Identification of a Spinal Circuit for Mechanical and Persistent Spontaneous Itch

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).

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.

Local keratinocyte-nociceptor interactions enhance obesity-mediated small fiber neuropathy via NGF-TrkA-PI3K signaling axis

The pathology of diabetic small fiber neuropathy, characterized by neuropathic pain and axon degeneration, develops locally within the skin during the stages of obesity and pre-diabetes. However, the initiation and progression of morphological and functional abnormalities in skin sensory nerves remains elusive. To address this, we utilized ear skin from mice with diet-induced obesity (DIO), the mouse models for obesity and pre-type 2 diabetes. We evaluated pain-associated wiping behavior and conducted ex vivo live Ca 2+ imaging of the DIO ear skin to detect sensory hypersensitivity. Our findings reveal sensory hypersensitivity in skin nociceptive axons followed by axon degeneration. Further mechanistic analysis identified keratinocytes as a major source of nerve growth factor (NGF) in DIO skin, which locally sensitizes nociceptors through NGF-mediated signaling. Indeed, the local inactivation of NGF and its receptor TrkA-mediated downstream signaling, including the phosphoinositide 3-kinases (PI3K) pathway, suppresses sensory hypersensitivity in DIO skin. Thus, targeting these local interactions between keratinocytes and nociceptors offers a therapeutic strategy for managing neuropathic pain, avoiding the adverse effects associated with systemic interventions.

High-speed imaging of evoked rodent mechanical behaviors yields variable results that are not predictive of inflammatory injury

ABSTRACT

Few analgesics identified using preclinical models have successfully translated to clinical use. These translational limitations may be due to the unidimensional nature of behavioral response measures used to assess rodent nociception. Advances in high-speed videography for pain behavior allow for objective quantification of nuanced aspects of evoked paw withdrawal responses. However, whether videography-based assessments of mechanical hypersensitivity outperform traditional measurement reproducibility is unknown. First, we determined whether high-speed videography of paw withdrawal was reproducible across experimenters. Second, we examined whether this method distinguishes behavioral responses exhibited by naive mice and mice with complete Freund's adjuvant (CFA)-induced inflammation. Twelve experimenters stimulated naive C57BL/6 mice with varying mechanical stimuli. Paw withdrawal responses were recorded with high-speed videography and scored offline by one individual. Our group was unable to replicate the original findings produced by high-speed videography analysis. Surprisingly, ∼80% of variation was not accounted for by variables previously reported to distinguish between responses to innocuous and noxious stimuli (paw height, paw velocity, and pain score), or by additional variables (experimenter, time-of-day, and animal), but rather by unidentified factors. Similar high-speed videography assessments were performed in CFA- and vehicle-treated animals, and the cumulative data failed to reveal an effect of CFA injection on withdrawal as measured by high-speed videography. This study does not support using paw height, velocity, or pain score measurements from high-speed recordings to delineate behavioral responses to innocuous and noxious stimuli. Our group encourages the continued use of traditional mechanical withdrawal assessments until additional high-speed withdrawal measures are validated in established pain models.

Peripheral Mechanisms of Mechanical Itch

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

Rodent models of dermatological disorders

To assess the possible beneficial effects of drugs and drug candidates, different dermatological disease models are available in rodents. These models are able to mimic one or more characteristic features of the disorders, but not completely recapitulate the pathogenesis of the human skin diseases. Therefore, to improve the technology many new models have been developed both by genetic engineering and by chemical or physical induction. Currently the in vivo rodent models provide the physiologically most relevant approach to produce the pathology related to the majority of dermatological diseases. In this short review some widely used animal techniques (psoriasis, allergic contact dermatitis, atopic dermatitis, wound healing, melanoma and non-melanoma type skin cancers and UV erythema) are shown which are currently applied in pharmacological, pharmacokinetic, pharmaceutical and dermatological research. First the main points of the human pathomechanism are shown and afterwards the rodent models are briefly discussed. Finally critical evaluation is provided by the authors. However, according to the 3R rule the number of experimental animals is strongly suggested to be reduced, therefore the advanced in vitro and ex vivo techniques become more and more important contrary to in vivo preclinical methods also in dermatological research. As it is described in the outlook section, although the 2D/3D in vitro and skin on-a-chip techniques are promising and have many advantages they are not able to completely substitute the animal models in their vascular, immunological, secretory and neural complexity.

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.

Novel aspects of signal processing in lamina I

The most superficial layer of the spinal dorsal horn, lamina I, is a key element of the nociceptive processing system. It contains different types of projection neurons (PNs) and local-circuit neurons (LCNs) whose functional roles in the signal processing are poorly understood. This article reviews recent progress in elucidating novel anatomical features and physiological properties of lamina I PNs and LCNs revealed by whole-cell recordings in ex vivo spinal cord. This article is part of the Special Issue on "Ukrainian Neuroscience".

The kainate receptor GluK2 mediates cold sensing in mice

Thermosensors expressed in peripheral somatosensory neurons sense a wide range of environmental temperatures. While thermosensors detecting cool, warm and hot temperatures have all been extensively characterized, little is known about those sensing cold temperatures. Though several candidate cold sensors have been proposed, none has been demonstrated to mediate cold sensing in somatosensory neurons in vivo, leaving a knowledge gap in thermosensation. Here we characterized mice lacking the kainate-type glutamate receptor GluK2, a mammalian homolog of the Caenorhabditis elegans cold sensor GLR-3. While GluK2 knockout mice respond normally to heat and mechanical stimuli, they exhibit a specific deficit in sensing cold but not cool temperatures. Further analysis supports a key role for GluK2 in sensing cold temperatures in somatosensory DRG neurons in the periphery. Our results reveal that GluK2-a glutamate-sensing chemoreceptor mediating synaptic transmission in the central nervous system-is co-opted as a cold-sensing thermoreceptor in the periphery.

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.

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.

MrgprA3+ Primary Sensory Neurons Mediate Acute Allergic Itch Responses in Atopic Dermatitis Model Mice

Itch is a prominent symptom of atopic dermatitis (AD). However, the underlying mechanism remains complex and has not yet been fully elucidated. Mas-related G protein-coupled receptor A3 (MrgprA3) has emerged attention as a marker of primary sensory neurons that specifically transmit itch signals; however, its involvement in AD-related itch has not been extensively explored. In this study, we developed an AD itch mouse model by repeatedly applying house dust mite (HDM) extract to barrier-impaired skin via a special diet. To clarify the role of MrgprA3+ neurons in itch behavior in our AD model, we adopted a toxin receptor-mediated cell knockout strategy using transgenic mice in which the diphtheria toxin receptor (DTR) gene was placed under the control of the Mrgpra3 promoter. When the HDM extract was repeatedly applied to the face and back skin of special diet-fed mice, the mice exhibited AD-like dry and eczematous skin lesions accompanied by three types of itch-related behaviors:1) spontaneous scratching, 2) acute scratching after antigen challenge, and 3) light touch-evoked scratching. Upon diphtheria toxin administration, substantial depletion of DTR+/MrgprA3+ neurons was observed in the dorsal root ganglion. Ablation of MrgprA3+ neurons suppressed acute itch responses after HDM application, whereas spontaneous and touch-evoked itch behaviors remained unaffected. Our findings unequivocally demonstrate that in our AD model, MrgprA3+ primary sensory neurons mediate acute allergic itch responses, whereas these neurons are not involved in spontaneous itch or alloknesis.

Mechanisms of the PD-1/PD-L1 pathway in itch: From acute itch model establishment to the role in chronic itch in mouse

Programmed cell death receptor/ligand 1 (PD-1/PD-L1) blockade therapy for various cancers induces itch. However, few studies have evaluated the mechanism underlying PD-1/PD-L1 inhibitor-induced itch. This study aimed to establish and evaluate a mouse model of acute itch induced by PD-1/PD-L1 inhibitors and to explore the role of the PD-1/PD-L1 pathway in chronic itch. The intradermal injection of the PD-1/PD-L1 small molecule inhibitors, or anti-PD-1/PD-L1 antibodies in the nape of the neck in the mice elicited intense spontaneous scratches. The model was evaluated using pharmacological methods. The number of scratches was reduced by naloxone but not by antihistamines or the transient receptor potential (TRP) channel inhibitor. Moreover, the PD-1 receptor was detected in the spinal cord of the mouse models of chronic itch that exhibited acetone, diethyl ether, and water (AEW)-induced dry skin, imiquimod-induced psoriasis, and 1-fluoro-2,4-dinitrobenzene (DNFB)-induced allergic contact dermatitis. Intrathecal PD-L1 (1 μg, 4 times a week for 1 week) suppressed the activation of the microglia in the spinal dorsal horn to relieve the chronic itch that was elicited by imiquimod-induced psoriasis and DNFB-induced allergic contact dermatitis. Although the activation of the microglia in the spinal dorsal horn was not detected in the AEW-treated mice, intrathecal PD-L1 still reduced the number of scratches that were elicited by AEW. Our findings suggest that histamine receptor inhibitors or TRP channel inhibitors have limited effects on PD-1/PD-L1 inhibitor-induced itch and that spinal PD-1 is important for the spinal activation of the microglia, which may underlie chronic itch.

PTEN deletion in spinal pathways via retrograde transduction with AAV-RG enhances forelimb motor recovery after cervical spinal cord injury; Sex differences and late-onset pathophysiologies

Spinal cord injuries (SCI) cause permanent functional impairments due to interruption of motor and sensory pathways. Regeneration of axons does not occur due to lack of intrinsic growth capacity of adult neurons and extrinsic inhibitory factors, especially at the injury site. However, some regeneration can be achieved via deletion of the phosphatase and tensin homolog (PTEN) in cells of origin of spinal pathways. Here, we deployed an AAV variant that is retrogradely transported (AAV-rg) to deliver gene modifying cargos to the cells of origin of multiple pathways interrupted by SCI, testing whether this promoted recovery of motor function. PTENf/f;RosatdTomato mice and control RosatdTomato mice received injections of different doses (number of genome copies, GCs) of AAV-rg/Cre into the cervical spinal cord at the time of a C5 dorsal hemisection injury. Forelimb grip strength was tested over time using a grip strength meter. PTENf/f;RosatdTomato mice with AAV-rg/Cre (PTEN-deleted) exhibited substantial improvements in forelimb gripping ability in comparison to controls. Of note, there were major sex differences in the extent of recovery, with male mice exhibiting greater recovery than females. However, at around 5-7 weeks post-injury/injection, many mice with SCI and AAV-rg-mediated PTEN deletion began to exhibit pathophysiologies involving excessive scratching of the ears and back of the neck and rigid forward extension of the hindlimbs. These pathophysiologies increased in incidence and severity over time. Our results reveal that although intra-spinal injections of AAV-rg/Cre in PTENf/f;RosatdTomato mice can enhance forelimb motor recovery after SCI, late-developing functional abnormalities occur with the experimental conditions used here. Mechanisms underlying late-developing pathophysiologies remain to be defined.

Microglia are involved in regulating histamine-dependent and non-dependent itch transmissions with distinguished signal pathways

Although itch and pain have many similarities, they are completely different in perceptual experience and behavioral response. In recent years, we have a deep understanding of the neural pathways of itch sensation transmission. However, there are few reports on the role of non-neuronal cells in itch. Microglia are known to play a key role in chronic neuropathic pain and acute inflammatory pain. It is still unknown whether microglia are also involved in regulating the transmission of itch sensation. In the present study, we used several kinds of transgenic mice to specifically deplete CX3CR1+ microglia and peripheral macrophages together (whole depletion), or selectively deplete microglia alone (central depletion). We observed that the acute itch responses to histamine, compound 48/80 and chloroquine were all significantly reduced in mice with either whole or central depletion. Spinal c-fos mRNA assay and further studies revealed that histamine and compound 48/80, but not chloroquine elicited primary itch signal transmission from DRG to spinal Npr1- and somatostatin-positive neurons relied on microglial CX3CL1-CX3CR1 pathway. Our results suggested that microglia were involved in multiple types of acute chemical itch transmission, while the underlying mechanisms for histamine-dependent and non-dependent itch transmission were different that the former required the CX3CL1-CX3CR1 signal pathway.

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.

Antipruritic effects of geraniol on acute and chronic itch via modulating spinal GABA/GRPR signaling

BACKGROUND AND PURPOSE

Itch (pruritus) is a common unpleasant feeling, often accompanied by the urge of scratching the skin. It is the main symptom of many systemic and skin diseases, which can seriously affect the patient's quality of life. Geraniol (GE; trans-3,7-dimethyl-2,6-octadien-1-ol) is a natural monoterpene with diverse effects, including anti-inflammatory, antioxidant, neuroprotective, anti-nociceptive, and anticancer properties. The study aims to examine the effects of GE on acute and chronic itch, and explore the underlying mechanisms.

METHODS

Acute itch was investigated by using Chloroquine and compound 48/80 induced model, followed by manifestation of diphenylcyclopropenone (DCP)-induced allergic contact dermatitis and the acetone-ether-water (AEW)-induced dry skin model in mice. The scratching behavior, skin thickness, c-Fos expression, and GRPR protein expression in the spinal cord were subsequently monitored and evaluated by behavioral tests as well as pharmacological and pharmacogenetic technologies.

RESULTS

Dose-dependent intraperitoneal injection of GE alleviated the acute itch, induced by chloroquine and compound 48/80, as well as increased the spinal c-Fos expression. Intrathecal administration of GE suppressed the GABAA receptor inhibitor bicuculline-induced itch, GRP-induced itch, and the GABAergic neuron inhibition-induced itch. Furthermore, the subeffective dose of bicuculline blocked the anti-pruritic effect of GE on the chloroquine and compound 48/80 induced acute itch. GE also attenuated DCP and AEW-induced chronic itch, as well as the increase of spinal GRPR expression in DCP mice.

CONCLUSION AND IMPLICATIONS

GE alleviates both acute and chronic itch via modulating the spinal GABA/GRPR signaling in mice. Findings of this study reveal that GE may provide promising therapeutic options for itch management. Also, considering the pivotal role of essential oils in aromatherapy, GE has great application potential in aromatherapy for treating skin diseases, and especially the skin with severe pruritus.

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.

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.

PTEN deletion in spinal pathways via retrograde transduction with AAV-rg enhances forelimb motor recovery after cervical spinal cord injury; sex differences and late-onset pathophysiologies

Spinal cord injuries (SCI) cause permanent functional impairments due to interruption of motor and sensory pathways. Regeneration of axons does not occur due to lack of intrinsic growth capacity of adult neurons and extrinsic inhibitory factors, especially at the injury site. However, some regeneration can be achieved via deletion of the phosphatase and tensin homolog (PTEN) in cells of origin of spinal pathways. Here, we deployed an AAV variant that is retrogradely transported (AAV-rg) to deliver gene modifying cargos to the cells of origin of multiple pathways interrupted by SCI, testing whether this promoted recovery of motor function. PTEN f/f ;Rosa tdTomato mice and control Rosa tdTomato mice received injections of different doses (number of genome copies, GCs) of AAV-rg/Cre into the cervical spinal cord at the time of a C5 dorsal hemisection injury. Forelimb grip strength was tested over time using a grip strength meter. PTEN f/f ;Rosa tdTomato mice with AAV-rg/Cre (PTEN-deleted) exhibited substantial improvements in forelimb gripping ability in comparison to controls. Of note, there were major sex differences in the extent of recovery, with male mice exhibiting greater recovery than females. However, at around 5-7 weeks post-injury/injection, many mice with SCI and AAV-rg-mediated PTEN deletion began to exhibit pathophysiologies involving excessive scratching of the ears and back of the neck and rigid forward extension of the hindlimbs. These pathophysiologies increased in incidence and severity over time. Our results reveal that although intra-spinal injections of AAV-rg/Cre in PTEN f/f ;Rosa tdTomato mice can enhance forelimb motor recovery after SCI, late-developing functional abnormalities occur with the experimental conditions used here. Mechanisms underlying late-developing pathophysiologies remain to be defined.

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.

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.

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.

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.

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.

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.

Inflammation and Infection in Pain and the Role of GPR37

Inflammation is known to cause pain, and pain is of one of the cardinal signs of inflammation. Mounting evidence suggests that acute inflammation also resolves pain through specialized pro-resolving mediators (SPMs) and macrophage signaling. GPR37 is expressed by neurons and oligodendrocytes in the brain and has been implicated in multiple disorders, such as demyelination, Parkinson's disease, stroke, and cancer. Recent studies have demonstrated that GPR37 is expressed by macrophages and confers protection against infection by bacteria and parasites. Furthermore, GPR37 promotes the resolution of inflammatory pain and infection-induced pain, as the duration of pain after tissue injury and infection is prolonged in mice lacking Gpr37. Mechanistically, activation of GPR37 enhances macrophage phagocytosis, and Gpr37-deficient macrophages exhibit dysregulations of pro-inflammatory and anti-inflammatory cytokines, switching from M2- to M1-like phenotypes. We also discuss novel ligands of GPR37, including neuroprotectin D1 (NPD1), a SPM derived from docosahexaenoic acid (DHA), and bone-derived hormone osteocalcin (OCN), which can suppress oligodendrocyte differentiation and myelination. NPD1 stimulates macrophage phagocytosis via GPR37 and exhibits potent analgesic actions in various animal models of inflammatory and neuropathic pain. Targeting GPR37 may lead to novel therapeutics for treating inflammation, infection, pain, and neurological diseases.

The contribution of mouse models to understanding atopic dermatitis

Atopic dermatitis (AD) is a dermatological disease accompanied by dry and cracked skin with severe pruritus. Although various therapeutic strategies have been introduced to alleviate AD, it remains challenging to cure the disorder. To achieve such a goal, understanding the pathophysiological mechanisms of AD is a prerequisite, requiring mouse models that properly reflect the AD phenotypes. Currently, numerous AD mouse models have been established, but each model has its own advantages and weaknesses. In this review, we categorized and summarized mouse models of AD and described their characteristics from a researcher's perspective.

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.

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.

Dorsal Root Ganglion Stimulation for Chronic Pain: Hypothesized Mechanisms of Action

Dorsal root ganglion stimulation (DRGS) is a neuromodulation therapy for chronic pain that is refractory to conventional medical management. Currently, the mechanisms of action of DRGS-induced pain relief are unknown, precluding both our understanding of why DRGS fails to provide pain relief to some patients and the design of neurostimulation technologies that directly target these mechanisms to maximize pain relief in all patients. Due to the heterogeneity of sensory neurons in the dorsal root ganglion (DRG), the analgesic mechanisms could be attributed to the modulation of one or many cell types within the DRG and the numerous brain regions that process sensory information. Here, we summarize the leading hypotheses of the mechanisms of DRGS-induced analgesia, and propose areas of future study that will be vital to improving the clinical implementation of DRGS. PERSPECTIVE: This article synthesizes the evidence supporting the current hypotheses of the mechanisms of action of DRGS for chronic pain and suggests avenues for future interdisciplinary research which will be critical to fully elucidate the analgesic mechanisms of the therapy.

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.

Central, but not peripheral, nervous system ERK2 is essential for itch signals in murine allergic skin inflammation

BACKGROUND

Itch is a common cutaneous symptom in a variety of dermatological diseases, but detailed neuropathological mechanisms remain to be fully elucidated. This study aimed to assess in vivo ERK2 functions in the nervous system for itch responses.

METHODS

We generated conditional knockout mice deficient in ERK2 of the central nervous system (CNS) or peripheral nervous system (PNS), respectively, and assessed chemical and mechanical itch responses in vivo.

RESULTS

Chemical itch responses to histamine, but not to BAM8-22, were alleviated in CNS Erk2-deficient mice. In contrast, both histamine- and BAM8-22-induced mechanical itch (alloknesis) were alleviated in CNS Erk2-deficient mice. Neither chemical itch nor mechanical itch induced by these pruritogens was affected by PNS ERK2 deficiency. Spontaneous scratching behaviors during acute and chronic contact hypersensitivity were impaired in CNS Erk2-deficient mice, but not PNS Erk2-deficient mice. In addition, CNS ERK2 deficiency attenuated mechanical itch responses during chronic contact hypersensitivity. Again, PNS Erk2-deficient mice showed comparable responses of mechanical itch to control mice. In addition, alleviated mechanical itch in CNS Erk2-deficient mice was observed in IgE-mediated prurigo-like allergic skin inflammation. Mechanical itch induced by IL-31 was also alleviated by CNS ERK2 deficiency. Phosphorylated ERK1/2 was detected in neurokinin B-expressing cells of the spinal dorsal horn of control mice; these cells accumulated during the induction of chronic contact hypersensitivity. Notably, phosphorylated ERK1/2 was also localized in spinal urocortin3-expressing neurons that are known to transmit mechanical itch.

CONCLUSIONS

Spinal cord ERK2 could be a potential therapeutic target for intractable itch in pruritic skin diseases.

A neuropeptide code for itch

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

Circuit Mechanisms of Itch in the Brain

Itch is an unpleasant somatic sensation with the desire to scratch, and it consists of sensory, affective, and motivational components. Acute itch serves as a critical protective mechanism because an itch-evoked scratching response will help to remove harmful substances invading the skin. Recently, exciting progress has been made in deciphering the mechanisms of itch at both the peripheral nervous system and the CNS levels. Key neuronal subtypes and circuits have been revealed for ascending transmission and the descending modulation of itch. In this review, we mainly summarize the current understanding of the central circuit mechanisms of itch in the brain.

A harmonized atlas of mouse spinal cord cell types and their spatial organization

Single-cell RNA sequencing data can unveil the molecular diversity of cell types. Cell type atlases of the mouse spinal cord have been published in recent years but have not been integrated together. Here, we generate an atlas of spinal cell types based on single-cell transcriptomic data, unifying the available datasets into a common reference framework. We report a hierarchical structure of postnatal cell type relationships, with location providing the highest level of organization, then neurotransmitter status, family, and finally, dozens of refined populations. We validate a combinatorial marker code for each neuronal cell type and map their spatial distributions in the adult spinal cord. We also show complex lineage relationships among postnatal cell types. Additionally, we develop an open-source cell type classifier, SeqSeek, to facilitate the standardization of cell type identification. This work provides an integrated view of spinal cell types, their gene expression signatures, and their molecular organization.

Chronic itch: emerging treatments following new research concepts

Until recently, itch pathophysiology was poorly understood and treatments were poorly effective in relieving itch. Current progress in our knowledge of the itch processing, the numerous mediators and receptors involved has led to a large variety of possible therapeutic pathways. Currently, inhibitors of IL-31, IL-4/13, NK₁ receptors, opioids and cannabinoids, JAK, PDE4 or TRP are the main compounds involved in clinical trials. However, many new targets, such as Mas-related GPCRs and unexpected new pathways need to be also explored.

Cinnamaldehyde elicits itch behavior via TRPV1 and TRPV4 but not TRPA1

Introduction:

Cinnamaldehyde (CA) elicits itch sensation in humans. We investigated if CA elicits scratching behavior in mice and determined the roles for TRPV1, TRPA1, and TRPV4.

Materials and Methods:

Scratching behavior elicited by intradermal injection of CA was assessed in wildtype (WT) mice and knockout (KO) mice lacking TRPV1, TRPA1, TRPV4, or deficient in mast cells. We also assessed scratching and wet dog shakes elicited by low-threshold mechanical stimulation of skin treated topically with CA or vehicle. Using calcium imaging we tested if CA activates dorsal root ganglion (DRG) neurons of each genotype.

Results:

Intradermal cheek injection of CA elicited dose-dependent hindlimb scratch bouts, with fewer forelimb wipes and facial groom bouts that were not dose-dependent. CA elicited significantly fewer scratch bouts in TRPV1 and TRPV4 KO mice, but not TRPA1KOs, compared with WTs. There were no sex differences across genotypes. The histamine H1 antagonist cetirizine did not affect CA-evoked scratching, which was normal in mast cell deficient mice, indicating lack of histamine involvement. Scores for alloknesis were significantly greater following topical application of CA compared with vehicle. Post-CA alloknesis scores were significantly higher in TRPV4KOs of both sexes and in female TRPV1 and TRPA1KOs, compared with WTs. Low threshold mechanical stimuli also elicited significantly more wet dog shakes in mice treated topically with 20% CA, with significantly fewer in TRPV1, TRPA1, and TRPV4KOs compared with WTs. In calcium imaging studies, CA excited 24% of WT DRG cells, significantly fewer (11.5%) in cells from TRPV4KOs, and none in TRPA1KOs. Responses of cells of all genotypes exhibited significant sensitization to repeated CA stimulation. Sensitization was significantly enhanced by IL-4, which itself excited 16% of WT DRG cells and none from TRPA1KOs.

Discussion:

The results indicate that TRPA1 is dispensable for CA-evoked scratching, which depends partly on TRPV1 and TRPV4.