Peripheral and Central Mechanisms of Itch

P2 purinergic signaling and pruritus

Pruritus is a common sensation that triggers scratching. Extracellular nucleotides and nucleosides, along with their receptors, primarily compose the purinergic signaling. The purinergic signaling mechanism in itch remains incompletely understood. Keratinocytes, fibroblasts, Langerhans cells, primary sensory nerve endings in the skin, and neurons and satellite glial cells in primary sensory ganglia (dorsal root ganglia and trigeminal ganglia) have been confirmed to express multiple subtypes of P2X and P2Y receptors. Purinergic signaling in the skin and primary sensory ganglia is involved in the pathological changes of skin pruritus, including atopic dermatitis, psoriasis, systemic sclerosis, diabetes complicated with pruritus, or other pruritus disorders. The interaction between P2 purinergic signaling and histamine receptors, transient receptor potential (TRP) channel receptors, and Mas-related G protein-coupled receptor member A3 (MrgprA3) receptors, which mediate itch signaling, is involved in the pathological process of skin pruritus. P2 purinergic receptor agonists can induce itching behaviors in animals. Targeted antagonism or inhibition of P2 purinergic receptors in the skin and primary sensory ganglia can alleviate pathological changes in skin pruritus. This review summarizes studies concluding that P2 receptors are involved in the pathogenesis of pruritus, with several showing potential as novel therapeutic options for alleviating pruritus.

Immune cells in dorsal root ganglia are associated with pruritus in a mouse model of allergic contact dermatitis and co-culture study

The interaction between the neuroimmune system plays a crucial role in itch sensation, yet most research has focused on immune cells within the skin. Our study seeks to explore the presence and functions of immune cells within the dorsal root ganglia (DRG) in the context of allergic contact dermatitis (ACD). Immunofluorescence and histological staining techniques were employed to identify immune cells, including T-cells, basophils, mast cells, and dendritic cells (DCs), within the DRG of BALB/c mice sensitized and challenged with toluene diisocyanate (TDI). Our findings revealed an increase in mast cells and DCs within the DRG under ACD condition. Additionally, when DRG neurons were cultured with mast cells, a higher proportion of neurons exhibited responses to non-histaminergic pruritogens compared to neurons cultured alone. This suggests that mast cells may contribute to heightened sensitivity to non-histaminergic pruritogens. Furthermore, we conducted transcriptomic analysis of DCs within the DRG using RNA sequencing, followed by pathway enrichment analysis. Our analysis revealed that sorted DCs are implicated in immune responses, inflammation, and itch, with notable upregulation of Cathepsin S (Ctss) and sphingosine-1-phosphate (S1P) phosphatase 2 (Sgpp2). Subsequent functional experiments targeting CTSS in co-culture studies validated suppressed response to pruritogen and agonists of TRPA1 and TRPV1, indicating a potential role in peripheral sensitization. Additionally, the co-culture study indicated that the neuroimmune interaction between DCs and DRG neurons might involve S1P metabolism and S1P receptor signaling. In conclusion, targeting DCs and exploring the non-histaminergic functions of mast cells within the DRG, holds promise as novel targets for treating pruritus.

Sphingosylphosphorylcholine induces itch via activation of TRPM3 and TRPA1 in mice

Itch is a prevalent symptom in atopic dermatitis (AD), often leading to a strong urge to scratch. Elevated levels of sphingosylphosphorylcholine (SPC) are found in the stratum corneum of AD patients, and while SPC is known to induce itch, its molecular targets are not well understood. This study aims to identify the signaling pathway of SPC-induced itch under AD conditions. We demonstrate that SPC specifically activates the Transient Receptor Potential Melastatin 3 (TRPM3) channel in sensory neurons. In HEK293T cells expressing TRPM3, SPC treatment caused a significant increase in intracellular calcium, which was inhibited by TRPM3 antagonists. Among various TRP channels tested, TRPM3 exhibited the highest reactivity to SPC, followed by TRPA1. Molecular docking analysis also supported interactions between SPC and both TRPM3 and TRPA1. In an AD mouse model, SPC-induced responses were dependent on TRPM3 and TRPA1, and the expression of these channels increased in dorsal root ganglion neurons. SPC-induced scratching behaviors were significantly reduced by TRPM3 and TRPA1 antagonists, with TRPM3 playing a critical role in spontaneous scratching. This study identifies TRPM3 and TRPA1 as key mediators of SPC-induced itch, providing potential therapeutic targets for treating itch in AD patients.

Systemic contact dermatitis following spinal cord stimulation in a patient with complex regional pain syndrome: A case report

RATIONALE

Spinal cord stimulation (SCS) has been widely used since the 1960s to treat intractable pain. However, hypersensitivity related to inserted devices such as implantable pulse generators, leads, and electrode contacts has rarely been reported as a side effect. We report a case of systemic contact dermatitis following SCS insertion in a patient with complex regional pain syndrome and suggest preventive and therapeutic strategies.

PATIENTS CONCERNS

A 50-year-old man was diagnosed with complex regional pain syndrome due to persistent severe pain following surgery for a left ankle fracture and sprain. The patient had an SCS inserted because his leg pain was not controlled despite medication and nerve block for several years, and he was discharged without any side effects.

DIAGNOSES

A dermatologist diagnosed the patient with contact dermatitis and prescribed medications.

OUTCOMES

Itching worsened near the arms, neck, and scrotum; hence, 4 mg methylon and 0.3% difuco ointment (difucortolone valerate) were additionally prescribed. The patient was undergoing continuous treatment in the pain and dermatology department for these symptoms.

LESSONS

Hypersensitivity after SCS insertion is difficult to treat and may require the removal of the insertion device. Therefore, the patient's medical history and screening tests, such as patch tests, are important before the procedure.

The neural basis of affective empathy: What is known from rodents

Empathy is the cornerstone of social interactions between conspecies for human beings and other social animals. Human beings with empathy defects might either suffer unpleasant or failed social interactions as ASD patients, or even display antisocial behaviors. To find efficient cure for empathy defects, first of all, the neural mechanisms underpinning various empathy behaviors should be well studied and understood. And the research in the field of affective empathy thrives fast in recent years. It is necessary to review the important contributions in this field, especially for understanding the delicate neural mechanisms of diverse forms of affective empathy. Here, we have summarized the characteristics of various types of affective empathy. We also discuss the distinctions between empathy for pain and fear, as well as instinctive and experienced empathy. Our analysis further highlights the findings in the complex neural mechanisms and potential brain regions underlying different affective empathy behaviors. Above all, this work is expected to help enhance our comprehension of behavioral dynamics and neural basis of affective empathy along with its role in emotional regulation and social behavior.

Corneal Sensory Receptors and Pharmacological Therapies to Modulate Ocular Pain

Nociceptors respond to noxious stimuli and transmit pain signals to the central nervous system. In the cornea, the nociceptors located in the most external layer provide a myriad of sensation modalities. Damage to these corneal nerve fibers can induce neuropathic pain. In response, corneal nerves become sensitized to previously non-noxious stimuli. Assessing corneal pain origin is a complex ophthalmic challenge due to variations in its causes and manifestations. Current FDA-approved therapies for corneal nociceptive pain, such as acetaminophen and NSAIDs, provide only broad-acting relief with unwanted side effects, highlighting the need for precision medicine for corneal nociceptive pain. A few targeted treatments, including perfluorohexyloctane (F6H8) eye drops and Optive Plus (TRPV1 antagonist), are FDA-approved, while others are in preclinical development. Treatments that target signaling pathways related to neurotrophic factors, such as nerve growth factors and ion channels, such as the transient receptor potential (TRP) family or tropomyosin receptor kinase A, may provide a potential combinatory therapeutic approach. This review describes the roles of nociceptors in corneal pain. In addition, it evaluates molecules within nociceptor signaling pathways for their potential to serve as targets for efficient therapeutic strategies for corneal nociceptive pain aimed at modulating neurotrophic factors and nociceptive channel sensitivity.

Different subregions of the anterior cingulate cortex differentially regulate acute itch-scratching behavior in rats

Numerous studies in both humans and rodent models have consistently demonstrated the important role of the anterior cingulate cortex (ACC) in emotion perception, pain processing, memory formation, etc. The ACC can be anatomically subdivided into rostral and caudal subregions, each exhibiting distinct functional profiles. Previous studies have provided robust evidence supporting the involvement of the ACC as a whole in pruritus regulation; however, the specific regulatory effects exerted by the distinct subregions of the ACC on pruritus remain unknown. In this study, we investigated the impact of rostral ACC and caudal ACC neuronal intervention on scratching behavior in rats with acute experimental histaminergic and nonhistaminergic itch. The results demonstrated that activation of ACC neurons occurs during acute itching. Moreover, pharmacogenetic inhibition of the rostral ACC increased acute itch scratching induced by 5-hydroxytryptamine (5-HT), whereas inhibition of the caudal ACC reduced both 5-HT- and Compound 48/80-induced acute itch-scratching behavior. In conclusion, the rostral ACC exerts a negative modulatory effect on itch, whereas the caudal ACC plays a positive modulatory role. Both subregions are implicated in the modulation of nonhistaminergic itch, whereas only the caudal ACC is involved in the modulation of histaminergic itch.

A critical view of silk fibroin for non-viral gene therapy

Exogenous genes are inserted into target cells during gene therapy in order to compensate or rectify disorders brought on by faulty or aberrant genes. However, gene therapy is still in its early stages because of its unsatisfactory therapeutic effects which are mainly due to low transfection efficiency of vectors, high toxicity, and poor target specificity. A natural polymer with numerous bioactive sites, good mechanical qualities, biodegradability, biocompatibility, and processability called silk fibroin has gained attention as a possible gene therapy vector. Using silk fibroin as a gene vector can reduce cell toxicity, extend the duration of gene expression, and allow further release even in the bloodstream, thereby expanding its therapeutic scope. This review outlines the advancements made with regard to gene delivery methods based on silk fibroin materials in the fields of malignant tumors, bone tissue regeneration, neural tissue, and vascular tissue engineering. Silk fibroin exhibits remarkable repair and therapeutic effects in gene therapy and can be employed in numerous forms, such as a vector (nanoparticles, microcapsules) or a matrix (hydrogel, scaffold) for gene delivery.

Differential regulation of pruritic sensation and emotion by cannabinoid type 1 receptors on mPFC glutamatergic and GABAergic neurons

Itch causes a strong urge to scratch and induces negative emotions, such as aversion and anxiety. Antihistamine medications are key in the clinical management of pruritus, but their therapeutic efficacy in controlling moderate and severe itching remains limited. The neural circuits in the brain that process itching and itch-induced aversion and anxiety remain unclear so far. Human brain imaging suggests that the medial prefrontal cortex (mPFC) is involved in processing the emotional and motivational components of itching. In this study, we investigated the mechanisms by which glutamatergic and GABAergic neurons in mPFC differentially regulated pruritic sensation and emotion through cannabinoid type 1 receptors (CB1Rs). Chloroquinoline (CQ)-induced acute and calcipotriol (MC903)-induced chronic itch models were established. Fiberoptic calcium imaging was used to detect the activity of the two types of neurons in response to itching. The CB1R antagonist AM251 (0.5 mg in 200 nL) was microinjected into the mPFC through the implanted cannula. We showed that chemogenetic activation of glutamatergic neurons and inhibition of GABAergic neurons in the mPFC reduced scratching and chronic itch-induced anxiety. GABAergic, but not glutamatergic, neurons were involved in acute itch-induced aversion. CB1Rs on glutamatergic and GABAergic neurons modulated chronic itch-induced scratching and anxiety in divergent manners. However, CB1Rs did not affect acute itch-induced scratching. CB1Rs on GABAergic, but not glutamatergic, neurons regulated acute itch-induced aversion. These results may guide the development of therapeutic strategies targeting CB1Rs to treat itch-induced sensory and emotional responses.

Characterization and targeting of the endosomal signaling of the gastrin releasing peptide receptor in pruritus

Chronic pruritus is a major unmet clinical problem affecting one in four adults. G protein-coupled receptors (GPCRs) are key receptors driving itch signaling and are a therapeutic target for itch relief. The endosomal signaling of GPCRs provides new challenges for understanding how GPCR signaling is regulated, how endosomal signaling of GPCRs contributes to disease states like chronic pruritus and opens new targets for therapeutic development. The Gastrin releasing peptide receptor (GRPR) is a key mediator of pruritus in the spinal cord. Yet, little is known about the molecular mechanisms regulating GRPR signaling in pruritus, if GRPR can signal from endosomes, or the role of endosomal GRPR in the development of pruritus. Here we show the importance of internalization and endosomal signaling of GRPR in pruritus. Agonist induced GRPR internalization and trafficking was quantified using BRET or microscopy while endosomal-mediated ERK signaling was measured using compartmentalized FRET biosensors. Recruitment of G proteins to endosomes was measured with NanoBit BRET. pH sensitive mesoporous silica nanoparticles (MSN) which accumulated in endosomes were used to deliver RC-3095, a GRPR specific antagonist, intracellularly to block endosomal signaling of GRPR. MSN-RC proved more effective than free RC-3095 at inhibiting chloroquine scratching in mice. Our results demonstrate a critical role for GRPR endosomal signaling in itch sensation. These results highlight the ability of endosomally targeted antagonist to inhibit GRPR signaling and provide a new target for developing therapeutics that block GRPR mediated pruritus.

Beyond the itch: the complex interplay of immune, neurological, and psychological factors in chronic urticaria

Chronic urticaria (CU) arises from a multifaceted interplay of immunological, neurological, and psychological components. Immune dysregulation, mediated through both immunoglobulin E (IgE)-dependent and IgE-independent pathways, plays a pivotal role in CU pathogenesis, involving key effector cells such as mast cells (MCs), basophils, and eosinophils. This dysregulation culminates in the release of histamine, prostaglandins, and other mediators, which precipitate pruritus. The chronicity of the disease leads to sustained pruritic symptoms, contributing to both central and peripheral sensitization. The excitation of the itch circuit is augmented, leading to the release of neurotransmitters and neuropeptides, which subsequently interact with immune cells. Psychological factors such as depression, anxiety, and stress exacerbate CU symptoms and diminish quality of life. These factors disrupt the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system (ANS). Furthermore, the act of scratching activates the reward circuit, resulting in the manifestation of the itch-scratching cycle. Current treatments, such as antihistamines, omalizumab, and cyclosporine, demonstrate variable efficacy and are often associated with adverse effects. A holistic approach addressing both psychological and physiological aspects is advocated. This review highlights the critical importance of understanding neuroimmune interactions and the influence of psychosomatic factors in CU. It aims to enhance diagnostic and therapeutic strategies by integrating psychological, neurological, and immunological perspectives.

Glutamatergic Projections from the Basolateral Amygdala to Medial Prefrontal Cortex Contribute to Acute Itch Sensation Processing

Itch refers to an aversive sensation that generates a desire to scratch. The basolateral amygdala (BLA) activity is crucial in driving motivation, sensation, and emotional responses. Excitatory projections from the BLA play a vital role in regulating neuronal activity throughout the brain, including the medial prefrontal cortex (mPFC). Nevertheless, whether the BLA neurons and BLA-mPFC circuit contribute to itch sensation remains elusive. In this study, fluoro-gold retrograde tracing, morphological staining, and neuronal manipulation approaches were employed to investigate the role of BLA-mPFC projections in itch processing. Results showed that glutamatergic neurons in the BLA were activated in response to histamine- and chloroquine-induced acute itch stimuli. Chemogenetic activation of these neurons significantly mitigated the scratching behavior, whereas their inhibition increased the number of scratching bouts. The percentages of fluoro-gold-labeled CaMKII+ neurons expressing FOS in the BLA, which project to the mPFC, were 40.10 ± 2.26% and 73.84 ± 6.48% in acute itch models induced by histamine and chloroquine, respectively. Optogenetic activation of the BLA-mPFC pathway reduced histamine- or chloroquine-induced scratching bouts, whereas its inhibition increased the scratching bouts. These results provide evidence that BLA-mPFC projections are implicated in the acute itch processing, expanding our understanding to the circuit mechanism underlying the modulation of itch.

The Fungal Secretory Peptide Micasin Induces Itch by Activating MRGPRX1/C11/A1 on Peripheral Neurons

Pruritus is the leading symptom of dermatophytosis. Microsporium canis is one of the predominant dermatophytes causing dermatophytosis. However, the pruritogenic agents and the related molecular mechanisms of the dermatophyte M canis remain poorly understood. In this study, the secretion of the dermatophyte M canis was found to dose-dependently evoke itch in mice. The fungal peptide micasin secreted from M canis was then identified to elicit mouse significant scratching and itching responses. The peptide micasin was further revealed to directly activate mouse dorsal root ganglia neurons to mediate the nonhistaminergic itch. Knockout and antagonistic experiments demonstrated that MRGPRX1/C11/A1 rather than MRGPRX2/b2 activated by micasin contributed to pruritus. The chimeras and single-amino acid variants of MRGPRX1 showed that 3 domains (extracellular loop 3, transmembrane helical domain 3, and transmembrane helical domain 6) and 4 hydrophobic residues (Y99, F237, L240, and W241) of MRGPRX1 played the key role in micasin-triggered MRGPRX1 activation. Our study sheds light on the dermatophytosis-associated pruritus and may provide potential therapeutic targets and strategies against pruritus caused by dermatophytes.

Neuroimaging Characteristics of Pruritus Induced by Eczema: An fMRI Study

OBJECTIVE

To explore the neuroimaging characteristics of eczema-induced pruritus with resting-state functional magnetic resonance imaging (rs-fMRI).

METHODS

A total of 42 patients with eczema were recruited in the PE group, and 42 healthy participants were included in the HC group. The Visual Analogue Score (VAS), 12-Item Pruritus Severity Scale (12-PSS), Pittsburgh Sleep Quality Index (PSQI), and Self-Rating Anxiety Scale (SAS) were recorded in the PE group. The different values of fraction amplitude of low-frequency fluctuation (fALFF) and functional connectivity (FC) were compared after rs-fMRI scanning.

RESULTS

Compared with the HC group, the fALFF values of the left precentral gyrus, left postcentral gyrus, left supplementary motor area (SMA), and left midcingulate cortex in the PE group were increased. The FC values between the left precentral gyrus, bilateral superior temporal gyrus, bilateral hippocampus, and left inferior occipital gyrus in the PE group were decreased. The FC values between left SMA and bilateral superior temporal gyrus in the PE group were decreased. The 12-PSS score was positively correlated with fALFF value of the left precentral gyrus and left postcentral gyrus.

CONCLUSION

Pruritus caused increased spontaneous activity in given cerebral regions, involving the perception of itch, control of scratching movements, and expression of itch-related emotions. Meanwhile, there is a correlation between fALFF values of given cerebral regions and clinical scales, which provided potential neurobiological markers for the future study of pruritus.

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

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

Baicalein ameliorates chronic itch in ACD mice by suppressing the spinal astrocytic STAT3-LCN2 cascade

Chronic itch is a maladaptive and debilitating symptom in patients with allergic contact dermatitis (ACD), adversely affecting their quality of life. There is a lack of effective treatments for ACD-associated uncontrollable itch. In this study, we explored the antipruritic effects of baicalein (BE), a bioactive flavonoid extracted from the root of Scutellaria baicalensis Georgi, and the underlying mechanisms in alleviating chronic itch triggered by diphenylcyclopropenone (DCP) in a mouse model of ACD. The ACD mice were intraperitoneally injected with BE (5, 30, and 60 mg·kg-1·d-1) for 7 days during the DCP challenge phase. The results showed that DCP-treated mice exhibited severe spontaneous scratching behaviors that was reduced after BE injections in a dose-dependent manner accompanied by inhibition of spinal astrocyte activation. We observed that the spinal astrocytic STAT3-LCN2 cascade plays a crucial role in controlling the activation of astrocytes in chronic itch. Intrathecal injection of the STAT3 inhibitor AG490 or Lcn2 siRNA significantly reduced scratching behavior and astrocyte activation in ACD mice. Moreover, BE markedly attenuated the increased phosphorylation of STAT3 (p-STAT3) and LCN2 expression in the spinal cords of ACD mice and in lipopolysaccharide-stimulated primary spinal astrocytes. Altogether, BE relieved chronic itch by suppressing the spinal astrocytic STAT3-LCN2 cascade. These findings provide a potential avenue for the management of chronic itch. Schematic summary of the main findings illustrating that BE alleviates chronic itch through suppressing the spinal astrocytic STAT3-LCN2 cascade. Specifically, BE suppresses the expression of p-STAT3 to inhibit the reactive state of astrocytes in spinal dorsal horn, and then decreases the expression of astrocytic LCN2 to alleviate chronic itch in ACD mice.

Molecular hydrogen reduces dermatitis-induced itch, diabetic itch and cholestatic itch by inhibiting spinal oxidative stress and synaptic plasticity via SIRT1-β-catenin pathway in mice

Chronic itch which is primarily associated with dermatologic, systemic, or metabolic disorders is often refractory to most current antipruritic medications, thus highlighting the need for improved therapies. Oxidative damage is a novel determinant of spinal pruriceptive sensitization and synaptic plasticity. The resolution of oxidative insult by molecular hydrogen has been manifested. Herein, we strikingly report that both hydrogen gas (2 %) inhalation and hydrogen-rich saline (5 mL/kg, intraperitoneal) injection prevent and alleviate persistent dermatitis-induced itch, diabetic itch and cholestatic itch. Hydrogen therapy reverses the decrease of spinal SIRT1 expression and antioxidant enzymes (SOD, GPx and CAT) activity after dermatitis, diabetes and cholestasis. Furthermore, hydrogen reduces spinal ROS generation, oxidation products (MDA, 8-OHdG and 3-NT) accumulation, β-catenin acetylation and dendritic spine density in persistent itch models. Spinal SIRT1 inhibition eliminates antipruritic and antioxidative effects of hydrogen, while SIRT1 agonism attenuates chronic itch phenotype, spinal β-catenin acetylation and mitochondrial damage. β-catenin inhibitors are effective against chronic itch via reducing β-catenin acetylation, blocking ERK phosphorylation and elevating antioxidant enzymes activity. Hydrogen treatment suppressed dermatitis and cholestasis mediated spontaneous excitatory postsynaptic currents in vitro. Additionally, hydrogen impairs cholestasis-induced the enhancement of cerebral functional connectivity between the right primary cingulate cortex and bilateral sensorimotor cortex, as well as bilateral striatum. Taken together, this study uncovers that molecular hydrogen protects against chronic pruritus and spinal pruriceptive sensitization by reducing oxidative damage via up-regulation of SIRT1-dependent β-catenin deacetylation in mice, implying a promising strategy in translational development for itch control.

Scratching promotes allergic inflammation and host defense via neurogenic mast cell activation

Itch is a dominant symptom in dermatitis, and scratching promotes cutaneous inflammation, thereby worsening disease. However, the mechanisms through which scratching exacerbates inflammation and whether scratching provides benefit to the host are largely unknown. We found that scratching was required for skin inflammation in mouse models dependent on FcεRI-mediated mast cell activation. Scratching-induced inflammation required pain-sensing nociceptors, the neuropeptide substance P, and the mast cell receptor MrgprB2. Scratching also increased cutaneous inflammation and augmented host defense to superficial Staphylococcus aureus infection. Thus, through the activation of nociceptor-driven neuroinflammation, scratching both exacerbated allergic skin disease and provided protection from S. aureus, reconciling the seemingly paradoxical role of scratching as a pathological process and evolutionary adaptation.

Promising Strategies for the Management of Burn-Wound-Associated Pruritus

Patients who have been injured by burns often suffer from persistent and debilitating post burn pruritus. Despite a myriad of therapeutic interventions and medications, this complex condition remains particularly difficult to ameliorate. Recently, a new generation of antipruritic medications has demonstrated clinical success in managing pruritus in a number of dermatologic, nephritic and hepatic disease states, targeting unique aspects of the pruritic pathways. While specific trials demonstrating efficacy and safety are currently lacking, the purported mechanisms of action and similarities to the targeted inflammatory markers, pruritogens and neural pathways of these new medications, in concert with clinical evidence, hold promise for burn patients.

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

Fibroblast-derived IL-33 exacerbates orofacial neuropathic pain via the activation of TRPA1 in trigeminal ganglion neurons

Damage to the peripheral nerves of trigeminal ganglion (TG) neurons leads to intractable orofacial neuropathic pain through the induction of neuroinflammation. However, the details of this process are not yet fully understood. Here, we found that fibroblast-derived interleukin (IL)-33 was required for the development of mechanical allodynia in whisker pad skin following infraorbital nerve injury (IONI). The amount of IL-33 in the TG increased after IONI when the mice exhibited mechanical allodynia. Neutralization of IL-33 in the TG inhibited the development of IONI-induced mechanical allodynia. Conversely, intra-TG administration of recombinant human IL-33 (rhIL-33) elicited mechanical allodynia in naïve mice. IL-33 and its receptor were exclusively expressed in fibroblasts and neurons, respectively, in the TG. Fibroblast ablation caused the loss of IL-33 in the TG and delayed the development of mechanical allodynia after IONI. rhIL-33 elicited an increase in intracellular Ca2+ concentration and subsequent enhancement of Ca2+ influx via transient receptor potential ankyrin 1 (TRPA1) in primary cultured TG neurons. Additionally, rhIL-33 facilitated membrane translocation of TRPA1 in the TG. Mechanical allodynia caused by intra-TG administration of rhIL-33 was significantly inhibited by pharmacological blockade or gene silencing of TRPA1 in the TG. Inhibition of protein kinase A abrogated TRPA1 membrane translocation and delayed mechanical allodynia after IONI. Substance P stimulation caused upregulation of IL-33 expression in primary cultured fibroblasts. Preemptive administration of a neurokinin-1 receptor antagonist in the TG attenuated mechanical allodynia and IL-33 expression following IONI. Taken together, these results indicate that fibroblast-derived IL-33 exacerbates TG neuronal excitability via suppression of tumorigenicity 2 (ST2)-TRPA1 signaling, ultimately leading to orofacial neuropathic pain.

[Mechanism of transduction of itch and strategy of treatment for itch]

Itch is an unpleasant sense to evoke desire to scratch skin. Itch not only disturbs daily lives, but also exacerbates inflammation in case of atopic dermatitis (AD). It had been thought that both itch and pain are transduced by the same neurons; however, it is now known that neutrons transducing either itch or pain are distinct. Moreover, TRP channels, a family of calcium channels, play an important role for transducing itch as well as pain, temperature, and pressure. Development of neuroscience and molecular biology has dramatically advanced our understanding of how itch is transduced in recent years. On the other hand, development of immunology has revealed that there exist several immune types in our host defense mechanism and that type 2 immune reaction is dominant in the pathogenesis of allergic diseases including AD. Although it had been already known that type 2 cytokines contribute to the pathogenesis of AD by binding to their receptors on both immune cells and tissue resident cells, it has been recently found that several type 2 cytokines directly transduce the itch signals by binding to peripheral nerves. Due to this discovery, we can understand more deeply the itch mechanism of AD and can develop molecularly targeted drugs for AD targeting type 2 cytokines, which has dramatically changed the treatment of AD. In this review article, we describe the progress of our recent understanding of the itch mechanism and the strategy of treatment against it.

Correlation between dietary calcium intake and eczema in American adult population

Eczema is a common chronic skin condition. Previous studies indicated the dietary factors, such as calcium intake, might influence the onset and progression of eczema in the population of gravidas and infants. However, there was no studies on the correlation between dietary calcium and the adult population. In this study, we aim to investigate the correlation between dietary calcium intake and the prevalence of eczema in adults. The characteristics of adults (≥ 18 years) were collected from the National Health and Nutrition Examination Survey (NHANES) 2005-2006 database. Dietary calcium intake was assessed using the 24-hour dietary recall method. The prevalence of eczema was determined through an allergy questionnaire. Logistic regression modeling was applied to analyze the correlation between dietary calcium intake and eczema prevalence. Restricted cubic spline (RCS) was used to investigate the nonlinear relationship between calcium intake and eczema. A two-stage linear regression model was used to calculate the critical effect of calcium intake on the prevalence of eczema by smoothed curve fitting. Subgroup analyses were performed to explore the effect of different demographic characteristics on the relationship between dietary calcium intake and eczema. Results In this cross-sectional study, we collected 4086 adult samples. There were 1930 males (46.9%) and 2156 females (53.1%), at the average age of 46.7 years, and 266 participants (7.6%) were diagnosed with eczema. Logistic regression results showed there was a significant difference between the third quartile group and eczema compared to the 1st quartile group of dietary calcium (OR: 1.913, 95% CI: 1.024-3.576, P = 0.043). The RCS showed an inverted U-shaped correlation between dietary calcium intake and eczema prevalence (non-linear P-value < 0.05). An increase in calcium intake was associated with an increase in eczema prevalence when the logarithmic value of dietary calcium intake was below 7.089 (OR: 1.790, 95% CI: 1.006-3.183, P = 0.048). These data indicated there was an inverted U-shaped correlation between dietary calcium intake and the prevalence of eczema, which suggested moderate reduction of calcium intake might be beneficial in the incidence of eczema. Further prospective studies are needed to explore causal relationships and optimal calcium intake levels to prevent eczema.

Structure-guided discovery of bile acid derivatives for treating liver diseases without causing itch

Chronic itch is a debilitating symptom profoundly impacting the quality of life in patients with liver diseases like cholestasis. Activation of the human G-protein coupled receptor, MRGPRX4 (hX4), by bile acids (BAs) is implicated in promoting cholestasis itch. However, the detailed underlying mechanisms remain elusive. Here, we identified 3-sulfated BAs that are elevated in cholestatic patients with itch symptoms. We solved the cryo-EM structure of hX4-Gq in a complex with 3-phosphated deoxycholic acid (DCA-3P), a mimic of the endogenous 3-sulfated deoxycholic acid (DCA-3S). This structure revealed an unprecedented ligand-binding pocket in MRGPR family proteins, highlighting the crucial role of the 3-hydroxyl (3-OH) group on BAs in activating hX4. Guided by this structural information, we designed and developed compound 7 (C7), a BA derivative lacking the 3-OH. Notably, C7 effectively alleviates hepatic injury and fibrosis in liver disease models while significantly mitigating the itch side effects.

Beyond classical immunity: Mast cells as signal converters between tissues and neurons

Mast cells are regarded as effectors in immune defense against parasites and venoms and play an essential role in the pathology of allergic diseases. More recently, mast cells have been shown to receive stimuli derived from type 2 immunity, tissue damage, stress, and inflammation. Mast cells then rapidly convert these diverse signals into appropriate, organ-specific protective reflexes that can limit inflammation or reduce tissue damage. In this review, we consider functions of mast cells in sensations-such as pain, itch, and nausea-arising from tissue insults and inflammation and the ensuing protective responses. In light of emerging data highlighting the involvement of mast cells in neuroimmune communication, we also propose that mast cells are "signal converters" linking immunological and tissue states with nervous system responses.

Neutrophils in Atopic Dermatitis

Neutrophils have a critical role in inflammation. Recent studies have identified their distinctive presence in certain types of atopic dermatitis (AD), yet their exact function remains unclear. This review aims to compile studies elucidating the role of neutrophils in AD pathophysiology. Proteins released by neutrophils, including myeloperoxidase, elastase, and lipocalin, contribute to pruritus progression in AD. Neutrophilic oxidative stress and the formation of neutrophil extracellular traps may further worsen AD. Elevated neutrophil elastase and high-mobility group box 1 protein expression in AD patients' skin exacerbates epidermal barrier defects. Neutrophil-mast cell interactions in allergic inflammation steer the immunological response toward Th2 imbalance and activate the Th17 pathway, particularly in response to allergens or infections linked to AD. Notably, drugs alleviating pruritic symptoms in AD inhibit neutrophilic inflammation. In conclusion, these findings underscore that neutrophils may be therapeutic targets for AD symptoms, emphasizing their inclusion in AD treatment strategies.

Microneedle patches incorporating zinc-doped mesoporous silica nanoparticles loaded with betamethasone dipropionate for psoriasis treatment

Treating psoriasis presents a major clinical challenge because of the limitations associated with traditional topical glucocorticoid therapy. This study introduced a drug delivery system utilizing zinc-doped mesoporous silica nanoparticle (Zn-MSN) and microneedle (MN), designed to enhance drug utilization for prolonged anti-inflammatory and anti-itch effects. The MN system facilitated the transdermal delivery of betamethasone dipropionate (BD), allowing its slow release. The BD@Zn-MSN-MN system promoted the polarization of macrophages towards the anti-inflammatory M2 phenotype, achieving superior anti-inflammatory effects compared to the clinically used BD cream. Additionally, this study demonstrated that BD@Zn-MSN-MN could further alleviate itching in psoriasis-afflicted mice by decreasing the excitability of the transient receptor potential vanilloid V1 (TRPV1) ion channel positive neurons and reducing the release of calcitonin gene-related peptide (CGRP) in the dorsal root ganglion (DRG). These findings offer new insights and effective therapeutic options for the future design of transdermal drug delivery for psoriasis.

Neuroanatomy of the nociceptive system: From nociceptors to brain networks

This chapter reviews the neuroanatomy of the nociceptive system and its functional organization. We describe three main compartments of the nervous system that underlie normal nociception and the resulting pain percept: Peripheral, Spinal Cord, and Brain. We focus on how ascending nociceptive processing streams traverse these anatomical compartments, culminating in the multidimensional experience of pain. We also describe neuropathic pain conditions, in which nociceptive processing is abnormal, not only because of the primary effects of a lesion or disease affecting peripheral nerves or the central nervous system (CNS), but also due to secondary effects on ascending pathways and brain networks. We discuss how the anatomical components (circuits/networks) reorganize under various etiologies of neuropathic pain and how these changes can give rise to pathological pain states.

Cell Type-Specific Modulation of Acute Itch Processing in the Anterior Cingulate Cortex

Despite remarkable progress in understanding the fundamental bases of itching, its cortical mechanisms remain poorly understood. Herein, the causal contributions of defined anterior cingulate cortex (ACC) neuronal populations to acute itch modulation in mice are established. Using cell type-specific manipulations, the opposing functions of ACC glutamatergic and GABAergic neurons in regulating acute itching are demonstrated. Photometry studies indicated that ACC glutamatergic neurons are activated during scratching induced by both histamine and chloroquine, whereas the activation pattern of GABAergic neurons is complicated by GABAergic subpopulations and acute itch modalities. By combining cell type- and projection-specific techniques, a thalamocortical circuit is further identified from the mediodorsal thalamus driving the itch-scratching cycle related to histaminergic and non-histaminergic itching, which is contingent on the activation of postsynaptic parvalbumin-expressing neurons in the ACC. These findings reveal a cellular and circuit signature of ACC neurons orchestrating behavioral responses to itching and may provide insights into therapies for itch-related diseases.

Concerning neuromodulation as treatment of neurological and neuropsychiatric disorder: Insights gained from selective targeting of the subthalamic nucleus, para-subthalamic nucleus and zona incerta in rodents

Neuromodulation such as deep brain stimulation (DBS) is advancing as a clinical intervention in several neurological and neuropsychiatric disorders, including Parkinson's disease, dystonia, tremor, and obsessive-compulsive disorder (OCD) for which DBS is already applied to alleviate severely afflicted individuals of symptoms. Tourette syndrome and drug addiction are two additional disorders for which DBS is in trial or proposed as treatment. However, some major remaining obstacles prevent this intervention from reaching its full therapeutic potential. Side-effects have been reported, and not all DBS-treated individuals are relieved of their symptoms. One major target area for DBS electrodes is the subthalamic nucleus (STN) which plays important roles in motor, affective and associative functions, with impact on for example movement, motivation, impulsivity, compulsivity, as well as both reward and aversion. The multifunctionality of the STN is complex. Decoding the anatomical-functional organization of the STN could enhance strategic targeting in human patients. The STN is located in close proximity to zona incerta (ZI) and the para-subthalamic nucleus (pSTN). Together, the STN, pSTN and ZI form a highly heterogeneous and clinically important brain area. Rodent-based experimental studies, including opto- and chemogenetics as well as viral-genetic tract tracings, provide unique insight into complex neuronal circuitries and their impact on behavior with high spatial and temporal precision. This research field has advanced tremendously over the past few years. Here, we provide an inclusive review of current literature in the pre-clinical research fields centered around STN, pSTN and ZI in laboratory mice and rats; the three highly heterogeneous and enigmatic structures brought together in the context of relevance for treatment strategies. Specific emphasis is placed on methods of manipulation and behavioral impact.

The neuroscience of itch in relation to transdiagnostic psychological approaches

The experience of itch and its associated chronic conditions (i.e., atopic dermatitis) form a significant burden of disease. Knowledge of how the brain processes itch, that might occur uniquely for chronic itch populations, could be used to guide more effective psychotherapeutic interventions for these groups. To build the evidence base for such approaches, we conducted a series of coordinates-based fMRI analyses, to identify the shared neural mechanisms for itch across the published literature. Upon so doing, we identified a core "itch network" that spans the Basal Ganglia/Thalamus, Claustrum and Insula. Additionally, we found evidence that the Paracentral Lobule and Medial Frontal Gyrus, regions associated with cognitive control and response inhibition, deactivate during itch. Interestingly, a separate analysis for chronic itch populations identified significant recruitment of the Left Paracentral Lobule, potentially suggesting the recruitment of cognitive control mechanisms to resist the urge to scratch. We position these results in light of further integrative studies that could use neuroimaging alongside clinical studies, to explore how transdiagnostic psychological approaches-such as mindfulness and compassion training-might help to improve quality of life for individuals who experience chronic itch.

Functional Role of Piezo1 in the Human Eosinophil Cell Line AML14.3D10: Implications for the Immune and Sensory Nervous Systems

Mechanosensitive ion channels, particularly Piezo channels, are widely expressed in various tissues. However, their role in immune cells remains underexplored. Therefore, this study aimed to investigate the functional role of Piezo1 in the human eosinophil cell line AML14.3D10. We detected Piezo1 mRNA expression, but not Piezo2 expression, in these cells, confirming the presence of the Piezo1 protein. Activation of Piezo1 with Yoda1, its specific agonist, resulted in a significant calcium influx, which was inhibited by the Piezo1-specific inhibitor Dooku1, as well as other nonspecific inhibitors (Ruthenium Red, Gd3+, and GsMTx-4). Further analysis revealed that Piezo1 activation modulated the expression and secretion of both pro-inflammatory and anti-inflammatory cytokines in AML14.3D10 cells. Notably, supernatants from Piezo1-activated AML14.3D10 cells enhanced capsaicin and ATP-induced calcium responses in the dorsal root ganglion neurons of mice. These findings elucidate the physiological role of Piezo1 in AML14.3D10 cells and suggest that factors secreted by these cells can modulate the activity of transient receptor potential 1 (TRPV1) and purinergic receptors, which are associated with pain and itch signaling. The results of this study significantly advance our understanding of the function of Piezo1 channels in the immune and sensory nervous systems.

TRPA1-Related Diseases and Applications of Nanotherapy

Transient receptor potential (TRP) channels, first identified in Drosophila in 1969, are multifunctional ion channels expressed in various cell types. Structurally, TRP channels consist of six membrane segments and are classified into seven subfamilies. Transient receptor potential ankyrin 1 (TRPA1), the first member of the TRPA family, is a calcium ion affinity non-selective cation channel involved in sensory transduction and responds to odors, tastes, and chemicals. It also regulates temperature and responses to stimuli. Recent studies have linked TRPA1 to several disorders, including chronic pain, inflammatory diseases, allergies, and respiratory problems, owing to its activation by environmental toxins. Mutations in TRPA1 can affect the sensory nerves and microvasculature, potentially causing nerve pain and vascular problems. Understanding the function of TRPA1 is important for the development of treatments for these diseases. Recent developments in nanomedicines that target various ion channels, including TRPA1, have had a significant impact on disease treatment, providing innovative alternatives to traditional disease treatments by overcoming various adverse effects.

A thalamic nucleus reuniens-lateral septum-lateral hypothalamus circuit for comorbid anxiety-like behaviors in chronic itch

Chronic itch often clinically coexists with anxiety symptoms, creating a vicious cycle of itch-anxiety comorbidities that are difficult to treat. However, the neuronal circuit mechanisms underlying the comorbidity of anxiety in chronic itch remain elusive. Here, we report anxiety-like behaviors in mouse models of chronic itch and identify γ-aminobutyric acid-releasing (GABAergic) neurons in the lateral septum (LS) as the key player in chronic itch-induced anxiety. In addition, chronic itch is accompanied with enhanced activity and synaptic plasticity of excitatory projections from the thalamic nucleus reuniens (Re) onto LS GABAergic neurons. Selective chemogenetic inhibition of the Re → LS circuit notably alleviated chronic itch-induced anxiety, with no impact on anxiety induced by restraint stress. Last, GABAergic neurons in lateral hypothalamus (LH) receive monosynaptic inhibition from LS GABAergic neurons to mediate chronic itch-induced anxiety. These findings underscore the potential significance of the Re → LS → LH pathway in regulating anxiety-like comorbid symptoms associated with chronic itch.

The effect of photobiomodulation on histamine and Mucuna pruriens-induced pruritus, hyperknesis and alloknesis in healthy volunteers: A double-blind, randomized, sham-controlled study

BACKGROUND

Photobiomodulation, also referred to as Low-Level Light Therapy (LLLT), has emerged as a promising intervention for pruritus, a prevalent and often distressing symptom.

OBJECTIVES

This study investigated the efficacy of low-level light therapy (LLLT) in alleviating pruritus, hyperknesis, and alloknesis induced by histamine and Mucuna pruriens.

METHODS

In a double-blind, randomized, sham-controlled trial with a split-body design, healthy volunteers underwent 6 minutes of LLLT and sham treatments in separate upper back quadrants. The histamine model was applied to the upper quadrants, and Mucuna pruriens to the lower quadrants. Pruritus intensity, alloknesis, hyperknesis, flare area, and skin temperature were measured pre and post treatment.

RESULTS

Seventeen individuals (eight females, nine males) participated in the study. In the histamine model, LLLT notably reduced itch intensity (difference = 13.9 (95% CI: 10.5 - 17.4), p = 0.001), alloknesis (difference = 0.80 (95% CI: 0.58-1.02), p = 0.001), and hyperknesis (difference = 0.48 (95% CI: 0.09-0.86), p = 0.01). Skin temperature changes were not significantly different between the two groups (difference = -2.0 (95% CI: -6.7-2.6), p = 0.37). For the Mucuna pruriens model, no significant differences were observed in any measures, including itch intensity (difference = 0.8 (95% CI: -2.3 - 3.8), p = 0.61) hyperknesis (difference = 0.08 (95% CI: -0.06-0.33), p = 0.16) and alloknesis (difference = 0. 0.09 (95% CI: -0.08-0.256), p = 0.27).

CONCLUSIONS

LLLT effectively reduced histamine-induced pruritus, alloknesis, and hyperknesis; however, LLLT was ineffective against Mucuna pruriens-induced pruritus. Further investigations are required to determine LLLT's effectiveness of LLLT in various pruritus models.

Peripheral Mechanisms of Mechanical Itch

Mechanical itch, which is defined as an itch sensation caused by innocuous mechanical force, may warn of the potential risk in the skin. The increased mechanosensitivity in sensory neurons may cause scratch-induced itch and promote the transition from acute itch to chronic itch. Recent studies have not only expanded our knowledge about the neuronal circuits in the CNS but have also highlighted the importance of the peripheral epithelia-immune-neuronal crosstalk in the development of mechanical itch. In this review, we will summarize related findings about the molecular and cellular mechanisms of mechanical itch in the skin.

Prelimbic cortex-nucleus accumbens core projection positively regulates itch and itch-related aversion

Itch is one of the most common clinical symptoms in patients with diseases of the skin, liver, or kidney, and it strongly triggers aversive emotion and scratching behavior. Previous studies have confirmed the role of the prelimbic cortex (Prl) and the nucleus accumbens core (NAcC), which are reward and motivation regulatory centers, in the regulation of itch. However, it is currently unclear whether the Prl-NAcC projection, an important pathway connecting these two brain regions, is involved in the regulation of itch and its associated negative emotions. In this study, rat models of acute neck and cheek itch were established by subcutaneous injection of 5-HT, compound 48/80, or chloroquine. Immunofluorescence experiments determined that the number of c-Fos-immunopositive neurons in the Prl increased during acute itch. Chemogenetic inhibition of Prl glutamatergic neurons or Prl-NAcC glutamatergic projections can inhibit both histaminergic and nonhistaminergic itch-scratching behaviors and rectify the itch-related conditioned place aversion (CPA) behavior associated with nonhistaminergic itch. The Prl-NAcC projection may play an important role in the positive regulation of itch-scratching behavior by mediating the negative emotions related to itch.

Morgellons disease: a narrative review

Morgellons disease is characterized by the persistent delusion of skin infestation, ultimately inflicting wounds and impairing quality of life. There is insufficient and conflicting research pertaining to this condition, imposing challenges on clinicians in understanding, diagnosing, and treating it. In this review, we summarize the available literature on Morgellons disease including its historical evolution, epidemiology, proposed pathophysiology, underlying structural and functional brain pathologies, typical and atypical clinical presentations, diagnosis, and treatment. A comprehensive review of the literature was conducted on PubMed, Embase, and Scopus using specified keywords. Selected articles were screened by two independent reviewers based on set inclusion and exclusion criteria. Conflicts were resolved by a third reviewer as needed. No limit to the date of selected articles was set due to the scarce literature available on the subject. Morgellons disease is an underdiagnosed entity, owing mostly to the lack of an established pathophysiology and treatment guidelines. While many authors classify it as a type of delusional infestation (DI), others correlate MD with an underlying spirochetal infection, namely Lyme disease. Neuroimaging studies have revealed abnormalities in the "fronto-striato-thalamo-parietal network", a finding common to patients with DI, in addition to alterations in structures related to the "Itch Processing Pathway". Patients tend to extract fibers from their skin lesions and place them in a match box hence the term "match box sign". The diagnosis is that of exclusion, requiring extensive work up to rule out secondary causes and differential diagnoses. Treatment is largely based on the use of antipsychotics, with or without cognitive behavioral therapy. Despite being a diagnosis of exclusion, clinicians must be aware of this entity and have a profound understanding of the pathogenesis underlying it. Upon clinical suspicion, secondary Morgellons should always be ruled out through a thorough history taking, physical examination, and laboratory exams. Despite the challenges brought by the heterogeneous presentation of the condition and the paucity of research revolving around it, the great impact that Morgellons disease has on patients' quality of life forms a pressing need for its adequate detection, diagnosis, and treatment.

Prediction of cell-cell communication patterns of dorsal root ganglion cells: single-cell RNA sequencing data analysis

Dorsal root ganglion neurons transmit peripheral somatic information to the central nervous system, and dorsal root ganglion neuron excitability affects pain perception. Dorsal root ganglion stimulation is a new approach for managing pain sensation. Knowledge of the cell-cell communication among dorsal root ganglion cells may help in the development of new pain and itch management strategies. Here, we used the single-cell RNA-sequencing (scRNA-seq) database to investigate intercellular communication networks among dorsal root ganglion cells. We collected scRNA-seq data from six samples from three studies, yielding data on a total of 17,766 cells. Based on genetic profiles, we identified satellite glial cells, Schwann cells, neurons, vascular endothelial cells, immune cells, fibroblasts, and vascular smooth muscle cells. Further analysis revealed that eight types of dorsal root ganglion neurons mediated proprioceptive, itch, touch, mechanical, heat, and cold sensations. Moreover, we predicted several distinct forms of intercellular communication among dorsal root ganglion cells, including cell-cell contact, secreted signals, extracellular matrix, and neurotransmitter-mediated signals. The data mining predicted that Mrgpra3 -positive neurons robustly express the genes encoding the adenosine Adora2b (A2B) receptor and glial cell line-derived neurotrophic factor family receptor alpha 1 (GFRα-1). Our immunohistochemistry results confirmed the coexpression of the A2B receptor and GFRα-1. Intrathecal injection of the A2B receptor antagonist PSB-603 effectively prevented histamine-induced scratching behaviour in a dose-dependent manner. Our results demonstrate the involvement of the A2B receptor in the modulation of itch sensation. Furthermore, our findings provide insight into dorsal root ganglion cell-cell communication patterns and mechanisms. Our results should contribute to the development of new strategies for the regulation of dorsal root ganglion excitability.

FGF13 enhances the function of TRPV1 by stabilizing microtubules and regulates acute and chronic itch

Itching is an aversive somatosensation that triggers the desire to scratch. Transient receptor potential (TRP) channel proteins are key players in acute and chronic itch. However, whether the modulatory effect of fibroblast growth factor 13 (FGF13) on acute and chronic itch is associated with TRP channel proteins is unclear. Here, we demonstrated that conditional knockout of Fgf13 in dorsal root ganglion neurons induced significant impairment in scratching behaviors in response to acute histamine-dependent and chronic dry skin itch models. Furthermore, FGF13 selectively regulated the function of the TRPV1, but not the TRPA1 channel on Ca2+ imaging and electrophysiological recordings, as demonstrated by a significant reduction in neuronal excitability and current density induced by TRPV1 channel activation, whereas TRPA1 channel activation had no effect. Changes in channel currents were also verified in HEK cell lines. Subsequently, we observed that selective modulation of TRPV1 by FGF13 required its microtubule-stabilizing effect. Furthermore, in FGF13 knockout mice, only the overexpression of FGF13 with a tubulin-binding domain could rescue TRP channel function and the impaired itch behavior. Our findings reveal a novel mechanism by which FGF13 is involved in TRPV1-dependent itch transduction and provide valuable clues for alleviating pathological itch syndrome.

MRGPRX4 mediates phospho-drug-associated pruritus in a humanized mouse model

The phosphate modification of drugs is a common chemical strategy to increase solubility and allow for parenteral administration. Unfortunately, phosphate modifications often elicit treatment- or dose-limiting pruritus through an unknown mechanism. Using unbiased high-throughput drug screens, we identified the Mas-related G protein-coupled receptor X4 (MRGPRX4), a primate-specific, sensory neuron receptor previously implicated in itch, as a potential target for phosphate-modified compounds. Using both Gq-mediated calcium mobilization and G protein-independent GPCR assays, we found that phosphate-modified compounds potently activate MRGPRX4. Furthermore, a humanized mouse model expressing MRGPRX4 in sensory neurons exhibited robust phosphomonoester prodrug-evoked itch. To characterize and confirm this interaction, we further determined the structure of MRGPRX4 in complex with a phosphate-modified drug through single-particle cryo-electron microscopy (cryo-EM) and identified critical amino acid residues responsible for the binding of the phosphate group. Together, these findings explain how phosphorylated drugs can elicit treatment-limiting itch and identify MRGPRX4 as a potential therapeutic target to suppress itch and to guide future drug design.

Staphylococcus aureus: The Bug Behind the Itch in Atopic Dermatitis

Pruritus or itch is a defining symptom of atopic dermatitis (AD). The origins of itch are complex, and it is considered both a defense mechanism and a cause of disease that leads to inflammation and psychological stress. Considerable progress has been made in understanding the processes that trigger itch, particularly the pruritoceptive origins that are generated in the skin. This perspective review discusses the implications of a recent observation that the V8 protease expressed by Staphylococcus aureus can directly trigger sensory neurons in the skin through activation of protease-activated receptor 1. This may be a key to understanding why itch is so common in AD because S. aureus commonly overgrows in this disease owing to deficient antimicrobial defense from both the epidermis and the cutaneous microbiome. Increased understanding of the role of microbes in AD provides increased opportunities for safely improving the treatment of this disorder.

Annexin 1 Reduces Dermatitis-Induced Itch and Cholestatic Itch through Inhibiting Neuroinflammation and Iron Overload in the Spinal Dorsal Horn of Mice

The unclear pathogenesis of chronic itch originating from several systemic disorders poses challenges to clinical intervention. Recent studies recapitulate the spinal neurocircuits associated with neuroinflammation and synaptic plasticity responsible for pruriceptive sensations. The resolution of nociception and inflammation by Annexin 1 (ANXA1) has been identified. Given that pain and itch share many neural mechanisms, we employed two mice models of chronic itch to study the underlying targets and therapeutic potential of ANXA1, comprising allergic contact dermatitis-induced itch and cholestatic itch. Herein, we report that spinal expression of ANXA1 is down-regulated in mice with dermatitis-induced itch and cholestatic itch. Repetitive injections of ANXA1-derived peptide Ac2-26 (intrathecal, 10 μg) reduce itch-like scratching behaviors following dermatitis and cholestasis. Single exposure to Ac2-26 (intrathecal, 10 μg) alleviates the established itch phenotypes. Moreover, systemic delivery of Ac2-26 (intravenous, 100 μg) is effective against chronic dermatitis-induced itch and cholestatic itch. Strikingly, Ac2-26 therapy inhibits transferrin receptor 1 over-expression, iron accumulation, cytokine IL-17 release and the production of its receptor IL-17R, as well as astrocyte activation in the dorsal horn of spinal cord in mouse with dermatitis and cholestasis. Pharmacological intervention with iron chelator deferoxamine impairs chronic itch behaviors and spinal iron accumulation after dermatitis and cholestasis. Also, spinal IL-17/IL-17R neutralization attenuates chronic itch. Taken together, this current research indicates that ANXA1 protects against the beginning and maintenance of long-term dermatitis-induced itch and cholestatic itch, which may occur via the spinal suppression of IL-17-mediated neuroinflammation, astrocyte activation and iron overload.

Sensory neurons: An integrated component of innate immunity

The sensory nervous system possesses the ability to integrate exogenous threats and endogenous signals to mediate downstream effector functions. Sensory neurons have been shown to activate or suppress host defense and immunity against pathogens, depending on the tissue and disease state. Through this lens, pro- and anti-inflammatory neuroimmune effector functions can be interpreted as evolutionary adaptations by host or pathogen. Here, we discuss recent and impactful examples of neuroimmune circuitry that regulate tissue homeostasis, autoinflammation, and host defense. Apparently paradoxical or conflicting reports in the literature also highlight the complexity of neuroimmune interactions that may depend on tissue- and microbe-specific cues. These findings expand our understanding of the nuanced mechanisms and the greater context of sensory neurons in innate immunity.

An insular cortical circuit required for itch sensation and aversion

Itch encompasses both sensory and emotional dimensions, with the two dimensions reciprocally exacerbating each other. However, whether a shared neural circuit mechanism governs both dimensions remains elusive. Here, we report that the anterior insular cortex (AIC) is activated by both histamine-dependent and -independent itch stimuli. The activation of AIC elicits aversive emotion and exacerbates pruritogen-induced itch sensation and aversion. Mechanistically, AIC excitatory neurons project to the GABAergic neurons in the dorsal bed nucleus of the stria terminalis (dBNST). Manipulating the activity of the AIC → dBNST pathway affects both itch sensation and itch-induced aversion. Our study discovers the shared neural circuit (AIC → dBNST pathway) underlying the itch sensation and aversion, highlights the critical role of the AIC as a central hub for the itch processing, and provides a framework to understand the neural mechanisms underlying the sensation and emotion interaction.

Similarities and differences in peripheral itch and pain pathways in atopic dermatitis

Atopic dermatitis (AD) is predominantly characterized by intense itching, but concomitant skin pain is experienced by more than 40% of patients. Patients with AD display considerable somatosensory aberrations, including increased nerve sensitivity to itch stimuli (hyperknesis), perception of itch from innocuous stimuli (alloknesis), or perception of pain from innocuous stimuli (allodynia). This review summarizes the current understanding of the similarities and differences in the peripheral mechanisms underlying itch and pain in AD. These distinct yet reciprocal sensations share many similarities in the peripheral nervous system, including common mediators (such as serotonin, endothelin-1, IL-33, and thymic stromal lymphopoietin), receptors (such as members of the G protein-coupled receptor family and Toll-like receptors), and ion channels for signal transduction (such as certain members of the transient receptor potential [TRP] cation channels). Itch-responding neurons are also sensitive to pain stimuli. However, there are distinct differences between itch and pain signaling. For example, specific immune responses are associated with pain (type 1 and/or type 3 cytokines and certain chemokine C-C [CCL2, CCL5] and C-X-C [CXCL] motif ligands) and itch (type 2 cytokines, including IL-31, and periostin). The TRP melastatin channels TRPM2 and TRPM3 have a role in pain but no known role in itch. Activation of μ-opioid receptors is known to alleviate pain but exacerbate itch. Understanding the connection between itch and pain mechanisms may offer new insights into the treatment of chronic pain and itch in AD.

Mechanical epilation exerts complex biological effects on human hair follicles and perifollicular skin: An ex vivo study approach

OBJECTIVE

Electrical epilation of unwanted hair is a widely used hair removal method, but it is largely unknown how this affects the biology of human hair follicles (HF) and perifollicular skin. Here, we have begun to explore how mechanical epilation changes selected key biological read-out parameters ex vivo within and around the pilosebaceous unit.

METHODS

Human full-thickness scalp skin samples were epilated ex vivo using an electro-mechanical device, organ-cultured for up to 6 days in serum-free, supplemented medium, and assessed at different time points by quantitative (immuno-)histomorphometry for selected relevant read-out parameters in epilated and sham-epilated control samples.

RESULTS

Epilation removed most of the hair shafts, often together with fragments of the outer and inner root sheath and hair matrix. This was associated with persistent focal thinning of the HF basal membrane, decreased melanin content of the residual HF epithelium, and increased HF keratinocyte apoptosis, including in the bulge, yet without affecting the number of cytokeratin 15+ HF epithelial stem cells. Sebocyte apoptosis in the peripheral zone was increased, albeit without visibly altering sebum production. Epilation transiently perturbed HF immune privilege, and increased the expression of ICAM-1 in the bulge and bulb mesenchyme, and the number of perifollicular MHC class II+ cells as well as mast cells around the distal epithelium and promoted mast cell degranulation around the suprabulbar and bulbar area. Moreover, compared to controls, several key players of neurogenic skin inflammation, itch, and/or thermosensation (TRPV1, TRPA1, NGF, and NKR1) were differentially expressed in post-epilation skin.

CONCLUSION

These data generated in denervated, organ-cultured human scalp skin demonstrate that epilation-induced mechanical HF trauma elicits surprisingly complex biological responses. These may contribute to the delayed re-growth of thinner and lighter hair shafts post-epilation and temporary post-epilation discomfort. Our findings also provide pointers regarding the development of topically applicable agents that minimize undesirable sequelae of epilation.

Current and emerging drugs for the treatment of pruritus: an update of the literature

INTRODUCTION

Pruritus, particularly in its chronic form, often imposes significant suffering and reductions in patients' quality of life. The pathophysiology of itch is varied depending on disease context, creating opportunities for unique drug development and multimodal therapy.

AREAS COVERED

The purpose of this article is to provide an update of the literature regarding current and emerging therapeutics in itch. We review the multitudes of drug targets available and corresponding drugs that have shown efficacy in clinical trials, with a particular emphasis on phase 2 and 3 trials and beyond. Broadly, these targets include therapies directed against type 2 inflammation (i.e. Th2 cytokines, JAK/STAT, lipid mediators, T-cell mediators, and other enzymes and receptors) and neural receptors and targets (i.e. PARs, TRP channels, opioid receptors, MRGPRs, GABA receptors, and cannabinoid receptors).

EXPERT OPINION

Therapeutics for itch are emerging at a remarkable pace, and we are entering an era with more and more specialized therapies. Increasingly, these treatments are able to relieve itch beyond their effect on inflammation by directly targeting the neurosensory system.

Molecular and cellular pruritus mechanisms in the host skin

Pruritus, also known as itching, is a complex sensation that involves the activation of specific physiological and cellular receptors. The skin is innervated with sensory nerves as well as some receptors for various sensations, and its immune system has prominent neurological connections. Sensory neurons have a considerable impact on the sensation of itching. However, immune cells also play a role in this process, as they release pruritogens. Disruption of the dermal barrier activates an immune response, initiating a series of chemical, physical, and cellular reactions. These reactions involve various cell types, including keratinocytes, as well as immune cells involved in innate and adaptive immunity. Collective activation of these immune responses confers protection against potential pathogens. Thus, understanding the molecular and cellular mechanisms that contribute to pruritus in host skin is crucial for the advancement of effective treatment approaches. This review provides a comprehensive analysis of the present knowledge concerning the molecular and cellular mechanisms underlying itching signaling in the skin. Additionally, this review explored the integration of these mechanisms with the broader context of itch mediators and the expression of their receptors in the skin.

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

AIMS

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.