TRPV1 mediates histamine-induced itching via the activation of phospholipase A2 and 12-lipoxygenase

Inhibition of temperature-sensitive TRPV3 channel by two natural isochlorogenic acid isomers for alleviation of dermatitis and chronic pruritus

Genetic gain-of-function mutations of warm temperature-sensitive transient receptor potential vanilloid 3 (TRPV3) channel cause Olmsted syndrome characterized by severe itching and keratoderma, indicating that pharmacological inhibition of TRPV3 may hold promise for therapy of chronic pruritus and skin diseases. However, currently available TRPV3 tool inhibitors are either nonselective or less potent, thus impeding the validation of TRPV3 as therapeutic target. Using whole-cell patch-clamp and single-channel recordings, we report the identification of two natural dicaffeoylquinic acid isomers isochlorogenic acid A (IAA) and isochlorogenic acid B (IAB) that selectively inhibit TRPV3 currents with IC₅₀ values of 2.7 ± 1.3 and 0.9 ± 0.3 μmol/L, respectively, and reduce the channel open probability to 3.7 ± 1.2% and 3.2 ± 1.1% from 26.9 ± 5.5%, respectively. In vivo evaluation confirms that both IAA and IAB significantly reverse the ear swelling of dermatitis and chronic pruritus. Furthermore, the isomer IAB is able to rescue the keratinocyte death induced by TRPV3 agonist carvacrol. Molecular docking combined with site-directed mutations reveals two residues T636 and F666 critical for the binding of the two isomers. Taken together, our identification of isochlorogenic acids A and B that act as specific TRPV3 channel inhibitors and gating modifiers not only provides an essential pharmacological tool for further investigation of the channel pharmacology and pathology, but also holds developmental potential for treatment of dermatitis and chronic pruritus.

A brain-to-spinal sensorimotor loop for repetitive self-grooming

Self-grooming is a complex behavior with important biological functions and pathological relevance. How the brain coordinates with the spinal cord to generate the repetitive movements of self-grooming remains largely unknown. Here, we report that in the caudal part of the spinal trigeminal nucleus (Sp5C), neurons that express Cerebellin-2 (Cbln2⁺) form a neural circuit to the cervical spinal cord to maintain repetitive orofacial self-grooming. Inactivation of Cbln2⁺ Sp5C neurons blocked both sensory-evoked and stress-induced repetitive orofacial self-grooming. Activation of these neurons triggered short-latency repetitive forelimb movements that resembled orofacial self-grooming. The Cbln2⁺ Sp5C neurons were monosynaptically innervated by both somatosensory neurons in the trigeminal ganglion and paraventricular hypothalamic neurons. Among the divergent projections of Cbln2⁺ Sp5C neurons, a descending pathway that innervated motor neurons and interneurons in the cervical spinal cord was necessary and sufficient for repetitive orofacial self-grooming. These data reveal a brain-to-spinal sensorimotor loop for repetitive self-grooming in mice.

Transcriptome Analysis of Host Inflammatory Responses to the Ectoparasitic Mite Sarcoptes scabiei var. hominis

Scabies, a human skin infestation caused by the ectoparasitic mite Sarcoptes scabiei var. hominis, affects more than 200 million people globally. The prevailing knowledge of the disease process and host immune response mechanisms is limited. A better understanding of the host-parasite relationship is essential for the identification of novel vaccine and drug targets. Here we aimed to interrogate the transcriptomic profiles of mite-infested human skin biopsies with clinical manifestations of ordinary scabies subjects ("OS"; n = 05) and subjects naive to scabies ("control"; n = 03) using RNASeq data analysis. A combined clustering, network, and pathway mapping approach enabled us to identify key signaling events in the host immune and pro-inflammatory responses to S. scabiei infestation. The clustering patterns showed various differentially expressed genes including inflammatory responses and innate immunity genes (DEFB4A, IL-19, CXCL8, CSF3, SERPINB4, S100A7A, HRNR) and notably upregulation of the JAK-STAT pathway in scabies-infested samples. Mite-infested human skin biopsies (GSE178563) were compared with an ex-vivo porcine infested model (E-MTAB-6433) and human skin equivalents (GSE48459). Marked enrichment of immune response pathways (JAK-STAT signaling, IL-4 and IL-13 pathway, and Toll receptor cascade), chemokine ligands and receptors (CCL17, CCL18, CCL3L1, CCL3L3, CCR7), and cytokines (IL-13 and IL-20) were observed. Additionally, genes known for their role in psoriasis and atopic dermatitis were upregulated, e.g., IL-19. The detailed transcriptomic profile has provided an insight into molecular functions, biological processes, and immunological responses and increased our understanding about transcriptomic regulation of scabies in human.

Peripheral Mechanisms of Itch

Itch is a universally experienced sensation, and chronic itch can be as diabolically debilitating as pain. Recent advances have not only identified the neuronal itch sensing circuitry, but also have uncovered the intricate interactions between skin and immune cells that work together with neurons to identify itch-inducing irritants. In this review, we will summarize the fundamental mechanisms of acute itch detection in the skin, as well as highlight the recent discoveries relating to this topic.

Connections between Immune-Derived Mediators and Sensory Nerves for Itch Sensation

Although histamine is a well-known itch mediator, histamine H1-receptor blockers often lack efficacy in chronic itch. Recent molecular and cellular based studies have shown that non-histaminergic mediators, such as proteases, neuropeptides and cytokines, along with their cognate receptors, are involved in evocation and modulation of itch sensation. Many of these molecules are produced and secreted by immune cells, which act on sensory nerve fibers distributed in the skin to cause itching and sensitization. This understanding of the connections between immune cell-derived mediators and sensory nerve fibers has led to the development of new treatments for itch. This review summarizes current knowledge of immune cell-derived itch mediators and neuronal response mechanisms, and discusses therapeutic agents that target these systems.

Glucosylsphingosine evokes pruritus via activation of 5-HT2A receptor and TRPV4 in sensory neurons

BACKGROUND AND PURPOSE

Glucosylsphingosine (GS), an endogenous sphingolipid, is highly accumulated in the epidermis of patients with atopic dermatitis (AD) due to abnormal ceramide metabolism. More importantly, GS can evoke scratching behaviors. However, the precise molecular mechanism by which GS induces pruritus has been elusive. Thus, the present study aimed to elucidate the molecular signaling pathway of GS, especially at the peripheral sensory neuronal levels.

EXPERIMENTAL APPROACH

Calcium imaging was used to investigate the responses of HEK293T cells or mouse dorsal root ganglion (DRG) neurons to application of GS. Scratching behavior tests were also performed with wild-type and Trpv4 knockout mice.

KEY RESULTS

GS activated DRG neurons in a manner involving both the 5-HT_2A receptor and TRPV4. Furthermore, GS-induced responses were significantly suppressed by various inhibitors, including ketanserin (5-HT_2A receptor antagonist), YM254890 (Gα_q/11 inhibitor), gallein (Gβγ complex inhibitor), U73122 (phospholipase C inhibitor), bisindolylmaleimide I (PKC inhibitor), and HC067047 (TRPV4 antagonist). Moreover, DRG neurons from Trpv4 knockout mice exhibited significantly reduced responses to GS. Additionally, GS-evoked scratching behaviors were greatly decreased by pretreatment with inhibitors of either 5-HT_2A receptor or TRPV4. As expected, GS-evoked scratching behavior was also significantly decreased in Trpv4 knockout mice.

CONCLUSION AND IMPLICATIONS

Overall, the present study provides evidence for a novel molecular signaling pathway for GS-evoked pruritus, which utilizes both 5-HT_2A receptor and TRPV4 in mouse sensory neurons. Considering the high accumulation of GS in the epidermis of patients with AD, GS could be another pruritogen in patients with AD.

Neuron‒Mast Cell Cross-Talk in the Skin

Skin-resident mast cells (MCs) and cutaneous sensory neurons both play crucial roles in microbial‒host defense and inflammatory diseases. MCs can be directly activated by pathogens or their products, resulting in the release of numerous mediators that promote innate immune responses and also activate sensory neurons. Cutaneous sensory neurons can also directly detect the presence of pathogens, resulting in the release of neuropeptides that modulate MC function. In this review, we will focus on the reciprocal interactions between cutaneous sensory neurons and MCs and the importance of this cross-talk in skin diseases.

Desensitization of TRPV1 Involved in the Antipruritic Effect of Osthole on Histamine-Induced Scratching Behavior in Mice

Osthole has been isolated from the fruits of Cnidium monnieri (L.) Cusson, which has been used in Chinese traditional medicine to treat pruritic disorders for a long time. However, the antipruritic mechanism of osthole is not fully understood. In the present study, using calcium imaging, molecular docking, and animal scratching behavior, we analyzed the pharmacological effects of osthole on transient receptor potential vanilloid 1 (TRPV1). The results showed that osthole significantly induced calcium influx in a dose-dependent manner in dorsal root ganglion (DRG) neurons. Osthole-induced calcium influx was inhibited by AMG9810, an antagonist of TRPV1. Osthole and the TRPV1 agonist capsaicin-induced calcium influx were desensitized by pretreatment with osthole. Furthermore, molecular docking results showed that osthole could bind to TRPV1 with a hydrogen bond by anchoring to the amino acid residue ARG557 in the binding pocket of TRPV1. In addition, TRPV1 is a downstream ion channel for the histamine H1 and H4 receptors to transmit itch signals. Osthole attenuated scratching behavior induced by histamine, HTMT (histamine H1 receptor agonist), and VUF8430 (histamine H4 receptor agonist) in mice. These results suggest that osthole inhibition of histamine-dependent itch may be due to the activation and subsequent desensitization of TRPV1 in DRG neurons.

Itch: Pathogenesis and treatment

Itch pathogenesis is broadly characterized into histaminergic and nonhistaminergic pathways and transmitted via 2 main receptor families: G protein-coupled receptors and transient receptor potential channels. In the skin, itch is primarily transmitted by unmyelinated type C and thinly myelinated type Aδ nerve fibers. Crosstalk between the immune and neural systems modulates itch transmission at the skin, spinal cord, and brain. Among the many known pruritogens, Th2 cytokines, such as interleukin-4, interleukin-13, interleukin-31, and thymic stromal lymphopoietin, are particularly important mediators that signal through shared Janus kinase pathways, representing novel targets for novel itch therapeutics. Emerging evidence has also revealed that the opioidergic system is a potent modulator of itch transmission, with increased μ-opioid activity and decreased κ-opioid activity contributing to itch pathogenesis. Optimal management of itch requires that treatment approaches be tailored to specific etiologic itch subtypes. When the etiology is unknown and patients are given a diagnosis of chronic pruritus of unknown origin, treatment should be guided by the presence of Th2 polarization, often reflected by increased blood eosinophils. In the second article of this 2-part series, we outline our current understanding of itch pathogenesis and discuss available and emerging treatments for itch.

Dissecting the precise nature of itch-evoked scratching

Itch is a discrete and irritating sensation tightly coupled to a drive to scratch. Acute scratching developed evolutionarily as an adaptive defense against skin irritants, pathogens, or parasites. In contrast, the itch-scratch cycle in chronic itch is harmful, inducing escalating itch and skin damage. Clinically and preclinically, scratching incidence is currently evaluated as a unidimensional motor parameter and believed to reflect itch severity. We propose that scratching, when appreciated as a complex, multidimensional motor behavior, will yield greater insight into the nature of itch and the organization of neural circuits driving repetitive motor patterns. We outline the limitations of standard measurements of scratching in rodent models and present new approaches to observe and quantify itch-evoked scratching. We argue that accurate quantitative measurements of scratching are critical for dissecting the molecular, cellular, and circuit mechanisms underlying itch and for preclinical development of therapeutic interventions for acute and chronic itch disorders.

Asivatrep, a TRPV1 antagonist, for the topical treatment of atopic dermatitis: Phase 3, randomized, vehicle-controlled study (CAPTAIN-AD)

BACKGROUND

Asivatrep is a potent and selective antagonist of transient receptor potential vanilloid subfamily V member 1 (TRPV1), which plays an important role in itch and inflammation in atopic dermatitis (AD).

OBJECTIVE

This current study aimed to evaluate the efficacy and safety of asivatrep cream in patients with AD.

METHODS

For this phase 3 double-blind, vehicle-controlled study, patients aged ≥12 years with mild to moderate AD were enrolled and randomly assigned 2:1 to the 1.0% asivatrep or vehicle group for 8 weeks of twice-daily application (n = 240). The primary end point was the proportion of patients with an Investigator's Global Assessment score (IGA) of 0 or 1 at week 8. Standard safety assessments were conducted.

RESULTS

At week 8, significantly more patients in the asivatrep group (36.0%) than in the vehicle group (12.8%) had IGA scores of 0 or 1 (P < .001); significantly more had ≥2 points of improvement on the IGA from baseline score (20.3% vs 7.7%; P = .01). The mean percentage reduction in the Eczema Area and Severity Index (EASI) score was 44.3% for the asivatrep group and 21.4% for the vehicle group at week 8 (P < .001). Significantly more asivatrep-treated patients experienced an improvement of at least 50%, 75%, and 90% on the EASI than the vehicle group. The mean ± SD change in the pruritus visual analog scale score at week 8 was -2.3 ± 2.4 for the asivatrep group and -1.5 ± 2.4 for the vehicle group (P = .02). No significant safety issues were reported.

CONCLUSION

Asivatrep improved clinical signs and symptoms of AD and was well tolerated.

Irradiation of the Normal Murine Tongue Causes Upregulation and Activation of Transient Receptor Potential (TRP) Ion Channels

Signal transduction at sensory neurons occurs via transmembrane flux of cations, which is largely governed by the transient receptor potential (TRP) family of ion channels. It is unknown whether TRP channel activation contributes to the pain that accompanies radiation-induced oral mucositis. This study sought to characterize changes in TRP channel expression and function that occur in the locally irradiated tissues and afferent neurons of mice. Female CD-1 mice received single high-dose (27 Gy) tongue irradiation, or sham irradiation. Animals were euthanized either before overt glossitis developed (days 1 and 5 postirradiation), when glossitis was severe (day 11), or after mice had recovered (days 21 and 45). Tongue irradiation caused upregulation of the Trpv1 gene in trigeminal ganglia (TG) neurons. Other TRP genes (Trpv2, Trpv4, Trpa1, Trpm8) and Gfrα3 (which acts upstream of several TRP channels) were also upregulated in TGs and/or tongue tissue, in response to radiation. Ex vivo calcium imaging experiments demonstrated that the proportions of TG neurons responding to histamine (an activator of TRPV1, TRPV4 and TRPA1), TNF-α (an activator of TRPV1, TRPV2 and TRPV4), and capsaicin (a TRPV1 agonist), were increased as early as one day after tongue irradiation; these changes persisted for at least 21 days. In a subsequent experiment, we found that genetic deletion of TRPV1 mitigated weight loss (a surrogate marker of pain severity) in mice with severe glossitis. The results intimate that various TRP channels, and TRPV1 in particular, should be explored as analgesic targets for patients experiencing pain after oral irradiation.

Lipid-dependent sequential allosteric activation of heat-sensing TRPV1 channels by anchor-stereoselective "hot" vanilloid compounds and analogs

Both a silent resident phosphatidylinositol lipid and a “hot” vanilloid agonist capsaicin or resiniferatoxin have been shown to share the same inter-subunit binding pocket between a voltage sensor like domain and a pore domain in TRPV1. However, how the vanilloid competes off the resident lipid for allosteric TRPV1 activation is unknown. Here, the in silico research suggested that anchor-stereoselective sequential cooperativity between an initial recessive transient silent weak ligand binding site and a subsequent dominant steady-state strong ligand binding site in the vanilloid pocket may facilitate the lipid release for allosteric activation of TRPV1 by vanilloids or analogs upon non-covalent interactions. Thus, the resident lipid may play a critical role in allosteric activation of TRPV1 by vanilloid compounds and analogs.

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Four active vanilloid binding pockets as revealed by the cryo-EM structure of TRPV1 have no cooperativity between subunits.

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Allosteric activation of TRPV1 by vanilloid compounds and analogs is lipid-dependent and anchor-stereoselective.

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The resident and occluded lipid must be competed off for allosteric activation of TRPV1 by vanilloid compounds and analogs.

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The first ligand binding is needed to release the resident lipid for the second ligand binding in the vanilloid pocket.

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A lipid-free anchor facilitates the vanilloid ligand binding to remove the resident lipid from the active site in TRPV1.

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Site accessibility controls sequential cooperative interactions of TRPV1 with vanilloids or analogs to open the channel.

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The anchor stereoselectivity depends on the formation of a vanilloid bridge between two separated residues at the active site.

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Different anchor stereoselectivities produce diverse recessive transient reaction intermediates or steps for TRPV1 opening.

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Membrane hyperpolarization and depolarization may stabilize and loosen the resident lipid for TRPV1 gating, respectively.

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Phospholipids at N- and C- terminal domains may affect the cooperativity or the potency of the vanilloid ligands.

Cinnamaldehyde elicits itch behavior via TRPV1 and TRPV4 but not TRPA1

Introduction:

Cinnamaldehyde (CA) elicits itch sensation in humans. We investigated if CA elicits scratching behavior in mice and determined the roles for TRPV1, TRPA1, and TRPV4.

Materials and Methods:

Scratching behavior elicited by intradermal injection of CA was assessed in wildtype (WT) mice and knockout (KO) mice lacking TRPV1, TRPA1, TRPV4, or deficient in mast cells. We also assessed scratching and wet dog shakes elicited by low-threshold mechanical stimulation of skin treated topically with CA or vehicle. Using calcium imaging we tested if CA activates dorsal root ganglion (DRG) neurons of each genotype.

Results:

Intradermal cheek injection of CA elicited dose-dependent hindlimb scratch bouts, with fewer forelimb wipes and facial groom bouts that were not dose-dependent. CA elicited significantly fewer scratch bouts in TRPV1 and TRPV4 KO mice, but not TRPA1KOs, compared with WTs. There were no sex differences across genotypes. The histamine H1 antagonist cetirizine did not affect CA-evoked scratching, which was normal in mast cell deficient mice, indicating lack of histamine involvement. Scores for alloknesis were significantly greater following topical application of CA compared with vehicle. Post-CA alloknesis scores were significantly higher in TRPV4KOs of both sexes and in female TRPV1 and TRPA1KOs, compared with WTs. Low threshold mechanical stimuli also elicited significantly more wet dog shakes in mice treated topically with 20% CA, with significantly fewer in TRPV1, TRPA1, and TRPV4KOs compared with WTs. In calcium imaging studies, CA excited 24% of WT DRG cells, significantly fewer (11.5%) in cells from TRPV4KOs, and none in TRPA1KOs. Responses of cells of all genotypes exhibited significant sensitization to repeated CA stimulation. Sensitization was significantly enhanced by IL-4, which itself excited 16% of WT DRG cells and none from TRPA1KOs.

Discussion:

The results indicate that TRPA1 is dispensable for CA-evoked scratching, which depends partly on TRPV1 and TRPV4.

TRPV1 and TRPA1 Channels Are Both Involved Downstream of Histamine-Induced Itch

Two histamine receptor subtypes (HR), namely H1R and H4R, are involved in the transmission of histamine-induced itch as key components. Although exact downstream signaling mechanisms are still elusive, transient receptor potential (TRP) ion channels play important roles in the sensation of histaminergic and non-histaminergic itch. The aim of this study was to investigate the involvement of TRPV1 and TRPA1 channels in the transmission of histaminergic itch. The potential of TRPV1 and TRPA1 inhibitors to modulate H1R- and H4R-induced signal transmission was tested in a scratching assay in mice in vivo as well as via Ca²⁺ imaging of murine sensory dorsal root ganglia (DRG) neurons in vitro. TRPV1 inhibition led to a reduction of H1R- and H4R- induced itch, whereas TRPA1 inhibition reduced H4R- but not H1R-induced itch. TRPV1 and TRPA1 inhibition resulted in a reduced Ca²⁺ influx into sensory neurons in vitro. In conclusion, these results indicate that both channels, TRPV1 and TRPA1, are involved in the transmission of histamine-induced pruritus.

MRGPRX4 in Cholestatic Pruritus

Pruritus (itch) is a debilitating symptom in liver diseases with cholestasis, which severely affects patients' quality of life. Limited treatment options are available for cholestatic itch, largely due to the incomplete understanding of the underlying molecular mechanisms. Several factors have been proposed as pruritogens for cholestatic itch, such as bile acids, bilirubin, lysophosphatidic acid, and endogenous opioids. Recently, two research groups independently identified Mas-related G protein-coupled receptor X4 (MRGPRX4) as a receptor for bile acids and bilirubin and demonstrated its likely role in cholestatic itch. This discovery not only opens new avenues for understanding the molecular mechanisms in cholestatic itch but provides a promising target for developing novel anti-itch treatments. In this review, we summarize the current theories and knowledge of cholestatic itch, emphasizing MRGPRX4 as a bile acid and bilirubin receptor mediating cholestatic itch in humans. We also discuss some future perspectives in cholestatic itch research.

Immunopharmacology in Vernal Keratoconjunctivitis: Current and Future Perspectives

Vernal keratoconjunctivitis (VKC) is a complex and chronic, multifactorial Th2 cell-mediated chronic ocular surface inflammatory condition that typically affects predominantly male children in hot or warm climates. The primary symptom is intense ocular pruritus, often significant enough to affect activities of daily living. Clinical features differ from simple forms of allergic conjunctivitis in that they are more-or-less confined to the superior tarsus and limbus. There is also a risk of corneal involvement, which leads to irreversible vision loss in approximately 6% of patients. Right now, there is no standardized treatment protocol, and many of the currently available options are not effective in severe and recurrent cases. As such, it is imperative to understand this complex allergic immune response in order to identify future therapeutic targets. This review will focus on potential drug targets in VKC, with particular emphasis on immunomodulators and immunobiologic agents.

Sneezing reflex is mediated by a peptidergic pathway from nose to brainstem

Sneezing is a vital respiratory reflex frequently associated with allergic rhinitis and viral respiratory infections. However, its neural circuit remains largely unknown. A sneeze-evoking region was discovered in both cat and human brainstems, corresponding anatomically to the central recipient zone of nasal sensory neurons. Therefore, we hypothesized that a neuronal population postsynaptic to nasal sensory neurons mediates sneezing in this region. By screening major presynaptic neurotransmitters/neuropeptides released by nasal sensory neurons, we found that neuromedin B (NMB) peptide is essential for signaling sneezing. Ablation of NMB-sensitive postsynaptic neurons in the sneeze-evoking region or deficiency in NMB receptor abolished the sneezing reflex. Remarkably, NMB-sensitive neurons further project to the caudal ventral respiratory group (cVRG). Chemical activation of NMB-sensitive neurons elicits action potentials in cVRG neurons and leads to sneezing behavior. Our study delineates a peptidergic pathway mediating sneezing, providing molecular insights into the sneezing reflex arc.

Quercetin inhibits histamine-induced calcium influx in human keratinocyte via histamine H4 receptors

Histamine is released from mast cells when tissues are inflamed or stimulated by allergens. Activation of histamine receptors and calcium influx via TRPV1 could be related to histamine-induced itch and skin inflammation. Quercetin is known to have anti-inflammatory and anti-itching effects. This study aims to understand whether quercetin can directly affect histamine-induced calcium influx in human keratinocyte. In it, we investigated quercetin, which acts on histamine-induced intracellular free calcium ([Ca2+]i) elevation in human keratinocyte. Changes in [Ca2+]i were measured using spectrofluorometry and confocal Imaging. We detected the expression of IL-8 after treatment of quercetin using qRT-PCR and evaluated its anti-itching effect in BALB/c mice. We also performed a docking study to estimate the binding affinity of quercetin to H4 receptors. We found that quercetin pretreatment decreased histamine-induced [Ca2+]i elevation in a concentration-dependent manner. The inhibitory effect of quercetin on histamine-induced [Ca2+]i elevation was blocked by JNJ7777120, a selective H4 antagonist, as well as by U73122, a PLC inhibitor, and by GF109203X, a PKC inhibitor. We also found that H4 agonist (4-methylhistamine)-induced [Ca2+]i elevation could be inhibited by quercetin. Moreover, the selective TRPV1 blocker capsazepine significantly suppressed the quercetin-mediated inhibition of histamine-induced [Ca2+]i elevation, whereas the TRPV4 blocker GSK2193874 had no effect. Last, quercetin decreased histamine and H4 agonist-induced IL-8 expression in keratinocyte and inhibited the scratching behavior-induced compound 48/80 in BALB/c mice. The molecular docking study also showed that quercetin exhibited high binding affinities with H4 receptors (autodock scores for H4 = -8.7 kcal/mol). These data suggest that quercetin could decrease histamine 4 receptor-induced calcium influx through the TRPV1 channel and could provide a molecular mechanism of quercetin in anti-itching, anti-inflammatory, and unpleasant sensations.

Limosilactobacillus reuteri DSM-17938 for preventing cough in adults with mild allergic asthma: A double-blind randomized placebo-controlled cross-over study

BACKGROUND

Cough is a common troublesome symptom in asthma which is neuronally mediated. Limosilactobacillus reuteri DSM-17938 (L. reuteri DSM-17938) is a probiotic shown to be effective in pre-clinical models at suppressing neuronal responses to capsaicin, a transient receptor potential vanilloid agonist (TRPV1).

OBJECTIVE

Investigate the effects of DSM-17938 versus matched placebo on capsaicin-evoked coughs in mild allergic asthmatics.

METHODS

We performed a 4-visit, randomized, double-blind, placebo-controlled, two-way cross-over study comparing full dose cough responses with inhaled capsaicin in mild allergic asthmatics after 1 month of treatment with DSM-17938 compared with matched placebo. Randomization and allocation to trial group were carried out by a central computer system. Histamine skin prick testing, airway hyper-responsiveness and inflammatory cells in induced sputum were measured at every visit. Blood was collected to extract PBMCs and stimulated with CD3/CD28 to ascertain the effects of DSM-17938 /placebo on T-cell cytokine responses.

RESULTS

Seventeen subjects were recruited and 15 completed the study (8 females, mean age 27.3 years). There was no difference in the change in maximum capsaicin-evoked coughs (Emax) after treatment with L. reuteri DSM-17938 compared with placebo [mean difference 2.07 coughs (95% CI -2.77 to 6.91, p = .38) or relative changes in geometric mean ratios for the dose evoking at least half the Emax (ED50) [1.05 (95% CI 0.31-3.58, p = .94)], concentration evoking 2 coughs (C2) [0.63 (0.26-1.53), p = .28] and 5 coughs (C5) [0.79 (0.25-2.50), p = .67]. There was no effect on histamine skin prick wheal size, intensity of itch sensation, methacholine PC20, airway inflammation or T-cell responses after stimulation with CD3/CD28. There were no serious adverse events. One subject developed a mild upper respiratory tract infection and another mild transient nausea whilst on DSM-17938.

CONCLUSION

In this small study in adults with mild allergic asthma, we found no evidence that L. reuteri DSM-17938 has any systemic effects on airway nerves, smooth muscle, sputum inflammatory cells, skin responses or T-cell responses after oral consumption.

TRIAL REGISTRATION

Clinicaltrials.gov Identifier: NCT03603522.

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.

Riboflavin Inhibits Histamine-Dependent Itch by Modulating Transient Receptor Potential Vanilloid 1 (TRPV1)

Riboflavin, also known as vitamin B₂, isfound in foods and is used as a dietary supplement. Its deficiency (also called ariboflavinosis) results in some skin lesions and inflammations, such as stomatitis, cheilosis, oily scaly skin rashes, and itchy, watery eyes. Various therapeutic effects of riboflavin, such as anticancer, antioxidant, anti-inflammatory, and anti-nociceptive effects, are well known. Although some studies have identified the clinical effect of riboflavin on skin problems, including itch and inflammation, its underlying mechanism of action remains unknown. In this study, we investigated the molecular mechanism of the effects of riboflavin on histamine-dependent itch based on behavioral tests and electrophysiological experiments. Riboflavin significantly reduced histamine-induced scratching behaviors in mice and histamine-induced discharges in single-nerve fiber recordings, while it did not alter motor function in the rotarod test. In cultured dorsal root ganglion (DRG) neurons, riboflavin showed a dose-dependent inhibitory effect on the histamine- and capsaicin-induced inward current. Further tests wereconducted to determine whether two endogenous metabolites of riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), have similar effects to those of riboflavin. Here, FMN, but not FAD, significantly inhibited capsaicin-induced currents and itching responses caused by histamine. In addition, in transient receptor potential vanilloid 1 (TRPV1)-transfected HEK293 cells, both riboflavin and FMN blocked capsaicin-induced currents, whereas FAD did not. These results revealed that riboflavin inhibits histamine-dependent itch by modulating TRPV1 activity. This study will be helpful in understanding how riboflavin exerts antipruritic effects and suggests that it might be a useful drug for the treatment of histamine-dependent itch.

Ovalicin attenuates atopic dermatitis symptoms by inhibiting IL-31 signaling and intracellular calcium influx

Atopic dermatitis (AD) is a common skin disorder difficult to be treated with medication. This study investigated the potential of ovalicin extracted from Cordyceps militaris for the treatment of AD using in vitro and in vivo models. We found that, in canine macrophage cell line DH82, lipopolysaccharide (LPS) upregulated the expression of genes associated with inflammation and pruritic responses through activating calcium and interleukin-31 (IL-31) signaling, and the upregulation could be suppressed by ovalicin, with an effect significantly stronger than dexamethasone. Ovalicin also reduced the expression of IL-31 downstream genes, including JAK2 (Janus kinase 2), TRPV1 (transient receptor potential vanilloid receptor-1), and HRH2 (histamine receptor H2). Ovalicin significantly alleviated the allergic symptoms in the AD mouse model. Histologically, the number of macrophages and mast cells infiltrated in the dermis was significantly reduced by ovalicin treatment. In the skin tissue of AD mice, reduction of IL-31 receptor was observed in the ovalicin treated group compared to the group without ovalicin treatment. To our knowledge, this is the first study to elucidate the anti-atopic mechanism of ovalicin, which could be an alternative to steroidal drugs commonly used for AD treatment.

Pruritogenic mechanisms and gut sensation: putting the "irritant" into irritable bowel syndrome

Chronic abdominal pain is a common clinical condition experienced by patients with irritable bowel syndrome (IBS). A general lack of suitable treatment options for the management of visceral pain is the major contributing factor to the debilitating nature of the disease. Understanding the underlying causes of chronic visceral pain is pivotal to identifying new effective therapies for IBS. This review provides the current evidence, demonstrating that mediators and receptors that induce itch in the skin also act as "gut irritants" in the gastrointestinal tract. Activation of these receptors triggers specific changes in the neuronal excitability of sensory pathways responsible for the transmission of nociceptive information from the periphery to the central nervous system leading to visceral hypersensitivity and visceral pain. Accumulating evidence points to significant roles of irritant mediators and their receptors in visceral hypersensitivity and thus constitutes potential targets for the development of more effective therapeutic options for IBS.

TRPV1 Ion Channel: Structural Features, Activity Modulators, and Therapeutic Potential

Although TRPV1 ion channel has been attracting researchers' attention for many years, its functions in animal organisms, the principles of regulation, and the involvement in pathological processes have not yet been fully clarified. Mutagenesis experiments and structural studies have identified the structural features of the channel and binding sites for its numerous ligands; however, these studies are far from conclusion. This review summarizes recent achievements in the TRPV1 research with special focus on structural and functional studies of the channel and on its ligands, which are extremely diverse in their nature and interaction specificity to TRPV1. Particular attention was given to the effects of numerous endogenous agonists and antagonists that can fine-tune the channel sensitivity to its usual activators, such as capsaicin, heat, acids, or their combination. In addition to the pain sensing not covered in this review, the TRPV1 channel was found to be involved in the regulation of many important physiological and pathological processes and, therefore, can be considered as a promising therapeutic target in the treatment of various diseases, such as pneumonia, ischemia, diabetes, epilepsy, schizophrenia, psoriasis, etc.

Periostin Activation of Integrin Receptors on Sensory Neurons Induces Allergic Itch

Chronic allergic itch is a common symptom affecting millions of people and animals, but its pathogenesis is not fully explained. Herein, we show that periostin, abundantly expressed in the skin of patients with atopic dermatitis (AD), induces itch in mice, dogs, and monkeys. We identify the integrin α_Vβ3 expressed on a subset of sensory neurons as the periostin receptor. Using pharmacological and genetic approaches, we inhibited the function of neuronal integrin α_Vβ3, which significantly reduces periostin-induced itch in mice. Furthermore, we show that the cytokine TSLP, the application of AD-causing MC903 (calcipotriol), and house dust mites all induce periostin secretion. Finally, we establish that the JAK/STAT pathway is a key regulator of periostin secretion in keratinocytes. Altogether, our results identify a TSLP-periostin reciprocal activation loop that links the skin to the spinal cord via peripheral sensory neurons, and we characterize the non-canonical functional role of an integrin in itch.

Mishra et al. demonstrate periostin-induced itch in mice, dogs, and monkeys and identify the integrin α_Vβ3 as the periostin neuronal receptor. They find that keratinocytes release periostin in response to TSLP, thus identifying a possible reciprocal vicious circle implicating the cytokine TSLP and periostin in chronic allergic itch.

Activation of V1a vasopressin receptors excite subicular pyramidal neurons by activating TRPV1 and depressing GIRK channels

Arginine vasopressin (AVP) is a nonapeptide that serves as a neuromodulator in the brain and a hormone in the periphery that regulates water homeostasis and vasoconstriction. The subiculum is the major output region of the hippocampus and an integral component in the networks that processes sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whereas the subiculum expresses high densities of AVP-binding sites and AVP has been shown to increase the synaptic excitability of subicular pyramidal neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of V_1a receptors increased the excitability of subicular pyramidal neurons via activation of TRPV1 channels and depression of the GIRK channels. V_1a receptor-induced excitation of subicular pyramidal neurons required the function of phospholipase Cβ, but was independent of intracellular Ca²⁺ release. Protein kinase C was responsible for AVP-mediated depression of GIRK channels, whereas degradation of phosphatidylinositol 4,5-bisphosphate was involved in V_1a receptor-elicited activation of TRPV1 channels. Our results may provide one of the cellular and molecular mechanisms to explain the physiological functions of AVP in the brain.

The CysLT2R receptor mediates leukotriene C4-driven acute and chronic itch

Interactions between the nervous system and immune system are central regulators of chronic itch, a key feature of pathologies like atopic dermatitis and allergic contact dermatitis. Cysteinyl leukotrienes (LTC₄, LTD₄, and LTE₄) are eicosanoid lipids known for mediating inflammation, bronchoconstriction, and vascular leakage. We demonstrate here that CysLTs are potent itch inducers and that this effect depends on the specific coupling of LTC₄ with its receptor CysLT₂R, which is expressed in a population of peripheral sensory neurons in the mouse and in human. We show that the LTC₄/CysLT₂R pathway contributes to a model of chronic itch, suggesting that CysLT₂R could be a new therapeutic target for intractable chronic itch.

Acute and chronic itch are burdensome manifestations of skin pathologies including allergic skin diseases and atopic dermatitis, but the underlying molecular mechanisms are not well understood. Cysteinyl leukotrienes (CysLTs), comprising LTC₄, LTD₄, and LTE₄, are produced by immune cells during type 2 inflammation. Here, we uncover a role for LTC₄ and its signaling through the CysLT receptor 2 (CysLT₂R) in itch. Cysltr2 transcript is highly expressed in dorsal root ganglia (DRG) neurons linked to itch in mice. We also detected CYSLTR2 in a broad population of human DRG neurons. Injection of leukotriene C₄ (LTC₄) or its nonhydrolyzable form NMLTC₄, but neither LTD₄ nor LTE₄, induced dose-dependent itch but not pain behaviors in mice. LTC₄-mediated itch differed in bout duration and kinetics from pruritogens histamine, compound 48/80, and chloroquine. NMLTC₄-induced itch was abrogated in mice deficient for Cysltr2 or when deficiency was restricted to radioresistant cells. Itch was unaffected in mice deficient for Cysltr1, Trpv1, or mast cells (W^Sh mice). CysLT₂R played a role in itch in the MC903 mouse model of chronic itch and dermatitis, but not in models of dry skin or compound 48/80- or Alternaria-induced itch. In MC903-treated mice, CysLT levels increased in skin over time, and Cysltr2^−/− mice showed decreased itch in the chronic phase of inflammation. Collectively, our study reveals that LTC₄ acts through CysLT₂R as its physiological receptor to induce itch, and CysLT₂R contributes to itch in a model of dermatitis. Therefore, targeting CysLT signaling may be a promising approach to treat inflammatory itch.

ZBTB20 in Nociceptive Neurons of the Trigeminal Ganglia Regulates Pruritus

Recent studies have shown that ZBTB20, a zinc-finger protein containing transcription factor, is highly expressed in small-diameter primary sensory neurons in mice, and modulates pain through regulating TRP channels. However, whether ZBTB20 regulates itch sensation has not been demonstrated. In this study, small-diameter primary sensory neuron-specific ZBTB20 knockout (PN-ZB20KO) mice were used to investigate the role of ZBTB20 in the regulation of itch sensation. First, both histamine-dependent and non-histamine-dependent itch behaviors induced by injection of histamine and chloroquine (CQ) into the cheek were significantly diminished in PN-ZB20KO mice. Second, double immunohistochemistry showed that ZBTB20 was mainly expressed in CGRP-labeled small peptidergic neurons and was expressed at low levels in IB4-labeled small non-peptidergic and NF200-labeled large neurons in the trigeminal ganglia (TG). ZBTB20 was also expressed in most TRPV1⁺ and TRPA1⁺ neurons and to a lesser extent in TRPM8⁺ neurons in the TG. Furthermore, cheek injection of histamine and CQ enhanced the mRNA expression of TRPV1 and TRPA1 but not TRPM8 in the TG. Moreover, TRPV1 and TRPA1 knockout (KO) mice exhibited attenuation of itch behavior induced by histamine and CQ, respectively. Finally, silencing endogenous ZBTB20 with recombinant lentivirus expressing a short hairpin RNA against ZBTB20 (LV-shZBTB20) in TG neurons attenuated histamine- and non-histamine-induced itch and downregulated TRP channels in the TG. Our study suggests that ZBTB20 plays an important role in mediating itch in small primary sensory neurons.

TRPM3 in Brain (Patho)Physiology

Already for centuries, humankind is driven to understand the physiological and pathological mechanisms that occur in our brains. Today, we know that ion channels play an essential role in the regulation of neural processes and control many functions of the central nervous system. Ion channels present a diverse group of membrane-spanning proteins that allow ions to penetrate the insulating cell membrane upon opening of their channel pores. This regulated ion permeation results in different electrical and chemical signals that are necessary to maintain physiological excitatory and inhibitory processes in the brain. Therefore, it is no surprise that disturbances in the functions of cerebral ion channels can result in a plethora of neurological disorders, which present a tremendous health care burden for our current society. The identification of ion channel-related brain disorders also fuel the research into the roles of ion channel proteins in various brain states. In the last decade, mounting evidence has been collected that indicates a pivotal role for transient receptor potential (TRP) ion channels in the development and various physiological functions of the central nervous system. For instance, TRP channels modulate neurite growth, synaptic plasticity and integration, and are required for neuronal survival. Moreover, TRP channels are involved in numerous neurological disorders. TRPM3 belongs to the melastatin subfamily of TRP channels and represents a non-selective cation channel that can be activated by several different stimuli, including the neurosteroid pregnenolone sulfate, osmotic pressures and heat. The channel is best known as a peripheral nociceptive ion channel that participates in heat sensation. However, recent research identifies TRPM3 as an emerging new player in the brain. In this review, we summarize the available data regarding the roles of TRPM3 in the brain, and correlate these data with the neuropathological processes in which this ion channel may be involved.

Involvement of Neuro-Immune Interactions in Pruritus With Special Focus on Receptor Expressions

Pruritus is a common, but very challenging symptom with a wide diversity of underlying causes like dermatological, systemic, neurological and psychiatric diseases. In dermatology, pruritus is the most frequent symptom both in its acute and chronic form (over 6 weeks in duration). Treatment of chronic pruritus often remains challenging. Affected patients who suffer from moderate to severe pruritus have a significantly reduced quality of life. The underlying physiology of pruritus is very complex, involving a diverse network of components in the skin including resident cells such as keratinocytes and sensory neurons as well as transiently infiltrating cells such as certain immune cells. Previous research has established that there is a significant crosstalk among the stratum corneum, nerve fibers and various immune cells, such as keratinocytes, T cells, basophils, eosinophils and mast cells. In this regard, interactions between receptors on cutaneous and spinal neurons or on different immune cells play an important role in the processing of signals which are important for the transmission of pruritus. In this review, we discuss the role of various receptors involved in pruritus and inflammation, such as TRPV1 and TRPA1, IL-31RA and OSMR, TSLPR, PAR-2, NK1R, H1R and H4R, MRGPRs as well as TrkA, with a focus on interaction between nerve fibers and different immune cells. Emerging evidence shows that neuro-immune interactions play a pivotal role in mediating pruritus-associated inflammatory skin diseases such as atopic dermatitis, psoriasis or chronic spontaneous urticaria. Targeting these bidirectional neuro-immune interactions and the involved pruritus-specific receptors is likely to contribute to novel insights into the underlying pathogenesis and targeted treatment options of pruritus.

Formalin Itch Test: Low-Dose Formalin Induces Histamine-Independent, TRPA1-Mediated Itch in Mice

Chronic itch is a common distressing symptom of many diseases, which reduced patient's quality of life. The mechanistic study on itch and screening for new anti-itch drugs require the development of new pre-clinical itch animal models. Herein, we established an acute itch model by intradermal (i.d.) injection of low-dose formalin into the neck or cheek in mice. In mice, i.d. injection of formalin (0.1–5%) in the nape of the neck evoked robust scratching behavior in a dose-dependent manner and the dose–response curves showed an inverted “U” shape. I.d. injection of formalin (0.3–0.6%) into the cheek evoked scratching in mice but wiping in rats, while formalin (1.25–5%) induced mixed wiping and scratching behavior in both mice and rats. Further, we found that 0.3% formalin-induced scratching was histamine-independent and significantly attenuated by transient receptor potential ion channel A1 (TRPA1) inhibitor (HC030031) or in TRPA1 knockout (KO) mice, but not affected by transient receptor potential ion channel V1 (TRPV1) inhibitor (capsazepine) or in TRPV1 KO mice. Additionally, 0.3% formalin-induced up-regulation of phosphorylation of extracellular regulated protein kinases (p-ERK) in the dorsal root ganglion (DRG) and scratching were suppressed by intrathecal injection of MEK inhibitor U0126 in mice. Incubation of 0.03% formalin induced the accumulation of intracellular reactive oxygen species (ROS) in the cultured DRG-derived cell line ND7-23, and formalin-induced itch was suppressed by antioxidants in mice. Finally, perfusion of 0.03% formalin induced elevation of intracellular calcium in a subset of primary cultured DRG neurons of mice. Thus, these results indicate that low-dose formalin induced non-histaminergic itch by activation of TRPA1 in mice, which may be employed as a useful acute itch model for screening potential anti-itch drugs.

A capsaicinoid-based soft drug, AG1529, for attenuating TRPV1-mediated histaminergic and inflammatory sensory neuron excitability

TRPV1, a member of the transient receptor potential (TRP) family, is a nonselective calcium permeable ion channel gated by physical and chemical stimuli. In the skin, TRPV1 plays an important role in neurogenic inflammation, pain and pruritus associated to many dermatological diseases. Consequently, TRPV1 modulators could represent pharmacological tools to respond to important patient needs that still represent an unmet medical demand. Previously, we reported the design of capsaicinoid-based molecules that undergo dermal deactivation (soft drugs), thus preventing their long-term dermal accumulation. Here, we investigated the pharmacological properties of the lead antagonist, 2-((4-hydroxy-2-iodo-5-methoxybenzyl) amino)-2-oxoethyl dodecanoate (AG1529), on heterologously expressed human TRPV1 (hTRPV1), on nociceptor excitability and on an in vivo model of acute pruritus. We report that AG1529 competitively blocked capsaicin-evoked activation of hTRPV1 with micromolar potency, moderately affected pH-induced gating, and did not alter voltage- and heat-mediated responses. AG1529 displays modest receptor selectivity as it mildly blocked recombinant hTRPA1 and hTRPM8 channels. In primary cultures of rat dorsal root ganglion (DRG) neurons, AG1529 potently reduced capsaicin-evoked neuronal firing. AG1529 exhibited lower potency on pH-evoked TRPV1 firing, and TRPA1-elicited nociceptor excitability. Furthermore, AG1529 abolished histaminergic and inflammation mediated TRPV1 sensitization in primary cultures of DRG neurons. Noteworthy, dermal wiping of AG1529, either in an acetone-based formulation or in an anhydrous ointment, dose-dependently attenuated acute histaminergic itch in a rodent model. This cutaneous anti-pruritic effect was devoid of the normal nocifensive action evoked by the burning sensation of capsaicin. Taken together, these preclinical results unveil the mode of action of AG1529 on TRPV1 channels and substantiate the tenet that this capsaicinoid-based soft drug is a promising candidate for drug development as a topical anti-pruritic and anti-inflammatory medication.

Optical Assessment of Nociceptive TRP Channel Function at the Peripheral Nerve Terminal

Free nerve endings are key structures in sensory transduction of noxious stimuli. In spite of this, little is known about their functional organization. Transient receptor potential (TRP) channels have emerged as key molecular identities in the sensory transduction of pain-producing stimuli, yet the vast majority of our knowledge about sensory TRP channel function is limited to data obtained from in vitro models which do not necessarily reflect physiological conditions. In recent years, the development of novel optical methods such as genetically encoded calcium indicators and photo-modulation of ion channel activity by pharmacological tools has provided an invaluable opportunity to directly assess nociceptive TRP channel function at the nerve terminal.

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.

Activation of Oxytocin Receptors Excites Subicular Neurons by Multiple Signaling and Ionic Mechanisms

Oxytocin (OXT) is a nonapeptide that serves as a neuromodulator in the brain and a hormone participating in parturition and lactation in the periphery. The subiculum is the major output region of the hippocampus and an integral component in the networks that process sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whilst the subiculum expresses the highest OXT-binding sites and is the first brain region to be activated by peripheral application of OXT, the precise actions of OXT in the subiculum have not been determined. Our results demonstrate that application of the selective OXT receptor (OXTR) agonist, [Thr4,Gly7]-oxytocin (TGOT), excited subicular neurons via activation of TRPV1 channels, and depression of K+ channels. The OXTR-mediated excitation of subicular neurons required the functions of phospholipase Cβ, protein kinase C, and degradation of phosphatidylinositol 4,5-bisphosphate (PIP2). OXTR-elicited excitation of subicular neurons enhanced long-term potentiation via activation of TRPV1 channels. Our results provide a cellular and molecular mechanism to explain the physiological functions of OXT in the brain.

FGF13 Is Required for Histamine-Induced Itch Sensation by Interaction with NaV1.7

Itch can be induced by activation of small-diameter DRG neurons, which express abundant intracellular fibroblast growth factor 13 (FGF13). Although FGF13 is revealed to be essential for heat nociception, its role in mediating itch remains to be investigated. Here, we reported that loss of FGF13 in mouse DRG neurons impaired the histamine-induced scratching behavior. Calcium imaging showed that the percentage of histamine-responsive DRG neurons was largely decreased in FGF13-deficient mice; and consistently, electrophysiological recording exhibited that histamine failed to evoke action potential firing in most DRG neurons from these mice. Given that the reduced histamine-evoked neuronal response was caused by knockdown of FGF13 but not by FGF13A deficiency, FGF13B was supposed to mediate this process. Furthermore, overexpression of histamine Type 1 receptor H1R, but not H2R, H3R, nor H4R, increased the percentage of histamine-responsive DRG neurons, and the scratching behavior in FGF13-deficient mice was highly reduced by selective activation of H1R, suggesting that H1R is mainly required for FGF13-mediated neuronal response and scratching behavior induced by histamine. However, overexpression of H1R failed to rescue the histamine-evoked neuronal response in FGF13-deficient mice. Histamine enhanced the FGF13 interaction with Na_V1.7. Disruption of this interaction by a membrane-permeable competitive peptide, GST-Flag-Na_V1.7CT-TAT, reduced the percentage of histamine-responsive DRG neurons, and impaired the histamine-induced scratching, indicating that the FGF13/Na_V1.7 interaction is a key molecular determinant in the histamine-induced itch sensation. Therefore, our study reveals a novel role of FGF13 in mediating itch sensation via the interaction of Na_V1.7 in the peripheral nervous system.SIGNIFICANCE STATEMENT Scratching induced by itch brings serious tissue damage in chronic itchy diseases, and targeting itch-sensing molecules is crucial for its therapeutic intervention. Here, we reveal that FGF13 is required for the neuronal excitation and scratching behavior induced by histamine. We further provide the evidence that the histamine-evoked neuronal response is mainly mediated by histamine Type 1 receptor H1R, and is largely attenuated in FGF13-deficent mice. Importantly, we identify that histamine enhances the FGF13/Na_V1.7 interaction, and disruption of this interaction reduces histamine-evoked neuronal excitation and highly impairs histamine-induced scratching behavior. Additionally, we also find that FGF13 is involved in 5-hydroxytryptamine-induced scratching behavior and hapten 1-fluoro-2,4-dinitrobenzene-induced chronic itch.

Thermal Hyperalgesia and Mechanical Allodynia Elicited by Histamine and Non-histaminergic Itch Mediators: Respective Involvement of TRPV1 and TRPA1

Acute itch is elicited by histamine, as well as non-histaminergic itch mediators including chloroquine, BAM8-22 and Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL). When injected intradermally, histamine binds to histamine H1 and H4 receptors that activate transient receptor potential vanilloid 1 (TRPV1) to depolarize pruriceptors. Chloroquine, BAM8-22, and SLIGRL, respectively, bind to Mas-related G-protein-coupled receptors MrgprA3, MrgprC11, and MrgprC11/PAR2 that in turn activate transient receptor potential ankyrin 1 (TRPA1). In this study we tested if histamine, chloroquine, BAM8-22 and SLIGRL elicit thermal hyperalgesia and mechanical allodynia in adult male mice. We measured the latency of hindpaw withdrawal from a noxious heat stimulus, and the threshold for hindpaw withdrawal from a von Frey mechanical stimulus. Intraplantar injection of histamine resulted in significant thermal hyperalgesia (p < 0.001) and mechanical allodynia (p < 0.001) ipsilaterally that persisted for 1 h. Pretreatment with the TRPV1 antagonist AMG-517 (10 or 20 μg), but not the TRPA1 antagonist HC-030031 (50 or 100 μg), significantly attenuated the magnitude and time course of thermal hyperalgesia and mechanical allodynia elicited by histamine (p < 0.001 for both), indicating that these effects are mediated by TRPV1. In contrast, pretreatment with the TRPA1 antagonist significantly reduced thermal hyperalgesia and mechanical allodynia elicited by chloroquine (p < 0.001 for both ), BAM-822 (p < 0.01, p < 0.001, respectively) and SLGRL (p < 0.05, p < 0.001, respectively), indicating that effects elicited by these non-histaminergic itch mediators require TRPA1. TRPV1 and TRPA1 channel inhibitors thus may have potential use in reducing hyperalgesia and allodynia associated with histaminergic and non-histaminergic itch, respectively.

Itch: A Paradigm of Neuroimmune Crosstalk

Although the medical definition of itch has been in existence for 360 years, only in the last 20 years have we begun to understand the basic mechanisms that underlie this unique sensation. Therapeutics that specifically target chronic itch as a pathologic entity are currently still not available. Recent seminal advances in itch circuitry within the nervous system have intersected with discoveries in immunology in unexpected ways to rapidly inform emerging treatment strategies. The current review aims to introduce these basic concepts in itch biology and highlight how distinct immunologic pathways integrate with recently identified itch-sensory circuits in the nervous system to inform a major new paradigm of neuroimmunology and therapeutic development for chronic itch.

TRPV1-Targeted Drugs in Development for Human Pain Conditions

The transient receptor potential vanilloid-1 (TRPV1) is a non-specific cation channel known for its sensitivity to pungent vanilloid compound (i.e. capsaicin) and noxious stimuli, including heat, low pH or inflammatory mediators. TRPV1 is found in the somatosensory system, particularly primary afferent neurons that respond to damaging or potentially damaging stimuli (nociceptors). Stimulation of TRPV1 evokes a burning sensation, reflecting a central role of the channel in pain. Pharmacological and genetic studies have validated TRPV1 as a therapeutic target in several preclinical models of chronic pain, including cancer, neuropathic, postoperative and musculoskeletal pain. While antagonists of TRPV1 were found to be a valuable addition to the pain therapeutic toolbox, their clinical use has been limited by detrimental side effects, such as hyperthermia. In contrast, capsaicin induces a prolonged defunctionalisation of nociceptors and thus opened the door to the development of a new class of therapeutics with long-lasting pain-relieving effects. Here we review the list of TRPV1 agonists undergoing clinical trials for chronic pain management, and discuss new indications, formulations or combination therapies being explored for capsaicin. While the analgesic pharmacopeia for chronic pain patients is ancient and poorly effective, modern TRPV1-targeted drugs could rapidly become available as the next generation of analgesics for a broad spectrum of pain conditions.

Crotamiton, an Anti-Scabies Agent, Suppresses Histamine- and Chloroquine-Induced Itch Pathways in Sensory Neurons and Alleviates Scratching in Mice

Crotamiton is an anti-scabies drug, but it was recently found that crotamiton also suppresses non-scabietic itching in mice. However, the underlying mechanism is largely unclear. Therefore, aim of the study is to investigate mechanisms of the anti-pruritic effect of crotamiton for non-scabietic itching. Histamine and chloroquine are used as non-scabietic pruritogens. The effect of crotamiton was identified using fluorometric intracellular calcium assays in HEK293T cells and primary cultured dorsal root ganglion (DRG) neurons. Further in vivo effect was evaluated by scratching behavior tests. Crotamiton strongly inhibited histamine-induced calcium influx in HEK293T cells, expressing both histamine receptor 1 (H1R) and transient receptor potential vanilloid 1 (TRPV1), as a model of histamine-induced itching. Similarly, it also blocked chloroquine-induced calcium influx in HEK293T cells, expressing both Mas-related G-protein-coupled receptor A3 (MRGPRA3) and transient receptor potential A1 (TRPA1), as a model of histamine-independent itching. Furthermore, crotamiton also suppressed both histamine- and chloroquine-induced calcium influx in primary cultures of mouse DRG. Additionally, crotamiton strongly suppressed histamine- and chloroquine-induced scratching in mice. Overall, it was found that crotamiton has an anti-pruritic effect against non-scabietic itching by histamine and chloroquine. Therefore, crotamiton may be used as a general anti-pruritic agent, irrespective of the presence of scabies.

Involvement of Neural Transient Receptor Potential Channels in Peripheral Inflammation

Transient receptor potential (TRP) channels are a superfamily of non-selective cation channels that act as polymodal sensors in many tissues throughout mammalian organisms. In the context of ion channels, they are unique for their broad diversity of activation mechanisms and their cation selectivity. TRP channels are involved in a diverse range of physiological processes including chemical sensing, nociception, and mediating cytokine release. They also play an important role in the regulation of inflammation through sensory function and the release of neuropeptides. In this review, we discuss the functional contribution of a subset of TRP channels (TRPV1, TRPV4, TRPM3, TRPM8, and TRPA1) that are involved in the body’s immune responses, particularly in relation to inflammation. We focus on these five TRP channels because, in addition to being expressed in many somatic cell types, these channels are also expressed on peripheral ganglia and nerves that innervate visceral organs and tissues throughout the body. Activation of these neural TRP channels enables crosstalk between neurons, immune cells, and epithelial cells to regulate a wide range of inflammatory actions. TRP channels act either through direct effects on cation levels or through indirect modulation of intracellular pathways to trigger pro- or anti-inflammatory mechanisms, depending on the inflammatory disease context. The expression of TRP channels on both neural and immune cells has made them an attractive drug target in diseases involving inflammation. Future work in this domain will likely yield important new pathways and therapies for the treatment of a broad range of disorders including colitis, dermatitis, sepsis, asthma, and pain.

Recent advances in understanding the molecular mechanisms of cholestatic pruritus: A review

Cholestasis, a condition characterized by an abnormal decrease in bile flow, is accompanied by various symptoms such as pruritus. Although cholestatic pruritus is a prominent condition, its precise mechanisms have largely been elusive. Recently, advancements have been made for understanding the etiology and pathogenesis of cholestatic pruritus. The current review therefore focuses on summarizing the overall progress made in the elucidation of its molecular mechanisms. We have reviewed the available animal models on cholestasis to compare the differences between them, characterized potential pruritogens involved in cholestatic pruritus, and have summarized the receptor and ion channels implicated in the condition. Finally, we have discussed the available treatment options for alleviation of cholestatic pruritus. As our understanding of the mechanisms of cholestatic pruritus deepens, novel strategies to cure this condition are awaited.

Cav3.2 T-type calcium channels control acute itch in mice

Cav3.2 T-type calcium channels are important mediators of nociceptive signaling, but their roles in the transmission of itch remains poorly understood. Here we report a key involvement of these channels as key modulators of itch/pruritus-related behavior. We compared scratching behavior responses between wild type and Cav3.2 null mice in models of histamine- or chloroquine-induced itch. We also evaluated the effect of the T-type calcium channel blocker DX332 in male and female wild-type mice injected with either histamine or chloroquine. Cav3.2 null mice exhibited decreased scratching responses during both histamine- and chloroquine-induced acute itch. DX332 co-injected with the pruritogens inhibited scratching responses of male and female mice treated with either histamine or chloroquine. Altogether, our data provide strong evidence that Cav3.2 T-type channels exert an important role in modulating histamine-dependent and -independent itch transmission in the primary sensory afferent pathway, and highlight these channels as potential pharmacological targets to treat pruritus.

The Cytokine TGF-β Induces Interleukin-31 Expression from Dermal Dendritic Cells to Activate Sensory Neurons and Stimulate Wound Itching

Cutaneous wound healing is associated with the unpleasant sensation of itching. Here we investigated the mechanisms underlying this type of itch, focusing on the contribution of soluble factors released during healing. We found high amounts of interleukin 31 (IL-31) in skin wound tissue during the peak of itch responses. Il31^-/- mice lacked wound-induced itch responses. IL-31 was released by dermal conventional type 2 dendritic cells (cDC2s) recruited to wounds and increased itch sensory neuron sensitivity. Transfer of cDC2s isolated from late-stage wounds into healthy skin was sufficient to induce itching in a manner dependent on IL-31 expression. Addition of the cytokine TGF-β1, which promotes wound healing, to dermal DCs in vitro was sufficient to induce Il31 expression, and Tgfbr1^f/f CD11c-Cre mice exhibited reduced scratching and decreased Il31 expression in wounds in vivo. Thus, cDC2s promote itching during skin would healing via a TGF-β-IL-31 axis with implications for treatment of wound itching.

Neural Mechanisms of Itch

Itch is a unique sensation that helps organisms scratch away external threats; scratching itself induces an immune response that can contribute to more itchiness. Itch is induced chemically in the peripheral nervous system via a wide array of receptors. Given the superficial localization of itch neuron terminals, cells that dwell close to the skin contribute significantly to itch. Certain mechanical stimuli mediated by recently discovered circuits also contribute to the itch sensation. Ultimately, in the spinal cord, and likely in the brain, circuits that mediate touch, pain, and itch engage in cross modulation. Much of itch perception is still a mystery, but we present in this review the known ligands and receptors associated with itch. We also describe experiments and findings from investigations into the spinal and supraspinal circuitry responsible for the sensation of itch.

Inhibition of M/Kv7 Currents Contributes to Chloroquine-Induced Itch in Mice

M/K_v7 potassium channels play a key role in regulation of neuronal excitability. Modulation of neuronal excitability of primary sensory neurons determines the itch sensation induced by a variety of itch-causing substances including chloroquine (CQ). In the present study, we demonstrate that suppression of M/K_v7 channel activity contributes to generation of itch in mice. CQ enhances excitability of the primary sensory neurons through inhibiting M/K_v7 potassium currents in a Ca²⁺ influx-dependent manner. Specific M/K_v7 channel opener retigabine (RTG) or tannic acid (TA) not only reverses the CQ-induced enhancement of neuronal excitability but also suppresses the CQ-induced itch behavior. Systemic application of RTG or TA also significantly inhibits the itch behavior induced by a variety of pruritogens. Taken together, our findings provide novel insight into the molecular basis of CQ-induced itch sensation in mammals that can be applied to the development of strategies to mitigate itch behavior.

Central circuit mechanisms of itch

Itch, in particular chronic forms, has been widely recognized as an important clinical problem, but much less is known about the mechanisms of itch in comparison with other sensory modalities such as pain. Recently, considerable progress has been made in dissecting the circuit mechanisms of itch at both the spinal and supraspinal levels. Major components of the spinal neural circuit underlying both chemical and mechanical itch have now been identified, along with the circuits relaying ascending transmission and the descending modulation of itch. In this review, we summarize the progress in elucidating the neural circuit mechanism of itch at spinal and supraspinal levels.

Endogenous and Exogenous Vanilloids Evoke Disparate TRPV1 Activation to Produce Distinct Neuronal Responses

Neuronal signals are processed along the nociceptive pathway to convey discriminative information, which would manifest in the produced pain sensation. The transient receptor potential vanilloid 1 (TRPV1), an important signaling complex in nociceptors termini, is activated by different noxious stimuli that underlie distinct pain sensations. For example, while endovanilloids are associated with inflammatory pain and hypersensitivity through TRPV1 activation, the exovanilloid toxin, capsaicin, evokes an acute pain by activating this channel. Differences in the TRPV1 activation profile evoked by exogenous and endogenous vanilloids were suggested to underlie this disparity in pain sensations. However, the cellular processes that lead to these differences in pain sensation mediated by the same channel are not fully understood. Here, we sought to describe the neuronal response of TRPV1-expressing nociceptors to exo-and endovanilloids. To this end, we performed current-clamp recordings in rat trigeminal neurons exposed to either capsaicin or intracellular endovanilloids produced downstream of the bradykinin receptor BK2. Our results show that lipoxygenase metabolites generate persistent TRPV1-dependent action potential firing while capsaicin evokes robust depolarization and high-frequency firing that is quickly terminated by depolarization block. Additionally, we found that a weak TRPV1 activation prolongs action potential firing. Overall, our results indicate different firing patterns evoked by inflammatory mediators and capsaicin via TRPV1 that correlate with the respective subsequent pain sensation. These findings also suggest that differences in neuronal activation stem from the variable degree of TRPV1 activation they produce.