A subpopulation of nociceptors specifically linked to itch

Mechanisms and treatments of neuropathic itch in a mouse model of lymphoma

Our understanding of neuropathic itch is limited due to a lack of relevant animal models. Patients with cutaneous T cell lymphoma (CTCL) experience severe itching. Here, we characterize a mouse model of chronic itch with remarkable lymphoma growth, immune cell accumulation, and persistent pruritus. Intradermal CTCL inoculation produced time-dependent changes in nerve innervations in lymphoma-bearing skin. In the early phase (20 days), CTCL caused hyperinnervations in the epidermis. However, chronic itch was associated with loss of epidermal nerve fibers in the late phases (40 and 60 days). CTCL was also characterized by marked nerve innervations in mouse lymphoma. Blockade of C-fibers reduced pruritus at early and late phases, whereas blockade of A-fibers only suppressed late-phase itch. Intrathecal (i.t.) gabapentin injection reduced late-phase, but not early-phase, pruritus. IL-31 was upregulated in mouse lymphoma, whereas its receptor Il31ra was persistently upregulated in Trpv1-expressing sensory neurons in mice with CTCL. Intratumoral anti-IL-31 treatment effectively suppressed CTCL-induced scratching and alloknesis (mechanical itch). Finally, i.t. administration of a TLR4 antagonist attenuated pruritus in early and late phases and in both sexes. Collectively, we have established a mouse model of neuropathic and cancer itch with relevance to human disease. Our findings also suggest distinct mechanisms underlying acute, chronic, and neuropathic itch.

Somatosensory neurons express specific sets of lincRNAs, and lincRNA CLAP promotes itch sensation in mice

Somatosensory neurons are highly heterogeneous with distinct types of neural cells responding to specific stimuli. However, the distribution and roles of cell-type-specific long intergenic noncoding RNAs (lincRNAs) in somatosensory neurons remain largely unexplored. Here, by utilizing droplet-based single-cell RNA-seq (scRNA-seq) and full-length Smart-seq2, we show that lincRNAs, but not coding mRNAs, are enriched in specific types of mouse somatosensory neurons. Profiling of lincRNAs from single neurons located in dorsal root ganglia (DRG) identifies 200 lincRNAs localized in specific types or subtypes of somatosensory neurons. Among them, the conserved cell-type-specific lincRNA CLAP associates with pruritus and is abundantly expressed in somatostatin (SST)-positive neurons. CLAP knockdown reduces histamine-induced Ca2+ influx in cultured SST-positive neurons and in vivo reduces histamine-induced scratching in mice. In vivo knockdown of CLAP also decreases the expression of neuron-type-specific and itch-related genes in somatosensory neurons, and this partially depends on the RNA binding protein MSI2. Our data reveal a cell-type-specific landscape of lincRNAs and a function for CLAP in somatosensory neurons in sensory transmission.

[From ocular itching to eye rubbing: a review of the literature]

Ocular itching and eye rubbing are frequent complaints in an ophthalmology practice. Numerous studies address the consequences of eye rubbing, such as keratoconus. However, there are few studies concerning the pathophysiology of itching, its transmission pathways, or its interactions with eye rubbing. Through this literature review, we will address the various clinical, physiological and therapeutic aspects of this pair of symptoms with a variety of ocular consequences. We will then describe the state of the art in itching and scratching in dermatology, in order to draw a parallel between these two vicious cycles. A better understanding of the pathophysiology of ocular itching and eye rubbing, as well as new studies based on dermatological data, might allow more appropriate clinical management of our patients and their symptoms.

The Delta-Opioid Receptor Bidirectionally Modulates Itch

Opioid signaling has been shown to be critically important in the neuromodulation of sensory circuits in the superficial spinal cord. Agonists of the mu-opioid receptor (MOR) elicit itch, whereas agonists of the kappa-opioid receptor (KOR) have been shown to inhibit itch. Despite the clear roles of MOR and KOR for the modulation itch, whether the delta-opioid receptor (DOR) is involved in the regulation of itch remained unknown. Here, we show that intrathecal administration of DOR agonists suppresses chemical itch and that intrathecal application of DOR antagonists is sufficient to evoke itch. We identify that spinal enkephalin neurons co-express neuropeptide Y (NPY), a peptide previously implicated in the inhibition of itch. In the spinal cord, DOR overlapped with both the NPY receptor (NPY1R) and KOR, suggesting that DOR neurons represent a site for convergent itch information in the dorsal horn. Lastly, we found that neurons co-expressing DOR and KOR showed significant Fos induction following pruritogen-evoked itch. These results uncover a role for DOR in the modulation of itch in the superficial dorsal horn. PERSPECTIVE: This article reveals the role of the delta-opioid receptor in itch. Intrathecal administration of delta agonists suppresses itch whereas the administration of delta antagonists is sufficient to induce itch. These studies highlight the importance of delta-opioid signaling for the modulation of itch behaviors, which may represent new targets for the management of itch disorders.

Dictamnine ameliorates chronic itch in DNFB-induced atopic dermatitis mice via inhibiting MrgprA3

Chronic itch is the most prominent feature of atopic dermatitis (AD), and antihistamine treatment is often less effective in reducing clinical pruritus severity in AD. Multiple studies have shown that histamine-independent itch pathway is thought to predominate in AD-induced chronic itch. Mas-related G-protein-coupled receptor (Mrgpr) A3⁺ sensory neurons have been identified as one of the major itch-sensing neuron populations, and transient receptor potential (TRP) channel A1 is the key downstream of MrgprA3-mediated histamine-independent itch. MrgprA3-TRPA1 signal pathway is necessary for the development of chronic itch and may be the potentially promising target of chronic itch in AD. Dictamnine is one of the main quinoline alkaloid components of Cortex Dictamni (a traditional Chinese medicine widely used in clinical treatment of skin diseases). However, the anti-inflammatory and anti-pruritic effect of dictamnine on AD have not been reported. In this study, we used the 2,4-dinitrofluorobenzene (DNFB)-induced AD mouse model to observe the scratching behavior, inflammatory manifestations, and to detect the expression of MrgprA3 and TRPA1 in skin and DRG. The data demonstrated that dictamnine effectively inhibited AD-induced chronic itch, inflammation symptoms, epidermal thickening, inflammatory cell infiltration, and downregulated the expression of MrgprA3 and TRPA1. Furthermore, dictamnine restrained the excitability of MrgprA3⁺ and TRPA1⁺ neurons. Molecular docking also indicated that dictamnine has better binding affinity with MrgprA3. These results suggest that dictamnine may inhibit chronic itch caused by AD through the MrgprA3-TRPA1 mediated histamine-independent itch pathway, and may have a potential utility in AD treatment.

The opposite roles of orexin neurons in pain and itch neural processing

Pain and itch are antagonistically regulated sensations; pain suppresses itch, and inhibition of pain enhances itch. Understanding the central neural circuit of antagonistic regulation between pain and itch is required to develop new therapeutics better to manage these two feelings in a clinical situation. However, evidence of the neural mechanism underlying the pain-itch interaction in the central nervous system (CNS) is still insufficient. To pave the way for this research area, our laboratory has focused on orexin (ORX) producing neurons in the hypothalamus, which is known as a master switch that induces various defense responses when animals face a stressful environment. This review article summarized the previous evidence and our latest findings to argue the neural regulation between pain and itch and the bidirectional roles of ORX neurons in processing these two sensations. i.e., pain relief and itch exacerbation. Further, we discussed the possible neural circuit mechanism for the opposite controlling of pain and itch by ORX neurons. Focusing on the roles of ORX neurons would provide a new perspective to understand the antagonistic regulation of pain and itch in CNS.

Acid-sensing ion channel 3 is required for agmatine-induced histamine-independent itch in mice

Introduction

Itch is a common symptom of many skin and systemic diseases. Identifying novel endogenous itch mediators and the downstream signaling pathways involved will contribute to the development of new strategies for the treatment of chronic itch. In the present study, we adopted behavioral testing, patch clamp recording and metabonomics analysis to investigate the role of agmatine in itch and the underlying mechanism.

Methods

Behavioral analysis was used to evaluate the establishing of acute and chronic itch mice model, and to test the effects of different drugs or agents on mice itch behavior. Western blotting analysis was used to test the effect of agmatine on phosphorylation of ERK (p-ERK) expression in the spinal cord. Patch clamp recording was used to determine the effect agmatine on the excitability of DRG neurons and the role of ASIC3. Finally, the metabonomics analysis was performed to detect the concentration of agmatine in the affected skin under atopic dermatitis or psoriasis conditions.

Results

We fused a mouse model and found that an intradermal injection of agmatine (an endogenous polyamine) into the nape of the neck or cheek induced histamine-independent scratching behavior in a dose-dependent manner. In addition, the ablation of nociceptive C-fibers by resiniferatoxin (RTX) abolished agmatine-induced scratching behavior. However, agmatine-induced itch was not affected by the pharmacological inhibition of either transient receptor potential vanilloid 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1); similar results were obtained from TRPV1^-/- or TRPA1^-/- mice. Furthermore, agmatine-induced itch was significantly suppressed by the administration of acid-sensing ion channel 3 (ASIC3) inhibitors, APETx2 or amiloride. Agmatine also induced the upregulation of p-ERK in the spinal cord; this effect was inhibited by amiloride. Current clamp recording showed that the acute perfusion of agmatine reduced the rheobase and increased the number of evoked action potentials in acute dissociated dorsal root ganglion (DRG) neurons while amiloride reversed agmatine-induced neuronal hyperexcitability. Finally, we identified significantly higher levels of agmatine in the affected skin of a mouse model of atopic dermatitis (AD) when compared to controls, and the scratching behavior of AD mice was significantly attenuated by blocking ASIC3.

Discussion

Collectively, these results provide evidence that agmatine is a novel mediator of itch and induces itch via the activation of ASIC3. Targeting neuronal ASIC3 signaling may represent a novel strategy for the treatment of itch.

Sensory neuron-expressed TRPC3 mediates acute and chronic itch

ABSTRACT

Chronic pruritus is a prominent symptom of allergic contact dermatitis (ACD) and represents a huge unmet health problem. However, its underlying cellular and molecular mechanisms remain largely unexplored. TRPC3 is highly expressed in primary sensory neurons and has been implicated in peripheral sensitization induced by proinflammatory mediators. Yet, the role of TRPC3 in acute and chronic itch is still not well defined. Here, we show that, among mouse trigeminal ganglion (TG) neurons, Trpc3 mRNA is predominantly expressed in nonpeptidergic small diameter TG neurons of mice. Moreover, Trpc3 mRNA signal was present in most presumptively itch sensing neurons. TRPC3 agonism induced TG neuronal activation and acute nonhistaminergic itch-like and pain-like behaviors in naive mice. In addition, genetic deletion of Trpc3 attenuated acute itch evoked by certain common nonhistaminergic pruritogens, including endothelin-1 and SLIGRL-NH2. In a murine model of contact hypersensitivity (CHS), the Trpc3 mRNA expression level and function were upregulated in the TG after CHS. Pharmacological inhibition and global knockout of Trpc3 significantly alleviated spontaneous scratching behaviors without affecting concurrent cutaneous inflammation in the CHS model. Furthermore, conditional deletion of Trpc3 in primary sensory neurons but not in keratinocytes produced similar antipruritic effects in this model. These findings suggest that TRPC3 expressed in primary sensory neurons may contribute to acute and chronic itch through a histamine independent mechanism and that targeting neuronal TRPC3 might benefit the treatment of chronic itch associated with ACD and other inflammatory skin disorders.

Intensity-adjustable pain management with prolonged duration based on phase-transitional nanoparticles-assisted ultrasound imaging-guided nerve blockade

Background

The lack of a satisfactory strategy for postoperative pain management significantly impairs the quality of life for many patients. However, existing nanoplatforms cannot provide a longer duration of nerve blockage with intensity-adjustable characteristics under imaging guidance for clinical applications.

Results

To overcome this challenge, we proposed a biocompatible nanoplatform that enables high-definition ultrasound imaging-guided, intensity-adjustable, and long-lasting analgesia in a postoperative pain management model in awake mice. The nanoplatform was constructed by incorporating perfluoropentane and levobupivacaine with red blood cell membranes decorated liposomes. The fabricated nanoplatform can achieve gas-producing and can finely escape from immune surveillance in vivo to maximize the anesthetic effect. The analgesia effect was assessed from both motor reactions and pain-related histological markers. The findings demonstrated that the duration of intensity-adjustable analgesia in our platform is more than 20 times longer than free levobupivacaine injection with pain relief for around 3 days straight. Moreover, the pain relief was strengthened by repeatable ultrasound irradiation to effectively manage postoperative pain in an intensity-adjustable manner. No apparent systemic and local tissue injury was detected under different treatments.

Conclusion

Our results suggest that nanoplatform can provide an effective strategy for ultrasound imaging-guided intensity-adjustable pain management with prolonged analgesia duration and show considerable transformation prospects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01707-z.

Evaluation and Management of Pruritus in Primary Biliary Cholangitis

Chronic pruritus is a classic symptom in patients with primary biliary cholangitis. It affects up to two-thirds of patients in the course of the disease. Efficient therapy consists of topical treatment combined with systemic options such as anion exchangers, rifampicin, bezafibrate, μ-opioid receptor antagonists, selective-serotonin receptor uptake inhibitors, and gabapentinoids. Future therapeutic approaches may contain the selective blockade of the enterohepatic cycle by inhibiting the ileal bile acid transporter, the agonism at κ-opioid receptors, and antagonism of the mas-related G protein-coupled receptor X4. As nondrug treatment, ultraviolet B therapy, albumin dialysis, and biliary drainage are available at specialized centers.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Neurobiology, Stem Cell Biology, and Immunology: An Emerging Triad for Understanding Tissue Homeostasis and Repair

The peripheral nervous system (PNS) endows animals with the remarkable ability to sense and respond to a dynamic world. Emerging evidence shows the PNS also participates in tissue homeostasis and repair by integrating local changes with organismal and environmental changes. Here, we provide an in-depth summary of findings delineating the diverse roles of peripheral nerves in modulating stem cell behaviors and immune responses under steady-state conditions and in response to injury and duress, with a specific focus on the skin and the hematopoietic system. These examples showcase how elucidating neuro-stem cell and neuro-immune cell interactions provides a conceptual framework that connects tissue biology and local immunity with systemic bodily changes to meet varying demands. They also demonstrate how changes in these interactions can manifest in stress, aging, cancer, and inflammation, as well as how these findings can be harnessed to guide the development of new therapeutics.

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.

Hypothalamic orexinergic neurons modulate pain and itch in an opposite way: pain relief and itch exacerbation

Pain and itch are recognized as antagonistic sensations; pain suppresses itch and inhibition of pain generates itch. There is still a lack of evidence about the neural mechanism of the interaction between pain and itch in the central nervous system. In this study, we focused on the orexin (ORX) neurons in the lateral hypothalamus (LH), which mediate various "defense responses" when animals confront stressors. We found that the scratching behaviors induced by the pruritogen were significantly suppressed in ORX-neuron-ablated (ORX-abl) mice. The exaggerated pain behavior and attenuated itch behavior observed in ORX-abl mice indicated that ORX neurons modulate pain and itch in an opposite way, i.e., pain relief and itch exacerbation. In addition, most of the ORX neurons responded to both pain and itch input. Our results suggest that ORX neurons inversely regulate pain- and itch-related behaviors, which could be understood as a defense response to cope with stress environment.

Distinct neural networks derived from galanin-containing nociceptors and neurotensin-expressing pruriceptors

Pain and itch are distinct sensations arousing evasion and compulsive desire for scratching, respectively. It's unclear whether they could invoke different neural networks in the brain. Here, we use the type 1 herpes simplex virus H129 strain to trace the neural networks derived from two types of dorsal root ganglia (DRG) neurons: one kind of polymodal nociceptors containing galanin (Gal) and one type of pruriceptors expressing neurotensin (Nts). The DRG microinjection and immunosuppression were performed in transgenic mice to achieve a successful tracing from specific types of DRG neurons to the primary sensory cortex. About one-third of nuclei in the brain were labeled. More than half of them were differentially labeled in two networks. For the ascending pathways, the spinothalamic tract was absent in the network derived from Nts-expressing pruriceptors, and the two networks shared the spinobulbar projections but occupied different subnuclei. As to the motor systems, more neurons in the primary motor cortex and red nucleus of the somatic motor system participated in the Gal-containing nociceptor-derived network, while more neurons in the nucleus of the solitary tract (NST) and the dorsal motor nucleus of vagus nerve (DMX) of the emotional motor system was found in the Nts-expressing pruriceptor-derived network. Functional validation of differentially labeled nuclei by c-Fos test and chemogenetic inhibition suggested the red nucleus in facilitating the response to noxious heat and the NST/DMX in regulating the histamine-induced scratching. Thus, we reveal the organization of neural networks in a DRG neuron type-dependent manner for processing pain and itch.

Itch-specific neurons in the ventrolateral orbital cortex selectively modulate the itch processing

Itch is a cutaneous sensation that is critical in driving scratching behavior. The long-standing question of whether there are specific neurons for itch modulation inside the brain remains unanswered. Here, we report a subpopulation of itch-specific neurons in the ventrolateral orbital cortex (VLO) that is distinct from the pain-related neurons. Using a Tet-Off cellular labeling system, we showed that local inhibition or activation of these itch-specific neurons in the VLO significantly suppressed or enhanced itch-induced scratching, respectively, whereas the intervention did not significantly affect pain. Conversely, suppression or activation of pain-specific neurons in the VLO significantly affected pain but not itch. Moreover, fiber photometry and immunofluorescence verified that these itch- and pain-specific neurons are distinct in their functional activity and histological location. In addition, the downstream targets of itch- and pain-specific neurons were different. Together, the present study uncovers an important subpopulation of neurons in the VLO that specifically modulates itch processing.

Meis1 Regulates Nociceptor Development and Behavioral Response to Tactile Stimuli

Nociceptors in the dorsal root ganglia (DRG) and trigeminal ganglia (TG) are necessary for transmitting pain and itch signals. However, the molecular mechanism regulating nociceptor development remains largely unknown. This study identifies that the transcription factor Meis1 is generally expressed in two groups of sensory neurons in the developing DRG. During prenatal and neonatal stages, approximately 2/3 of Meis1+ neurons are Runx1+ nociceptors, while 1/3 of Meis1+ neurons are NF200+ myelinated neurons. At postnatal stages, Meis1 expression in nociceptors is gradually reduced. Here, we constructed a Meis1 conditional knockout mouse line to selectively delete Meis1 in Nav1.8 lineage nociceptors. Microarray analyses showed that differentially expressed genes in the Meis1 mutant DRG were enriched in pathways related to sensory perception of pain and nervous system development. In addition, Meis1 regulates the expression of some marker genes of Nppb+ neurons and C-LTMRs. Furthermore, Meis1 mutant mice exhibit behavioral deficits in response to light mechanical pain, static touch and chemical itch. Therefore, this study reveals that Meis1 is required to regulate the development of nociceptors.

Druggable Targets and Compounds with Both Antinociceptive and Antipruritic Effects

Pain and itch are both important manifestations of various disorders, such as herpes zoster, atopic dermatitis, and psoriasis. Growing evidence suggests that both sensations have shared mediators, overlapping neural circuitry, and similarities in sensitization processes. In fact, pain and itch coexist in some disorders. Determining pharmaceutical agents and targets for treating pain and itch concurrently is of scientific and clinical relevance. Here we review the neurobiology of pain and itch and discuss the pharmaceutical targets as well as novel compounds effective for the concurrent treatment of these sensations.

Miswiring of Merkel cell and pruriceptive C fiber drives the itch-scratch cycle

Itch sensation provokes the scratch reflex to protect us from harmful stimuli in the skin. Although scratching transiently relieves acute itch through activation of mechanoreceptors, it propagates the vicious itch-scratch cycle in chronic itch by further aggravating itch over time. Although well recognized clinically, the peripheral mechanisms underlying the itch-scratch cycle remain poorly understood. Here, we show that mechanical stimulation of the skin results in activation of the Piezo2 channels on Merkel cells that pathologically promotes spontaneous itch in experimental dry skin. Three-dimensional reconstruction and immunoelectron microscopy revealed structural alteration of MRGPRA3⁺ pruriceptor nerve endings directed toward Merkel cells in the setting of dry skin. Our results uncover a functional miswiring mechanism under pathologic conditions, resulting in touch receptors triggering the firing of pruriceptors in the skin to drive the itch-scratch cycle.

GPR15L is an epithelial inflammation-derived pruritogen

Itch is an unpleasant sensation that often accompanies chronic dermatological conditions. Although many of the itch receptors and the neural pathways underlying this sensation are known, the identity of endogenous ligands is still not fully appreciated. Using an unbiased bioinformatic approach, we identified GPR15L as a candidate pruritogen whose expression is robustly up-regulated in psoriasis and atopic dermatitis. Although GPR15L was previously shown to be a cognate ligand of the receptor GPR15, expressed in dermal T cells, here we show that it also contributes to pruritogenesis by activating Mas-related G protein-coupled receptors (MRGPRs). GPR15L can selectively stimulate mouse dorsal root ganglion neurons that express Mrgpra3 and evokes intense itch responses. GPR15L causes mast cell degranulation through stimulation of MRGPRX2 and Mrgprb2. Genetic disruption of GPR15L expression attenuates scratch responses in a mouse model of psoriasis. Our study reveals unrecognized features of GRP15L, showing that it is a potent itch-inducing agent.

Mechanisms of bone pain: Progress in research from bench to bedside

The field of research on pain originating from various bone diseases is expanding rapidly, with new mechanisms and targets asserting both peripheral and central sites of action. The scope of research is broadening from bone biology to neuroscience, neuroendocrinology, and immunology. In particular, the roles of primary sensory neurons and non-neuronal cells in the peripheral tissues as important targets for bone pain treatment are under extensive investigation in both pre-clinical and clinical settings. An understanding of the peripheral mechanisms underlying pain conditions associated with various bone diseases will aid in the appropriate application and development of optimal strategies for not only managing bone pain symptoms but also improving bone repairing and remodeling, which potentially cures the underlying etiology for long-term functional recovery. In this review, we focus on advances in important preclinical studies of significant bone pain conditions in the past 5 years that indicated new peripheral neuronal and non-neuronal mechanisms, novel targets for potential clinical interventions, and future directions of research.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

Topical Capsaicin in Poly(lactic-co-glycolic)acid (PLGA) Nanoparticles Decreases Acute Itch and Heat Pain

BACKGROUND

Capsaicin, the hot pepper agent, produces burning followed by desensitization. To treat localized itch or pain with minimal burning, low capsaicin concentrations can be repeatedly applied. We hypothesized that alternatively controlled release of capsaicin from poly(lactic-co-glycolic acid) (PLGA) nanoparticles desensitizes superficially terminating nociceptors, reducing burning.

METHODS

Capsaicin-loaded PLGA nanoparticles were prepared (single-emulsion solvent evaporation) and characterized (size, morphology, capsaicin loading, encapsulation efficiency, in vitro release profile). Capsaicin-PLGA nanoparticles were applied to murine skin and evaluated in healthy human participants (n = 21) for 4 days under blinded conditions, and itch and nociceptive sensations evoked by mechanical, heat stimuli and pruritogens cowhage, β-alanine, BAM8-22 and histamine were evaluated.

RESULTS

Nanoparticles (loading: 58 µg capsaicin/mg) released in vitro 23% capsaicin within the first hour and had complete release at 72 h. In mice, 24 h post-application Capsaicin-PLGA nanoparticles penetrated the dermis and led to decreased nociceptive behavioral responses to heat and mechanical stimulation (desensitization). Application in humans produced a weak to moderate burning, dissipating after 3 h. A loss of heat pain up to 2 weeks was observed. After capsaicin nanoparticles, itch and nociceptive sensations were reduced in response to pruritogens cowhage, β-alanine or BAM8-22, but were normal to histamine.

CONCLUSIONS

Capsaicin nanoparticles could be useful in reducing pain and itch associated with pruritic diseases that are histamine-independent.

A novel spinal neuron connection for heat sensation

Heat perception enables acute avoidance responses to prevent tissue damage and maintain body thermal homeostasis. Unlike other modalities, how heat signals are processed in the spinal cord remains unclear. By single-cell gene profiling, we identified ErbB4, a transmembrane tyrosine kinase, as a novel marker of heat-sensitive spinal neurons in mice. Ablating spinal ErbB4+ neurons attenuates heat sensation. These neurons receive monosynaptic inputs from TRPV1+ nociceptors and form excitatory synapses onto target neurons. Activation of ErbB4+ neurons enhances the heat response, while inhibition reduces the heat response. We showed that heat sensation is regulated by NRG1, an activator of ErbB4, and it involves dynamic activity of the tyrosine kinase that promotes glutamatergic transmission. Evidence indicates that the NRG1-ErbB4 signaling is also engaged in hypersensitivity of pathological pain. Together, these results identify a spinal neuron connection consisting of ErbB4+ neurons for heat sensation and reveal a regulatory mechanism by the NRG1-ErbB4 signaling.

The role of PTEN in primary sensory neurons in processing itch and thermal information in mice

PTEN is known as a tumor suppressor and plays essential roles in brain development. Here, we report that PTEN in primary sensory neurons is involved in processing itch and thermal information in adult mice. Deletion of PTEN in the dorsal root ganglia (DRG) is achieved in adult Drg11-Cre^ER: PTEN^flox/flox (PTEN CKO) mice with oral administration of tamoxifen, and CKO mice develop pathological itch and elevated itch responses on exposure to various pruritogens. PTEN deletion leads to ectopic expression of TRPV1 and MrgprA3 in IB4⁺ non-peptidergic DRG neurons, and the TRPV1 is responsive to capsaicin. Importantly, the elevated itch responses are no longer present in Drg11-Cre^ER: PTEN^flox/flox: TRPV1^flox/flox (PTEN: TRPV1 dCKO) mice. In addition, thermal stimulation is enhanced in PTEN CKO mice but blunted in dCKO mice. PTEN-involved regulation of itch-related gene expression in DRG neurons provides insights for understanding molecular mechanism of itch and thermal sensation at the spinal level.

Bridging skin, brain, and behavior to understand pleasurable social touch

Social touch-the affiliative skin-to-skin contact between individuals-can rapidly evoke emotions of comfort, pleasure, or calm, and is essential for mental and physical well-being. Physical isolation from social support can be devastating. During the COVID-19 pandemic, we observed a global increase in suicidal ideation, anxiety, domestic violence, and worsening of pre-existing physical conditions, alerting society to our need to understand the neurobiology of social touch and how it promotes normal health. Gaining a mechanistic understanding of how sensory neuron stimulation induces pleasure, calm, and analgesia may reveal untapped therapeutic targets in the periphery for treatment of anxiety and depression, as well as social disorders and traumas in which social touch becomes aversive. Bridging the gap between stimulation in the skin and positive affect in the brain-especially during naturally occurring social touch behaviors-remains a challenge to the field. However, with advances in mouse genetics, behavioral quantification, and brain imaging approaches to measure neuronal firing and neurochemical release, completing this mechanistic picture may be on the horizon. Here, we summarize some exciting new findings about social touch in mammals, emphasizing both the peripheral and central nervous systems, with attempts to bridge the gap between external stimulation and internal representations in the brain.

Punctate Administration of Ficin as a Human and Animal Model of Non-Histaminergic Itch

BACKGROUND

Ficin, a cysteine protease derived from fig-tree latex, has been reported to elicit itch and nociceptive sensations, though quantitative sensory studies are lacking. Cowhage containing the pruritic cysteine Mucunain, on the other hand, has been widely studied as activating polymodal nociceptors and eliciting a histamine-independent itch.

OBJECTIVES

We tested whether ficin in heat-inactivated cowhage spicules would elicit itch and nociceptive sensations in humans, and analogous behaviors in mice, that are similar to those evoked by native cowhage and, whether these behaviors in mice were dose-dependent when ficin was injected intradermally.

METHODS

Human volunteers rated the magnitude of itch and nociceptive sensations evoked by either native cowhage spicules or heat-inactivated spicules soaked in 1, 10 or 100 mg/ml ficin (0.03, 0.3, and 3 ng of ficin in spicule tip), applied to forearm. In mice, itch-like scratching and nociceptive-like wiping were recorded in response to either native cowhage, to heat-inactivated spicules that were either inactive or contained 100 mg/ml ficin, or to intradermal injections of 1.25, 2.5, or 5 μg/ 5 μl, each treatment applied to the cheek.

RESULTS

The dose of 100 mg/ml ficin in spicules evoked comparable magnitudes of itch, nociceptive sensations and areas of cutaneous dysesthesia as native cowhage in humans and comparable itch-like scratching and pain-like wiping behaviors in mice. But to elicit similar behaviors when injected intradermally in mice a greater amount of ficin (1.25 μg) was required.

CONCLUSION

Spicule-delivery or intradermal injection of ficin elicits behaviors in mice that model itch and nociceptive sensations in humans, suggesting that ficin may be useful in translating mechanistic research on the neural mechanisms of pruritic and nociceptive effects of cysteine proteases between the two species.

Analgesic effect of gastrin-releasing peptide in the dorsal horn

Itch and pain are both unpleasant, but they are discrete sensations. Both of these sensations are transmitted by C-fibers and processed in laminae I-II of the dorsal horn. To examine whether pruriception modulates pain, we first confirmed the activation of cells in the itch-related circuits that were positive for gastrin-releasing peptide (GRP) and GRP receptor (GRPR) using a paw formalin injection model. This pain model with typical biphasic pain behavior increased c-Fos but did not affect the expressions of GRP and GRPR mRNAs in the dorsal horn. Using c-Fos expression as a marker for activated cells, we confirmed that formalin injection increased the number of cells double-labeled for c-Fos and GRP or GRPR in the dorsal horn. The emergence of these neurons indicates the activation of itch-related circuits by acute pain signals. The effect of an antagonist for a GRPR was examined in the paw formalin injection model. Intrathecal chronic antagonization of spinal GRPR enhanced the onset of phase II of paw formalin injection-induced pain behavior. Exogenous intrathecal GRP infusion to the paw-formalin injection model not only showed significant reduction of pain behavior but also increased c-Fos in the inhibitory neurons in the dorsal horn. The anti-nociceptive effect of spinal GRP infusion was observed in the peripheral inflammation model (complete Freund’s adjuvant injection model). In this study we suggest that painful stimuli activated itch-related neuronal circuits and uncovered the spinal activation of the itch-induced analgesic effect on acute and established inflammatory pain.

Nociceptors: Their Role in Body’s Defenses, Tissue Specific Variations and Anatomical Update

The human body is constantly under the influence of numerous pathological factors: both external and internal. These factors can be potentially harmful and are perceived as such with a specialized nervous system subunit: the nociceptive system. The functional unit of the nociceptive system is the nociceptor. Recent studies have shown that nociceptors play a crucial role in maintaining of defensive homeostasis (responsive, immune, behavioral). Nociceptors respond to potentially harmful stimuli within viscera, bones, muscles, skin and specialized sensory organs. They function as complex predictors of harm through formation of pain stimulus. Their function and structures vary within different tissues. This variability reflects the anatomical and pathological peculiarities of varying tissues. Nociceptors play a significant role in adaptive, protective and behavioral reactions. Their functional capabilities and vast spread throughout the body make them the main units of the body’s defense system, allowing us to interact with the inner and outer environments.

Acupuncture for the Treatment of Itch: Peripheral and Central Mechanisms

Despite the widespread clinical use of acupuncture in the treatment of pruritus caused by psoriasis, urticaria, uremic, and other diseases, insights into the mechanism of action of acupuncture are still emerging. For the above reasons, a beneficial effect of acupuncture on pruritus was not recommended or reported in recent clinical practice guidelines. Acupuncture is a kind of physical stimulation, which has the characteristics of multi-channel and multi-target effects. The biomechanical stimulation signal of acupuncture needling can be transformed into bioelectric and chemical signals; interfere with kinds of cells and nerve fibers in the skin and muscle; alter signaling pathways and transcriptional activity of cells, mediators, and receptors; and result in inhibition of peripheral and central transmission of pruritus. Available mechanistic data give insights into the biological regulation potency of acupuncture for pruritus and provide a basis for more in-depth and comprehensive mechanism research.

Mitochondrial Reactive Oxygen Species Elicit Acute and Chronic Itch via Transient Receptor Potential Canonical 3 Activation in Mice

Mitochondrial reactive oxygen species (mROS) that are overproduced by mitochondrial dysfunction are linked to pathological conditions including sensory abnormalities. Here, we explored whether mROS overproduction induces itch through transient receptor potential canonical 3 (TRPC3), which is sensitive to ROS. Intradermal injection of antimycin A (AA), a selective inhibitor of mitochondrial electron transport chain complex III for mROS overproduction, produced robust scratching behavior in naïve mice, which was suppressed by MitoTEMPO, a mitochondria-selective ROS scavenger, and Pyr10, a TRPC3-specific blocker, but not by blockers of TRPA1 or TRPV1. AA activated subsets of trigeminal ganglion neurons and also induced inward currents, which were blocked by MitoTEMPO and Pyr10. Besides, dry skin-induced chronic scratching was relieved by MitoTEMPO and Pyr10, and also by resveratrol, an antioxidant. Taken together, our results suggest that mROS elicit itch through TRPC3, which may underlie chronic itch, representing a potential therapeutic target for chronic itch.

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.

Glutamate in primary afferents is required for itch transmission

Whether glutamate or itch-selective neurotransmitters are used to confer itch specificity is still under debate. We focused on an itch-selective population of primary afferents expressing MRGPRA3, which highly expresses Vglut2 and the neuropeptide neuromedin B (Nmb), to investigate this question. Optogenetic stimulation of MRGPRA3⁺ afferents triggers scratching and other itch-related avoidance behaviors. Using a combination of optogenetics, spinal cord slice recordings, Vglut2 conditional knockout mice, and behavior assays, we showed that glutamate is essential for MRGPRA3⁺ afferents to transmit itch. We further demonstrated that MRGPRA3⁺ afferents form monosynaptic connections with both NMBR⁺ and NMBR^- neurons and that NMB and glutamate together can enhance the activity of NMBR⁺ spinal DH neurons. Moreover, Nmb in MRGPRA3⁺ afferents and NMBR⁺ DH neurons are required for chloroquine-induced scratching. Together, our results establish a new model in which glutamate is an essential neurotransmitter in primary afferents for itch transmission, whereas NMB signaling enhances its activities.

Neuronal FcεRIα directly mediates ocular itch via IgE-immune complex in a mouse model of allergic conjunctivitis

Background

Classical understanding of allergic conjunctivitis (ACJ) suggests that ocular itch results from a mast cell-dependent inflammatory process. However, treatments that target inflammatory mediators or immune cells are often unsatisfying in relieving the stubborn itch symptom. This suggests that additional mechanisms are responsible for ocular itch in ACJ. In this study, we aim to determine the role of neuronal FcεRIa in allergic ocular itch.

Methods

Calcium imaging was applied to observe the effect of IgE-immune complex in trigeminal neurons. Genomic FcεRIa knockout mice and adeno-associated virus (AAV) mediated sensory neuron FcεRIa knockdown mice were used in conjunction with behavioral tests to determine ocular itch. In addition, immunohistochemistry, Western blot and quantitative RT-PCR were used for in vitro experiments.

Results

We found that FcεRIα was expressed in a subpopulation of conjunctiva sensory neurons. IgE-IC directly activated trigeminal neurons and evoked acute ocular itch without detectible conjunctival inflammation. These effects were attenuated in both a global FcεRIa-knockout mice and after sensory neuronal-specific FcεRIa-knockdown in the mouse trigeminal ganglion. In an ovalbumin (OVA) induced murine ACJ model, FcεRIα was found upregulated in conjunctiva-innervating CGRP+ sensory neurons. Sensory neuronal-specific knockdown of FcεRIa significantly alleviated ocular itch in the ACJ mice without affecting the immune cell infiltration and mast cell activation in conjunctiva. Although FcεRIα mRNA expression was not increased by IgE in dissociated trigeminal ganglion neurons, FcεRIα protein level was enhanced by IgE in a cycloheximide-resistance manner, with concordant enhancement of neuronal responses to IgE-IC. In addition, incremental sensitization gradually enhanced the expression of FcεRIα in small-sized trigeminal neurons and aggravated OVA induced ocular itch.

Conclusions

Our study demonstrates that FcεRIα in pruriceptive neurons directly mediates IgE-IC evoked itch and plays an important role in ocular itch in a mouse model of ACJ. These findings reveal another axis of neuroimmune interaction in allergic itch condition independent to the classical IgE-mast cell pathway, and might suggest novel therapeutic strategies for the treatment of pruritus in ACJ and other immune-related disorders.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02417-x.

TRPV1 in Pain and Itch

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel that is intensively expressed in the peripheral nerve system and involved in a variety of physiological and pathophysiological processes in mammals. Its activity is of great significance in transmitting pain or itch signals from peripheral sensory neurons to the central nervous system. The alteration or hypersensitivity of TRPV1 channel is well evidenced under various pathological conditions. Moreover, accumulative studies have revealed that TRPV1-expressing (TRPV1⁺) sensory neurons mediate the neuroimmune crosstalk by releasing neuropeptides to innervated tissues as well as immune cells. In the central projection, TRPV1⁺ terminals synapse with the secondary neurons for the transmission of pain and itch signalling. The intense involvement of TRPV1 and TRPV1⁺ neurons in pain and itch makes it a potential pharmaceutical target. Over decades, the basis of TRPV1 channel structure, the nature of its activity, and its modulation in pathological processes have been broadly studied and well documented. Herein, we highlight the role of TRPV1 and its associated neurons in sensing pain and itch. The fundamental understandings of TRPV1-involved nociception, pruriception, neurogenic inflammation, and cell-specific modulation will help bring out more effective strategies of TRPV1 modulation in treating pain- and itch-related diseases.

A compendium of validated pain genes

The development of novel pain therapeutics hinges on the identification and rigorous validation of potential targets. Model organisms provide a means to test the involvement of specific genes and regulatory elements in pain. Here we provide a list of genes linked to pain-associated behaviors. We capitalize on results spanning over three decades to identify a set of 242 genes. They support a remarkable diversity of functions spanning action potential propagation, immune response, GPCR signaling, enzymatic catalysis, nucleic acid regulation, and intercellular signaling. Making use of existing tissue and single-cell high-throughput RNA sequencing datasets, we examine their patterns of expression. For each gene class, we discuss archetypal members, with an emphasis on opportunities for additional experimentation. Finally, we discuss how powerful and increasingly ubiquitous forward genetic screening approaches could be used to improve our ability to identify pain genes. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Molecular and Cellular Physiology.

Mas-Related G Protein-Coupled Receptors (Mrgprs) as Mediators of Gut Neuro-Immune Signaling

Over the past 15 years, the research field on Mas-related G protein-coupled receptors (Mrgprs), a relatively new family of rhodopsin A-like G protein-coupled receptors, has expanded enormously, and a plethora of recent studies have provided evidence that several of these Mrgpr family members play an important role in the underlying mechanisms of itch and pain, as well as in the initiation and modulation of inflammatory/allergic responses. Initial studies mainly focused on the skin, but more recently also visceral organs such as the respiratory and gastrointestinal (GI) tracts emerged as sites for Mrgpr involvement. It has become clear that the gastrointestinal tract and its innervation in close association with the immune system represent a novel expression site for Mrgprs where they contribute to the interoceptive mechanisms maintaining homeostasis and might constitute promising targets in chronic abdominal pain disorders. In this short review, we provide an update of our current knowledge on the expression, distribution, and function of members of this Mrgpr family in intrinsic and extrinsic neuro-immune pathways related to the gastrointestinal tract, their mediatory role(s) in gut neuro-immune signaling, and their involvement in visceral afferent (nociceptive) pathways.

Molecular Genetics of Kappa Opioids in Pain and Itch Sensations

The opioid peptides and their receptors have been linked to multiple key biological processes in the nervous system. Here we review the functions of the kappa opioid receptor (KOR) and its endogenous agonists dynorphins (Goldstein A, Tachibana S, Lowney LI, Hunkapiller M, Hood L, Proc Natl Acad Sci U S A 76:6666-6670, 1979) in modulating itch and pain (nociception). Specifically, we discuss their roles relative to recent findings that tell us more about the cells and circuits which are impacted by this opioid and its receptor and present reanalysis of single-cell sequencing data showing the expression profiles of these molecules. Since the KOR is relatively specifically activated by peptides derived from the prodynorphin gene and other opioid peptides that show lower affinities, this will be the only interactions we consider (Chavkin C, Goldstein A, Nature 291:591-593, 1981; Chavkin C, James IF, Goldstein A, Science 215:413-415, 1982), although it was noted that at higher doses peptides other than dynorphins might stimulate KOR (Lai J, Luo MC, Chen Q, Ma S, Gardell LR, Ossipov MH, Porreca F, Nat Neurosci 9:1534-1540, 2006). This review has been organized based on anatomy with each section describing the effect of the kappa opioid system in a specific location but let us not forget that most of these circuits are interconnected and are therefore interdependent.

Interactions of the Neuro‒Immune‒Stromal Triad in Itch

This review focuses on recent advances in understanding the mechanisms involved in itch signaling in the skin and how these new findings fit into the wider picture of the expression of itch mediators and their receptors in the dermal layer. Because at present studies mostly concentrate on single cellular compartments (e.g., neural alone), we suggest that they may miss important interactions with other compartments. Therefore, to fully appreciate pruritus, we propose that studies should consider (e.g., using transcriptomic information) signal transmission within the entire neuro‒immune‒stromal triad.

Spinal glial cells in itch modulation

Glial cells are non-neuronal cells in the nervous system that are electrically non-excitable and outnumber neurons in humans. Glial cells have attracted attention in recent years for their active involvement in the regulation of neuronal activity, suggesting their contribution to the pathogenesis and progression of neurological diseases. Studies have shown that astrocytes, a type of glial cell, are activated in the spinal cord in response to skin inflammation and contribute to the exacerbation of chronic itch. This review summarizes the current knowledge about the role of astrocytes and other glial cells in the modulation of itch processing and the mechanism of their activation under itch conditions.

Peripheral Mechanisms of Itch

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

Single-nucleus transcriptomic analysis of human dorsal root ganglion neurons

Somatosensory neurons with cell bodies in the dorsal root ganglia (DRG) project to the skin, muscles, bones, and viscera to detect touch and temperature as well as to mediate proprioception and many types of interoception. In addition, the somatosensory system conveys the clinically relevant noxious sensations of pain and itch. Here, we used single nuclear transcriptomics to characterize transcriptomic classes of human DRG neurons that detect these diverse types of stimuli. Notably, multiple types of human DRG neurons have transcriptomic features that resemble their mouse counterparts although expression of genes considered important for sensory function often differed between species. More unexpectedly, we identified several transcriptomic classes with no clear equivalent in the other species. This dataset should serve as a valuable resource for the community, for example as means of focusing translational efforts on molecules with conserved expression across species.

The interaction between the immune system and the peripheral sensory nerves in pruritus

Pruritus is a skin-specific sensation that is observed in various skin diseases, especially in inflammatory skin diseases such as atopic dermatitis, and is deeply involved in their pathogenesis. Pruritus also adversely affects patients' sleep and mental health, placing a heavy burden on daily life. As such, pruritus control is important to the maintenance of health. The mechanism of pruritus has recently been clarified and the discovery of various pruritus mediators, the identification of specific nerves that transmit pruritus and the accumulation of knowledge on pruritus perception have led to a better understanding of all aspects of pruritus generation, transmission and recognition. In the case of pruritus caused by dermatitis, immune cells infiltrating the skin secrete inflammatory cytokines, which also act on peripheral nerves as pruritus mediators and induce an inflammatory response. Interestingly, there has been accumulating evidence that peripheral nerves are also involved in the inflammation via neuropeptides. In this article, we summarize the findings on pruritus mediators secreted by immune cells and the roles of peripheral nerves in pruritus in terms of their interactions with immunity.

Oncostatin M can sensitize sensory neurons in inflammatory pruritus

[Figure: see text].

Neuron‒Mast Cell Cross-Talk in the Skin

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

Patch-seq of mouse DRG neurons reveals candidate genes for specific mechanosensory functions

A variety of mechanosensory neurons are involved in touch, proprioception, and pain. Many molecular components of the mechanotransduction machinery subserving these sensory modalities remain to be discovered. Here, we combine recordings of mechanosensitive (MS) currents in mechanosensory neurons with single-cell RNA sequencing. Transcriptional profiles are mapped onto previously identified sensory neuron types to identify cell-type correlates between datasets. Correlation of current signatures with single-cell transcriptomes provides a one-to-one correspondence between mechanoelectric properties and transcriptomically defined neuronal populations. Moreover, a gene-expression differential comparison provides a set of candidate genes for mechanotransduction complexes. Piezo2 is expectedly found to be enriched in rapidly adapting MS current-expressing neurons, whereas Tmem120a and Tmem150c, thought to mediate slow-type MS currents, are uniformly expressed in all mechanosensory neuron subtypes. Further knockdown experiments disqualify them as mediating MS currents in sensory neurons. This dataset constitutes an open resource to explore further the cell-type-specific determinants of mechanosensory properties.

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.

TRPC3 Antagonizes Pruritus in a Mouse Contact Dermatitis Model

Contact dermatitis (CD), including allergic and irritant CD, are common dermatological diseases and are characterized by an erythematous rash and severe itch. In this study, we investigated the function of TRPC3, a canonical transient receptor potential channel highly expressed in type 1 nonpeptidergic (NP1) nociceptive primary afferents and other cell types, in a mouse CD model. Although TrpC3 null mice had little deficits in acute somatosensation, they showed significantly increased scratching with CD. In addition, TrpC3 null mice displayed no differences in mechanical and thermal hypersensitivity in an inflammatory pain model, suggesting that this channel preferentially functions to antagonize CD-induced itch. Using dorsal root ganglia and panimmune-specific TrpC3 conditional knockout mice, we determined that TrpC3 in dorsal root ganglia neurons but not in immune cells is required for this phenotype. Furthermore, the number of MRGPRD⁺ NP1 afferents in CD-affected dorsal root ganglia is significantly reduced in TrpC3-mutant mice. Taken together, our results suggest that TrpC3 plays a critical role in NP1 afferents to cope with CD-induced excitotoxicity and that the degeneration of NP1 fibers may lead to an increased itch of CD. Our study identified a role of TrpC3 and NP1 afferents in CD pathology.