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

S1P/S1PRs-TRPV4 axis is a novel therapeutic target for persistent pain and itch in chronic dermatitis

BACKGROUND AND PURPOSE

While pain and itch are both commonly associated with chronic dermatitis (CD), the molecular mechanisms underlying these debilitating symptoms is not well understood. This study aims to identify novel, endogenous compounds that mediate CD-associated pain and itch.

EXPERIMENTAL APPROACH

Lesional skin of CD model mice was examined using unbiased metabolomic analysis to identify candidate pain or itch inducing compounds in CD. Sphingosine-1-phosphate (S1P) concentration in CD model skin was analysed using UPLC/MS/MS. Behaviour, calcium imaging and immunofluorescence staining were used to determine the pain and itch effects and mechanisms of the identified CD-related compounds.

KEY RESULTS

In the lesional skin of CD model mice, 136 compounds were significantly changed. These compounds are predominately associated with the sphingolipids metabolism pathway. S1P is significantly increased in the lesional skin . The TRPV4 channel was critical for S1P induced itch and pain. Sphingosine kinase 2 (SPHK2), the key enzyme controlling S1P synthesis, was significantly increased in lesional skin. ABC294640, a SPHK2 inhibitor, significantly decreased S1P concentration in lesional CD model skin, as well as in model associated epidermal hyperplasia and chronic pain and itch. In CD patients, SPHK2 expression and S1P concentration were significantly elevated compared to healthy control skin.

CONCLUSION AND IMPLICATIONS

Our results indicate that, in CD, increased S1P induces chronic pain and itch partly through TRPV4. Inhibition of S1P synthesis or the S1P/S1P receptor-TRPV4 pathway are promising treatment strategies for CD-associated pain and itch.

Molecular and cellular mechanisms of itch sensation and the anti-itch drug targets

Itch is an uncomfortable feeling that evokes a desire to scratch. This protective reflex can effectively eliminate parasites that invade the skin. When itchy skin becomes severe or lasts for more than six weeks, it has deleterious effects on both quality of life and productivity. Despite decades of research, the complete molecular and cellular coding of chronic itch remains elusive. This persistent condition often defies treatment, including with antihistamines, and poses a significant societal challenge. Obtaining pathophysiological insights into the generation of chronic itch is essential for understanding its mechanisms and the development of innovative anti-itch medications. In this review we provide a systematic overview of the recent advancement in itch research, alongside the progress made in drug discovery within this field. We have examined the diversity and complexity of the classification and mechanisms underlying the complex sensation of itch. We have also delved into recent advancements in the field of itch mechanism research and how these findings hold potential for the development of new itch treatment medications. But the treatment of clinical itch symptoms still faces significant challenges. Future research needs to continue to delve deeper, not only to discover more itch-related pathways but also to explore how to improve treatment efficacy through multitarget or combination therapy.

Itching for Answers: A Comprehensive Review of Cholestatic Pruritus Treatments

Cholestasis is a clinical and laboratory syndrome indicating impaired bile production or excretion. One of the hallmark symptoms of cholestasis is pruritus. Itch can be severe and debilitating for patients, impacting their quality of life similarly to pain, and, in some cases, it can be refractory. Current therapies like anion exchange resins and rifampicin, offer partial relief but with side effects. Effective, well-tolerated treatments are urgently needed. This literature review examines existing options (bile acid sequestrants, antihistamines, opioid antagonists, sertraline, and rifampicin) and explores novel therapies (monoclonal antibodies, PPAR agonists, and bile-acid-based therapies). We analyze mechanisms, limitations, and adverse effects to aid clinicians and researchers. Novel approaches include monoclonal antibodies to inhibit bile recirculation and PPAR agonists targeting pruritus signaling. Despite the limited current options, ongoing research promises better treatments for cholestatic pruritus, addressing its distressing impact. In summary, cholestasis-associated pruritus poses a significant challenge with limited treatments. Advancements in understanding its pathophysiology offer hope for more effective therapies in the future.

Cannabinoids and Multiple Sclerosis: A Critical Analysis of Therapeutic Potentials and Safety Concerns

Multiple sclerosis (MS) is a complicated condition in which the immune system attacks myelinated axons in the central nervous system (CNS), destroying both myelin and axons to varying degrees. Several environmental, genetic, and epigenetic factors influence the risk of developing the disease and how well it responds to treatment. Cannabinoids have recently sparked renewed interest in their therapeutic applications, with growing evidence for their role in symptom control in MS. Cannabinoids exert their roles through the endogenous cannabinoid (ECB) system, with some reports shedding light on the molecular biology of this system and lending credence to some anecdotal medical claims. The double nature of cannabinoids, which cause both positive and negative effects, comes from their actions on the same receptor. Several mechanisms have been adopted to evade this effect. However, there are still numerous limitations to using cannabinoids to treat MS patients. In this review, we will explore and discuss the molecular effect of cannabinoids on the ECB system, the various factors that affect the response to cannabinoids in the body, including the role of gene polymorphism and its relation to dosage, assessing the positive over the adverse effects of cannabinoids in MS, and finally, exploring the possible functional mechanism of cannabinoids in MS and the current and future progress of cannabinoid therapeutics.

Transient Receptor Potential Channels and Itch

Transient Receptor Potential (TRP) channels are multifunctional sensory molecules that are abundant in the skin and are involved in the sensory pathways of itch, pain, and inflammation. In this review article, we explore the complex physiology of different TRP channels, their role in modulating itch sensation, and their contributions to the pathophysiology of acute and chronic itch conditions. We also cover small molecule and topical TRP channel agents that are emerging as potential anti-pruritic treatments; some of which have shown great promise, with a few treatments advancing into clinical trials-namely, TRPV1, TRPV3, TRPA1, and TRPM8 targets. Lastly, we touch on possible ethnic differences in TRP channel genetic polymorphisms and how this may affect treatment response to TRP channel targets. Further controlled studies on the safety and efficacy of these emerging treatments is needed before clinical use.

Elevated Level of Serum Neurotrophin-4, but Not of Brain-Derived Neurotrophic Factor, in Patients with Chronic Kidney Disease-Associated Pruritus

Chronic kidney disease-associated pruritus (CKD-aP) is a bothersome condition that occurs in patients with advanced chronic kidney disease (CKD) and severely reduces their quality of life. Recently, much research has focused on the search for markers that are involved in the pathogenesis of CKD-aP and may become a therapeutic target. One of the suggested hypotheses is the increased activation of sensory neurons by molecules such as neurotrophins (NTs). An increased serum concentration of NTs has been demonstrated in pruritic patients, which may suggest their involvement in the pathogenesis of itch. The purpose of this study is to assess the serum concentration of neurotrophin-4 (NT-4) and brain-derived neurotrophic factor (BDNF) in hemodialysis patients. The study enrolled 126 patients undergoing dialysis. Participants were divided into 2 groups: with and without CKD-aP. NRS scale was used to evaluate itch severity. Serum levels of NT-4 and BDNF have been assessed using ELISA. The results showed a significantly higher level of NT-4 in the group with pruritus. No significant difference was reported in the serum level of BDNF between the two groups of patients. There was also no correlation between serum NT-4 nor BDNF levels and the severity of pruritus. In summary, NT-4 may play an important role in the pathophysiology of pruritus in dialysis patients. More research is needed to understand the exact mechanism by which NTs influence the pathogenesis of CKD-aP.

Safety and Efficacy of Lenabasum, a Cannabinoid Receptor Type 2 Agonist, in Patients with Dermatomyositis with Refractory Skin Disease: A Randomized Clinical Trial

BACKGROUND

Treatment options are limited for skin disease in dermatomyositis. Lenabasum is a cannabinoid receptor type 2 agonist that triggers the resolution of inflammation.

OBJECTIVE

The objective of this study was to evaluate the safety and efficacy of lenabasum in patients with refractory cutaneous dermatomyositis.

DESIGN

This study was a single-center, double-blind, randomized, placebo-controlled phase 2 study conducted from July 2015 to August 2017.

POPULATION

The population included subjects aged ≥18 years with at least moderately active dermatomyositis skin activity by Cutaneous Dermatomyositis Disease Area and Severity Index activity ≥ 14 and failure or intolerance to hydroxychloroquine.

INTERVENTION

Participants received 20 mg lenabasum daily for 28 days and then 20 mg twice per day for 56 days or placebo.

MAIN OUTCOMES AND MEASURES

The primary outcome was a change in Cutaneous Dermatomyositis Disease Area and Severity Index activity. Safety and other secondary efficacy assessments were performed till day 113.

RESULTS

A total of 22 subjects were randomized to lenabasum (n = 11) or placebo (n = 11). No serious or severe adverse events were related to lenabasum, and no participants discontinued the study. The adjusted least-squares mean for Cutaneous Dermatomyositis Disease Area and Severity Index activity decreased more for lenabasum, and the difference was significant on day 113 (least-squares mean [standard error] difference = ‒6.5 [3.1], P = 0.038). Numerically greater improvements were seen in multiple secondary efficacy outcomes and biomarkers with lenabasum.

CONCLUSION

Lenabasum treatment was well tolerated and was associated with greater improvement in Cutaneous Dermatomyositis Disease Area and Severity Index activity and multiple efficacy outcomes.

TRIAL REGISTRATION

This study was registered at ClinicalTrials.gov, NCT02466243.

Critical Players and Therapeutic Targets in Chronic Itch

Chronic itch is one of the most prominent clinical characteristics of diverse systematic diseases. It is a devastating sensation in pathological diseases. Despite its importance, there are no FDA-labelled drugs specifically geared toward chronic itch. The associated complex pathogenesis and diverse causes escalate chronic itch to being one of the top challenges in healthcare. Humanized antibodies against IL-13, IL-4, and IL-31 proved effective in treatment of itch-associated atopic dermatitis but remain to be validated in chronic itch. There are still no satisfactory anti-itch therapeutics available toward itch-related neuropeptides including GRP, BNP, SST, CGRP, and SP. The newly identified potential itch targets including OSM, NMB, glutamate, periostin, and Serpin E1 have opened new avenues for therapeutic development. Proof-of-principle studies have been successfully performed on antagonists against these proteins and their receptors in itch treatment in animal models. Their translational interventions in humans need to be evaluated. It is of great importance to summarize and compare the newly emerging knowledge on chronic itch and its pathways to promote the development of novel anti-itch therapeutics. The goal of this review is to analyze the different physiologies and pathophysiologies of itch mediators, whilst assessing their suitability as new targets and discussing future therapeutic development.

Portulaca oleracea extract relieves skin barrier damage induced by increased photosensitivity after GA peeling

Purpose We aimed to find active substances to help relieve the symptoms caused by increased photosensitivity after alpha hydroxy acid (AHA) peeling. Methods: A questionnaire survey was provided to 66 patients who received AHA peeling therapy to understand if increased photosensitivity existed and its specific symptoms. We verified increased photosensitivity after AHA peeling by monitoring cell viability to detect the combined toxicity of glycolic acid (GA) and UVB in HaCaT cells. The ELISA method was used to determine the expression of KLK7, FLG, IL-1β, and IL-8 to correlate damage to the skin barrier and inflammation induced by GA and UVB and the relieving effects of Portulaca oleracea extract. Results: Our survey results showed that 6.06% of people were more sensitive to sunlight after AHA peeling than before. Experiments at the cellular level showed that UVB induced cytotoxicity on HaCaT cells pre-treated with GA. Combined exposure of GA and UVB induced up-regulation of KLK7 and down-regulation of FLG and increased inflammatory cytokines of IL-1β and IL-8. Portulaca oleracea extract inhibited the reduction of FLG and increased KLK7, IL-1β, and IL-8 expression caused by combined exposure. Conclusions: Our study found that combined exposure to GA and UV disrupted the skin barrier and induced significant inflammation. These results provided a theoretical basis for increased photosensitivity after chemical peeling. Portulaca oleracea extract ameliorated GA and UVB-induced impaired skin barrier function and inflammation in HaCaT cells and may have the potential to relieve photosensitivity after AHA peeling.

An LPAR5-antagonist that reduces nociception and increases pruriception

Introduction

The G-protein coupled receptor LPAR₅ plays a prominent role in LPA-mediated pain and itch signaling. In this study we focus on the LPAR₅-antagonist compound 3 (cpd3) and its ability to affect pain and itch signaling, both in vitro and in vivo.

Methods

Nociceptive behavior in wild type mice was induced by formalin, carrageenan or prostaglandin E2 (PGE₂) injection in the hind paw, and the effect of oral cpd3 administration was measured. Scratch activity was measured after oral administration of cpd3, in mice overexpressing phospholipase A2 (sPLA2tg), in wild type mice (WT) and in TRPA1-deficient mice (Trpa1 KO). In vitro effects of cpd3 were assessed by measuring intracellular calcium release in HMC-1 and HEK-TRPA1 cells.

Results

As expected, nociceptive behavior (induced by formalin, carrageenan or PGE₂) was reduced after treatment with cpd3. Unexpectedly, cpd3 induced scratch activity in mice. In vitro addition of cpd3 to HEK-TRPA1 cells induced an intracellular calcium wave that could be inhibited by the TRPA1-antagonist A-967079. In Trpa1 KO mice, however, the increase in scratch activity after cpd3 administration was not reduced.

Conclusions

Cpd3 has in vivo antinociceptive effects but induces scratch activity in mice, probably by activation of multiple pruriceptors, including TRPA1. These results urge screening of antinociceptive candidate drugs for activity with pruriceptors.

Pruritus: A Sensory Symptom Generated in Cutaneous Immuno-Neuronal Crosstalk

Pruritus or itch generated in the skin is one of the most widespread symptoms associated with various dermatological and systemic (immunological) conditions. Although many details about the molecular mechanisms of the development of both acute and chronic itch were uncovered in the last 2 decades, our understanding is still incomplete and the clinical management of pruritic conditions is one of the biggest challenges in daily dermatological practice. Recent research revealed molecular interactions between pruriceptive sensory neurons and surrounding cutaneous cell types including keratinocytes, as well as resident and transient cells of innate and adaptive immunity. Especially in inflammatory conditions, these cutaneous cells can produce various mediators, which can contribute to the excitation of pruriceptive sensory fibers resulting in itch sensation. There also exists significant communication in the opposite direction: sensory neurons can release mediators that maintain an inflamed, pruritic tissue-environment. In this review, we summarize the current knowledge about the sensory transduction of pruritus detailing the local intercellular interactions that generate itch. We especially emphasize the role of various pruritic mediators in the bidirectional crosstalk between cutaneous non-neuronal cells and sensory fibers. We also list various dermatoses and immunological conditions associated with itch, and discuss the potential immune-neuronal interactions promoting the development of pruritus in the particular diseases. These data may unveil putative new targets for antipruritic pharmacological interventions.

Inhibitory Effects of Luteolin 7-Methyl Ether Isolated from Wikstroemia ganpi on Tnf-A/Ifn-Γ Mixture-Induced Inflammation in Human Keratinocyte

Plants of the genus Wikstroemia are traditionally used in China to treat various inflammatory diseases. The purpose of this study was to isolate the components of Wikstroemia ganpi (Siebold & Zucc.) Maxim., to evaluate their anti-atopic activities and to identify candidates with anti-atopic therapeutics. A total of 24 compounds were isolated by bioassay-guided separation, including one novel compound, which was tilianin 5-methyl ether. The anti-atopic activities of the isolated compounds were determined using TNF-α-treated RBL-2H3 cells and HaCaT cells. The mRNA expressions of IL-4, IL-6, GM-CSF, G-CSF and TRPV1 were reduced by luteolin 7-methyl ether. The study shows that the luteolin 7-methyl ether isolated from W. ganpi is a potential therapeutic agent for the treatment of atopic dermatitis.

Emerging Role of Transient Receptor Potential Vanilloid 4 (TRPV4) Ion Channel in Acute and Chronic Itch

Itch is a clinical problem that leaves many sufferers insufficiently treated, with over 20 million cases in the United States. This is due to incomplete understanding of its molecular, cellular, and cell-to-cell signaling mechanisms. Transient receptor potential (TRP) ion channels are involved in several sensory modalities including pain, vision, taste, olfaction, hearing, touch, and thermosensation, as well as itch. Relative to the extensive studies on TRPV1 and TRPA1 ion channels in itch modulation, TRPV4 has received relatively little research attention and its mechanisms have remained poorly understood until recently. TRPV4 is expressed in ganglion sensory neurons and a variety of skin cells. Growing evidence in the past few years strongly suggests that TRPV4 in these cells contributes to acute and chronic disease-associated itch. This review focuses on the current experimental evidence involving TRPV4 in itch under pathophysiological conditions and discusses its possible cellular and molecular mechanisms.

Th2 Modulation of Transient Receptor Potential Channels: An Unmet Therapeutic Intervention for Atopic Dermatitis

Atopic dermatitis (AD) is a multifaceted, chronic relapsing inflammatory skin disease that affects people of all ages. It is characterized by chronic eczema, constant pruritus, and severe discomfort. AD often progresses from mild annoyance to intractable pruritic inflammatory lesions associated with exacerbated skin sensitivity. The T helper-2 (Th2) response is mainly linked to the acute and subacute phase, whereas Th1 response has been associated in addition with the chronic phase. IL-17, IL-22, TSLP, and IL-31 also play a role in AD. Transient receptor potential (TRP) cation channels play a significant role in neuroinflammation, itch and pain, indicating neuroimmune circuits in AD. However, the Th2-driven cutaneous sensitization of TRP channels is underappreciated. Emerging findings suggest that critical Th2-related cytokines cause potentiation of TRP channels, thereby exaggerating inflammation and itch sensation. Evidence involves the following: (i) IL-13 enhances TRPV1 and TRPA1 transcription levels; (ii) IL-31 sensitizes TRPV1 via transcriptional and channel modulation, and indirectly modulates TRPV3 in keratinocytes; (iii) The Th2-cytokine TSLP increases TRPA1 synthesis in sensory neurons. These changes could be further enhanced by other Th2 cytokines, including IL-4, IL-25, and IL-33, which are inducers for IL-13, IL-31, or TSLP in skin. Taken together, this review highlights that Th2 cytokines potentiate TRP channels through diverse mechanisms under different inflammatory and pruritic conditions, and link this effect to distinct signaling cascades in AD. This review strengthens the notion that interrupting Th2-driven modulation of TRP channels will inhibit transition from acute to chronic AD, thereby aiding the development of effective therapeutics and treatment optimization.

The TRPM3 ion channel mediates nociception but not itch evoked by endogenous pruritogenic mediators

During the molecular transduction of itch, the stimulation of pruriceptors on sensory fibers leads to the activation or sensitization of ion channels, which results in a consequent depolarization of the neurons. These ion channels mostly belong to the transient receptor potential (TRP) channels, which are involved in nociception and thermosensation. In particular, TRPV1 and TRPA1 were described in the transduction of both thermal nociception as well as histaminergic and non-histaminergic itch. The thermosensitive TRPM3 plays an indispensable role in heat nociception together with TRPV1 and TRPA1. However, the role of TRPM3 in the development of pruritus has not been studied yet. Therefore, in this study we aimed at investigating the potential role of TRPM3 in the transduction of pruritus and pain by investigating itch- and nociception-related behavior of Trpm3+/+ and Trpm3-/- mice, and by studying the activation of somatosensory neurons isolated from trigeminal ganglia upon application of algogenic and pruritogenic substances. Activators of TRPM3 evoked only nocifensive responses, but not itch in Trpm3+/+ animals, and these nocifensive responses were abolished in the Trpm3-/- strain. Histamine and endogenous non-histaminergic pruritogens induced itch in both Trpm3+/+ and Trpm3-/- mice to a similar extent. Genetic deletion or pharmacological blockade diminished TRPM3 mediated Ca2+ responses of sensory neurons, but did not affect responses evoked by pruritogenic substances. Our results demonstrate that, in contrast to other thermosensitive TRP channels, TRPM3 selectively mediates nociception, but not itch sensation, and suggest that TRPM3 is a promising candidate to selectively target pain sensation.

The Role of TRP Channels in Allergic Inflammation and its Clinical Relevance

Allergy refers to an abnormal adaptive immune response to non-infectious environmental substances (allergen) that can induce various diseases such as asthma, atopic dermatitis, and allergic rhinitis. In this allergic inflammation, various immune cells, such as B cells, T cells, and mast cells, are involved and undergo complex interactions that cause a variety of pathophysiological conditions. In immune cells, calcium ions play a crucial role in controlling intracellular Ca2+ signaling pathways. Cations, such as Na+, indirectly modulate the calcium signal generation by regulating cell membrane potential. This intracellular Ca2+ signaling is mediated by various cation channels; among them, the Transient Receptor Potential (TRP) family is present in almost all immune cell types, and each channel has a unique function in regulating Ca2+ signals. In this review, we focus on the role of TRP ion channels in allergic inflammatory responses in T cells and mast cells. In addition, the TRP ion channels, which are attracting attention in clinical practice in relation to allergic diseases, and the current status of the development of therapeutic agents that target TRP channels are discussed.

Pain and Pruritus: a study of their similarities and differences

Pruritus is one of the most common dermatologic complaints and, as the most common dermatologic symptom, is a major contributor to frequent dermatology visits. Chronic pruritus mirrors another major medical condition faced by millions of Americans each year - chronic pain. In older literature, pain and pruritus were thought to have been conveyed by the same C fiber, and the proportion contributing to pruritus was just a small subset of this general fiber. Overall, pain and pruritus share many integral similarities. Although these sensations both initiate the body's awareness to injury, pain and itch may have evolved for sensing different damages such as a burrowing parasite or a noxious stimulus, respectively. This seems to have been validated through analyses of their pathophysiology, acute and chronic conditions, and treatment modalities. However, their symptoms and intrinsic mechanisms vary considerably. It is important to view pruritus in more of an overall, whole body experience, rather than just the sensory aspect. Future studies should investigate the psychological treatment of chronic pruritus, considering the immense similarities with its chronic pain counterpart.

Activation of TRPV3 Regulates Inflammatory Actions of Human Epidermal Keratinocytes

Transient receptor potential (TRP) ion channels were first characterized on neurons, where they are classically implicated in sensory functions; however, research in recent decades has shown that many of these channels are also expressed on nonneuronal cell types. Emerging findings have highlighted the role of TRP channels in the skin, where they have been shown to be important in numerous cutaneous functions. Of particular interest is TRPV3, which was first described on keratinocytes. Its functional importance was supported when its gain-of-function mutation was linked to Olmsted syndrome, which is characterized by palmoplantar keratoderma, periorifacial hyperkeratosis, diffuse hypotrichosis and alopecia, and itch. Despite these exciting results, we have no information about the role and functionality of TRPV3 on keratinocytes at the cellular level. In this study, we identified TRPV3 expression both on human skin and cultured epidermal keratinocytes. TRPV3 stimulation was found to function as a Ca²⁺-permeable ion channel that suppresses proliferation of epidermal keratinocytes and induces cell death. Stimulation of the channel also triggers a strong proinflammatory response via the NF-κB pathway. Collectively, our data show that TRPV3 is functionally expressed on human epidermal keratinocytes and that it plays a role in cutaneous inflammatory processes.

The aryl hydrocarbon receptor AhR links atopic dermatitis and air pollution via induction of the neurotrophic factor artemin

Atopic dermatitis is increasing worldwide in correlation with air pollution. Various organic components of pollutants activate the transcription factor AhR (aryl hydrocarbon receptor). Through the use of AhR-CA mice, whose keratinocytes express constitutively active AhR and that develop atopic-dermatitis-like phenotypes, we identified Artn as a keratinocyte-specific AhR target gene whose product (the neurotrophic factor artemin) was responsible for epidermal hyper-innervation that led to hypersensitivity to pruritus. The activation of AhR via air pollutants induced expression of artemin, alloknesis, epidermal hyper-innervation and inflammation. AhR activation and ARTN expression were positively correlated in the epidermis of patients with atopic dermatitis. Thus, AhR in keratinocytes senses environmental stimuli and elicits an atopic-dermatitis pathology. We propose a mechanism of air-pollution-induced atopic dermatitis via activation of AhR.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

Transient Receptor Potential Vanilloid 4 Ion Channel Functions as a Pruriceptor in Epidermal Keratinocytes to Evoke Histaminergic Itch

TRPV4 ion channels function in epidermal keratinocytes and in innervating sensory neurons; however, the contribution of the channel in either cell to neurosensory function remains to be elucidated. We recently reported TRPV4 as a critical component of the keratinocyte machinery that responds to ultraviolet B (UVB) and functions critically to convert the keratinocyte into a pain-generator cell after excess UVB exposure. One key mechanism in keratinocytes was increased expression and secretion of endothelin-1, which is also a known pruritogen. Here we address the question of whether TRPV4 in skin keratinocytes functions in itch, as a particular form of “forefront” signaling in non-neural cells. Our results support this novel concept based on attenuated scratching behavior in response to histaminergic (histamine, compound 48/80, endothelin-1), not non-histaminergic (chloroquine) pruritogens in Trpv4 keratinocyte-specific and inducible knock-out mice. We demonstrate that keratinocytes rely on TRPV4 for calcium influx in response to histaminergic pruritogens. TRPV4 activation in keratinocytes evokes phosphorylation of mitogen-activated protein kinase, ERK, for histaminergic pruritogens. This finding is relevant because we observed robust anti-pruritic effects with topical applications of selective inhibitors for TRPV4 and also for MEK, the kinase upstream of ERK, suggesting that calcium influx via TRPV4 in keratinocytes leads to ERK-phosphorylation, which in turn rapidly converts the keratinocyte into an organismal itch-generator cell. In support of this concept we found that scratching behavior, evoked by direct intradermal activation of TRPV4, was critically dependent on TRPV4 expression in keratinocytes. Thus, TRPV4 functions as a pruriceptor-TRP in skin keratinocytes in histaminergic itch, a novel basic concept with translational-medical relevance.

Neural Ablation and Regeneration in Pain Practice

A nerve block is an effective tool for diagnostic and therapeutic methods. If a diagnostic nerve block is successful for pain relief and the subsequent therapeutic nerve block is effective for only a limited duration, the next step that should be considered is a nerve ablation or modulation. The nerve ablation causes iatrogenic neural degeneration aiming only for sensory or sympathetic denervation without motor deficits. Nerve ablation produces the interruption of axonal continuity, degeneration of nerve fibers distal to the lesion (Wallerian degeneration), and the eventual death of axotomized neurons. The nerve ablation methods currently available for resection/removal of innervation are performed by either chemical or thermal ablation. Meanwhile, the nerve modulation method for interruption of innervation is performed using an electromagnetic field of pulsed radiofrequency. According to Sunderland's classification, it is first and foremost suggested that current neural ablations produce third degree peripheral nerve injury (PNI) to the myelin, axon, and endoneurium without any disruption of the fascicular arrangement, perineurium, and epineurium. The merit of Sunderland's third degree PNI is to produce a reversible injury. However, its shortcoming is the recurrence of pain and the necessity of repeated ablative procedures. The molecular mechanisms related to axonal regeneration after injury include cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules, and their receptors. It is essential to establish a safe, long-standing denervation method without any complications in future practices based on the mechanisms of nerve degeneration as well as following regeneration.

TRPV1 and PLC Participate in Histamine H4 Receptor-Induced Itch

Histamine H4 receptor has been confirmed to play a role in evoking peripheral pruritus. However, the ionic and intracellular signaling mechanism of activation of H4 receptor on the dorsal root ganglion (DRG) neurons is still unknown. By using cell culture and calcium imaging, we studied the underlying mechanism of activation of H4 receptor on the DRG neuron. Immepip dihydrobromide (immepip)—a histamine H4 receptor special agonist under cutaneous injection—obviously induced itch behavior of mice. Immepip-induced scratching behavior could be blocked by TRPV1 antagonist AMG9810 and PLC pathway inhibitor U73122. Application of immepip (8.3–50 μM) could also induce a dose-dependent increase in intracellular Ca²⁺ ([Ca²⁺]_i) of DRG neurons. We found that 77.8% of the immepip-sensitized DRG neurons respond to the TRPV1 selective agonist capsaicin. U73122 could inhibit immepip-induced Ca²⁺ responses. In addition, immepip-induced [Ca²⁺]_i increase could be blocked by ruthenium red, capsazepine, and AMG9810; however it could not be blocked by TRPA1 antagonist HC-030031. These results indicate that TRPV1 but not TRPA1 is the important ion channel to induce the DRG neurons' responses in the downstream signaling pathway of histamine H4 receptor and suggest that TRPV1 may be involved in the mechanism of histamine-induced itch response by H4 receptor activation.

TRP channels in skin: from physiological implications to clinical significances

TRP channels are expressed in various cells in skin. As an organ system to border the host and environment, many nonneuronal cells, including epidermal keratinocytes and melanocytes, express several TRP channels functionally distinct from sensory processing. TRPV1 and TRPV3 in keratinocytes of the epidermis and hair apparatus inhibit proliferation, induce terminal differentiation, induce apoptosis, and promote inflammation. Activation of TRPV4, 6, and TRPA1 promotes regeneration of the severed skin barriers. TRPA1 also enhances responses in contact hypersensitivity. TRPCs in keratinocytes regulate epidermal differentiation. In human diseases with pertubered epidermal differentiation, the expression of TRPCs are altered. TRPMs, which contribute to melanin production in melanocytes, serve as significant prognosis markers in patients with metastatic melanoma. In summary, not only act in sensory processing, TRP channels also contribute to epidermal differentiation, proliferation, barrier integration, skin regeneration, and immune responses. In diseases with aberrant TRP channels, TRP channels might be good therapeutic targets.

3-iodothyroacetic acid, a metabolite of thyroid hormone, induces itch and reduces threshold to noxious and to painful heat stimuli in mice

BACKGROUND AND PURPOSE

Itch is associated with increased sensitization to nociceptive stimuli. We investigated whether 3-iodothyroacetic acid (TA1), by releasing histamine, induces itch and increases sensitization to noxious and painful heat stimuli.

EXPERIMENTAL APPROACH

Itch was evaluated after s.c. administration of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Mice threshold to noxious (NHT) and to painful heat stimuli were evaluated by the increasing-temperature hot plate (from 45.5 to 49.5°C) or by the hot plate (51.5°C) test, respectively, 15 min after i.p. injection of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Itch, NHT and pain threshold evaluation were repeated in mice pretreated with pyrilamine. Itch and NHT were also measured in HDC(+/+) and HDC(-/-) following injection of saline or TA1 (1.32, 4 and 11 μg·kg(-1) ; s.c. and i.p.). pERK1/2 levels were determined by Western blot in dorsal root ganglia (DRG) isolated from CD1 mice 15 min after they received (i.p.): saline, saline and noxious heat stimulus (46.5°C), TA1 (0.1, 0.4, 1.32, 4 μg·kg(-1) ) or TA1 1.32 μg·kg(-1) and noxious heat stimulus.

KEY RESULTS

TA1 0.4 and 1.32 μg·kg(-1) induced itch and reduced NHT; pyrilamine pretreatment prevented both of these effects. TA1 4 μg·kg(-1) (i.p.) reduced pain threshold without inducing itch or modifying NHT. In HDC(-/-) mice, TA1 failed to induce itch and to reduce NHT. In DRG, pERK1/2 levels were significantly increased by noxious heat stimuli and by TA1 0.1, 0.4 and 1.32 μg·kg(-1) ; i.p.

CONCLUSIONS AND IMPLICATIONS

Increased TA1 levels induce itch and an enhanced sensitivity to noxious heat stimuli suggesting that TA1 might represent a potential cause of itch in thyroid diseases.

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.

A randomized, investigator-blinded efficacy assessment study of stand-alone emollient use in mild to moderately severe atopic dermatitis flares

BACKGROUND

Whereas emollients are integral to the long-term management of atopic dermatitis (AD), the evidence for their efficacy in disease flares is limited.

OBJECTIVE

We aimed to investigate the stand-alone efficacy of an emollient formulation with regard to improvement of the clinical symptoms, skin barrier function and reduction of pathogenic bacterial colonization in acute stage of AD.

MATERIALS AND METHODS

Twenty AD volunteers aged 12-65 years with symmetric, mild to moderately severe inflammatory lesions on the forearms/arms were recruited for the study. At inclusion, the forearms/arms of each volunteer were randomized to receive for 1 week either an o/w formulation containing licochalcone A (Glycyrrhiza Inflata root extract), decanediol, menthoxypropanediol and ω-6-fatty acids (emollient arm) or 1% hydrocortisone (HC arm); after 1 week, the application of the emollient and HC were discontinued and the volunteers applied a w/o emollient containing licochalcone A and ω-6-fatty acids on both arms for further 3 weeks. The outcomes included reduction of the clinical and itch severity, decrease in S.aureus colonization, improvement of the barrier function, skin hydration and skin tolerability assessed after 1 week (D7) and after 4 weeks (D28) respectively.

RESULTS

In both arms, there was a significant decrease in the severity score, itch intensity, erythema and TEWL on D7 and D28 compared to baseline. In addition, emollient use resulted in pronounced decrease in S.aureus colonization and significant increase of skin hydration on D7. The comparison of the outcomes, based on percentage change from baseline, showed no significant differences between the emollient and HC arm at any time point.

CONCLUSIONS

The results of the study indicate that the 1-week stand-alone application of an emollient, tailored to target inflammation, pruritus, compromised barrier function and pathogenic bacterial colonization may offer benefit for the improvement of mild to moderately severe localized flares of AD.

A randomised trial evaluating the effects of the TRPV1 antagonist SB705498 on pruritus induced by histamine, and cowhage challenge in healthy volunteers

BACKGROUND

Transient receptor potential vanilloid type 1 (TRPV1) is a non-selective cation channel widely expressed in skin tissues, and peripheral sensory nerve fibres. Activation of TRPV1 releases neuropeptides; the resulting neurogenic inflammation is believed to contribute to the development of pruritus. A TRPV1 antagonist has the potential to perform as an anti-pruritic agent. SB705498 is a TRPV1 antagonist that has demonstrated in vitro activity against cloned TRPV1 human receptors and when orally administered has demonstrated pharmacodynamic activity in animal models and clinical studies.

OBJECTIVES

To select a topical dose of SB705498 using the TRPV1 agonist capsaicin; to confirm engagement of the TRPV1 antagonistic action of SB705498 and assess whether the dose selected has an effect on itch induced by two challenge agents.

METHODS

A clinical study was conducted in 16 healthy volunteers to assess the effects of 3 doses of SB705498 on skin flare induced by capsaicin. Subjects with a robust capsaicin response were chosen to determine if the selected topical formulation of SB705498 had an effect on challenge agent induced itch.

RESULTS

Following capsaicin challenge the greatest average reduction in area of flare was seen for the 3% formulation. This dose was selected for further investigation. Itch intensity induced by two challenge agents (cowhage and histamine) was assessed on the Computerised Visual Analogue Scale. The difference in average itch intensity (Weighted Mean Over 15 Mins) between the 3% dose of SB705498 and placebo for the cowhage challenge was -0.64, whilst the histamine challenge showed on average a -4.65 point change.

CONCLUSIONS

The 3% topical formulation of SB705498 cream was clinically well tolerated and had target specific pharmacodynamic activity. However there were no clinically significant differences on pruritus induced by either challenge agent in comparison to placebo. SB705498 is unlikely to be of symptomatic benefit for histaminergic or non-histaminergic induced itch.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01673529.

Pruritus in cholestasis: facts and fiction

Pruritus is a common symptom in patients with cholestatic liver diseases such as primary biliary cirrhosis, primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy, or hereditary pediatric cholestatic disorders and may accompany, although less frequently, many other liver diseases. Recent findings indicate that lysophosphatidic acid (LPA), a potent neuronal activator, and autotaxin (ATX; ectonucleotide pyrophosphatase/phosphodiesterase 2), the enzyme which forms LPA, may form a key element of the long-sought pruritogenic signaling cascade in cholestatic patients suffering from itch. Serum ATX, but no other pruritogen candidate studied so far, correlates with pruritus intensity and responds to therapeutic interventions. In this comprehensive review, we provide a short update on actual insights in signal transmission related to pruritus and discuss pruritogen candidates in cholestasis. We also summarize evidence-based and guideline-approved as well as experimental therapeutic approaches for patients suffering from pruritus in cholestasis.

Itch sensation through transient receptor potential channels: a systematic review and relevance to manual therapy

OBJECTIVE

Patients may present with a complaint of "itchiness" or an "odd sensation" that can be relieved by manual therapy treatment options, which demonstrates the relevance of transient receptor potential (TRP) channels. There are studies that identify the role of various TRP channels as modulators of the itch sensation; however, discrepancies in the literature exist with respect to the overall neural pathway of the itch sensation, musculoskeletal implications, and decisive therapeutic implications. The purpose of this study was to review the literature and rate the quality of published articles regarding the role of TRP channels in the itch sensation.

METHODS

A systematic search of relevant literature that was published in English by a peer-reviewed journal between January 2000 and June 2012 was performed in PubMed. Studies that met the predetermined inclusion criteria regarding the relationship between TRP channels and itch were identified and then evaluated for methodological quality by the Downs and Black Quality Index score system and were summarized.

RESULTS

Nine studies were identified that met the inclusion criteria, all of which had fair methodological quality from the perspective of the modified Downs and Black Quality Index. TRPA1, TRPM8, and TRPV1-4 were indicated as key channels responsible for the transmission of the itch sensation. TRPV1 channels convey histamine-dependent itch, and TRPA1 channels convey histamine-independent itch. Temperature, nerve growth factor, and substance-P were also described as important itch modulators. There are similarities between the neural pathways responsible for itch, pain, and temperature, which explain the ability of noxious temperature to suppress the desire to scratch. Although transcutaneous electrical nerve stimulation, innocuous vibration, and cutaneous field stimulation have demonstrated relatively weak attenuation of itch, the use of topical capsaicin, noxious heat, and noxious cold have been demonstrated as effective therapies.

CONCLUSIONS

The findings of this review show that studies have assessed the function of TRP channels and itch, rather than identifying the relationship between itch and effective noninvasive treatment options. Therefore, TRP channels could serve as important, complex clinical targets for manual therapists.

Investigational drugs for pruritus

INTRODUCTION

Chronic pruritus (CP), defined as itch lasting for > 6 weeks, is a burdensome symptom of several different diseases, dermatological and systemic, with a high negative impact on the quality of life of patients. Given the manifold aetiologies of CP, therapy is often difficult. In recent years, however, novel substances have been developed for treatment of certain CP entities and identified targets.

AREAS COVERED

In this review, the authors present a survey of targets currently believed to be promising (H4R, IL-31, MOR, KOR, GRPR, NGF, NK-1R, TRP channels) and related investigational drugs that are in the preclinical or clinical stage of development. Some substances have already undergone clinical testing, but only one of them (nalfurafine) has been licensed so far. Many of them are most likely to exert their effects on the skin and interfere there with the cutaneous neurobiology of CP.

EXPERT OPINION

Currently, the most promising candidates for new therapeutic agents in CP are neurokinin-1 receptor antagonists and substances targeting the kappa- or mu-opioid receptor, or both. They have the potential to target the neuronal pathway of CP and are thus of interest for several CP entities. The goal for the coming years is to validate these concepts and move forward in developing new drugs for the therapy of CP.

Excitation and modulation of TRPA1, TRPV1, and TRPM8 channel-expressing sensory neurons by the pruritogen chloroquine

The sensations of pain, itch, and cold often interact with each other. Pain inhibits itch, whereas cold inhibits both pain and itch. TRPV1 and TRPA1 channels transduce pain and itch, whereas TRPM8 transduces cold. The pruritogen chloroquine (CQ) was reported to excite TRPA1, leading to the sensation of itch. It is unclear how CQ excites and modulates TRPA1(+), TRPV1(+), and TRPM8(+) neurons and thus affects the sensations of pain, itch, and cold. Here, we show that only 43% of CQ-excited dorsal root ganglion neurons expressed TRPA1; as expected, the responses of these neurons were completely prevented by the TRPA1 antagonist HC-030031. The remaining 57% of CQ-excited neurons did not express TRPA1, and excitation was not prevented by either a TRPA1 or TRPV1 antagonist but was prevented by the general transient receptor potential canonical (TRPC) channel blocker BTP2 and the selective TRPC3 inhibitor Pyr3. Furthermore, CQ caused potent sensitization of TRPV1 in 51.9% of TRPV1(+) neurons and concomitant inhibition of TRPM8 in 48.8% of TRPM8(+) dorsal root ganglion neurons. Sensitization of TRPV1 is caused mainly by activation of the phospholipase C-PKC pathway following activation of the CQ receptor MrgprA3. By contrast, inhibition of TRPM8 is caused by a direct action of activated Gαq independent of the phospholipase C pathway. Our data suggest the involvement of the TRPC3 channel acting together with TRPA1 to mediate CQ-induced itch. CQ not only elicits itch by directly exciting itch-encoding neurons but also exerts previously unappreciated widespread actions on pain-, itch-, and cold-sensing neurons, leading to enhanced pain and itch.

New insights into the mechanisms of itch: are pain and itch controlled by distinct mechanisms?

Itch and pain are closely related but distinct sensations. They share largely overlapping mediators and receptors, and itch-responding neurons are also sensitive to pain stimuli. Itch-mediating primary sensory neurons are equipped with distinct receptors and ion channels for itch transduction, including Mas-related G protein-coupled receptors (Mrgprs), protease-activated receptors, histamine receptors, bile acid receptor, toll-like receptors, and transient receptor potential subfamily V1/A1 (TRPV1/A1). Recent progress has indicated the existence of an itch-specific neuronal circuitry. The MrgprA3-expressing primary sensory neurons exclusively innervate the epidermis of skin, and their central axons connect with gastrin-releasing peptide receptor (GRPR)-expressing neurons in the superficial spinal cord. Notably, ablation of MrgprA3-expressing primary sensory neurons or GRPR-expressing spinal cord neurons results in selective reduction in itch but not pain. Chronic itch results from dysfunction of the immune and nervous system and can manifest as neural plasticity despite the fact that chronic itch is often treated by dermatologists. While differences between acute pain and acute itch are striking, chronic itch and chronic pain share many similar mechanisms, including peripheral sensitization (increased responses of primary sensory neurons to itch and pain mediators), central sensitization (hyperactivity of spinal projection neurons and excitatory interneurons), loss of inhibitory control in the spinal cord, and neuro-immune and neuro-glial interactions. Notably, painful stimuli can elicit itch in some chronic conditions (e.g., atopic dermatitis), and some drugs for treating chronic pain are also effective in chronic itch. Thus, itch and pain have more similarities in pathological and chronic conditions.

Potentiation of the transient receptor potential vanilloid 1 channel contributes to pruritogenesis in a rat model of liver disease

Persistent pruritus is a common disabling dermatologic symptom associated with different etiologic factors. These include primary skin conditions, as well as neuropathic, psychogenic, or systemic disorders like chronic liver disease. Defective clearance of potential pruritogenic substances that activate itch-specific neurons innervating the skin is thought to contribute to cholestatic pruritus. However, because the underlying disease-specific pruritogens and itch-specific neuronal pathways and mechanism(s) are unknown, symptomatic therapeutic intervention often leads to no or only limited success. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a model for hepatic pruritus and then to evaluate the contribution of inflammation, peripheral neuronal sensitization, and specific signaling pathways and subpopulations of itch-responsive neurons to scratching behavior and thermal hypersensitivity. Chronic BDL rats displayed enhanced scratching behavior and thermal hyperalgesia indicative of peripheral neuroinflammation. BDL-induced itch and hypersensitivity involved a minor contribution of histaminergic/serotonergic receptors, but significant activation of protein-activated receptor 2 (PAR2) receptors, prostaglandin PGE2 formation, and potentiation of transient receptor potential vanilloid 1 (TRPV1) channel activity. The sensitization of dorsal root ganglion nociceptors in BDL rats was associated with increased surface expression of PAR2 and TRPV1 proteins and an increase in the number of PAR2- and TRPV1-expressing peptidergic neurons together with a shift of TRPV1 receptor expression to medium sized dorsal root ganglion neurons. These results suggest that pruritus and hyperalgesia in chronic cholestatic BDL rats are associated with neuroinflammation and involve PAR2-induced TRPV1 sensitization. Thus, pharmacological modulation of PAR2 and/or TRPV1 may be a valuable therapeutic approach for patients with chronic liver pruritus refractory to conventional treatments.

Superoxide generation and leukocyte accumulation: key elements in the mediation of leukotriene B₄-induced itch by transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1

The underlying mechanisms of itch are poorly understood. We have investigated a model involving the chemoattractant leukotriene B₄ (LTB₄) that is up-regulated in common skin diseases. Intradermal injection of LTB4 (0.1 nmol/site) into female CD1 mice induced significant scratching movements (used as an itch index) compared with vehicle-injected (0.1% bovine serum albumin-saline) mice. Intraperitoneal transient receptor potential (TRP) channel antagonist treatment significantly inhibited itch as follows: TRP vanilloid 1 (TRPV1) antagonist SB366791 (0.5 mg/kg, by 97%) and the TRP ankyrin 1 (TRPA1) antagonists TCS 5861528 (10 mg/kg; 82%) and HC-030031 (100 mg/kg; 76%). Leukotriene B₄ receptor 2 antagonism by LY255283 (5 mg/kg i.p.; 62%) reduced itch. Neither TRPV1-knockout (TRPV1-KO) nor TRPA1-knockout (TRPA1-KO mice exhibited LTB₄-induced itch compared with their wild-type counterparts. The reactive oxygen species scavengers N-acetylcysteine (NAC; 204 mg/kg i.p.; 86%) or superoxide dismutase (SOD; 10 mg/kg i.p.; 83%) also inhibited itch. LTB4-induced superoxide release was attenuated by TCS 5861528 (56%) and HC-030031 (66%), NAC (58%), SOD (50%), and LY255283 (59%) but not by the leukotriene B4 receptor 1 antagonist U-75302 (9 nmol/site) or SB366791. Itch, superoxide, and myeloperoxidase generation were inhibited by the leukocyte migration inhibitor fucoidan (10 mg/kg i.v.) by 80, 61, and 34%, respectively. Myeloperoxidase activity was also reduced by SB366791 (35%) and SOD (28%). TRPV1-KO mice showed impaired myeloperoxidase release, whereas TRPA1-KO mice exhibited diminished production of superoxide. This result provides novel evidence that TRPA1 and TRPV1 contribute to itch via distinct mechanisms.

Itch pathophysiology may differ among ethnic groups

The prevalence of itch is higher in individuals with darker skin types. In this paper, we review the systems involved in the physiology of itch and how they may differ across the races. Current data point out that the differences may be explained by barrier function, mast cell physiology, and itch receptor polymorphisms.

Oxidative stress induces itch via activation of transient receptor potential subtype ankyrin 1 in mice

OBJECTIVE

To investigate the role of oxidative stress in itch-indicative scratching behavior in mice, and furthermore, to define the cellular and molecular mechanisms underlying oxidative stress-mediated itch.

METHODS

Scratching behavior was induced by intradermal injection of the oxidants hydrogen peroxide (H₂O₂) or tert-butylhydroperoxide (tBHP) into the nape of the neck in mice. The mice were observed for 30 min.

RESULTS

Intradermal H₂O₂ (0.03%-1%) or tBHP (1-30 μmol) elicited robust scratching behavior, displaying an inverted U-shaped dose-response curve. Naloxone, an opioid receptor antagonist, but not morphine, largely suppressed the oxidant-induced scratching. Chlorpheniramine, a histamine H1 receptor antagonist, blocked histamine- but not oxidant-induced scratching, indicating the involvement of a histamine-independent mechanism in oxidant-evoked itch. Further, resiniferatoxin treatment abolished oxidant-induced scratching, suggesting an essential role of C-fibers. Notably, blockade of transient receptor potential subtype ankyrin 1 (TRPA1) with the selective TRPA1 antagonist HC-030031, or genetic deletion of Trpa1 but not Trpv1 (subfamily V, member 1) resulted in a profound reduction in H₂O₂-evoked scratching. Finally, systemic administration of the antioxidant N-acetyl-L-cysteine or trolox (a water-soluble vitamin E analog) attenuated scratching induced by the oxidants.

CONCLUSION

Oxidative stress by different oxidants induces profound scratching behavior, which is largely histamine- and TRPV1-independent but TRPA1-dependent. Antioxidants and TRPA1 antagonists may be used to treat human itch conditions associated with oxidative stress.

The itchy scalp--scratching for an explanation

Scalp pruritus is a common complaint that is considered a diagnostically and therapeutically challenging situation. Scalp skin has a unique neural structure that contains densely innervated hair follicles and dermal vasculature. In spite of the recent advances in our understanding of itch pathophysiology, scalp itching has not been studied as yet. In this review, we summarize the current knowledge on the neurobiology of scalp and hair follicles as well as itch mediators and provide a putative mechanism for scalp itch with special emphasis on neuroanatomy and pathophysiology.

Cannabinoid actions at TRPV channels: effects on TRPV3 and TRPV4 and their potential relevance to gastrointestinal inflammation

AIM

Plant cannabinoids, like Δ(9)-tetrahydrocannabinol (THC) and cannabidiol (CBD), activate/desensitize thermosensitive transient receptor potential (TRP) channels of vanilloid type-1 or -2 (TRPV1 or TRPV2). We investigated whether cannabinoids also activate/desensitize two other 'thermo-TRP's', the TRP channels of vanilloid type-3 or -4 (TRPV3 or TRPV4), and if the TRPV-inactive cannabichromene (CBC) modifies the expression of TRPV1-4 channels in the gastrointestinal tract.

METHODS

TRP activity was assessed by evaluating elevation of [Ca(2+)](i) in rat recombinant TRPV3- and TRPV4-expressing HEK-293 cells. TRP channel mRNA expression was measured by quantitative RT-PCR in the jejunum and ileum of mice treated with vehicle or the pro-inflammatory agent croton oil.

RESULTS

(i) CBD and tetrahydrocannabivarin (THCV) stimulated TRPV3-mediated [Ca(2+)](i) with high efficacy (50-70% of the effect of ionomycin) and potency (EC(50∼) 3.7 μm), whereas cannabigerovarin (CBGV) and cannabigerolic acid (CBGA) were significantly more efficacious at desensitizing this channel to the action of carvacrol than at activating it; (ii) cannabidivarin and THCV stimulated TRPV4-mediated [Ca(2+)](i) with moderate-high efficacy (30-60% of the effect of ionomycin) and potency (EC(50) 0.9-6.4 μm), whereas CBGA, CBGV, cannabinol and cannabigerol were significantly more efficacious at desensitizing this channel to the action of 4-α-phorbol 12,13-didecanoate (4α-PDD) than at activating it; (iii) CBC reduced TRPV1β, TRPV3 and TRPV4 mRNA in the jejunum, and TRPV3 and TRPV4 mRNA in the ileum of croton oil-treated mice.

CONCLUSIONS

Cannabinoids can affect both the activity and the expression of TRPV1-4 channels, with various potential therapeutic applications, including in the gastrointestinal tract.

The channel physiology of the skin

During embryonic development, the skin, the largest organ of the human body, and nervous system are both derived from the neuroectoderm. Consequently, several key factors and mechanisms that influence and control central or peripheral nervous system activities are also present and hence involved in various regulatory mechanisms of the skin. Apparently, this is the case for the ion and non-ion selective channels as well. Therefore, in this review, we shall focus on delineating the regulatory roles of the channels in skin physiology and pathophysiology. First, we introduce key cutaneous functions and major characteristics of the channels in question. Then, we systematically detail the involvement of a multitude of channels in such skin processes (e.g. skin barrier formation, maintenance, and repair, immune mechanisms, exocrine secretion) which are mostly defined by cutaneous non-neuronal cell populations. Finally, we close by summarizing data suggesting that selected channels are also involved in skin diseases such as e.g. atopic dermatitis, psoriasis, non-melanoma cancers and malignant melanoma, genetic and autoimmune diseases, etc., as well as in skin ageing.

Topical therapies for pruritus

Itch, or pruritus, is the predominant symptom associated with acute and chronic cutaneous disease, and in some cases may be debilitating. To date, there is no single universally effective anti-itch treatment. Because the pathophysiology of itch in most cutaneous or systemic disorders remains unclear, antipruritic therapy is often directed against a variety of targets, including the epidermal barrier, immune system, or the nervous system. Topical therapy is the mainstay of dermatologic management of acute or localized itch or in patients with contraindications to systemic therapies. This review will summarize current topical therapies to treat pruritus and discuss potential future therapies.