Frontiers in pruritus research: scratching the brain for more effective itch therapy

Mechanical and chemical itch regulated by neuropeptide Y-Y1 signaling

Itch is a somatosensory sensation to remove potential harmful stimulation with a scratching desire, which could be divided into mechanical and chemical itch according to diverse stimuli, such as wool fiber and insect biting. It has been reported that neuropeptide Y (NPY) neurons, a population of spinal inhibitory interneurons, could gate the transmission of mechanical itch, with no effect on chemical itch. In our study, we verified that chemogenetic activation of NPY neurons could inhibit the mechanical itch as well as the chemical itch, which also attenuated the alloknesis phenomenon in the chronic dry skin model. Afterwards, intrathecal administration of NPY1R agonist, [Leu31, Pro34]-NPY (LP-NPY), showed the similar inhibition effect on mechanical itch, chemical itch and alloknesis as chemo-activation of NPY neurons. Whereas, intrathecal administration of NPY1R antagonist BIBO 3304 enhanced mechanical itch and reversed the alloknesis phenomenon inhibited by LP-NPY treatment. Moreover, selectively knocking down NPY1R by intrathecal injection of Npy1r siRNA enhanced mechanical and chemical itch behavior as well. These results indicate that NPY neurons in spinal cord regulate mechanical and chemical itch, and alloknesis in dry skin model through NPY1 receptors.

Brain network mechanism of acupuncture for chronic spontaneous urticaria: a functional magnetic resonance imaging study protocol

Introduction

Chronic spontaneous urticaria (CSU) is a common skin condition that can significantly impact patients' quality of life. Although studies have demonstrated the efficacy of acupuncture in treating CSU, the underlying mechanisms remain unclear. Dysfunction within the brain's default mode network (DMN) represents a fundamental characteristic of central pathological changes associated with CSU. Therefore, it is hypothesized that improving brain network dysfunction could serve as a key mechanism through which acupuncture exerts its therapeutic effects. This study aims to provide evidence supporting this hypothesis.

Methods and analysis

This study, a parallel, randomized, sham-controlled functional neuroimaging investigation will be conducted in China. We aim to enroll 50 patients with CSU and 25 healthy controls, distributing them evenly between the acupuncture and sham acupuncture groups in a 1:1 ratio. The total observation period will span 6 weeks, including 2 weeks designated for the baseline phase and 4 weeks allocated for the clinical treatment phase. Prior to treatment, all participants will undergo magnetic resonance scanning, clinical index detection, and microbiota collection. Following treatment, the patients with CSU will be retested for these indicators. Using resting-state functional connectivity (rsFC) analysis, dynamic Functional Connection (dFC) analysis, and brain microstate extraction technology combined with correlation analysis of microbiota and clinical indicators, the regulatory mechanism of acupuncture on the brain network of CSU will be evaluated from multiple dimensions.

Ethics and dissemination

This trial was approved by the Biomedical Ethics Review Committee of the West China Hospital, Sichuan University (No. 2022-1255). Each participant will provide written informed consent to publish any potentially identifiable images or data.Clinical trial registrationhttps://www.chictr.org.cn/, identifier: ChiCTR2200064563.

Inhibitory effect of Sanguisorba hakusanensis Makino ethanol extract on atopic dermatitis-like responses in NC/Nga mice and human keratinocytes

Atopic dermatitis (AD) is an allergic, inflammatory skin disease caused by immune dysregulation. In this study, we investigated anti-atopic and anti-inflammatory activities of Sanguisorba hakusanensis ethanol extract (SHE) both in vivo using NC/Nga mice and in vitro using human HaCaT keratinocytes. Oral administration of SHE suppressed several atopic symptoms associated with house dust mites (induced with Dermatophagoides farinae extract) in NC/Nga mice and decreased serum levels of inflammatory mediators such as immunoglobulin E, histamine, and inflammatory chemokines. Additionally, SHE treatment reduced the infiltration of immune cells such as mast cells and macrophages in AD skin lesions. In vitro, interferon-γ- and tumor necrosis factor-α-stimulated HaCaT cells exhibited increased expression of T helper 1 and 2 chemokines; their expression was inhibited by SHE treatment. The anti-inflammatory effects of SHE treatment involved blocking of the mitogen-activated protein kinase and signal transducer and activator of transcription 1 signaling pathways. In conclusion, SHE exerts potent anti-atopic and anti-inflammatory effects and should be considered for the clinical treatment of AD.

Single-cell RNA sequencing defines disease-specific differences between chronic nodular prurigo and atopic dermatitis

BACKGROUND

Chronic nodular prurigo (CNPG) is an inflammatory skin disease that is maintained by a chronic itch-scratch cycle likely rooted in neuroimmunological dysregulation. This condition may be associated with atopy in some patients, and there are now promising therapeutic results from blocking type 2 cytokines such as IL-4, IL-13, and IL-31.

OBJECTIVES

This study aimed to improve the understanding of pathomechanisms underlying CNPG as well as molecular relationships between CNPG and atopic dermatitis (AD).

METHODS

We profiled skin lesions from patients with CNPG in comparison with AD and healthy control individuals using single-cell RNA sequencing combined with T-cell receptor sequencing.

RESULTS

We found type 2 immune skewing in both CNPG and AD, as evidenced by CD4+ helper T cells expressing IL13. However, only AD harbored an additional, oligoclonally expanded CD8A+IL9R+IL13+ cytotoxic T-cell population, and immune activation pathways were highly upregulated in AD, but less so in CNPG. Conversely, CNPG showed signatures of extracellular matrix organization, collagen synthesis, and fibrosis, including a unique population of CXCL14-IL24+ secretory papillary fibroblasts. Besides known itch mediators such as IL31 and oncostatin M, we also detected increased levels of neuromedin B in fibroblasts of CNPG lesions compared with AD and HC, with neuromedin B receptors detectable on some nerve endings.

CONCLUSIONS

These data show that CNPG does not harbor the strong disease-specific immune activation pathways that are typically found in AD but is rather characterized by upregulated stromal remodeling mechanisms that might have a direct impact on itch fibers.

Clinical aspects and management of chronic itch

Chronic pruritus (CP) (ie, itch that persists for more than 6 weeks) poses significant challenges to patients' health and quality of life. It is a common reason for visits to dermatologists and general practitioners and can be caused by a range of conditions, including systemic diseases such as chronic kidney disease or liver diseases, malignancies, neuropathic conditions, and dermatoses such as atopic dermatitis. CP often does not develop in parallel with the course of the disease and can become an entity of its own, which must be treated with antipruritic drugs, even if the underlying cause is already under therapy. Depending on the etiology of CP, different pathways in the pathogenesis have been analyzed recently, following which new treatments have been developed and tested in randomized controlled trials. This article discusses the recent results of these studies and highlights how best to manage health care for patients with CP.

Mechanism of agonist-induced activation of the human itch receptor MRGPRX1

Mas-related G-protein-coupled receptors X1-X4 (MRGPRX1-X4) are 4 primate-specific receptors that are recently reported to be responsible for many biological processes, including itch sensation, pain transmission, and inflammatory reactions. MRGPRX1 is the first identified human MRGPR, and its expression is restricted to primary sensory neurons. Due to its dual roles in itch and pain signaling pathways, MRGPRX1 has been regarded as a promising target for itch remission and pain inhibition. Here, we reported a cryo-electron microscopy (cryo-EM) structure of Gq-coupled MRGPRX1 in complex with a synthetic agonist compound 16 in an active conformation at an overall resolution of 3.0 Å via a NanoBiT tethering strategy. Compound 16 is a new pain-relieving compound with high potency and selectivity to MRGPRX1 over other MRGPRXs and opioid receptor. MRGPRX1 was revealed to share common structural features of the Gq-mediated receptor activation mechanism of MRGPRX family members, but the variable residues in orthosteric pocket of MRGPRX1 exhibit the unique agonist recognition pattern, potentially facilitating to design MRGPRX1-specific modulators. Together with receptor activation and itch behavior evaluation assays, our study provides a structural snapshot to modify therapeutic molecules for itch relieving and analgesia targeting MRGPRX1.

Representation and control of pain and itch by distinct prefrontal neural ensembles

Pain and itch are two closely related but essentially distinct sensations that elicit different behavioral responses. However, it remains mysterious how pain and itch information is encoded in the brain to produce differential perceptions. Here, we report that nociceptive and pruriceptive signals are separately represented and processed by distinct neural ensembles in the prelimbic (PL) subdivision of the medial prefrontal cortex (mPFC) in mice. Pain- and itch-responsive cortical neural ensembles were found to significantly differ in electrophysiological properties, input-output connectivity profiles, and activity patterns to nociceptive or pruriceptive stimuli. Moreover, these two groups of cortical neural ensembles oppositely modulate pain- or itch-related sensory and emotional behaviors through their preferential projections to specific downstream regions such as the mediodorsal thalamus (MD) and basolateral amygdala (BLA). These findings uncover separate representations of pain and itch by distinct prefrontal neural ensembles and provide a new framework for understanding somatosensory information processing in the brain.

Possibilities of Anxiolytic Therapy in the Elimination of Stress Skin Manifestations: A Case Report

The case of a 42-year-old female patient with pronounced itching and exanthema, mainly in the area of the trunk and lower limbs, is presented. Previously, the patient took antihistamines without effect, was treated for scabies, but the itching remained pronounced and led to rash and excoriations. From the anamnesis, it was found that the patient has a high level of stress. According to the Hospital Anxiety and Depression Scale, the anxiety of the patient reached 14 points, and depression 1 point. Functional (psychogenic) itching was suspected. Since the patient refused dermatologist consultation, therapy with the anxiolytic temgicoluril, topical antipruritic agents and nonpharmacological methods of treatment were recommended at the initial stage. The patient felt a significant relief of itching symptoms on the first day of anxiolytic usage, she withdrew topical antipruritic agents after 5 days of anxiolytic treatment, in 15 days she began to reduce the dose of temgicoluril, and at the end of the third week she stopped treatment with anxiolytic due to a significant positive effect. In three weeks, practically all elements of the rash, except for the largest wounds from scratching, disappeared. The peculiarity of the case is that functional itching was completely eliminated during anxiolytic therapy without other systemic medications, which emphasizes the importance of eliminating the component of stress and anxiety in the treatment of such patients.

Novel insight on GRP/GRPR axis in diseases

The gastrin-releasing peptide receptor (GRPR), a member of the G protein-coupled receptors (GPCRs), binds to ligands such as gastrin-releasing peptide (GRP) and plays a variety of biological roles. GRP/GRPR signalling is involved in the pathophysiological processes of many diseases, including inflammatory diseases, cardiovascular diseases, neurological diseases, and various cancers. In the immune system, the unique function of GRP/GRPR in neutrophil chemotaxis suggests that GRPR can be directly stimulated through GRP-mediated neutrophils to activate selective signalling pathways, such as PI3K, PKC, and MAPK, and participate in the occurrence and development of inflammation-related diseases. In the cardiovascular system, GRP increases intercellular adhesion molecule 1 (ICAM-1) and induces vascular cell adhesion molecule-1 (VCAM-1). GRP activates ERK1/2, MAPK, and AKT, leading to cardiovascular diseases, including myocardial infarction. Central nervous system signal transduction mediated by the GRP/GRPR axis plays a vital role in emotional responses, social interaction, and memory. The GRP/GRPR axis is elevated in various cancers, including lung, cervical, colorectal, renal cell, and head and neck squamous cell carcinomas. GRP is a mitogen in a variety of tumour cell lines. Its precursor, pro-gastrin-releasing peptide (ProGRP), may play an important role as an emerging tumour marker in early tumour diagnosis. GPCRs serve as therapeutic targets for drug development, but their function in each disease remains unclear, and their involvement in disease progression has not been well explored or summarised. This review lays out the above mentioned pathophysiological processes based on previous research conclusions. The GRP/GRPR axis may be a potential target for treating multiple diseases, and the study of this signalling axis is particularly important.

An Anterior Cingulate Cortex-to-Midbrain Projection Controls Chronic Itch in Mice

Itch is an unpleasant sensation that provokes the desire to scratch. While acute itch serves as a protective system to warn the body of external irritating agents, chronic itch is a debilitating but poorly-treated clinical disease leading to repetitive scratching and skin lesions. However, the neural mechanisms underlying the pathophysiology of chronic itch remain mysterious. Here, we identified a cell type-dependent role of the anterior cingulate cortex (ACC) in controlling chronic itch-related excessive scratching behaviors in mice. Moreover, we delineated a neural circuit originating from excitatory neurons of the ACC to the ventral tegmental area (VTA) that was critically involved in chronic itch. Furthermore, we demonstrate that the ACC→VTA circuit also selectively modulated histaminergic acute itch. Finally, the ACC neurons were shown to predominantly innervate the non-dopaminergic neurons of the VTA. Taken together, our findings uncover a cortex–midbrain circuit for chronic itch-evoked scratching behaviors and shed novel insights on therapeutic intervention.

Functional connectivity impairment of thalamus-cerebellum-scratching neural circuits in pruritus of chronic spontaneous urticaria

Pruritus of chronic spontaneous urticaria (CSU) is one of the most common and irritating sensations that severely affects the quality of life. However, the changes in the functional connectivity (FC) between thalamic subregions and other brain regions have not been fully elucidated. This study aimed to explore the potential changes in brain neural circuits by focusing on various subregions of the thalamus in patients with CSU pruritus to contribute to the understanding of chronic pruritus from the perspective of central mechanisms. A total of 56 patients with CSU and 30 healthy controls (HCs) completed the data analysis. Urticaria Activity Score 7 (UAS7), pruritus visual analog score (VAS-P), Dermatological Life Quality Index (DLQI), and immunoglobulin E (IgE) values were collected to assess clinical symptoms. Seed-based resting-state functional connectivity (rs-FC) analysis was used to assess relevant changes in the neural circuits of the brain. Compared to HCs, seeds within the caudal temporal thalamus (cTtha) on the right side of patients with CSU showed increased rs-FC with the cerebellum anterior lobe (CAL). Seeds within the lateral prefrontal thalamus (lPFtha) on the right side showed increased rs-FC with both CAL and pons, while those within the medial prefrontal thalamus (mPFtha) on the right side showed increased rs-FC with both CAL and the dorsal lateral prefrontal cortex (dlPFC) on the right side. Seeds within the posterior parietal thalamus (PPtha) on the right side showed increased rs-FC with the cerebellum posterior lobe (CPL) on the left side. The UAS7 values and IgE levels were positively correlated with the rs-FC of the right dlPFC. Our results suggest that patients with CSU may exhibit stronger rs-FC alterations between certain thalamic subregions and other brain regions. These changes affect areas of the brain involved in sensorimotor and scratching.

Trial registration number

[http://www.chictr.org.cn], identifier [ChiCTR1900022994].

The potential action of SSRIs in the treatment of skin diseases including atopic dermatitis and slow-healing wounds

Selective serotonin reuptake inhibitors (SSRIs) are used to treat affective and anxiety disorders. Antidepressants have also been shown to have antimicrobial and immunomodulatory effects, which may affect the microbiota-intestinal-brain axis. Studies show that SSRIs have antimicrobial activity both in vivo and in vitro and influence bacteria by inhibiting biofilm, affecting efflux pumps, among others. A huge challenge today is the prevention and treatment of skin diseases, including atopic dermatitis (AD) and slow-healing wounds. Skin diseases including AD and non-healing wounds are serious medical problem. People suffering from these conditions feel constant discomfort, which also affects their psychological state. Research on new treatments for AD and slow-healing wounds is essential because current medications are not fully effective and have many side effects. Exploring new drug groups for AD and slow-healing wounds will allow for the creation of an alternative treatment for these diseases. SSRIs represent a hope for the treatment of skin diseases due to their immunomodulatory and antimicrobial properties.

The exploration of the potential mechanism of oxymatrine-mediated antipruritic effect based on network pharmacology and weighted gene co-expression network analysis

The treatment of chronic itch is considered to be a challenge for its non-histamine dependence and the search for alternative medicine is still striving. The pathology of the chronic itch is closely related to immune system regulation and inflammatory response. Oxymatrine (OMT) is a traditional Chinese medicine ingredient extracted from the roots of Sophora flavescens Aiton with significant antitumor, analgesic, and anti-inflammatory effects. However, the underlying mechanism of OMT on chronic itch is obscure, which limits clinical application. Hence, this study is aimed to clarify the pruritus alleviation mechanism of OMT by combining network pharmacology analysis, weighted gene co-expression analysis (WGCNA), and molecular docking. We screened 125 common targets of OMT regulating inflammation and pruritus with pharmacology technology, the GO enrichment function analysis and KEGG signaling pathway analysis to demonstrate the close relation to the signaling pathways regulating inflammation such as MAPK signaling pathway and PI3K-AKT signaling pathway. We adopted the most relevant templates for pruritus diseases, combined with network pharmacology to preliminarily screen out 3 OMT functions and regulatory targets, exerting a good connection and correlation with the target at the screened disease targets. Further experiments were conducted to explore the potential mechanism of OMT using the LPS-induced RAW264.7 cell inflammation model. The results showed that pretreatment with different concentrations of OMT (25 μM, 50 μM, and 100 μM) for 24 h, inhibited expression of IL-6, iNOS TLR4 and TGFR-1 as well as apoptosis of Raw264.7 cells induced by LPS. Moreover, OMT effectively inhibited LPS-induced MAPK pathway activation and the expression of related sites MAP2K1, MAPK8 MAP2K4, and MAPKAP-K2 in RAW 264.7 cells. The OMT also reduced the phosphorylation of p-38, associated with site in the activation of MAPK signaling pathway. These results could contribute to a better understanding of the mechanisms underlying how OMT alleviates inflammation to treat chronic pruritic diseases and provide a potential drug for the treatment of chronic itch.

Critical Players and Therapeutic Targets in Chronic Itch

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

Treatment Options for Troublesome Itch

Itch (or pruritus) is an unpleasant sensation, inducing the desire to scratch. It is also a major and distressing symptom of many skin and systemic diseases. The involvement of histamine, which is a major itch mediator, has been extensively examined. Recent studies suggest that histamine-independent pathways may play roles in chronic itch. Therefore, antihistamines are not always effective in the treatment of patients with chronic itch. The development of biologics and κ-opioid receptor (KOR) agonists has contributed to advances in the treatment of itch; however, since biologics are expensive for patients to purchase, some patients may limit or discontinue their use of these agents. Furthermore, KOR agonists need to be prescribed with caution due to risks of side effects in the central nervous system. Janus kinase (JAK) inhibitors are sometimes associated with side effects, such as infection. In this review, we summarize antidepressants, antineuralgics, cyclosporine A, antibiotics, crotamiton, phosphodiesterase 4 inhibitor, botulinum toxin type A, herbal medicines, phototherapy, and acupuncture therapy as itch treatment options other than antihistamines, biologics, opioids, and JAK inhibitors; we also explain their underlying mechanisms of action.

TRPV1 SUMOylation suppresses itch by inhibiting TRPV1 interaction with H1 receptors

The molecular mechanism underlying the functional interaction between H1R and TRPV1 remains unclear. We show here that H1R directly binds to the carboxy-terminal region of TRPV1 at residues 715-725 and 736-749. Cell-penetrating peptides containing these sequences suppress histamine-induced scratching behavior in a cheek injection model. The H1R-TRPV1 binding is kept at a minimum at rest in mouse trigeminal neurons due to TRPV1 SUMOylation and it is enhanced upon histamine treatment through a transient TRPV1 deSUMOylation. The knockin of the SUMOylation-deficient TRPV1^K823R mutant in mice leads to constitutive enhancement of H1R-TRPV1 binding, which exacerbates scratching behaviors induced by histamine. Conversely, SENP1 conditional knockout in sensory neurons enhances TRPV1 SUMOylation and suppresses the histamine-induced scratching response. In addition to interfering with binding, TRPV1 SUMOylation promotes H1R degradation through ubiquitination. Our work unveils the molecular mechanism of histaminergic itch by which H1R directly binds to deSUMOylated TRPV1 to facilitate the transduction of the pruritogen signal to the scratching response.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

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

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

Indigo Pulverata Levis (Chung-Dae, Persicaria tinctoria) Alleviates Atopic Dermatitis-like Inflammatory Responses In Vivo and In Vitro

Atopic dermatitis (AD) is a chronic inflammatory skin disease associated with a type 2 T helper cell (Th2) immune response. The IndigoPulverata Levis extract (CHD) is used in traditional Southeast Asian medicine; however, its beneficial effects on AD remain uninvestigated. Therefore, we investigated the therapeutic effects of CHD in 2,4-dinitrochlorobenzene (DNCB)-induced BALB/c mice and tumor necrosis factor (TNF)-α- and interferon gamma (IFN)-γ-stimulated HaCaT cells. We evaluated immune cell infiltration, skin thickness, and the serum IgE and TNF-α levels in DNCB-induced AD mice. Moreover, we measured the expression levels of pro-inflammatory cytokines, mitogen-activated protein kinase (MAPK), and the nuclear factor-kappa B (NF-κB) in the mice dorsal skin. We also studied the effect of CHD on the translocation of NF-κB p65 and inflammatory chemokines in HaCaT cells. Our in vivo results revealed that CHD reduced the dermis and epidermis thicknesses and inhibited immune cell infiltration. Furthermore, it suppressed the proinflammatory cytokine expression and MAPK and NF-κB phosphorylations in the skin tissue and decreased serum IgE and TNF-α levels. In vitro results indicated that CHD downregulated inflammatory chemokines and blocked NF-κB p65 translocation. Thus, we deduced that CHD is a potential drug candidate for AD treatment.

Itch perception is reflected by neuronal ignition in the primary somatosensory cortex

Multiple cortical areas including the primary somatosensory cortex (S1) are activated during itch signal processing, yet cortical representation of itch perception remains unknown. Using novel miniature two-photon microscopic imaging in free-moving mice, we investigated the coding of itch perception in S1. We found that pharmacological inactivation of S1 abolished itch-induced scratching behavior, and the itch-induced scratching behavior could be well predicted by the activity of a fraction of layer 2/3 pyramidal neurons, suggesting that a subpopulation of S1 pyramidal neurons encoded itch perception, as indicated by immediate subsequent scratching behaviors. With a newly established optogenetics-based paradigm that allows precisely controlled pruritic stimulation, we found that a small fraction of S1 neurons exhibited an ignition-like pattern at the detection threshold of itch perception. Our study revealed the neural mechanism underlying itch perceptual coding in S1, thus paving the way for the study of cortical representation of itch perception at the single-neuron level in freely moving animals.

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.

Neuropathic agents in the management of pruritus in burn injuries: a systematic review and meta-analysis

Objectives

Pruritus is a common and often distressing complication after a burn injury. The purpose of this review is to explore the efficacy of drugs classically used to treat neuropathic pain in the management of pruritus after burn injury.

Methods

A systematic literature search of medical databases was conducted to find studies investigating drugs listed in the National Institute for Health and Care Excellence (NICE) guideline (CG173, "neuropathic pain in adults") for the management of pruritus after burn injury in patients of any age. Controlled studies were stratified by the drug class studied and their risk of bias before conducting meta-analysis. A narrative review of case series or observational studies was presented. Severity of pruritus at any time point, with all quantitative and qualitative measures, was included.

Results

Fifteen studies were included in the final analysis, 10 investigated the use of gabapentinoids, 4 studied doxepin, and 1 local anesthetic agents. Meta-analysis of three randomized controlled trials (RCTs) demonstrated that the use of gabapentinoids was associated with an improvement in mean VAS (Visual Analog Scale) 0-10 scores of 2.96 (95% confidence interval (95% CI) 1.20 to 4.73, p<0.001) when compared with placebo or antihistamine. A meta-analysis of four RCTs investigating topical doxepin showed an improvement in mean VAS scores of 1.82 (95% CI 0.55 to 3.09, p<0.001). However, when excluding two studies found to be at high risk of bias, no such improvement was found (-0.32, 95% CI -1.64 to -0.99, p=0.83).

Conclusion

This study suggests that gabapentinoids are beneficial in the management of burn-related pruritus. There is a lack of evidence to suggest that doxepin is an effective treatment. Topical local anesthetic agents may be safe and beneficial, but studies are scarce.

Level of evidence

Systematic review, level II.

Maximizing the Utility of Transcriptomics Data in Inflammatory Skin Diseases

Inflammatory skin diseases are induced by disorders of the host defense system of the skin, which is composed of a barrier, innate and acquired immunity, as well as the cutaneous microbiome. These disorders are characterized by recurrent cutaneous lesions and intense itch, which seriously affecting life quality of people across all ages and ethnicities. To elucidate molecular factors for typical inflammatory skin diseases (such as psoriasis and atopic dermatitis), transcriptomic profiling assays have been largely performed. Additionally, single-cell RNA sequencing (scRNA-seq) as well as spatial transcriptomic profiling have revealed multiple potential translational targets and offered guides to improve diagnosis and treatment strategies for inflammatory skin diseases. High-throughput transcriptomics data has shown unprecedented power to disclose the complex pathophysiology of inflammatory skin diseases. Here, we will summarize discoveries from transcriptomics data and discuss how to maximize the transcriptomics data to propel the development of diagnostic biomarkers and therapeutic targets in inflammatory skin diseases.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Chronic itch induced by thalamic deep brain stimulation: a case for a central itch centre

Background

Central itch syndrome has been previously described in conditions such as stroke. The neurophysiology of central itch syndrome has been investigated in non-human primates but remains incompletely understood.

Methods

We report an observational study of a rare case of severe central itch following thalamic deep brain stimulation and postulate the location of the central itch centre in humans.

Results

The patient was a 47-year-old female, with congenital spinal malformations, multiple previous corrective spinal surgeries and a 30-year history of refractory neuropathic pain in her back and inferior limbs. Following multidisciplinary pain assessment and recommendation, she was referred for spinal cord stimulation, but the procedure failed technically due to scarring related to her multiple previous spinal surgeries. She was therefore referred to our centre and underwent bilateral deep brain stimulation (DBS) of the ventral posterolateral nucleus of the thalamus for management of her chronic pain. Four weeks after switching on the stimulation, the patient reported significant improvement in her pain but developed a full body progressive itch which was then complicated with a rash. Common causes of skin eczema were ruled out by multiple formal dermatological evaluation. A trial of unilateral “off stimulation” was performed showing improvement of the itchy rash. Standard and normalized brain atlases were used to localize the active stimulating contact within the thalamus at a location we postulate as the central itch centre.

Conclusions

Precise stereotactic imaging points to the lateral portion of the ventral posterolateral and posteroinferior nuclei of the thalamus as critical in the neurophysiology of itch in humans.

Interleukin-31: The "itchy" cytokine in inflammation and therapy

The cytokine interleukin-31 has been implicated in the pathophysiology of multiple atopic disorders such as atopic dermatitis (AD), allergic rhinitis, and airway hyper-reactivity. In AD, IL-31 has been identified as one of the main "drivers" of its cardinal symptom, pruritus. Here, we summarize the mechanisms by which IL-31 modulates inflammatory and allergic diseases. T_H 2 cells play a central role in AD and release high levels of T_H 2-associated cytokines including IL-31, thereby mediating inflammatory responses, initiating immunoregulatory circuits, stimulating itch, and neuronal outgrowth through activation of the heterodimeric receptor IL-31 receptor A (IL31RA)/Oncostatin M receptor (OSMRβ). IL31RA expression is found on human and murine dorsal root ganglia neurons, epithelial cells including keratinocytes and various innate immune cells. IL-31 is a critical cytokine involved in neuroimmune communication, which opens new avenues for cytokine modulation in neuroinflammatory diseases including AD/pruritus, as validated by recent clinical trials using an anti-IL-31 antibody. Accordingly, inhibition of IL-31-downstream signaling may be a beneficial approach for various inflammatory diseases including prurigo. However, as to whether downstream JAK inhibitors directly block IL-31-mediated-signaling needs to be clarified. Targeting the IL-31/IL31RA/OSMRβ axis appears to be a promising approach for inflammatory, neuroinflammatory, and pruritic disorders in the future.

Lycopus lucidus Turcz ameliorates DNCB‑induced atopic dermatitis in BALB/c mice

Atopic dermatitis (AD) is a chronic inflammatory allergic skin disease, characterized by pruritic and eczematous skin lesions. Lycopus lucidus Turcz (LLT) is a perennial herb that has been reported to have various biological properties, including effects on blood circulation, as well as anti-inflammatory, antioxidant, anti-vascular inflammation and wound-healing effects. However, whether LLT improves dermatitis and the underlying mechanisms has yet to be determined. The aim of the present study was to determine whether LLT can improve 2,4-dinitrochlorobenzene (DNCB)-induced dermatitis and to verify the inhibitory effect of LLT on the expression of chemokines and pro-inflammatory cytokines in the HaCaT immortalized keratinocyte cell line. In addition, the anti-inflammatory function of LLT in RAW264.7 mouse macrophages was investigated. In the DNCB-induced AD mouse model, LLT inhibited infiltration by mast cells, eosinophils and CD8⁺ cells in the dorsal skin tissue of AD mice, and suppressed the expression of IgE and IL-6 in serum. In addition, LLT inhibited the phosphorylation of ERK and JNK, as well as NF-κB in skin tissue. In the HaCaT cell model induced by TNF-α/IFN-γ, LLT inhibited the expression of thymus and activation-regulated chemokine, granulocyte-macrophage colony-stimulating factor, monocyte chemoattractant protein-1, TNF-α and IL-1β, whilst inhibiting the phosphorylation of NF-κB. In addition, in the lipopolysaccharide-induced RAW 264.7 cell inflammation model, LLT inhibited the expression of TNF-α and IFN-γ, the nuclear translocation of NF-κB and the phosphorylation of ERK and JNK. These results suggested that LLT may be a promising candidate for the treatment of inflammatory dermatitis.

A folliculocentric perspective of dandruff pathogenesis: Could a troublesome condition be caused by changes to a natural secretory mechanism?

Dandruff is a common scalp condition, which frequently causes psychological distress in those affected. Dandruff is considered to be caused by an interplay of several factors. However, the pathogenesis of dandruff remains under-investigated, especially with respect to the contribution of the hair follicle. As the hair follicle exhibits unique immune-modulatory properties, including the creation of an immunoinhibitory, immune-privileged milieu, we propose a novel hypothesis taking into account the role of the hair follicle. We hypothesize that the changes and imbalance of yeast and bacterial species, along with increasing proinflammatory sebum by-products, leads to the activation of immune response and inflammation. Hair follicle keratinocytes may then detect these changes in scalp microbiota resulting in the recruitment of leukocytes to the inflammation site. These changes in the scalp skin immune-microenvironment may impact hair follicle immune privilege status, which opens new avenues into exploring the role of the hair follicle in dandruff pathogenesis. Also see the video abstract here: https://youtu.be/mEZEznCYtNs.

Attentional Bias Modification Training for Itch: A Proof-of-Principle Study in Healthy Individuals

Itch draws our attention to allow imposing action against bodily harm (e.g., remove insects). At the same time, itch is found to interfere with ongoing tasks and daily life goals. Despite the key role of attention in itch processing, interventions that train individuals to automatically disengage attention from itch cues are lacking. The present proof-of-principle attention bias modification (ABM) training study was aimed at investigating whether attention to itch as well as sensitivity to mild itch can be changed. Healthy volunteers were randomized over three ABM-training conditions. Training was done via a modified pictorial dot-probe task. In particular, participants were trained to look away from itch stimuli (n = 38), toward itch stimuli (n = 40) or not trained toward or away from itch at all (sham training, n = 38). The effects of the ABM-training were tested primarily on attention to itch pictures. Secondarily, it was investigated whether training effects generalized to alterations in attention to itch words and mechanical itch sensitivity. The ABM-training did not alter attention toward the itch pictures, and there was no moderation by baseline levels of attention bias for itch. Also, attention bias to the itch words and itch sensitivity were not affected by the ABM-training. This study was a first step toward trainings to change attention toward itch. Further research is warranted to optimize ABM-training methodology, for example increasing motivation of participants. Eventually, an optimized training could be used in patient populations who suffer most from distraction by their symptoms of itch.

Clinical Trial Registration: Identifier: NL6134 (NTR6273). The website URL is: https://www.trialregister.nl/

Chronic Kidney Disease-Associated Itch (CKD-aI) in Children-A Narrative Review

Chronic kidney disease (CKD) is a condition of widespread epidemiology and serious consequences affecting all organs of the organism and associated with significant mortality. The knowledge on CKD is rapidly evolving, especially concerning adults. Recently, more data is also appearing regarding CKD in children. Chronic itch (CI) is a common symptom appearing due to various underlying dermatological and systemic conditions. CI may also appear in association with CKD and is termed chronic kidney disease-associated itch (CKD-aI). CKD-aI is relatively well-described in the literature concerning adults, yet it also affects children. Unfortunately, the data on paediatric CKD-aI is particularly scarce. This narrative review aims to describe various aspects of CKD-aI with an emphasis on children, based on the available data in this population and the data extrapolated from adults. Its pathogenesis is described in details, focusing on the growing role of uraemic toxins (UTs), as well as immune dysfunction, altered opioid transmission, infectious agents, xerosis, neuropathy and dialysis-associated aspects. Moreover, epidemiological and clinical aspects are reviewed based on the few data on CKD-aI in children, whereas treatment recommendations are proposed as well, based on the literature on CKD-aI in adults and own experience in managing CI in children.

The Disruptiveness of Itchiness from Psoriasis: A Qualitative Study of the Impact of a Single Symptom on Quality of Life

BACKGROUND

Pruritus is the most common symptom of psoriasis, with a significant impact on patient quality of life. In spite of this, the severity, persistence, and overall impact of itchiness has only been rarely formally assessed during standard psoriasis clinic visits. Objectives: We sought to understand the far-reaching impacts of itchiness on the lives of those with psoriasis and their families.

METHODS

We conducted a qualitative study with five focus groups and 10 semi-structured interviews from August 2018 to January 2019. We enrolled 25 individuals with a diagnosis of at least moderate plaque psoriasis and 11 family members (primarily significant others). Views and experiences were analyzed thematically via content analysis.

RESULTS

Itchiness considerably impacts those with plaque psoriasis and their families. Our narrative analysis produced three main themes relating to itchiness: the triggers of itchiness, including climate, emotions, and behaviors; the physical consequences of itchiness, including disruption of emotional well-being, sleep disturbance, and daily activities; and the prevention and treatment strategies used to alleviate itchiness.

CONCLUSION

Itchiness impacts the quality of life in those with psoriasis and their family members. We strongly urge clinicians to inquire about and monitor the severity and impact of itchiness in psoriasis patients.

Extract of Scutellaria baicalensis induces semaphorin 3A production in human epidermal keratinocytes

In a disease-state-dependent manner, the histamine-resistant itch in dry skin-based skin diseases such as atopic dermatitis (AD) and xerosis is mainly due to hyperinnervation in the epidermis. Semaphorin 3A (Sema3A) is a nerve repulsion factor expressed in keratinocytes and it suppresses nerve fiber elongation in the epidermis. Our previous studies have shown that Sema3A ointment inhibits epidermal hyperinnervation and scratching behavior and improves dermatitis scores in AD model mice. Therefore, we consider Sema3A as a key therapeutic target for improving histamine-resistant itch in AD and xerosis. This study was designed to screen a library of herbal plant extracts to discover compounds with potential to induce Sema3A in normal human epidermal keratinocytes (NHEKs) using a reporter gene assay, so that positive samples were found. Among the positive samples, only the extract of S. baicalensis was found to consistently increase Sema3A levels in cultured NHEKs in assays using quantitative real-time PCR and ELISA. In evaluation of reconstituted human epidermis models, the level of Sema3A protein in culture supernatants significantly increased by application of the extract of S. baicalensis. In addition, we investigated which components in the extract of S. baicalensis contributed to Sema3A induction and found that baicalin and baicalein markedly increased the relative luciferase activity, and that baicalein had higher induction activity than baicalin. Thus, these findings suggest that S. baicalensis extract and its compounds, baicalin and baicalein, may be promising candidates for improving histamine-resistant itch via the induction of Sema3A expression in epidermal keratinocytes.

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.

Vulvar Pruritus: A Review of Clinical Associations, Pathophysiology and Therapeutic Management

Vulvar pruritus is an unpleasant sensation and frequent symptom associated with many dermatologic conditions, including infectious, inflammatory and neoplastic dermatoses affecting the female genitalia. It can lead to serious impairment of quality of life, impacting sexual function, relationships, sleep and self-esteem. In this review, common conditions associated with vulvar itch are discussed including atopic and contact dermatitis, lichen sclerosus, psoriasis and infectious vulvovaginitis. We review the potential physiologic, environmental and infectious factors that contribute to the development of vulvar itch and emphasize the importance of addressing their complex interplay when managing this disruptive and challenging symptom.

Pruriplastic Itch-A Novel Pathogenic Concept in Chronic Pruritus

The International Association for the Study of Pain (IASP) defined three descriptors for pain: nociceptive pain is “pain that arises from actual or threatened damage to non neural tissue and is due to the activation of nociceptors”; neuropathic pain is “pain caused by a lesion or disease of the somatosensory nervous system”; and nociplastic pain is “pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.” Based on clinical and pathophysiological arguments, a similar definition of “pruriplastic pruritus” should be made. Pruriplastic pruritus would include psychogenic pruritus, as well as some cases of pruritus ani, vulvar pruritus, sensitive skin or other poorly understood cases of pruritus. This new descriptor of itch could serve as systematic screening for altered pruriceptive function in patients who suffer from chronic itch and it may also help in defining better tailored treatment by identifying patients who are likely to respond better to centrally rather than to peripherally targeted therapies.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

A spinal neural circuitry for converting touch to itch sensation

Touch and itch sensations are crucial for evoking defensive and emotional responses, and light tactile touch may induce unpleasant itch sensations (mechanical itch or alloknesis). The neural substrate for touch-to-itch conversion in the spinal cord remains elusive. We report that spinal interneurons expressing Tachykinin 2-Cre (Tac2^Cre) receive direct Aβ low threshold mechanoreceptor (LTMR) input and form monosynaptic connections with GRPR neurons. Ablation or inhibition markedly reduces mechanical but not acute chemical itch nor noxious touch information. Chemogenetic inhibition of Tac2^Cre neurons also displays pronounced deficit in chronic dry skin itch, a type of chemical itch in mice. Consistently, ablation of gastrin-releasing peptide receptor (GRPR) neurons, which are essential for transmitting chemical itch, also abolishes mechanical itch. Together, these results suggest that innocuous touch and chemical itch information converge on GRPR neurons and thus map an exquisite spinal circuitry hard-wired for converting innocuous touch to irritating itch.

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 ambiguous pruritogenic role of interleukin-31 in cutaneous T-cell lymphomas in comparison to atopic dermatitis: a review

Cutaneous T-cell lymphomas (CTCLs) comprise a group of chronic heterogeneous diseases of unknown pathogenesis, characterized by non-specific skin lesions such as patches, plaques and tumours. CTCL is accompanied by persistent pruritus poorly responding to antihistamines and therefore significantly reducing quality of life in patients with lymphomas. According to research data, interleukin-31 (IL-31) contributes to initiation and maintenance of the inflammatory process of the skin and pruritus in inflammatory dermatoses such as atopic dermatitis (AD), which is well established. The studies of a similar role of IL-31 in CTCLs are less homogenous. Due to contradictory reports concerning IL-31 and CTCL we have analysed available literature to summarize its role, focusing on CTCL and AD.

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.

Pathogenesis of psoriasis in the "omic" era. Part I. Epidemiology, clinical manifestation, immunological and neuroendocrine disturbances

Psoriasis is a common, chronic, inflammatory, immune-mediated skin disease affecting about 2% of the world's population. According to current knowledge, psoriasis is a complex disease that involves various genes and environmental factors, such as stress, injuries, infections and certain medications. The chronic inflammation of psoriasis lesions develops upon epidermal infiltration, activation, and expansion of type 1 and type 17 Th cells. Despite the enormous progress in understanding the mechanisms that cause psoriasis, the target cells and antigens that drive pathogenic T cell responses in psoriatic lesions are still unproven and the autoimmune basis of psoriasis still remains hypothetical. However, since the identification of the Th17 cell subset, the IL-23/Th17 immune axis has been considered a key driver of psoriatic inflammation, which has led to the development of biologic agents that target crucial elements of this pathway. Here we present the current understanding of various aspects in psoriasis pathogenesis.

Functional Connectivity Evoked by Orofacial Tactile Perception of Velocity

The cortical representations of orofacial pneumotactile stimulation involve complex neuronal networks, which are still unknown. This study aims to identify the characteristics of functional connectivity (FC) evoked by three different saltatory velocities over the perioral and buccal surface of the lower face using functional magnetic resonance imaging in twenty neurotypical adults. Our results showed a velocity of 25 cm/s evoked stronger connection strength between the right dorsolateral prefrontal cortex and the right thalamus than a velocity of 5 cm/s. The decreased FC between the right secondary somatosensory cortex and right posterior parietal cortex for 5-cm/s velocity versus all three velocities delivered simultaneously (“All ON”) and the increased FC between the right thalamus and bilateral secondary somatosensory cortex for 65 cm/s vs “All ON” indicated that the right secondary somatosensory cortex might play a role in the orofacial tactile perception of velocity. Our results have also shown different patterns of FC for each seed (bilateral primary and secondary somatosensory cortex) at various velocity contrasts (5 vs 25 cm/s, 5 vs 65 cm/s, and 25 vs 65 cm/s). The similarities and differences of FC among three velocities shed light on the neuronal networks encoding the orofacial tactile perception of velocity.

Role of neuroimmune circuits and pruritus in psoriasis

Psoriasis is a chronic inflammatory skin disease presenting with an array of clinical phenotypes, often associated with pruritus. Environmental and psychological stressors can exacerbate psoriasis symptoms and provoke flares. Recent studies suggest a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis in some patients with psoriasis that can result in immune dysregulation. The immune system, in turn, can communicate with the nervous system to induce, maintain or aggravate psoriasis. In the skin, peripheral sensory as well as autonomic nerves control release of inflammatory mediators from dendritic cells, mast cells, T cells or keratinocytes, thereby modulating inflammatory responses and, in case of sensory nerves, pruritus. In response to the environment or stress, cytokines, chemokines, proteases, and neuropeptides fluctuate in psoriasis and influence immune responses as well as nerve activity. Furthermore, immune cells communicate with sensory nerves which control release of cytokines, such as IL-23, that are ultimately involved in psoriasis pathogenesis. Nerves also communicate with keratinocytes to induce epidermal proliferation. Notably, in contrast to recent years the debilitating problem of pruritus in psoriasis has been increasingly appreciated. Thus, investigating neuroimmune communication in psoriasis will not only expand our knowledge about the impact of sensory nerves in inflammation and pruritus and give new insights into the impact of environmental factors activating neuroimmune circuits or of stress in psoriasis, but may also lead to novel therapies. This review summarizes the relevant literature on the role of neuroimmune circuits, stress and how the central HPA axis and its peripheral equivalent in the skin, impact psoriasis.

Effects of Stress on Itch

PURPOSE

Psychological stress and ensuing modulation of the immune and nervous systems can have a significant impact on itch. Stress can exacerbate itch and vice versa, resulting in a vicious cycle that can greatly impair a patient's quality of life. This review summarizes the association between stress and itch, elucidates the mechanism by which these two phenomena influence one another, and explores treatment modalities that aim to reduce stress-induced itch.

METHODS

A complete search of the PubMed and Google Scholar databases was completed and literature pertinent to this review was compiled.

FINDINGS

Both acute and chronic stress can significantly affect itch in healthy individuals and in those diagnosed with itchy skin diseases as well as systemic diseases, thus resulting in a vicious cycle in which stress exacerbates itch and vice versa. The mechanisms by which stress induces or aggravates itch include both central and peripheral activation of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system. Activation of these systems, in turn, affects the mast cells, keratinocytes, and nerves that secrete neuropeptides, such as substance P, nerve growth factor, acetylcholine, histamine, and itchy cytokines. A dysfunctional parasympathetic response is thought to be involved in the chronic stress/itch response. Brain structures associated with emotion, such as the limbic system and periaqueductal gray, which work on the descending facilitation of itch, play a significant role in stress-induced itch.

IMPLICATIONS

As specific brain structures are associated with stress, drug treatments targeting these areas (ie, γ-aminobutyric acid-ergic drugs, serotonin and norepinephrine reuptake inhibitors) may help to modulate itch. Stress can also be combatted using nonpharmacologic treatments such as cognitive-behavioral therapies and stress-relieving holistic approaches (eg, yoga, acupuncture).

Calcium imaging of primary canine sensory neurons: Small-diameter neurons responsive to pruritogens and algogens

INTRODUCTION

Rodent primary sensory neurons are commonly used for studying itch and pain neurophysiology, but translation from rodents to larger mammals and humans is not direct and requires further validation to make correlations.

METHODS

This study developed a primary canine sensory neuron culture from dorsal root ganglia (DRG) excised from cadaver dogs. Additionally, the canine DRG cell cultures developed were used for single-cell ratiometric calcium imaging, with the activation of neurons to the following pruritogenic and algogenic substances: histamine, chloroquine, canine protease-activated receptor 2 (PAR2) activating peptide (SLIGKT), compound 48/80, 5-hydroxytryptamine receptor agonist (5-HT), bovine adrenal medulla peptide (BAM8-22), substance P, allyl isothiocyanate (AITC), and capsaicin.

RESULTS

This study demonstrates a simple dissection and rapid processing of DRG collected from canine cadavers used to create viable primary sensory neuron cultures to measure responses to pruritogens and algogens.

CONCLUSION

Ratiometric calcium imaging demonstrated that small-diameter canine sensory neurons can be activated by multiple stimuli, and a single neuron can react to both a pruritogenic stimulation and an algogenic stimulation.

Optimizing audiovisual itch induction: the role of attention and expectancy

Linked Article: Marzell et al. Br J Dermatol 2020; 182:1253–1261.

Neuroimmune interactions in chronic itch of atopic dermatitis

Itch is a defining symptom of atopic dermatitis. Crosstalk between keratinocytes, the immune system and non‐histaminergic sensory nerves is responsible for the pathophysiology of chronic itch in atopic dermatitis. An expanding understanding of the contribution of the nervous system and its interaction with immune pathways in atopic itch are helping to identify new therapeutic strategies.