Deletion of μ- and κ-Opioid Receptors in Mice Changes Epidermal Hypertrophy, Density of Peripheral Nerve Endings, and Itch Behavior

Angelica Yinzi Alleviates Pruritus-Related Atopic Dermatitis through Skin Repair, Antioxidation, and Balancing Peripheral μ- and κ-opioid Receptors

Background

Angelica Yinzi (AYZ) is a Chinese traditional herbal formula reported to attenuate itches and inflammation caused by atopic dermatitis (AD). However, the underlying mechanism of AYZ in the attenuation of itchiness and inflammation remains unknown.

Objective

This study investigated the mechanism of AYZ in reducing itchiness in mice with 1-chloro-2,4-dinitrobenzene- (DNCB-)-induced atopic dermatitis.

Methods

Hematoxylin and eosin (H&E) and toluidine blue staining were used to evaluate pathological changes in skin tissue, while an enzyme-linked immunosorbent assay (ELISA) was used to assess the cytokine levels in the skin. After that, qRT-PCR was performed to determine the mRNA levels of cytokines in the skin. Immunofluorescence and western blotting analysis were further used to assess µ-opioid receptor (MOR) expression and immunohistochemistry to assess the p-ERK, p-AKT, and κ-opioid receptor (KOR).

Results

The AYZ treatment alleviated the AD clinical symptoms, including decreasing the scratching frequency, the ear thickness, and the infiltration of mast cells, lymphocytes, inflammatory cells, and mononuclear cells. In addition, AYZ inhibited the expression of interleukin (IL)-13, thymic stromal lymphopoietin (TSLP), and reduced neuraminidase (NA), corticotropin-releasing factor (CRF), and reactive oxygen species (ROS) expression. Markers involved in itches, such as p-ERK and p-AKT, were significantly downregulated following AYZ treatment. Besides, AYZ significantly increased MOR expression and downregulated KOR in the epidermis and spinal cord.

Conclusion

Our findings imply that AYZ ameliorates pruritus-related AD through skin repair, antioxidation, and balancing peripheral MOR and KOR. The findings in this study lay a theoretical foundation for the control mechanism of peripheral itch.

Arsenic exposure and pruritus: Evidence from observational, interventional, and mendelian randomization studies

BACKGROUND

Pruritus is identified as an adverse drug reaction to arsenic trioxide, but the association of arsenic exposure with pruritus has not been investigated.

METHODS

A cross-sectional study was conducted in Shimen, China. A Mendelian randomization analysis was conducted to confirm the causal relationship between genetically predicted percentages of monomethylated arsenic (MMA%) and dimethylated arsenic (DMA%) in urine with chronic pruritus in UK Biobank. A case-control study was then conducted to determine the biomarker for pruritus. Arsenite-treated mice were used to confirm the biomarker, and von Frey test was used to induce scratching bouts. Last, a randomized, double-blind, placebo-controlled trial was conducted to test the efficacy of naloxone in arsenic-exposed patients with pruritus in Shimen.

RESULTS

Hair arsenic (μg/g) showed a dose-response relationship with the intensity of itch in 1079 participants, with odds ratios (OR) of 1.11 for moderate-to-severe itch (p = 0.012). The Mendelian randomization analysis confirmed the causal relationship, with ORs of 1.043 for MMA% (p = 0.029) and 0.904 for DMA% (p = 0.077) above versus under median. Serum β-endorphin was identified as a significant biomarker for the intensity of itch (p < 0.001). Consistently, treatment with arsenite upregulated the level of β-endorphin (p = 0.002) and induced scratching bouts (p < 0.001) in mice. The randomized controlled trial in 126 participants showed that treatment with sublingual naloxone significantly relieved the intensity of itch in arsenic-exposed participants in 2 weeks (β = -0.98, p = 0.04).

CONCLUSION

Arsenic exposure is associated with pruritus, and β-endorphin serves as a biomarker of pruritus. Naloxone relieves pruritus in patients with arseniasis.

The Kappa Opioid Receptor: A Promising Therapeutic Target for Multiple Pathologies

Kappa-opioid receptors (KOR) are widely expressed throughout the central nervous system, where they modulate a range of physiological processes depending on their location, including stress, mood, reward, pain, inflammation, and remyelination. However, clinical use of KOR agonists is limited by adverse effects such as dysphoria, aversion, and sedation. Within the drug-development field KOR agonists have been extensively investigated for the treatment of many centrally mediated nociceptive disorders including pruritis and pain. KOR agonists are potential alternatives to mu-opioid receptor (MOR) agonists for the treatment of pain due to their anti-nociceptive effects, lack of abuse potential, and reduced respiratory depressive effects, however, dysphoric side-effects have limited their widespread clinical use. Other diseases for which KOR agonists hold promising therapeutic potential include pruritis, multiple sclerosis, Alzheimer’s disease, inflammatory diseases, gastrointestinal diseases, cancer, and ischemia. This review highlights recent drug-development efforts targeting KOR, including the development of G-protein–biased ligands, mixed opioid agonists, and peripherally restricted ligands to reduce side-effects. We also highlight the current KOR agonists that are in preclinical development or undergoing clinical trials.

Opioidergic Signaling-A Neglected, Yet Potentially Important Player in Atopic Dermatitis

Atopic dermatitis (AD) is one of the most common skin diseases, the prevalence of which is especially high among children. Although our understanding about its pathogenesis has substantially grown in recent years, and hence, several novel therapeutic targets have been successfully exploited in the management of the disease, we still lack curative treatments for it. Thus, there is an unmet societal demand to identify further details of its pathogenesis to thereby pave the way for novel therapeutic approaches with favorable side effect profiles. It is commonly accepted that dysfunction of the complex cutaneous barrier plays a central role in the development of AD; therefore, the signaling pathways involved in the regulation of this quite complex process are likely to be involved in the pathogenesis of the disease and can provide novel, promising, yet unexplored therapeutic targets. Thus, in the current review, we aim to summarize the available potentially AD-relevant data regarding one such signaling pathway, namely cutaneous opioidergic signaling.

β-Neoendorphin Enhances Wound Healing by Promoting Cell Migration in Keratinocyte

The skin is the largest and a remarkably plastic organ that serves as a protective barrier against environmental stimuli and injuries throughout life. Skin injuries are serious health problems, and wound healing is a critical process to replace devitalized cellular and tissue structures. Although some endogenous opioids are known to be involved in the modulation of wound healing, it remains to be determined whether the β-neoendorphin (β-NEP), an endogenous opioid, has beneficial effects on wound repair in human keratinocyte. In this study, we found that β-NEP accelerated wound repair through activation of mitogen-activated protein kinase (MAPK)/Erk1/2 signaling pathways in human keratinocytes. Moreover, the wound healing effect of β-NEP is mainly through the acceleration of keratinocyte migration without affecting cell proliferation. Therefore, our studies reveal that β-NEP plays an important role in the regulation of wound repair and suggest a therapeutic strategy to promote wound healing using β-NEP.

Substance use disorders and chronic itch

Chronic pruritus is one dermatologic manifestation of an underlying substance use disorder. Recent literature has uncovered similarities between the general neurologic mechanisms of addiction and chronic itch, largely involving activation of the dopaminergic reward circuits within the brain and imbalances between mu and kappa opioid receptor activation. It is likely that the use of specific drugs, like central nervous system stimulants and opioids, results in further activation and imbalances within these pathways, perpetuating both addiction and pruritus simultaneously. Opioid users often present to dermatology clinics with a generalized pruritus, whereas individuals using central nervous system stimulants like cocaine and methylenedioxymethamphetamine (MDMA), as well as legally prescribed drugs like treatments for attention deficit hyperactivity disorder, frequently complain of crawling, delusional infestation-like sensations underneath the skin. Because of these overlapping mechanisms and similar clinical presentations to many other chronically itchy conditions, it is necessary for dermatologists to consider and investigate an underlying substance use disorder to effectively treat these patients.

Opioid receptors expression in the skin of haemodialysis patients suffering from uraemic pruritus

BACKGROUND

Uraemic pruritus is a common symptom in patients with chronic kidney disease undergoing haemodialysis (HD) treatment. Etiopathogenesis of uraemic pruritus is complex and not fully explained, but there are reports indicating a possible role of peripheral opioid system in its pathomechanism.

OBJECTIVES

This study was undertaken to analyse the expression of mu- and kappa-opioid receptors in the skin of HD patients with and without uraemic pruritus.

METHODS

Forty patients suffering from chronic kidney disease, treated with haemodialysis (21 pruritic and 19 non-pruritic), were included. The expression of mu- and kappa-opioid receptors in their skin was evaluated with immunohistochemistry.

RESULTS

The significant (P < 0.02) decrease in kappa-opioid receptor expression was shown in the skin of pruritic patients in comparison with the skin of those without itch (1.07 ± 0.62 and 1.66 ± 0.85, respectively). Moreover, there was a significant negative correlation between intensity of pruritus and expression of kappa-opioid receptors (r = -0.63, P = 0.002). There was no difference in the expression of mu-opioid receptor expression in the skin of pruritic and non-pruritic HD patients.

CONCLUSIONS

This study indicates that changes in peripheral opioid system may play an important role in the uraemic itch pathogenesis and that future studies are necessary to fully understand this mechanism.

The analgesic hybrid of dermorphin/substance P and analog of enkephalin improve wound healing in streptozotocin-induced diabetic rats

The purpose of this study was to investigate the effect of the peptide analgesic hybrid compounds: AWL3106 analog of dermorphin and substance P (7-11), and biphalin enkephalin analog on wound healing in streptozotocin-induced diabetic rats. The diabetes was induced in 6-7 week-old male Wistar rats by intraperitoneal injection of streptozotocin. After 70 days, the wounds were created on the back of the rats and then, once a day for 21 days, the dressing containing lanolin ointment, 10% of keratin scaffolds, and 1 mM of AWL3106 or biphalin was applied. The wounds histology were analyzed by hematoxylin and eosin staining. The orientation and organization of collagen was analyzed by Masson's trichome staining. The number of macrophages, blood vessels, and fibroblasts were visualized by CD68, CD34, and vimentin immunoreactivity, respectively. Our results demonstrated that the wound area of AWL3106- and biphalin-treated groups was greatly reduced (up to 47% on the 7 day) in comparison with untreated diabetic groups. The immunohistochemical staining of macrophages demonstrated that AWL3106 and biphalin accelerated inflammatory progression and subsequently decreased persistent inflammation. The histological analysis showed that the structure of tissue in the groups under the study was very similar to the one of wound tissue in N-DM group. The H&E and Masson's trichome staining demonstrated that the orientation and organization of collagen as well as the number and shape of blood vessels were better in 3106- and BIF-treated group than in DM group. In conclusion, the obtained data suggested that our hybrid peptides enhanced wound healing, particularly by accelerating the inflammatory phase and promoted the wound closure.

Non-canonical Opioid Signaling Inhibits Itch Transmission in the Spinal Cord of Mice

Chronic itch or pruritus is a debilitating disorder that is refractory to conventional anti-histamine treatment. Kappa opioid receptor (KOR) agonists have been used to treat chronic itch, but the underlying mechanism remains elusive. Here, we find that KOR and gastrin-releasing peptide receptor (GRPR) overlap in the spinal cord, and KOR activation attenuated GRPR-mediated histamine-independent acute and chronic itch in mice. Notably, canonical KOR-mediated Gαi signaling is not required for desensitizing GRPR function. In vivo and in vitro studies suggest that KOR activation results in the translocation of Ca2+-independent protein kinase C (PKC)δ from the cytosol to the plasma membrane, which in turn phosphorylates and inhibits GRPR activity. A blockade of phospholipase C (PLC) in HEK293 cells prevented KOR-agonist-induced PKCδ translocation and GRPR phosphorylation, suggesting a role of PLC signaling in KOR-mediated GRPR desensitization. These data suggest that a KOR-PLC-PKCδ-GRPR signaling pathway in the spinal cord may underlie KOR-agonists-induced anti-pruritus therapies.

Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides

Opioids are the most powerful analgesics. As pain is driven by sensory transmission and opioid receptors couple to inhibitory G proteins, according to the classical concept, opioids alleviate pain by activating receptors on neurons and blocking the release of excitatory mediators (e.g., substance P). Here we show that analgesia can be mediated by opioid receptors in immune cells. We propose that activation of leukocyte opioid receptors leads to the secretion of opioid peptides Met-enkephalin, β-endorphin and dynorphin A (1-17), which subsequently act at local neuronal receptors, to relieve pain. In a mouse model of neuropathic pain induced by a chronic constriction injury of the sciatic nerve, exogenous agonists of δ-, μ- and κ-opioid receptors injected at the damaged nerve infiltrated by opioid peptide- and receptor-expressing leukocytes, produced analgesia, as assessed with von Frey filaments. The analgesia was attenuated by pharmacological or genetic inactivation of opioid peptides, and by leukocyte depletion. This decrease in analgesia was restored by the transfer of wild-type, but not opioid receptor-lacking leukocytes. Ex vivo, exogenous opioids triggered secretion of opioid peptides from wild-type immune cells isolated from damaged nerves, which was diminished by blockade of Gαi/o or Gβγ (but not Gαs) proteins, by chelator of intracellular (but not extracellular) Ca(2+), by blockers of phospholipase C (PLC) and inositol 1,4,5-trisphosphate (IP3) receptors, and was partially attenuated by protein kinase C inhibitor. Similarly, the leukocyte depletion-induced decrease in exogenous opioid analgesia was re-established by transfer of immune cells ex vivo pretreated with extracellular Ca(2+) chelator, but was unaltered by leukocytes pretreated with intracellular Ca(2+) chelator or blockers of Gαi/o and Gβγ proteins. Thus, both ex vivo opioid peptide release and in vivo analgesia were mediated by leukocyte opioid receptors coupled to the Gαi/o-Gβγ protein-PLC-IP3 receptors-intracellular Ca(2+) pathway. Our findings suggest that opioid receptors in immune cells are important targets for the control of pathological pain.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Primary afferent neurons express functional delta opioid receptors in inflamed skin

Peripherally-restricted opiate compounds attenuate hyperalgesia in experimental models of inflammatory pain, but have little discernable effect on nociceptive behavior in normal animals. This suggests that activation of opioid receptors on peripheral sensory axons contributes to decreased afferent activity after injury. Previously, we reported that direct application of morphine to cutaneous receptive fields decreased mechanical and heat-evoked responses in a population of C-fiber nociceptors in inflamed skin. Consistent with reported behavioral studies, direct application of morphine had no effect on fiber activity in control skin. The aim of the present study was to determine whether mechanical responsiveness of nociceptors innervating inflamed skin was attenuated by direct activation of delta opioid receptors (DORs) on peripheral terminals. An ex vivo preparation of rat plantar skin and tibial nerve was used to examine effects of a selective DOR agonist, deltorphin II, on responsiveness of single fibers innervating inflamed skin. Electrical recordings were made eighteen hours after injection of complete Freund's adjuvant into the hindpaw. Deltorphin II produced an inhibition of the mechanical responsiveness of single fibers innervating inflamed skin; an effect blocked by the DOR-selective antagonist, naltrindole. The population of units responsive to deltorphin II was identified as consisting of C fiber mechanical nociceptors.

TRP channels in skin: from physiological implications to clinical significances

TRP channels are expressed in various cells in skin. As an organ system to border the host and environment, many nonneuronal cells, including epidermal keratinocytes and melanocytes, express several TRP channels functionally distinct from sensory processing. TRPV1 and TRPV3 in keratinocytes of the epidermis and hair apparatus inhibit proliferation, induce terminal differentiation, induce apoptosis, and promote inflammation. Activation of TRPV4, 6, and TRPA1 promotes regeneration of the severed skin barriers. TRPA1 also enhances responses in contact hypersensitivity. TRPCs in keratinocytes regulate epidermal differentiation. In human diseases with pertubered epidermal differentiation, the expression of TRPCs are altered. TRPMs, which contribute to melanin production in melanocytes, serve as significant prognosis markers in patients with metastatic melanoma. In summary, not only act in sensory processing, TRP channels also contribute to epidermal differentiation, proliferation, barrier integration, skin regeneration, and immune responses. In diseases with aberrant TRP channels, TRP channels might be good therapeutic targets.

Morphine for the treatment of pain in sickle cell disease

Pain is a hallmark of sickle cell disease (SCD) and its treatment remains challenging. Opioids are the major family of analgesics that are commonly used for treating severe pain. However, these are not always effective and are associated with the liabilities of their own. The pharmacology and multiorgan side effects of opioids are rapidly emerging areas of investigation, but there remains a scarcity of clinical studies. Due to opioid-induced endothelial-, mast cell-, renal mesangial-, and epithelial-cell-specific effects and proinflammatory as well as growth influencing signaling, it is likely that when used for analgesia, opioids may have organ specific pathological effects. Experimental and clinical studies, even though extremely few, suggest that opioids may exacerbate existent organ damage and also stimulate pathologies of their own. Because of the recurrent and/or chronic use of large doses of opioids in SCD, it is critical to evaluate the role and contribution of opioids in many complications of SCD. The aim of this review is to initiate inquiry to develop strategies that may prevent the inadvertent effect of opioids on organ function in SCD, should it occur, without compromising analgesia.

The Mu opioid receptor promotes opioid and growth factor-induced proliferation, migration and Epithelial Mesenchymal Transition (EMT) in human lung cancer

Recent epidemiologic studies implying differences in cancer recurrence based on anesthetic regimens raise the possibility that the mu opioid receptor (MOR) can influence cancer progression. Based on our previous observations that overexpression of MOR in human non-small cell lung cancer (NSCLC) cells increased tumor growth and metastasis, this study examined whether MOR regulates growth factor receptor signaling and epithelial mesenchymal transition (EMT) in human NSCLC cells. We utilized specific siRNA, shRNA, chemical inhibitors and overexpression vectors in human H358 NSCLC cells that were either untreated or treated with various concentrations of DAMGO, morphine, fentanyl, EGF or IGF. Cell function assays, immunoblot and immunoprecipitation assays were then performed. Our results indicate MOR regulates opioid and growth factor-induced EGF receptor signaling (Src, Gab-1, PI3K, Akt and STAT3 activation) which is crucial for consequent human NSCLC cell proliferation and migration. In addition, human NSCLC cells treated with opioids, growth factors or MOR overexpression exhibited an increase in snail, slug and vimentin and decrease ZO-1 and claudin-1 protein levels, results consistent with an EMT phenotype. Further, these effects were reversed with silencing (shRNA) or chemical inhibition of MOR, Src, Gab-1, PI3K, Akt and STAT3 (p<0.05). Our data suggest a possible direct effect of MOR on opioid and growth factor-signaling and consequent proliferation, migration and EMT transition during lung cancer progression. Such an effect provides a plausible explanation for the epidemiologic findings.

Targeting inflammation and wound healing by opioids

Opioid receptors are expressed on peripheral sensory nerve endings, cutaneous cells, and immune cells; and local application of opioids is used for the treatment of inflammatory pain in arthritis, burns, skin grafts, and chronic wounds. However, peripherally active opioids can also directly modulate the inflammatory process and wound healing. Here, we discuss the underlying mechanisms of opioid action and the conceivable therapeutic approaches for opioid treatment, as investigated in experimental and clinical studies. A large number of in vitro experiments and animal model investigations have produced evidence that peripherally active opioids can reduce plasma extravasation, vasodilation, proinflammatory neuropeptides, immune mediators, and tissue destruction. In contrast to currently available anti-inflammatory agents, opioids have not demonstrated organ toxicity, thus making them interesting candidates for drug development. Few clinical studies have tapped into this potential to date.

Immunohistochemistry detection of kappa-opioid receptors in human skin

The imbalance of p- and kappa-opioid receptors in the skin or central nervous system is currently deemed to be one of the reasons of chronic pruritus. A number of studies demonstrated a positive effect of system agonists of kappa-opioid receptors in the treatment of uremic pruritus, nodular pruritus, paraneoplastic and cholestatic pruritus. This research demonstrates an expression of kappa-opioid receptors in human skin (basal keratinocytes, dendritic cells, epidermal melanocytes and fibroblasts of the upper dermis) detected with the use of different immunochemistry methods.

Changes in hind paw epidermal thickness, peripheral nerve distribution and mechanical sensitivity after immobilization in rats

This study was designed to investigate histological changes in skin tissue accompanying immobilization-induced hypersensitivity. Changes in mechanical sensitivity, epidermal thickness, and peripheral nerve profiles in the upper dermis were examined in glabrous skin of rat hind paw after 1, 2, and 4 weeks of ankle joint immobilization by plaster casts. Induction of mechanical hypersensitivity was confirmed after 2 and 4 weeks of joint immobilization. Epidermal thinning and increase in peripheral nerve profiles were observed in skin tissues in immobilized rats. The time course of epidermal thinning and increase in peripheral nerve profiles were similar closely to that of hypersensitivity, with significant differences between the immobilized and control rats after 2 weeks of immobilization, which became even more remarkable at 4 weeks of immobilization. These findings suggest that joint immobilization by cast induces epidermal thinning and increases peripheral nerve profiles in the upper dermis and that these changes might be partly responsible for immobilization-induced hypersensitivity.

Mouse models of acute, chemical itch and pain in humans

In psychophysical experiments, humans use different verbal responses to pruritic and algesic chemical stimuli to indicate the different qualities of sensation they feel. A major challenge for behavioural models in the mouse of chemical itch and pain in humans is to devise experimental protocols that provide the opportunity for the animal to exhibit a multiplicity of responses as well. One basic criterion is that chemicals that evoke primarily itch or pain in humans should elicit different types of responses when applied in the same way to the mouse. Meeting this criterion is complicated by the fact that the type of behavioural responses exhibited by the mouse depends in part on the site of chemical application such as the nape of the neck that evokes only scratching with the hind paw versus the hind limb that elicits licking and biting. Here, we review to what extent mice behaviourally differentiate chemicals that elicit itch versus pain in humans.

Engineering endomorphin drugs: state of the art

INTRODUCTION

Although endomorphins-1 (EM-1; H-Tyr-Pro-Phe-Trp-NH(2)) and -2 (EM-2; H-Tyr-Pro-Phe-Phe-NH(2)) are primarily considered agonists for the μ-opioid receptor (MOR), systematic alterations to specific residues provided antagonists and ligands with mixed μ/δ-opioid properties, suitable for application to health-related topics. While the application of endomorphins as antinociceptive agents and numerous biological endpoints were experimentally delineated in laboratory animals and in vitro, clinical use is currently absent. However, structural alterations provide enhanced stability; formation of MOR antagonists or mixed and dual μ/δ-acting ligands could find considerable therapeutic potential.

AREAS COVERED

This review attempts to succinctly provide insight on the development and bioactivity of endomorphin analogues during the past decade. Rational design approaches will focus on the engineering of endomorphin agonists, antagonists and mixed ligands for their application as a multi-target ligand.

EXPERT OPINION

Aside from alleviating pain, EM analogues open new horizons in the treatment of medical syndromes involving neural reward mechanisms and extraneural regulation effects on homeostasis. Highly selective MOR antagonists may be promising to reduce inflammation, attenuate addiction to drugs and excess consumption of high-caloric food, ameliorate alcoholism, affect the immune system and combat opioid bowel dysfunction.

Evaluation of epidermal nerve density and opioid receptor levels in psoriatic itch

BACKGROUND

Psoriasis is a complex, multifactorial inflammatory skin disease with genetic and environmental interactions. Patients with psoriasis exhibit erythematous plaques with itch, but the mechanisms of psoriatic itch are poorly understood.

OBJECTIVES

This study was performed to investigate epidermal nerve density and opioid receptor levels in psoriatic skin with or without itch.

METHODS

Twenty-four patients with psoriasis aged between 39 and 82 years were included in this study. The number of epidermal nerve fibres, the levels of semaphorin 3A (Sema3A) and the expression patterns of μ- and κ-opioid systems were examined immunohistologically in skin biopsies from psoriatic patients with or without itch and healthy volunteers as controls.

RESULTS

The number of epidermal nerve fibres tended to increase in approximately 40% of psoriatic patients with itch compared with healthy controls, while such intraepidermal nerves were not observed in other itchy patients. In comparison with healthy controls, Sema3A levels also tended to decrease in the epidermis of psoriatic patients with itch. However, no relationship was found between nerve density and Sema3A levels in the epidermis of psoriatic patients with itch. The levels of μ-opioid receptor and β-endorphin in the epidermis were the same in healthy controls and psoriatic patients with or without itch. The levels of κ-opioid receptor and dynorphin A were significantly decreased in the epidermis of psoriatic patients with itch compared with healthy controls.

CONCLUSIONS

Based on Sema3A levels in the epidermis, epidermal opioid systems, rather than hyperinnervation, may be involved in the pathogenesis of psoriatic itch.

Surface rejuvenating effect of Achillea millefolium extract

Proopiomelanocortin is a precursor peptide that gives rise to several neuropeptides including adrenocorticotrophic hormone (ACTH) and β-endorphin. POMC-derived peptides have been shown to be synthesized in human epidermis where they modulate numerous skin functions. Because we previously observed that melanocortin receptor-2 and μ-opioid receptor 1, the respective receptors for ACTH and β-endorphin decreased with ageing in human epidermis, we have selected an active ingredient (INCI name: Achillea millefolium extract) able to upregulate receptor expressions. The aim of the present work was first to evaluate the effect of A. millefolium extract on the expression pattern of various epidermal differentiation markers ex vivo in normal human skin biopsies using quantitative image analysis and second to evaluate its capacity to rejuvenate the appearance of skin surface in vivo. Results show an improved expression profile of cytokeratin 10, transglutaminase-1 and filaggrin in cultured skin biopsies as well as an increased epidermal thickness. In vivo, a 2-month treatment with A. millefolium extract at 2% significantly improved the appearance of wrinkles and pores compared with placebo. Results were also directionally better than those of glycolic acid that was chosen as reference resurfacing molecule.

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.

Pain-related behaviors and neurochemical alterations in mice expressing sickle hemoglobin: modulation by cannabinoids

Sickle cell disease causes severe pain. We examined pain-related behaviors, correlative neurochemical changes, and analgesic effects of morphine and cannabinoids in transgenic mice expressing human sickle hemoglobin (HbS). Paw withdrawal threshold and withdrawal latency (to mechanical and thermal stimuli, respectively) and grip force were lower in homozygous and hemizygous Berkley mice (BERK and hBERK1, respectively) compared with control mice expressing human hemoglobin A (HbA-BERK), indicating deep/musculoskeletal and cutaneous hyperalgesia. Peripheral nerves and blood vessels were structurally altered in BERK and hBERK1 skin, with decreased expression of mu opioid receptor and increased calcitonin gene-related peptide and substance P immunoreactivity. Activators of neuropathic and inflammatory pain (p38 mitogen-activated protein kinase, STAT3, and mitogen-activated protein kinase/extracellular signal-regulated kinase) showed increased phosphorylation, with accompanying increase in COX-2, interleukin-6, and Toll-like receptor 4 in the spinal cord of hBERK1 compared with HbA-BERK. These neurochemical changes in the periphery and spinal cord may contribute to hyperalgesia in mice expressing HbS. In BERK and hBERK1, hyperalgesia was markedly attenuated by morphine and cannabinoid receptor agonist CP 55940. We show that mice expressing HbS exhibit characteristics of pain observed in sickle cell disease patients, and neurochemical changes suggestive of nociceptor and glial activation. Importantly, cannabinoids attenuate pain in mice expressing HbS.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Skin biopsy for the diagnosis of peripheral neuropathy

Skin biopsy has become an accepted tool for investigating small nerve fibres, which are invisible to conventional neurophysiological tests even though they are affected early on in peripheral neuropathies of varying aetiology. Morphometric analysis of epidermal and dermal nerves has proved to be reliable, reproducible and unaffected by the severity of neuropathy, making skin biopsy useful for diagnosing small fibre neuropathy (SFN) in clinical practice. The possibility of obtaining skin biopsy specimens from different sites of the body, to repeat them within the area of the same sensory nerve, to distinguish between somatic and autonomic nerves and to investigate the expression of nerve-related proteins has widened the potential applications of this technique to clinical research. Skin biopsy performed using a minimally invasive disposable punch is a safe and painless procedure. Using specific antibodies with bright-field immunohistochemistry or immunofluorescence technique, it is possible to investigate unmyelinated fibres innervating the epidermis of hairy and glabrous skin, large myelinated fibres supplying specialized corpuscles in glabrous skin, and autonomic fibres innervating sweat glands, blood vessels and arrector pilorum muscles. This review discusses the features of skin innervation in hairy and glabrous skin, the functional properties of skin nerve fibres and their changes in peripheral neuropathies.

Endogenous opioids and cannabinoids: system interactions in the regulation of appetite, grooming and scratching

Growing evidence suggests substantial crosstalk between endogenous opioid and cannabinoid systems in the regulation of appetite. Not only is cannabinoid-induced hyperphagia abolished by opioid receptor antagonists (and vice versa), but several laboratories have reported supra-additive anorectic responses following co-administration of opioid and CB1 receptor antagonists. In the present study, videoanalysis has been used to characterise the acute effects of sub-anorectic doses of rimonabant (0.25, 0.75 mg/kg) and naloxone (0.1 mg/kg), alone and in combination, on mash intake, ingestive and non-ingestive behaviour, and post-treatment weight gain in male rats. The results confirmed that, when administered alone, none of these treatments significantly altered mash consumption, various measures of feeding behaviour, or weight gain. Although most non-ingestive behaviours were also unaffected, 0.75 mg/kg rimonabant induced compulsive scratching and grooming. However, when naloxone was given in combination with either dose of rimonabant, both food intake and time spent feeding were significantly decreased while the behavioural satiety sequence (BSS) was accelerated. On further analysis, the co-treatment reductions in food intake and feeding behaviour were found to be of an additive rather than supra-additive nature. Intriguingly, the co-administration of naloxone also virtually abolished the compulsive scratching response to the higher dose of rimonabant. Findings are discussed in relation to current views on the molecular bases of opioid-cannabinoid system interactions and the unexpected 'dual' advantage (reduction in appetite plus attenuation of side-effect) of low-dose combinations of opioid and cannabinoid CB1 receptor antagonists.

Neuroimmune interactions in allergic skin diseases

PURPOSE OF REVIEW

Recent studies have advanced our understanding that allergic inflammation triggers neuronal dysfunction, thereby modulating inflammation-related changes in affected tissues including the skin. Vice versa, evidence has emerged that inflammatory responses are controlled by neurons. Moreover, structural cells and invading immune cells express neuronal receptors and release mediators which directly communicate with nerve endings in the skin.

RECENT FINDINGS

During the allergic response, skin cells do not only represent a significant source of neuromediators but also represent targets for neuropeptides or neurotrophins as well as neurotransmitters in the inflamed tissue. During the last decade, it has become obvious that a large variety of molecules influence the adaptive as well as the innate immune response. Beside neuropeptide receptors, proteinase-activated receptors, novel histamine receptors, different cytokine or chemokine receptors play a role in the pathophysiology of atopic and allergic diseases.

SUMMARY

Peripheral sensory and autonomic nerves are critically involved in many pathways of the innate and adoptive immune system during allergic and atopic skin diseases. Further dissection of receptor-mediated and intracellular signal pathways will help to develop more effective therapeutic approaches for allergic and inflammatory skin diseases.

Possible roles of epidermal opioid systems in pruritus of atopic dermatitis

The micro-opioid (beta-endorphin/micro-opioid receptor) and kappa-opioid (dynorphin A (DynA)/kappa-opioid receptor) systems play pivotal roles in the modulation of pruritus in the central nervous system. The micro-opioid system has also been identified in human epidermis, raising the possibility that the system controls the peripheral itch. However, the precise distribution of the kappa-opioid system has not yet been clarified in human epidermis. To address this issue, reverse transcription-PCR and immunohistochemical analyses were performed on cultured keratinocytes and normal skins from humans. The analyses revealed that epidermal keratinocytes express kappa-opioid receptor and its ligands, DynA (1-17) and DynA (1-8). Moreover, expression for micro- and kappa-opioid systems was examined immunohistochemically in skin biopsies from healthy volunteers and patients with atopic dermatitis (AD) before and after psoralen-ultraviolet A (PUVA) therapy. Our expression analyses showed that only the kappa-opioid system, not the micro-opioid system, was downregulated in the epidermis of AD patients. The downregulation of the micro-opioid system and the restoration of the kappa-opioid system by PUVA therapy were observed in the AD patients, concomitant with a decrease of VAS (visual analogue scale) scores. These results suggest epidermal opioid systems are associated with the modulation of pruritus in AD. This new finding may help us to understand the control mechanism of peripheral itch.

Opioids and the Skin: “Itchy” Perspectives beyond Analgesia and Abuse

Opioids are intimately linked to central pain inhibition and their abuse potential. Thus, peripheral opioid receptors in the skin have been studied initially with a focus on their peripheral analgesic properties. Recent results, however, clearly indicate that opioids play a specific role in skin homeostasis by modulating keratinocyte differentiation, wound healing, and inflammatory responses.