T lymphocytes containing β-endorphin ameliorate mechanical hypersensitivity following nerve injury

Trends and hotspots in burns-related pain research: A bibliometric analysis

OBJECTIVE

The aim of this investigation was to conduct a thorough synthesis of the extant scholarly discourse and to delineate the prevailing global trends in the domain of burn pain, employing a bibliometric analysis.

METHODS

A bibliometric analysis was performed utilizing the Web of Science Core Collection database. Articles were selected based on titles or abstracts containing keywords associated with burns and pain. Both quantitative and qualitative methodologies were applied to examine the retrieved data, encompassing an analysis of publication trends, research themes, and collaboration networks.

RESULTS

The number of articles on this topic has been increasing, averaging an annual growth rate of 6.9 % from 1997 to 2023. Contributions have come from 645 institutions across 53 countries, resulting in 446 papers that span areas such as nursing, anesthesia, and immunology. Key journals include Burns, Journal of Burn Care & Research, and Pain. The United States has demonstrated a significant research output in this field, with active international collaboration, notably with Washington University leading in contributions. Patterson DR was the most prolific author in terms of published papers, while Choiniere M was the most frequently co-cited author. The focus of research has shifted from symptom management to exploring pain mechanisms. Current research priorities in burn pain include "quality of life," "music therapy," and "psychological state." Recent analysis has highlighted key areas in neuropathic pain mechanisms, novel analgesic therapies, and specific groups such as pediatric burn patients. Influential studies have advanced our understanding of pathophysiology, while psychological interventions and inflammation are increasingly receiving attention. Emerging topics include non-pharmacological interventions, psychological support, technology in pain assessment and management, quality of life, and personalized pain management.

CONCLUSION

Research on burn pain is advancing rapidly; however, collaboration among countries and institutions remains limited. Increased cooperation and communication across these entities could significantly advance the field in the future. Future research should prioritize placebo-controlled trials of targeted therapeutic drugs and innovative pain management approaches, with a strong emphasis on patient outcomes and quality of life.

Exploring neuroinflammation: A key driver in neuropathic pain disorders

Inflammation is a fundamental part of the body's natural defense mechanism, involving immune cells and inflammatory mediators to promote healing and protect against harm. In the event of a lesion or disease of the somatosensory nervous system, inflammation, however, triggers a cascade of changes in both the peripheral and central nervous systems, ultimately contributing to chronic neuropathic pain. Substantial evidence links neuroinflammation to various conditions associated with neuropathic pain. This chapter will explore the role of neuroinflammation in the initiation, maintenance, and resolution of peripheral and central neuropathic pain. Additionally, biomarkers of neuroinflammation in humans will be examined, emphasizing their relevance in different neuropathic pain disorders.

Infraorbital nerve injury triggers sex-specific neuroimmune responses in the peripheral trigeminal pathway and common pain behaviours

Trigeminal neuropathic pain is emotionally distressing and disabling. It presents with allodynia, hyperalgesia and dysaesthesia. In preclinical models it has been assumed that cephalic nerve constriction injury shows identical molecular, cellular, and sex dependent neuroimmune changes as observed in extra-cephalic injury models. This study sought empirical evidence for such assumptions using the infraorbital nerve chronic constriction model (ION-CCI). We compared the behavioural consequences of nerve constriction with: (i) the temporal patterns of recruitment of macrophages and T-lymphocytes at the site of nerve injury and in the trigeminal ganglion; and (ii) the degree of demyelination and axonal reorganisation in the injured nerve. Our data demonstrated that simply testing for allodynia and hyperalgesia as is done in extra-cephalic neuropathic pain models does not provide access to the range of injury-specific nociceptive responses and behaviours reflective of the experience of trigeminal neuropathic pain. Similarly, trigeminal neuroimmune changes evoked by nerve injury are not the same as those identified in models of extra-cephalic neuropathy. Specifically, the timing, magnitude, and pattern of ION-CCI evoked macrophage and T-lymphocyte activity differs between the sexes.

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.

Deciphering the Molecular Mechanism of Escin against Neuropathic Pain: A Network Pharmacology Study

Background

Escin is the main active component in Aesculus hippocastanum. It has been demonstrated that escin has anti-inflammatory properties. This study combined the methods of network pharmacology, molecular docking, and molecular dynamics to explore the molecular mechanism of escin against neuropathic pain (NP).

Methods

The Swiss Target Prediction and the Pharm Mapper database were employed for predicting the targets of escin. Also, the candidate targets of NP were gathered via the databases including Therapeutic Targets, DisGeNet, GeneCards, DrugBank, and OMIM. Subsequently, the network of protein-protein interaction was screened for the key targets by the software Cytoscape 3.8.0. Then, the intersection of these targets was analysed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment. Additionally, we further investigated the ligand-target interactions by molecular docking and molecular dynamics simulations.

Results

In total, 94 escin targets were predicted by network pharmacology. Among them, SRC, MMP9, PTGS2, and MAPK1 were the core candidate targets. Subsequently, the analysis of GO and KEGG enrichment revealed that escin affected NP by regulating protein kinase C, MAP kinase, TRP channels, the T-cell receptors signaling pathway, and the TNF signaling pathway. The results of molecular docking and molecular dynamics simulation confirmed that escin not only had a strong binding activity with the four core target proteins but also stably combined in 50 ns.

Conclusions

Our study revealed that escin acts on the core targets SRC, MMP9, PTGS2, MAPK1, and associated enrichment pathways to alleviate neuronal inflammation and regulate the immune response, thus exerting anti-NP efficacy. This study provided innovative ideas and methods for the promising treatment of escin in relieving NP.

Systemic and Peripheral Mechanisms of Cortical Stimulation-Induced Analgesia and Refractoriness in a Rat Model of Neuropathic Pain

Epidural motor cortex stimulation (MCS) is an effective treatment for refractory neuropathic pain; however, some individuals are unresponsive. In this study, we correlated the effectiveness of MCS and refractoriness with the expression of cytokines, neurotrophins, and nociceptive mediators in the dorsal root ganglion (DRG), sciatic nerve, and plasma of rats with sciatic neuropathy. MCS inhibited hyperalgesia and allodynia in two-thirds of the animals (responsive group), and one-third did not respond (refractory group). Chronic constriction injury (CCI) increased IL-1β in the nerve and DRG, inhibited IL-4, IL-10, and IL-17A in the nerve, decreased β-endorphin, and enhanced substance P in the plasma, compared to the control. Responsive animals showed decreased NGF and increased IL-6 in the nerve, accompanied by restoration of local IL-10 and IL-17A and systemic β-endorphin. Refractory animals showed increased TNF-α and decreased IFNγ in the nerve, along with decreased TNF-α and IL-17A in the DRG, maintaining low levels of systemic β-endorphin. Our findings suggest that the effectiveness of MCS depends on local control of inflammatory and neurotrophic changes, accompanied by recovery of the opioidergic system observed in neuropathic conditions. So, understanding the refractoriness to MCS may guide an improvement in the efficacy of the technique, thus benefiting patients with persistent neuropathic pain.

Pain-resolving immune mechanisms in neuropathic pain

Interactions between the immune and nervous systems are of central importance in neuropathic pain, a common and debilitating form of chronic pain caused by a lesion or disease affecting the somatosensory system. Our understanding of neuroimmune interactions in pain research has advanced considerably. Initially considered as passive bystanders, then as culprits in the pathogenesis of neuropathic pain, immune responses in the nervous system are now established to underpin not only the initiation and progression of pain but also its resolution. Indeed, immune cells and their mediators are well-established promoters of neuroinflammation at each level of the neural pain pathway that contributes to pain hypersensitivity. However, emerging evidence indicates that specific subtypes of immune cells (including antinociceptive macrophages, pain-resolving microglia and T regulatory cells) as well as immunoresolvent molecules and modulators of the gut microbiota-immune system axis can reduce the pain experience and contribute to the resolution of neuropathic pain. This Review provides an overview of the immune mechanisms responsible for the resolution of neuropathic pain, including those involved in innate, adaptive and meningeal immunity as well as interactions with the gut microbiome. Specialized pro-resolving mediators and therapeutic approaches that target these neuroimmune mechanisms are also discussed.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Contribution of CD4+ cells in the emotional alterations induced by endometriosis in mice

Endometriosis is a disease defined by the presence of endometrial tissue in extrauterine locations. This chronic condition is frequently associated with pain and emotional disorders and has been related with altered immune function. However, the specific involvement of immune cells in pain and behavioral symptoms of endometriosis has not been yet elucidated. Here, we implement a mouse model of non-surgical endometriosis in which immunocompetent mice develop abdomino-pelvic hypersensitivity, cognitive deficits, anxiety and depressive-like behaviors. This behavioral phenotype correlates with expression of inflammatory markers in the brain, including the immune cell marker CD4. Depletion of CD4 + cells decreases the anxiety-like behavior of mice subjected to the endometriosis model, whereas abdomino-pelvic hypersensitivity, depressive-like behavior and cognitive deficits remain unaltered. The present data reveal the involvement of the immune response characterized by CD4 + white blood cells in the anxiety-like behavior induced by endometriosis in mice. This model, which recapitulates the symptoms of human endometriosis, may be a useful tool to study the immune mechanisms involved in pain and behavioral alterations associated to endometriosis.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

Positive Long-Term Effects of Third Molar Extraction on Taste Function

Taste and other neurosensory defects have been reported postoperatively in a number of patients who have undergone mandibular third molar extraction (TME). Although the taste deficits are generally believed to resolve within a year, the long-term effects of TME remain unknown. We retrospectively examined the whole-mouth taste function of 891 individuals who had received TMEs, on average, more than 2 decades earlier, and 364 individuals who had not undergone TME. All had been extensively tested for chemosensory function at the University of Pennsylvania Smell and Taste Center over the course of the last 20 years. The whole-mouth identification test incorporated 2 presentations each of 5 different concentrations of sucrose, sodium chloride, citric acid, and caffeine. Analyses of covariance (age = covariate) found those with histories of TME to exhibit better overall test scores for all 4 taste qualities than nonoperated controls. Such scores were not associated with the time since the TME. In both groups, women outperformed men and function declined with age. The basis of this phenomenon, which requires confirmation from prospective studies, is unknown, but could reflect sensitization of CN VII nerve afferents or the partial release of the tonic inhibition that CN VII exerts on CN IX via central nervous system processes.

T cell transgressions: Tales of T cell form and function in diverse disease states

Insights into T cell form, function, and dysfunction are rapidly evolving. T cells have remarkably varied effector functions including protecting the host from infection, activating cells of the innate immune system, releasing cytokines and chemokines, and heavily contributing to immunological memory. Under healthy conditions, T cells orchestrate a finely tuned attack on invading pathogens while minimizing damage to the host. The dark side of T cells is that they also exhibit autoreactivity and inflict harm to host cells, creating autoimmunity. The mechanisms of T cell autoreactivity are complex and dynamic. Emerging research is elucidating the mechanisms leading T cells to become autoreactive and how such responses cause or contribute to diverse disease states, both peripherally and within the central nervous system. This review provides foundational information on T cell development, differentiation, and functions. Key T cell subtypes, cytokines that create their effector roles, and sex differences are highlighted. Pathological T cell contributions to diverse peripheral and central disease states, arising from errors in reactivity, are highlighted, with a focus on multiple sclerosis, rheumatoid arthritis, osteoarthritis, neuropathic pain, and type 1 diabetes.

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.

T Cells as Guardians of Pain Resolution

Despite successful research efforts aimed at understanding pain mechanisms, there is still no adequate treatment for many patients suffering from chronic pain. The contribution of neuroinflammation to chronic pain is widely acknowledged. Here, we summarize findings indicating that T cells play a key role in the suppression of pain. An active contribution of the immune system to resolution of pain may explain why immunosuppressive drugs are often not sufficient to control pain. This would also imply that dysregulation of certain immune functions promote transition to chronic pain. Conversely, stimulating the endogenous immune-mediated resolution pathways may provide a potent approach to treat chronic pain.

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.

Pain Management in a Model of Interstitial Cystitis/Bladder Pain Syndrome by a Vaccinal Strategy

Current analgesic treatments for Interstitial Cystitis/Bladder Pain Syndrome (IC/BPS) are limited. Here, we propose a novel antinociceptive strategy exploiting the opioid-mediated analgesic properties of T lymphocytes to relieve from bladder pain. In a chronic model of IC/BPS in rats, we show that a secondary T cell response against intravesically administered ovalbumin prevents from visceral pain in OVA-primed animals. The analgesic effect is associated with the recruitment of T lymphocytes within the inflamed mucosa and is reversed by naloxone-methiodide, a peripheral opioid receptor antagonist. Similarly, intravesical instillation of BCG or tetanus toxoid antigens in vaccinated rats protects from pain in the same model. We show opioid-dependent analgesic properties of local vaccine antigen recall in a preclinical rat model of chronic cystitis. Since BCG bladder instillation is regularly used in humans (as anticancer therapy), our results open it as a new therapeutic positioning for a pain management indication for IC/BPS patients.

The Role of Corticotropin-Releasing Hormone at Peripheral Nociceptors: Implications for Pain Modulation

Peripheral nociceptors and their synaptic partners utilize neuropeptides for signal transmission. Such communication tunes the excitatory and inhibitory function of nociceptor-based circuits, eventually contributing to pain modulation. Corticotropin-releasing hormone (CRH) is the initiator hormone for the conventional hypothalamic-pituitary-adrenal axis, preparing our body for stress insults. Although knowledge of the expression and functional profiles of CRH and its receptors and the outcomes of their interactions has been actively accumulating for many brain regions, those for nociceptors are still under gradual investigation. Currently, based on the evidence of their expressions in nociceptors and their neighboring components, several hypotheses for possible pain modulations are emerging. Here we overview the historical attention to CRH and its receptors on the peripheral nociception and the recent increases in information regarding their roles in tuning pain signals. We also briefly contemplate the possibility that the stress-response paradigm can be locally intrapolated into intercellular communication that is driven by nociceptor neurons. Such endeavors may contribute to a more precise view of local peptidergic mechanisms of peripheral pain modulation.

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.

Heme attenuates beta-endorphin levels in leukocytes of HIV positive individuals with chronic widespread pain

The prevalence of chronic widespread pain (CWP) in people with HIV is high, yet the underlying mechanisms are elusive. Leukocytes synthesize the endogenous opioid, β-endorphin, within their endoplasmic reticulum (ER). When released into plasma, β-endorphin dampens nociception by binding to opioid receptors on sensory neurons. We hypothesized that the heme-dependent redox signaling induces ER stress, which attenuates leukocyte β-endorphins levels/release, thereby increasing pain sensitivity in people with HIV. Results demonstrated that HIV positive individuals with CWP had increased plasma methemoglobin, erythrocytes membrane oxidation, hemolysis, and low plasma heme scavenging enzyme, hemopexin, compared to people with HIV without CWP and HIV-negative individuals with or without pain. In addition, the leukocytes from people with HIV with CWP had attenuated levels of the heme metabolizing enzyme, heme oxygenase-1, which metabolizes free heme to carbon-monoxide and biliverdin. These individuals also had elevated ER stress, and low β-endorphin in leukocytes. In vitro, heme exposure or heme oxygenase-1 deletion, decreased β-endorphins in murine monocytes/macrophages. Treating cells with a carbon-monoxide donor or an ER stress inhibitor, increased β-endorphins. To mimic hemolytic effects in a preclinical model, C57BL/6 mice were injected with phenylhydrazine hydrochloride (PHZ). PHZ increased cell-free heme and ER stress, decreased leukocyte β-endorphin levels and hindpaw mechanical sensitivity thresholds. Treatment of PHZ-injected mice with hemopexin blocked these effects, suggesting that heme-induced ER stress and a subsequent decrease in leukocyte β-endorphin is responsible for hypersensitivity in people with HIV.

Regulatory T cells counteract neuropathic pain through inhibition of the Th1 response at the site of peripheral nerve injury

The inflammatory/immune response at the site of peripheral nerve injury participates in the pathophysiology of neuropathic pain. Nevertheless, little is known about the local regulatory mechanisms underlying peripheral nerve injury that counteracts the development of pain. Here, we investigated the contribution of regulatory T (Treg) cells to the development of neuropathic pain by using a partial sciatic nerve ligation model in mice. We showed that Treg cells infiltrate and proliferate in the site of peripheral nerve injury. Local Treg cells suppressed the development of neuropathic pain mainly through the inhibition of the CD4 Th1 response. Treg cells also indirectly reduced neuronal damage and neuroinflammation at the level of the sensory ganglia. Finally, we identified IL-10 signaling as an intrinsic mechanism by which Treg cells counteract neuropathic pain development. These results revealed Treg cells as important inhibitory modulators of the immune response at the site of peripheral nerve injury that restrains the development of neuropathic pain. In conclusion, the boosting of Treg cell function/activity might be explored as a possible interventional approach to reduce neuropathic pain development after peripheral nerve damage.

A novel immunocompetent model of metastatic prostate cancer-induced bone pain

BACKGROUND

Over 70% to 85% of men with advanced prostate cancer (PCa) develop bone metastases characterized by severe bone pain and increased likelihood of bone fracture. These clinical features result in decreased quality of life and act as a predictor of higher mortality. Mechanistically, the skeletal pathologies such as osteolytic lesions and abnormal osteoblastic activity drive these symptoms. The role of immune cells in bone cancer pain remains understudied, here we sought to recapitulate this symptomology in a murine model.

METHODS

The prostate cancer bone metastasis-induced pain model (CIBP) was established by transplanting a mouse prostate cancer cell line into the femur of immunocompetent mice. Pain development, gait dynamics, and the changes in emotional activities like depression and anxiety were evaluated. Animal tissues including femurs, dorsal root ganglion (DRG), and spinal cord were collected at killing and microcomputed tomography (μCT), histology/immunohistochemistry, and quantitative immunofluorescent analysis were performed.

RESULTS

Mice receiving prostate cancer cells showed a significantly lower threshold for paw withdrawal responses induced by mechanical stimulation compared with their control counterparts. Zero maze and DigiGait analyses indicated reduced and aberrant movement associated emotional activity compared with sham control at 8-weeks postinjection. The μCT analysis showed osteolytic and osteoblastic changes and a 50% reduction of the trabecular volumes within the prostate cancer group. Neurologically we demonstrated, increased calcitonin gene-related peptide (CGRP) and neuronal p75NTR immune-reactivities in both the projected terminals of the superficial dorsal horn and partial afferent neurons in DRG at L2 to L4 level in tumor-bearing mice. Furthermore, our data show elevated nerve growth factor (NGF) and TrkA immunoreactivities in the same segment of the superficial dorsal horn that were, however, not colocalized with CGRP and p75NTR .

CONCLUSIONS

This study describes a novel immunocompetent model of CIBP and demonstrates the contribution of NGF and p75NTR to chronic pain in bone metastasis.

Neuropeptide and cytokine regulation of pain in the context of substance use disorders

Substance use disorders (SUDs) are frequently accompanied by affective symptoms that promote negative reinforcement mechanisms contributing to SUD maintenance or progression. Despite their widespread use as analgesics, chronic or excessive exposure to alcohol, opioids, and nicotine produces heightened nociceptive sensitivity, termed hyperalgesia. This review focuses on the contributions of neuropeptide (CRF, melanocortin, opioid peptide) and cytokine (IL-1β, TNF-α, chemokine) systems in the development and maintenance of substance-induced hyperalgesia. Few effective therapies exist for either chronic pain or SUD, and the common interaction of these disease states likely complicates their effective treatment. Here we highlight promising new discoveries as well as identify gaps in research that could lead to more effective and even simultaneous treatment of SUDs and co-morbid hyperalgesia symptoms.

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.

Bidirectional Regulation of Opioid and Chemokine Function

The opioid family of GPCRs consists of the classical opioid receptors, designated μ-, κ-, and δ-opioid receptors, and the orphanin-FQ receptor, and these proteins are expressed on both neuronal and hematopoietic cells. A number of laboratories have reported that an important degree of cross-talk can occur between the opioid receptors and the chemokine and chemokine receptor families. As a part of this, the opioid receptors are known to regulate the expression of certain chemokines and chemokine receptors, including those that possess strong pro-inflammatory activity. At the level of receptor function, it is clear that certain members of the chemokine family can mediate cross-desensitization of the opioid receptors. Conversely, the opioid receptors are all able to induce heterologous desensitization of some of the chemokine receptors. Consequently, activation of one or more of the opioid receptors can selectively cross-desensitize chemokine receptors and regulate chemokine function. These cross-talk processes have significant implications for the inflammatory response, since the regulation of both the recruitment of inflammatory cells, as well as the sensation of pain, can be controlled in this way.

CD3+ T cells are critical for the resolution of comorbid inflammatory pain and depression-like behavior

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T cells are necessary for resolution of CFA-induced mechanical allodynia and spontaneous pain.

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T cells are required for the resolution of inflammation-induced depression-like behavior.

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T cells did not contribute to onset or severity of indicators of pain and depression-like behavior.

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T cells did not affect cytokine expression in the paw, spinal cord and brain.

Background

Chronic pain and depression often co-occur. The mechanisms underlying this comorbidity are incompletely understood. Here, we investigated the role of CD3⁺ T cells in an inflammatory model of comorbid persistent mechanical allodynia, spontaneous pain, and depression-like behavior in mice.

Methods

C57Bl/6 wt and Rag2^−/− mice were compared in their response to intraplantar administration of complete Freund’s adjuvant (CFA). Mechanical allodynia, spontaneous pain and depression-like behavior were assessed by von Frey, conditioned place preference and forced swim test respectively.

Results

Resolution of mechanical allodynia, spontaneous pain, and depression-like behavior was markedly delayed in Rag2^−/− mice that are devoid of adaptive immune cells. Reconstitution of Rag2^−/− mice with CD3⁺ T cells from WT mice before CFA injection normalized the resolution of indicators of pain and depression-like behavior. T cells did not contribute to onset or severity of indicators of pain and depression-like behavior. The lack of T cells did not affect cytokine expression in the paw, spinal cord and brain, indicating that the delayed resolution was not resulting from prolonged (neuro)inflammation.

Conclusions

Our findings show that T cells are critical for the natural resolution of mechanical allodynia, spontaneous pain, and depression-like behavior after an inflammatory challenge. Dysregulation of this T cell-mediated resolution pathway could contribute to the comorbidity of chronic pain and depression.

Significance

Chronic pain and depression are frequently associated with signs of inflammation. However, general immunosuppression is not sufficient to resolve comorbid pain and depression. Here we demonstrate that T cells are required for resolution of comorbid persistent mechanical allodynia, spontaneous pain, and depression in a model of peripheral inflammation, indicating the immune system can contribute to both onset and resolution of these comorbidities. Enhancing pro-resolution effects of T cells may have a major impact to treat patients with comorbid persistent pain and depression.

MHCII-restricted T helper cells: an emerging trigger for chronic tactile allodynia after nerve injuries

Nerve injury-induced chronic pain has been an urgent problem for both public health and clinical practice. While transition to chronic pain is not an inevitable consequence of nerve injuries, the susceptibility/resilience factors and mechanisms for chronic neuropathic pain after nerve injuries still remain unknown. Current preclinical and clinical studies, with certain notable limitations, have shown that major histocompatibility complex class II–restricted T helper (Th) cells is an important trigger for nerve injury-induced chronic tactile allodynia, one of the most prevalent and intractable clinical symptoms of neuropathic pain. Moreover, the precise pathogenic neuroimmune interfaces for Th cells remain controversial, not to mention the detailed pathogenic mechanisms. In this review, depending on the biology of Th cells in a neuroimmunological perspective, we summarize what is currently known about Th cells as a trigger for chronic tactile allodynia after nerve injuries, with a focus on identifying what inconsistencies are evident. Then, we discuss how an interdisciplinary perspective would improve the understanding of Th cells as a trigger for chronic tactile allodynia after nerve injuries. Finally, we hope that the expected new findings in the near future would translate into new therapeutic strategies via targeting Th cells in the context of precision medicine to either prevent or reverse chronic neuropathic tactile allodynia.

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.

Mobilization of CD4+ T lymphocytes in inflamed mucosa reduces pain in colitis mice: toward a vaccinal strategy to alleviate inflammatory visceral pain

T lymphocytes play a pivotal role in endogenous regulation of inflammatory visceral pain. The analgesic activity of T lymphocytes is dependent on their production of opioids, a property acquired on antigen activation. Accordingly, we investigated whether an active recruitment of T lymphocytes within inflamed colon mucosa via a local vaccinal strategy may counteract inflammation-induced visceral pain in mice. Mice were immunized against ovalbumin (OVA). One month after immunization, colitis was induced by adding 3% (wt/vol) dextran sulfate sodium into drinking water containing either cognate antigen OVA or control antigen bovine serum albumin for 5 days. Noncolitis OVA-primed mice were used as controls. Visceral sensitivity was then determined by colorectal distension. Oral administration of OVA but not bovine serum albumin significantly reduced dextran sulfate sodium-induced abdominal pain without increasing colitis severity in OVA-primed mice. Analgesia was dependent on local release of enkephalins by effector anti-OVA T lymphocytes infiltrating the inflamed mucosa. The experiments were reproduced with the bacillus Calmette-Guerin vaccine as antigen. Similarly, inflammatory visceral pain was dramatically alleviated in mice vaccinated against bacillus Calmette-Guerin and then locally administered with live Mycobacterium bovis. Together, these results show that the induction of a secondary adaptive immune response against vaccine antigens in inflamed mucosa may constitute a safe noninvasive strategy to relieve from visceral inflammatory pain.

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.

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.

CD4+ αβ T cell infiltration into the leptomeninges of lumbar dorsal roots contributes to the transition from acute to chronic mechanical allodynia after adult rat tibial nerve injuries

Background

Antigen-specific and MHCII-restricted CD4+ αβ T cells have been shown or suggested to play an important role in the transition from acute to chronic mechanical allodynia after peripheral nerve injuries. However, it is still largely unknown where these T cells infiltrate along the somatosensory pathways transmitting mechanical allodynia to initiate the development of chronic mechanical allodynia after nerve injuries. Therefore, the purpose of this study was to ascertain the definite neuroimmune interface for these T cells to initiate the development of chronic mechanical allodynia after peripheral nerve injuries.

Methods

First, we utilized both chromogenic and fluorescent immunohistochemistry (IHC) to map αβ T cells along the somatosensory pathways for the transmission of mechanical allodynia after modified spared nerve injuries (mSNIs), i.e., tibial nerve injuries, in adult male Sprague-Dawley rats. We further characterized the molecular identity of these αβ T cells selectively infiltrating into the leptomeninges of L4 dorsal roots (DRs). Second, we identified the specific origins in lumbar lymph nodes (LLNs) for CD4+ αβ T cells selectively present in the leptomeninges of L4 DRs by two experiments: (1) chromogenic IHC in these lymph nodes for CD4+ αβ T cell responses after mSNIs and (2) fluorescent IHC for temporal dynamics of CD4+ αβ T cell infiltration into the L4 DR leptomeninges after mSNIs in prior lymphadenectomized or sham-operated animals to LLNs. Finally, following mSNIs, we evaluated the effects of region-specific targeting of these T cells through prior lymphadenectomy to LLNs and chronic intrathecal application of the suppressive anti-αβTCR antibodies on the development of mechanical allodynia by von Frey hair test and spinal glial or neuronal activation by fluorescent IHC.

Results

Our results showed that during the sub-acute phase after mSNIs, αβ T cells selectively infiltrate into the leptomeninges of the lumbar DRs along the somatosensory pathways responsible for transmitting mechanical allodynia. Almost all these αβ T cells are CD4 positive. Moreover, the temporal dynamics of CD4+ αβ T cell infiltration into the lumbar DR leptomeninges are specifically determined by LLNs after mSNIs. Prior lymphadenectomy to LLNs specifically reduces the development of mSNI-induced chronic mechanical allodynia. More importantly, intrathecal application of the suppressive anti-αβTCR antibodies reduces the development of mSNI-induced chronic mechanical allodynia. In addition, prior lymphadenectomy to LLNs attenuates mSNI-induced spinal activation of glial cells and PKCγ⁺ excitatory interneurons.

Conclusions

The noteworthy results here provide the first evidence that CD4+ αβ T cells selectively infiltrate into the DR leptomeninges of the somatosensory pathways transmitting mechanical allodynia and contribute to the transition from acute to chronic mechanical allodynia after peripheral nerve injuries.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1115-7) contains supplementary material, which is available to authorized users.

Chinese herb cinobufagin-reduced cancer pain is associated with increased peripheral opioids by invaded CD3/4/8 lymphocytes

Objectives

To investigate the mechanism of cinobufagin-reduced cancer pain in mouse cancer pain model and in vitro cell co-culture system.

Methods

Female Kunming mice were randomly divided into 4 groups. One group of animals was set as normal control without any treatment. Other three groups of animals received H22 hepatoma cell inoculation in right hind paw. At day 9 after inoculation, mice in other three groups were injected intraperitoneally once a day for 8 days with the solvent, morphine or cinobufagin, respectively. The pain behavior was recorded daily. On the last day, all mice were sacrificed and xenograft tissues homogenate and plasma levels of β-endorphin (β-END), corticotropin-releasing factor (CRF) and interleukin-1β (IL-1β) were assessed by ELISA assay. Immunohistochemistry was performed to determine the expression of β-END, pro-opiomelanocortin (POMC) and the μ-opioid receptor (μ-OR) in the xenograft tissues. Immunofluorescence was used to localize lymphocytes with expression of CD3⁺, CD4⁺ and CD8⁺ in xenograft tumors and adjacent tissues. Mice splenic lymphocytes and H22 hepatoma carcinoma ascites cells were prepared for co-culture. β-END and CRF were detected in co-culture supernatants. The MTT assay and cytometry were used to assess cell proliferation. RT-PCR was conducted to determine the gene expression of POMC and Cathepsin L (CTSL). Chemotaxis was examined using a transwell-based migration assay.

Results

Compared to the model group, the thermal and mechanical pain thresholds were increased in mice after cinobufagin treatment. The expression of β-END and CRF in the plasma and tumor tissues of cinobufagin group were much higher than that of the model group mice, but the expression of IL-1β in the plasma and tumor tissues was much lower than that in the model group mice. Meanwhile, the expression of β-END, POMC and μ-OR proteins was significantly increased in the xenograft tissues from cinobufagin group. Lymphocyte population of CD3⁺, CD4⁺, CD8⁺ were also elevated in xenograft tumors and adjacent tissues. In the cell co-culture assays, the content of β-END in the supernatant was significantly increased by cinobufagin in a dose-dependent manner. Cinobufagin also largely increased the proliferation of immune cells and inhibited H22 hepatoma carcinoma cell proliferation in single or co-culture cell assays. Gene expression of POMC and CTSL in cinobufagin group was significantly up-regulated comparing to the control group. Finally, cinobufagin addition enhanced the migration of immune cells in transwell assay.

Conclusions

Cinobufagin-induced local analgesic effect might be associated with increased activity of POMC/β-END/μ-OR pathway released from invaded CD^3/4/8 lymphocytes in cancer tissues.

Flow cytometry analysis of inflammatory cells isolated from the sciatic nerve and DRG after chronic constriction injury in mice

BACKGROUND

Cellular responses to nerve injury play a central role in the pathogenesis of neuropathic pain. However, the analysis of site specific cellular responses to nerve injury and neuropathic pain is limited to immunohistochemistry staining with numerous limitations.

NEW METHODS

We proposed to apply flow cytometry to overcome some of the limitations and developed two protocols for isolation of cells from small specimens of the sciatic nerve and dorsal root ganglion (DRG) in mice. RESULTS AND COMPARASION WITH EXISTING: methods We found that both the non-enzymatic and enzymatic approaches were highly effective in harvesting a sufficient number of cells for flow cytometry analysis in normal and pathological conditions. The total number of cells in the injury site of the sciatic and its DRGs increased significantly 14days after chronic constriction injury (CCI) of the sciatic nerve, compared to sham surgery control or the contralateral control. The enzymatic approach yielded a significantly higher total number of cells and CD45 negative cells, suggesting that this approach allows for harvest of more resident cells, compared to the non-enzymatic method. The percentage of CD45⁺/CD11b⁺ cells was significantly increased in the sciatic nerve but not in the DRG. These results were consistent with both protocols.

CONCLUSIONS

We thus offer two simple and effective protocols that allow for application of flow cytometry to the investigation of cellular and molecular mechanisms of neuropathic pain.

Cytotoxic T cells modulate inflammation and endogenous opioid analgesia in chronic arthritis

Background

This study examined the development of chronic pain, a cardinal symptom of rheumatoid arthritis (RA), in mice with antigen- and collagen-induced arthritis (ACIA). Since the role of CD8⁺ T cells in arthritis is controversial, we investigated the consequences of CD8-depletion on arthritis development and opioid modulation of pain in this novel model of chronic autoimmune arthritis.

Methods

Disease severity in control and CD8-depleted animals was determined by histological assessment of knee-joint sections and measurement of autoantibody formation. Pain was evaluated by measuring mechanical allodynia and thermal hyperalgesia in von Frey and Hargreaves tests, respectively. The production and release of endogenous opioids and inflammatory cytokines was assessed in immunoassays.

Results

In ACIA, mice display persistent mechanical allodynia and thermal hyperalgesia for more than 2 months after induction of arthritis. The blockade of peripheral opioid receptors with naloxone-methiodide (NLXM) transiently increased thermal hyperalgesia, indicating that endogenous opioid peptides were released in the arthritic joint to inhibit pain. CD8⁺ T cell depletion did not affect autoantibody formation or severity of joint inflammation, but serum levels of the pro-inflammatory cytokines TNFα and IL-17 were increased. The release of opioid peptides from explanted arthritic knee cells and the NLXM effect were significantly reduced in the absence of CD8⁺ T cells.

Conclusions

We have successfully modeled the development of chronic pain, a hallmark of RA, in ACIA. Furthermore, we detected a yet unknown protective role of CD8⁺ T cells in chronic ACIA since pro-inflammatory cytokines rose and opioid peptide release decreased in the absence of these cells.

Adoptive transfer of M2 macrophages reduces neuropathic pain via opioid peptides

Background

During the inflammation which occurs following nerve damage, macrophages are recruited to the site of injury. Phenotypic diversity is a hallmark of the macrophage lineage and includes pro-inflammatory M1 and anti-inflammatory M2 populations. Our aim in this study was to investigate the ability of polarized M0, M1, and M2 macrophages to secrete opioid peptides and to examine their relative contribution to the modulation of neuropathic pain.

Methods

Mouse bone marrow-derived cells were cultured as unstimulated M0 macrophages or were stimulated into an M1 phenotype using lipopolysaccharide and interferon-γ or into an M2 phenotype using interleukin-4. The macrophage phenotypes were verified using flow cytometry for surface marker analysis and cytokine bead array for cytokine profile assessment. Opioid peptide levels were measured by radioimmunoassay and enzyme immunoassay. As a model of neuropathic pain, a chronic constriction injury (CCI) of the sciatic nerve was employed. Polarized M0, M1, and M2 macrophages (5 × 10⁵ cells) were injected perineurally twice, on days 14 and 15 following CCI or sham surgery. Mechanical and heat sensitivity were measured using the von Frey and Hargreaves tests, respectively. To track the injected macrophages, we also transferred fluorescently stained polarized cells and analyzed the surface marker profile of endogenous and injected cells in the nerves ex vivo.

Results

Compared to M0 and M1 cells, M2 macrophages contained and released higher amounts of opioid peptides, including Met-enkephalin, dynorphin A (1–17), and β-endorphin. M2 cells transferred perineurally at the nerve injury site reduced mechanical, but not heat hypersensitivity following the second injection. The analgesic effect was reversed by the perineurally applied opioid receptor antagonist naloxone methiodide. M2 cells did not affect sensitivity following sham surgery. Neither M0 nor M1 cells altered mechanical and heat sensitivity in CCI or sham-operated animals. Tracing the fluorescently labeled M0, M1, and M2 cells ex vivo showed that they remained in the nerve and preserved their phenotype.

Conclusions

Perineural transplantation of M2 macrophages resulted in opioid-mediated amelioration of neuropathy-induced mechanical hypersensitivity, while M1 macrophages did not exacerbate pain. Therefore, rather than focusing on macrophage-induced pain generation, promoting opioid-mediated M2 actions may be more relevant for pain control.

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

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

Distinct roles of exogenous opioid agonists and endogenous opioid peptides in the peripheral control of neuropathy-triggered heat pain

Neuropathic pain often results from peripheral nerve damage, which can involve immune response. Local leukocyte-derived opioid peptides or exogenous opioid agonists inhibit neuropathy-induced mechanical hypersensitivity in animal models. Since neuropathic pain can also be augmented by heat, in this study we investigated the role of opioids in the modulation of neuropathy-evoked heat hypersensitivity. We used a chronic constriction injury of the sciatic nerve in wild-type and opioid peptide-knockout mice, and tested opioid effects in heat and mechanical hypersensitivity using Hargreaves and von Frey tests, respectively. We found that although perineural exogenous opioid agonists, including peptidergic ligands, were effective, the endogenous opioid peptides β-endorphin, Met-enkephalin and dynorphin A did not alleviate heat hypersensitivity. Specifically, corticotropin-releasing factor, an agent triggering opioid peptide secretion from leukocytes, applied perineurally did not attenuate heat hypersensitivity in wild-type mice. Exogenous opioids, also shown to release opioid peptides via activation of leukocyte opioid receptors, were equally analgesic in wild-type and opioid peptide-knockout mice, indicating that endogenous opioids do not contribute to exogenous opioid analgesia in heat hypersensitivity. Furthermore, exogenously applied opioid peptides were ineffective as well. Conversely, opioid peptides relieved mechanical hypersensitivity. Thus, both opioid type and sensory modality may determine the outcome of neuropathic pain treatment.

Endogenous analgesia mediated by CD4(+) T lymphocytes is dependent on enkephalins in mice

Background

T cell-derived opioids play a key role in the control of inflammatory pain. However, the nature of opioids produced by T cells is still matter of debate in mice. Whereas β-endorphin has been found in T lymphocytes by using antibody-based methods, messenger RNA (mRNA) quantification shows mainly mRNA encoding for enkephalins. The objective of the study is to elucidate the nature of T cell-derived opioids responsible for analgesia and clarify discrepancy of the results at the protein and genetic levels.

Methods

CD4⁺ T lymphocytes were isolated from wild-type and enkephalin-deficient mice. mRNA encoding for β-endorphin and enkephalin was quantified by RT-qPCR. The binding of commercially available polyclonal anti-endorphin antibodies to lymphocytes from wild-type or enkephalin knockout mice was assessed by cytofluorometry. Opioid-mediated analgesic properties of T lymphocytes from wild-type and enkephalin-deficient mice were compared in a model of inflammation-induced somatic pain by measuring sensitivity to mechanical stimuli using calibrated von Frey filaments.

Results

CD4⁺ T lymphocytes expressed high level of mRNA encoding for enkephalins but not for β-endorphin in mice. Anti-β-endorphin polyclonal IgG antibodies are specific for β-endorphin but cross-react with enkephalins. Anti-β-endorphin polyclonal antibodies bound to wild-type but not enkephalin-deficient CD4⁺ T lymphocytes. Endogenous regulation of inflammatory pain by wild-type T lymphocytes was completely abolished when T lymphocytes were deficient in enkephalins. Pain behavior of immune-deficient (i.e., without B and T lymphocytes) mice was superimposable to that of mice transferred with enkephalin-deficient lymphocytes.

Conclusions

Rabbit polyclonal anti-β-endorphin serum IgG bind to CD4⁺ T lymphocytes because of their cross-reactivity towards enkephalins. Thus, staining of T lymphocytes by anti-β-endorphin polyclonal IgG reported in most of studies in mice is because of their binding to enkephalins. In mice, CD4⁺ T lymphocytes completely lose their analgesic opioid-mediated activity when lacking enkephalins.

An early granulocyte colony-stimulating factor treatment attenuates neuropathic pain through activation of mu opioid receptors on the injured nerve

Several studies have shown that the mu opioid receptor (MOR) located in the peripheral nerves can be activated after nerve injury and that it attenuates peripheral nociceptive signals to the spinal dorsal horn. Various cytokines and phosphorylated-p38 (p-p38) activation in the dorsal horn also play an important role in neuropathic pain development. Granulocyte-colony stimulating factor (GCSF) is a growth factor that can stimulate granulocyte formation and has been shown to exert an analgesic effect on neuropathic pain through recruiting opioid-containing leukocytes to the injured nerve. However, the underlying mechanisms are not well understood. Herein, the results of behavior tests in addition to MOR levels in the injured sciatic nerve and the levels of p-p38 and various cytokines in the spinal dorsal horn were studied in vehicle-treated or GCSF-treated chronic constriction injured (CCI) rats at different time points (i.e., 1, 3, and 7 days, respectively) after nerve injury. The results showed that a single early systemic GCSF treatment after nerve injury can up-regulate MORs in the injured nerve, which can decrease peripheral nociceptive signals. Thereafter, those changes suppress the pro-inflammatory cytokine IL-6 but enhance the anti-inflammatory cytokine IL-4, followed by decreases in p-p38 in the dorsal horn, and thus further attenuate neuropathic pain.

Opioids and TRPV1 in the peripheral control of neuropathic pain--Defining a target site in the injured nerve

Targeting peripheral neuropathic pain at its origin may prevent the development of hypersensitivity. Recently we showed this can be mediated by opioid receptors at the injured nerve trunk. Here, we searched for the most relevant peripheral site to block transient receptor potential vanilloid 1 (TRPV1), and investigated analgesic interactions between TRPV1 and opioids in neuropathy. In a chronic constriction injury (CCI) of the sciatic nerve in mice, we assessed the effects of μ-, δ- and κ-opioid receptor agonists and TRPV1 antagonist (SB366791) injected at the CCI site or into the injured nerve-innervated paw on spontaneous paw lifting, heat and mechanical sensitivity. We also examined TRPV1 expression in total membrane and plasma membrane fractions from nerves and paws. We found that opioids and SB366791 co-injected in per se nonanalgesic doses at the CCI site or into the paw diminished heat and mechanical sensitivity. SB366791 alone dose-dependently alleviated heat and mechanical sensitivity. TRPV1 blockade in the paw was more effective than at the CCI site. None of the treatments diminished spontaneous paw lifting. TRPV1 expression analysis suggests that the levels of functional TRPV1 do not critically determine the TRPV1 antagonist-mediated analgesia. Together, the identification of the primary action site in damaged nerves is crucial for effective pain control. Contrary to opioids, the TRPV1 blockade in the injured nerve peripheral terminals, rather than at the nerve trunk, appears promising against heat pain. Opioid/TRPV1 antagonist combinations at both locations partially reduced neuropathy-triggered heat and mechanical pain.

Variability in C-type lectin receptors regulates neuropathic pain-like behavior after peripheral nerve injury

INTRODUCTION

Neuropathic pain is believed to be influenced in part by inflammatory processes. In this study we examined the effect of variability in the C-type lectin gene cluster (Aplec) on the development of neuropathic pain-like behavior after ligation of the L5 spinal nerve in the inbred DA and the congenic Aplec strains, which carries seven C-type lectin genes originating from the PVG strain.

RESULTS

While both strains displayed neuropathic pain behavior early after injury, the Aplec strain remained sensitive throughout the whole study period. Analyses of several mRNA transcripts revealed that the expression of Interleukin-1β, Substance P and Cathepsin S were more up-regulated in the dorsal part of the spinal cord of Aplec rats compared to DA, indicating a stronger inflammatory response. This notion was supported by flow cytometric analysis revealing increased infiltration of activated macrophages into the spinal cord. In addition, macrophages from the Aplec strain stimulated in vitro displayed higher expression of inflammatory cytokines compared to DA cells. Finally, we bred a recombinant congenic strain (R11R6) comprising only four of the seven Aplec genes, which displayed similar clinical and immune phenotypes as the Aplec strain.

CONCLUSION

We here for the first time demonstrate that C-type lectins, a family of innate immune receptors with largely unknown functions in the nervous system, are involved in regulation of inflammation and development of neuropathic pain behavior after nerve injury. Further experimental and clinical studies are needed to dissect the underlying mechanisms more in detail as well as any possible relevance for human conditions.

Toll like receptor (TLR)-4 as a regulator of peripheral endogenous opioid-mediated analgesia in inflammation

BACKGROUND

Leukocytes containing opioid peptides locally control inflammatory pain. In the early phase of complete Freund's adjuvant (CFA)-induced hind paw inflammation, formyl peptides (derived e.g. from Mycobacterium butyricum) trigger the release of opioid peptides from neutrophils contributing to tonic basal antinociception. In the later phase we hypothesized that toll-like-receptor-(TLR)-4 activation of monocytes/macrophages triggers opioid peptide release and thereby stimulates peripheral opioid-dependent antinociception.

RESULTS

In Wistar rats with CFA hind paw inflammation in the later inflammatory phase (48-96 h) systemic leukocyte depletion by cyclophosphamide (CTX) or locally injected naloxone (NLX) further decreased mechanical and thermal nociceptive thresholds. In vitro β-endorphin (β-END) content increased during human monocyte differentiation as well as in anti-inflammatory CD14+CD16- or non-classical M2 macrophages. Monocytes expressing TLR4 dose-dependently released β-END after stimulation with lipopolysaccharide (LPS) dependent on intracellular calcium. Despite TLR4 expression proinflammatory M1 and anti-inflammatory M2 macrophages only secreted opioid peptides in response to ionomycin, a calcium ionophore. Intraplantar injection of LPS as a TLR4 agonist into the inflamed paw elicited an immediate opioid- and dose-dependent antinociception, which was blocked by TAK-242, a small-molecule inhibitor of TLR4, or by peripheral applied NLX. In the later phase LPS lowered mechanical and thermal nociceptive thresholds. Furthermore, local peripheral TLR4 blockade worsened thermal and mechanical nociceptive pain thresholds in CFA inflammation.

CONCLUSION

Endogenous opioids from monocytes/macrophages mediate endogenous antinociception in the late phase of inflammation. Peripheral TLR4 stimulation acts as a transient counter-regulatory mechanism for inflammatory pain in vivo, and increases the release of opioid peptides from monocytes in vitro. TLR4 antagonists as new treatments for sepsis and neuropathic pain might unexpectedly transiently enhance pain by impairing peripheral opioid analgesia.

μ-Opioid receptor antibody reveals tissue-dependent specific staining and increased neuronal μ-receptor immunoreactivity at the injured nerve trunk in mice

Neuropathic pain is a debilitating chronic disease often resulting from damage to peripheral nerves. Activation of opioid receptors on peripheral sensory neurons can attenuate pain without central nervous system side effects. Here we aimed to analyze the distribution of neuronal μ-opioid receptors, the most relevant opioid receptors in the control of clinical pain, along the peripheral neuronal pathways in neuropathy. Hence, following a chronic constriction injury of the sciatic nerve in mice, we used immunohistochemistry to quantify the μ-receptor protein expression in the dorsal root ganglia (DRG), directly at the injured nerve trunk, and at its peripheral endings in the hind paw skin. We also thoroughly examined the μ-receptor antibody staining specificity. We found that the antibody specifically labeled μ-receptors in human embryonic kidney 293 cells as well as in neuronal processes of the sciatic nerve and hind paw skin dermis, but surprisingly not in the DRG, as judged by the use of μ/δ/κ-opioid receptor knockout mice. Therefore, a reliable quantitative analysis of μ-receptor expression in the DRG was not possible. However, we demonstrate that the μ-receptor immunoreactivity was strongly enhanced proximally to the injury at the nerve trunk, but was unaltered in paws, on days 2 and 14 following injury. Thus, μ-opioid receptors at the site of axonal damage might be a promising target for the control of painful neuropathies. Furthermore, our findings suggest a rigorous tissue-dependent characterization of antibodies' specificity, preferably using knockout animals.

Targeting peripheral opioid receptors to promote analgesic and anti-inflammatory actions

Mechanisms of endogenous pain control are significant. Increasing studies have clearly produced evidence for the clinical usefulness of opioids in peripheral analgesia. The immune system uses mechanisms of cell migration not only to fight pathogens but also to control pain and inflammation within injured tissue. It has been demonstrated that peripheral inflammatory pain can be effectively controlled by an interaction of immune cell-derived opioid peptides with opioid receptors on peripheral sensory nerve terminals. Experimental and clinical studies have clearly shown that activation of peripheral opioid receptors with exogenous opioid agonists and endogenous opioid peptides are able to produce significant analgesic and anti-inflammatory effects, without central opioid mediated side effects (e.g., respiratory depression, sedation, tolerance, dependence). This article will focus on the role of opioids in peripheral inflammatory conditions and the clinical implications of targeting peripheral opioid receptors.

Stronger antinociceptive efficacy of opioids at the injured nerve trunk than at its peripheral terminals in neuropathic pain

Activation of opioid receptors on peripheral sensory neurons has the potential for safe pain control, as it lacks centrally mediated side effects. While this approach often only partially suppressed neuropathic pain in animal models, opioids were mostly applied to animal paws although neuropathy was induced at the nerve trunk. Here we aimed to identify the most relevant peripheral site of opioid action for efficient antinociception in neuropathy. On days 2 and 14 following a chronic constriction injury (CCI) of the sciatic nerve in mice, we evaluated dose and time relationships of the effects of μ-, δ-, and κ-opioid receptor agonists injected either at the CCI site or intraplantarly (i.pl.) into the lesioned nerve-innervated paw, on spontaneous paw lifting and heat and mechanical hypersensitivity (using Hargreaves and von Frey tests, respectively). We found that neither agonist diminished spontaneous paw lifting, despite the application site. Heat hypersensitivity was partially attenuated by i.pl. μ-receptor agonist only, while it was improved by all three agonists applied at the CCI site. Mechanical hypersensitivity was slightly diminished by all agonists administered i.pl., whereas it was completely blocked by all opioids injected at the CCI site. These antinociceptive effects were opioid receptor type-selective and site-specific. Thus, opioids might not be effective against spontaneous pain, but they improve heat and mechanical hypersensitivity in neuropathy. Importantly, efficient alleviation of hypersensitivity is governed by peripheral opioid receptors at the injured nerve trunk rather than at its peripheral terminals. Identifying the primary action site of analgesics is important for the development of adequate pain therapies.