Detection of kappa and delta opioid receptors in skin--outside the nervous system

Limitations and potential of κOR biased agonists for pain and itch management

The concept of ligand bias is based on the premise that different agonists can elicit distinct responses by selectively activating the same receptor. These responses often determine whether an agonist has therapeutic or undesirable effects. Therefore, it would be highly advantageous to have agonists that specifically trigger the therapeutic response. The last two decades have seen a growing trend towards the consideration of ligand bias in the development of ligands to target the κ-opioid receptor (κOR). Most of these ligands selectively favor G-protein signaling over β-arrestin signaling to potentially provide effective pain and itch relief without adverse side effects associated with κOR activation. Importantly, the specific role of β-arrestin 2 in mediating κOR agonist-induced side effects remains unknown, and similarly the therapeutic and side-effect profiles of G-protein-biased κOR agonists have not been established. Furthermore, some drugs previously labeled as G-protein-biased may not exhibit true bias but may instead be either low-intrinsic-efficacy or partial agonists. In this review, we discuss the established methods to test ligand bias, their limitations in measuring bias factors for κOR agonists, as well as recommend the consideration of other systematic factors to correlate the degree of bias signaling and pharmacological effects. This article is part of the Special Issue on "Ligand Bias".

Delta opioid receptor expression correlates to skin ageing and melanin expression in Asian women

While the evidence for the implication of opioid receptors (OPr) in ageing is growing, there is, to our knowledge, no study focusing directly on changes in vivo cutaneous OPr expression with increasing age. We thus investigated OPr expression in 30 healthy female Asian volunteers in Southern China whose ages range from the early 20s to the early 60s. Excisional biopsies were taken from the sun-exposed extensor area of the lower arm and the photo-protected area of the upper inner arm. The thickness of the epidermal layers, melanin content, as well as expression of mu-opioid receptors (MOPr) and delta-opioid receptors (DOPr) were compared between different age ranges and photo-exposure status. Significant increased epidermal hypertrophy on the extensor surface was observed. There was significant reduction of DOPr in the epidermis with increasing age, independent of photo-ageing. The increase of melanin was significantly correlated with epidermal DOPr expression, not with MOPr expression. DOPr expression could thus serve as a marker for real biological ageing unaffected by chronic photo-exposure. Additionally, DOPr expression was inversely correlated with the deposition of melanin. Based on these results, we hypothesise that regulation of DOPr expression could be used to improve aged skin, including hyperpigmentation.

Xylazine is an agonist at kappa opioid receptors and exhibits sex-specific responses to opioid antagonism

Xylazine is in the unregulated drug supply at increasing rates, usually combined with fentanyl, necessitating understanding of its pharmacology. Despite commentary from politicians, and public health officials, it is unknown how xylazine impacts naloxone efficacy, and. few studies have examined it alone. Here, we examine the impact of xylazine alone and in combination with fentanyl on several behaviors in mice. Surprisingly, naloxone precipitates withdrawal from xylazine and fentanyl/xylazine coadministration, with enhanced sensitivity in females. Further, xylazine is a full agonist at kappa opioid receptors, a potential mechanism for its naloxone sensitivity. Finally, we demonstrate surprising effects of xylazine to kappa opioid antagonism, which are relevant for public health considerations. These data address an ongoing health crisis and will help inform critical policy and healthcare decisions.

Oral difelikefalin reduces moderate to severe pruritus and expression of pruritic and inflammatory biomarkers in subjects with atopic dermatitis

BACKGROUND

Pruritus is the most common and burdensome symptom of atopic dermatitis (AD). Pruritus-targeted treatments in AD are lacking, particularly for patients with milder skin disease.

OBJECTIVE

We sought to evaluate the impact of the selective κ-opioid receptor agonist difelikefalin (DFK) on pruritus intensity and pruritus- and immune-related biomarkers in subjects with moderate to severe AD-related pruritus.

METHODS

A phase 2 clinical trial investigated the efficacy and safety of oral DFK 0.25, 0.5, and 1.0 mg in subjects with moderate to severe AD-related pruritus. A biomarker substudy evaluated the effects of DFK on the expression of pruritus, TH2-associated genes, and skin barrier-related genes.

RESULTS

In the clinical trial (N = 401), all DFK doses reduced itch versus placebo; however, the results were not statistically significant at week 12. In a subgroup of subjects in the trial with mild to moderate skin inflammation and moderate to severe itch (itch-dominant AD phenotype), DFK reduced itch at week 12 versus placebo. In the biomarker substudy, DFK downregulated the expression of key pruritus-related genes (eg, IL-31 and TRPV1) and the AD phenotype (eg, CCL17). Gene set variation analysis confirmed that DFK, but not placebo, downregulated pruritus-related genes and TH2 pathways. DFK improved skin barrier integrity markers and upregulated the expression of claudins and lipid metabolism-associated genes (eg, SEC14L6, ELOVL3, CYP1A2, and AKR1D1).

CONCLUSIONS

DFK treatment reduced itch in subjects with moderate to severe AD-related pruritus, particularly those with an "itch-dominant" AD phenotype, and had an impact on the expression of pruritus, TH2-associated genes, and skin barrier-related genes. DFK is a promising therapy for AD-related pruritus; further clinical studies are warranted.

The translational revolution in atopic dermatitis: the paradigm shift from pathogenesis to treatment

Atopic dermatitis (AD) is the most common inflammatory skin disease, and it is considered a complex and heterogeneous condition. Different phenotypes of AD, defined according to the patient age at onset, race, and ethnic background; disease duration; and other disease characteristics, have been recently described, underlying the need for a personalized treatment approach. Recent advancements in understanding AD pathogenesis resulted in a real translational revolution and led to the exponential expansion of the therapeutic pipeline. The study of biomarkers in clinical studies of emerging treatments is helping clarify the role of each cytokine and immune pathway in AD and will allow addressing the unique immune fingerprints of each AD subset. Personalized medicine will be the ultimate goal of this targeted translational research. In this review, we discuss the changes in the concepts of both the pathogenesis of and treatment approach to AD, highlight the scientific rationale behind each targeted treatment and report the most recent clinical efficacy data.

Effect of Analgesic Opioid Drugs on Opioid Receptor Genes Expression in HL-1 Mouse Cardiac Myocytes

BACKGROUND: The opioid system was mainly involved three types of opioid receptors (ORs): μ (MOR), δ (DOR) and κ (KOR). These ORs are activated by its agonist, a family of endogenous peptides: Endorphins, enkephalins, and dynorphins, respectively. AIM: This study determined the OR mRNA on effects of agonists exogenous morphine, fentanyl, D-penicillamine (2,5) enkephalin, and ketazocine in HL-1 mouse cardiac myocytes. MATERIALS AND METHODS: HL-1 mouse cardiac myocytes were treated with 10 μM morphine sulfate, 1 μM fentanyl,1 μM D-penicillamine (2,5) enkephalin, and 1 μM ketazocine. Total mRNAs were extracted and cDNA was synthesized and quantitative real-time polymerase chain reaction was used to analyze gene expression. RESULTS: The data analysis of MOR, DOR and KOR mRNA expression on effect of morphine was shown less level than control (0.61-fold, 0.67-fold, and 0.65-fold), respectively. The morphine-induced ORs down-regulation, whereas enkephalin treatment demonstrated highly significantly increased in mRNA of DOR (6.3-fold, p = 0.002). As well as, KOR mRNA expression was found highly significant increased under effect of Ketazocine (7.16-fold, p = 0.004). CONCLUSION: This study found DOR and KOR, but not MOR expressed in HL-1 mouse cardiac myocytes under activation of exogenous opioid analogists. These findings suggested that exogenous analogist’s opioids mimeses the endogenous analogist’s opioids.

Nalfurafine Hydrochloride, a κ-Opioid Receptor Agonist, Induces Melanophagy via PKA Inhibition in B16F1 Cells

Selective autophagy controls cellular homeostasis by degrading unnecessary or damaged cellular components. Melanosomes are specialized organelles that regulate the biogenesis, storage, and transport of melanin in melanocytes. However, the mechanisms underlying melanosomal autophagy, known as the melanophagy pathway, are poorly understood. To better understand the mechanism of melanophagy, we screened an endocrine-hormone chemical library and identified nalfurafine hydrochlorides, a κ-opioid receptor agonist, as a potent inducer of melanophagy. Treatment with nalfurafine hydrochloride increased autophagy and reduced melanin content in alpha-melanocyte-stimulating hormone (α-MSH)-treated cells. Furthermore, inhibition of autophagy blocked melanosomal degradation and reversed the nalfurafine hydrochloride-induced decrease in melanin content in α-MSH-treated cells. Consistently, treatment with other κ-opioid receptor agonists, such as MCOPPB or mianserin, inhibited excessive melanin production but induced autophagy in B16F1 cells. Furthermore, nalfurafine hydrochloride inhibited protein kinase A (PKA) activation, which was notably restored by forskolin, a PKA activator. Additionally, forskolin treatment further suppressed melanosomal degradation as well as the anti-pigmentation activity of nalfurafine hydrochloride in α-MSH-treated cells. Collectively, our data suggest that stimulation of κ-opioid receptors induces melanophagy by inhibiting PKA activation in α-MSH-treated B16F1 cells.

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.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

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.

Optimization of Influential Variables in the Development of Buprenorphine and Bupivacaine Loaded Invasome for Dermal Delivery

Purpose: Hydrophilic drugs are extensively applied in clinical applications. Inadequate dermal penetration of these drugs is a great challenge. Incorporation of drugs into nano-carrier systems overcomes lower penetration drawbacks. Invasomes are novel nano-carrier systems which enhance transdermal penetration by using terpene and ethanol in their structures. buprenorphine and bupivacaine hydrochlorides are two potent analgesic drugs that are loaded simultaneously in the nano-invasome structure as opioid and non-opioid drugs.

Methods: The full factorial experimental design was used for planning and estimating optimum formulations of invasome systems. Three influential factors like terpene type, terpene concentration and preparation method were comprehensively analyzed for achieving high encapsulation efficiency (EE) and optimum size.

Results: The mean sizes of designed invasomes were in the range of 0.39-5.86 µm and high values of EE and loading capacity (LC) were reported as 98.77 and 19.75 for buprenorphine-loaded invasome, respectively. Zeta potential measurements confirmed that the obtained high value of EE might be as a result of reversible ionic interactions between positively charged drugs and negatively charged phospholipidic part of invasome structure. Another characterization of the prepared formulations was carried out by Fourier transform infrared (FTIR), X-ray diffraction (XRD) and dynamic light scattering (DLS) technique.

Conclusion: The satisfactory obtained results of formulations encourage researchers to get optimum topical analgesic formulations with potent and rapid onset time properties required in invasive cutaneous procedures.

Advances in Understanding the Initial Steps of Pruritoceptive Itch: How the Itch Hits the Switch

Pruritoceptive (dermal) itch was long considered an accompanying symptom of diseases, a side effect of drug applications, or a temporary sensation induced by invading pruritogens, as produced by the stinging nettle. Due to extensive research in recent years, it was possible to provide detailed insights into the mechanism of itch mediation and modulation. Hence, it became apparent that pruritus is a complex symptom or disease in itself, which requires particular attention to improve patients’ health. Here, we summarize recent findings in pruritoceptive itch, including how this sensation is triggered and modulated by diverse endogenous and exogenous pruritogens and their receptors. A differentiation between mediating pruritogen and modulating pruritogen seems to be of great advantage to understand and decipher the molecular mechanism of itch perception. Only a comprehensive view on itch sensation will provide a solid basis for targeting this long-neglected adverse sensation accompanying numerous diseases and many drug side effects. Finally, we identify critical aspects of itch perception that require future investigation.

Drosophila SMN2 minigene reporter model identifies moxifloxacin as a candidate therapy for SMA

Spinal muscular atrophy is a rare and fatal neuromuscular disorder caused by the loss of alpha motor neurons. The affected individuals have mutated the ubiquitously expressed SMN1 gene resulting in the loss or reduction in the survival motor neuron (SMN) protein levels. However, an almost identical paralog exists in humans: SMN2. Pharmacological activation of SMN2 exon 7 inclusion by small molecules or modified antisense oligonucleotides is a valid approach to treat SMA. Here we describe an in vivo SMN2 minigene reporter system in Drosophila motor neurons that serves as a cost-effective, feasible, and stringent primary screening model for identifying chemicals capable of crossing the conserved Drosophila blood-brain barrier and modulating exon 7 inclusion. The model was used for the screening of 1100 drugs from the Prestwick Chemical Library, resulting in 2.45% hit rate. The most promising candidate drugs were validated in patient-derived fibroblasts where they proved to increase SMN protein levels. Among them, moxifloxacin modulated SMN2 splicing by promoting exon 7 inclusion. The recovery of SMN protein levels was confirmed by increased colocalization of nuclear gems with Cajal Bodies. Thus, a Drosophila-based drug screen allowed the discovery of an FDA-approved small molecule with the potential to become a novel therapy for SMA.

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.

A thermosensitive morphine-containing hydrogel for the treatment of large-scale skin wounds

PURPOSE

Topically applied opioids are an option to induce efficient analgesia in patients with severe skin wounds. For ongoing pain reduction, the vehicle should provide sustained drug release in order to increase the intervals during the regular wound dressing changes. In addition, the formulation should not impair wound healing. Hydrogels provide a moist wound environment, which is known to facilitate the healing process.

METHODS AND RESULTS

Investigating poloxamer hydrogels as a carrier system for morphine in terms of release behavior and (per-)cutaneous absorption, poloxamer 407 25wt.% hydrogel sustained morphine release up to 24h. The drug release rate decreased with increasing concentration of the gel forming triblock copolymer. Poloxamer 407 25wt.% hydrogel retarded morphine uptake into reconstructed human skin and percutaneous drug absorption compared to a hydroxyethyl cellulose reference gel.

CONCLUSIONS

The results of our in vitro study indicate that the thermosensitive poloxamer 407 25wt.% hydrogel is an appropriate carrier system for the topical application of morphine with regard to sustained drug release and ongoing analgesia.

Morphine stimulates cell migration of oral epithelial cells by delta-opioid receptor activation

Oral mucositis is one of the most common side effects of chemoradiation regimens and manifestation can be dose-limiting for the therapy, can impair the patient's nutritional condition and quality of life due to severe pain. The therapeutic options are limited; often only an alleviation of the symptoms such as pain reduction by using systemic opioids is possible. Stimulating opioid receptors on peripheral neurons and dermal tissue, potent analgesic effects are induced e.g. in skin grafted patients. Advantageous effects on the cell migration and, thus, on the wound healing process are described, too. In this study, we investigated whether opioid receptors are also expressed on oral epithelial cells and if morphine can modulate their cell migration behavior. The expression of the opioid receptors MOR, DOR and KOR on primary human oral epithelial cells was verified. Furthermore, a significantly accelerated cell migration was observed following incubation with morphine. The effect even slightly exceeded the cell migration stimulating effect of TGF-ß: After 14 h of morphine treatment about 86% of the wound area was closed, whereas TGF-ß application resulted in a closed wound area of 80%. With respect to morphine stimulated cell migration we demonstrate that DOR plays a key role and we show the involvement of the MAPK members Erk 1/2 and p38 using Western blot analysis.Further studies in more complex systems in vitro and in vivo are required. Nevertheless, these findings might open up a new therapeutic option for the treatment of oral mucositis.

Opioid receptor expression in human brain and peripheral tissues using absolute quantitative real-time RT-PCR

BACKGROUND

The actions of endogenous opioid peptides are mediated by 3 main classes of opioid receptors; mu (MOR), kappa (KOR), and delta (DOR).

METHODS

We developed an absolute quantitative real-time reverse transcriptase PCR (AQ-rt-RT-PCR) assay to quantify MOR, DOR, and KOR mRNA in 22 human tissues.

RESULTS

MOR mRNA was greatly enriched (12-20×10(6)copies/μg) in the cerebellum, nucleus accumbens, and caudate nucleus; moderate (6×10(6)copies/μg) in the dorsal root ganglion, spinal cord, and adrenal gland; low (2×10(4)copies/μg) in the pancreas and small intestine; and absent in the lung, spleen, kidney, heart, skeletal muscle, liver, and thymus. High levels (>8.8×10(6)copies/μg) of DOR mRNA were expressed in the brain and dorsal root ganglion; moderate (1.5×10(6)copies/μg) in the adrenal gland and pancreas; low (2×10(4)-6.5×10(5)copies/μg in the cerebellum, spinal cord, small intestine, skeletal muscle, thymus, lung, and kidney); and very low (3.8×10(3)copies/μg) in the heart. DOR mRNA was not detected in the spleen or liver. KOR mRNA was moderate (1×10(6)copies/μg) in brain regions and dorsal root ganglion, but low (1.6-7×10(5)copies/μg) in the cerebellum, temporal lobe and all other peripheral tissues.

CONCLUSIONS

Our data demonstrate that the AQ-rt-RT-PCR is a highly reproducible and precise method to study the expression of opioid receptors in various tissues and under different disease conditions.

Preservation of retina ganglion cell function by morphine in a chronic ocular-hypertensive rat model

PURPOSE

The current study examined if opioid-receptor-activation by morphine can improve retinal function and retinal ganglion cell (RGC) integrity in a chronic glaucoma rat model.

METHODS

IOP was raised in Brown Norway rats by injecting hypertonic saline into the limbal venous system. Rats were treated daily with 1 mg/kg morphine for 28 days at 24-hour intervals; animals were examined for changes in IOP by a TonoLab tonometer. Pattern-ERG (PERG) was obtained in response to contrast-reversal of patterned visual stimuli. RGCs were visualized by fluorogold retrograde-labeling. Changes in the expression pattern of TNF-α and caspases were measured by Western blotting.

RESULTS

A significant IOP elevation was seen as early as 7 days, and maintained for up to 8 weeks, after surgery. PERG amplitudes were significantly reduced in ocular-hypertensive eyes (15.84±0.74 μvolts) when compared with normal eyes (19±0.86 μvolts). PERG deficits in hypertensive eyes were reversed by morphine treatment (18.23±0.78 μvolts; P<0.05). In untreated rats, a 24% reduction in labeled RGCs was measured in the hypertensive eye compared with the normal eye. This reduction in RGC labeling was significantly ameliorated in the presence of morphine. In retinal samples, TNF-α, caspase-8, and caspase-3 expressions were significantly upregulated in ocular hypertensive eyes, but completely inhibited in the morphine-treated animals.

CONCLUSIONS

These data provide evidence that activation of opioid receptors can provide significant improvement in PERG and RGC integrity against glaucomatous injury. Mechanistic data provide clues that activation of one or more opioid receptors can reduce glaucomatous-injury via suppression of TNF-α and caspase activation.

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.

Topical treatment with the opioid antagonist naltrexone facilitates closure of full-thickness wounds in diabetic rats

A major problem associated with diabetes is the complication of chronic non-healing wounds that can lead to the formation of debilitating ulcers, and can progress to more serious problems including amputation. There is no fully effective prevention of these complications, constituting an unmet medical need to understand the pathophysiology and treatment of wound healing in diabetes. This study determined whether blockade of opioid receptors from opioid peptides, known to inhibit cell proliferation and be overexpressed in diabetes, by topical application of the opioid antagonist naltrexone (NTX) reverses delays in wound closure. Rats with streptozotocin-induced type 1 diabetes (T1D) received topical applications of NTX (10(-4)-10(-6) mol/L) or vehicle in a variety of carriers; DNA synthesis was evaluated 12 h later. DNA synthesis in the epithelium of T1D rats was significantly reduced from normal animals. Both systemic and topical application of NTX increased DNA synthesis (up to 2-fold higher) within 12 h of administration. In a second study, diabetic and normal rats received full-thickness cutaneous wounds and were treated three times daily with either 10(-5) mol/L NTX or vehicle in topical carriers. Wound sizes were analyzed, and BrdU (5-bromo-2'-deoxyuridine) labeling in the skin was evaluated to determine DNA synthesis. Application of NTX in a variety of carriers to rats with full-thickness wounds resulted in significantly smaller wound areas relative to T1D animals receiving vehicle, and comparable to that of normal rats. Wound contraction in T1D animals was 50% of that in normal rats, with NTX-treated wounds restoring wound contraction to that of normal cohorts. DNA synthesis was also enhanced in NTX-treated T1D animals compared with T1D vehicle controls. These data suggest that topical application of NTX is a non-toxic and efficacious facilitator for healing full thickness wounds in T1D, with wound contraction serving as a particular target of NTX action.

3D-wound healing model: influence of morphine and solid lipid nanoparticles

For efficient pain reduction in severe skin wounds, topically applied opioids may be a new option. Moreover, by stimulating keratinocyte migration opioids may also accelerate wound healing. Yet, conventional formulations failed to consistently provide sufficient pain control in patients which may be due to local drug degradation or insufficient concentrations at the target site. After having excluded major morphine glucuronidation by keratinocytes and fibroblasts, we next aimed for an optimised formulation. Since long intervals for painful wound dressing changes are intended, the formulations should allow for prolonged opioid release and should not impair the healing process. We developed morphine-loaded solid lipid nanoparticles (SLN, mean size about 180 nm), and tested improvement of wound closure in a new human-based 3D-wound healing model. Standardised wounds were induced by CO(2)-laser irradiation of reconstructed human full-thickness skin equivalents (EpiDermFT). Morphine, morphine-loaded and unloaded SLN accelerated reepithelialization. Keratinocytes almost completely covered the dermis equivalent after 4 days, which was not the case when applying the vehicle. In conclusion, acceleration of wound closure, low cytotoxicity and irritation as well as possible prolonged morphine release make SLN an interesting approach for innovative wound management.

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.

Deletion of delta-opioid receptor in mice alters skin differentiation and delays wound healing

In addition to their well-known antinociceptive action, opioids can modulate non-neuronal functions, such as immune activity and physiology of different cell types. Several findings suggest that the delta-opioid receptor (DOR) and its endogenous ligands (enkephalins) are important players in cell differentiation and proliferation. Here we show the expression of DOR in mouse skin and human skin cultured fibroblasts and keratinocytes using RT-PCR. In DOR knock-out (KO) mice, a phenotype of thinner epidermis and higher expression of cell differentiation marker cytokeratin 10 (CK 10) were observed compared with wild type (WT). Using a burn wound model, significant wound healing delay (about 2 days) and severe epidermal hypertrophy were shown at the wound margin of DOR KO mice. This wound healing delay was further investigated by immunohistochemistry using markers for proliferation, differentiation, re-epithelialization, and dermal repair (CK 6, CK 10, and collagen IV). The levels of all these markers were increased in wounds of KO mice compared with WT. During the wound healing, the epidermal thickness in KO mice augments faster and exceeds that of the WT by day 3. These results suggest an essential role of DOR in skin differentiation, proliferation, and migration, factors that are important for wound healing.

Endogenous opiates and behavior: 2005

This paper is the 28th 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 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).