Gate control of mechanical itch by a subpopulation of spinal cord interneurons

Neuropeptide Y Y2 Receptors in Sensory Neurons Tonically Suppress Nociception and Itch but Facilitate Postsurgical and Neuropathic Pain Hypersensitivity

BACKGROUND

Neuropeptide Y (NPY) Y2 receptor (Y2) antagonist BIIE0246 can both inhibit and facilitate nociception. The authors hypothesized that Y2 function depends on inflammation or nerve injury status.

METHODS

The authors implemented a battery of behavioral tests in mice of both sexes that received (1) no injury; (2) an incision model of postoperative pain; (3) a spared nerve injury model of neuropathic pain; and (4) a latent sensitization model of chronic postsurgical pain. In addition to Y2 gene expression assays, spinal Y2 G-protein coupling was studied with guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assays.

RESULTS

The authors report that intrathecal BIIE0246 increased mechanical and cold hypersensitivity, produced behavioral signs of spontaneous nociception and itch, and produced conditioned place aversion and preference in normal, uninjured mice. BIIE0246 did not change heat hypersensitivity or motor coordination. Conditional (sensory neuron-specific) Y2 deletion prevented BIIE0246-induced mechanical and cold hypersensitivity, nocifensive behaviors, and aversion. Both conditional deletion and pharmacologic blockade of Y2 reduced mechanical and thermal hypersensitivity after incision or nerve injury. SNI did not change the sensitivity of Y2 G-protein coupling with the Y2 agonist peptide YY (3-36) (PYY3-36), but increased the population of Y2 that effectively coupled G-proteins. Intrathecal PYY3-36 failed to reduce spared nerve injury- or incision-induced hypersensitivity in C57BL/6N mice. Incision did not change Npy2r gene expression in dorsal root ganglion.

CONCLUSIONS

The authors conclude that Y2 at central terminals of primary afferent neurons provides tonic inhibition of mechanical and cold nociception and itch. This switches to the promotion of mechanical and thermal hyperalgesia in models of acute and chronic postsurgical and neuropathic pain, perhaps due to an increase in the population of Y2 that effectively couples to G-proteins. These results support the development of Y2 antagonists for the treatment of chronic postsurgical and neuropathic pain.

EDITOR’S PERSPECTIVE

Neuropeptide Y Y2 receptors in acute and chronic pain and itch

Pain and itch are regulated by a diverse array of neuropeptides and their receptors in superficial laminae of the spinal cord dorsal horn (DH). Neuropeptide Y (NPY) is normally expressed on DH neurons but not sensory neurons. By contrast, the Npy2r receptor (Y2) is expressed on the central and peripheral terminals of sensory neurons but not on DH neurons. Neurophysiological slice recordings indicate that Y2-selective agonists inhibits spinal neurotransmitter release from sensory neurons. However, behavioral pharmacology studies indicate that Y2 agonists exert minimal changes in nociception, even after injury. Additional discrepancies in the behavioral actions of the Y2-antagonist BIIE0246 - reports of either pronociception or antinociception - have now been resolved. In the normal state, spinally-directed (intrathecal) administration of BIIE0246 elicits ongoing nociception, hypersensitivity to sensory stimulation, and aversion. Conversely, in the setting of nerve injury and inflammation, intrathecal BIIE024 reduced not only mechanical and thermal hypersensitivity, but also a measure of the affective dimension of pain (conditioned place preference). When administered in chronic pain models of latent sensitization, BIIE0246 produced a profound reinstatement of pain-like behaviors. We propose that tissue or nerve injury induces a G protein switch in the action of NPY-Y2 signaling from antinociception in the naïve state to the inhibition of mechanical and heat hyperalgesia in the injured state, and then a switch back to antinociception to keep LS in a state of remission. This model clarifies the pharmacotherapeutic potential of Y2 research, pointing to the development of a new non-opioid pharmacotherapy for chronic pain using Y2 antagonists in patients who do not develop LS.

Spinal lumbar Urocortin 3-expressing neurons are associated with both scratching and Compound 48/80-induced sensations

ABSTRACT

Urocortin 3 is a neuropeptide that belongs to the corticotropin-releasing hormone family and is involved in mechanosensation and stress regulation. In this study, we show that Urocortin 3 marks a population of excitatory neurons in the mouse spinal cord, divided into 2 nonoverlapping subpopulations expressing protein kinase C gamma or calretinin/calbindin 2, populations previously associated with mechanosensation. Electrophysiological experiments demonstrated that lumbar spinal Urocortin 3 neurons receive both glycinergic and GABAergic local tonic inhibition, and monosynaptic inputs from both Aβ and C fibers, which could be confirmed by retrograde trans-synaptic rabies tracing. Furthermore, fos analyses showed that subpopulations of lumbar Urocortin 3 neurons are activated by artificial scratching or Compound 48/80-induced sensations. Chemogenetic activation of lumbar Urocortin 3-Cre neurons evoked a targeted biting/licking behavior towards the corresponding dermatome and chemogenetic inhibition decreased Compound 48/80-induced behavior. Hence, spinal lumbar Urocortin 3 neurons represent a mechanically associated population with roles in both scratching and Compound 48/80-induced sensations.

Diospyros lotus leaf extract and its main component myricitrin inhibit itch‑related IL‑6 and IL‑31 by suppressing microglial inflammation and microglial‑mediated astrocyte activation

Diospyros lotus has been traditionally used in Asia for medicinal purposes, exhibiting a broad spectrum of pharmacological effects including antioxidant, neuroprotective and anti‑inflammatory properties. While the anti‑itch effect of D. lotus leaves has been reported, studies on the detailed mechanism of action in microglia and astrocytes, which are members of the central nervous system, have yet to be revealed. The present study aimed to investigate effects of D. lotus leaf extract (DLE) and its main component myricitrin (MC) on itch‑related cytokines and signaling pathways in lipopolysaccharide (LPS)‑stimulated microglia. The effect of DLE and MC on activation of astrocyte stimulated by microglia was also examined. Cytokine production was evaluated through reverse transcription PCR and western blot analysis. Signaling pathway was analyzed by performing western blotting and immunofluorescence staining. The effect of microglia on astrocytes activation was evaluated via western blotting for receptors, signaling molecules and itch mediators and confirmed through gene silencing using short interfering RNA. DLE and MC suppressed the production of itch‑related cytokine IL‑6 and IL‑31 in LPS‑stimulated microglia. These inhibitory effects were mediated through the blockade of NF‑κB, MAPK and JAK/STAT pathways. In astrocytes, stimulation by microglia promoted the expression of itch‑related molecules such as oncostatin M receptor, interleukin 31 receptor a, inositol 1,4,5‑trisphosphate receptor 1, lipocalin‑2 (LCN2), STAT3 and glial fibrillary acidic protein. However, DLE and MC significantly inhibited these receptors. Additionally, astrocytes stimulated by microglia with IL‑6, IL‑31, or both genes silenced did not show activation of LCN2 or STAT3. The findings of the present study demonstrated that DLE and MC could suppress pruritic activity in astrocytes induced by microglia‑derived IL‑6 and IL‑31. This suggested the potential of DLE and MC as functional materials capable of alleviating pruritus.

Influence of Early-Life Stress on the Excitability of Dynorphin Neurons in the Adult Mouse Dorsal Horn

While early-life adversity has been associated with a higher risk of developing chronic pain in adulthood, the cellular and molecular mechanisms by which chronic stress during the neonatal period can persistently sensitize developing nociceptive circuits remain poorly understood. Here, we investigate the effects of early-life stress (ELS) on synaptic integration and intrinsic excitability in dynorphin-lineage (DYN) interneurons within the adult mouse superficial dorsal horn (SDH), which are important for inhibiting mechanical pain and itch. The administration of neonatal limited bedding between postnatal days (P)2 and P9 evoked sex-dependent effects on spontaneous glutamatergic signaling, as female SDH neurons exhibited a higher amplitude of miniature excitatory postsynaptic currents (mEPSCs) after ELS, while mEPSC frequency was reduced in DYN neurons of the male SDH. Furthermore, ELS decreased the frequency of miniature inhibitory postsynaptic currents selectively in female DYN neurons. As a result, ELS increased the balance of spontaneous excitation versus inhibition (E:I ratio) in mature DYN neurons of the female, but not male, SDH network. Nonetheless, ELS weakened the total primary afferent-evoked glutamatergic drive onto adult DYN neurons selectively in females, without modifying afferent-evoked inhibitory signaling onto the DYN population. Finally, ELS failed to significantly change the intrinsic membrane excitability of mature DYN neurons in either males or females. Collectively, these data suggest that ELS exerts a long-term influence on the properties of synaptic transmission onto DYN neurons within the adult SDH, which includes a reduction in the overall strength of sensory input onto this important subset of inhibitory interneurons. PERSPECTIVE: This study suggests that chronic stress during the neonatal period influences synaptic function within adult spinal nociceptive circuits in a sex-dependent manner. These findings yield new insight into the potential mechanisms by which early-life adversity might shape the maturation of pain pathways in the central nervous system (CNS).

Activation of NPY2R-expressing amygdala neurons inhibits itch behavior in mice without lateralization

The central amygdala (CeA) is a crucial hub in the processing of affective itch, containing a diverse array of neuronal populations. Among these components, Neuropeptide Y (NPY) and its receptors, such as NPY2R, affect various physiological and psychological processes. Despite this broad impact, the precise role of NPY2R+ CeA neurons in itch modulation remains unknown, particularly concerning any potential lateralization effects. To address this, we employed optogenetics to selectively stimulate NPY2R+ CeA neurons in mice, investigating their impact on itch modulation. Optogenetic activation of NPY2R+ CeA neurons reduced scratching behavior elicited by pruritogens without exhibiting any lateralization effects. Electrophysiological recordings confirmed increased neuronal activity upon stimulation. However, this modulation did not affect thermal sensitivity, mechanical sensitivity, or formalin-induced hyperalgesia. Additionally, no alterations in anxiety-like behaviors or locomotion were observed upon stimulation. Projection tracing revealed connections of NPY2R+ CeA neurons to brain regions implicated in itch processing. Overall, this comprehensive study highlights the role of NPY2R+ CeA neurons in itch regulation without any lateralization effects.

Spinal Nmur2-positive Neurons Play a Crucial Role in Mechanical Itch

The dorsal spinal cord is crucial for the transmission and modulation of multiple somatosensory modalities, such as itch, pain, and touch. Despite being essential for the well-being and survival of an individual, itch and pain, in their chronic forms, have increasingly been recognized as clinical problems. Although considerable progress has been made in our understanding of the neurochemical processing of nociceptive and chemical itch sensations, the neural substrate that is crucial for mechanical itch processing is still unclear. Here, using genetic and functional manipulation, we identified a population of spinal neurons expressing neuromedin U receptor 2 (Nmur2+) as critical elements for mechanical itch. We found that spinal Nmur2+ neurons are predominantly excitatory neurons, and are enriched in the superficial laminae of the dorsal horn. Pharmacogenetic activation of cervical spinal Nmur2+ neurons evoked scratching behavior. Conversely, the ablation of these neurons using a caspase-3-based method decreased von Frey filament-induced scratching behavior without affecting responses to other somatosensory modalities. Similarly, suppressing the excitability of cervical spinal Nmur2+ neurons via the overexpression of functional Kir2.1 potassium channels reduced scratching in response to innocuous mechanical stimuli, but not to pruritogen application. At the lumbar level, pharmacogenetic activation of these neurons evoked licking and lifting behaviors. However, ablating these neurons did not affect the behavior associated with acute pain. Thus, these results revealed the crucial role of spinal Nmur2+ neurons in mechanical itch. Our study provides important insights into the neural basis of mechanical itch, paving the way for developing novel therapies for chronic itch. PERSPECTIVE: Excitatory Nmur2+ neurons in the superficial dorsal spinal cord are essential for mechanical but not chemical itch information processing. These spinal Nmur2+ neurons represent a potential cellular target for future therapeutic interventions against chronic itch. Spinal and supraspinal Nmur2+ neurons may play different roles in pain signal processing.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP mouse, in which humanized renilla green fluorescent protein (hrGFP) expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on several factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY on the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.NEW & NOTEWORTHY Across brain regions, neuropeptide Y (NPY) expression is dynamic and influenced by extrinsic and intrinsic factors. We previously showed that NPY is expressed by a class of inhibitory neurons in the auditory midbrain. Here, we find that this neuron class also includes neurons that previously expressed NPY, suggesting that NPY expression is dynamically regulated in the auditory midbrain. We also provide functional evidence that NPY neurons contribute to local inhibitory circuits in the auditory midbrain.

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.

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

The Dorsal Column Nuclei Scale Mechanical Sensitivity in Naive and Neuropathic Pain States

Tactile perception relies on reliable transmission and modulation of low-threshold information as it travels from the periphery to the brain. During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. Two main drivers of peripheral tactile information, low-threshold mechanoreceptors (LTMRs) and postsynaptic dorsal column neurons (PSDCs), terminate in the brainstem dorsal column nuclei (DCN). Activity within the DRG, spinal cord, and DCN have all been implicated in mediating allodynia, yet the DCN remains understudied at the cellular, circuit, and functional levels compared to the other two. Here, we show that the gracile nucleus (Gr) of the DCN mediates tactile sensitivity for low-threshold stimuli and contributes to mechanical allodynia during neuropathic pain in mice. We found that the Gr contains local inhibitory interneurons in addition to thalamus-projecting neurons, which are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations resulted in bidirectional changes to tactile sensitivity, but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, silencing Gr projection neurons or activating Gr inhibitory neurons was able to reduce tactile hypersensitivity, and enhancing inhibition was able to ameliorate paw withdrawal signatures of neuropathic pain, like shaking. Collectively, these results suggest that the Gr plays a specific role in mediating hypersensitivity to low-threshold, innocuous mechanical stimuli during neuropathic pain, and that Gr activity contributes to affective, pain-associated phenotypes of mechanical allodynia. Therefore, these brainstem circuits work in tandem with traditional spinal circuits underlying allodynia, resulting in enhanced signaling of tactile stimuli in the brain during neuropathic pain.

A novel Nav1.8-FLPo driver mouse for intersectional genetics to uncover the functional significance of primary sensory neuron diversity

The recent development of single-cell and single-nucleus RNA sequencing has highlighted the extraordinary diversity of dorsal root ganglia neurons. However, the few available genetic tools limit our understanding of the functional significance of this heterogeneity. We generated a new mouse line expressing the flippase recombinase from the scn10a locus. By crossing Nav1.8Ires-FLPo mice with the AdvillinCre and RC::FL-hM3Dq mouse lines in an intersectional genetics approach, we were able to obtain somatodendritic expression of hM3Dq-mCherry selectively in the Nav1.8 lineage. The bath application of clozapine N-oxide triggered strong calcium responses selectively in mCherry+ neurons. The intraplantar injection of CNO caused robust flinching, shaking, and biting responses accompanied by strong cFos activation in the ipsilateral lumbar spinal cord. The Nav1.8Ires-FLPo mouse model will be a valuable tool for extending our understanding of the in vivo functional specialization of neuronal subsets of the Nav1.8 lineage for which inducible Cre lines are available.

Two inhibitory neuronal classes govern acquisition and recall of spinal sensorimotor adaptation

Spinal circuits are central to movement adaptation, yet the mechanisms within the spinal cord responsible for acquiring and retaining behavior upon experience remain unclear. Using a simple conditioning paradigm, we found that dorsal inhibitory neurons are indispensable for adapting protective limb-withdrawal behavior by regulating the transmission of a specific set of somatosensory information to enhance the saliency of conditioning cues associated with limb position. By contrast, maintaining previously acquired motor adaptation required the ventral inhibitory Renshaw cells. Manipulating Renshaw cells does not affect the adaptation itself but flexibly alters the expression of adaptive behavior. These findings identify a circuit basis involving two distinct populations of spinal inhibitory neurons, which enables lasting sensorimotor adaptation independently from the brain.

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.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP reporter mouse, in which hrGFP expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on a range of factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY proximal to the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons routinely provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.

Pathophysiological Roles of Ion Channels in Epidermal Cells, Immune Cells, and Sensory Neurons in Psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by the rapid abnormal growth of skin cells in the epidermis, driven by an overactive immune system. Consequently, a complex interplay among epidermal cells, immune cells, and sensory neurons contributes to the development and progression of psoriasis. In these cellular contexts, various ion channels, such as acetylcholine receptors, TRP channels, Ca2+ release-activated channels, chloride channels, and potassium channels, each serve specific functions to maintain the homeostasis of the skin. The dysregulation of ion channels plays a major role in the pathophysiology of psoriasis, affecting various aspects of epidermal cells, immune responses, and sensory neuron signaling. Impaired function of ion channels can lead to altered calcium signaling, inflammation, proliferation, and sensory signaling, all of which are central features of psoriasis. This overview summarizes the pathophysiological roles of ion channels in epidermal cells, immune cells, and sensory neurons during early and late psoriatic processes, thereby contributing to a deeper understanding of ion channel involvement in the interplay of psoriasis and making a crucial advance toward more precise and personalized approaches for psoriasis treatment.

Mechanical and chemical itch regulated by neuropeptide Y-Y1 signaling

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

The plasticity of neuropeptide Y-Y1 receptor system on Tac2 neurons contributes to mechanical hyperknesis during chronic itch

Rationale: In the physiological states, the act of scratching protects the person from harmful substances, while in certain pathological conditions, the patient suffers from chronic itch, both physically and mentally. Chronic itch sufferers are more sensitive to mechanical stimuli, and mechanical hyperknesis relief is essential for chronic itch treatment. While neuropeptide Y-Y1 receptor (NPY-Y1R) system is known to play a crucial role in modulating mechanical itch in physiological conditions, it is elusive how they are altered during chronic itch. We hypothesize that the negative regulatory effect of Y1Rs on Tac2 neurons, the key neurons that transmit mechanical itch, declines during chronic itch. Methods: We combined transgenic mice, chemogenetic manipulation, immunofluorescence, rabies virus circuit tracing, and electrophysiology to investigate the plasticity of Y1Rs on Tac2 neurons during chronic itch. Results: We found that Tac2 neurons receive direct input from Npy neurons and that inhibition of Npy neurons induces activation of Tac2 neurons. Moreover, the expression of Y1Rs on Tac2 neurons is reduced, and the regulatory effect is also reduced during chronic itch. Conclusion: Our study clarifies the plasticity of Y1Rs on Tac2 neurons during chronic itch and further elucidates the mechanism by which NPY-Y1R system is responsible for modulating mechanical itch. We highlight Y1Rs as a promising therapeutic target for mechanical hyperknesis during chronic itch.

A tactile twist: decoding the phenomena of mechanical itch and alloknesis

Itch is a sensation in the skin which provokes the desire to scratch. In the past few decades there has been a significant elucidation of the immune and neural pathways which underly the sensation of itch. An interesting divergence in the itch pathway relates to the type of stimulation used to evoke an itchy sensation. Commonly, chemical mediators of itch such as histamine are injected into the skin where they activate their cognate receptors on sensory neurons. Another way to evoke itch, particularly in patients with chronic itch, is to use light mechanical stimulation. Investigation into these pathways utilizing the mouse model have shown that the neuronal pathways which underly chemical itch are distinct from those which mediate itch in response to mechanical stimulation. Specific populations of primary sensory neurons, spinal interneurons and transmission neurons have been identified which suggests a labeled line for itch transmission. Additionally, Piezo channels, which underly mechanosensation, were discovered to play an important role in the mechanical itch pathway. Given these novel findings relating to the mechanical itch pathway, the purpose of this review is to summarize the reports from human subjects and animal studies to highlight the advances in our understanding of mechanical itch and alloknesis.

Targeting Peripheral μ-opioid Receptors or μ-opioid Receptor-Expressing Neurons Does not Prevent Morphine-induced Mechanical Allodynia and Anti-allodynic Tolerance

The chronic use of morphine and other opioids is associated with opioid-induced hypersensitivity (OIH) and analgesic tolerance. Among the different forms of OIH and tolerance, the opioid receptors and cell types mediating opioid-induced mechanical allodynia and anti-allodynic tolerance remain unresolved. Here we demonstrated that the loss of peripheral μ-opioid receptors (MORs) or MOR-expressing neurons attenuated thermal tolerance, but did not affect the expression and maintenance of morphine-induced mechanical allodynia and anti-allodynic tolerance. To confirm this result, we made dorsal root ganglia-dorsal roots-sagittal spinal cord slice preparations and recorded low-threshold Aβ-fiber stimulation-evoked inputs and outputs in superficial dorsal horn neurons. Consistent with the behavioral results, peripheral MOR loss did not prevent the opening of Aβ mechanical allodynia pathways in the spinal dorsal horn. Therefore, the peripheral MOR signaling pathway may not be an optimal target for preventing mechanical OIH and analgesic tolerance. Future studies should focus more on central mechanisms.

A new Hoxb8FlpO mouse line for intersectional approaches to dissect developmentally defined adult sensorimotor circuits

Improvements in the speed and cost of expression profiling of neuronal tissues offer an unprecedented opportunity to define ever finer subgroups of neurons for functional studies. In the spinal cord, single cell RNA sequencing studies support decades of work on spinal cord lineage studies, offering a unique opportunity to probe adult function based on developmental lineage. While Cre/Flp recombinase intersectional strategies remain a powerful tool to manipulate spinal neurons, the field lacks genetic tools and strategies to restrict manipulations to the adult mouse spinal cord at the speed at which new tools develop. This study establishes a new workflow for intersectional mouse-viral strategies to dissect adult spinal function based on developmental lineages in a modular fashion. To restrict manipulations to the spinal cord, we generate a brain-sparing Hoxb8FlpO mouse line restricting Flp recombinase expression to caudal tissue. Recapitulating endogenous Hoxb8 gene expression, Flp-dependent reporter expression is present in the caudal embryo starting day 9.5. This expression restricts Flp activity in the adult to the caudal brainstem and below. Hoxb8FlpO heterozygous and homozygous mice do not develop any of the sensory or locomotor phenotypes evident in Hoxb8 heterozygous or mutant animals, suggesting normal developmental function of the Hoxb8 gene and protein in Hoxb8FlpO mice. Compared to the variability of brain recombination in available caudal Cre and Flp lines, Hoxb8FlpO activity is not present in the brain above the caudal brainstem, independent of mouse genetic background. Lastly, we combine the Hoxb8FlpO mouse line with dorsal horn developmental lineage Cre mouse lines to express GFP in developmentally determined dorsal horn populations. Using GFP-dependent Cre recombinase viruses and Cre recombinase-dependent inhibitory chemogenetics, we target developmentally defined lineages in the adult. We show how developmental knock-out versus transient adult silencing of the same ROR𝛃 lineage neurons affects adult sensorimotor behavior. In summary, this new mouse line and viral approach provides a blueprint to dissect adult somatosensory circuit function using Cre/Flp genetic tools to target spinal cord interneurons based on genetic lineage.

Neuropeptide Y-expressing dorsal horn inhibitory interneurons gate spinal pain and itch signalling

Somatosensory information is processed by a complex network of interneurons in the spinal dorsal horn. It has been reported that inhibitory interneurons that express neuropeptide Y (NPY), either permanently or during development, suppress mechanical itch, with no effect on pain. Here, we investigate the role of interneurons that continue to express NPY (NPY-INs) in the adult mouse spinal cord. We find that chemogenetic activation of NPY-INs reduces behaviours associated with acute pain and pruritogen-evoked itch, whereas silencing them causes exaggerated itch responses that depend on cells expressing the gastrin-releasing peptide receptor. As predicted by our previous studies, silencing of another population of inhibitory interneurons (those expressing dynorphin) also increases itch, but to a lesser extent. Importantly, NPY-IN activation also reduces behavioural signs of inflammatory and neuropathic pain. These results demonstrate that NPY-INs gate pain and itch transmission at the spinal level, and therefore represent a potential treatment target for pathological pain and itch.

Calretinin-expressing islet cells are a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Shank2 identifies a subset of glycinergic neurons involved in altered nociception in an autism model

BACKGROUND

Autism Spectrum Disorders (ASD) patients experience disturbed nociception in the form of either hyposensitivity to pain or allodynia. A substantial amount of processing of somatosensory and nociceptive stimulus takes place in the dorsal spinal cord. However, many of these circuits are not very well understood in the context of nociceptive processing in ASD.

METHODS

We have used a Shank2^-/- mouse model, which displays a set of phenotypes reminiscent of ASD, and performed behavioural and microscopic analysis to investigate the role of dorsal horn circuitry in nociceptive processing of ASD.

RESULTS

We determined that Shank2^-/- mice display increased sensitivity to formalin pain and thermal preference, but a sensory specific mechanical allodynia. We demonstrate that high levels of Shank2 expression identifies a subpopulation of neurons in murine and human dorsal spinal cord, composed mainly by glycinergic interneurons and that loss of Shank2 causes the decrease in NMDAR in excitatory synapses on these inhibitory interneurons. In fact, in the subacute phase of the formalin test, glycinergic interneurons are strongly activated in wild type (WT) mice but not in Shank2^-/- mice. Consequently, nociception projection neurons in laminae I are activated in larger numbers in Shank2^-/- mice.

LIMITATIONS

Our investigation is limited to male mice, in agreement with the higher representation of ASD in males; therefore, caution should be applied to extrapolate the findings to females. Furthermore, ASD is characterized by extensive genetic diversity and therefore the findings related to Shank2 mutant mice may not necessarily apply to patients with different gene mutations. Since nociceptive phenotypes in ASD range between hyper- and hypo-sensitivity, diverse mutations may affect the circuit in opposite ways.

CONCLUSION

Our findings prove that Shank2 expression identifies a new subset of inhibitory interneurons involved in reducing the transmission of nociceptive stimuli and whose unchecked activation is associated with pain hypersensitivity. We provide evidence that dysfunction in spinal cord pain processing may contribute to the nociceptive phenotypes in ASD.

Identification of an essential spinoparabrachial pathway for mechanical itch

The sensation of itch is a protective response that is elicited by either mechanical or chemical stimuli. The neural pathways for itch transmission in the skin and spinal cord have been characterized previously, but the ascending pathways that transmit sensory information to the brain to evoke itch perception have not been identified. Here, we show that spinoparabrachial neurons co-expressing Calcrl and Lbx1 are essential for generating scratching responses to mechanical itch stimuli. Moreover, we find that mechanical and chemical itch are transmitted by separate ascending pathways to the parabrachial nucleus, where they engage separate populations of FoxP2PBN neurons to drive scratching behavior. In addition to revealing the architecture of the itch transmission circuitry required for protective scratching in healthy animals, we identify the cellular mechanisms underlying pathological itch by showing the ascending pathways for mechanical and chemical itch function cooperatively with the FoxP2PBN neurons to drive chronic itch and hyperknesis/alloknesis.

Calretinin-expressing islet cells: a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Novel proresolving lipid mediator mimetic 3-oxa-PD1n-3 docosapentaenoic acid reduces acute and chronic itch by modulating excitatory and inhibitory synaptic transmission and astroglial secretion of lipocalin-2 in mice

ABSTRACT

Specialized proresolving mediators (SPMs) have demonstrated potent analgesic actions in animal models of pathological pain. The actions of SPMs in acute and chronic itch are currently unknown. Recently, n-3 docosapentaenoic acid (DPA) was found to be a substrate for the biosynthesis of several novel families of SPMs and 3-oxa-PD1 n-3 DPA (3-oxa-PD1) is an oxidation-resistant metabolic stable analogue of the n-3 DPA-derived protectin D1 (PD1). In this article, we demonstrate that 3-oxa-PD1 effectively reduces both acute and chronic itch in mouse models. Intrathecal injection of 3-oxa-PD1 (100 ng) reduced acute itch induced by histamine, chloroquine, or morphine. Furthermore, intrathecal 3-oxa-PD1 effectively reduced chronic itch, induced by cutaneous T-cell lymphoma (CTCL), allergic contact dermatitis with dinitrofluorobenzene, and psoriasis by imiquimod. Intratumoral injection of 3-oxa-PD1 also suppressed CTCL-induced chronic itch. Strikingly, the antipruritic effect lasted for several weeks after 1-week intrathecal 3-oxa-PD1 treatment. Whole-cell recordings revealed significant increase in excitatory postsynaptic currents in spinal dorsal horn (SDH) neurons of CTCL mice, but this increase was blocked by 3-oxa-PD1. 3-oxa-PD1 further increased inhibitory postsynaptic currents in SDH neurons of CTCL mice. Cutaneous T-cell lymphoma increased the spinal levels of lipocalin-2 (LCN2), an itch mediator produced by astrocytes. 3-oxa-PD1 suppressed LCN2 production in CTCL mice and LCN2 secretion in astrocytes. Finally, CTCL-induced anxiety was alleviated by intrathecal 3-oxa-PD1. Our findings suggest that 3-oxa-PD1 potently inhibits acute and chronic itch through the regulation of excitatory or inhibitory synaptic transmission and astroglial LCN2 production. Therefore, stable SPM analogs such as 3-oxa-PD1 could be useful to treat pruritus associated with different skin injuries.

MrgprA3-expressing pruriceptors drive pruritogen-induced alloknesis through mechanosensitive Piezo2 channel

Although touch and itch are coded by distinct neuronal populations, light touch also provokes itch in the presence of exogenous pruritogens, resulting in a phenomenon called alloknesis. However, the cellular and molecular mechanisms underlying the initiation of pruritogen-induced mechanical itch sensitization are poorly understood. Here, we show that intradermal injections of histamine or chloroquine (CQ) provoke alloknesis through activation of TRPV1- and MrgprA3-expressing prurioceptors, and functional ablation of these neurons reverses pruritogen-induced alloknesis. Moreover, genetic ablation of mechanosensitive Piezo2 channel function from MrgprA3-expressing prurioceptors also dampens pruritogen-induced alloknesis. Mechanistically, histamine and CQ sensitize Piezo2 channel function, at least in part, through activation of the phospholipase C (PLC) and protein kinase C-δ (PKCδ) signaling. Collectively, our data find a TRPV1+/MrgprA3+ prurioceptor-Piezo2 signaling axis in the initiation of pruritogen-induced mechanical itch sensitization in the skin.

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

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.

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.

Role of kappa-opioid and mu-opioid receptors in pruritus: Peripheral and central itch circuits

Modern genetic approaches in animal models have unveiled novel itch-specific neural pathways, emboldening a paradigm in which drugs can be developed to selectively and potently target itch in a variety of chronic pruritic conditions. In recent years, kappa-opioid receptors (KORs) and mu-opioid receptors (MORs) have been implicated in both the suppression and promotion of itch, respectively, by acting on both the peripheral and central nervous systems. The precise mechanisms by which agents that modulate these pathways alleviate itch remains an active area of investigation. Notwithstanding this, a number of agents have demonstrated efficacy in clinical trials that influence both KOR and MOR signalling. Herein, we summarize a number of opioid receptor modulators in development and their promising efficacy across a number of chronic pruritic conditions, such as atopic dermatitis, uremic pruritus and beyond.

Electroacupuncture reduces chronic itch via cannabinoid CB1 receptors in the ventrolateral periaqueductal gray

Chronic itch severely reduces the quality of life of patients. Electroacupuncture (EA) is widely used to treat chronic itch. However, the underlying mechanism of this therapeutic action of EA is largely unknown. Cannabinoid CB1 receptors in the ventrolateral periaqueductal gray (vlPAG) mediate the analgesic effect of EA. Using a dry skin-induced itch model in mice, we determined whether EA treatment reduces chronic itch via CB1 receptors in the vlPAG. We showed that the optimal inhibitory effect of EA on chronic itch was achieved at the high frequency and high intensity (100 Hz and 3 mA) at “Quchi” (LI11) and “Hegu” (LI14) acupoints, which are located in the same spinal dermatome as the cervical skin lesions. EA reversed the increased expression of CB1 receptors in the vlPAG and decreased the concentration of 5-hydroxytryptamine (5-HT) in the medulla oblongata and the expression of gastrin-releasing peptide receptors (GRPR) in the cervical spinal cord. Furthermore, knockout of CB1 receptors on GABAergic neurons in the vlPAG attenuated scratching behavior and the 5-HT concentration in the medulla oblongata. In contrast, knockout of CB1 receptors on glutamatergic neurons in the vlPAG blocked the antipruritic effects of EA and the inhibitory effect of EA on the 5-HT concentration in the medulla oblongata. Our findings suggest that EA treatment reduces chronic itch by activation of CB1 receptors on glutamatergic neurons and inhibition of CB1 receptors on GABAergic neurons in the vlPAG, thereby inhibiting the 5-HT release from the medulla oblongata to GRPR-expressing neurons in the spinal cord. Our findings suggest that EA attenuates chronic itch via activating CB1 receptors expressed on glutamatergic neurons and downregulating CB1 receptors on GABAergic neurons in the vlPAG, leading to the reduction in 5-HT release in the rostroventral medulla and GRPR signaling in the spinal cord. Our study not only advances our understanding of the mechanisms of the therapeutic effect of EA on chronic itch but also guides the selection of optimal parameters and acupoints of EA for treating chronic itch.

Cell type-specific calcium imaging of central sensitization in mouse dorsal horn

Allodynia is a state in which pain is elicited by innocuous stimuli. Capsaicin applied to the skin results in an allodynia that extends to a broad region beyond the application site. This sensitization is thought to be mediated by spinal networks; however, we do not have a clear picture of which spinal neurons mediate this phenomenon. To address this gap, we used two-photon calcium imaging of excitatory interneurons and spinal projection neurons in the mouse spinal dorsal horn. To distinguish among neuronal subtypes, we developed CICADA, a cell profiling approach to identify cell types during calcium imaging. We then identified capsaicin-responsive and capsaicin-sensitized neuronal populations. Capsaicin-sensitized neurons showed emergent responses to innocuous input and increased receptive field sizes consistent with psychophysical reports. Finally, we identified spinal output neurons that showed enhanced responses from innocuous input. These experiments provide a population-level view of central sensitization and a framework with which to model somatosensory integration in the dorsal horn.

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.

Molecular Determinants of Mechanical Itch Sensitization in Chronic Itch

Chronic itch is associated with sensitization of the somatosensory nervous system. Recent studies have identified the neural circuits transmitting acute itch; however, the mechanisms by which itch transforms into a pathological state remain largely unknown. We have previously shown that Aβ low-threshold mechanoreceptors, together with spinal urocortin 3-positive (Ucn3+) excitatory interneurons and neuropeptide Y-positive (NPY+) inhibitory interneurons, form a microcircuit that transmits and gates acute mechanical itch. Here, using whole-cell patch-clamp recordings, we observed increased excitability in spinal Ucn3+ neurons under chronic itch conditions. In contrast to Ucn3+ neurons, the excitability of spinal NPY+ neurons was largely reduced under chronic itch conditions. To explore the molecular mechanisms underlying sensitization of this microcircuit, we examined the mRNA expression levels of voltage-gated ion channels in recorded spinal Ucn3+ and NPY+ neurons by single-cell quantitative real-time PCR (qRT-PCR). We found that the expression levels of Nav1.6 and Cav2.3 channels were increased in spinal Ucn3+ neurons in chronic itch mice, while the expression level of SK3 channels was decreased. By contrast, the expression levels of Nav1.6 and BK channels were decreased in spinal NPY+ neurons in chronic itch mice. To determine the contribution of different ion channels in chronic itch sensitization, we then used a Markov Chain Monte Carlo method to parameterize a large number of biophysically distinct multicompartment models of Ucn3+ and NPY+ neurons. These models included explicit representations of the ion channels that we found to be up- or down-regulated under chronic itch conditions. Our models demonstrated that changes in Nav1.6 conductance are predominantly responsible for the changes in excitability of both Ucn3+ and NPY+ neurons during chronic itch pathogenesis. Furthermore, when simulating microcircuits of our Ucn3+ and NPY+ models, we found that reduced Nav1.6 conductance in NPY+ models played a major role in opening the itch gate under chronic itch conditions. However, changing SK, BK, or R-type calcium channel conductance had negligible effects on the sensitization of this circuit. Therefore, our results suggest that Nav1.6 channels may play an essential role in mechanical itch sensitization. The findings presented here may open a new avenue for developing pharmaceutical strategies to treat chronic itch.

Spinal ascending pathways for somatosensory information processing

The somatosensory system processes diverse types of information including mechanical, thermal, and chemical signals. It has an essential role in sensory perception and body movement and, thus, is crucial for organism survival. The neural network for processing somatosensory information comprises multiple key nodes. Spinal projection neurons represent the key node for transmitting somatosensory information from the periphery to the brain. Although the anatomy of spinal ascending pathways has been characterized, the mechanisms underlying somatosensory information processing by spinal ascending pathways are incompletely understood. Recent studies have begun to reveal the diversity of spinal ascending pathways and their functional roles in somatosensory information processing. Here, we review the anatomic, molecular, and functional characteristics of spinal ascending pathways.

Mechanisms and therapeutic targets for neuropathic itch

Neuropathic pruritus conditions arise from structural and/or functional damage of the peripheral or central nervous system. Novel findings of pruritus specific mediators and pathways strengthen the specificity theory of pruritus transmission, however electrophysiological studies suggest that focal activation of nociceptors and distinct discharge patterns of primary afferents also contribute to the development of the sensation of pruritus. A complex interplay between excitatory and inhibitory interneurons at spinal level, non-neuronal cells and descending modulation from upper centers contributes to neuronal sensitization and clinically to the chronicity of pruritus, as well as accompanying phenomena such as alloknesis and hyperknesis. Several topical, systemic and non-pharmacological therapeutic approaches directed at distinct targets are currently available.

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.

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.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.

The Spino-Parabrachial Pathway for Itch

Itch-induced scratching is an evolutionarily conserved behavioral response that protects organisms from potential parasites/irritants in their immediate vicinity. How the exposure to a pruritogen is translated to the perception of itch and how that perception drives scratching directed towards the site of exposure remains poorly understood. In this review, we focus on the recent findings that shed light on the neural pathways in the brain that underlie itch-induced scratching. We compare the molecularly defined itch pathways with the known pain circuits as they have anatomical and functional overlap. We review the roles played by the neurons in the spinoparabrachial pathway-comprising of the neurons in the spinal cord and the parabrachial nucleus (PBN), which acts as a hub for transmitting itch information across the brain. Lastly, we deliberate on scratching as a behavioral measure of the intensity of itch and its implication in unraveling the underlying supraspinal mechanisms. In summary, we provide a resource on the recent advances and discuss a path forward on our understanding of the neural circuits for itch.

Topical Treatment of Colquhounia Root Relieves Skin Inflammation and Itch in Imiquimod-Induced Psoriasiform Dermatitis in Mice

Itch is one of the major clinical manifestations of psoriasis, which is closely related with neurogenic inflammation and difficult to control. Colquhounia Root (CR) is a Chinese herb exhibiting broad bioactivities on anti-inflammation. This study was designed to explore the antipsoriatic and anti-itch potential of CR and its underlying mechanisms. Mice in a model of imiquimod-induced psoriasiform dermatitis were treated topically with CR for 7 days, and the severity of skin lesions and itch was significantly ameliorated. CR reduced the inflammatory cell infiltration, as well as mast cells in skins. Particularly, the expression of inflammatory cytokines and chemokine including Il17a, Il22, and Ccl20 and itch-related molecules such as SP, CGRP, and NGF in lesions were decreased in diseased mice upon application with CR. The normal human epidermal keratinocytes were stimulated with the M5 cytokine cocktail, the mixture of IL-17A, IL-22, Oncostatin M, IL-1α, and TNF-α, and cell viability and mRNA expression levels of inflammatory factors and itch-related molecules were measured after being treated with CR. We found that CR inhibited both cell hyperproliferation and overexpression of inflammatory cytokines and itch-related molecules in vitro. Altogether, we conclude that CR relieves psoriatic lesions and itch via controlling immunological and neurogenic inflammation.

Discrete field potentials produced by coherent activation of spinal dorsal horn neurons

In addition to the action potentials generated by the ongoing activation of single dorsal horn neurons in the anesthetized cat, we often recorded small negative field potentials with a fast-rising phase and a slow decay (dIFPs). These potentials could be separated in different classes, each with a specific and rather constant shape and amplitude. They were largest in spinal laminae III-V and gradually faded at deeper locations, without showing the polarity reversal displayed at these depths by the focal potentials produced by stimulation of muscle and cutaneous afferents. We propose that the dIFPs are postsynaptic field potentials generated by strongly coupled sets of dorsal horn neurons displaying a spatial orientation that generates closed field potentials in response to stimulation of high-threshold cutaneous and muscle afferents. These neuronal sets could form part of the spinal inhibitory circuitry that mediates presynaptic inhibition and Ib non-reciprocal postsynaptic inhibition and could be involved in the sensory-motor transformations activated by stimulation of high-threshold cutaneous afferents.

Study of the Involvement of the P2Y12 Receptor in Chronic Itching in Type 2 Diabetes Mellitus

Itching is a common clinical symptom in diabetic patients. This research is to carry out experiments on the pathological changes in the P2Y12 receptor in type 2 diabetes mellitus complicated with chronic itching. Changes in body weight, fasting blood glucose (FBG), thermal hyperalgesia, cold hyperalgesia, spontaneous itching, and sciatic nerve conduction velocity were detected. The content of reactive oxygen species (ROS) in the dorsal root ganglion was detected by chemical fluorescence. The expression of the P2Y12 receptor, NLRP3, ASC, interleukin-1β (IL-1β), and IL-18 was detected by Western blotting, real-time quantitative PCR, immunofluorescence double labelling, and enzyme-linked immunosorbent assay. Itching and pain behaviours of the mice in the type 2 diabetes mellitus + itch group were significantly increased, and the expression of P2Y12 and NLRP3 as well as the content of ROS increased, and these changes were significantly reversed by treatment with P2Y12 short hairpin RNA (shRNA) or P2Y12 antagonist ticagrelor. Upregulated P2Y12 receptor expression after the activation of satellite glial cells contributes to the increase in ROS content in vivo, followed by NLRP3 inflammasome activation, increased inflammatory cytokine release, and damage to peripheral nerves, which leads to chronic itching. Treatment with P2Y12 shRNA or ticagrelor can inhibit these pathological changes, thus improving itching behaviour. Development mechanism of diabetes mellitus complicated with chronic itching. Notes: The upregulation of P2Y12 receptor expression and the activation of SGCs lead to the increase of ROS content in vivo, followed by the activation of NLRP3 inflammasome, the increase of inflammatory cytokine release, the abnormal excitation of DRG neurons, and the damage of peripheral nerves, resulting in chronic itching. P2Y12 receptor-related inflammatory injury involves chronic itching in type 2 diabetes mellitus. Treatment with P2Y12 receptor shRNA or P2Y12 antagonist ticagrelor can inhibit these pathological changes and improve itching behaviour.

Selective activation of TRPA1 ion channels by nitrobenzene skin sensitizers DNFB and DNCB

2, 4-dinitrofluorobenzene (DNFB) and 2, 4-dinitrochlorobenzene (DNCB) are well known as skin sensitizers that can cause dermatitis. DNFB has shown to more potently sensitize skin; however, how DNFB and DNCB cause skin inflammation at a molecular level and why this difference in their sensitization ability is observed remains unknown. In this study, we aimed to identify the molecular targets and mechanisms on which DNFB and DNCB act. We used a fluorescent calcium imaging plate reader in an initial screening assay before patch-clamp recordings for validation. Molecular docking in combination with site-directed mutagenesis was then carried out to investigate DNFB and DNCB binding sites in the TRPA1 ion channel that may be selectively activated by these tow sensitizers. We found that DNFB and DNCB selectively activated TRPA1 channel with EC₅₀ values of 2.3 ± 0.7 μM μM and 42.4 ± 20.9 μM, respectively. Single-channel recordings revealed that DNFB and DNCB increase the probability of channel opening and acts on three residues (C621, E625 and Y658) critical for TRPA1 activation. Our findings may not only help explain the molecular mechanism underlying the dermatitis and pruritus caused by chemicals like DNFB and DNCB, but also provide a molecular tool 7.5-fold more potent than the current TRPA1 activator allyl isothiocyanate (AITC) used for investigating TRPA1 channel pharmacology and pathology.