Site-Specific Regulation of P2X7 Receptor Function in Microglia Gates Morphine Analgesic Tolerance

Diversity of microglial transcriptional responses during opioid exposure and neuropathic pain

ABSTRACT

Microglia take on an altered morphology during chronic opioid treatment. This morphological change is broadly used to identify the activated microglial state associated with opioid side effects, including tolerance and opioid-induced hyperalgesia (OIH). Microglia display similar morphological responses in the spinal cord after peripheral nerve injury (PNI). Consistent with this observation, functional studies have suggested that microglia activated by opioids or PNI engage common molecular mechanisms to induce hypersensitivity. In this article, we conducted deep RNA sequencing (RNA-seq) and morphological analysis of spinal cord microglia in male mice to comprehensively interrogate transcriptional states and mechanistic commonality between multiple models of OIH and PNI. After PNI, we identify an early proliferative transcriptional event across models that precedes the upregulation of histological markers of microglial activation. However, we found no proliferative transcriptional response associated with opioid-induced microglial activation, consistent with histological data, indicating that the number of microglia remains stable during morphine treatment, whereas their morphological response differs from PNI models. Collectively, these results establish the diversity of pain-associated microglial transcriptomic responses and point towards the targeting of distinct insult-specific microglial responses to treat OIH, PNI, or other central nervous system pathologies.

Pannexin-1 channel inhibition alleviates opioid withdrawal in rodents by modulating locus coeruleus to spinal cord circuitry

Opioid withdrawal is a liability of chronic opioid use and misuse, impacting people who use prescription or illicit opioids. Hyperactive autonomic output underlies many of the aversive withdrawal symptoms that make it difficult to discontinue chronic opioid use. The locus coeruleus (LC) is an important autonomic centre within the brain with a poorly defined role in opioid withdrawal. We show here that pannexin-1 (Panx1) channels expressed on microglia critically modulate LC activity during opioid withdrawal. Within the LC, we found that spinally projecting tyrosine hydroxylase (TH)-positive neurons (LCspinal) are hyperexcitable during morphine withdrawal, elevating cerebrospinal fluid (CSF) levels of norepinephrine. Pharmacological and chemogenetic silencing of LCspinal neurons or genetic ablation of Panx1 in microglia blunted CSF NE release, reduced LC neuron hyperexcitability, and concomitantly decreased opioid withdrawal behaviours in mice. Using probenecid as an initial lead compound, we designed a compound (EG-2184) with greater potency in blocking Panx1. Treatment with EG-2184 significantly reduced both the physical signs and conditioned place aversion caused by opioid withdrawal in mice, as well as suppressed cue-induced reinstatement of opioid seeking in rats. Together, these findings demonstrate that microglial Panx1 channels modulate LC noradrenergic circuitry during opioid withdrawal and reinstatement. Blocking Panx1 to dampen LC hyperexcitability may therefore provide a therapeutic strategy for alleviating the physical and aversive components of opioid withdrawal.

The interplay between the microbiota and opioid in the treatment of neuropathic pain

Neuropathic pain (NP) is characterized by its complex and multifactorial nature and limited responses to opioid therapy; NP is associated with risks of drug resistance, addiction, difficulty in treatment cessation, and psychological disorders. Emerging research on gut microbiota and their metabolites has demonstrated their effectiveness in alleviating NP and augmenting opioid-based pain management, concurrently mitigating the adverse effects of opioids. This review addresses the following key points: (1) the current advances in gut microbiota research and the challenges in using opioids to treat NP, (2) the reciprocal effects and benefits of gut microbiota on NP, and (3) the interaction between opioids with gut microbiota, as well as the benefits of gut microbiota in opioid-based treatment of NP. Through various intricate mechanisms, gut microbiota influences the onset and progression of NP, ultimately enhancing the efficacy of opioids in the management of NP. These insights pave the way for further pragmatic clinical research, ultimately enhancing the efficacy of opioid-based pain management.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Focal mu-opioid receptor activation promotes neuroinflammation and microglial activation in the mesocorticolimbic system: Alterations induced by inflammatory pain

Microglia participates in the modulation of pain signaling. The activation of microglia is suggested to play an important role in affective disorders that are related to a dysfunction of the mesocorticolimbic system (MCLS) and are commonly associated with chronic pain. Moreover, there is evidence that mu-opioid receptors (MORs), expressed in the MCLS, are involved in neuroinflammatory events, although the way by which they do it remains to be elucidated. In this study, we propose that MOR pharmacological activation within the MCLS activates and triggers the local release of proinflammatory cytokines and this pattern of activation is impacted by the presence of systemic inflammatory pain. To test this hypothesis, we used in vivo microdialysis coupled with flow cytometry to measure cytokines release in the nucleus accumbens and immunofluorescence of IBA1 in areas of the MCLS on a rat model of inflammatory pain. Interestingly, the treatment with DAMGO, a MOR agonist locally in the nucleus accumbens, triggered the release of the IL1α, IL1β, and IL6 proinflammatory cytokines. Furthermore, MOR pharmacological activation in the ventral tegmental area (VTA) modified the levels of IBA1-positive cells in the VTA, prefrontal cortex, the nucleus accumbens and the amygdala in a dose-dependent way, without impacting mechanical nociception. Additionally, MOR blockade in the VTA prevents DAMGO-induced effects. Finally, we observed that systemic inflammatory pain altered the IBA1 immunostaining derived from MOR activation in the MSCLS. Altogether, our results indicate that the microglia-MOR relationship could be pivotal to unravel some inflammatory pain-induced comorbidities related to MCLS dysfunction.

Chronic Morphine Modulates PDGFR-β and PDGF-B Expression and Distribution in Dorsal Root Ganglia and Spinal Cord in Male Rats

The analgesic effect of opioids decreases over time due to the development of analgesic tolerance. We have shown that inhibition of the platelet-derived growth factor beta (PDGFR-β) signaling eliminates morphine analgesic tolerance in rats. Although the PDGFR-β and its ligand, the platelet-derived growth factor type B (PDGF-B), are expressed in the substantia gelatinosa of the spinal cord (SG) and in the dorsal root ganglia (DRG), their precise distribution within different cell types of these structures is unknown. Additionally, the impact of a tolerance-mediating chronic morphine treatment, on the expression and distribution of PDGF-B and PDGFR-β has not yet been studied. Using immunohistochemistry (IHC), we found that in the spinal cord, PDGFR-β and PDGF-B were expressed in neurons and oligodendrocytes and co-localized with the mu-opioid receptor (MOPr) in opioid naïve rats. PDGF-B was also found in microglia and astrocytes. Both PDGFR-β and PDGF-B were detected in DRG neurons but not in spinal primary afferent terminals. Chronic morphine exposure did not change the cellular distribution of PDGFR-β or PDGF-B. However, PDGFR-β expression was downregulated in the SG and upregulated in the DRG. Consistent with our previous finding that morphine caused tolerance by inducing PDGF-B release, PDGF-B was upregulated in the spinal cord. We also found that chronic morphine exposure caused a spinal proliferation of oligodendrocytes. The changes in PDGFR-β and PDGF-B expression induced by chronic morphine treatment suggest potential mechanistic substrates underlying opioid tolerance.

Remifentanil does not affect human microglial immune activation in response to pro-inflammatory cytokines

Remifentanil is a potent ultra-short acting μ-opioid analgesic drug, frequently used in anaesthesia due to its favorable pharmacodynamic and pharmacokinetic profile. It may be associated with the occurrence of hyperalgesia. Preclinical studies suggest a potential role of microglia, although the molecular mechanisms have not been fully elucidated. Considering the role of microglia in brain inflammation and the relevant differences among species, the effects of remifentanil were studied on the human microglial C20 cells. The drug was tested at clinically relevant concentrations under basal and inflammatory conditions. In the C20 cells, the expression and secretion of interleukin 6, interleukin 8 and the monocyte chemotactic protein 1 were rapidly induced by a mixture of pro-inflammatory cytokines. This stimulatory effect was sustained up to 24 h. Remifentanil did not exert any toxic effect nor modify the production of these inflammatory mediators, thus suggesting the lack of direct immune modulatory actions on human microglia.

Frequent Low-Dose Δ9-Tetrahydrocannabinol in Adolescence Disrupts Microglia Homeostasis and Disables Responses to Microbial Infection and Social Stress in Young Adulthood

BACKGROUND

During adolescence, microglia are actively involved in neocortical maturation while concomitantly undergoing profound phenotypic changes. Because the teenage years are also a time of experimentation with cannabis, we evaluated whether adolescent exposure to the drug's psychotropic constituent, Δ9-tetrahydrocannabinol (THC), might persistently alter microglia function.

METHODS

We administered THC (5 mg/kg, intraperitoneal) once daily to male and female mice from postnatal day (PND) 30 to PND44 and examined the transcriptome of purified microglia in adult animals (PND70 and PND120) under baseline conditions or following either of two interventions known to recruit microglia: lipopolysaccharide injection and repeated social defeat. We used high-dimensional mass cytometry by time-of-flight to map brain immune cell populations after lipopolysaccharide challenge.

RESULTS

Adolescent THC exposure produced in mice of both sexes a state of microglial dyshomeostasis that persisted until young adulthood (PND70) but receded with further aging (PND120). Key features of this state included broad alterations in genes involved in microglia homeostasis and innate immunity along with marked impairments in the responses to lipopolysaccharide- and repeated social defeat-induced psychosocial stress. The endocannabinoid system was also dysfunctional. The effects of THC were prevented by coadministration of either a global CB1 receptor inverse agonist or a peripheral CB1 neutral antagonist and were not replicated when THC was administered in young adulthood (PND70-84).

CONCLUSIONS

Daily low-intensity CB1 receptor activation by THC during adolescence may disable critical functions served by microglia until young adulthood with potentially wide-ranging consequences for brain and mental health.

An analgesic peptide H-20 attenuates chronic pain via the PD-1 pathway with few adverse effects

The lack of effective and safe analgesics for chronic pain management has been a health problem associated with people's livelihoods for many years. Analgesic peptides have recently shown significant therapeutic potential, as they are devoid of opioid-related adverse effects. Programmed cell death protein 1 (PD-1) is widely expressed in neurons. Activation of PD-1 by PD-L1 modulates neuronal excitability and evokes significant analgesic effects, making it a promising target for pain treatment. However, the research and development of small molecule analgesic peptides targeting PD-1 have not been reported. Here, we screened the peptide H-20 using high-throughput screening. The in vitro data demonstrated that H-20 binds to PD-1 with micromolar affinity, evokes Src homology 2 domain-containing tyrosine phosphatase 1 (SHP-1) phosphorylation, and diminishes nociceptive signals in dorsal root ganglion (DRG) neurons. Preemptive treatment with H-20 effectively attenuates perceived pain in naïve WT mice. Spinal H-20 administration displayed effective and longer-lasting analgesia in multiple preclinical pain models with a reduction in or absence of tolerance, abuse liability, constipation, itch, and motor coordination impairment. In summary, our findings reveal that H-20 is a promising candidate drug that ameliorates chronic pain in the clinic.

P2X7 receptor in inflammation and pain

Different studies have confirmed P2X7 receptor-mediated inflammatory mediators play a key role in the development of pain. P2X7 receptor activation can induce the development of pain by mediating the release of inflammatory mediators. In view of the fact that P2X7 receptor is expressed in the nervous system and immune system, it is closely related to the stability and maintenance of the nervous system function. ATP activates P2X7 receptor, opens non-selective cation channels, activates multiple intracellular signaling, releases multiple inflammatory cytokines, and induces pain. At present, the role of P2X7 receptor in inflammatory response and pain has been widely recognized and affirmed. Therefore, in this paper, we discussed the pathological mechanism of P2X7 receptor-mediated inflammation and pain, focused on the internal relationship between P2X7 receptor and pain. Moreover, we also described the effects of some antagonists on pain relief by inhibiting the activities of P2X7 receptor. Thus, targeting to inhibit activation of P2X7 receptor is expected to become another potential target for the relief of pain.

Impaired Calcium Signaling and Neuropsychiatric Disorders in Darier Disease: An Exploratory Review

Darier (Darier-White) disease (DD) is an autosomal dominant skin disorder caused by pathogenic mutations in the ATP2A2 gene which encodes a calcium ATPase in the sarco-endoplasmic reticulum (SERCA2). Defects in the SERCA2 protein leads to an impairment of cellular calcium homeostasis, which in turn, triggers cell death pathways. There is a high prevalence of neuropsychiatric disorders in patients affected by this condition, namely intellectual disability, bipolar disorder, schizophrenia, and suicidality. Though these associations have been well-documented over the years, little has been discussed or investigated regarding the pathophysiological mechanisms. The goal of this article is to review the literature related to the most commonly associated neuropsychiatric disorders found in patients with DD, highlight the pathophysiological mechanisms underlying each condition, and examine potential interventions that may be of interest for future development. A literature search was performed using PubMed to access and review relevant articles published in the last 40 years. Keywords searched included Darier disease neuropsychiatric, Darier disease pathophysiology, SERCA2 central nervous system, SERCA 2 skin, ATP2A2 central nervous system, ATP2A2 skin, sphingosine-1-phosphate signaling skin, sphingosine-1-phosphate signaling central nervous system, P2X7 receptor skin, and P2X7 receptor central nervous system. Our search resulted in 2,692 articles, of which 61 articles were ultimately included in this review.

A cAMP-Related Gene Network in Microglia Is Inversely Regulated by Morphine Tolerance and Withdrawal

Background

Microglia have recently been implicated in opioid dependence and withdrawal. Mu Opioid (MOR) receptors are expressed in microglia, and microglia form intimate connections with nearby neurons. Accordingly, opioids have both direct (MOR mediated) and indirect (neuron-interaction mediated) effects on microglia function.

Methods

To investigate this directly, we used RNA sequencing of ribosome-associated RNAs from striatal microglia (RiboTag-Seq) after the induction of morphine tolerance and followed by naloxone precipitated withdrawal (n=16). We validated the RNA-Seq data by combining fluorescent in-situ hybridization with immunohistochemistry for microglia (n=18). Finally, we expressed and activated the Gi/o-coupled hM4Di DREADD receptor in CX3CR1-expressing cells during morphine withdrawal (n=18).

Results

We detected large, inverse changes in RNA translation following opioid tolerance and withdrawal. WGCNA analysis revealed an intriguing network of cAMP-associated genes that are known to be involved in microglial motility, morphology, and interactions with neurons that were downregulated with morphine tolerance and upregulated rapidly by withdrawal. Three-dimensional histological reconstruction of microglia allowed for volumetric, visual colocalization of mRNA within individual microglia that validated our bioinformatics results. Direct activation of Gi/o-coupled DREADD receptors in CX3CR1-expressing cells exacerbated signs of opioid withdrawal rather than mimicking the effects of morphine.

Conclusions

These results indicate that Gi-signaling and cAMP-associated gene networks are inversely engaged during opioid tolerance and early withdrawal, perhaps revealing a role of microglia in mitigating the consequences of opioids.

Src family kinases activity is required for transmitting purinergic P2X7 receptor signaling in cortical spreading depression and neuroinflammation

BACKGROUND

Purinergic P2X7 receptor plays an important role in migraine pathophysiology. Yet precise molecular mechanism underlying P2X7R signaling in migraine remains unclear. This study explores the hypothesis that P2X7 receptor transmits signaling to Src family kinases (SFKs) during cortical spreading depression (CSD) and neuroinflammation after CSD.

METHODS

CSD was recorded using electrophysiology in rats and intrinsic optical imaging in mouse brain slices. Cortical IL-1β and TNFα mRNA levels were detected using qPCR. Glutamate release from mouse brain slices was detected using glutamate assay.

RESULTS

The data showed that deactivation of SFKs by systemic injection of PP2 reduced cortical susceptibility to CSD in rats and CSD-induced IL-1β and TNF-α gene expression in rat ipsilateral cortices. Consistently, in mouse brain slices, inhibition of SFKs activity by saracatinib and P2X7 receptor by A740003 similarly reduced cortical susceptibility to CSD. When the interaction of P2X7 receptor and SFKs was disrupted by TAT-P2X7, a marked reduction of cortical susceptibility to CSD, IL-1β gene expression and glutamate release after CSD induction were observed in mouse brain slices. The reduced cortical susceptibility to CSD by TAT-P2X7 was restored by NMDA, and disrupting the Fyn-NMDA interaction using TAT-Fyn (39-57) but not disrupting Src-NMDA receptor interaction using TAT-Src (40-49) reduced cortical susceptibility to CSD. Furthermore, activation of P2X7 receptor by BzATP restored the TAT-Fyn (39-57)-reduced cortical susceptibility to CSD.

CONCLUSION

This study reveals that SFKs activity transmits P2X7 receptor signaling to facilitate CSD propagation via glutamatergic pathway and promote neuroinflammation, which is of particular relevance to migraine.

Involvement of P2X7 receptors in chronic pain disorders

Chronic pain is caused by cellular damage with an obligatory inflammatory component. In response to noxious stimuli, high levels of ATP leave according to their concentration gradient, the intracellular space through discontinuities generated in the plasma membrane or diffusion through pannexin-1 hemichannels, and activate P2X7Rs localized at peripheral and central immune cells. Because of the involvement of P2X7Rs in immune functions and especially the initiation of macrophage/microglial and astrocytic secretion of cytokines, chemokines, prostaglandins, proteases, reactive oxygen, and nitrogen species as well as the excitotoxic glutamate/ATP, this receptor type has a key role in chronic pain processes. Microglia are equipped with a battery of pattern recognition receptors that detect pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS) from bacterial infections or danger associated molecular patterns (DAMPs) such as ATP. The co-stimulation of these receptors leads to the activation of the NLRP3 inflammasome and interleukin-1β (IL-1β) release. In the present review, we invite you to a journey through inflammatory and neuropathic pain, primary headache, and regulation of morphine analgesic tolerance, in the pathophysiology of which P2X7Rs are centrally involved. P2X7R bearing microglia and astrocyte-like cells playing eminent roles in chronic pain will be also discussed.

Opioid Modulation of the Gut-Brain Axis in Opioid-Associated Comorbidities

Growing evidence from animal and human studies show that opioids have a major impact on the composition and function of gut microbiota. This leads to disruption in gut permeability and altered microbial metabolites, driving both systemic and neuroinflammation, which in turn impacts central nervous system (CNS) homeostasis. Tolerance and dependence are the major comorbidities associated with prolonged opioid use. Inflammatory mediators and signaling pathways have been implicated in both opioid tolerance and dependence. We provide evidence that targeting the gut microbiome during opioid use through prebiotics, probiotics, antibiotics, and fecal microbial transplantation holds the greatest promise for novel treatments for opioid abuse. Basic research and clinical trials are required to examine what is more efficacious to yield new insights into the role of the gut-brain axis in opioid abuse.

Repeated Endomorphin Analogue MEL-0614 Reduces Tolerance and Improves Chronic Postoperative Pain without Modulating the P2X7R Signaling Pathway

The clinical treatment of chronic postoperative pain (CPSP) remains challenging. The side effects of chronic morphine treatment limit its clinical application. MEL-0614, a novel endomorphin analogue that is highly selective and agonistic for μ opioid receptor (MOR), produces a more powerful analgesic effect than that of morphine. In this study, we explored the difference in antinociceptive tolerance and related mechanisms between MEL-0614 and morphine in CPSP induced in a skin/muscle incision and retraction (SMIR) mice model. We found that acute administration of MEL-0614 (1, 3, 5, and 10 nmol, i.t.) produced a dose-dependent analgesic effect that was superior to that of morphine in the SMIR mice model. Long-term MEL-0614 treatment (10 nmol, i.t.) did not induce tolerance compared with morphine. Notably, tolerance induced by morphine could be greatly prevented and/or inhibited via cross-administration or coadministration between MEL-0614 and morphine. In addition, MEL-0614 accelerated the recovery of postoperative pain, whereas morphine aggravated postoperative pain and prolonged its recovery time regardless of preoperative or postoperative treatment. In addition, MEL-0614 did not activate microglia and the P2X7R signaling pathway and showed reduced expression iba1 and P2X7R compared with that observed after morphine administration. Release of inflammatory factors was induced by continued administration of morphine during SMIR surgery, but MEL-0614 did not promote the activation of inflammatory factors. Our results showed that MEL-0614 has superior analgesic effects in CPSP and leads to tolerance to a lesser degree than morphine. Further, MEL-0614 may be used as a promising treatment option for the long-term treatment in CPSP.

The effects of anaesthetics and sedatives on brain inflammation

Microglia are involved in many dynamic processes in the central nervous system (CNS) including the development of inflammatory processes and neuromodulation. Several sedative, analgesic or anaesthetic drugs, such as opioids, ∝2-adrenergic agonists, ketamine, benzodiazepines and propofol can cause both neuroprotective and harmful effects on the brain. The purpose of this review is to present the main findings on the use of these drugs and the mechanisms involved in microglial activation. Alpha 2-adrenergic agonists, propofol and benzodiazepines have several pro- or anti-inflammatory effects on microglia. Long-term use of benzodiazepines and propofol causes neuroapoptotic effects and α2-adrenergic agonists may attenuate these effects. Conversely, morphine and fentanyl may have proinflammatory effects, causing behavioural changes in patients and changes in cell viability in vitro. Conversely, chronic administration of morphine induces CCL5 chemokine expression in microglial cells that promotes their survival.

Spinal A3 adenosine receptor activation acutely restores morphine antinociception in opioid tolerant male rats

Opioids are potent analgesics, but their pain-relieving effects diminish with repeated use. The reduction in analgesic potency is a hallmark of opioid analgesic tolerance, which hampers opioid pain therapy. In the central nervous system, opioid analgesia is critically modulated by adenosine, a purine nucleoside implicated in the beneficial and detrimental actions of opioid medications. Here, we focus on the A₃ adenosine receptor (A₃ AR) in opioid analgesic tolerance. Intrathecal administration of the A₃ AR agonist MRS5698 with daily systemic morphine in male rats attenuated the reduction in morphine antinociception over 7 days. In rats with established morphine tolerance, intrathecal MRS5698 partially restored the antinociceptive effects of morphine. However, when MRS5698 was discontinued, these animals displayed a reduced antinociceptive response to morphine. Our results suggest that MRS5698 acutely and transiently potentiates morphine antinociception in tolerant rats. By contrast, in morphine-naïve rats MRS5698 treatment did not impact thermal nociceptive threshold or affect antinociceptive response to a single injection of morphine. Furthermore, we found that morphine-induced adenosine release in cerebrospinal fluid was blunted in tolerant animals, but total spinal A₃ AR expression was not affected. Collectively, our findings indicate that spinal A₃ AR activation acutely potentiates morphine antinociception in the opioid tolerant state.

Antagonism of the ATP-gated P2X7 receptor: a potential therapeutic strategy for cancer

The P2X receptor 7 (P2X7R) is a plasma membrane receptor sensing extracellular ATP associated with a wide variety of cellular functions. It is most commonly expressed on immune cells and is highly upregulated in a number of human cancers where it can play a trophic role in tumorigenesis. Activation of this receptor leads to the formation of a non-selective cation channel, which has been associated with several cellular functions mediated by the PI3K/Akt pathway and protein kinases. Due to its broad range of functions, the receptor represents a potential therapeutic target for a number of cancers. This review describes the range of mechanisms associated with P2X7R activation in cancer settings and highlights the potential of targeted inhibition of P2X7R as a therapy. It also describes in detail a number of key P2X7R antagonists currently in pre-clinical and clinical development, including oxidised ATP, Brilliant Blue G (BBG), KN-62, KN-04, A740003, A438079, GSK1482160, CE-224535, JNJ-54175446, JNJ-55308942, and AZ10606120. Lastly, it summarises the in vivo studies and clinical trials associated with the use and development of these P2X7R antagonists in different disease contexts.

Can Src protein tyrosine kinase inhibitors be combined with opioid analgesics? Src and opioid-induced tolerance, hyperalgesia and addiction

The clinical application of opioids may be accompanied by a series of adverse consequences, such as opioid tolerance, opioid-induced hyperalgesia, opioid dependence or addiction. In view of this issue, clinicians are faced with the dilemma of treating various types of pain with or without opioids. In this review, we discuss that Src protein tyrosine kinase plays an important role in these adverse consequences, and Src inhibitors can solve these problems well. Therefore, Src inhibitors have the potential to be used in combination with opioids to achieve synergy. How to combine them together to maximize the analgesic effect while avoiding unnecessary trouble provides a topic for follow-up research.

Decreased Level of Exosomal miR-5121 Released from Microglia Suppresses Neurite Outgrowth and Synapse Recovery of Neurons Following Traumatic Brain Injury

Activated microglia can suppress neurite outgrowth and synapse recovery in the acute stage following traumatic brain injury (TBI). However, the underlying mechanism has not been clearly elucidated. Exosomes derived from microglia have been reported to play a critical role in microglia-neuron interaction in healthy and pathological brains. Here, we aimed to investigate the role of microglia-derived exosomes in regulating neurite outgrowth and synapse recovery following TBI. In our study, exosomes derived from microglia were co-cultured with stretch-injured neurons in vitro and intravenously injected into mice that underwent fluid percussion injury (FPI) by tail vein injection in vivo. The results showed that microglia-derived exosomes could be absorbed by neurons in vitro and in vivo. Moreover, exosomes derived from stretch-injured microglia decreased the protein levels of GAP43, PSD-95, GluR1, and Synaptophysin and dendritic complexity in stretch-injured neurons in vitro, and reduced GAP43+ NEUN cell percentage and apical dendritic spine density in the pericontusion region in vivo. Motor coordination was also impaired in mice treated with stretch-injured microglia-derived exosomes after FPI. A microRNA microarray showed that the level of miR-5121 was decreased most greatly in exosomes derived from stretch-injured microglia. Overexpression of miR-5121 in stretch-injured microglia-derived exosomes partly reversed the suppression of neurite outgrowth and synapse recovery of neurons both in vitro and in vivo. Moreover, motor coordination in miR-5121 overexpressed exosomes treated mice was significantly improved after FPI. Following mechanistic study demonstrated that miR-5121 might promote neurite outgrowth and synapse recovery by directly targeting RGMa. In conclusion, our finding revealed a novel exosome-mediated mechanism of microglia-neuron interaction that suppressed neurite outgrowth and synapse recovery of neurons following TBI.

The role of microglia in chronic pain and depression: innocent bystander or culprit?

Clinical evidence shows that chronic pain and depression often accompany each other, but the underlying pathogenesis of comorbid chronic pain and depression remains mostly undetermined. Biotechnology is gradually revealing the phenotype and function of microglia, with great progress regarding microglia's role in neurodegeneration, depression, chronic pain, and other conditions. This article summarizes the role of microglia in chronic pain, depression, and comorbidities, which is conducive to finding new targets to treat chronic pain and depression.

Spinal TLR4/P2X7 Receptor-Dependent NLRP3 Inflammasome Activation Contributes to the Development of Tolerance to Morphine-Induced Antinociception

Background

Long-term use of morphine induces antinociceptive tolerance and limits its clinical efficacy. Neuroinflammation in the spinal cord is thought to play a pivotal role in the development of morphine tolerance. Toll-like receptor 4 (TLR4) and P2X7 receptor (P2X7R) are key modulators of neuroinflammation. Recent studies show that the Nod-like receptor protein 3 (NLRP3) inflammasome play a crucial role in microglia-mediated neuroinflammation. Thus far, the mechanism underlying NLRP3 inflammasome activation during morphine-induced tolerance is not yet fully understood. Therefore, we sought to investigate the mechanisms of NLRP3 inflammasome activation and its role in the development of morphine-induced tolerance.

Methods

Repeated morphine treatment through intrathecal injection (15 μg once daily for 7 days) was given to establish antinociceptive tolerance in mice. Tail-flick latency was used to evaluate morphine-induced antinociception. NLRP3 knockout mice were used to assess the role of NLRP3 inflammasome in morphine tolerance. TLR4 knockout mice and A438079, a P2X7R antagonist, were used to assess the role of TLR4 and P2X7R in chronic morphine-induced NLRP3 inflammasome activation. Western blot and immunofluorescence were used for quantitative comparison.

Results

Repeated morphine treatment increased the expression of NLRP3. Knockout of NLRP3 attenuated morphine-induced tolerance and suppressed morphine-induced activation of microglia. Knockout of TLR4 alleviated morphine tolerance and chronic morphine-induced upregulation of spinal NLRP3. Inhibition of spinal P2X7R with A438079 not only prevented the development of morphine-induced tolerance but also inhibited repeated morphine treatment-induced upregulation of spinal NLRP3. Furthermore, spinal NLRP3, TLR4 and P2X7R were collectively colocalized with the microglia marker Iba1.

Conclusion

This study demonstrates that the NLRP3 inflammasome in microglia plays a crucial role in morphine tolerance and that both TLR4- and P2X7R-dependent pathways are required for NLRP3 inflammasome activation over the course of the development of morphine-induced tolerance. Our results provide a new perspective for the targeted treatment of morphine-induced tolerance.

Fundamental sex differences in morphine withdrawal-induced neuronal plasticity

ABSTRACT

Withdrawal from systemic opioids can induce long-term potentiation (LTP) at spinal C-fibre synapses ("opioid-withdrawal-LTP"). This is considered to be a cellular mechanism underlying opioid withdrawal-induced hyperalgesia, which is a major symptom of the opioid withdrawal syndrome. Opioids can activate glial cells leading to the release of proinflammatory mediators. These may influence synaptic plasticity and could thus contribute to opioid-withdrawal-LTP. Here, we report a sexual dimorphism in the mechanisms of morphine-withdrawal-LTP in adult rats. We recorded C-fibre-evoked field potentials in the spinal cord dorsal horn from deeply anaesthetised male and female rats. In both sexes, we induced a robust LTP through withdrawal from systemic morphine infusion (8 mg·kg-1 bolus, followed by a 1-hour infusion at a rate of 14 mg·kg-1·h-1). This paradigm also induced mechanical hypersensitivity of similar magnitude in both sexes. In male rats, systemic but not spinal application of (-)naloxone blocked the induction of morphine-withdrawal-LTP, suggesting the involvement of descending pronociceptive pathways. Furthermore, we showed that in male rats, the induction of morphine-withdrawal-LTP required the activation of spinal astrocytes and the release of the proinflammatory cytokines tumour necrosis factor and interleukin-1. In striking contrast, in female rats, the induction of morphine-withdrawal-LTP was independent of spinal glial cells. Instead, blocking µ-opioid receptors in the spinal cord was sufficient to prevent a facilitation of synaptic strength. Our study revealed fundamental sex differences in the mechanisms underlying morphine-withdrawal-LTP at C-fibre synapses: supraspinal and gliogenic mechanisms in males and a spinal, glial cell-independent mechanism in females.

Withdrawal from an opioid induces a transferable memory trace in the cerebrospinal fluid

Opioids are the most powerful analgesics available to date. However, they may also induce adverse effects including paradoxical opioid-induced hyperalgesia. A mechanism that might underlie opioid-induced hyperalgesia is the amplification of synaptic strength at spinal C-fibre synapses after withdrawal from systemic opioids such as remifentanil ("opioid-withdrawal long-term potentiation [LTP]"). Here, we show that both the induction as well as the maintenance of opioid-withdrawal LTP were abolished by pharmacological blockade of spinal glial cells. By contrast, the blockade of TLR4 had no effect on the induction of opioid-withdrawal LTP. D-serine, which may be released upon glial cell activation, was necessary for withdrawal LTP. D-serine is the dominant coagonist for neuronal NMDA receptors, which are required for the amplification of synaptic strength on remifentanil withdrawal. Unexpectedly, opioid-withdrawal LTP was transferable through the cerebrospinal fluid between animals. This suggests that glial-cell-derived mediators accumulate in the extracellular space and reach the cerebrospinal fluid at biologically active concentrations, thereby creating a soluble memory trace that is transferable to another animal ("transfer LTP"). When we enzymatically degraded D-serine in the superfusate, LTP could no longer be transferred. Transfer LTP was insensitive to pharmacological blockade of glial cells in the recipient animal, thus representing a rare form of glial cell-independent LTP in the spinal cord.

P2X7 Receptors Amplify CNS Damage in Neurodegenerative Diseases

ATP is a (co)transmitter and signaling molecule in the CNS. It acts at a multitude of ligand-gated cationic channels termed P2X to induce rapid depolarization of the cell membrane. Within this receptor-channel family, the P2X7 receptor (R) allows the transmembrane fluxes of Na⁺, Ca²⁺, and K⁺, but also allows the slow permeation of larger organic molecules. This is supposed to cause necrosis by excessive Ca²⁺ influx, as well as depletion of intracellular ions and metabolites. Cell death may also occur by apoptosis due to the activation of the caspase enzymatic cascade. Because P2X7Rs are localized in the CNS preferentially on microglia, but also at a lower density on neuroglia (astrocytes, oligodendrocytes) the stimulation of this receptor leads to the release of neurodegeneration-inducing bioactive molecules such as pro-inflammatory cytokines, chemokines, proteases, reactive oxygen and nitrogen molecules, and the excitotoxic glutamate/ATP. Various neurodegenerative reactions of the brain/spinal cord following acute harmful events (mechanical CNS damage, ischemia, status epilepticus) or chronic neurodegenerative diseases (neuropathic pain, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, amyotrophic lateral sclerosis) lead to a massive release of ATP via the leaky plasma membrane of neural tissue. This causes cellular damage superimposed on the original consequences of neurodegeneration. Hence, blood-brain-barrier permeable pharmacological antagonists of P2X7Rs with excellent bioavailability are possible therapeutic agents for these diseases. The aim of this review article is to summarize our present state of knowledge on the involvement of P2X7R-mediated events in neurodegenerative illnesses endangering especially the life quality and duration of the aged human population.

Chronic morphine induces cyclic adenosine monophosphate formation and hyperpolarization-activated cyclic nucleotide-gated channel expression in the spinal cord of mice

Chronic morphine exposure persistently activates Gαi/o protein-coupled receptors and enhances adenylyl cyclase (AC) activity, which can increase cyclic adenosine monophosphate (cAMP) production. Direct binding of cAMP to the cytoplasmic site on hyperpolarization-activated cyclic nucleotide-gated (HCN) channels increases the probability of channel opening. HCN channels play a prominent role in chronic pain the disease that shares some common mechanisms with opioid tolerance. This compensatory AC activation may be responsible for the induction of morphine-induced analgesic tolerance. We investigated spinal cAMP formation and expression of HCN2 in the spinal cord, and observed the effect of AC inhibition on the induction of morphine analgesic tolerance. We found that chronic morphine-induced antinociceptive tolerance increased spinal cAMP formation and the expression of spinal HCN2. Inhibition of spinal AC partially blocked chronic morphine-induced cAMP formation and prevented the induction of morphine-induced analgesic tolerance. Inhibition of HCN2 also showed a partial preventive effect on morphine-induced tolerance, hypothermia tolerance and also the right-shift of the dose-response curve. We conclude that repeated morphine treatment increases AC activity and cAMP formation, and also spinal HCN2 expression, blockade of AC or HCN2 can prevent the development of morphine-induced analgesic tolerance.

Inhibition of leucine-rich repeats and calponin homology domain containing 1 accelerates microglia-mediated neuroinflammation in a rat traumatic spinal cord injury model

BACKGROUND

Spinal cord injury (SCI) triggers the primary mechanical injury and secondary inflammation-mediated injury. Neuroinflammation-mediated insult causes secondary and extensive neurological damage after SCI. Microglia play a pivotal role in the initiation and progression of post-SCI neuroinflammation.

METHODS

To elucidate the significance of LRCH1 to microglial functions, we applied lentivirus-induced LRCH1 knockdown in primary microglia culture and tested the role of LRCH1 in microglia-mediated inflammatory reaction both in vitro and in a rat SCI model.

RESULTS

We found that LRCH1 was downregulated in microglia after traumatic SCI. LRCH1 knockdown increased the production of pro-inflammatory cytokines such as IL-1β, TNF-α, and IL-6 after in vitro priming with lipopolysaccharide and adenosine triphosphate. Furthermore, LRCH1 knockdown promoted the priming-induced microglial polarization towards the pro-inflammatory inducible nitric oxide synthase (iNOS)-expressing microglia. LRCH1 knockdown also enhanced microglia-mediated N27 neuron death after priming. Further analysis revealed that LRCH1 knockdown increased priming-induced activation of p38 mitogen-activated protein kinase (MAPK) and Erk1/2 signaling, which are crucial to the inflammatory response of microglia. When LRCH1-knockdown microglia were adoptively injected into rat spinal cords, they enhanced post-SCI production of pro-inflammatory cytokines, increased SCI-induced recruitment of leukocytes, aggravated SCI-induced tissue damage and neuronal death, and worsened the locomotor function.

CONCLUSION

Our study reveals for the first time that LRCH1 serves as a negative regulator of microglia-mediated neuroinflammation after SCI and provides clues for developing novel therapeutic approaches against SCI.

The "Culture" of Pain Control: A Review of Opioid-Induced Dysbiosis (OID) in Antinociceptive Tolerance

It is increasingly recognized that chronic opioid use leads to maladaptive changes in the composition and localization of gut bacteria. Recently, this "opioid-induced dysbiosis" (OID) has been linked to antinociceptive tolerance development in preclinical models and may therefore identify promising targets for new opioid-sparing strategies. Such developments are critical to curb dose escalations in the clinical setting and combat the ongoing opioid epidemic. In this article, we review the existing literature that pertains to OID, including the current evidence regarding its qualitative nature, influence on antinociceptive tolerance, and future prospects. PERSPECTIVE: This article reviews the current literature on OID of gut bacteria, including its qualitative nature, influence on antinociceptive tolerance, and future prospects. This work may help identify targets for new opioid-sparing strategies.

Morphine and HIV-1 gp120 cooperatively promote pathogenesis in the spinal pain neural circuit

Opioids are common analgesics for pain relief in HIV patients. Ironically, emerging clinical data indicate that repeated use of opioid analgesics in fact leads to a heightened chronic pain state. To understand the underlying pathogenic mechanism, we generated a mouse model to study the interactive effect of morphine and HIV-1 gp120 on pain pathogenesis. We simulated chronic pain in the model by showing that repeated morphine administrations potentiated HIV-1 intrathecal gp120-induced pain. Several spinal cellular and molecular pathologies that are implicated in the development of HIV-associated pain are exacerbated by morphine, including astroglial activation, pro-inflammatory cytokine expression and Wnt5a signaling. We further demonstrated that inhibition of Wnt5a not only reversed the glial activation and cytokine upregulation but also the exacerbation of gp120-induced pain. These studies establish a mouse model for the opioid exacerbation of HIV-associated pain and reveal potential cellular and molecular mechanisms by which morphine enhances the pain.

Microglia-Mediated Regulation of Neuropathic Pain: Molecular and Cellular Mechanisms

Pain is a defense system that responds rapidly to harmful internal and external stimuli through the somatosensory neuronal pathway. However, damage to the nervous system through cancer, diabetes, infection, autoimmune disease, chemotherapy or trauma often leads to neuropathic pain, a debilitating chronic pain condition. Neuropathic pain is not simply a temporal continuum of acute nociceptive signals from the periphery, but rather due to pathologically altered functions in the nervous system, which shift the net neuronal excitatory balance toward excitation. Although alterations were long thought to be a result of changes in neurons, but an increasing body of evidence over the past decades indicates the necessity and sufficiency of microglia, the tissue-resident macrophages of the spinal cord and brain, for nerve injury-induced malfunction of the nervous system. In this review article, I describe our current understanding of the molecular and cellular mechanisms underlying the role of microglia in the pathogenesis of neuropathic pain and discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia.

Mu opioid receptor in microglia contributes to morphine analgesic tolerance, hyperalgesia, and withdrawal in mice

A major challenge in medicine is developing potent pain therapies without the adverse effects of opiates. Neuroinflammation and in particular microglial activation have been shown to contribute to these effects. However, the implication of the microglial mu opioid receptor (MOR) is not known. We developed a novel conditional knockout (cKO) mouse line, wherein MOR is deleted in microglia. Morphine analgesic tolerance was delayed in both sexes in cKO mice in the hot plate assay. Opioid-induced hyperalgesia (OIH) as measured in the tail immersion assay was abolished in male cKO mice, and physical dependence to morphine as assessed by naloxone-induced withdrawal was attenuated in female cKO mice. Our results show a sex-dependent contribution of microglial MOR in morphine analgesic tolerance, OIH, and physical dependence. In conclusion, our data suggest that blockade of microglial MOR could represent a therapeutic target for opiate analgesia without the opiate adverse effects.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Pellino1 Contributes to Morphine Tolerance by Microglia Activation via MAPK Signaling in the Spinal Cord of Mice

Chronic morphine-induced antinociceptive tolerance is a major unresolved issue in clinical practices, which is associated with microglia activation in the spinal cord. E3 ubiquitin ligase Pellino1 (Peli1) is known to be an important microglia-specific regulator. However, it is unclear whether Peli1 is involved in morphine tolerance. Here, we found that Peli1 levels in the spinal cord were significantly elevated in morphine tolerance mouse model. Notably, Peli1 was expressed in a great majority of microglia in the spinal dorsal horn, while downregulation of spinal Peli1 attenuated the development of morphine tolerance and associated hyperalgesia. Our biochemical data revealed that morphine tolerance-induced increase in Peli1 was accompanied by spinal microglia activation, activation of mitogen-activated protein kinase (MAPK) signaling, and production of proinflammatory cytokines. Peli1 additionally was found to promote K63-linked ubiquitination of tumor necrosis factor receptor-associated factor 6 (TRAF6) in the spinal cord after repeated morphine treatment. Furthermore, knocking down Peli1 in cultured BV2 microglial cells significantly attenuated inflammatory reactions in response to morphine challenge. Therefore, we conclude that the upregulation of Peli1 in the spinal cord plays a curial role in the development of morphine tolerance via Peli1-dependent mobilization of spinal microglia, activation of MAPK signaling, and production of proinflammatory cytokines. Modulation of Peli1 may be a potential strategy for the prevention of morphine tolerance.

Repeated Morphine Prolongs Postoperative Pain in Male Rats

BACKGROUND

Opioids are effective postoperative analgesics. Disturbingly, we have previously reported that opioids such as morphine can worsen inflammatory pain and peripheral and central neuropathic pain. These deleterious effects are mediated by immune mediators that promote neuronal hyperexcitability in the spinal dorsal horn. Herein, we tested whether perioperative morphine could similarly prolong postoperative pain in male rats.

METHODS

Rats were treated with morphine for 7 days, beginning immediately after laparotomy, while the morphine was tapered in a second group. Expression of genes for inflammatory mediators was quantified in the spinal dorsal horn. In the final experiment, morphine was administered before laparotomy for 7 days.

RESULTS

We found that morphine treatment after laparotomy extended postoperative pain by more than 3 weeks (time × treatment: P < .001; time: P < .001; treatment: P < .05). Extension of postoperative pain was not related to morphine withdrawal, as it was not prevented by dose tapering (time × treatment: P = .8; time: P < .001; treatment: P = .9). Prolonged postsurgical pain was associated with increased expression of inflammatory genes, including those encoding Toll-like receptor 4, NOD like receptor protein 3 (NLRP3), nuclear factor kappa B (NFκB), caspase-1, interleukin-1β, and tumor necrosis factor (P < .05). Finally, we showed that of preoperative morphine, concluding immediately before laparotomy, similarly prolonged postoperative pain (time × treatment: P < .001; time: P < .001; treatment: P < .001). There is a critical window for morphine potentiation of pain, as a 7-day course of morphine that concluded 1 week before laparotomy did not prolong postsurgical pain.

CONCLUSIONS

These studies indicate the morphine can have a deleterious effect on postoperative pain. These studies further suggest that longitudinal studies could be performed to test whether opioids similarly prolong postoperative pain in the clinic.

Microglia in Pain: Detrimental and Protective Roles in Pathogenesis and Resolution of Pain

The previous decade has seen a rapid increase in microglial studies on pain, with a unique focus on microgliosis in the spinal cord after nerve injury and neuropathic pain. Numerous signaling molecules are altered in microglia and contribute to the pathogenesis of pain. Here, we discuss how microglial signaling regulates spinal cord synaptic plasticity in acute and chronic pain conditions with different degrees and variations of microgliosis. We highlight that microglial mediators such as pro- and anti-inflammatory cytokines are powerful neuromodulators that regulate synaptic transmission and pain via neuron-glial interactions. We also reveal an emerging role of microglia in the resolution of pain, in part via specialized pro-resolving mediators including resolvins, protectins, and maresins. We also discuss a possible role of microglia in chronic itch.

P2X7 Interactions and Signaling - Making Head or Tail of It

Extracellular adenine nucleotides play important roles in cell-cell communication and tissue homeostasis. High concentrations of extracellular ATP released by dying cells are sensed as a danger signal by the P2X7 receptor, a non-specific cation channel. Studies in P2X7 knockout mice and numerous disease models have demonstrated an important role of this receptor in inflammatory processes. P2X7 activation has been shown to induce a variety of cellular responses that are not usually associated with ion channel function, for example changes in the plasma membrane composition and morphology, ectodomain shedding, activation of lipases, kinases, and transcription factors, as well as cytokine release and apoptosis. In contrast to all other P2X family members, the P2X7 receptor contains a long intracellular C-terminus that constitutes 40% of the whole protein and is considered essential for most of these effects. So far, over 50 different proteins have been identified to physically interact with the P2X7 receptor. However, few of these interactions have been confirmed in independent studies and for the majority of these proteins, the interaction domains and the physiological consequences of the interactions are only poorly described. Also, while the structure of the P2X7 extracellular domain has recently been resolved, information about the organization and structure of its C-terminal tail remains elusive. After shortly describing the structure and assembly of the P2X7 receptor, this review gives an update of the identified or proposed interaction domains within the P2X7 C-terminus, describes signaling pathways in which this receptor has been involved, and provides an overlook of the identified interaction partners.

Microglial alterations induced by psychoactive drugs: A possible mechanism in substance use disorder?

Recently, the xenobiotic hypothesis has implicated the immune system in targeting substances of abuse as foreign molecules and stimulating inflammatory responses. Microglial cells are the resident immune cells of the central nervous system and function in homeostatic surveillance. Microglial changes that are induced by exposure to substances of abuse appear to mediate in part the establishment of addiction and the persistence of drug-mediated biological and behavioral changes. In this context, interest in the study of drug-microglia interactions has increased recently. This review summarizes the most recent preclinical rodent and clinical studies on the interaction between microglia and various classes of drugs of abuse, such as ethanol, psychostimulants, and opioids. The principal biological mechanisms of the communication between substances of abuse and microglia will be described to consider putative mechanisms of the establishment of drug addiction and future potential targets for treating substance use disorder.

Perioperative opioid analgesia-when is enough too much? A review of opioid-induced tolerance and hyperalgesia

Opioids are a mainstay of acute pain management but can have many adverse effects, contributing to problematic long-term use. Opioid tolerance (increased dose needed for analgesia) and opioid-induced hyperalgesia (paradoxical increase in pain with opioid administration) can contribute to both poorly controlled pain and dose escalation. Hyperalgesia is particularly problematic as further opioid prescribing is largely futile. The mechanisms of opioid tolerance and hyperalgesia are complex, involving μ opioid receptor signalling pathways that offer opportunities for novel analgesic alternatives. The intracellular scaffold protein β-arrestin-2 is implicated in tolerance, hyperalgesia, and other opioid side-effects. Development of agonists biased against recruitment of β-arrestin-2 could provide analgesic efficacy with fewer side-effects. Alternative approaches include inhibition of peripheral μ opioid receptors and blockade of downstream signalling mechanisms, such as the non-receptor tyrosine kinase Src or N-methyl-D-aspartate receptors. Furthermore, it is prudent to use multimodal analgesic regimens to reduce reliance on opioids during the perioperative period. In the third paper in this Series we focus on clinical and mechanism-based understanding of tolerance and opioid-induced hyperalgesia, and discuss current and future strategies for pain management.

TRPV1 promotes opioid analgesia during inflammation

Pain and inflammation are inherently linked responses to injury, infection, or chronic diseases. Given that acute inflammation in humans or mice enhances the analgesic properties of opioids, there is much interest in determining the inflammatory transducers that prime opioid receptor signaling in primary afferent nociceptors. Here, we found that activation of the transient receptor potential vanilloid type 1 (TRPV1) channel stimulated a mitogen-activated protein kinase (MAPK) signaling pathway that was accompanied by the shuttling of the scaffold protein β-arrestin2 to the nucleus. The nuclear translocation of β-arrestin2 in turn prevented its recruitment to the μ-opioid receptor (MOR), the subsequent internalization of agonist-bound MOR, and the suppression of MOR activity that occurs upon receptor desensitization. Using the complete Freund's adjuvant (CFA) inflammatory pain model to examine the role of TRPV1 in regulating endogenous opioid analgesia in mice, we found that naloxone methiodide (Nal-M), a peripherally restricted, nonselective, and competitive opioid receptor antagonist, slowed the recovery from CFA-induced hypersensitivity in wild-type, but not TRPV1-deficient, mice. Furthermore, we showed that inflammation prolonged morphine-induced antinociception in a mouse model of opioid receptor desensitization, a process that depended on TRPV1. Together, our data reveal a TRPV1-mediated signaling pathway that serves as an endogenous pain-resolution mechanism by promoting the nuclear translocation of β-arrestin2 to minimize MOR desensitization. This previously uncharacterized mechanism may underlie the peripheral opioid control of inflammatory pain. Dysregulation of the TRPV1-β-arrestin2 axis may thus contribute to the transition from acute to chronic pain.

ATP-Gated P2X7 Receptors Require Chloride Channels To Promote Inflammation in Human Macrophages

Immune cells of myeloid origin show robust expression of ATP-gated P2X7 receptors, two-transmembrane ion channels permeable to Na⁺, K⁺, and Ca²⁺ Receptor activation promotes inflammasome activation and release of the proinflammatory cytokines IL-1β and IL-18. In this study, we show that ATP generates facilitating cationic currents in monocyte-derived human macrophages and permeabilizes the plasma membrane to polyatomic cationic dyes. We find that antagonists of PLA₂ and Cl^- channels abolish P2X7 receptor-mediated current facilitation, membrane permeabilization, blebbing, phospholipid scrambling, inflammasome activation, and IL-1β release. Our data demonstrate significant differences in the actions of ATP in murine and human macrophages and suggest that PLA₂ and Cl^- channels mediate innate immunity downstream of P2X7 receptors in human macrophages.

Regulation of P2X7 receptor expression and function in the brain

Because of its prominent role in driving inflammatory processes, the ATP-gated purinergic P2X7 receptor has attracted much attention over the past decade as a potential therapeutic target for numerous human conditions, particularly diseases of the central nervous system, including neurodegenerative diseases (e.g. Alzheimer's and Huntington's disease), psychiatric disorders (e.g. schizophrenia and depression) and the neurological disease, epilepsy. Evidence stems from studies using experimental models and patient tissue showing changes in P2X7 expression and function under pathological conditions and beneficial effects provided by P2X7 antagonism. Apart from promoting neuroinflammation, P2X7, however, also impacts on other pathological processes in the brain, including cell death, hyperexcitability, changes in neurotransmitter release and neurogenesis. Reports also suggest a role for P2X7 in the maintenance of blood-brain-barrier integrity. It therefore comes as no surprise that the regulation of P2X7 expression and function is complex, providing tight control on P2X7 activation. Much progress has been made in understanding how P2X7 is regulated during physiological and pathological conditions and what the consequences are of pathological P2X7 expression and function. Regulatory mechanisms altering P2X7 expression include transcriptional and post-translational regulation including nucleotide polymorphisms, promoter regulation via DNA methylation, transcription factors (e.g. Sp1 and HIF-1α), the generation of different splice variants and receptor phosphorylation, glycosylation and palmitoylation. Finally, more recently, reports have also shown P2X7-targeting by microRNAs, blocking P2X7 translation into functional proteins. The present review provides a broad overview of what is known to-date about the complex regulation of P2X7 expression with a particular emphasis on the brain and how each of these regulatory mechanisms impacts on receptor function and pathology.

Neuroimmune signaling in alcohol use disorder

Alcohol use disorder (AUD) is a widespread disease with limited treatment options. Targeting the neuroimmune system is a new avenue for developing or repurposing effective pharmacotherapies. Alcohol modulates innate immune signaling in different cell types in the brain by altering gene expression and the molecular pathways that regulate neuroinflammation. Chronic alcohol abuse may cause an imbalance in neuroimmune function, resulting in prolonged perturbations in brain function. Likewise, manipulating the neuroimmune system may change alcohol-related behaviors. Psychiatric disorders that are comorbid with AUD, such as post-traumatic stress disorder, major depressive disorder, and other substance use disorders, may also have underlying neuroimmune mechanisms; current evidence suggests that convergent immune pathways may be involved in AUD and in these comorbid disorders. In this review, we provide an overview of major neuroimmune cell-types and pathways involved in mediating alcohol behaviors, discuss potential mechanisms of alcohol-induced neuroimmune activation, and present recent clinical evidence for candidate immune-related drugs to treat AUD.

Intrathecal delivery of a palmitoylated peptide targeting Y382-384 within the P2X7 receptor alleviates neuropathic pain

Pain hypersensitivity resulting from peripheral nerve injury depends on pathological microglial activation in the dorsal horn of the spinal cord. This microglial activity is critically modulated by P2X7 receptors (P2X7R) and ATP stimulation of these receptors produces mechanical allodynia, a defining feature of neuropathic pain. Peripheral nerve injury increases P2X7R expression and potentiates its cation channel function in spinal microglia. Here, we report a means to preferentially block the potentiation of P2X7R function by delivering a membrane permeant small interfering peptide that targets Y_382-384, a putative tyrosine phosphorylation site within the P2X7R intracellular C-terminal domain. Intrathecal administration of this palmitoylated peptide (P2X7R_379-389) transiently reversed mechanical allodynia caused by peripheral nerve injury in both male and female rats. Furthermore, targeting Y_382-384 suppressed P2X7R-mediated release of cytokine tumor necrosis factor alpha and blocked the adoptive transfer of mechanical allodynia caused by intrathecal injection of P2X7R-stimulated microglia. Thus, Y_382-384 site-specific modulation of P2X7R is an important microglial mechanism in neuropathic pain.

Lipopolysaccharide from Rhodobacter sphaeroides (TLR4 antagonist) attenuates hypersensitivity and modulates nociceptive factors

CONTEXT

Accumulating evidence has demonstrated that Toll-like receptors (TLRs), especially TLR4 localized on microglia/macrophages, may play a significant role in nociception.

OBJECTIVE

We examine the role of TLR4 in a neuropathic pain model. Using behavioural/biochemical methods, we examined the influence of TLR4 antagonist on levels of hypersensitivity and nociceptive factors whose contribution to neuropathy development has been confirmed.

MATERIALS AND METHODS

Behavioural (von Frey's/cold plate) tests were performed with Wistar male rats after intrathecal administration of a TLR4 antagonist (LPS-RS ULTRAPURE (LPS-RSU), 20 μG: lipopolysaccharide from Rhodobacter sphaeroides, InvivoGen, San Diego, CA) 16 H and 1 h before chronic constriction injury (cci) to the sciatic nerve and then daily for 7 d. three groups were used: an intact group and two cci-exposed groups that received vehicle or LPS-RSU. tissue [spinal cord/dorsal root ganglia (DRG)] for western blot analysis was collected on day 7.

RESULTS

The pharmacological blockade of TLR4 diminished mechanical (from ca. 40% to 16% that in the INTACT group) and thermal (from ca. 51% to 32% that in the INTACT group) hypersensitivity despite the enhanced activation of IBA-1-positive cells in DRG. Moreover, LPS-RSU changed the ratio between IL-18/IL-18BP and MMP-9/TIMP-1 in favour of the increase of antinociceptive factors IL-18BP (25%-spinal; 96%-DRG) and TIMP-1 (15%-spinal; 50%-DRG) and additionally led to an increased IL-6 (40%-spinal; 161%-DRG), which is known to have analgesic properties in neuropathy.

CONCLUSIONS

Our results provide evidence that LPS-RSU influences pain through the expression of TLR4. TLR4 blockade has analgesic properties and restores the balance between nociceptive factors, which indicates its engagement in neuropathy development.