Histamine-induced Itch Research Articles

A Diterpenoid of the Medicinal Plant Andrographis paniculata Targets Cutaneous TRPV3 Channel and Relieves Itch.

Transient receptor potential vanilloid subtype 3 (TRPV3) is an ion channel implicated in skin physiology and itch. TRPV3 inhibitors can present a novel strategy for combating debilitating itch conditions, and medicinal plants are a natural pool of such compounds. Here, we report the isolation of a TRPV3-inhibiting compound from Andrographis paniculata, a medicinal plant with anti-inflammatory properties whose bioactive components are poorly characterized in terms of molecular targets. Using 1H and 13C NMR and high-resolution mass spectrometry, the compound was identified as a labdane-type diterpenoid, 14-deoxy-11,12-didehydroandrographolide (ddA). The activity of the compound was evaluated by fluorescent calcium assay and manual whole-cell patch-clamp technique. ddA inhibited human TRPV3 in stably expressing CHO and HaCaT keratinocytes, acting selectively among other TRP channels implicated in itch and inflammation and not showing toxicity to HaCaT cells. Antipruritic effects of the compound were evaluated in scratching behavior models on ICR mice. ddA suppressed itch induced by the TRPV3 activator carvacrol. Additionally, ddA potently suppressed histamine-induced itch with efficacy comparable to loratadine, a clinically used antihistamine drug. These results suggest the potential of ddA as a possible safe and efficacious alternative for antipruritic therapy.

A phytosphingosine derivative mYG-II-6 inhibits histamine-mediated TRPV1 activation and MRGPRX2-dependent mast cell degranulation

BackgroundPhytosphingosine and its derivative are known for their skin-protective properties. While mYG-II-6, a phytosphingosine derivative, has shown anti-inflammatory and antipsoriatic effects, its potential antipruritic qualities have yet to be explored. This study aimed to investigate mYG-II-6′s antipruritic properties. MethodsThe calcium imaging technique was employed to investigate the activity of ion channels and receptors. Mast cell degranulation was confirmed through the β-hexosaminidase assay. Additionally, in silico molecular docking and an in vivo mouse scratching behavior test were utilized. ResultsUsing HEK293T cells transfected with H1R and TRPV1, we examined the impact of mYG-II-6 on histamine-induced intracellular calcium rise, a key signal in itch-mediating sensory neurons. Pretreatment with mYG-II-6 significantly reduced histamine-induced calcium levels and inhibited TRPV1 activity, suggesting its role in blocking the calcium influx channel. Additionally, mYG-II-6 suppressed histamine-induced calcium increase in primary cultures of mouse dorsal root ganglia, indicating its potential antipruritic effect mediated by histamine. Interestingly, mYG-II-6 exhibited inhibitory effects on human MRGPRX2, a G protein-coupled receptor involved in IgE-independent mast cell degranulation. However, it did not inhibit mouse MrgprB2, the ortholog of human MRGPRX2. Molecular docking analysis revealed that mYG-II-6 selectively interacts with the binding pocket of MRGPRX2. Importantly, mYG-II-6 suppressed histamine-induced scratching behaviors in mice. ConclusionsOur findings show that mYG-II-6 can alleviate histamine-induced itch sensation through dual mechanisms. This underscores its potential as a versatile treatment for various pruritic conditions.

The Effect of Osthole on Transient Receptor Potential Channels: A Possible Alternative Therapy for Atopic Dermatitis.

Chronic recurrent skin inflammation and severe itching in patients with atopic dermatitis (AD) significantly impair their quality of life. The H4 histamine receptor plays a key role in histamine-induced itching. During the skin inflammation associated with AD, pro-inflammatory mediators (interleukins, cytokines) are released from neurons. Ultimately, a cascade of reactions leads to the activation and sensitization of transient receptor potential channels (TRP), which exacerbate the inflammation and itching associated with AD. Osthole (OST) is a natural coumarin with a proven versatile pharmacological effect: anti-cancer, anti-inflammatory and immunomodulatory. However, the molecular mechanism of OST in relieving inflammation in histamine-mediated itching is not yet clear. In the studies presented, the possible effect of the OST action on the inhibition of the gene expression of the histamine H4 receptor and the key genes of the TRP channels as well as on the concentration of proinflammatory interleukins was analyzed. Inflammation was induced in a 3D skin model and a keratinocyte cell line Normal Human Epidermal Keratinocytes (NHEK) identical to that of AD, and then OST was administered at various doses. The concentrations of IL-4/-13 were determined by ELISA. RNA was isolated from the 3D skin cells and the NHEK cell line, and the qPCR method was used to determine the expression of: IL-4α, H4R, TRPV1, TRPV4, TRPM8 analyzed. The study showed that OST significantly reduced the secretion of IL-4/-13 in a keratinocyte cell line and in a 3D skin model. In addition, OST was found to significantly decrease the gene expression of IL-4α, H4R, TRPV1, TRPV4 and increase TRPM8 in both the NHEK cell line and the organotypic 3D skin model. The data obtained provide the first in vitro evidence of itch relief following the application of OST to atopic skin. Research on the use of OST as an active component of emollients in the treatment of AD should be continued in the future.

Clinically uninvolved but not healthy-The skin of patients with atopic dermatitis is primed for itch and inflammation.

Atopic dermatitis (AD) is a highly prevalent inflammatory skin disorder characterized by episodic exacerbations and remissions. Why the clinically healthy skin of AD patients becomes rapidly inflamed and very pruritic is poorly understood. To investigate cowhage- and histamine-induced itch and skin expression levels of their target receptors in lesional and non-lesional skin of AD, compared to the skin of patients with psoriasis, chronic spontaneous urticaria (CSU) and healthy subjects. Patients with AD, psoriasis and chronic spontaneous urticaria (CSU) as well as healthy control subjects (HC) (n = 20 each) were assessed for differences in itch parameters, neurogenic flare reaction and local blood flow responses to skin provocations with cowhage and histamine. Skin biopsies from 10 AD, 10 psoriasis,11 CSU and 12 HC were obtained to assess expression of protease-activated receptors 2 and 4 (PAR-2, PAR-4), histamine H1 and H4 receptors (H1R, H4R), and mast cells. Provocation of non-lesional skin of AD patients with cowhage resulted in prolonged itch (p = 0.020), which was not observed in psoriasis and CSU. Significantly prolonged and more intense cowhage- and histamine-induced itch (for duration, peak and overall intensity) was also observed in lesional AD skin. Diminished neurogenic flare reaction and blood flow after histamine provocation were shown in AD and psoriasis patients. Non-lesional AD skin along with lesional AD and psoriasis skin showed an increased expression of PAR-2 and PAR-4, H1R and H4R. Mast cell number was higher in lesional AD and psoriasis skin (p = 0.006 and p = 0.006, respectively). The non-lesional skin of AD patients markedly differs from healthy skin in cowhage-induced itch responses and the expression of receptors for proteases and histamine. Proactive therapeutic interventions that downregulate these receptors may prevent episodic exacerbation in AD.

IL-33 potentiates histaminergic itch

Itch is a common symptom that can greatly diminish quality of life. Histamine is a potent endogenous pruritogen, and while antihistamines are often the first-line treatment for itch, in conditions like chronic spontaneous urticaria (CSU), many patients remain symptomatic while receiving maximal doses. Mechanisms that drive resistance to antihistamines are poorly defined. Signaling of the alarmin cytokine IL-33 in sensory neurons is postulated to drive chronic itch by inducing neuronal sensitization to pruritogens. Thus, we sought to determine if IL-33 can augment histamine-induced (histaminergic) itch. Itch behavior was assessed in response to histamine after IL-33 or saline administration. Various stimuli and conditional and global knockout mice were utilized to dissect cellular mechanisms. Multiple existing transcriptomic data sets were evaluated, including single-cell RNA sequencing of human and mouse skin, microarrays of isolated mouse mast cells at steady state and after stimulation with IL-33, and microarrays of skin biopsy samples from subjects with CSU and healthy controls. IL-33 amplifies histaminergic itch independent of IL-33 signaling in sensory neurons. Mast cells are the top expressors of the IL-33 receptor in both human and mouse skin. When stimulated by IL-33, mouse mast cells significantly increase IL-13 levels. Enhancement of histaminergic itch by IL-33 relies on a mast cell- and IL-13-dependent mechanism. IL-33 receptor expression is increased in lesional skin of subjects with CSU compared to healthy controls. Our findings suggest that IL-33 signaling may be a key driver of histaminergic itch in mast cell-associated pruritic conditions such as CSU.

Emerging trends and hotspots of the itch research: A bibliometric and visualized analysis.

Itch, a common uncomfortable sensory experience, occurs frequently in inflammatory or allergic disorders. In recent years, with the discovery of itch-specific pathways in the peripheral and central nervous system, the association between immunology and neural pathways has gradually emerged as the main mechanism of itch. Although many studies have been conducted on itch, no bibliometric analysis study focusing on this topic has been conducted. This study aimed to explore the research hotspots and trends in the itch field from a bibliometric perspective. Publications relevant to itch, published from 2003 to 2022, were retrieved from the Science Citation Index-Expanded of Web of Science Core Collection. Publications were critically reviewed and analyzed with CiteSpace software, Vosviewer, and the bibliometric online analysis platform. Visual maps were conducted in terms of annual production, collaborating countries or institutions, productive authors, core journals, co-cited references, and keyword bursts. 2395 articles on itch that met our criteria were identified and the quantity of publications has been increasing rapidly since 2012. The USA was the most influential country. University Hospital Münster was the institution with the most publications. Gil Yosipovitch was the most prolific author. Atopic dermatitis (AD), intradermal serotonin, chronic pruritus, mechanical itch, gastrin-releasing peptide, substance p, interleukin-31 receptor, histamine-induced itch, bile acid, scratching behavior, and h-4 receptor were the top 11 clusters in co-citation cluster analysis. Keyword burst analysis suggested that treatment, inflammation, and AD are current research hotspots. Global publications on itch research have increased steadily and rapidly over the past 20 years. Inflammation and AD are current research hotspots. The neuroimmunological and neuroinflammatory mechanisms of itch, as well as clinical assessment methods and therapeutic targets, will be novel research directions in the future. This study provides guidance for further itch research.

Sophorolipid inhibits histamine-induced itch by decreasing PLC/IP3R signaling pathway activation and modulating TRPV1 activity

Biosurfactants are attracting much interest due to their potential application as therapeutic agents in the medical and cosmetic field. Previous studies have demonstrated that biosurfactant such as sophorolipid (SL) exhibits immunomodulatory effects. In this article, we found the potential of sophorolipid for inhibiting histamine-induced itch and preliminarily explored its molecular basis. First, behavioral tests indicated that SL can remit the histamine-induced scratching behaviors of mice. Second, SL can suppress the the calcium influx induced by histamine, HTMT and VUF8430 in HaCaT cells. RT-PCR analysis showed that the histamine-induced upregulation of mRNA levels of phospholipase Cγ1, 1,4,5-trisphosphate receptor (IP3R), and protein kinase Cα can be inhibted by SL, suggesting that SL may impede the PLC/IP3R signaling pathway activated by histamine. In further tests, the capsaicin-induced calcium influx can also be inhibited by SL. The immunofluorescence and molecular docking analysis indicated that SL acts as an inhibitor of transient receptor potential vanilloid-1 (TRPV1) activation to decrease calcium influx against stimuli. In summary, these results revealed that SL may inhibit histamine-induced itch by decreasing PLC/IP3R signaling pathway activation and modulating TRPV1 activity. This paper indicates that SL can be a useful treatment for histamine-dependent itch.

The effect of repetitive topical applications of local anesthetics (EMLA) on experimental pain and itch (histaminergic and nonhistaminergic)

Background: The effects of repeated topical applications of local anesthetics are poorly investigated as they may, in addition to analgesia, impact peripheral nerve endings in a cumulative manner. In the present study, the effects of 6 repetitive applications of eutectic mixture of lidocaine (EMLA 2.5% and prilocaine 2.5%) were investigated on experimentally induced pain, histaminergic and nonhistaminergic itch, and neurogenic inflammation. Methods: Four skin areas on the forearms of 24 subjects were randomized to receive 3 hours of application of EMLA or placebo twice a day for 3 consecutive days. After each application, superficial blood perfusion (SBP), mechanical (mechanically evoked itch, mechanical pain threshold, and mechanical pain sensitivity), and thermal sensitivity (warm detection threshold, heat pain threshold, and suprathreshold heat sensitivity) were assessed. After the last application of EMLA/placebo, histamine and cowhage was applied (2 areas each) and itch and pain intensity and SBP were assessed. Results: After 3 hours of EMLA application, significant mechanical and thermal hypoalgesia were found with no cumulative efficacy over the 3 days. EMLA alone had no effect on SBP. Significantly increased SBP, reduced cowhage-induced itch, but the unaffected histamine-induced itch was found when applying EMLA ahead of histamine and cowhage. Conclusions: EMLA induced a reduction of mechanical and thermal sensitivity without a cumulative-dose effect. EMLA reduced nonhistaminergic itch and pain but not the experimentally provoked histaminergic itch. Selective action of EMLA on polymodal C-fibers could explain these effects.

Adriforant is a functional antagonist of histamine receptor 4 and attenuates itch and skin inflammation in mice

BackgroundHistamine has been postulated to play a role in atopic dermatitis via histamine receptor 4, mediating pruritic and inflammatory effects. The H4R antagonist adriforant (PF-3893787 or ZPL389) indicated clinical efficacy in a Ph2a study in atopic dermatitis. Preclinical investigations of adriforant had been scarce as experiments in transfectants with H4R from several species suggested partial agonism, not seen in human cells. ObjectiveDuring the Ph2b trial in AD, we performed experiments to understand the pharmacology of adriforant in primary murine cells and in vivo models. We assessed its effects on ERK phosphorylation and transcriptional changes in bone marrow-derived mast cells, histamine-dependent Ca2+ flux in neurons and histamine-induced itch response. In addition, its impact on MC903-induced skin inflammation was evaluated. ResultsWe show that, contrary to transfectants, adriforant is a competitive antagonist of the murine histamine receptor 4, antagonizes histamine-induced ERK phosphorylation, normalizes histamine-induced transcriptional changes in mast cells and reduces histamine-dependent Ca2+ flux in neurons. Administration to mice reduces acute histamine-induced itch response. In addition, adriforant ameliorates inflammation in the mouse MC903 model. ConclusionsOur results suggest that functional inhibition of histamine receptor 4 by adriforant reduces itch and inflammation in vivo. The effects observed in mice, however, did not translate to clinical efficacy in patients as the Ph2b clinical trial with adriforant did not meet pre-specified efficacy endpoints. Given the complex pathogenesis of AD, antagonism of histamine receptor 4 alone appears insufficient to reduce disease severity in AD patients, despite the effects seen in mouse models.

GABAergic neurons in the ventral lateral geniculate nucleus and intergeniculate leaflet are involved in itch processing in mice

Itch and pain are two closely related sensations that receiving similar encodings at multiple levels. Accumulated evidences suggest that activation of the ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL)-to-lateral and ventrolateral periaqueductal gray (l/vlPAG) projections mediates the antinociceptive effects of bright light therapy. Clinical study showed that bright light therapy may ameliorate cholestasis-induced pruritus. However, the underlying mechanism and whether this circuit participates in itch modulation remains unclear. In this study, chloroquine and histamine were utilized to induce acute itch models in mice. Neuronal activities in vLGN/IGL nucleus were evaluated with c-fos immunostaining as well as fiber photometry. Optogenetic manipulations were performed to activate or inhibit GABAergic neurons in the vLGN/IGL nucleus. Our results showed that the expressions of c-fos in vLGN/IGL were significantly increased upon both chloroquine- and histamine-induced acute itch stimuli. GABAergic neurons in vLGN/IGL were activated during histamine and chloroquine-induced scratching. Optogenetic activation of the vLGN/IGL GABAergic neurons exerts antipruritic effect, while inhibiting these neurons exerts pruritic effect. Our results provide evidence that GABAergic neurons in vLGN/IGL nucleus might play a crucial role in modulating itch, which may provide clue for application of bright light as an antipruritic treatment in clinic.

Antipruritic Effects of Doxepin Cream on Experimentally Induced Histaminergic and Nonhistaminergic Itch

Background. Itch can be transmitted by two parallel pathways, histaminergic and nonhistaminergic. Histaminergic itch is transmitted by a subgroup of mechano‐insensitive C‐fibers, while nonhistaminergic itch by a subgroup of polymodal C‐fibers. Experimental models are used to study pruritus: histamine for the histaminergic itch by antagonizing the histamine H1‐receptors, and BAM8‐22 and cowhage for the nonhistaminergic by activating Mas‐related G protein‐coupled receptors and protease‐activated receptors, respectively. This study aims to evaluate the antipruritic effects of topical doxepin (H1‐receptor antagonistic effect) on histaminergic and nonhistaminergic itch induced by histamine, BAM8‐22, and cowhage. Methods. This study was conducted on 22 healthy subjects. Histamine, BAM8‐22, cowhage, and vehicle were applied to 4 areas on the forearms. After 7 days, the same substances were applied after 11/2 hour‐pretreatment with doxepin. After the application of pruritogens, itch and pain intensities were assessed for 9 minutes, followed by the measurement of superficial blood perfusion (SBP), mechanical and thermal sensitivities. Results. Application of histamine, BAM8‐22, and cowhage all induced itch as compared to a vehicle. The pretreatment with doxepin almost abolished the histamine‐induced itch and modestly reduced BAM8‐22‐ and cowhage‐induced itch. Histamine induced a higher SBP compared to the other conditions. Doxepin reduced SBP induced by each pruritogen, even though SBP of histamine remains the highest. Conclusion. Doxepin cream abolished histaminergic itch by antagonizing the peripheral H1‐histamine receptors. Moreover, doxepin reduced nonhistaminergic itch and related neurogenic inflammation. Further studies are needed to elucidate the molecular mechanisms underlying this peripheral modulation of nonhistaminergic itch by a topically applied H1‐antagonist. This trial is registered with NCT04588532.

Associations Between Interindividual Differences, Expectations and Placebo and Nocebo Effects in Itch.

Introduction: Placebo and nocebo effects are positive and negative health outcomes that can be elicited by the psychosocial context. They can be mediated by expectations, and may emerge in somatic symptoms even when people are aware of these effects. Interindividual differences (e.g., in personality, affective states) could impact placebo and nocebo responding, but findings are inconsistent.Methods: The current work examined expectation as a mediator of the association between verbal placebo and nocebo suggestions (VSs) and histamine-induced itch across three experimental studies. Moreover, we examined whether interindividual differences (e.g., in optimism, neuroticism, behavioral activation system (BAS), body ignorance) modulated: (1) the direct association between VSs and itch (direct moderation), and (2) the indirect, expectation-mediated association between VSs and itch (moderated mediation). Positive VSs were compared to neutral instructions (Study 1; n = 92) or negative VSs (Studies 2+3; n = 203) in an open-label (i.e., explaining placebo and nocebo effects) or closed-label (concealed) context using PROCESS. First, mediation of VSs effects on itch by expectations was tested. Next, moderation by individual traits was explored using conditional process analyses.Results: The effects of VSs on itch were significantly mediated by expectation in Study 1 and in the open-label (but not closed-label) contexts of Studies 2 and 3. Ignorance of bodily signals marginally moderated the direct effects of VSs on itch when closed-label suggestions were given: at low levels of body ignorance, effects of positive and negative VSs were stronger. Moreover, moderated mediation was observed in the open-label groups of Studies 2 and 3: The expectation-mediated effects of VSs on itch were stronger when BAS drive was lower.Conclusion: Overall, the effects of VSs on itch were mediated by expectations in the open-label, but not the closed-label context. Moreover, the current work suggests that placebo and nocebo effects may be moderated by ignorance of bodily signals and the BAS. There was limited evidence that other interindividual differences modulated placebo and nocebo responding in itch.

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.

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.

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 NaV1.7. Disruption of this interaction by a membrane-permeable competitive peptide, GST-Flag-NaV1.7CT-TAT, reduced the percentage of histamine-responsive DRG neurons, and impaired the histamine-induced scratching, indicating that the FGF13/NaV1.7 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 NaV1.7 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/NaV1.7 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.

Flare Size but Not Intensity Reflects Histamine-Induced Itch

Introduction: Flare reactions arise due to the release of vasodilators from sensory nerves caused by antidromic transmission of action potentials after the induction of itch. Objective: We investigated the link between flare and itch using 3 models of itch. Methods: Skin provocations with histamine, capsaicin, and cowhage were performed in 31 subjects. Itch was quantified using the visual analog scale. Flare was assessed using laser speckle contrast imaging (LSCI) and digital photography. Results: The duration, intensity, and area under the curve of histamine-induced itch correlated with the area of increased blood flow measured with LSCI (r = 0.545, p = 0.002; r = 0.575, p = 0.001; and r = 0.649, p < 0.001, respectively). Itch and skin blood flow in response to capsaicin or cowhage did not correlate. Conclusion: In histamine-induced skin inflammation, itch and increased blood flow are linked. Thus, the area of histamine-induced flare may be used as a surrogate marker for histamine-induced itch.

Targeting spinal neuropeptide Y1 receptor-expressing interneurons to alleviate chronic pain and itch.

An accelerating basic science literature is providing key insights into the mechanisms by which spinal neuropeptide Y (NPY) inhibits chronic pain. A key target of pain inhibition is the Gi-coupled neuropeptide Y1 receptor (Y1). Y1 is located in key sites of pain transmission, including the peptidergic subpopulation of primary afferent neurons and a dense subpopulation of small, excitatory, glutamatergic/somatostatinergic interneurons (Y1-INs) that are densely expressed in the dorsal horn, particularly in superficial lamina I-II. Selective ablation of spinal Y1-INs with an NPY-conjugated saporin neurotoxin attenuates the development of peripheral nerve injury-induced mechanical and cold hypersensitivity. Conversely, conditional knockdown of NPY expression or intrathecal administration of Y1 antagonists reinstates hypersensitivity in models of chronic latent pain sensitization. These and other results indicate that spinal NPY release and the consequent inhibition of pain facilitatory Y1-INs represent an important mechanism of endogenous analgesia. This mechanism can be mimicked with exogenous pharmacological approaches (e.g. intrathecal administration of Y1 agonists) to inhibit mechanical and thermal hypersensitivity and spinal neuron activity in rodent models of neuropathic, inflammatory, and postoperative pain. Pharmacological activation of Y1 also inhibits mechanical- and histamine-induced itch. These immunohistochemical, pharmacological, and cell type-directed lesioning data, in combination with recent transcriptomic findings, point to Y1-INs as a promising therapeutic target for the development of spinally directed NPY-Y1 agonists to treat both chronic pain and itch.

Computer-aided Discovery of a New Nav1.7 Inhibitor for Treatment of Pain and Itch.

Voltage-gated sodium channel Nav1.7 has been validated as a perspective target for selective inhibitors with analgesic and anti-itch activity. The objective of this study was to discover new candidate compounds with Nav1.7 inhibitor properties. The authors hypothesized that their approach would yield at least one new compound that inhibits sodium currents in vitro and exerts analgesic and anti-itch effects in mice. In silico structure-based similarity search of 1.5 million compounds followed by docking to the Nav1.7 voltage sensor of Domain 4 and molecular dynamics simulation was performed. Patch clamp experiments in Nav1.7-expressing human embryonic kidney 293 cells and in mouse and human dorsal root ganglion neurons were conducted to test sodium current inhibition. Formalin-induced inflammatory pain model, paclitaxel-induced neuropathic pain model, histamine-induced itch model, and mouse lymphoma model of chronic itch were used to confirm in vivo activity of the selected compound. After in silico screening, nine compounds were selected for experimental assessment in vitro. Of those, four compounds inhibited sodium currents in Nav1.7-expressing human embryonic kidney 293 cells by 29% or greater (P < 0.05). Compound 9 (3-(1-benzyl-1H-indol-3-yl)-3-(3-phenoxyphenyl)-N-(2-(pyrrolidin-1-yl)ethyl)propanamide, referred to as DA-0218) reduced sodium current by 80% with a 50% inhibition concentration of 0.74 μM (95% CI, 0.35 to 1.56 μM), but had no effects on Nav1.5-expressing human embryonic kidney 293 cells. In mouse and human dorsal root ganglion neurons, DA-0218 reduced sodium currents by 17% (95% CI, 6 to 28%) and 22% (95% CI, 9 to 35%), respectively. The inhibition was greatly potentiated in paclitaxel-treated mouse neurons. Intraperitoneal and intrathecal administration of the compound reduced formalin-induced phase II inflammatory pain behavior in mice by 76% (95% CI, 48 to 100%) and 80% (95% CI, 68 to 92%), respectively. Intrathecal administration of DA-0218 produced acute reduction in paclitaxel-induced mechanical allodynia, and inhibited histamine-induced acute itch and lymphoma-induced chronic itch. This study's computer-aided drug discovery approach yielded a new Nav1.7 inhibitor that shows analgesic and anti-pruritic activity in mouse models.

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.

Exploration of sensory and spinal neurons expressing gastrin-releasing peptide in itch and pain related behaviors

Gastrin-releasing peptide (GRP) functions as a neurotransmitter for non-histaminergic itch, but its site of action (sensory neurons vs spinal cord) remains controversial. To determine the role of GRP in sensory neurons, we generated a floxed Grp mouse line. We found that conditional knockout of Grp in sensory neurons results in attenuated non-histaminergic itch, without impairing histamine-induced itch. Using a Grp-Cre knock-in mouse line, we show that the upper epidermis of the skin is exclusively innervated by GRP fibers, whose activation via optogeneics and chemogenetics in the skin evokes itch- but not pain-related scratching or wiping behaviors. In contrast, intersectional genetic ablation of spinal Grp neurons does not affect itch nor pain transmission, demonstrating that spinal Grp neurons are dispensable for itch transmission. These data indicate that GRP is a neuropeptide in sensory neurons for non-histaminergic itch, and GRP sensory neurons are dedicated to itch transmission.