Itch--mediators and mechanisms

Apigenin: A Therapeutic Agent for Treatment of Skin Inflammatory Diseases and Cancer

The skin is the main barrier between the body and the environment, protecting it from external oxidative stress induced by ultraviolet rays. It also prevents the entrance of infectious agents such as viruses, external antigens, allergens, and bacteria into our bodies. An overreaction to these agents causes severe skin diseases, including atopic dermatitis, pruritus, psoriasis, skin cancer, and vitiligo. Members of the flavonoid family include apigenin, quercetin, luteolin, and kaempferol. Of these, apigenin has been used as a dietary supplement due to its various biological activities and has been shown to reduce skin inflammation by downregulating various inflammatory markers and molecular targets. In this review, we deal with current knowledge about inflammatory reactions in the skin and the molecular mechanisms by which apigenin reduces skin inflammation.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

Pruritus: Progress toward Pathogenesis and Treatment

Pruritus, the most common cutaneous symptom, is widely seen in many skin complaints. It is an uncomfortable feeling on the skin and sometimes impairs patients' quality of life. At present, the specific mechanism of pruritus still remains unclear. Antihistamines, which are usually used to relieve pruritus, ineffectively work in some patients with itching. Recent evidence has suggested that, apart from histamine, many mediators and signaling pathways are involved in the pathogenesis of pruritus. Various therapeutic options for itching correspondingly have been developed. In this review, we summarize the updated pathogenesis and therapeutic strategies for pruritus.

An update on the physiopathology and therapeutic management of cholestatic pruritus in children

Pruritus is a disabling symptom accompanying chronic cholestasis. In extreme cases, the refractory nature of pruritus can result in a need for invasive therapies including liver transplantation. The pathogenesis of pruritus in cholestatic disease is poorly understood. It may involve a specific neural pathway (similar to that associated with pain) regulated by several pruritogenic substances such as bile acids, opioids, serotonin, and the more recently identified lysophosphatidic acid. While the therapeutic management of cholestatic pruritus is well established in adults, there is no consensus in children, in light of the difficulty of conducting controlled clinical studies. The currently recommended strategy to manage cholestatic pruritus in children is based on several lines of specific therapies that should be associated with skin hydration and with non-specific treatment of cholestasis including ursodeoxycholic acid. Pruritus should be assessed as objectively as possible between each line of therapy. Rifampicin, a potent CYP3A4 inducer, is the first-line treatment of cholestatic pruritus. Second-line therapies require evaluation of the child in an expert center and are discussed on a case-by-case basis depending on the underlying disease and the experience of the center. These include inhibitors of serotonin reuptake (sertraline), opioid antagonists (naloxone), or ASBT inhibitors. Invasive therapies such as biliary diversion or liver transplantation can also be proposed in the most severe cases. The aim of the current update is to review the physiopathologic mechanisms implicated in cholestatic pruritus and to propose potential therapeutic strategies in children.

Itch and pain differences and commonalities

Pain and itch are generally regarded antagonistic as painful stimuli such as scratching suppresses itch. Moreover, inhibition of pain processing by opioids generates itch further supporting their opposing role. Separate specific pathways for itch and pain processing have been uncovered, and several molecular markers have been established in mice that identify neurons involved in the processing of histaminergic and non-histaminergic itch on primary afferent and spinal level. These results are in agreement with the specificity theory for itch and might suggest that pain and itch should be investigated separately on the level of neurons, mediators, and mechanisms. However, in addition to broadly overlapping mediators of itch and pain, there is also evidence for overlapping functions in primary afferents: nociceptive primary afferents can provoke itch when activated very locally in the epidermis, and sensitization of both nociceptors and pruriceptors has been found following local nerve growth factor application in volunteers. Thus, also mechanisms that underlie the development of chronic itch and pain including spontaneous activity and sensitization of primary afferents as well as spinal cord sensitization may well overlap to a great extent. Rather than separating itch and pain, research concepts should therefore address the common mechanisms. Such an approach appears most appropriate for clinical conditions of neuropathic itch and pain and also chronic inflammatory conditions. While itch researchers can benefit from the large body of information of the pain field, pain researchers will find behavioral readouts of spontaneous itch much simpler than those for spontaneous pain in animals and the skin as source of the pruritic activity much more accessible even in patients.

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.

Updated neurophysiology of itch

The unique physiological features of histamine-sensitive C-fibers and spinothalamic tract neurons support the hypothesis of itch specific pathway, whereas subsequent studies on cowhage-induced itch have provided evidence against it, suggesting the presence of multiple neural pathways for itch. Not only peripheral pruritogens but also spinal neural receptors are involved in the control of itch, and will be the target of treatment. Itch sensitization in chronic pruritus is another crucial factor that needs to be considered in the treatment. Neuropathic itch is the type of itch that occurs when nerve fibers are damaged or injured and spontaneous firing of nerves takes place, and plays a major role in itch accompanying some pathological conditions such as herpes zoster. The complexity of itch is due to the broad range of mediators involved and the large variety of neural mechanisms behind.

How to Psychologically Minimize Scratching Impulses

Research in recent years, especially in the analgesic field, has intensively studied the placebo effect and its mechanisms. It has been shown that physical complaints can be efficiently reduced via learning and cognitive processes (conditioning and expectancies). However, despite evidence demonstrating a large variety of physiological similarities between pain and itch, the possible transfer of the analgesic placebo model to itch has not yet been widely discussed in research. This review therefore aims at highlighting potential transfers of placebo mechanisms to itch processes by demonstrating the therapeutic issues in pharmacological treatments for pruritus on a physiological basis and by discussing the impact of psychological mechanisms and psychological factors influencing itch sensations.

Ticks and tick-borne pathogens at the cutaneous interface: host defenses, tick countermeasures, and a suitable environment for pathogen establishment

Ticks are unique among hematophagous arthropods by continuous attachment to host skin and blood feeding for days; complexity and diversity of biologically active molecules differentially expressed in saliva of tick species; their ability to modulate the host defenses of pain and itch, hemostasis, inflammation, innate and adaptive immunity, and wound healing; and, the diverse array of infectious agents they transmit. All of these interactions occur at the cutaneous interface in a complex sequence of carefully choreographed host defense responses and tick countermeasures resulting in an environment that facilitates successful blood feeding and establishment of tick-borne infectious agents within the host. Here, we examine diverse patterns of tick attachment to host skin, blood feeding mechanisms, salivary gland transcriptomes, bioactive molecules in tick saliva, timing of pathogen transmission, and host responses to tick bite. Ticks engage and modulate cutaneous and systemic immune defenses involving keratinocytes, natural killer cells, dendritic cells, T cell subpopulations (Th1, Th2, Th17, Treg), B cells, neutrophils, mast cells, basophils, endothelial cells, cytokines, chemokines, complement, and extracellular matrix. A framework is proposed that integrates tick induced changes of skin immune effectors with their ability to respond to tick-borne pathogens. Implications of these changes are addressed. What are the consequences of tick modulation of host cutaneous defenses? Does diversity of salivary gland transcriptomes determine differential modulation of host inflammation and immune defenses and therefore, in part, the clades of pathogens effectively transmitted by different tick species? Do ticks create an immunologically modified cutaneous environment that enhances specific pathogen establishment? Can tick saliva molecules be used to develop vaccines that block pathogen transmission?

Spanish transcultural adaptation of the Leuven Itch Scale

Itching is one of the most frequent symptoms, and most resistant to treatment, relating to scar formation after burns. The Leuven Itch Scale is a questionnaire which has been validated for use in burns, analysing different clinical dimensions of pruritus. However, until now there was no Spanish version available for use with Spanish-speaking patients. This article presents the process of the academic translation of the questionnaire in order to obtain a Spanish version, and the final version of the questionnaire. We followed a translation/back-translation methodology with a final pilot test on a sample of burn patients. The final result fills a major gap in the quality-of-life questionnaires available in Spanish.

Artemin causes hypersensitivity to warm sensation, mimicking warmth-provoked pruritus in atopic dermatitis

BACKGROUND

Itch impairs the quality of life for many patients with dermatoses, especially atopic dermatitis (AD), and is frequently induced by a warm environment.

OBJECTIVE

To determine the mechanism underlying itch induction by warmth, we focused on artemin, a member of glial cell line-derived neurotrophic factors (GDNFs).

METHODS

A gene array assay revealed that artemin was expressed in substance P-treated dermal fibroblasts. The expression of artemin in healthy and AD-lesional skin was evaluated with immunohistochemistry and in situ hybridization. The impact of fibroblast-derived artemin on the proliferation and morphology of neural cell was investigated in vitro. To confirm the involvement of artemin in skin sensibility, wild-type and GDNF family receptor α3 knockout mice were employed for sensory examination.

RESULTS

Artemin-expressing fibroblasts accumulated in skin lesions of patients with AD. Artemin induced cell proliferation of a neuroblastoma cell line in vitro, and intradermal injection of artemin in mice resulted in peripheral nerve sprouting and thermal hyperalgesia. Artemin-treated mice demonstrated scratching behavior in a warm environment, but mice deficient for GDNF family receptor α3, a potent artemin receptor, did not show this behavior. Furthermore, the escaping response to heat stimulus was attenuated in GDNF family receptor α3 knockout mice, suggesting that artemin may contribute to sensitivity to heat.

CONCLUSION

These data suggest that dermal fibroblasts secrete artemin in response to substance P, leading to abnormal peripheral innvervation and thermal hyperalgesia. We hypothesize that artemin lowers the threshold of temperature-dependent itch sensation and might therefore be a novel therapeutic target for treating pruritic skin disorders, including AD.

Opioid side effects and their treatment in patients with chronic cancer and noncancer pain

Opioids are the foundation of standard analgesic regimens for moderate to severe pain due to life-threatening illnesses such as cancer, and are increasingly employed in chronic noncancer pain of the same severity. Opioids are frequently used for long periods in these populations, sometimes for years. However, side effects are common and may reduce quality of life, or become life threatening, and frequently cause patients to discontinue opioid therapy. Successful opioid therapy dictates that benefits of analgesia outweigh safety concerns. The mechanisms, incidence, and treatment or prevention of commonly reported side effects in chronic pain populations are reviewed, employing best available evidence along with empiric practice. General management strategies include switching opioids ("opioid rotation"), discontinuation of concurrent medications that exacerbate side effects, and symptomatic treatment. In addition, recently recognized adverse events that occur after long-term opioid therapy are discussed. High-quality evidence is lacking for the treatment of most side effects, and the true incidence, underlying mechanisms, and clinical implications of long-term responses to opioid therapy are not yet fully understood.

Treatment of pruritic diseases with topical calcineurin inhibitors

The introduction of topical calcineurin inhibitors resulted in a significant improvement in the treatment of atopic dermatitis. In addition, rapid amelioration of pruritus could be observed. In case reports, other pruritic dermatoses such as chronic irritative hand dermatitis, rosacea, graft-versus-host-disease, and lichen sclerosus were also treated successfully with pimecrolimus and tacrolimus. Twenty patients were treated with tacrolimus and pimecrolimus in a surveillance study to evaluate efficacy in pruritus and prurigo. Eighteen of 20 patients responded to therapy. Best results were obtained in localized and generalized pruritus while in prurigo nodularis only a subgroup of patients showed an improvement of pruritus. Further controlled studies are necessary to confirm these results.

Anatomy and neurophysiology of pruritus

Itch has been described for many years as an unpleasant sensation that evokes the urgent desire to scratch. Studies of the neurobiology, neurophysiology, and cellular biology of itch have gradually been clarifying the mechanism of itch both peripherally and centrally. The discussion has been focused on which nerves and neuroreceptors play major roles in itch induction. The "intensity theory" hypothesizes that signal transduction on the same nerves leads to either pain (high intensity) or itch (low intensity), depending on the signal intensity. The "labeled-line coding theory" hypothesizes the complete separation of pain and itch pathways. Itch sensitization must also be considered in discussions of itch. This review highlights anatomical and functional properties of itch pathways and their relation to understanding itch perception and pruritic diseases.

Transcutaneous electrical nerve stimulation for reduction of pruritus in macular amyloidosis and lichen simplex

Lichen simplex (LS) is characterized by circumscribed, lichenified, pruritic patches that may develop on any part of the body. Macular amyloidosis (MA) is the form of primary localized cutaneous amyloidosis. Transcutaneous electrical nerve stimulation (TENS) uses a pulsed electric current generated transcutaneously by a device to cause impulses to be carried along large-diameter afferent nerves. In this article, we report the effects of TENS on the Dermatology Life Quality Index (DLQI) measures and visual analogue scale (VAS) scores in patients with pruritus, in whom LS and MA were diagnosed. All patients with MA and six (75%) patients with LS had relief of their pruritus with TENS therapy. At week 2, there was a significant difference in median VAS scores between baseline in the group of LS (P = 0.007). At 4 weeks of therapy, statistically significant differences were observed compared with the baseline and week 2 in the median VAS scores in the group of MA (P < 0.001). There was also a statistically significant improvement in median DLQI total scores with respect to baseline, which was achieved as early as week 2 in patients with LS and MA who were on the TENS treatment (P = 0.006, P = 0.001, respectively).

Pruritus: an overview of current concepts

Pruritus is an integral part of the patient's symptoms in numerous dermatological and systemic diseases in humans and animals. Comparable to chronic pain, pruritus can have a dramatic impact on the quality of life of the patient. In recent years, pruritus has been defined as an autonomous, pain-independent sensation, and itch-specific neurons, mediators, spinal neurons and cortical areas have been identified. These observations have not only improved our understanding of the neurobiology of itch but will also lead to improved diagnosis and to the development of new and more efficient therapeutic options. This article reviews the role of itch fibres and their response to various mediators of pruritus including histamine, vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), and substance P (SP), and opioids. Substances that may be involved in the induction or modulation of itch may be termed pruritogenic mediators and examples discussed include proteases, lipid mediators, neuropeptides, opioids and various cytokines. There is no single, generally accepted clinical classification of chronic pruritus. In the past pruritus has been classified on the basis of the neuroanatomical origin and on the potential underlying disease. Therapeutic options for the management of pruritus are discussed including topical and systemic therapies, assuming that trigger factors have been eliminated where possible. Topical agents may include capsaicin, the calcineurin inhibitors tacrolimus and pimecrolimus, and cannabinoid agonists such as N-palmitoyl ethanolamine. Systemic therapies may include antihistamines, anticonvulsants, opiate receptor antagonist or agonists, antidepressants, ciclosporin, and UV light.

Development of an in vitro coculture of primary sensitive pig neurons and keratinocytes for the study of cutaneous neurogenic inflammation

Cutaneous neurogenic inflammation (CNI) is often associated with skin disorders. Activated sensory neurons secrete neuropeptides, such as substance P (SP), which initiate or aggravate inflammation in the skin. The discovery of new molecules acting on these neurons is hampered by the difficulty of reproducing the interactions between nerve endings and skin in vitro. We developed an in vitro model based on the coculture of porcine primary keratinocytes and sensory neurons, which mimics skin innervation. To test the relevance of this model, we compared the effects of different substances on CNI by measuring SP secretion in vitro using a sensitive enzyme immunoassay. Collectively, our results indicate that the use of porcine cells could be very useful to perform an in vitro model of CNI. By adding capsaicin, which induces the secretion of SP by neurons, to the culture, we show that our model mimics CNI in vitro, allowing us to screen for molecules that inhibit this inflammatory response. Such a model can be used to test the effects of different substances on CNI and may be useful for dermatological or cosmetic applications. Based on our screen, we found that extracts of Laminaria digitata and Vernonia sublutea inhibit CNI.

A C-fiber-mediated neuropathic brachioradial pruritus

A 56-year-old man presented to the outpatient clinic with a 3-year history of itch within the innervation territory of C-6 of the left arm. Sudden neck movements induced intermittent paresthesias in the same dermatome. No dermatological diseases, allergies, or trauma to the affected extremity or the spine or a history of familial pruritus were reported. Neurological physical examination and electromyography revealed normal findings. Quantitative sensory testing demonstrated selective C-fiber dysfunction at C6-8 on the left, and cervical MR imaging revealed multilevel degenerative cervical spine pathology with neuroforaminal stenoses. Brachioradial neuropathic pruritus caused by cervical neuroforaminal stenosis was the final diagnosis. Treatment consisted of 2 cervical epidural steroid applications that resulted in clinical disappearance of the itch and improvement in C-fiber function on quantitative sensory testing.

Response of human oral mucosa and skin to histamine provocation: laser Doppler perfusion imaging discloses differences in the nociceptive nervous system

OBJECTIVE

To investigate the existence of histamine-excitable nerve fibers in the oral mucosa and to compare the response to histamine provocation in healthy volunteers with that in a small group of patients with chronic oral pain.

MATERIAL AND METHODS

Thirteen healthy volunteers and six patients suffering from chronic oral pain took part in the study. Blood perfusion was monitored in the hard palate, the tongue, and the skin of the cheek using laser Doppler perfusion imaging (Perimed; Sweden). Baseline scannings were performed, followed by 15 scannings after iontophoresis of histamine (1%). A free description of the sensations was then obtained from the participants after finishing the measurements.

RESULTS

Compared to pre-histamine scanning, histamine application resulted in a considerable increase in blood perfusion in all regions (p<0.001) that was significantly higher in skin than in oral mucosa (p<0.001). There were no significant differences between the healthy volunteers and the patients regarding baseline blood flow, increased blood perfusion, or flare size after histamine provocation. The sensory impression was reported to be more persistent and intense in the skin than in the oral mucosa. No effect on mucosa could be detected by visual inspection.

CONCLUSIONS

Intra-oral flare could be induced by activating histamine-excitable nerve fibers. Both duration and intensity of the flare were considerably less pronounced than in the control skin site. Histamine application was not clearly associated with itch.

Botulinum toxin type A reduces histamine-induced itch and vasomotor responses in human skin

BACKGROUND

Clinical evidence has revealed the antipruritic effect of botulinum toxin type A (BoNT/A). BoNT/A is believed to be effective against itch as it inhibits the release of acetylcholine as well as some other substances that may be involved in itch.

OBJECTIVES

To investigate the effect of subcutaneous administration of BoNT/A on experimentally histamine-induced itch in human skin.

METHODS

In this double-blind, placebo-controlled study, 14 healthy men (mean +/- SD age 26.3 +/- 2.6 years) received BoNT/A (Botox; Allergan, Irvine, CA, U.S.A.; 5 U) and isotonic saline on the volar surface of either forearm. Histamine prick tests were performed four times at the treatment sites (before treatment, and days 1, 3 and 7 after treatment). The itch intensity (as rated on a 0-10 visual analogue scale), itch area, neurogenic inflammation (visible flare area), blood flow (laser Doppler) and cutaneous temperature (thermographic images) were measured over the course of the trials.

RESULTS

BoNT/A reduced the histamine-induced itch intensity (F(1,39) = 30.2, P < 0.001) and itch area (F(1,39) = 8.8, P = 0.011) compared with saline at all time points after treatment. The duration of itch was also shorter for BoNT/A-treated areas (F(1,39) = 19.4, P < 0.001), with a peak effect at day 7. The flare area was smaller in the BoNT/A-treated arm compared with the saline-treated arm at all time points after treatment (F(1,39) = 15.4, P = 0.002). Findings from blood flow (F(1,26) = 177.3, P < 0.001) and temperature measurements (F(1,26) = 27.6, P < 0.001) clearly showed the suppressive effect of BoNT/A on vasomotor reactions, with the maximal effect on days 3 and 7.

CONCLUSIONS

BoNT/A reduced the itch intensity, blood flow and neurogenic inflammation in response to the histamine prick test in human skin. The findings could be applicable in the treatment of some pruritic conditions that can be difficult to treat with conventional treatments.

Pathogenesis and treatment of pruritus in cholestasis

Pruritus is an enigmatic, seriously disabling symptom accompanying cholestatic liver diseases and a broad range of other disorders. Most recently, novel itch-specific neuronal pathways, itch mediators and their relevant receptors have been identified. In addition, new antipruritic therapeutic strategies have been developed and/or are under evaluation. This review highlights recent experimental and clinical findings focusing on the pathogenesis and actual treatment of pruritus in cholestatic liver disease. Evidence-based therapeutic recommendations, including the use of anion exchange resins cholestyramine, colestipol and colesevelam, the microsomal enzyme inducer rifampicin, the opioid receptor antagonists naltrexone and naloxone, and the serotonin reuptake inhibitor sertraline, are provided.

Intracisternal, but not intrathecal, injection of naloxone inhibits cutaneous itch-related response in mice

The present study was conducted to determine whether cutaneous itch involves mu-opioid receptors in either of the spinal cord or lower brainstem or in both regions in mice. An intraplantar injection of serotonin hydrochloride (100 nmol/site) induced biting, an itch-related behavior. The behavior was inhibited by subcutaneous (0.3-1 mg/kg) and intracisternal (1--10 nmol/site), but not intrathecal (1--10 nmol/site), injections of naloxone hydrochloride. An intradermal injection of serotonin (100 nmol/site) to the rostral back induced scratching, an itch-related behavior, which was inhibited by subcutaneous (1 mg/kg) and intracisternal (10 nmol/site) injections of naloxone. These results suggest that mu-opioid receptor in the lower brainstem, but not spinal cord, is a site of central pruritogenic action of opioids and is involved in the facilitatory regulation of itch signaling.

Histamine-induced itch and its relationship with pain

Itch is one of the major complications of skin diseases. Although there are various substances that induce itch or pruritus, it is evident that histamine is the best known endogenous agent that evokes itch. Even though histamine-induced itch has been studied for some time, the underlying mechanism of itch is just beginning to emerge. Although various downstream signaling pathways of histamine receptors have been revealed, more studies are required to determine the cause of histamine-induced itch. It appears that itch and pain involve different neuronal pathways. Pain generally inhibits itch, which indicates an inter-communication between the two. Complex interactions between itch and pain may be expected based on reports on disease states and opioids. In this review, we discuss the molecular mechanism and the pharmacological aspects of histamine-induced itch. Especially, the underlying mechanism of TRPV1 (an anti-pruritus target) has been determined to some extent.

Prurigo nodularis and lichen simplex chronicus

Emotional tensions in predisposed subjects may play a key role in inducing a pruritic sensation, leading to a scratching that, becoming a self-perpetuating pathomechanism, may represent the main feature of two distinct cutaneous clinical entities: prurigo nodularis and lichen simplex chronicus. Psychogenic factors play a relevant role in both conditions, and they are often associated with depression and dissociative experiences. Hence, the importance of the evaluation of these patients from the point of view of psychodermatology, which may analyze the relationship between skin disease and psychological factors. Patients with real or perceived imperfections in particular areas of the body (face, scalp, hands, and genital area) are more prone to psychologic distress, whereas cutaneous diseases may lead to experience a heightened level of distress. As psychosomatic factors have been estimated to be present in at least one-third of dermatologic patients, effective management of skin conditions involves consideration of the associated emotional factors.

TRP channels as novel players in the pathogenesis and therapy of itch

Itch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including itch and pain. In particular, we argue that - due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities - certain thermosensitive TRP channels are key players in pruritus pathogenesis. We close by proposing a novel, TRP-centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch.

Cognitive, behavioral, and physiological reactivity to chronic itching: analogies to chronic pain

It is well-known that cognitive, behavioral, and physiological reactivity to pain, such as catastrophizing, avoidance of activity, and increased physiological responses, can unfavorably affect long-term outcomes in patients with chronic pain. In line with similarities between the psychophysiology of pain and itching, corresponding mechanisms may be relevant for the maintenance of chronic itching. The goal of this study was to examine the role of self-reported cognitive, behavioral, and physiological reactivity factors on itching-related outcomes in 235 patients with chronic skin diseases suffering from chronic itching. Sequential regression analyses indicate that all 3 reactivity systems predicted itching-related outcomes. Specifically, more catastrophizing, higher levels of avoidance of activity, and heightened self-reported physiological reactivity predicted more itching, more scratching, and a reduced disease-related quality of life. The results suggest that a psychological model as described for chronic pain is a useful starting point for study of the maintaining mechanisms of chronic itching.

Cytokines and chemokines orchestrate atopic skin inflammation

Atopic dermatitis (AD) is a common pruritic and chronically relapsing inflammatory skin disease. The pathophysiology of AD includes disturbed skin barrier functions, frequent allergic responses against allergens, defects in the antimicrobial immune defense, and a genetic predisposition. In this review we summarize advances in our understanding of the complex interdependent network of members of the rapidly growing protein superfamilies of cytokines and chemokines that lead to the development of AD.

Endogenous opioid-mediated antinociception in cholestatic mice is peripherally, not centrally, mediated

BACKGROUND/AIMS

Cholestasis is associated with naloxone reversible antinociception and opiate receptor antagonists are used clinically to treat pruritus. Pain and pruritus are closely interrelated and opioids modulate both sensations. Therefore, we undertook a series of experiments to characterize opioid-mediated antinociception in cholestasis and determine if it occurs inside or outside the CNS.

METHODS

Antinociception scores to both thermal and mechanical stimuli were determined in mice with cholestasis due to bile duct resection vs sham controls.

RESULTS

Cholestatic mice demonstrated significant antinociception to both stimuli compared to controls, which was reversible by the opiate receptor antagonist naloxone. The experiments were repeated with a naloxone derivative, which does not cross the blood-brain-barrier (i.e. naloxone methiodide) with similar results, indicating an opioid antinociceptive effect mediated outside of the CNS. Experiments with intraplantar injections of low dose naloxone methiodide confirmed that cholestasis-associated antinociception occurs at the level of cutaneous nerve endings. These findings were supported by findings of increased dermal met-enkephalin expression in cholestatic mice.

CONCLUSIONS

Cholestasis in mice is associated with antinociception due to local effects of endogenous opioids (i.e. met-enkephalin) at the level of sensory nerve endings. These findings may have direct implications in the management of cholestasis associated pruritus.

Klinische Klassifikation von chronischem Pruritus

Chronic pruritus is a frequent symptom in dermatology caused by a variety of dermatologic, neurologic and other systemic diseases. For adequate, economically-appropriate medical care of patients with chronic pruritus, a clinical classification of pruritus which also serves as a diagnostic algorithm is necessary. Currently suggested classifications are etiologic and not helpful in daily clinical routine. The proposed clinical classification is oriented at the clinical appearance of the patients and differentiates between disorders with or without primary or secondary skin lesions.

Interaktionen von Juckreiz und Schmerz

The discovery of specific pathways for the processing of itch has greatly enhanced our understanding of the physiology of pruritus. However, the complex interactions between itch and pain are only partly understood. This review focuses on the neurophysiological basis of itch under experimental and clinical conditions. Chronic inflammatory diseases can locally sensitize nerve endings and thereby contribute to itch. In addition, there is increasing evidence that also central processing of itch can be sensitized in pruritus patients. Interestingly, this pattern of peripheral and central sensitization in pruritus has striking similarities to the one observed in chronic pain patients. The presumed similarities in underlying sensitizing mechanisms between itch and pain has major therapeutic consequences as successful therapies for chronic pain might be used also in chronic itch.

Itch and pain

The discovery of a specialized neuronal pathway for itch has markedly improved our understanding of itch processing under physiological conditions. However, the complex interactions of pain and itch are only partly understood. This review focuses on the neurophysiological mechanisms involved in clinical and experimental itch conditions. There is emerging evidence that similar patterns of peripheral and central sensitization occur in chronic pain and chronic itch conditions. It will be of major interest to reveal whether the underlying mechanism for sensitization in the itch and pain pathways are also similar, as this might have major implications for therapy.

Topical anti-itch therapy

Topical anti-itch therapy could be causative (antiviral, antimycotic, and/or antiparasitic preparations) or symptomatic. Symptomatic therapy includes substituting some other sensation by cooling, heating, and/or counterirritation, by anesthesia of sensory nerve endings with local anesthetics, blocking mediators of pruritus (to deplete substance P or to block acetylcholine release), and reducing inflammation of the skin with corticosteroids or topical immunomodulators (pimecrolimus and tacrolimus). In addition to drugs, the patient should be taught to use emollients and to avoid skin dryness and vasodilatation by contact with irritants. Topical anti-itch preparations can be recommended not only for treatment of localized pruritus, but also for therapy of generalized pruritus when general measures are not effective, systemic drugs are contraindicated, and/or as addition to causative or systemic therapy. Topical anti-itch preparations should be prescribed after the diagnosis is made and used as the first-choice treatment together with general measures.

Itch, pain, and metaesthetic sensation

Itch is one of the alarm sensations that human beings have phylogenetically evolved for a defense purpose. Many theories and evidences regarding the biological, pathophysiological, and clinical aspects have been given, but an update on the neuroanatomy paths and consequent treatments is required. Some chemicals that are released after skin injury and facilitate the inflammatory process can induce itch or pain or basically lead to a sensitization of the nociceptor response. In clinical practice, the present authors note a continuum of sensations from touch to pain, among which many metaesthetic sensations can be described, even if the patients themselves cannot precisely define them. The specificity of itch neurons is therefore based on their spinal connections to the itch pathway rather than on unique peripheral receptors. The ambiguity of "itch unit" discharge to pruritics and algogens may be solved by the central inhibition of itch by pain: it is common knowledge that scratching relieves itching. Conversely, centrally acting pain-inhibiting opioids enhance itch by disinhibition. The relation between itch and pain is interesting in its clinical and physiopathological aspects in order to select appropriate treatment.

κ-Opioid System in Uremic Pruritus: Multicenter, Randomized, Double-Blind, Placebo-Controlled Clinical Studies

Uremic pruritus is a very common and frustrating condition for both patients and clinicians because no treatment has been demonstrated to be effective in relieving the itch. In this report, nalfurafine, a new kappa-opioid receptor agonist, was used to treat uremic pruritus in patients who were undergoing routine hemodialysis. Two multicenter, randomized, double-blind, placebo-controlled studies enrolled 144 patients with uremic pruritus to postdialysis intravenous treatment with either nalfurafine or placebo for 2 to 4 wk. A meta-analysis approach was used to assess the efficacy of nalfurafine. Statistically significant reductions in worst itching (P = 0.0212), itching intensity (P = 0.0410), and sleep disturbances (P = 0.0003) were noted in the nalfurafine group as compared with placebo. Improvements in itching (P = 0.0025) and excoriations (P = 0.0060) were noted for the nalfurafine-treated patients. Nalfurafine showed similar types and incidences of drug-related adverse events as did placebo. Nalfurafine was shown to be an effective and safe compound for use in this severely ill patient population.

Pruritus-associated response mediated by cutaneous histamine H3 receptors

BACKGROUND

Histamine is one of the most common chemical mediators causing pruritus, and H1 receptor antagonists have been used as a first choice in its treatment. On the other hand, although the presence of H3 receptors has been identified in the skin, few studies have investigated the involvement of H3 receptors on pruritus.

OBJECTIVE

The purpose of this study was to examine whether H3 receptor agonist or antagonist influences the incidence of scratching behaviour in ICR or mast cell-deficient WBB6F1-W/WV mice.

METHODS

The mice were given an intradermal injection of H3 receptor agonist or antagonist into the rostral part of the back, and the occurrence of scratching behaviour at the injected site by the hind paws was counted over 60 min.

RESULTS

H3 receptor antagonists, thioperamide and AQ0145 significantly increased the incidence of scratching behaviour in ICR mice. H3 receptor agonist, (R)-alpha-methylhistamine, had no effect. On the other hand, (R)-alpha-methylhistamine significantly inhibited thioperamide or AQ0145-induced scratching behaviour. In addition, both thioperamide and AQ0145 elicited scratching behaviour in mast cell-deficient WBB6F1-W/WV mice.

CONCLUSION

From these results, it may be concluded that H3 receptors are involved in the modulation of pruritus in the skin, and mast cells are not essential in this response. In addition, H3 receptor agonists can be useful as a novel therapeutic approach against pruritus.

Herpes zoster itch: preliminary epidemiologic data

The best-known complication of shingles (herpes zoster) is postherpetic neuralgia (PHN). PHN is commonly studied to investigate causes of and treatments for neuropathic pain. However, many patients with shingles experience neuropathic itch accompanying, or instead of, pain. Some report severe disabling postherpetic itch (PHI), and though it is rare, some patients injure themselves by scratching itchy skin that has lost protective sensation. To date, there is virtually no mention of PHI in the medical literature; neither epidemiologic, anatomic, physiologic, nor treatment studies. We analyzed 3 independent existing sets of data from 586 adults with shingles or PHN to glean epidemiologic information about pruritus during and after shingles. All data refer to itch localized to shingles-affected areas and initiated by shingles. They indicate that pruritus, usually mild or moderate, commonly accompanies both acute zoster and PHN. There was no significant difference in age between subjects with and without PHI. In one group, but not in another, there was an increased number of women with PHI. Subjects whose shingles affected the head, face, and neck were more likely to experience PHI than those whose shingles affected the torso. These findings indicate a need for research on zoster-associated itch, including prospective studies on frequency, impact, and treatment.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Management of opioid side effects in cancer-related and chronic noncancer pain: a systematic review

Side effects can limit opioid dosage and reduce quality of life. The purpose of this systematic review was to assess the management of opioid side effects in the context of cancer pain management or, in the event that no evidence was available for cancer pain, for chronic noncancer pain. The side effects studied were constipation, pruritus, nausea and vomiting, myoclonus, sedation, respiratory depression, and delirium. Opioid rotation to manage side effects was also studied. For each side effect, we searched MEDLINE and the Cochrane Controlled Trials Register and identified 657 possible titles for inclusion. Of these, 67 studies met inclusion criteria for analysis. The lack of well-designed, randomized controlled trials and the heterogeneity of populations and study designs made the drawing of firm conclusions difficult and precluded performance of meta-analysis. The type, strength, and consistency of evidence for available interventions to manage opioid side effects vary from strong (eg, on the use of naloxone to reverse respiratory depression or constipation) to weak (eg, changing from the oral to epidural route of morphine administration to manage sedation). Well-designed trials in the specified populations are required to furnish clinicians with secure evidence on managing opioid side effects successfully.

Opioid and non-opioid analgesics

Opioids are the most potent analgesics. Toxicity results either from effects mediated by variation in affinity and intrinsic efficacy at specific opioid receptors or, rarely, from a direct toxic effect of the drugs. For some adverse effects, opioids exhibit a 'dual pharmacology' whereby these effects are usually observed only in pain-free individuals, and are not seen in patients in pain. Paracetamol, although generally very safe in therapeutic doses, displays potentially fatal toxicity in overdose requiring specific treatment. Non-steroidal anti-inflammatory drugs (NSAIDs) are known to act by inhibiting COX-1 and COX-2 isoenzymes to various degrees. Toxicity arises primarily from undesired inhibition at these enzyme sites. Knowledge of the mechanism of action of these drugs is fundamental to the understanding of their potential for toxicity, the details of which are still emerging.

Pruritus - Pathophysiologie, Klinik und Therapie - Eine Ubersicht. Pruritus - pathophysiology, clinical features and therapy - an overview

Pruritus is an unpleasant sensory perception of the skin associated with the desire to scratch. As a physiological nociception, pruritus leads to the removal of harmful agents such as parasites and plants from the skin surface. More often, pruritus occurs as a severe and therapy-refractory symptom of various underlying dermatological and systemic diseases. Comparable to chronic pain, chronic pruritus worsens the general condition and may lead to physical and psychological exhaustion. Until the 1990s, pruritus had been regarded as an incomplete pain sensation. Only recently, itch was defined as a separate, pain-independent sensation with its own mediators, spinal neurons and cortical areas. These observations led to the development of new therapeutic modalities. This paper gives an overview of itch pathophysiology, clinical types and therapies.