New insights into the mechanisms behind mechanical itch

Similarities and differences in the response and molecular characteristics of peripheral sensory neurons associated with pain and itch

Dorsal root ganglion (DRG) neurons are responsible for the primary detection and transmission of peripheral noxious stimuli, mainly pain and itch. However, as two distinct noxious sensations, how DRG neurons respond differently to and code pain and itch is still an attractive topic. Here, we investigate the response and activation spectrum of DRG neurons under peripheral pain and itch stimuli using in vivo two-photon calcium imaging and find differences in the response intensity to pain and itch between multisensory neurons (both pain and itch) and single-sensory neurons (either pain or itch). In addition, single-cell RNA sequencing (scRNA-seq) is used to reveal the heterogeneity of distinct subpopulations on the basis of their expressions of pain- or itch-related marker genes and to determine the similarities and differences in their transcriptomic changes under chronic pain and itch. Our results show that primary sensory neurons with different sensory patterns respond differently to the same nociceptive stimuli. Additionally, distinct clusters of neurons exhibit unique transcriptomic changes in the development of chronic pain and itch, which may offer new insights for treating these conditions.

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.

Cav3.2 T-Type Calcium Channel Mediates Acute Itch and Contributes to Chronic Itch and Inflammation in Experimental Atopic Dermatitis

Voltage-gated calcium channels regulate neuronal excitability. The Cav3.2 isoform of the T-type voltage-activated calcium channel is expressed in sensory neurons and is implicated in pain transmission. However, its role in itch remains unclear. In this study, we demonstrated that Cav3.2 is expressed by mechanosensory and peptidergic subsets of mouse dorsal root ganglion neurons and colocalized with TRPV1 and receptors for type 2 cytokines. Cav3.2-positive neurons innervate human skin. A deficiency of Cav3.2 reduces histamine, IL-4/IL-13, and TSLP-induced itch in mice. Cav3.2 channels were upregulated in the dorsal root ganglia of an atopic dermatitis (AD)-like mouse model and mediated neuronal excitability. Genetic knockout of Cav3.2 or T-type calcium channel blocker mibefradil treatment reduced spontaneous and mechanically induced scratching behaviors and skin inflammation in an AD-like mouse model. Substance P and vasoactive intestinal polypeptide levels were increased in the trigeminal ganglia from AD-like mouse model, and genetic ablation or pharmacological inhibition of Cav3.2 reduced their gene expression. Cav3.2 knockout also attenuated the pathologic changes in ex vivo skin explants cocultured with trigeminal ganglia neurons from AD-induced mice. Our study identifies the role of Cav3.2 in both histaminergic and nonhistaminergic acute itch. Cav3.2 channel also contributes to AD-related chronic itch and neuroinflammation.

Aetiology, pathogenesis and management of neuropathic itch: A narrative review with recent updates

Neuropathic itch is a relatively common yet under-reported cause of systemic pruritus. It is a debilitating condition often associated with pain, which impairs the patient's quality of life. Although much literature exists about renal and hepatic pruritus, there is a dearth of information and awareness about neuropathic itch. The pathogenesis of neuropathic itch is complex and can result from an insult at any point along the itch pathway, ranging from the peripheral receptors and nerves until the brain. There are several causes of neuropathic itch, many of which do not produce any skin lesions and are thus, often missed. A detailed history and clinical examination are necessary for the diagnosis, while laboratory and radiologic investigations may be needed in select cases. Several therapeutic strategies currently exist involving both non-pharmacological and pharmacological measures, the latter including topical, systemic, and invasive options. Further research is ongoing to clarify its pathogenesis and to design newer targeted therapies with minimal adverse effects. This narrative review highlights the current understanding of this condition, focusing on its causes, pathogenesis, diagnosis, and management, along with newer investigational drugs.

Itch: from the skin to the brain - peripheral and central neural sensitization in chronic itch

Similar to chronic pain, chronic itch is frequently linked to neural sensitization, a phenomenon wherein the nervous system becomes hypersensitive to stimuli. This process of neural sensitization of chronic itch is orchestrated by various signaling pathways and mediators in both the peripheral and central nervous systems. At the level of the peripheral nervous system, inflammation and neuroimmune interactions induce plastic changes to peripheral nerve fibers, thereby amplifying the transmission of itch signaling. Neural sensitization in the central nervous system occurs at both the spinal cord and brain levels. At the level of the spinal cord, it involves hyperactivity of itch-activating spinal pathways, dysfunction of spinal inhibitory circuits, and attenuation of descending supraspinal inhibitory pathways. In the brain, neural sensitization manifests as structural and functional changes to itch-associated brain areas and networks. Currently, we have a diverse array of neuroimmune-modulating therapies targeting itch neural sensitization mechanisms to help with providing relief to patients with chronic itch. Itch research is a dynamic and continually evolving field, and as we grow in our understanding of chronic itch mechanisms, so will our therapeutic toolbox. Further studies exploring the peripheral and central neural sensitization mechanisms in the context of chronic itch are needed.

Comparison of brain functional response to mechanical prickling stimuli to the glabrous and hairy skin

BACKGROUND

A kind of prickle sensation, which is a composite feeling of pain and itch, can be evoked by mechanical stimulation of fiber ends from fabric surface against to human hairy skin, rather than glabrous skin. Now, a functional magnetic resonance imaging (fMRI) study was conducted to investigate the cognitive differences in the brain for mechanical prickling stimuli to the two types of skin.

MATERIALS AND METHODS

A nylon filament with the diameter of 205 μm and the length of 8 mm was used to deliver mechanical prickling stimuli respectively to two skin sites, fingertip (glabrous skin) and volar forearm (hairy skin), of eight healthy male subjects. Simultaneously, the technology of fMRI was adopted to acquire BOLD (Blood Oxygen Level-Dependent) signals of brain functional response of the subjects.

RESULTS

Somatosensory areas, emotional areas, and the posterior parietal cortex (especially the precuneus) are important brain regions that distinguish between the two conditions. The representation of mechanical prickling stimulation to glabrous skin in the brain favors much more the tactile information of the stimulation and contains no itch, while the key brain area, precuneus, involved in itch was activated by the same mechanical prickling stimulation to hairy skin, and brain response for the condition of hairy skin contains more emotional information, which plays an important role in pain processing.

CONCLUSION

Therefore, it can be inferred that a kind of stronger prickle sensation, which contains both pain and itch, was evoked by mechanical stimulation to hairy skin than glabrous skin.

Do Merkel complexes initiate mechanical itch?

Itch is a common sensation which is amenable to disabling patients' life under pathological and chronic conditions. Shared assertion easily limits itch to chemical itch, without considering mechanical itch and alloknesis, its pathological counterpart. However, in recent years, our understanding of the mechanical itch pathway, particularly in the central nervous system, has been enhanced. In addition, Merkel complexes, conventionally considered as tactile end organs only responsible for light touch perception due to Piezo2 expressed by both Merkel cells and SA1 Aβ-fibres - low threshold mechanical receptors (LTMRs) -, have recently been identified as modulators of mechanical itch. However, the tactile end organs responsible for initiating mechanical itch remain unexplored. The consensus is that some LTMRs, either SA1 Aβ- or A∂- and C-, are cutaneous initiators of mechanical itch, even though they are not self-sufficient to finely detect and encode light mechanical stimuli into sensory perceptions, which depend on the entire hosting tactile end organ. Consequently, to enlighten our understanding of mechanical itch initiation, this article discusses the opportunity to consider Merkel complexes as potential tactile end organs responsible for initiating mechanical itch, under both healthy and pathological conditions. Their unsuspected modulatory abilities indeed show that they are tuned to detect and encode light mechanical stimuli leading to mechanical itch, especially as they host not only SA1 Aβ-LTMRs but also A∂- and C-fibres.

PIEZO1 transduces mechanical itch in mice

Itch triggers scratching, a behavioural defence mechanism that aids in the removal of harmful irritants and parasites¹. Chemical itch is triggered by many endogenous and exogenous cues, such as pro-inflammatory histamine, which is released during an allergic reaction¹. Mechanical itch can be triggered by light sensations such as wool fibres or a crawling insect². In contrast to chemical itch pathways, which have been extensively studied, the mechanisms that underlie the transduction of mechanical itch are largely unknown. Here we show that the mechanically activated ion channel PIEZO1 (ref. ³) is selectively expressed by itch-specific sensory neurons and is required for their mechanically activated currents. Loss of PIEZO1 function in peripheral neurons greatly reduces mechanically evoked scratching behaviours and both acute and chronic itch-evoked sensitization. Finally, mice expressing a gain-of-function Piezo1 allele⁴ exhibit enhanced mechanical itch behaviours. Our studies reveal the polymodal nature of itch sensory neurons and identify a role for PIEZO1 in the sensation of itch.

Experiments in mice show that the mechanically activated ion channel PIEZO1 is expressed in itch-specific sensory neurons and has a role in transducing mechanical itch.

Neuroinflammation Involved in Diabetes-Related Pain and Itch

Diabetes mellitus (DM) is a global epidemic with increasing incidence, which results in diverse complications, seriously affects the patient quality of life, and brings huge economic burdens to society. Diabetic neuropathy is the most common chronic complication of DM, resulting in neuropathic pain and chronic itch. The precise mechanisms of diabetic neuropathy have not been fully clarified, hindering the exploration of novel therapies for diabetic neuropathy and its terrible symptoms such as diabetic pain and itch. Accumulating evidence suggests that neuroinflammation plays a critical role in the pathophysiologic process of neuropathic pain and chronic itch. Indeed, researchers have currently made significant progress in knowing the role of glial cells and the pro-inflammatory mediators produced from glial cells in the modulation of chronic pain and itch signal processing. Here, we provide an overview of the current understanding of neuroinflammation in contributing to the sensitization of the peripheral nervous system (PNS) and central nervous system (CNS). In addition, we also summarize the inflammation mechanisms that contribute to the pathogenesis of diabetic itch, including activation of glial cells, oxidative stress, and pro-inflammatory factors. Targeting excessive neuroinflammation may provide potential and effective therapies for the treatment of chronic neuropathic pain and itch in DM.

Epigenetic regulation and signalling pathways in Merkel cell development

Merkel cells are specialized epithelial cells connected to afferent nerve endings responsible for light-touch sensations, formed at specific locations in touch-sensitive regions of the mammalian skin. Although Merkel cells are descendants of the epidermal lineage, little is known about the mechanisms responsible for the development of these unique mechanosensory cells. Recent studies have highlighted that the Polycomb group (PcG) of proteins play a significant role in spatiotemporal regulation of Merkel cell formation. In addition, several of the major signalling pathways involved in skin development have been shown to regulate Merkel cell development as well. Here, we summarize the current understandings of the role of developmental regulators in Merkel cell formation, including the interplay between the epigenetic machinery and key signalling pathways, and the lineage-specific transcription factors involved in the regulation of Merkel cell development.

Interleukin-31 and Pruritic Skin

Skin inflammation often evokes pruritus, which is the major subjective symptom in many inflammatory skin diseases such as atopic dermatitis and prurigo nodularis. Pruritus or itch is a specific sensation found only in the skin. Recent studies have stressed the pivotal role played by interleukin-31 (IL-31) in the sensation of pruritus. IL-31 is produced by various cells including T helper 2 cells, macrophages, dendritic cells and eosinophils. IL-31 signals via a heterodimeric receptor composed of IL-31 receptor A (IL-31RA) and oncostatin M receptor β. Recent clinical trials have shown that the anti-IL-31RA antibody nemolizumab can successfully decrease pruritus in patients with atopic dermatitis and prurigo nodularis. The IL-31 pathway and pruritic skin are highlighted in this review article.

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

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