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

Enkephalins and Pain Modulation: Mechanisms of Action and Therapeutic Perspectives

Enkephalins, a subclass of endogenous opioid peptides, play a pivotal role in pain modulation. Enkephalins primarily exert their effects through opioid receptors located widely throughout both the central and peripheral nervous systems. This review will explore the mechanisms by which enkephalins produce analgesia, emotional regulation, neuroprotection, and other physiological effects. Furthermore, this review will analyze the involvement of enkephalins in the modulation of different pathologies characterized by severe pain. Understanding the complex role of enkephalins in pain processing provides valuable insight into potential therapeutic strategies for managing pain disorders.

Methadone Requires the Co-Activation of μ-Opioid and Toll-Like-4 Receptors to Produce Extracellular DNA Traps in Bone-Marrow-Derived Mast Cells

Methadone is an effective and long-lasting analgesic drug that is also used in medication-assisted treatment for people with opioid use disorders. Although there is evidence that methadone activates μ-opioid and Toll-like-4 receptors (TLR-4s), its effects on distinct immune cells, including mast cells (MCs), are not well characterized. MCs express μ-opioid and Toll-like receptors (TLRs) and constitute an important cell lineage involved in allergy and effective innate immunity responses. In the present study, murine bone-marrow-derived mast cells (BMMCs) were treated with methadone to evaluate cell viability by flow cytometry, cell morphology with immunofluorescence and scanning electron microscopy, reactive oxygen species (ROS) production, and intracellular calcium concentration ([Ca2+]i) increase. We found that exposure of BMMCs to 0.5 mM or 1 mM methadone rapidly induced cell death by forming extracellular DNA traps (ETosis). Methadone-induced cell death depended on ROS formation and [Ca2+]i. Using pharmacological approaches and TLR4-defective BMMC cultures, we found that µ-opioid receptors were necessary for both methadone-induced ROS production and intracellular calcium increase. Remarkably, TLR4 receptors were also involved in methadone-induced ROS production as it did not occur in BMMCs obtained from TLR4-deficient mice. Finally, confocal microscopy images showed a significant co-localization of μ-opioid and TLR4 receptors that increased after methadone treatment. Our results suggest that methadone produces MCETosis by a mechanism requiring a novel crosstalk pathway between μ-opioid and TLR4 receptors.

Interactions Between Endogenous Opioids and the Immune System

The endogenous opioid system, which consists of opioid receptors and their ligands, is widely expressed in the nervous system and also found in the immune system. As a part of the body's defense machinery, the immune system is heavily regulated by endogenous opioid peptides. Many types of immune cells, including macrophages, dendritic cells, neutrophils, and lymphocytes are influenced by endogenous opioids, which affect cell activation, differentiation, proliferation, apoptosis, phagocytosis, and cytokine production. Additionally, immune cells also synthesize and secrete endogenous opioid peptides and participate peripheral analgesia. This chapter is structured into two sections. Part one focuses on immunoregulatory functions of central endogenous opioids; and part two describes how opioid peptide-containing immune cells participate in local analgesia.

The Underlying Mechanisms of Sleep Deprivation Exacerbating Neuropathic Pain

Pain disrupts sleep, and sleep deprivation or interference can alter pain perception in animals and humans, for example by increasing sensitivity to pain. However, the mechanism by which sleep affects neuropathic pain remains unclear. In this review, we discuss the available evidence from the epidemiologic, clinical, and human, as well as laboratory studies. In previous studies, we have found that sleep deprivation affects various injurious systems, including opioids, dopaminergic, immune, orexins, hypothalamic-pituitary-adrenal axis, and adenosine. At the same time, these systems play a crucial role in neuropathic pain regulation. In the complex interactions between these neurobiological systems, there may be potential regulatory pathways through which sleep deprivation amplifies neuropathic pain. Because of the impact sleep problems and neuropathic pain can have on the patients' quality of life, studying the link between sleep and neuropathic pain is important for neuropathic pain prevention and public health.

Pain Biomarkers in Fibromyalgia Syndrome: Current Understanding and Future Directions

Fibromyalgia is a complex and heterogeneous clinical syndrome, mainly characterized by the presence of widespread pain, possibly associated with a variety of other symptoms. Fibromyalgia can have an extremely negative impact on the psychological, physical and social lives of people affected, sometimes causing patients to experience dramatically impaired quality of life. Nowadays, the diagnosis of fibromyalgia is still clinical, thus favoring diagnostic uncertainties and making its clear identification challenging to establish, especially in primary care centers. These difficulties lead patients to undergo innumerable clinical visits, investigations and specialist consultations, thus increasing their stress, frustration and even dissatisfaction. Unfortunately, research over the last 25 years regarding a specific biomarker for the diagnosis of fibromyalgia has been fruitless. The discovery of a reliable biomarker for fibromyalgia syndrome would be a critical step towards the early identification of this condition, not only reducing patient healthcare utilization and diagnostic test execution but also providing early intervention with guideline-based treatments. This narrative article reviews different metabolite alterations proposed as possible biomarkers for fibromyalgia, focusing on their associations with clinical evidence of pain, and highlights some new, promising areas of research in this context. Nevertheless, none of the analyzed metabolites emerge as sufficiently reliable to be validated as a diagnostic biomarker. Given the complexity of this syndrome, in the future, a panel of biomarkers, including subtype-specific biomarkers, could be considered as an interesting alternative research area.

Requirement for ER-mitochondria Ca2+ transfer, ROS production and mPTP formation in L-asparaginase-induced apoptosis of acute lymphoblastic leukemia cells

Acute lymphoblastic leukemia (aLL) is a malignant cancer in the blood and bone marrow characterized by rapid expansion of lymphoblasts. It is a common pediatric cancer and the principal basis of cancer death in children. Previously, we reported that L-asparaginase, a key component of acute lymphoblastic leukemia chemotherapy, causes IP3R-mediated ER Ca²⁺ release, which contributes to a fatal rise in [Ca²⁺]_cyt, eliciting aLL cell apoptosis via upregulation of the Ca²⁺-regulated caspase pathway (Blood, 133, 2222-2232). However, the cellular events leading to the rise in [Ca²⁺]_cyt following L-asparaginase-induced ER Ca²⁺ release remain obscure. Here, we show that in acute lymphoblastic leukemia cells, L-asparaginase causes mitochondrial permeability transition pore (mPTP) formation that is dependent on IP3R-mediated ER Ca²⁺ release. This is substantiated by the lack of L-asparaginase-induced ER Ca²⁺ release and loss of mitochondrial permeability transition pore formation in cells depleted of HAP1, a key component of the functional IP3R/HAP1/Htt ER Ca²⁺ channel. L-asparaginase induces ER Ca²⁺ transfer into mitochondria, which evokes an increase in reactive oxygen species (ROS) level. L-asparaginase-induced rise in mitochondrial Ca²⁺ and reactive oxygen species production cause mitochondrial permeability transition pore formation that then leads to an increase in [Ca²⁺]_cyt. Such rise in [Ca²⁺]_cyt is inhibited by Ruthenium red (RuR), an inhibitor of the mitochondrial calcium uniporter (MCU) that is required for mitochondrial Ca²⁺ uptake, and cyclosporine A (CsA), an mitochondrial permeability transition pore inhibitor. Blocking ER-mitochondria Ca²⁺ transfer, mitochondrial ROS production, and/or mitochondrial permeability transition pore formation inhibit L-asparaginase-induced apoptosis. Taken together, these findings fill in the gaps in our understanding of the Ca²⁺-mediated mechanisms behind L-asparaginase-induced apoptosis in acute lymphoblastic leukemia cells.

CD206+/MHCII- macrophage accumulation at nerve injury site correlates with attenuation of allodynia in TASTPM mouse model of Alzheimer's disease

Chronic pain is undertreated in people with Alzheimer's disease (AD) and better understanding of the underlying mechanisms of chronic pain in this neurodegenerative disease is essential. Neuropathic pain and AD share a significant involvement of the peripheral immune system. Therefore, we examined the development of nerve injury-induced allodynia in TASTPM (APPsweXPS1.M146V) mice and assessed monocytes/macrophages at injury site. TASTPM developed partial allodynia compared to WT at days 7, 14 and 21 days after injury, and showed complete allodynia only after treatment with naloxone methiodide, a peripheralized opioid receptor antagonist. Since macrophages are one of the sources of endogenous opioids in the periphery, we examined macrophage infiltration at injury site and observed that CD206+/MHCII- cells were more numerous in TASTPM than WT. Accordingly, circulating TASTPM Ly6Chigh (classical) monocytes, which are pro-inflammatory and infiltrate at the site of injury, were less abundant than in WT. In in vitro experiments, TASTPM bone marrow-derived macrophages showed efficient phagocytosis of myelin extracts containing amyloid precursor protein, acquired CD206+/MHCII- phenotype, upregulated mRNA expression of proenkephalin (PENK) and accumulated enkephalins in culture media. These data suggest that in TASTPM nerve-injured mice, infiltrating macrophages which derive from circulating monocytes and may contain amyloid fragments, acquire M2-like phenotype after myelin engulfment, and release enkephalins which are likely to inhibit nociceptive neuron activity via activation of opioid receptors.

Combination of paeoniflorin and liquiritin alleviates neuropathic pain by lipid metabolism and calcium signaling coordination

Neuropathic pain (NP) affects 7%–10% of the general population and is still hard to cure. Here, we validated the therapeutic effect and demonstrated the mechanism of paeoniflorin and liquiritin combination (PL) on NP from the perspective of integrated lipidomics and transcriptomics for the first time. SwissTargetPrediction indicated that PL mainly targets lipid metabolism. Notably, lipidomics revealed that imbalanced lipid levels in the NP model could be reprogrammed to normal levels by PL treatment. RNA-sequencing showed that PL treatment could also rebalance the lipid metabolism in an indirect manner. Pathway analysis highly enriched the calcium signaling pathway among the most significant categories. Altogether, these findings suggested that PL can not only balance the lipid metabolism in direct and indirect manners but also reverse the dysfunctional activation of the calcium signaling pathway, thereby alleviating NP. This helps to better understand the mechanisms of NP and provides a new important potential therapeutic option for NP.

The Clash of Two Epidemics: the Relationship Between Opioids and Glucose Metabolism

PURPOSE OF REVIEW

We are currently in the midst of a global opioid epidemic. Opioids affect many physiological processes, but one side effect that is not often taken into consideration is the opioid-induced alteration in blood glucose levels.

RECENT FINDINGS

This review shows that the vast majority of studies report that opioid stimulation increases blood glucose levels. In addition, plasma levels of the endogenous opioid β-endorphin rise in response to low blood glucose. In contrast, in hyperglycaemic baseline conditions such as in patients with type 2 diabetes mellitus (T2DM), opioid stimulation lowers blood glucose levels. Furthermore, obesity itself alters sensitivity to opioids, changes opioid receptor expression and increases plasma β-endorphin levels. Thus, opioid stimulation can have various side effects on glycaemia that should be taken into consideration upon prescribing opioid-based medication, and more research is needed to unravel the interaction between obesity, glycaemia and opioid use.

Specialized Pro-Resolving Lipid Mediators: The Future of Chronic Pain Therapy?

Chronic pain (CP) is a severe clinical entity with devastating physical and emotional consequences for patients, which can occur in a myriad of diseases. Often, conventional treatment approaches appear to be insufficient for its management. Moreover, considering the adverse effects of traditional analgesic treatments, specialized pro-resolving lipid mediators (SPMs) have emerged as a promising alternative for CP. These include various bioactive molecules such as resolvins, maresins, and protectins, derived from ω-3 polyunsaturated fatty acids (PUFAs); and lipoxins, produced from ω-6 PUFAs. Indeed, SPMs have been demonstrated to play a central role in the regulation and resolution of the inflammation associated with CP. Furthermore, these molecules can modulate neuroinflammation and thus inhibit central and peripheral sensitizations, as well as long-term potentiation, via immunomodulation and regulation of nociceptor activity and neuronal pathways. In this context, preclinical and clinical studies have evidenced that the use of SPMs is beneficial in CP-related disorders, including rheumatic diseases, migraine, neuropathies, and others. This review integrates current preclinical and clinical knowledge on the role of SPMs as a potential therapeutic tool for the management of patients with CP.

Delta Opioid Receptor in Astrocytes Contributes to Neuropathic Cold Pain and Analgesic Tolerance in Female Mice

Background: The delta opioid receptor (DOR) contributes to pain control, and a major challenge is the identification of DOR populations that control pain, analgesia, and tolerance. Astrocytes are known as important cells in the pathophysiology of chronic pain, and many studies report an increased prevalence of pain in women. However, the implication of astrocytic DOR in neuropathic pain and analgesia, as well as the influence of sex in this receptor activity, remains unknown. Experimental Approach: We developed a novel conditional knockout (cKO) mouse line wherein DOR is deleted in astrocytes (named GFAP-DOR-KO), and investigated neuropathic mechanical allodynia as well as analgesia and analgesic tolerance in mutant male and female mice. Neuropathic cold allodynia was also characterized in mice of both sexes lacking DOR either in astrocytes or constitutively. Results: Neuropathic mechanical allodynia was similar in GFAP-DOR-KO and floxed DOR control mice, and the DOR agonist SNC80 produced analgesia in mutant mice of both sexes. Interestingly, analgesic tolerance developed in cKO males and was abolished in cKO females. Cold neuropathic allodynia was reduced in mice with decreased DOR in astrocytes. By contrast, cold allodynia was exacerbated in full DOR KO females. Conclusions: These findings show that astrocytic DOR has a prominent role in promoting cold allodynia and analgesic tolerance in females, while overall DOR activity was protective. Altogether this suggests that endogenous- and exogenous-mediated DOR activity in astrocytes worsens neuropathic allodynia while DOR activity in other cells attenuates this form of pain. In conclusion, our results show a sex-specific implication of astrocytic DOR in neuropathic pain and analgesic tolerance. These findings open new avenues for developing tailored DOR-mediated analgesic strategies.

The Interplay between Chronic Pain, Opioids, and the Immune System

Chronic pain represents one of the most serious worldwide medical problems, in terms of both social and economic costs, often causing severe and intractable physical and psychological suffering. The lack of biological markers for pain, which could assist in forming clearer diagnoses and prognoses, makes chronic pain therapy particularly arduous and sometimes harmful. Opioids are used worldwide to treat chronic pain conditions, but there is still an ambiguous and inadequate understanding about their therapeutic use, mostly because of their dual effect in acutely reducing pain and inducing, at the same time, tolerance, dependence, and a risk for opioid use disorder. In addition, clinical studies suggest that opioid treatment can be associated with a high risk of immune suppression and the development of inflammatory events, worsening the chronic pain status itself. While opioid peptides and receptors are expressed in both central and peripheral nervous cells, immune cells, and tissues, the role of opioids and their receptors, when and why they are activated endogenously and what their exact role is in chronic pain pathways is still poorly understood. Thus, in this review we aim to highlight the interplay between pain and immune system, focusing on opioids and their receptors.

Potential Therapeutic Benefit of Low Dose Naltrexone in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Role of Transient Receptor Potential Melastatin 3 Ion Channels in Pathophysiology and Treatment

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a debilitating multi-systemic chronic condition of unknown aetiology classified as an immune dysfunction syndrome and neurological disorder. The discovery of the widely expressed Transient Receptor Potential Melastatin 3 (TRPM3) as a nociceptor channel substantially targeted by certain opioid receptors, and its implication in calcium (Ca²⁺)-dependent Natural Killer (NK) cell immune functions has raised the possibility that TRPM3 may be pharmacologically targeted to treat characteristic symptoms of ME/CFS. Naltrexone hydrochloride (NTX) acts as an antagonist to the mu (μ)-opioid receptor thus negating its inhibitory function on TRPM3. Based on the benefits reported by patients on their symptoms, low dose NTX (LDN, 3.0-5.0 mg/day) treatment seems to offer some potential benefit suggesting that its effect may be targeted towards the pathomechanism of ME/CFS. As there is no literature confirming the efficacy of LDN for ME/CFS patients in vitro, this study investigates the potential therapeutic effect of LDN in ME/CFS patients. TRPM3 ion channel activity was measured after modulation with Pregnenolone sulfate (PregS) and ononetin in NK cells on 9 ME/CFS patients taking LDN and 9 age- and sex-matched healthy controls using whole-cell patch-clamp technique. We report that ME/CFS patients taking LDN have restored TRPM3-like ionic currents in NK cells. Small ionic currents with a typical TRPM3-like outward rectification were measured after application of PregS, a TRPM3-agonist, in NK cells from patients taking LDN. Additionally, PregS-evoked ionic currents through TRPM3 were significantly modulated by ononetin, a TRPM3-antagonist, in NK cells from ME/CFS patients taking LDN. These data support the hypothesis that LDN may have potential as a treatment for ME/CFS by characterising the underlying regulatory mechanisms of LDN treatment involving TRPM3 and opioid receptors in NK cells. Finally, this study may serve for the repurpose of marketed drugs, as well as support the approval of prospective randomized clinical studies on the role and dose of NTX in treating ME/CFS patients.

Recent Advances in Peripheral Opioid Receptor Therapeutics

PURPOSE OF REVIEW

Although opioids are excellent analgesics, they are associated with severe short- and long-term side effects that are especially concerning for the treatment of chronic pain. Peripherally acting opioid receptor agonists promise to mitigate the more serious centrally mediated side effects of opioids, and the goal of this paper is to identify and elaborate on recent advances in these peripheral opioid receptor therapeutics.

RECENT FINDINGS

Peripheral opioid receptor agonists are effective analgesics that at the same time circumvent the problem of centrally mediated opioid side effects by (1) preferentially targeting peripheral opioid receptors that are often the source of the pain and (2) their markedly diminished permeability or activity across the blood-brain barrier. Recent novel bottom-up approaches have been notable for the design of therapeutics that are either active only at inflamed tissue, as in the case of fentanyl-derived pH-sensitive opioid ligands, or too bulky or hydrophilic to cross the blood-brain barrier, as in the case of morphine covalently bound to hyperbranched polyglycerols. Recent innovations in peripheral opioid receptor therapeutics of pH-sensitive opioid ligands and limiting opioid permeability across the blood-brain barrier have had promising results in animal models. While this is grounds for optimism that some of these therapeutics will be efficacious in human subjects at a future date, each drug must undergo individualized testing for specific chronic pain syndromes to establish not only the nuances of each drug's therapeutic effect but also a comprehensive safety profile.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

Interleukin-4 Induces the Release of Opioid Peptides from M1 Macrophages in Pathological Pain

Interleukin-4 (IL-4) is an anti-inflammatory cytokine, which can be protective in inflammatory and neurologic disorders, and can alleviate pain. Classically, IL-4 diminishes pain by blocking the production of proinflammatory cytokines. Here, we uncovered that IL-4 induces acute antinociception by IL-4 receptor α (IL-4Rα)-dependent release of opioid peptides from M1 macrophages at injured nerves. As a model of pathologic pain, we used a chronic constriction injury (CCI) of the sciatic nerve in male mice. A single application of IL-4 at the injured nerves (14 d following CCI) attenuated mechanical hypersensitivity evaluated by von Frey filaments, which was reversed by co-injected antibody to IL-4Rα, antibodies to opioid peptides such as Met-enkephalin (ENK), β-endorphin and dynorphin A 1-17, and selective antagonists of δ-opioid, µ-opioid, and κ-opioid receptors. Injured nerves were predominately infiltrated by proinflammatory M1 macrophages and IL-4 did not change their numbers or the phenotype, assessed by flow cytometry and qRT-PCR, respectively. Macrophages isolated from damaged nerves by immunomagnetic separation (IMS) and stimulated with IL-4 dose dependently secreted all three opioid peptides measured by immunoassays. The IL-4-induced release of ENK was diminished by IL-4Rα antibody, intracellular Ca²⁺ chelator, and inhibitors of protein kinase A (PKA), phosphoinositide 3-kinase (PI3K), and ryanodine receptors. Together, we identified a new opioid mechanism underlying the IL-4-induced antinociception that involves PKA-mediated, PI3K-mediated, ryanodine receptor-mediated, and intracellular Ca²⁺-mediated release from M1 macrophages of opioid peptides, which activate peripheral opioid receptors in injured tissue.SIGNIFICANCE STATEMENT Interleukin-4 (IL-4) is an anti-inflammatory cytokine, which can ameliorate pain. The IL-4-mediated effects are considered to mostly result from the inhibition of the production of proinflammatory mediators (e.g., IL-1β, tumor necrosis factor, prostaglandin E2). Here, we found that IL-4 injected at the injured nerves attenuates pain by releasing opioid peptides from the infiltrating macrophages in mice. The opioids were secreted by IL-4 in the intracellular Ca²⁺-dependent manner and activated local peripheral opioid receptors. These actions represent a novel mode of IL-4 action, since its releasing properties have not been so far reported. Importantly, our findings suggest that the IL-4-opioid system should be targeted in the peripheral damaged tissue, since this can be devoid of central and systemic side effects.

Loss of POMC-mediated antinociception contributes to painful diabetic neuropathy

Painful neuropathy is a frequent complication in diabetes. Proopiomelanocortin (POMC) is an endogenous opioid precursor peptide, which plays a protective role against pain. Here, we report dysfunctional POMC-mediated antinociception in sensory neurons in diabetes. In streptozotocin-induced diabetic mice the Pomc promoter is repressed due to increased binding of NF-kB p50 subunit, leading to a loss in basal POMC level in peripheral nerves. Decreased POMC levels are also observed in peripheral nervous system tissue from diabetic patients. The antinociceptive pathway mediated by POMC is further impaired due to lysosomal degradation of μ-opioid receptor (MOR). Importantly, the neuropathic phenotype of the diabetic mice is rescued upon viral overexpression of POMC and MOR in the sensory ganglia. This study identifies an antinociceptive mechanism in the sensory ganglia that paves a way for a potential therapy for diabetic neuropathic pain.

D,L-Methadone causes leukemic cell apoptosis via an OPRM1-triggered increase in IP3R-mediated ER Ca2+ release and decrease in Ca2+ efflux, elevating [Ca2+]i

The search continues for improved therapy for acute lymphoblastic leukemia (aLL), the most common malignancy in children. Recently, d,l-methadone was put forth as sensitizer for aLL chemotherapy. However, the specific target of d,l-methadone in leukemic cells and the mechanism by which it induces leukemic cell apoptosis remain to be defined. Here, we demonstrate that d,l-methadone induces leukemic cell apoptosis through activation of the mu1 subtype of opioid receptors (OPRM1). d,l-Methadone evokes IP3R-mediated ER Ca²⁺ release that is inhibited by OPRM1 loss. In addition, the rate of Ca²⁺ extrusion following d,l-methadone treatment is reduced, but is accelerated by loss of OPRM1. These d,l-methadone effects cause a lethal rise in [Ca²⁺]_i that is again inhibited by OPRM1 loss, which then prevents d,l-methadone-induced apoptosis that is associated with activation of calpain-1, truncation of Bid, cytochrome C release, and proteolysis of caspase-3/12. Chelating intracellular Ca²⁺ with BAPTA-AM reverses d,l-methadone-induced apoptosis, establishing a link between the rise in [Ca²⁺]_i and d,l-methadone-induced apoptosis. Altogether, our findings point to OPRM1 as a specific target of d,l-methadone in leukemic cells, and that OPRM1 activation by d,l-methadone disrupts IP3R-mediated ER Ca²⁺ release and rate of Ca²⁺ efflux, causing a rise in [Ca²⁺]_i that upregulates the calpain-1-Bid-cytochrome C-caspase-3/12 apoptotic pathway.

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Peripherally acting opioid analgesics and peripherally-induced analgesia

The management of pain, particularly chronic pain, is still an area of medical need. In this context, opioids remain a gold standard for the treatment of pain. However, significant side effects, mainly of central origin, limit their clinical use. Here, we review recent progress to improve the therapeutic and safety profiles of opioids for pain management. Characterization of peripheral opioid-mediated pain mechanisms have been a key component of this process. Several studies identified peripheral µ, δ, and κ opioid receptors (MOR, DOR, and KOR, respectively) and nociceptin/orphanin FQ (NOP) receptors as significant players of opioid-mediated antinociception, able to achieve clinically significant effects independently of any central action. Following this, particularly from a medicinal chemistry point of view, main efforts have been directed towards the peripheralization of opioid receptor agonists with the objective of optimizing receptor activity and minimizing central exposure and the associated undesired effects. These activities have allowed the characterization of a great variety of compounds and investigational drugs that show low central nervous system (CNS) penetration (and therefore a reduced side effect profile) yet maintaining the desired opioid-related peripheral antinociceptive activity. These include highly hydrophilic/amphiphilic and massive molecules unable to easily cross lipid membranes, substrates of glycoprotein P (a extrusion pump that avoids CNS penetration), nanocarriers that release the analgesic agent at the site of inflammation and pain, and pH-sensitive opioid agonists that selectively activate at those sites (and represent a new pharmacodynamic paradigm). Hopefully, patients with pain will benefit soon from the incorporation of these new entities.

Dual-Acting Peripherally Restricted Delta/Kappa Opioid (CAV1001) Produces Antinociception in Animal Models of Sub-Acute and Chronic Pain

Background

The development of highly efficacious alternatives to mu-opioid analgesics represents an urgent unmet medical and public health need. In the presence of inflammation both delta- and kappa-opioid agonists, acting on peripheral sensory neurons, mediate analgesia. The dual-acting, peripherally restricted kappa/delta-opioid agonist, CAV1001, was tested in four rodent pain models.

Methods

Experiment 1 – Formalin testing in mice. Three doses (1–10 mg/kg) of CAV1001 or ICI204448 at 30 minutes were tested after formalin injection. Spontaneous nocifensive responses were video recorded. Experiment 2 – Complete Freund’s Adjuvant (CFA)-induced arthritis. CFA was injected into the ankle joint of rats. Joint compression thresholds (JCT) were measured. CAV1001 was compared to celecoxib. Experiment 3 – Spinal nerve ligation (SNL) in rats. Paw compression thresholds (PCT) were measured. CAV1001 was compared to gabapentin. Experiment 4 – MMRT-1 bone cancer implantation into the rat tibia. Weight-bearing was assessed. CAV1001 was compared to morphine.

Results

In Phase 2 of the formalin model, CAV1001 (1 mg/kg) significantly reduced pain behaviors to a degree comparable to the peripherally restricted kappa-opioid agonist, ICI204448 (10 mg/kg). CAV1001 (10 mg/kg) effectively eliminated pain behaviors associated with phase 2. In the CFA-induced arthritis model, a significant increase in JCTs, similar to the comparator celecoxib, was observed with CAV1001 at 1 mg/kg at 2 hours; CAV1001 (10 mg/kg) was effective at 1 hour. In the SNL model, both the comparator gabapentin and CAV1001 (5 mg/kg) significantly reduced PCT at 2 hours, but at 4 hours, the CAV1001 thresholds improved to baseline. CAV1001 10 mg/kg significantly improved weight bearing at 4-hour post-dosing compared to baseline following MMRT-1 implantation.

Conclusion

CAV1001 demonstrated efficacy in several different preclinical pain models. Time- and dose-dependent differences in the efficacy of CAV1001 amongst these rodent pain models parallel the degree of underlying inflammation.

Real-Time Quantitative Reverse Transcription PCR for Detection of Opioid Receptors in Immune Cells

Real-time quantitative reverse transcription-PCR (qRT-PCR ) is a highly sensitive molecular biology method based on the amplification of the cDNA of mRNA to detect and quantify the levels of mRNA of interest. In this chapter, we describe real-time qRT-PCR to detect and quantify mRNA of opioid receptors in immune cells. Specifically, we analyze mouse immune cells isolated from the blood and sciatic nerves exposed to a chronic constriction injury, which represents a model of neuropathic pain. We describe in detail the requirements and techniques to induce the chronic constriction injury, to isolate immune cells from the blood and injured nerves, to isolate the total RNA from immune cells, to perform a cDNA reverse transcription from the total RNA, and to perform real-time qRT-PCR for μ-, δ-, and κ-opioid receptor mRNAs.

Immunohistochemical Analysis of Opioid Receptors in Peripheral Tissues

Immunohistochemical staining is widely used to identify opioid receptors in specific cell types throughout the nervous system. Opioid receptors are not restricted to the central nervous system, but are also present in peripheral sensory neurons, where their activation exerts analgesic effects without inducing centrally mediated side effects. Here, we describe immunohistochemical analysis of μ-opioid receptors in the peripheral sensory neuron cell bodies, along the axons and their peripheral endings in the hind paw skin, as well as in the spinal cord, under naïve and sciatic nerve damage conditions in mice. Importantly, we consider the ongoing debate on the specificity of antibodies.

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

Protective role of neuronal and lymphoid cannabinoid CB2 receptors in neuropathic pain

Cannabinoid CB₂ receptor (CB₂) agonists are potential analgesics void of psychotropic effects. Peripheral immune cells, neurons and glia express CB₂; however, the involvement of CB₂ from these cells in neuropathic pain remains unresolved. We explored spontaneous neuropathic pain through on-demand self-administration of the selective CB₂ agonist JWH133 in wild-type and knockout mice lacking CB₂ in neurons, monocytes or constitutively. Operant self-administration reflected drug-taking to alleviate spontaneous pain, nociceptive and affective manifestations. While constitutive deletion of CB₂ disrupted JWH133-taking behavior, this behavior was not modified in monocyte-specific CB₂ knockouts and was increased in mice defective in neuronal CB₂ knockouts suggestive of increased spontaneous pain. Interestingly, CB₂-positive lymphocytes infiltrated the injured nerve and possible CB₂transfer from immune cells to neurons was found. Lymphocyte CB₂depletion also exacerbated JWH133 self-administration and inhibited antinociception. This work identifies a simultaneous activity of neuronal and lymphoid CB₂that protects against spontaneous and evoked neuropathic pain.

DAMGO-induced μ opioid receptor internalization and recycling restore morphine sensitivity in tolerant rat

This study investigated the effect of DAMGO-induced μ opioid receptor (MOR) internalization on morphine tolerance. Male Sprague-Dawley rats (200-250 g) aged 6-8 weeks were administered morphine via intrathecal (i.t.) injection (15 μg/10 μl twice daily for 6 days) to induce antinociceptive tolerance, which was evaluated using the tail-flick and paw-withdrawal tests. Response latency was calculated as the percentage of maximum possible effect (%MPE). A bolus of DAMGO was administered by i.t. injection on day 6, and the tail-flick and paw-withdrawal tests were carried out 24, 48, and 72 h later. Membrane and cytosolic MOR expression was assessed by western blotting. HEK293 cells were transfected with MOR-FLAG plasmid and after 6 days of morphine treatment (10 μM), the cells were treated with 1 μM DAMGO, and MOR localization was examined by immunofluorescence analysis 30 and 60 min later. Repeated morphine treatment induced tolerance after 5 days; however, i.t. DAMGO administration restored morphine sensitivity and enhanced acute morphine-induced antinociception after 24, 48, and 72 h. In HEK293 cells, DAMGO treatment stimulated MOR internalization after 30 min and MOR recycling to the membrane after 1 h. Membrane and cytoplasmic MOR expression in vivo was unchanged 24, 48, and 72 h after i.t. DAMGO injection. Morphine does not cause significant MOR internalization or downregulation, and can readily induce tolerance. DAMGO counters this effect by enhancing receptor endocytosis, thereby reversing morphine-induced antinociceptive tolerance and restoring its analgesic efficacy.

Opioid Receptors in Immune and Glial Cells-Implications for Pain Control

Opioid receptors comprise μ (MOP), δ (DOP), κ (KOP), and nociceptin/orphanin FQ (NOP) receptors. Opioids are agonists of MOP, DOP, and KOP receptors, whereas nociceptin/orphanin FQ (N/OFQ) is an agonist of NOP receptors. Activation of all four opioid receptors in neurons can induce analgesia in animal models, but the most clinically relevant are MOP receptor agonists (e.g., morphine, fentanyl). Opioids can also affect the function of immune cells, and their actions in relation to immunosuppression and infections have been widely discussed. Here, we analyze the expression and the role of opioid receptors in peripheral immune cells and glia in the modulation of pain. All four opioid receptors have been identified at the mRNA and protein levels in immune cells (lymphocytes, granulocytes, monocytes, macrophages) in humans, rhesus monkeys, rats or mice. Activation of leukocyte MOP, DOP, and KOP receptors was recently reported to attenuate pain after nerve injury in mice. This involved intracellular Ca²⁺-regulated release of opioid peptides from immune cells, which subsequently activated MOP, DOP, and KOP receptors on peripheral neurons. There is no evidence of pain modulation by leukocyte NOP receptors. More good quality studies are needed to verify the presence of DOP, KOP, and NOP receptors in native glia. Although still questioned, MOP receptors might be expressed in brain or spinal cord microglia and astrocytes in humans, mice, and rats. Morphine acting at spinal cord microglia is often reported to induce hyperalgesia in rodents. However, most studies used animals without pathological pain and/or unconventional paradigms (e.g., high or ultra-low doses, pain assessment after abrupt discontinuation of chronic morphine treatment). Therefore, the opioid-induced hyperalgesia can be viewed in the context of dependence/withdrawal rather than pain management, in line with clinical reports. There is convincing evidence of analgesic effects mediated by immune cell-derived opioid peptides in animal models and in humans. Together, MOP, DOP, and KOP receptors, and opioid peptides in immune cells can ameliorate pathological pain. The relevance of NOP receptors and N/OFQ in leukocytes, and of all opioid receptors, opioid peptides and N/OFQ in native glia for pain control is yet to be clarified.

IL-4 induces M2 macrophages to produce sustained analgesia via opioids

IL-4 is a pleiotropic antiinflammatory cytokine, which can be neuroprotective after nervous system injury. The beneficial actions of IL-4 are thought to result from the blunting of action of inflammatory mediators, such as proinflammatory cytokines. Here, we demonstrate that IL-4 induces M2 macrophages to continuously produce opioid peptides and ameliorate pain. IL-4 application at injured nerves in mice shifted F4/80+ macrophages from the proinflammatory M1 to the antiinflammatory M2 phenotype, which synthesized opioid peptides (Met-enkephalin, β-endorphin, and dynorphin A 1-17). These effects were accompanied by a long-lasting attenuation of neuropathy-induced mechanical hypersensitivity, beyond the IL-4 treatment. This IL-4-induced analgesia was decreased by opioid peptide antibodies and opioid receptor (δ, μ, κ) antagonists applied at injured nerves, which confirms the involvement of the local opioid system. The participation of M2 macrophages was supported by analgesia in recipient mice injected at injured nerves with F4/80+ macrophages from IL-4-treated donors. Together, IL-4-induced M2 macrophages at injured nerves produced opioid peptides, which activated peripheral opioid receptors to diminish pain. Fostering the opioid-mediated actions of intrinsic M2 macrophages may be a strategy to tackle pathological pain.

Platelet Concentrates in Musculoskeletal Medicine

Platelet concentrates (PCs), mostly represented by platelet-rich plasma (PRP) and platelet-rich fibrin (PRF) are autologous biological blood-derived products that may combine plasma/platelet-derived bioactive components, together with fibrin-forming protein able to create a natural three-dimensional scaffold. These types of products are safely used in clinical applications due to the autologous-derived source and the minimally invasive application procedure. In this narrative review, we focus on three main topics concerning the use of platelet concentrate for treating musculoskeletal conditions: (a) the different procedures to prepare PCs, (b) the composition of PCs that is related to the type of methodological procedure adopted and (c) the clinical application in musculoskeletal medicine, efficacy and main limits of the different studies.

Peripheral Delta Opioid Receptors Mediate Formoterol Anti-allodynic Effect in a Mouse Model of Neuropathic Pain

Neuropathic pain is a challenging condition for which current therapies often remain unsatisfactory. Chronic administration of β2 adrenergic agonists, including formoterol currently used to treat asthma and chronic obstructive pulmonary disease, alleviates mechanical allodynia in the sciatic nerve cuff model of neuropathic pain. The limited clinical data currently available also suggest that formoterol would be a suitable candidate for drug repurposing. The antiallodynic action of β2 adrenergic agonists is known to require activation of the delta-opioid (DOP) receptor but better knowledge of the molecular mechanisms involved is necessary. Using a mouse line in which DOP receptors were selectively ablated in neurons expressing Nav1.8 sodium channels (DOP cKO), we showed that these DOP peripheral receptors were necessary for the antiallodynic action of the β2 adrenergic agonist formoterol in the cuff model. Using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP), we established in a previous study, that mechanical allodynia is associated with a smaller percentage of DOPeGFP positive small peptidergic sensory neurons in dorsal root ganglia (DRG), with a reduced density of DOPeGFP positive free nerve endings in the skin and with increased DOPeGFP expression at the cell surface. Here, we showed that the density of DOPeGFP positive free nerve endings in the skin is partially restored and no increase in DOPeGFP translocation to the plasma membrane is observed in mice in which mechanical pain is alleviated upon chronic oral administration of formoterol. This study, therefore, extends our previous results by confirming that changes in the mechanical threshold are associated with changes in peripheral DOP profile. It also highlights the common impact on DOP receptors between serotonin noradrenaline reuptake inhibitors such as duloxetine and the β2 mimetic formoterol.

Mechanisms for Reducing Neuropathic Pain

Injury typically results in the development of neuropathic pain, but the pain normally decreases and disappears in paralleled with wound healing. The pain results from cells resident at, and recruited to, the injury site releasing pro-inflammatory cytokines and other mediators leading to the development of pro-inflammatory environment and causing nociceptive neurons to develop chronic ectopic electrical activity, which underlies neuropathic pain. The pain decreases as some of the cells that induce pro-inflammation, changing their phenotype leading to the blocking the release of pro-inflammatory mediators while releasing anti-inflammatory mediators, and blocking nociceptive neuron chronic spontaneous electrical activity. Often, despite apparent wound healing, the neuropathic pain becomes chronic. This raises the question of how chronic pain can be eliminated. While many of the cells and mediators contributing to the development and maintenance of neuropathic pain are known, a better understanding is required of how the injury site environment can be controlled to permanently eliminate the pro-inflammatory environment and silence the chronically electrically active nociceptive neurons. This paper examines how methods that can promote the transition of the pro-inflammatory injury site to an anti-inflammatory state, by changing the composition of local cell types, modifying the activity of pro- and anti-inflammatory receptors, inducing the release of anti-inflammatory mediators, and silencing the chronically electrically active nociceptive neurons. It also examines the hypothesis that factors released from platelet-rich plasma applied to chronic pain sites can permanently eliminate chronic inflammation and its associated chronic pain.

An Experimental Study on Spinal Cord µ-Opioid and α2-Adrenergic Receptors mRNA Expression Following Stress-Induced Hyperalgesia in Male Rats

Background:

Intense stress can change pain perception and induce hyperalgesia; a phenomenon called stress-induced hyperalgesia (SIH). However, the neurobiological mechanism of this effect remains unclear. The present study aimed to investigate the effect of the spinal cord µ-opioid receptors (MOR) and α2-adrenergic receptors (α2-AR) on pain sensation in rats with SIH.

Methods:

Eighteen Sprague-Dawley male rats, weighing 200-250 g, were randomly divided into two groups (n=9 per group), namely the control and stress group. The stress group was evoked by random 1-hour daily foot-shock stress (0.8 mA for 10 seconds, 1 minute apart) for 3 weeks using a communication box. The tail-flick and formalin tests were performed in both groups on day 22. The real-time RT-PCR technique was used to observe MOR and α2-AR mRNA levels at the L4-L5 lumbar spinal cord. Statistical analysis was performed using the GraphPad Prism 5 software (San Diego, CA, USA). Student’s t test was applied for comparisons between the groups. P<0.05 was considered statistically significant.

Results:

There was a significant (P=0.0014) decrease in tail-flick latency in the stress group compared to the control group. Nociceptive behavioral responses to formalin-induced pain in the stress group were significantly increased in the acute (P=0.007) and chronic (P=0.001) phases of the formalin test compared to the control group. A significant reduction was also observed in MOR mRNA level of the stress group compared to the control group (P=0.003). There was no significant difference in α2-AR mRNA level between the stress and control group.

Conclusion:

The results indicate that chronic stress can affect nociception and lead to hyperalgesia. The data suggest that decreased expression of spinal cord MOR causes hyperalgesia.

T Cells as an Emerging Target for Chronic Pain Therapy

The immune system is critically involved in the development and maintenance of chronic pain. However, T cells, one of the main regulators of the immune response, have only recently become a focus of investigations on chronic pain pathophysiology. Emerging clinical data suggest that patients with chronic pain have a different phenotypic profile of circulating T cells compared to controls. At the preclinical level, findings on the function of T cells are mixed and differ between nerve injury, chemotherapy, and inflammatory models of persistent pain. Depending on the type of injury, the subset of T cells and the sex of the animal, T cells may contribute to the onset and/or the resolution of pain, underlining T cells as a major player in the transition from acute to chronic pain. Specific T cell subsets release mediators such as cytokines and endogenous opioid peptides that can promote, suppress, or even resolve pain. Inhibiting the pain-promoting functions of T cells and/or enhancing the beneficial effects of pro-resolution T cells may offer new disease-modifying strategies for the treatment of chronic pain, a critical need in view of the current opioid crisis.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Dual enkephalinase inhibitor PL265: a novel topical treatment to alleviate corneal pain and inflammation

Ocular pain is a core symptom of inflammatory or traumatic disorders affecting the anterior segment. To date, the management of chronic ocular pain remains a therapeutic challenge in ophthalmology. The main endogenous opioids (enkephalins) play a key role in pain control but exhibit only transient analgesic effects due to their rapid degradation. The aim of this study was to explore the antinociceptive and anti-inflammatory effects of topical administration of PL265 (a dual enkephalinase inhibitor) on murine models of corneal pain. On healthy corneas, chronic PL265 topical administration did not alter corneal integrity nor modify corneal mechanical and chemical sensitivity. Then, on murine models of corneal pain, we showed that repeated instillations of PL265 (10 mM) significantly reduced corneal mechanical and chemical hypersensitivity. PL265-induced corneal analgesia was completely antagonized by naloxone methiodide, demonstrating that PL265 antinociceptive effects were mediated by peripheral corneal opioid receptors. Moreover, flow cytometry (quantification of CD11b+ cells) and in vivo confocal microscopy analysis revealed that instillations of PL265 significantly decreased corneal inflammation in a corneal inflammatory pain model. Chronic PL265 topical administration also decreased Iba1 and neuronal injury marker (ATF3) staining in the nucleus of primary sensory neurons of ipsilateral trigeminal ganglion. These results open a new avenue for ocular pain treatment based on the enhancement of endogenous opioid peptides' analgesic effects in tissues of the anterior segment of the eye. Dual enkephalinase inhibitor PL265 seems to be a promising topical treatment for safe and effective alleviation of ocular pain and inflammation.

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

Analgesic effects of a novel pH-dependent μ-opioid receptor agonist in models of neuropathic and abdominal pain

Supplemental Digital Content is Available in the Text.

The newly designed, pH-dependent opioid agonist NFEPP induced analgesia exclusively through peripheral opioid receptors in models of neuropathic and abdominal pain.

Recently, (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide (NFEPP), a newly designed μ-opioid receptor (MOR) agonist with a low pKa, has been shown to produce injury-restricted analgesia in models of inflammatory and postoperative pain, without exhibiting typical opioid side effects. Here, we investigated MOR binding of NFEPP in brain and dorsal root ganglia, pH in injured tissues, and the analgesic efficacy of NFEPP compared with fentanyl in a chronic constriction injury model of neuropathic pain, and in the acetic acid–induced abdominal writhing assay in rats. Binding experiments revealed significantly lower affinity of NFEPP compared with fentanyl at pH 7.4. In vivo, pH significantly dropped both at injured nerves after chronic constriction injury and in the abdominal cavity after acetic acid administration. Intravenous NFEPP as well as fentanyl dose-dependently diminished neuropathy-induced mechanical and heat hypersensitivity, and acetic acid–induced abdominal constrictions. In both models, NFEPP-induced analgesia was fully reversed by naloxone methiodide, a peripherally restricted opioid receptor antagonist, injected at the nerve injury site or into the abdominal cavity. Our results indicate that NFEPP exerts peripheral opioid receptor–mediated analgesia exclusively in damaged tissue in models of neuropathic and abdominal pain.

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

Novel modulation on myeloid-derived suppressor cells (MDSCs) by methionine encephalin (MENK)

The purpose of this study was to identify the modulatory effect of MENK on MDSCs and to investigate the relationship between this modulation and the expression of opioid receptors. Our results showed that MENK could inhibit the proliferation of MDSCs from both marine bone marrow and spleen. MENK also could promote the expression of opioid receptors MOR and DOR. MENK suppressed the PMN-MDSCs generated from splenocyte while up-regulated M-MDSCs. The stimulation of MENK increased the production of IL-4 secreted by MDSCs from slpenocytes. Our currently data indicated that MENK could suppress the accumulation of MDSCs in tumor-bearing mice via binding to and up-regulating expressions of subunits of opioid receptors. Therefore, it is concluded that MENK, through triggering opioid receptors could exert inhibiting modulation on MDSCs.

Advances in Achieving Opioid Analgesia Without Side Effects

Opioids are the most effective drugs for the treatment of severe pain, but they also cause addiction and overdose deaths, which have led to a worldwide opioid crisis. Therefore, the development of safer opioids is urgently needed. In this article, we provide a critical overview of emerging opioid-based strategies aimed at effective pain relief and improved side effect profiles. These approaches comprise biased agonism, the targeting of (i) opioid receptors in peripheral inflamed tissue (by reducing agonist access to the brain, the use of nanocarriers, or low pH-sensitive agonists); (ii) heteromers or multiple receptors (by monovalent, bivalent, and multifunctional ligands); (iii) receptor splice variants; and (iv) endogenous opioid peptides (by preventing their degradation or enhancing their production by gene transfer). Substantial advancements are underscored by pharmaceutical development of new opioids such as peripheral κ-receptor agonists, and by treatments augmenting the action of endogenous opioids, which have entered clinical trials. Additionally, there are several promising novel opioids comprehensively examined in preclinical studies, but also strategies such as biased agonism, which might require careful rethinking.

Evaluation of the thermal antinociceptive effects and pharmacokinetics of hydromorphone hydrochloride after intramuscular administration to cockatiels (Nymphicus hollandicus)

OBJECTIVE To evaluate the thermal antinociceptive effects and pharmacokinetics of hydromorphone hydrochloride after IM administration to cockatiels (Nymphicus hollandicus). ANIMALS 16 healthy adult cockatiels. PROCEDURES During the first of 2 study phases, each cockatiel received each of 4 treatments (hydromorphone at doses of 0.1, 0.3, and 0.6 mg/kg and saline [0.9% NaCl] solution [0.33 mL/kg; control], IM), with a 14-day interval between treatments. For each bird, foot withdrawal to a thermal stimulus was determined following assignment of an agitation-sedation score at predetermined times before and for 6 hours after each treatment. During the second phase, a subset of 12 birds received hydromorphone (0.6 mg/kg, IM), and blood samples were collected at predetermined times for 9 hours after drug administration. Plasma hydromorphone concentration was determined by liquid chromatography-mass spectrometry. Noncompartmental analysis of sparse data was used to calculate pharmacokinetic parameters. RESULTS Thermal withdrawal response did not differ among the 4 treatment groups at any time. Agitation-sedation scores following administration of the 0.3-and 0.6-mg/kg doses of hydromorphone differed significantly from those treated with saline solution and suggested the drug had a sedative effect. Plasma hydromorphone concentrations were > 1 ng/mL for 3 to 6 hours after drug administration in all birds. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that IM administration of hydromorphone at the evaluated doses did not increase the thermal withdrawal threshold of cockatiels despite plasma drug concentrations considered therapeutic for other species. Further research is necessary to evaluate the analgesic effects of hydromorphone in cockatiels.

Rules to activate CD8+T cells through regulating subunits of opioid receptors by methionine enkephalin (MENK)

The goal of this work was to investigate how MENK could regulate the functions of CD8⁺T cells and to explore the relationship between this regulation and opioid receptor expression. Our results showed that the opioid receptors presented on the cell menbrane of CD8⁺T cells were MOR and DOR. MENK promoted the expression of opioid receptors as well as the elevation of the surface molecules such as CD28, PD-1, CTLA-4 and FasL and intracellular granzyme B. Selectively blocking the MOR by CTAP or DOR by NTI could result in inhibition of the corresponding CD8⁺T cells proliferation and the expressions of surface molecules. In addition, non-selectively blocking both MOR and DOR by NTX could further impair the functions and proliferation of CD8⁺T cells. Our currently data indicated that MENK could play a vital role in immune functions via precise regulation to subunits of opioid receptors.

New concepts in opioid analgesia

INTRODUCTION

Opioids are the oldest and most potent drugs for the treatment of severe pain, but they are burdened by detrimental side effects such as respiratory depression, addiction, sedation, nausea, and constipation. Their clinical application is undisputed in acute (e.g. perioperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny and has contributed to the current 'opioid crisis.'

AREAS COVERED

This article reviews pharmacological principles and research strategies aiming at novel opioids with reduced side effects. Basic mechanisms underlying pain, opioid analgesia, and other opioid actions are outlined. To illustrate the clinical situation and medical needs, plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic, and side effects are discussed.

EXPERT OPINION

The epidemic of opioid misuse has taught us that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be considered in the context of drug development, and that novel analgesics with less abuse liability are badly needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and selectively targeting pathological conformations of peripheral opioid receptors.

Peripheral delta opioid receptors mediate duloxetine antiallodynic effect in a mouse model of neuropathic pain

Peripheral delta opioid (DOP) receptors are essential for the antiallodynic effect of the tricyclic antidepressant nortriptyline. However, the population of DOP-expressing cells affected in neuropathic conditions or underlying the antiallodynic activity of antidepressants remains unknown. Using a mouse line in which DOP receptors were selectively ablated in cells expressing Nav1.8 sodium channels (DOP cKO), we established that these DOP peripheral receptors were mandatory for duloxetine to alleviate mechanical allodynia in a neuropathic pain model based on sciatic nerve cuffing. We then examined the impact of nerve cuffing and duloxetine treatment on DOP-positive populations using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP). Eight weeks postsurgery, we observed a reduced proportion of DOPeGFP-positive small peptidergic sensory neurons (calcitonin gene-related peptide (CGRP) positive) in dorsal root ganglia and a lower density of DOPeGFP-positive free nerve endings in the skin. These changes were not present in nerve-injured mice chronically treated with oral duloxetine. In addition, increased DOPeGFP translocation to the plasma membrane was observed in neuropathic conditions but not in duloxetine-treated neuropathic mice, which may represent an additional level of control of the neuronal activity by DOP receptors. Our results therefore established a parallel between changes in the expression profile of peripheral DOP receptors and mechanical allodynia induced by sciatic nerve cuffing.

Opioids and the immune system - friend or foe

Systemically administered opioids are among the most powerful analgesics for treating severe pain. Several negative side effects (respiratory depression, addiction, nausea and confusion) and the risk of opioid-induced hyperalgesia accompany opioid administration. One other side effect is the potential of opioids to suppress the immune response and thereby to increase the vulnerability to infections. The link between opioids and immunosuppression has been investigated both in vitro and in vivo as well as in patients. However, the results are inconsistent: Exogenous opioids such as morphine and fentanyl have been found to impair the function of macrophages, natural killer cells and T-cells and to weaken the gut barrier in vitro and in animal studies. In epidemiological studies, high doses and the initiation of opioid therapy for non-malignant pain have been correlated with a higher risk of infectious diseases such as pneumonia. However clear randomized controlled studies are missing. Furthermore, immune cells including neutrophils, macrophages and T-cells have been shown to secrete endogenous opioid peptides, which then bind to peripheral opioid receptors to relieve inflammatory and neuropathic pain. In addition to cytokines, hormones and bacterial products, the release of opioid peptides is stimulated by the application of exogenous opioids. In summary, there is a reciprocal interaction between the immune system and endogenous as well as exogenous opioids. Further to the existing epidemiological studies, controlled clinical studies are needed in the future to elucidate the role of the opioid-immune system interaction in patients and to determine its clinical relevance.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Up-regulation of μ-, δ- and κ-opioid receptors in concanavalin A-stimulated rat spleen lymphocytes

Regulation of μ-, δ- and κ-opioid receptor protein level in spleen lymphocytes when stimulated by mitogen is not known. To answer the question whether these cells do express opioid receptor (OR) proteins, primary, fresh rat spleen lymphocytes were prepared and stimulated for 48 h with mitogenic dose of Con A. The unstimulated lymphocytes did not express μ- and δ-OR proteins in detectable amounts, however, stimulation with Con A resulted in appearance of clearly detectable immunoblot signals of both μ-OR and δ-OR. κ-OR were detected already in primary cells and increased 2.4-fold in Con A-stimulated cells. These results were supported by data obtained by flow cytometry analysis indicating a dramatic increase in number of μ-, δ- and κ-OR expressing cells after mitogen stimulation. The newly synthesized μ-, δ- and κ-OR in Con A-stimulated spleen lymphocytes were present in the cells interior and not functionally mature, at least in terms of their ability to enhance activity of trimeric G proteins determined by three different protocols of agonist-stimulated, high-affinity [³⁵S]GTPγS binding assay. The up-regulation of μ-, δ- and κ-OR was associated with specific decrease of their cognate trimeric G proteins, G_i1α/G_i2α; the other Gα and Gβ subunits were unchanged. The level of β-arrestin-1/2 was also decreased in Con A-stimulated splenocytes. We conclude that up-regulation of OR expression level in spleen lymphocytes by Con A proceeds in conjunction with down-regulation of their intracellular signaling partners, G_i1α/G_i2α proteins and β-arrestin-1/2. These regulatory proteins are expressed in high amounts already in unstimulated cells and decreased by mitogen stimulation.

Targeting immune-driven opioid analgesia by sigma-1 receptors: Opening the door to novel perspectives for the analgesic use of sigma-1 antagonists

Immune cells have a known role in pronociception, since they release a myriad of inflammatory algogens which interact with neurons to facilitate pain signaling. However, these cells also produce endogenous opioid peptides with analgesic potential. The sigma-1 receptor is a ligand-operated chaperone that modulates neurotransmission by interacting with multiple protein partners, including the μ-opioid receptor. We recently found that sigma-1 antagonists are able to induce opioid analgesia by enhancing the action of endogenous opioid peptides of immune origin during inflammation. This opioid analgesia is seen only at the inflamed site, where immune cells naturally accumulate. In this article we review the difficulties of targeting the opioid system for selective pain relief, and discuss the dual role of immune cells in pain and analgesia. Our discussion creates perspectives for possible novel therapeutic uses of sigma-1 antagonists as agents able to maximize the analgesic potential of the immune system.

[Research consortium Neuroimmunology and pain in the research network musculoskeletal diseases]

BACKGROUND

The research consortium Neuroimmunology and Pain (Neuroimpa) explores the importance of the relationships between the immune system and the nervous system in musculoskeletal diseases for the generation of pain and for the course of fracture healing and arthritis.

MATERIAL AND METHODS

The spectrum of methods includes analyses at the single cell level, in vivo models of arthritis and fracture healing, imaging studies on brain function in animals and humans and analysis of data from patients.

RESULTS

Proinflammatory cytokines significantly contribute to the generation of joint pain through neuronal cytokine receptors. Immune cells release opioid peptides which activate opioid receptors at peripheral nociceptors and thereby evoke hypoalgesia. The formation of new bone after fractures is significantly supported by the nervous system. The sympathetic nervous system promotes the development of immune-mediated arthritis. The studies show a significant analgesic potential of the neutralization of proinflammatory cytokines and of opioids which selectively inhibit peripheral neurons. Furthermore, they show that the modulation of neuronal mechanisms can beneficially influence the course of musculoskeletal diseases.

DISCUSSION

Interventions in the interactions between the immune system and the nervous system hold a great therapeutic potential for the treatment of musculoskeletal diseases and pain.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 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, drug abuse and alcohol, 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.