Chronic itch and chronic pain: Analogous mechanisms

Atopic Dermatitis Itch: Scratching for an Explanation

Chronic pruritus is a cardinal symptom of atopic dermatitis (AD). The mechanisms underlying atopic itch involve intricate crosstalk among skin, immune components, and neural components. In this review, we explore these mechanisms, focusing on key players and interactions that induce and exacerbate itch. We discuss the similarities and differences between pruritus and pain in patients with AD as well as the relationship between pruritus and factors such as sweat and the skin microbiome. Furthermore, we explore novel targets that could provide significant itch relief in these patients as well as exciting future research directions to better understand atopic pruritus in darker skin types.

The G protein-coupled estrogen receptor of the trigeminal ganglion regulates acute and chronic itch in mice

AIMS

Itch is an unpleasant sensation that severely impacts the patient's quality of life. Recent studies revealed that the G protein-coupled estrogen receptor (GPER) may play a crucial role in the regulation of pain and itch perception. However, the contribution of the GPER in primary sensory neurons to the regulation of itch perception remains elusive. This study aimed to investigate whether and how the GPER participates in the regulation of itch perception in the trigeminal ganglion (TG).

METHODS AND RESULTS

Immunofluorescence staining results showed that GPER-positive (GPER+ ) neurons of the TG were activated in both acute and chronic itch. Behavioral data indicated that the chemogenetic activation of GPER+ neurons of the TG of Gper-Cre mice abrogated scratching behaviors evoked by acute and chronic itch. Conversely, the chemogenetic inhibition of GPER+ neurons resulted in increased itch responses. Furthermore, the GPER expression and function were both upregulated in the TG of the dry skin-induced chronic itch mouse model. Pharmacological inhibition of GPER (or Gper deficiency) markedly increased acute and chronic itch-related scratching behaviors in mouse. Calcium imaging assays further revealed that Gper deficiency in TG neurons led to a marked increase in the calcium responses evoked by agonists of the transient receptor potential ankyrin A1 (TRPA1) and transient receptor potential vanilloid V1 (TRPV1).

CONCLUSION

Our findings demonstrated that the GPER of TG neurons is involved in the regulation of acute and chronic itch perception, by modulating the function of TRPA1 and TRPV1. This study provides new insights into peripheral itch sensory signal processing mechanisms and offers new targets for future clinical antipruritic therapy.

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.

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.

GABAergic neurons in the ventral lateral geniculate nucleus and intergeniculate leaflet modulate itch processing in mice

Itch and pain are two closely related sensations that receiving similar encodings at multiple levels. Accumulated evidences suggest that activation of the ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL)-to-lateral and ventrolateral periaqueductal gray (l/vlPAG) projections mediates the antinociceptive effects of bright light therapy. Clinical study showed that bright light therapy may ameliorate cholestasis-induced pruritus. However, the underlying mechanism and whether this circuit participates in itch modulation remains unclear. In this study, chloroquine and histamine were utilized to induce acute itch models in mice. Neuronal activities in vLGN/IGL nucleus were evaluated with c-fos immunostaining as well as fiber photometry. Optogenetic manipulations were performed to activate or inhibit GABAergic neurons in the vLGN/IGL nucleus. Our results showed that the expressions of c-fos in vLGN/IGL were significantly increased upon both chloroquine- and histamine-induced acute itch stimuli. GABAergic neurons in vLGN/IGL were activated during histamine and chloroquine-induced scratching. Optogenetic activation of the vLGN/IGL GABAergic neurons exerts antipruritic effect, while inhibiting these neurons exerts pruritic effect. Our results provide evidence that GABAergic neurons in vLGN/IGL nucleus might play a crucial role in modulating itch, which may provide clue for application of bright light as an antipruritic treatment in clinic.

TRPV1 in chronic pruritus and pain: Soft modulation as a therapeutic strategy

Chronic pain and pruritus are highly disabling pathologies that still lack appropriate therapeutic intervention. At cellular level the transduction and transmission of pain and pruritogenic signals are closely intertwined, negatively modulating each other. The molecular and cellular pathways involved are multifactorial and complex, including peripheral and central components. Peripherally, pain and itch are produced by subpopulations of specialized nociceptors that recognize and transduce algesic and pruritogenic signals. Although still under intense investigation, cumulative evidence is pointing to the thermosensory channel TRPV1 as a hub for a large number of pro-algesic and itchy agents. TRPV1 appears metabolically coupled to most neural receptors that recognize algesic and pruritic molecules. Thus, targeting TRPV1 function appears as a valuable and reasonable therapeutic strategy. In support of this tenet, capsaicin, a desensitizing TRPV1 agonist, has been shown to exhibit clinically relevant analgesic, anti-inflammatory, and anti-pruritic activities. However, potent TRPV1 antagonists have been questioned due to an hyperthermic secondary effect that prevented their clinical development. Thus, softer strategies directed to modulate peripheral TRPV1 function appear warranted to alleviate chronic pain and itch. In this regard, soft, deactivatable TRPV1 antagonists for topical or local application appear as an innovative approach for improving the distressing painful and itchy symptoms of patients suffering chronic pain or pruritus. Here, we review the data on these compounds and propose that this strategy could be used to target other peripheral therapeutic targets.

Druggable Targets and Compounds with Both Antinociceptive and Antipruritic Effects

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

Associations Between Atopic Dermatitis and Anxiety, Aggression, and Fear-Based Behaviors in Dogs

The goal of this study was to determine if anxiety, aggression, and fear-related behaviors are more common in pruritic dogs with atopic dermatitis than nonpruritic, healthy dogs. One hundred forty-one pruritic dogs >1 yr of age with a clinical diagnosis of atopic dermatitis and a >3 mo history of pruritus were recruited. Dog owners completed a behavioral survey (canine behavioral assessment and research questionnaire) and a pruritus scale (pruritus visual analog scale). Pruritic, atopic dogs showed significant increases in fear- and anxiety-related behaviors as well as aggression compared with a large control group of healthy dogs. Stranger-directed aggression, owner-directed aggression, familiar-dog aggression, dog-directed fear, nonsocial fear, touch sensitivity, excitability, and attention-seeking behaviors were all increased in the study group. Trainability was decreased in the study group. Chronically pruritic dogs experience fear and anxiety and are more likely to display aggression. This is an important welfare issue for these animals. Early recognition of the behavioral derangements that can be associated with chronic pruritic skin disease could allow early intervention with a multidisciplinary approach for these patients, thus improving patient and owner quality of life and long-term treatment outcomes.

Trigeminal neuropathy: Two case reports of gasserian ganglion stimulation

This report describes the successful treatment of two patients with trigeminal neuropathy by using gasserian ganglion stimulation. Case reports: The first case report deals with a 53-year-old woman suffering from right-sided facial pain after a gamma knife lesion for schwannoma of the right inner ear. For 9 years, several interventions with the aim of relieving the pain were unsuccessful; in fact, they had aggravated the symptoms. A trial with a neurostimulator at the level of the Gasser ganglion had an immediately positive effect on her score for facial pain, which decreased from 7.3 to 0 on a visual analog scale, assessed during a period of 2 months. Additionally, the patient had weaned off all her medication by the end of the period. The second case report describes a 64-year-old man suffering from trigeminal neuropathy, which mainly manifested itself as an itch. For a period of 15 years, neither medication nor several interventions were effective. A trial with an electrode at the level of the Gasser ganglion reduced his pain score from 7.0 to 1.5 on a visual analog scale, assessed during a period of three months. His medication could be limited to pregabalin 150 mg bidaily. In contrast, prior to the implantation, his oral medication consisted of pregabalin 75 mg up to five times a day. Conclusion: These case reports show that stimulation of the gasserian ganglion is a successful, minimally invasive, and non-destructive treatment in refractory trigeminal neuropathy and should be considered earlier in the treatment algorithm of trigeminal neuropathy.

What Do We Know about Pruritus in Very Young Infants? A Literature Review

In infants, pruritus is frequently considered as absent because they do not scratch themselves. Because pruritus could induce severe adverse effects in this vulnerable population, we aimed to review existing evidence on the ability of young infants to experience itch and on how to assess itch-related discomfort in this population. A literature review was performed (Pubmed, Google Scholar). Neurological itch pathways are well described. Skin development starts early during gestation. At 34 weeks of gestation, skin is almost complete while skin adaptations occur after birth. Newborn skin is neurologically functional, including the ability for young infants to feel pain. Similarities and interactions between pain and pruritus support the hypothesis that infants could feel pruritus. However, the existence of pruritus in infants has never been evidenced. Many itchy conditions can affect them, suggesting non-negligible prevalence of infant pruritus among which atopic dermatitis (AD) is the most studied disease. Studies reported a negative impact of AD on children and their families. There is no existing validated method to assess pruritus in infants, although they may feel pruritus and chronic pruritus can lead to serious adverse effects. To appropriately diagnose pruritus appears of great interest among young infants. Development of a method is required to this aim.

Enhanced Itch Intensity Is Associated with Less Efficient Descending Inhibition Processing for Itch But Not Pain Attenuation in Chronic Dermatology Patients

Objectives

The study aims were 1) to investigate the direction of mutual inhibitory pathways on itch intensity by utilizing conditioned pain modulation paradigms for pain and itch attenuation and 2) to explore whether itch severity is affected by the individual pain sensitivity profile, as well as pain scores reported during the tests and the past week.

Design

Cross-sectional.

Setting

Testing was conducted at the Department of Dermatology, Rambam Health Care Campus.

Subjects

Forty patients suffering from chronic skin disorders associated with itch and treated in the Dermatology Clinic at Rambam Health Care Campus participated in the study.

Methods

Efficacy of descending inhibition was evaluated by two conditioned pain modulation (CPM) paradigms: by pruriception (CPMItch) induced by cold and heat as counterstimuli to inhibit itch intensity and by nociception (CPMPain). Severity and interference of clinical pain were assessed using the Brief Pain Inventory (BPI).

Results

Robust CPMItch responses were obtained following the various noxious stimulations. No associations were observed between CPMPain and CPMItch, itch severity, skin disease severity, and clinical pain symptoms. According to the linear regression model, itch severity was independently associated with less efficient CPMItch (B = –0.750, P < 0.001) and more efficient CPMPain (B = 0.031, P = 0.016), which affects itch in opposing manners.

Conclusions

Findings indicate that the intrinsic capacity to inhibit pain and itch by exposure to exogenous noxious stimuli autonomously affects itch intensity in an opposing manner. These findings may shed new light on the mutual mechanistic similarity and dissimilarity between pain and itch and their hierarchy.

The role of itch and pain modulation in the prediction of phototherapy outcomes: a prospective cohort study

OBJECTIVES

Ability to predict which chronic itch patients will benefit from particular treatments is a challenge. Common features in itch and pain in respect to sensory elicitation, and mechanisms of processing including sensitization and inhibition at the peripheral and central levels, may serve to understand variability in treatment outcomes. As such this study aimed to explore whether phototherapy outcomes can be predicted by psychophysical parameters of pain and itch modulatory processing.

METHODS

Prospective cohort study on chronic-itch patients (n = 44) assessed before 20 treatments of NB UVB. Level of itch and pain reduction following painful stimulation (reflecting the 'pain inhibits pain' phenomenon) used to assess the top-down modulation response efficacy. Magnitude of Conditioned Pain Modulation (CPM) for itch (CPM-itch) and for pain (CPM-pain) (reflecting inhibition) and magnitude of temporal summation (TS) of pain (reflecting ascending facilitation pathways) assessed to predict treatment effect.

RESULTS

Higher improvement of itch symptoms following phototherapy was correlated with more efficient CPM-itch (r = 0.62, p < .001), but not magnitude of CPM-pain or level of temporal summation.

DISCUSSION

Findings emphasize the role of descending inhibition pathways in determining phototherapy efficacy in chronic itch patients. Such an evaluation-based approach may contribute to better patient selection for phototherapy improving patients' disease outcomes.

Itch: From mechanism to (novel) therapeutic approaches

Itch is a common sensory experience that is prevalent in patients with inflammatory skin diseases, as well as in those with systemic and neuropathic conditions. In patients with these conditions, itch is often severe and significantly affects quality of life. Itch is encoded by 2 major neuronal pathways: histaminergic (in acute itch) and nonhistaminergic (in chronic itch). In the majority of cases, crosstalk existing between keratinocytes, the immune system, and nonhistaminergic sensory nerves is responsible for the pathophysiology of chronic itch. This review provides an overview of the current understanding of the molecular, neural, and immune mechanisms of itch: beginning in the skin, proceeding to the spinal cord, and eventually ascending to the brain, where itch is processed. A growing understanding of the mechanisms of chronic itch is expanding, as is our pipeline of more targeted topical and systemic therapies. Our therapeutic armamentarium for treating chronic itch has expanded in the last 5 years, with developments of topical and systemic treatments targeting the neural and immune systems.

Pain Education for Adolescents and Young Adults Living Beyond Cancer: An Interdisciplinary Meeting Report

Pain is an understudied and undertreated consequence of cancer survival. Pain education is now a recommended treatment approach for persistent non-cancer pain, yet it has not been well applied to the context of adolescent and young adult (AYA) cancer survival. In March 2018, an interdisciplinary meeting was held in Adelaide, South Australia to set a research agenda for pain education in AYA cancer survivors. We identified that AYAs with persistent pain and those with heightened pain-related fear have the potential to benefit from pain education. We identified a number of unique challenges of engaging AYA survivors in pain education, and point towards future research directions.

How Gastrin-Releasing Peptide Opens the Spinal Gate for Itch

Spinal transmission of pruritoceptive (itch) signals requires transneuronal signaling by gastrin-releasing peptide (GRP) produced by a subpopulation of dorsal horn excitatory interneurons. These neurons also express the glutamatergic marker vGluT2, raising the question of why glutamate alone is insufficient for spinal itch relay. Using optogenetics together with slice electrophysiology and mouse behavior, we demonstrate that baseline synaptic coupling between GRP and GRP receptor (GRPR) neurons is too weak for suprathreshold excitation. Only when we mimicked the endogenous firing of GRP neurons and stimulated them repetitively to fire bursts of action potentials did GRPR neurons depolarize progressively and become excitable by GRP neurons. GRPR but not glutamate receptor antagonism prevented this action. Provoking itch-like behavior by optogenetic activation of spinal GRP neurons required similar stimulation paradigms. These results establish a spinal gating mechanism for itch that requires sustained repetitive activity of presynaptic GRP neurons and postsynaptic GRP signaling to drive GRPR neuron output.

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Spinal itch relay requires effective communication from GRP to GRP receptor neurons

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Single action potentials in GRP neurons fail to release sufficient GRP

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Only burst firing releases enough GRP to prime GRP receptor neurons for activation

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GRP acts as a volume transmitter probably explaining why itch is hard to localize

Comparison of Characteristics of Neuropathic and Non-neuropathic Pruritus to Develop a Tool for the Diagnosis of Neuropathic Pruritus: The NP5

The diagnosis of neuropathic pruritus (NP) may be difficult. The aim of this study was to compare the characteristics of both neuropathic pruritus and non-neuropathic pruritus (NNP) in order to elaborate a tool to help the diagnosis of NP without clinical examination. One hundred and seven patients were included: Fifty three in the NP group and Fifty four in the NNP group. In multiple regression, presence of twinges, absence of burning, worsening with activity, no worsening with stress, and relief with cold ambient temperature were independent factors that were associated with NP. A score of two criteria out of five was optimal to discriminate NP from NNP with a sensitivity of 76% and a specificity of 77%. Alloknesis, hyperknesis, or the ice cube test were not included because their evaluation is based on clinical examination. Future high-powered studies are needed to confirm the results of the present study.

Neuropathic symptoms of the ocular surface: dryness, pain, and itch

PURPOSE OF REVIEW

This review aims to describe the recent findings on epidemiology, pathophysiology, and management of neuropathic symptoms of the ocular surface, with a focus on potential similarities between sensations of dry eye, pain and itch.

RECENT FINDINGS

A narrative review of the literature was undertaken. Key references from research in dry eye, neuropathic symptoms of the ocular surface, ocular pain and itch, as well as general references on itch and pain neurobiology were included. Recent findings suggest aspects of dry eye, chronic ocular pain and itch symptomatology are driven by neuropathic pain mechanisms involving peripheral and central sensitization processes.

SUMMARY

Ocular dryness, pain, and itch are prevalent complaints with several of shared features. Multiple lines of evidence suggest that peripheral and central neuronal sensitization processes are involved in generating and maintaining ocular sensory symptoms. Research is warranted on the epidemiology of ocular sensations, molecular mechanisms involved in nociception and pruriception in the eye, electrophysiological alterations in animal models of eye conditions, and therapeutic modalities that can alleviate unpleasant ocular sensations.

Brain Processing of Contagious Itch in Patients with Atopic Dermatitis

Several studies show that itch and scratching cannot only be induced by pruritogens like histamine or cowhage, but also by the presentation of certain (audio-) visual stimuli like pictures on crawling insects or videos showing other people scratching. This phenomenon is coined "Contagious itch" (CI). Due to the fact that CI is more profound in patients with the chronic itchy skin disease atopic dermatitis (AD), we believe that it is highly relevant to study brain processing of CI in this group. Knowledge on brain areas involved in CI in AD-patients can provide us with useful hints regarding non-invasive treatments that AD-patients could profit from when they are confronted with itch-inducing situations in daily life. Therefore, this study investigated the brain processing of CI in AD-patients. 11 AD-patients underwent fMRI scans during the presentation of an itch inducing experimental video (EV) and a non-itch inducing control video (CV). Perfusion based brain activity was measured using arterial spin labeling functional MRI. As expected, the EV compared to the CV led to an increase in itch and scratching (p < 0.05). CI led to a significant increase in brain activity in the supplementary motor area, left ventral striatum and right orbitofrontal cortex (threshold: p < 0.001; cluster size k > 50). Moreover, itch induced by watching the EV was by trend correlated with activity in memory-related regions including the temporal cortex and the (pre-) cuneus as well as the posterior operculum, a brain region involved in itch processing (threshold: p < 0.005; cluster size k > 50). These findings suggest that the fronto-striatal circuit, which is associated with the desire to scratch, might be a target region for non-invasive treatments in AD patients.

Toll-like receptor 4 contributes to chronic itch, alloknesis, and spinal astrocyte activation in male mice

Increasing evidence suggests that Toll-like receptor 4 (TLR4) contributes importantly to spinal cord glial activation and chronic pain sensitization; however, its unique role in acute and chronic itch is unclear. In this study, we investigated the involvement of TLR4 in acute and chronic itch models in male mice using both transgenic and pharmacological approaches. Tlr4 mice exhibited normal acute itch induced by compound 48/80 and chloroquine, but these mice showed substantial reductions in scratching in chronic itch models of dry skin, induced by acetone and diethylether followed by water (AEW), contact dermatitis, and allergic contact dermatitis on the neck. Intrathecal (spinal) inhibition of TLR4 with lipopolysaccharide Rhodobacter sphaeroides did not affect acute itch but suppressed AEW-induced chronic itch. Compound 48/80 and AEW also produced robust alloknesis, a touch-elicited itch in wild-type mice, which was suppressed by intrathecal lipopolysaccharide R sphaeroides and Tlr4 deletion. Acetone and diethylether followed by water induced persistent upregulation of Tlr4 mRNA and increased TLR4 expression in GFAP-expressing astrocytes in spinal cord dorsal horn. Acetone and diethylether followed by water also induced TLR4-dependent astrogliosis (GFAP upregulation) in spinal cord. Intrathecal injection of astroglial inhibitor L-α-aminoadipate reduced AEW-induced chronic itch and alloknesis without affecting acute itch. Spinal TLR4 was also necessary for AEW-induced chronic itch in the cheek model. Interestingly, scratching plays an essential role in spinal astrogliosis because AEW-induced astrogliosis was abrogated by putting Elizabethan collars on the neck to prevent scratching the itchy skin. Our findings suggest that spinal TLR4 signaling is important for spinal astrocyte activation and astrogliosis that may underlie alloknesis and chronic itch.

Antioxidants Attenuate Acute and Chronic Itch: Peripheral and Central Mechanisms of Oxidative Stress in Pruritus

Itch (pruritus) is one of the most disabling syndromes in patients suffering from skin, liver, or kidney diseases. Our previous study highlighted a key role of oxidative stress in acute itch. Here, we evaluated the effects of antioxidants in mouse models of acute and chronic itch and explored the potential mechanisms. The effects of systemic administration of the antioxidants N-acetyl-L-cysteine (NAC) and N-tert-butyl-α-phenylnitrone (PBN) were determined by behavioral tests in mouse models of acute itch induced by compound 48/80 or chloroquine, and chronic itch by treatment with a mixture of acetone-diethyl-ether-water. We found that systemic administration of NAC or PBN significantly alleviated compound 48/80- and chloroquine-induced acute itch in a dose-dependent manner, attenuated dry skin-induced chronic itch, and suppressed oxidative stress in the affected skin. Antioxidants significantly decreased the accumulation of intracellular reactive oxygen species directly induced by compound 48/80 and chloroquine in the cultured dorsal root ganglia-derived cell line ND7-23. Finally, the antioxidants remarkably inhibited the compound 48/80-induced phosphorylation of extracellular signal-regulated kinase in the spinal cord. These results indicated that oxidative stress plays a critical role in acute and chronic itch in the periphery and spinal cord and antioxidant treatment may be a promising strategy for anti-itch therapy.

The Impact of Pain and Itch on Functioning and Health-Related Quality of Life in Systemic Sclerosis: An Exploratory Study

CONTEXT

Pain and itch are common symptoms reported by patients with systemic sclerosis (SSc; scleroderma), which can markedly diminish function and health-related quality of life (HRQL).

OBJECTIVES

The aim of this exploratory study was to examine the impact that pain, itch, and the interaction of both have on function (depressive symptoms, overall disability, fatigue, sleep disturbance) and HRQL in patients with SSc.

METHODS

A total of 964 patients from the Canadian Scleroderma Reserch Group Registry completed questionnaires measuring itch and pain severity, function, and HRQL. Multiple regression analyses were performed to examine the impact that pain, itch, and pain × itch interaction have on each outcome variable while controlling for demographic measures. A P-value of ≤0.01 was required for a difference to be deemed statistically significant.

RESULTS

Our results revealed that patients with SSc who reported higher pain and itch severity were also more likely to have greater depressive symptoms, overall disability, sleep and fatigue problems, even when demographic measures were controlled for (P-values ≤0.001). Similar results were obtained for HRQL, regardless of the domains (P-values ≤0.001). A significant association between pain × itch interaction and sleep (P = 0.002), physical functioning (P = 0.003), and general health (P ≤ 0.001) variables also was found. Further investigation of the nature of the pain × itch interaction showed that the effect of pain severity on outcome variables diminishes as itch severity increases.

CONCLUSION

Both pain and itch appear to have a detrimental impact on functioning and HRQL in patients with SSc, suggesting that more targeted approaches to symptom management are warranted.

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

Experimental elicitation of itch: Evoking and evaluation techniques

Itch is a very common symptom of many skin diseases. Chronic itch may be very troublesome and may strongly impact on work ability, sleep and on the quality of life. Many studies have been conducted to define peripheral and central itch pathways, and to test the anti-pruritic effect of drugs, in which pruritus was experimentally evoked by chemical mediators and by physical stimuli. However, no objective methods to reproduce and to evaluate itch has been standardized. In this review we summarize the experimental techniques used to induce pruritus and to quantify it both in animal models and in humans.

Neurophysiology and itch pathways

As we all can easily differentiate the sensations of itch and pain, the most straightforward neurophysiologic concept would consist of two specific pathways that independently encode itch and pain. Indeed, a neuronal pathway for histamine-induced itch in the peripheral and central nervous system has been described in animals and humans, and recently several non-histaminergic pathways for itch have been discovered in rodents that support a dichotomous concept differentiated into a pain and an itch pathway, with both pathways being composed of different "flavors." Numerous markers and mediators have been found that are linked to itch processing pathways. Thus, the delineation of neuronal pathways for itch from pain pathways seemingly proves that all sensory aspects of itch are based on an itch-specific neuronal pathway. However, such a concept is incomplete as itch can also be induced by the activation of the pain pathway in particular when the stimulus is applied in a highly localized spatial pattern. These opposite views reflect the old dispute between specificity and pattern theories of itch. Rather than only being of theoretic interest, this conceptual problem has key implication for the strategy to treat chronic itch as key therapeutic targets would be either itch-specific pathways or unspecific nociceptive pathways.

Current topical and systemic therapies for itch

Itch is a common distressing symptom which may be caused by multifactorial aetiologies including inflammatory skin diseases, systemic diseases, neuropathic conditions and psychogenic disorders. Itch is a term used synonymously with pruritus and is defined as acute if it lasts less than 6 weeks or chronic if it persists for more than 6 weeks. It can have the same impact on the quality of life as chronic pain and shares many of the same pathophysiological pathways. Depending on the aetiology of the itch, different pathogenic mechanisms have been postulated with a number of mediators identified. These include histamine, leukotrienes, proteases, neuropeptides, cytokines and opioids, which may activate peripheral itch-mediating C-fibres via receptors on the nerve terminals and central neuronal pathways. Therefore, there is no single universally effective anti-itch treatment available. First-line treatments for itch include topical therapies, such as emollients, mild cleansers (low pH), topical anaesthetics, steroids, calcineurin inhibitors and coolants (menthol). Treatment with systemic therapies can vary according to the aetiology of the chronic itch. Non-sedating antihistamines are helpful in conditions such as urticaria where the itch is primarily histamine mediated. Although the itch of eczema is not mediated by histamine, sedating antihistamines at night are helpful to break the itch-scratch cycle. Chronic itch may also be treated with other systemic therapies, such as anticonvulsants, antidepressants as well as mu-opioid antagonists, kappa-opioid agonists and phototherapy, depending on the cause of the itch. This article summarises the topical and systemic therapies available with our current understanding of the pathophysiology of itch.

Anesthetic premedication: new horizons of an old practice

The practice of anesthetic premedication embarked upon soon after ether and chloroform were introduced as general anesthetics in the middle of the 19(th) century. By applying opioids and anticholinergics before surgery, the surgical patients could achieve a less anxious state, and more importantly, they would acquire a smoother course during the tedious and dangerous induction stage. Premedication with opioids and anticholinergics was not a routine practice in the 20(th) century when intravenous anesthetics were primarily used as induction agents that significantly shorten the induction time. The current practice of anesthetic premedication has evolved into a generalized scheme that incorporates several aspects of patient care: decreasing preoperative anxiety, dampening intraoperative noxious stimulus and its associated neuroendocrinological changes, and minimizing postoperative adverse effects of anesthesia and surgery. Rational use of premedication in modern anesthesia practice should be justified by individual needs, the types of surgery, and the anesthetic agents and techniques used. In this article, we will provide our readers with updated information about premedication of surgical patients with a focus on the recent application of second generation serotonin type 3 antagonist, antidepressants, and anticonvulsants.

Characteristics of chronic wounds that itch

OBJECTIVE

This study was designed to explore characteristics of chronic wounds that present with wound-related itch.

BACKGROUND

Although wound-related itch is recognized clinically, little is known about the phenomenon. Recent scientific advances have enabled the study of itch physiology, yet the clinical problem is not well described in the literature.

DESIGN

The study was observational, descriptive.

METHOD

Persons (N = 200) with wounds being followed up at a hospital-affiliated wound care center were interviewed and assessed. Instruments included a health history tool, Paul-Pieper Itching Questionnaire, Bates-Jensen Wound Assessment Tool, and 10-g monofilament for assessment of sensation in the area of the wound.

RESULTS

Participants were aged 21 to 98 years (mean, 66.82 [SD, 14.02] years); 56% of the participants were men, and 85% were white. One-fourth (56/200) of the participants reported wound-related itch. Wounds that itched were generally larger (t77.74 = -3.27; P = .002; d = 0.63; 95% confidence interval [CI], -1.01 to -0.25), had more tissue edema (t88.38 = -2.19; P = .031; 95% CI, -0.93 to -0.47), and demonstrated more granulation tissue in the wound base (t98.71 = 2.03; P = .045; 95% CI, 0.01-0.87), compared with wounds without itch. Greater itch was associated with wounds that had a moderate amount of exudate (P = .02) or necrotic tissue in the base.

CONCLUSIONS

Wound itch was present in more severe wounds as evidenced by larger size, more tissue edema, and necrotic tissue. Understanding wound itch could promote wound healing and improve quality of life for persons with chronic wounds.

Itch relief by mirror scratching. A psychophysical study

Objective

The goal of this study was to test whether central mechanisms of scratching-induced itch attenuation can be activated by scratching the limb contralateral to the itching limb when the participant is made to visually perceive the non-itching limb as the itching limb by means of mirror images.

Methods

Healthy participants were asked to assess the intensity of an experimentally induced itch at their right forearm while they observed externally guided scratch movements either at their right (itching) or left (non-itching) forearm which were either mirrored or not mirrored. In the first experiment, a mirror placed between the participant’s forearms was used to create the visual illusion that the participant’s itching (right) forearm was being scratched while in fact the non-itching (left) forearm was scratched. To control visibility of the left (non-mirrored) forearm, a second experiment was performed in which unflipped and flipped real-time video displays of the participant’s forearms were used to create experimental conditions in which the participant visually perceived scratching either on one forearm only, on both forearms, or no scratching at all.

Results

In both experiments, scratching the non-itching limb attenuated perceived itch intensity significantly and selectively in the mirror condition, i.e., when the non-itching forearm was visually perceived as the itching limb.

Discussion

These data provide evidence that the visual illusion that an itching limb is being scratched while in fact the non-itching limb contralateral to the itching limb is scratched, can lead to significant itch relief. This effect might be due to a transient illusionary intersensory perceptual congruency of visual, tactile and pruriceptive signals. “Mirror scratching” might provide an alternative treatment to reduce itch perception in focal skin diseases with persistent pruritus without causing additional harm to the affected skin and might therefore have significant clinical impact.

Clinical and psychological characteristics of patients with psoriasis reporting various frequencies of pruritus

BACKGROUND

Pruritus is a common subjective symptom of psoriasis whose levels may be affected by a range of variables. The objective of this study was to evaluate the frequency of pruritus and its associations with clinical and psychological characteristics of psoriasis patients.

MATERIALS AND METHODS

One-hundred and seventy-four patients with psoriasis took part in the study, and they were assessed on clinical (psoriasis area and severity index [PASI], localization of lesions, pruritus severity, duration of the disease) and psychological (coping, personality, cognitive appraisal of the disease, quality of life) variables. ANOVA, Pearson's χ(2)-tests, and correlational analyses were employed to analyze the data.

RESULTS

The frequency of reported pruritus was found to be related to PASI (P < 0.000) and localization of skin lesions on visible body areas (P < 0.05). Patients reporting more frequent pruritus were significantly older (P < 0.05), had lower quality of life (P < 0.000), more frequently used coping strategies of resignation (P < 0.05) and self-blame (P < 0.05), and tended to appraise their disease in terms of a threat (P < 0.05), obstacle/loss (P < 0.05), and harm (P < 0.01). No significant associations were found between pruritus and basic personality traits. After controlling for age, all these variables remained significantly correlated with pruritus, whereas after controlling for psoriasis severity, only age, quality of life, and a coping strategy of resignation still significantly correlated with pruritus frequency.

CONCLUSIONS

The findings emphasize the importance of pruritus in the majority of psoriasis patients and its significant association with compromised quality of life, older age, greater psoriasis severity, and a specific pattern of psychological responses to stress.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Pruritus: still a challenge

Pruritus is a symptom that may be associated with a wide array of diseases - skin diseases, systemic diseases or even those without a defined cause. According to its severity and evolution, it may compromise the patients' quality of life in a significant way. New therapeutic approaches, in which histamine release is not the main event, are being described for pruritus control. Therefore, taking into account the origin of the mediators of pruritus, we conducted a review of the available resources emphasizing that, although very important, anti-histamines are not the answer for every kind of pruritus.

Spontaneous behavioral responses in the orofacial region: a model of trigeminal pain in mouse

OBJECTIVES

To develop a translational mouse model for the study and measurement of non-evoked pain in the orofacial region by establishing markers of nociceptive-specific grooming behaviors in the mouse.

BACKGROUND

Some of the most prevalent and debilitating conditions involve pain in the trigeminal distribution. Although there are current therapies for these pain conditions, for many patients, they are far from optimal. Understanding the pathophysiology of pain disorders arising from structures innervated by the trigeminal nerve is still limited, and most animal behavioral models focus on the measurement of evoked pain. In patients, spontaneous (non-evoked) pain responses provide a more accurate representation of the pain experience than do responses that are evoked by an artificial stimulus. Therefore, the development of animal models that measure spontaneous nociceptive behaviors may provide a significant translational tool for a better understanding of pain neurobiology.

METHODS

C57BL/6 mice received either an injection of 0.9% saline solution or complete Freund's adjuvant into the right masseter muscle. Animals were video-recorded and then analyzed by an observer blind to the experiment group. The duration of different facial grooming patterns performed in the area of injection were measured. After 2 hours, mice were euthanized and perfused, and the brainstem was removed. Fos protein expression in the trigeminal nucleus caudalis was quantified using immunohistochemistry to investigate nociceptive-specific neuronal activation. A separate group of animals was treated with morphine sulfate to determine the nociceptive-specific nature of their behaviors.

RESULTS

We characterized and quantified 3 distinct patterns of acute grooming behaviors: forepaw rubbing, lower lip skin/cheek rubbing against enclosure floor, and hindpaw scratching. These behaviors occurred with a reproducible frequency and time course, and were inhibited by the analgesic morphine. Complete Freund's adjuvant-injected animals also showed Fos labeling consistent with neuronal activation in nociceptive-specific pathways of the trigeminal nucleus after 2 hours.

CONCLUSIONS

These behaviors and their correlated cellular responses represent a model of trigeminal pain that can be used to better understand basic mechanisms of orofacial pain and identify new therapeutic approaches to this common and challenging condition.

Oxidative stress induces itch via activation of transient receptor potential subtype ankyrin 1 in mice

OBJECTIVE

To investigate the role of oxidative stress in itch-indicative scratching behavior in mice, and furthermore, to define the cellular and molecular mechanisms underlying oxidative stress-mediated itch.

METHODS

Scratching behavior was induced by intradermal injection of the oxidants hydrogen peroxide (H₂O₂) or tert-butylhydroperoxide (tBHP) into the nape of the neck in mice. The mice were observed for 30 min.

RESULTS

Intradermal H₂O₂ (0.03%-1%) or tBHP (1-30 μmol) elicited robust scratching behavior, displaying an inverted U-shaped dose-response curve. Naloxone, an opioid receptor antagonist, but not morphine, largely suppressed the oxidant-induced scratching. Chlorpheniramine, a histamine H1 receptor antagonist, blocked histamine- but not oxidant-induced scratching, indicating the involvement of a histamine-independent mechanism in oxidant-evoked itch. Further, resiniferatoxin treatment abolished oxidant-induced scratching, suggesting an essential role of C-fibers. Notably, blockade of transient receptor potential subtype ankyrin 1 (TRPA1) with the selective TRPA1 antagonist HC-030031, or genetic deletion of Trpa1 but not Trpv1 (subfamily V, member 1) resulted in a profound reduction in H₂O₂-evoked scratching. Finally, systemic administration of the antioxidant N-acetyl-L-cysteine or trolox (a water-soluble vitamin E analog) attenuated scratching induced by the oxidants.

CONCLUSION

Oxidative stress by different oxidants induces profound scratching behavior, which is largely histamine- and TRPV1-independent but TRPA1-dependent. Antioxidants and TRPA1 antagonists may be used to treat human itch conditions associated with oxidative stress.

Galanin-expression and galanin-dependent sensory neurons are not required for itch

Background

Galanin is a key modulator of nociception, and it is also required for the developmental survival of a subset of C-fibre sensory neurons which are critical to the mediation of neuropathic and inflammatory pain. However, the potential modulatory roles played by galanin, or the galanin-dependent neurons, in pruritoceptive mechanisms underlying the sensation of itch have not been investigated.

Findings

Here we report that mice carrying a loss-of-function mutation in the galanin gene (Gal-KO) show no differences in spontaneous behavioural itch responses compared to wild-type (WT) controls. Similarly, the responses to a range of pruritogens are not significantly different between the two genotypes.

Conclusions

These results suggest that neither galanin expression, nor the galanin-dependent subpopulation of sensory neurons is required for itch-related behaviours.

Assessment of pruritus characteristics and impact on burn survivors

The goal of this cross-sectional study was to characterize and describe persistent postburn pruritus. Cause and treatment of postburn itch is elusive. It has been suggested that burn survivors with persistent pruritus should be divided into acute itch (≤6 months postinjury) and chronic itch (>6 months postinjury) because the cause of itch may be different. Cross-sectional data of itch characteristics reported here are from the baseline data of a prospective, randomized, double-blind, controlled pilot study of 23 subjects with frequent and bothersome postburn pruritus. Subjects self-completed validated scales for variables of itch sensation, affect of itch, and severity. Variables of quality of life, frequency, pain and itch intensity, skin condition, scar, and medication were also recorded. Itch frequency revealed that 87% of subjects experienced itching daily, 96% experienced three or more episodes a day, and 52% had episode durations lasting 5 to 30 minutes per incidence. Itch was reported as unbearable by 94% of subjects with chronic itch and by 86% of subjects with acute itch, whereas bothersome was 88 and 100%, respectively. Itch sensation dimension of stinging was 74% in both acute and chronic itch subjects. Crawling and burning sensations were often severe. Potential itch triggers and relief activities were identified. Differences in sensory and affective itch components were detected between acute and chronic itch subjects. Combinations of itch mechanisms probably contribute to the development of and changes in pruritus. Characterizing the sensation and affective itch dimensions in conjunction with inflammation, burn injury, recovery, scar maturation, medication, and psychological status should better elucidate postburn itch.

[Pain and pruritus : differences and similarities as revealed by the PainDetect questionnaire parameters]

Chronic pruritus and chronic pain are frequent symptoms of a variety of underlying diseases. Painful sensations usually suppress acute itching. In chronic states, both may be present in parallel and be a part of one event. Patients with chronic pruritus should be asked for the presence of pain, which can be identified and characterized using specific and validated questionnaires. The early detection of (neuropathic) pain in patients with chronic pruritus can be done using the PainDetect questionnaire. In general, patients suffering from both itch and pain have a highly impaired quality of life, high degree of objective health burden, need a more intensive health care and a complex analgetic and antipruritic therapy.

TLR3 deficiency impairs spinal cord synaptic transmission, central sensitization, and pruritus in mice

Itch, also known as pruritus, is a common, intractable symptom of several skin diseases, such as atopic dermatitis and xerosis. TLRs mediate innate immunity and regulate neuropathic pain, but their roles in pruritus are elusive. Here, we report that scratching behaviors induced by histamine-dependent and -independent pruritogens are markedly reduced in mice lacking the Tlr3 gene. TLR3 is expressed mainly by small-sized primary sensory neurons in dorsal root ganglions (DRGs) that coexpress the itch signaling pathway components transient receptor potential subtype V1 and gastrin-releasing peptide. Notably, we found that treatment with a TLR3 agonist induces inward currents and action potentials in DRG neurons and elicited scratching in WT mice but not Tlr3(-/-) mice. Furthermore, excitatory synaptic transmission in spinal cord slices and long-term potentiation in the intact spinal cord were impaired in Tlr3(-/-) mice but not Tlr7(-/-) mice. Consequently, central sensitization-driven pain hypersensitivity, but not acute pain, was impaired in Tlr3(-/-) mice. In addition, TLR3 knockdown in DRGs also attenuated pruritus in WT mice. Finally, chronic itch in a dry skin condition was substantially reduced in Tlr3(-/-) mice. Our findings demonstrate a critical role of TLR3 in regulating sensory neuronal excitability, spinal cord synaptic transmission, and central sensitization. TLR3 may serve as a new target for developing anti-itch treatment.

Trigeminal trophic syndrome from stroke: an under-recognized central neuropathic itch syndrome

Trigeminal trophic syndrome (TTS) is the historic name for neuropathic self-induced facial ulceration from abnormal sensory symptoms leading to uncontrolled scratching. Anatomic co-localization of sensory loss (numbness) plus neuropathic itch and pain permits painless scratching. If the itch is severe, some patients will scratch to the point of causing self-injury. Patients may be unaware or may conceal the fact that their lesions are self-induced and thus the diagnosis presents a clinical challenge. Many cases remain undiagnosed, leading to unnecessary and ineffective tests, procedures, and prescribing. We document a patient with a central cause of TTS - multiple cerebral vascular accidents - and summarize the presentation, pathogenesis, and treatment options.