1, transient receptor potential melastin 8 and transient receptor potential vanilloid 1 single nucleotide polymorphisms and their impact on somatosensory abnormalities in neuropathic pain patients. Within the German Research Network on Neuropathic Pain (Deutscher Forscbungsverbund Neuropathischer Schmerz) 371 neuropathic pain patients were phenotypically characterized using standardized quantitative sensory testing. Pyrosequencing was employed to determine a total of eleven single nucleotide polymorphisms in transient receptor potential channel genes of the neuropathic pain patients and a cohort of 253 German healthy volunteers. Associations of quantitative sensory testing parameters and single nucleotide polymorphisms between and within groups and subgroups, based on sensory phenotypes, were analyzed. Single nucleotide polymorphisms frequencies did not differ between both the cohorts. However, in neuropathic pain patients transient receptor potential ankyrin 1 710G>A (rs920829, E179K) was associated with the presence of paradoxical heat sensation (p = 0.03), and transient receptor potential vanilloid 1 1911A>G (rs8065080, I585V) with cold hypoalgesia (p = 0.0035). Two main subgroups characterized by preserved (1) and impaired (2) sensory function were identified. In subgroup 1 transient receptor potential vanilloid 1 1911A>G led to significantly less heat hyperalgesia, pinprick hyperalgesia and mechanical hypaesthesia (p = 0.006, p = 0.005 and p<0.001) and transient receptor potential vanilloid 1 1103C>G (rs222747, M315I) to cold hypaesthesia (p = 0.002), but there was absence of associations in subgroup 2. In this study we found no evidence that genetic variants of transient receptor potential channels are involved in the expression of neuropathic pain, but transient receptor potential channel polymorphisms contributed significantly to the somatosensory abnormalities of neuropathic pain patients.

Heat avoidance is regulated by transient receptor potential (TRP) channels and a neuropeptide signaling pathway in Caenorhabditis elegans

The ability to avoid noxious extremes of hot and cold is critical for survival and depends on thermal nociception. The TRPV subset of transient receptor potential (TRP) channels is heat activated and proposed to be responsible for heat detection in vertebrates and fruit flies. To gain insight into the genetic and neural basis of thermal nociception, we developed assays that quantify noxious heat avoidance in the nematode Caenorhabditis elegans and used them to investigate the genetic basis of this behavior. First, we screened mutants for 18 TRP channel genes (including all TRPV orthologs) and found only minor defects in heat avoidance in single and selected double and triple mutants, indicating that other genes are involved. Next, we compared two wild isolates of C. elegans that diverge in their threshold for heat avoidance and linked this phenotypic variation to a polymorphism in the neuropeptide receptor gene npr-1. Further analysis revealed that loss of either the NPR-1 receptor or its ligand, FLP-21, increases the threshold for heat avoidance. Cell-specific rescue of npr-1 implicates the interneuron RMG in the circuit regulating heat avoidance. This neuropeptide signaling pathway operates independently of the TRPV genes, osm-9 and ocr-2, since mutants lacking npr-1 and both TRPV channels had more severe defects in heat avoidance than mutants lacking only npr-1 or both osm-9 and ocr-2. Our results show that TRPV channels and the FLP-21/NPR-1 neuropeptide signaling pathway determine the threshold for heat avoidance in C. elegans.

Identification of a calcium permeable human acid-sensing ion channel 1 transcript variant

The acid-sensing ion channels (ASICs) are proton-gated cation channels activated when extracellular pH declines. In rodents, the Accn2 gene encodes transcript variants ASIC1a and ASIC1b, which differ in the first third of the protein and display distinct channel properties. In humans, ACCN2 transcript variant 2 (hVariant 2) is homologous to mouse ASIC1a. In this article, we study two other human ACCN2 transcript variants. Human ACCN2 transcript variant 1 (hVariant 1) is not present in rodents and contains an additional 46 amino acids directly preceding the proposed channel gate. We report that hVariant 1 does not produce proton-gated currents under normal conditions when expressed in heterologous systems. We also describe a third human ACCN2 transcript variant (hVariant 3) that is similar to rodent ASIC1b. hVariant 3 is more abundantly expressed in dorsal root ganglion compared with brain and shows basic channel properties analogous to rodent ASIC1b. Yet, proton-gated currents from hVariant 3 are significantly more permeable to calcium than either hVariant 2 or rodent ASIC1b, which shows negligible calcium permeability. hVariant 3 also displays a small acid-dependent sustained current. Such a sustained current is particularly intriguing as ASIC1b is thought to play a role in sensory transduction in rodents. In human DRG neurons, hVariant 3 could induce sustained calcium influx in response to acidic pH and make a major contribution to acid-dependent sensations, such as pain.

A multicenter, randomized, double-blind, controlled dose finding study of NGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia

Postherpetic neuralgia (PHN) is a painful complication of acute herpes zoster. This multicenter, double-blind, controlled study randomized 299 PHN patients to receive either NGX-4010, a high-concentration capsaicin (8%) patch, or a low-concentration capsaicin (0.04%) control patch for 30, 60, or 90 minutes. The mean percent reductions in NPRS score from baseline to weeks 2 through 8 were significantly greater in the total NGX-4010 group (26.5%, P = .0286) and the 90-minute NGX-4010 group (27.8%, P = .0438) compared to the pooled control group (17.3%). After review of the data suggested a difference between genders in reporting of pain scores and a higher proportion of males (61%) in the 60-minute NGX-4010 group, post hoc gender-stratified analyses were performed and showed that the 60-minute NGX-4010 group also had a significantly larger mean percent reduction in average pain scores (28.0%, P = .0331). Pain reduction in the 30-minute NGX-4010 group, although similar in magnitude to the other doses, was not significantly different from control in either of these analyses. Similar results were observed during weeks 2 through 12. Most treatment-emergent adverse events were application-site specific, transient and mostly mild to moderate in severity.

PERSPECTIVE

This article reports the safety and efficacy of NGX-4010 applied for 3 different durations (30, 60, or 90 minutes) in patients with PHN. The results identified the 60-minute duration as the dose to be evaluated in subsequent studies and identified a gender effect on reported changes in pain.

Odontoblast TRP Channels and Thermo/Mechanical Transmission

Odontoblasts function as mechanosensory receptors because of the expression of mechanosensitive channels in these cells. However, it is unclear if odontoblasts direct the signal transmission evoked by heat/cold or osmotic changes. This study investigated the effects of heat/cold or osmotic changes on calcium signaling and the functional expression of the thermo/mechanosensitive transient receptor potential (TRP) channels in primary cultured mouse odontoblastic cells, with the use of RT-PCR, fluorometric calcium imaging, and electrophysiology. TRPV1, TRPV2, TRPV3, TRPV4, and TRPM3 mRNA was expressed, but TRPM8 and TRPA1 mRNA was not. The receptor-specific stimulation of TRPV1-3 (heat-sensing receptors) and TRPV4/ TRPM3 (mechanic receptors) caused increases in the intracellular calcium concentration. Moreover, the channel activities of TRPV1-4 and TRPM3 were confirmed by a whole-cell patch-clamp technique. These results suggest that primary cultured mouse odontoblasts express heat/mechanosensitive TRP channels and play a role in the underlying mechanisms of thermo/mechanosensitive sensory transmission.

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

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

Transient receptor potential channels on sensory nerves

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

Transient receptor potential channels in sensory neurons are targets of the antimycotic agent clotrimazole

Clotrimazole (CLT) is a widely used drug for the topical treatment of yeast infections of skin, vagina, and mouth. Common side effects of topical CLT application include irritation and burning pain of the skin and mucous membranes. Here, we provide evidence that transient receptor potential (TRP) channels in primary sensory neurons underlie these unwanted effects of CLT. We found that clinically relevant CLT concentrations activate heterologously expressed TRPV1 and TRPA1, two TRP channels that act as receptors of irritant chemical and/or thermal stimuli in nociceptive neurons. In line herewith, CLT stimulated a subset of capsaicin-sensitive and mustard oil-sensitive trigeminal neurons, and evoked nocifensive behavior and thermal hypersensitivity with intraplantar injection in mice. Notably, CLT-induced pain behavior was suppressed by the TRPV1-antagonist BCTC [(N-(-4-tertiarybutylphenyl)-4-(3-cholorpyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide)] and absent in TRPV1-deficient mice. In addition, CLT inhibited the cold and menthol receptor TRPM8, and blocked menthol-induced responses in capsaicin- and mustard oil-insensitive trigeminal neurons. The concentration for 50% inhibition (IC50) of inward TRPM8 current was approximately 200 nM, making CLT the most potent known TRPM8 antagonist and a useful tool to discriminate between TRPM8- and TRPA1-mediated responses. Together, our results identify TRP channels in sensory neurons as molecular targets of CLT, and offer means to develop novel CLT preparations with fewer unwanted sensory side effects.