A role for proinflammatory cytokines and fractalkine in analgesia, tolerance, and subsequent pain facilitation induced by chronic intrathecal morphine

Psilocybin reduces heroin seeking behavior and modulates inflammatory gene expression in the nucleus accumbens and prefrontal cortex of male rats

Preclinical and human studies indicate psilocybin may reduce perseverant maladaptive behaviors, including nicotine and alcohol seeking. Such studies in the opioid field are lacking, though opioids are involved in more >50% of overdose deaths. Psilocybin is an agonist at the serotonin 2A receptor (5-HT2AR), a well-documented target for modulation of drug seeking, and evidence suggests 5-HT2AR agonists may dampen motivation for opioids. We sought to investigate the therapeutic efficacy of psilocybin in mediating cessation of opioid use and maintenance of long-lasting abstinence from opioid seeking behavior in a rat model of heroin self-administration (SA). Psilocybin or 5-HT2AR antagonists ketanserin and volinanserin were administered systemically to rats prior to SA of 0.075 mg/kg/infusion of heroin, or relapse following forced abstinence. Psilocybin did not alter heroin taking, but a single exposure to 3.0 mg/kg psilocybin 4-24 hours prior to a relapse test blunted cue-induced heroin seeking. Conversely, 5-HT2AR antagonists exacerbated heroin relapse. To begin to elucidate mechanisms of psilocybin, drug-naïve rats received psilocybin and/or ketanserin, and tissue was collected from the prefrontal cortex (PFC), a region critical for drug seeking and responsive to psilocybin, 24 hours later for RNA-sequencing. 3.0 mg/kg psilocybin regulated ~2-fold more genes in the PFC than 1.0 mg/kg, including genes involved in the cytoskeleton and cytokine signaling. Ketanserin blocked >90% of psilocybin-regulated genes, including the IL-17a cytokine receptor, Il17ra. Psychedelic compounds have reported anti-inflammatory properties, and therefore we performed a gene expression array to measure chemokine/cytokine molecules in the PFC of animals that displayed psilocybin-mediated inhibition of heroin seeking. Psilocybin regulated 4 genes, including Il17a, and a subset of genes correlated with relapse behavior. Selective inhibition of PFC IL-17a was sufficient to reduce heroin relapse. We conclude that psilocybin reduces heroin relapse and highlight IL-17a signaling as a potential downstream pathway of psilocybin that also reduces heroin seeking.

Morphine-Driven m6A Epitranscriptomic Neuroadaptations in Primary Cortical Cultures

Opioid overdose is the leading cause of accidental death in the United States and remains a major public health concern, despite significant resources aimed at combating opioid misuse. Neurobiological research to elucidate molecular and cellular consequences of opioid exposure is required to define avenues to explore for reversal of opioid-induced neuroadaptations. Opioids impart well-documented regulation of the transcriptome and epigenetic modifications in the brain, but opioid-induced epitranscriptomic posttranscriptional regulation of RNA is vastly understudied. N6-methyladenosine (m6A) RNA methylation is significantly enriched in the brain and involved in learning, memory, and reward. m6A modifications have not been studied in opioid use disorder, despite being the most common RNA modification. We detected significant regulation of m6A-modifying enzymes in rat primary cortical cultures following morphine treatment, including AlkB Homolog 5 (Alkbh5). The m6a demethylase ALKBH5 functions as an m6A eraser, removing m6A modifications from mRNA. We hypothesized that chronic opioid treatment regulates m6A modifications through modulation of Alkbh5 and profiled m6A modifications in primary cortical cultures following chronic morphine treatment and Alkbh5 knock-down. We observed differential regulation of m6A modifications for a common set of transcripts following morphine or Alkbh5 knock-down, and the two treatments elicited concordant m6A epitranscriptomic profiles, suggesting that a subset of morphine-driven m6A modifications may be mediated through downregulation of Alkbh5 in cortical cultures. Gene Ontology terms of commonly regulated transcripts included serotonin secretion, synapse disassembly, neuron remodeling, and immune response. Thus, we conclude that morphine can drive epitranscriptomic changes, a subset of which may occur in an Alkbh5-dependent manner.

Emodin relieves morphine-stimulated BV2 microglial activation and inflammation through the TLR4/NF-κB/NLRP3 pathway

The objective of this study is to disclose the role of emodin, a natural anthraquinone derivative that has been proposed to suppress microglial activation and inflammation, in morphine tolerance. Here, cell counting kit-8 method assayed the viability of BV2 microglial cells treated by ascending concentrations of emodin. In emodin-pretreated BV2 microglial cells challenged with morphine with or without transfection of toll-like receptor 4 (TLR4) overexpression plasmids, transwell assay measured cell migration. Immunofluorescence staining and western blot detected the expression of microglial markers. Inflammatory levels were subjected to ELISA and western blot. BODIPY 581/591 C11 assay estimated lipid reactive oxygen species activity. Iron assay kit examined total iron content. Western blot tested the expression of ferroptosis- and TLR4/nuclear factor-kappaB (NF-κB)/NOD-like receptor 3 (NLRP3) pathway-associated proteins. Molecular docking predicted the binding affinity of emodin to TLR4. Emodin was noted to obstruct the migration, activation, inflammatory response, and ferroptosis of BV2 microglial cells induced by morphine. In addition, emodin had a high binding affinity with TLR4 and inactivated TLR4/NF-κB/NLRP3 pathway in morphine-challenged BV2 microglial cells. Upregulation of TLR4 partially countervailed the protective role of emodin against morphine-elicited BV2 microglial cell migration, activation, inflammation, and ferroptosis. Accordingly, emodin might target TLR4 and act as an inactivator of TLR4/NF-κB/NLRP3 pathway, thus inhibiting BV2 microglial activation and inflammation to mitigate morphine tolerance.

Promising immunomodulators for management of substance and alcohol use disorders

INTRODUCTION

The neuroimmune system has emerged as a novel target for the treatment of substance use disorders (SUDs), with immunomodulation producing encouraging therapeutic benefits in both preclinical and clinical settings.

AREAS COVERED

In this review, we describe the mechanism of action and immune response to methamphetamine, opioids, cocaine, and alcohol. We then discuss off-label use of immunomodulators as adjunctive therapeutics in the treatment of neuropsychiatric disorders, demonstrating their potential efficacy in affective and behavioral disorders. We then discuss in detail the mechanism of action and recent findings regarding the use of ibudilast, minocycline, probenecid, dexmedetomidine, pioglitazone, and cannabidiol to treat (SUDs). These immunomodulators are currently being investigated in clinical trials described herein, specifically for their potential to decrease substance use, withdrawal severity, central and peripheral inflammation, comorbid neuropsychiatric disorder symptomology, as well as their ability to improve cognitive outcomes.

EXPERT OPINION

We argue that although mixed, findings from recent preclinical and clinical studies underscore the potential benefit of immunomodulation in the treatment of the behavioral, cognitive, and inflammatory processes that underlie compulsive substance use.

Inhibition of spinal BRD4 alleviates pyroptosis and M1 microglia polarization via STING-IRF3 pathway in morphine-tolerant rats

BACKGROUND

Morphine tolerance has been a challenging medical issue. Neuroinflammation is considered as a critical mechanism for the development of morphine tolerance. Bromodomain-containing protein 4 (BRD4), a key regulator in cell damage and inflammation, participates in the development of chronic pain. However, whether BRD4 is involved in morphine tolerance and the underlying mechanisms remain unknown.

METHODS

The morphine-tolerant rat model was established by intrathecal administration of morphine twice daily for 7 days. Behavior test was assessed by a tail-flick latency test. The roles of BRD4, pyroptosis, microglia polarization and related signaling pathways in morphine tolerance were elucidated by Western blot, real-time quantitative polymerase chain reaction, and immunofluorescence.

RESULTS

Repeated morphine administration upregulated BRD4 level, induced pyroptosis, and promoted microglia M1-polarization in spinal cord, accompanied by the release of proinflammatory cytokines, such as TNF-α and IL-1β. JQ-1, a BRD4 antagonist, alleviated the development of morphine tolerance, diminished pyroptosis and induced the switch of microglia from M1 to M2 phenotype. Mechanistically, stimulator of interferon gene (STING)- interferon regulatory factor 3 (IRF3) pathway was activated and the protective effect of JQ-1 against morphine tolerance was at least partially mediated by inhibition of STING-IRF3 pathway.

CONCLUSION

This study demonstrated for the first time that spinal BRD4 contributes to pyroptosis and switch of microglia polarization via STING-IRF3 signaling pathway during the development of morphine tolerance, which extend the understanding of the neuroinflammation mechanism of morphine tolerance and provide an alternative strategy for the precaution against of this medical condition.

CC Chemokine Family Members' Modulation as a Novel Approach for Treating Central Nervous System and Peripheral Nervous System Injury-A Review of Clinical and Experimental Findings

Despite significant progress in modern medicine and pharmacology, damage to the nervous system with various etiologies still poses a challenge to doctors and scientists. Injuries lead to neuroimmunological changes in the central nervous system (CNS), which may result in both secondary damage and the development of tactile and thermal hypersensitivity. In our review, based on the analysis of many experimental and clinical studies, we indicate that the mechanisms occurring both at the level of the brain after direct damage and at the level of the spinal cord after peripheral nerve damage have a common immunological basis. This suggests that there are opportunities for similar pharmacological therapeutic interventions in the damage of various etiologies. Experimental data indicate that after CNS/PNS damage, the levels of 16 among the 28 CC-family chemokines, i.e., CCL1, CCL2, CCL3, CCL4, CCL5, CCL6, CCL7, CCL8, CCL9, CCL11, CCL12, CCL17, CCL19, CCL20, CCL21, and CCL22, increase in the brain and/or spinal cord and have strong proinflammatory and/or pronociceptive effects. According to the available literature data, further investigation is still needed for understanding the role of the remaining chemokines, especially six of them which were found in humans but not in mice/rats, i.e., CCL13, CCL14, CCL15, CCL16, CCL18, and CCL23. Over the past several years, the results of studies in which available pharmacological tools were used indicated that blocking individual receptors, e.g., CCR1 (J113863 and BX513), CCR2 (RS504393, CCX872, INCB3344, and AZ889), CCR3 (SB328437), CCR4 (C021 and AZD-2098), and CCR5 (maraviroc, AZD-5672, and TAK-220), has beneficial effects after damage to both the CNS and PNS. Recently, experimental data have proved that blockades exerted by double antagonists CCR1/3 (UCB 35625) and CCR2/5 (cenicriviroc) have very good anti-inflammatory and antinociceptive effects. In addition, both single (J113863, RS504393, SB328437, C021, and maraviroc) and dual (cenicriviroc) chemokine receptor antagonists enhanced the analgesic effect of opioid drugs. This review will display the evidence that a multidirectional strategy based on the modulation of neuronal-glial-immune interactions can significantly improve the health of patients after CNS and PNS damage by changing the activity of chemokines belonging to the CC family. Moreover, in the case of pain, the combined administration of such antagonists with opioid drugs could reduce therapeutic doses and minimize the risk of complications.

Amiloride alleviates morphine tolerance by suppressing ASIC3-dependent neuroinflammation in the spinal cord

BACKGROUND

The use of morphine in clinical medicine is severely constrained by tolerance. Therefore, it is essential to examine pharmacological therapies that suppress the development of morphine tolerance. Amiloride suppressed the expression of inflammatory cytokines by inhibiting microglial activation. Microglia play a crucial role in the establishment of morphine tolerance. Thus, we anticipated that amiloride might suppress the development of morphine tolerance. During this investigation, we assessed the impact of amiloride on mouse morphine tolerance.

METHODS

Mice received morphine (10 mg/kg, s.c.) twice daily with intrathecally injected amiloride (0.3 μg/5 μl, 1 μg/5 μl, and 3 μg/5 μl) for nine continuous days. To assess morphine tolerance, mice underwent the tail-flick and hot plate tests. BV-2 cells were used to investigate the mechanism of amiloride. By using Western blotting, real-time PCR, and immunofluorescence labeling methods, the levels of acid-sensing ion channels (ASICs), nuclear factor kappa B (NF-kB) p65, p38 mitogen-activated protein kinase (MAPK) proteins, and neuroinflammation-related cytokines were determined.

RESULTS

The levels of ASIC3 in the spinal cord were considerably increased after long-term morphine administration. Amiloride was found to delay the development of tolerance to chronic morphine assessed via tail-flick and hot plate tests. Amiloride reduced microglial activation and downregulated the cytokines IL-1β and TNF-a by inhibiting ASIC3 in response to morphine. Furthermore, amiloride reduced p38 MAPK phosphorylation and inhibited NF-κB expression.

CONCLUSIONS

Amiloride effectively reduces chronic morphine tolerance by suppressing microglial activation caused by morphine by inhibiting ASIC3.

Efficacy of Naldemedine on Intestinal Hypomotility and Adhesions in Rodent Models of Postoperative Ileus

Postoperative ileus (POI) often decreases patients' QOL because of prolonged hospitalization and readmission. Alvimopan, a peripheral μ-opioid receptor antagonist, is currently the only therapeutic drug for POI. The aim of this study was to examine the efficacy of naldemedine (a peripheral μ-opioid receptor antagonist with a non-competitive pharmacological profile different from that of alvimopan) on postoperative intestinal hypomotility and adhesion in rodent models, and compare it with the effects of alvimopan. Oral administration of naldemedine (0.3 mg/kg) and alvimopan (3 mg/kg) significantly inhibited the decrease in intestinal motility induced by mechanical irritation in mice (p < 0.01, for both). Naldemedine (1 mg/kg) significantly shortened the adhesion length in chemical-induced postoperative adhesion model rats (p < 0.05). Alvimopan (3 mg/kg) also significantly reduced the adhesion ratio (p < 0.01). These findings suggest that naldemedine is effective for postoperative intestinal hypomotility and adhesions in rodents (i.e., as for alvimopan). Thus, naldemedine may be a useful option for the treatment of POI.

Toll-like Receptor 2-Melatonin Feedback Loop Regulates the Activation of Spinal NLRP3 Inflammasome in Morphine-Tolerant Rats

BACKGROUND AND PURPOSE

Morphine is amongst the most effective analgesics available for the management of severe pain. However, prolonged morphine treatment leads to analgesic tolerance which limits its clinical usage. Previous studies have demonstrated that melatonin ameliorates morphine tolerance by reducing neuroinflammation. However, little is known about the relationship between Toll like receptor 2 (TLR2) and neuroinflammation in morphine tolerance. The aim of this study was to explore the role of TLR2 in morphine tolerance and its connections with melatonin and Nod-like receptor protein 3 (NLRP3) inflammasome.

METHODS

Sprague-Dawley rats were treated with morphine for 7 days and tail-flick latency test was performed to identify the induction of analgesic tolerance. The roles of TLR2 in microglia activation and morphine tolerance were assessed pharmacologically, and the possible interactions between melatonin, TLR2 and NLRP3 inflammasome were investigated.

KEY RESULTS

Morphine tolerance was accompanied by increased TLR2 expression and NLRP3 inflammasome activation in spinal cord. whereas melatonin level was down-regulated. Chronic melatonin administration resulted in a reduced TLR2 expression and NLRP3 inflammasome activation. Moreover, the analgesic effect of morphine was partially restored. Inhibition of TLR2 suppressed the microglia and NLRP3 inflammasome activation, as well as restored the spinal melatonin level while attenuated the development of morphine tolerance. Furthermore, the inhibition of microglia activation ameliorated morphine tolerance via inhibiting TLR2-NLRP3 inflammasome signaling in spinal cord.

CONCLUSION

In this study, we directly demonstrate a TLR2-melatonin negative feedback loop regulating microglia and NLRP3 inflammasome activation during the development of morphine tolerance.

Advances in Neuropathic Pain Research: Selected Intracellular Factors as Potential Targets for Multidirectional Analgesics

Neuropathic pain is a complex and debilitating condition that affects millions of people worldwide. Unlike acute pain, which is short-term and starts suddenly in response to an injury, neuropathic pain arises from somatosensory nervous system damage or disease, is usually chronic, and makes every day functioning difficult, substantially reducing quality of life. The main reason for the lack of effective pharmacotherapies for neuropathic pain is its diverse etiology and the complex, still poorly understood, pathophysiological mechanism of its progression. Numerous experimental studies, including ours, conducted over the last several decades have shown that the development of neuropathic pain is based on disturbances in cell activity, imbalances in the production of pronociceptive factors, and changes in signaling pathways such as p38MAPK, ERK, JNK, NF-κB, PI3K, and NRF2, which could become important targets for pharmacotherapy in the future. Despite the availability of many different analgesics, relieving neuropathic pain is still extremely difficult and requires a multidirectional, individual approach. We would like to point out that an increasing amount of data indicates that nonselective compounds directed at more than one molecular target exert promising analgesic effects. In our review, we characterize four substances (minocycline, astaxanthin, fisetin, and peimine) with analgesic properties that result from a wide spectrum of actions, including the modulation of MAPKs and other factors. We would like to draw attention to these selected substances since, in preclinical studies, they show suitable analgesic properties in models of neuropathy of various etiologies, and, importantly, some are already used as dietary supplements; for example, astaxanthin and fisetin protect against oxidative stress and have anti-inflammatory properties. It is worth emphasizing that the results of behavioral tests also indicate their usefulness when combined with opioids, the effectiveness of which decreases when neuropathy develops. Moreover, these substances appear to have additional, beneficial properties for the treatment of diseases that frequently co-occur with neuropathic pain. Therefore, these substances provide hope for the development of modern pharmacological tools to not only treat symptoms but also restore the proper functioning of the human body.

Neuroprotective effects of Bauhinia variegata in ameliorating diabetic neuropathy and neurodegeneration

INTRODUCTION

In diabetic neuropathy with neurodegeneration (DNN), a serious diabetes consequence, extreme hyperglycemia destroys neurons in the brain and limbs. The main therapies for this condition are glucose control and pain management. Phytopharmacology is thought to be more successful in addressing the pain and blood sugar management issues associated with DNN. The objective of this study was to investigate how Bauhinia variegata (BV) could offer protection against streptozotocin (STZ)-induced diabetic neuropathy.

METHODOLOGY

STZ-associated DNN was induced in rats, and these diabetic rats were treated with BV at 200 mg/kg and 400 mg/kg doses for 28 days. Blood glucose (BG), serum nitrite, lipid peroxidation, antioxidants, C-reactive protein, behavioral, and histopathological parameters were assessed.

RESULTS

BV dramatically reduced BG and HbA1c levels in diabetic rats, according to the findings. The levels of superoxide dismutase and catalase both rose significantly. Both lipid peroxidation and serum nitrite levels were drastically decreased with BV treatment. In this study, it was found that BV has anti-hyperglycemic and anti-inflammatory effects on DNN. This was shown by a significant drop in C-reactive protein in diabetic rats, which was a key factor in diabetic neuropathy. Thermal hyperalgesia was significantly alleviated after BV therapy, and diabetic rats' pain thresholds improved.

CONCLUSION

Present study concluded that BV treatment has excellent glycemic control, a high antioxidant status, and relevant pain-relieving potential in diabetic neuropathy with neurodegeneration by reversing thermal hyperalgesia and decreasing hypeglycemia in diabetic rats.

Progress in Pathological and Therapeutic Research of HIV-Related Neuropathic Pain

HIV-related neuropathic pain (HRNP) is a neurodegeneration that gradually develops during the long-term course of acquired immune deficiency syndrome (AIDS) and manifests as abnormal sock/sleeve-like symmetrical pain and nociceptive hyperalgesia in the extremities, which seriously reduces patient quality of life. To date, the pathogenesis of HRNP is not completely clear. There is a lack of effective clinical treatment for HRNP and it is becoming a challenge and hot spot for medical research. In this study, we conducted a systematic review of the progress of HRNP research in recent years including (1) the etiology, classification and clinical symptoms of HRNP, (2) the establishment of HRNP pathological models, (3) the pathological mechanisms underlying HRNP from three aspects: molecules, signaling pathways and cells, (4) the therapeutic strategies for HRNP, and (5) the limitations of recent HRNP research and the future research directions and prospects of HRNP. This detailed review provides new and systematic insight into the pathological mechanism of HRNP, which establishes a theoretical basis for the future exploitation of novel target drugs. HIV infection, antiretroviral therapy and opioid abuse contribute to the etiology of HRNP with symmetrical pain in both hands and feet, allodynia and hyperalgesia. The pathogenesis involves changes in cytokine expression, activation of signaling pathways and neuronal cell states. The therapy for HRNP should be patient-centered, integrating pharmacologic and nonpharmacologic treatments into multimodal intervention.

Role of autophagy in the pathogenesis and regulation of pain

Pain is a ubiquitous and highly concerned clinical symptom, usually caused by peripheral or central nervous injury, tissue damage, or other diseases. The long-term existence of pain can seriously affect daily physical function and quality of life and produce great torture on the physiological and psychological levels. However, the complex pathogenesis of pain involving molecular mechanisms and signaling pathways has not been fully elucidated, and managing pain remains highly challenging. As a result, finding new targets to pursue effective and long-term pain treatment strategies is required and urgent. Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, which can be cytoprotective and is vital in maintaining neural plasticity and proper nervous system function. Much evidence has shown that autophagy dysregulation is linked to the emergence of neuropathic pain, such as postherpetic neuralgia and cancer-related pain. Autophagy has also been connected to pain caused by osteoarthritis and lumbar disc degeneration. It is worth noting that in recent years, studies on traditional Chinese medicine have also proved that several traditional Chinese medicine monomers involve autophagy in the mechanism of pain relief. Therefore, autophagy can serve as a potential regulatory target to provide new ideas and inspiration for pain management.

Therapeutic Effects of Combined Treatment with the AEA Hydrolysis Inhibitor PF04457845 and the Substrate Selective COX-2 Inhibitor LM4131 in the Mouse Model of Neuropathic Pain

Chronic neuropathic pain resulting from peripheral nerve damage is a significant clinical problem, which makes it imperative to develop the mechanism-based therapeutic approaches. Enhancement of endogenous cannabinoids by blocking their hydrolysis has been shown to reduce inflammation and neuronal damage in a number of neurological disorders and neurodegenerative diseases. However, recent studies suggest that inhibition of their hydrolysis can shift endocannabinoids 2-arachidonoyl glycerol (2-AG) and anandamide (AEA) toward the oxygenation pathway mediated by cyclooxygenase-2 (COX-2) to produce proinflammatory prostaglandin glycerol esters (PG-Gs) and prostaglandin ethanolamides (PG-EAs). Thus, blocking both endocannabinoid hydrolysis and oxygenation is likely to be more clinically beneficial. In this study, we used the chronic constriction injury (CCI) mouse model to explore the therapeutic effects of simultaneous inhibition of AEA hydrolysis and oxygenation in the treatment of neuropathic pain. We found that the fatty acid amide hydrolase (FAAH) inhibitor PF04457845 and the substrate-selective COX-2 inhibitor LM4131 dose-dependently reduced thermal hyperalgesia and mechanical allodynia in the CCI mice. In addition to ameliorating the pain behaviors, combined treatment with subeffective doses of these inhibitors greatly attenuated the accumulation of inflammatory cells in both sciatic nerve and spinal cord. Consistently, the increased proinflammatory cytokines IL-1β, IL-6, and chemokine MCP-1 in the CCI mouse spinal cord and sciatic nerve were also significantly reduced by combination of low doses of PF04457845 and LM4131 treatment. Therefore, our study suggests that simultaneous blockage of endocannabinoid hydrolysis and oxygenation by using the substrate-selective COX-2 inhibitor, which avoids the cardiovascular and gastrointestinal side effects associated with the use of general COX-2 inhibitors, might be a suitable strategy for the treatment of inflammatory and neuropathic pain.

Exploring the molecular pathways and therapeutic implications of angiogenesis in neuropathic pain

Recently, much attention has been paid to chronic neuro-inflammatory condition underlying neuropathic pain. It is generally linked with thermal hyperalgesia and tactile allodynia. It results due to injury or infection in the nervous system. The neuropathic pain spectrum covers a variety of pathophysiological states, mostly involved are ischemic injury viral infections associated neuropathies, chemotherapy-induced peripheral neuropathies, autoimmune disorders, traumatic origin, hereditary neuropathies, inflammatory disorders, and channelopathies. In CNS, angiogenesis is evident in inflammation of neurons and pain in bone cancer. The role of chemokines and cytokines is dualistic; their aggressive secretion produces detrimental effects, leading to neuropathic pain. However, whether the angiogenesis contributes and exists in neuropathic pain remains doubtful. In the present review, we elucidated summary of diverse mechanisms of neuropathic pain associated with angiogenesis. Moreover, an overview of multiple targets that have provided insights on the VEGF signaling, signaling through Tie-1 and Tie-2 receptor, erythropoietin pathway promoting axonal growth are also discussed. Because angiogenesis as a result of these signaling, results in inflammation, we focused on the mechanisms of neuropathic pain. These factors are mainly responsible for the activation of post-traumatic regeneration of the PNS and CNS. Furthermore, we also reviewed synthetic and herbal treatments targeting angiogenesis in neuropathic pain.

Pharmacological Modulation of the MIP-1 Family and Their Receptors Reduces Neuropathic Pain Symptoms and Influences Morphine Analgesia: Evidence from a Mouse Model

Neuropathic pain pathophysiology is not fully understood, but it was recently shown that MIP-1 family members (CCL3, CCL4, and CCL9) have strong pronociceptive properties. Our goal was to examine how pharmacological modulation of these chemokines and their receptors (CCR1 and CCR5) influence hypersensitivity after nerve injury in Albino Swiss male mice. The spinal changes in the mRNA/protein levels of the abovementioned chemokines and their receptors were measured using RT-qPCR and ELISA/Western blot techniques in a mouse model of chronic constriction injury of the sciatic nerve. Behavioral studies were performed using the von Frey and cold plate tests after pharmacological treatment with neutralizing antibodies (nAbs) against chemokines or antagonists (CCR1-J113863, CCR5-TAK-220/AZD-5672) alone and in coadministration with morphine on Day 7, when the hypersensitivity was fully developed. Our results showed enhanced protein levels of CCL3 and CCL9 1 and 7 days after nerve injury. The single intrathecal administration of CCL3 or CCL9 nAb, J113863, TAK-220, or AZD-5672 diminished neuropathic pain symptoms and enhanced morphine analgesia. These findings highlight the important roles of CCL3 and CCL9 in neuropathic pain and additionally indicate that these chemokines play essential roles in opioid analgesia. The obtained results suggest CCR1 and CCR5 as new, interesting targets in neuropathy treatment.

Crosstalk between Mu-Opioid receptors and neuroinflammation: Consequences for drug addiction and pain

Mu-Opioid Receptors (MORs) are well-known for participating in analgesia, sedation, drug addiction, and other physiological functions. Although MORs have been related to neuroinflammation their biological mechanism remains unclear. It is suggested that MORs work alongside Toll-Like Receptors to enhance the release of pro-inflammatory mediators and cytokines during pathological conditions. Some cytokines, including TNF-α, IL-1β and IL-6, have been postulated to regulate MORs levels by both avoiding MOR recycling and enhancing its production. In addition, Neurokinin-1 Receptor, also affected during neuroinflammation, could be regulating MOR trafficking. Therefore, inflammation in the central nervous system seems to be associated with altered/increased MORs expression, which might regulate harmful processes, such as drug addiction and pain. Here, we provide a critical evaluation on MORs' role during neuroinflammation and its implication for these conditions. Understanding MORs' functioning, their regulation and implications on drug addiction and pain may help elucidate their potential therapeutic use against these pathological conditions and associated disorders.

The YTHDF1-TRAF6 pathway regulates the neuroinflammatory response and contributes to morphine tolerance and hyperalgesia in the periaqueductal gray

Long-term use of opioids such as morphine has negative side effects, such as morphine analgesic tolerance and morphine-induced hyperalgesia (MIH). These side effects limit the clinical use and analgesic efficacy of morphine. Elucidation of the mechanisms and identification of feasible and effective methods or treatment targets to solve this clinical phenomenon are important. Here, we discovered that YTHDF1 and TNF receptor-associated factor 6 (TRAF6) are crucial for morphine analgesic tolerance and MIH. The m6A reader YTHDF1 positively regulated the translation of TRAF6 mRNA, and chronic morphine treatments enhanced the m6A modification of TRAF6 mRNA. TRAF6 protein expression was drastically reduced by YTHDF1 knockdown, although TRAF6 mRNA levels were unaffected. By reducing inflammatory markers such as IL-1β, IL-6, TNF-α and NF-κB, targeted reduction of YTHDF1 or suppression of TRAF6 activity in ventrolateral periaqueductal gray (vlPAG) slows the development of morphine analgesic tolerance and MIH. Our findings provide new insights into the mechanism of morphine analgesic tolerance and MIH indicating that YTHDF1 regulates inflammatory factors such as IL-1β, IL-6, TNF-α and NF-κB by enhancing TRAF6 protein expression.

Microglial knockdown does not affect acute withdrawal but delays analgesic tolerance from oxycodone in male and female C57BL/6J mice

Opioid Use Disorder (OUD) affects approximately 8%-12% of the population. In dependent individuals, abrupt cessation of opioid taking results in adverse withdrawal symptoms that reinforce drug taking behavior. Considerable unmet clinical need exists for new pharmacotherapies to treat opioid withdrawal as well as improve long-term abstinence. The neuroimmune system has received much scientific attention in recent years as a potential therapeutic target to combat various neurodegenerative and psychiatric disorders including addiction. However, the specific contribution of microglia has not been investigated in oxycodone dependence. Chronic daily treatment with the CSF1R inhibitor Pexidartinib (PLX3397) was administered to knockdown microglia expression and evaluate consequences on analgesia and on naloxone induced withdrawal from oxycodone. In vivo results indicated that an approximately 40% reduction in brain IBA1 staining was achieved in the PLX treatment group, which was associated with a delay in the development of analgesic tolerance to oxycodone and maintained antinociceptive efficacy. Acute withdrawal behavioral symptoms, brain astrocyte expression, and levels of many neuroinflammatory markers were not affected by PLX treatment. KC/GRO (also known as CXCL1) was significantly enhanced in the somatosensory cortex in oxycodone-treated mice receiving PLX. Microglial knock-down did not affect the expression of naloxoneinduced opioid withdrawal but affected antinociceptive responsivity. The consequences of increased KC/GRO expression within the somatosensory cortex due to microglial reduction during opioid dependence are unclear but may be important for neural pathways mediating opioid-induced analgesia.

A neuron-to-astrocyte Wnt5a signal governs astrogliosis during HIV-associated pain pathogenesis

Chronic pain is the most common neurological disorder of HIV patients. Multiple neuropathologies were identified in the pain pathway. Among them is the prominent astrocytic reaction (also know an astrogliosis). However, the pathogenic role and mechanism of the astrogliosis are unclear. Here, we show that the astrogliosis is crucial for the pain development induced by a key neurotoxic HIV protein gp120 and that a neuron-to-astrocyte Wnt5a signal controls the astrogliosis. Ablation of astrogliosis blocked the development of gp120-induced mechanical hyperalgesia, and concomitantly the expression of neural circuit polarization in the spinal dorsal horn. We demonstrated that conditional knockout of either Wnt5a in neurons or its receptor ROR2 in astrocytes abolished not only gp120-induced astrogliosis but also hyperalgesia and neural circuit polarization. Furthermore, we found that the astrogliosis promoted expression of hyperalgesia and NCP via IL-1β regulated by a Wnt5a-ROR2-MMP2 axis. Our results shed light on the role and mechanism of astrogliosis in the pathogenesis of HIV-associated pain.

Stress- and drug-induced neuroimmune signaling as a therapeutic target for comorbid anxiety and substance use disorders

Stress and substance use disorders remain two of the most highly prevalent psychiatric conditions and are often comorbid. While individually these conditions have a debilitating impact on the patient and a high cost to society, the symptomology and treatment outcomes are further exacerbated when they occur together. As such, there are few effective treatment options for these patients, and recent investigation has sought to determine the neural processes underlying the co-occurrence of these disorders to identify novel treatment targets. One such mechanism that has been linked to stress- and addiction-related conditions is neuroimmune signaling. Increases in inflammatory factors across the brain have been heavily implicated in the etiology of these disorders, and this review seeks to determine the nature of this relationship. According to the "dual-hit" hypothesis, also referred to as neuroimmune priming, prior exposure to either stress or drugs of abuse can sensitize the neuroimmune system to be hyperresponsive when exposed to these insults in the future. This review completes an examination of the literature surrounding stress-induced increases in inflammation across clinical and preclinical studies along with a summarization of the evidence regarding drug-induced alterations in inflammatory factors. These changes in neuroimmune profiles are also discussed within the context of their impact on the neural circuitry responsible for stress responsiveness and addictive behaviors. Further, this review explores the connection between neuroimmune signaling and susceptibility to these conditions and highlights the anti-inflammatory pharmacotherapies that may be used for the treatment of stress and substance use disorders.

Multi-chemokine receptor antagonist RAP-103 inhibits opioid-derived respiratory depression, reduces opioid reinforcement and physical dependence, and normalizes opioid-induced dysregulation of mesolimbic chemokine receptors in rats

Chemokine-opioid crosstalk is a physiological crossroads for influencing therapeutic and adverse effects of opioids. Activation of chemokine receptors, especially CCR2, CCR5 and CXCR4, reduces opioid-induced analgesia by desensitizing OPRM1 receptors. Chemokine receptor antagonists (CRAs) enhance opioid analgesia, but knowledge about how CRAs impact adverse opioid effects remains limited. We examined effects of RAP-103, a multi-CRA orally active peptide analog of "DAPTA", on opioid-derived dependence, reinforcement, and respiratory depression in male rats and on changes in chemokine and OPRM1 (µ opioid) receptor levels in mesolimbic substrates during opioid abstinence. In rats exposed to chronic morphine (75 mg pellet x 7 d), daily RAP-103 (1 mg/kg, IP) treatment reduced the severity of naloxone-precipitated withdrawal responses. For self-administration (SA) studies, RAP-103 (1 mg/kg, IP) reduced heroin acquisition (0.1 mg/kg/inf) and reinforcing efficacy (assessed by motivation on a progressive-ratio reinforcement schedule) but did not impact sucrose intake. RAP-103 (1-3 mg/kg, IP) also normalized the deficits in oxygen saturation and enhancement of respiratory rate caused by morphine (5 mg/kg, SC) exposure. Abstinence from chronic morphine elicited brain-region specific changes in chemokine receptor protein levels. CCR2 and CXCR4 were increased in the ventral tegmental area (VTA), whereas CCR2 and CCR5 were reduced in the nucleus accumbens (NAC). Effects of RAP-103 (1 mg/kg, IP) were focused in the NAC, where it normalized morphine-induced deficits in CCR2 and CCR5. These results identify CRAs as potential biphasic function opioid signaling modulators to enhance opioid analgesia and inhibit opioid-derived dependence and respiratory depression.

Is the Neuroimmune System a Therapeutic Target for Opioid Use Disorder? A Systematic Review

Opioid use disorder (OUD) is an epidemic in the United States. In the past 12 months alone, there have been 75,000+ deaths attributed to opioid overdose: more than any other year in American history. Current pharmacotherapies for the treatment of OUD effectively suppress opioid withdrawal symptoms, but long-term relapse rates remain unacceptably high. Novel treatments for OUD are desperately needed to curb this epidemic. One target that has received considerable recent interest is the neuroimmune system. The neuroimmune system is anchored by glial cells, i.e., microglia and astrocytes, but neuroimmune signaling is known to influence neurons, including altering neurotransmission, synapse formation, and ultimately, brain function. Preclinical studies have shown that experimental attenuation of pro-inflammatory neuroimmune signaling modulates opioid addiction processes, including opioid reward, tolerance, and withdrawal symptoms, which suggests potential therapeutic benefit in patients. Whereas the peripheral immune system in OUD patients has been studied for decades and is well-understood, little is known about the neuroimmune system in OUD patients or its viability as a treatment target. Herein, we review the literature describing relationships between opioid administration and the neuroimmune system, the influence of neuroimmune signaling on opioid addiction processes, and the therapeutic potential for targeting the neuroimmune system in OUD subjects using glial modulator medications.

Spinal Cord Stimulation Alleviates Neuropathic Pain by Attenuating Microglial Activation via Reducing Colony-Stimulating Factor 1 Levels in the Spinal Cord in a Rat Model of Chronic Constriction Injury

BACKGROUND

Spinal cord stimulation (SCS) is an emerging, minimally invasive procedure used to treat patients with intractable chronic pain conditions. Although several signaling pathways have been proposed to account for SCS-mediated pain relief, the precise mechanisms remain poorly understood. Recent evidence reveals that injured sensory neuron-derived colony-stimulating factor 1 (CSF1) induces microglial activation in the spinal cord, contributing to the development of neuropathic pain (NP). Here, we tested the hypothesis that SCS relieves pain in a rat model of chronic constriction injury (CCI) by attenuating microglial activation via blocking CSF1 to the spinal cord.

METHODS

Sprague-Dawley rats underwent sciatic nerve ligation to induce CCI and were implanted with an epidural SCS lead. SCS was delivered 6 hours per day for 5 days. Some rats received a once-daily intrathecal injection of CSF1 for 3 days during SCS.

RESULTS

Compared with naive rats, CCI rats had a marked decrease in the mechanical withdrawal threshold of the paw, along with increased microglial activation and augmented CSF1 levels in the spinal dorsal horn and dorsal root ganglion, as measured by immunofluorescence or Western blotting. SCS significantly increased the mechanical withdrawal threshold and attenuated microglial activation in the spinal dorsal horn in CCI rats, which were associated with reductions in CSF1 levels in the spinal dorsal horn and dorsal roots but not dorsal root ganglion. Moreover, intrathecal injection of CSF1 completely abolished SCS-induced changes in the mechanical withdrawal threshold and activation of microglia in the spinal dorsal horn in CCI rats.

CONCLUSIONS

SCS reduces microglial activation in the spinal cord and alleviates chronic NP, at least in part by inhibiting the release of CSF1 from the dorsal root ganglion ipsilateral to nerve injury.

Toll-like receptors change morphine-induced antinociception, tolerance and dependence: Studies using male and female TLR and signalling gene KO mice

Toll-like receptors (TLR) have been proposed as a site of action that alters opioid pharmacodynamics. However, a comprehensive assessment of acute opioid antinociception, tolerance and withdrawal behaviours in genetic null mutant strains with altered innate immune signalling has not been performed. Nor has the impact of genetic deletion of TLR2/4 on high-affinity opioid receptor binding. Here we show that diminished TLR signalling potentiates acute morphine antinociception equally in male and female mice. However, only male TIR8 null mutant mice showed reduced morphine analgesia. Analgesic tolerance was prevented in TLR2 and TLR4 null mutants, but not MyD88 animals. Withdrawal behaviours were only protected in TLR2^-/- mice. In silico docking simulations revealed opioid ligands bound preferentially to the LPS binding pocket of MD-2 rather than TLR4. There was no binding of [³H](-)-naloxone or [³H]diprenorphine to TLR4 in the concentrations explored. These data confirm that opioids have high efficacy activity at innate immune pattern recognition binding sites but do not bind to TLR4 and identify critical pathway and sex-specific effects of the complex innate immune signalling contributions to opioid pharmacodynamics. These data further support the behavioural importance of the TLR-opioid interaction but fail to demonstrate direct evidence for high-affinity binding of the TLR4 signalling complex to ligands.

Targeting Chemokines and Chemokine GPCRs to Enhance Strong Opioid Efficacy in Neuropathic Pain

Neuropathic pain (NP) originates from an injury or disease of the somatosensory nervous system. This heterogeneous origin and the possible association with other pathologies make the management of NP a real challenge. To date, there are no satisfactory treatments for this type of chronic pain. Even strong opioids, the gold-standard analgesics for nociceptive and cancer pain, display low efficacy and the paradoxical ability to exacerbate pain sensitivity in NP patients. Mounting evidence suggests that chemokine upregulation may be a common mechanism driving NP pathophysiology and chronic opioid use-related consequences (analgesic tolerance and hyperalgesia). Here, we first review preclinical studies on the role of chemokines and chemokine receptors in the development and maintenance of NP. Second, we examine the change in chemokine expression following chronic opioid use and the crosstalk between chemokine and opioid receptors. Then, we examine the effects of inhibiting specific chemokines or chemokine receptors as a strategy to increase opioid efficacy in NP. We conclude that strong opioids, along with drugs that block specific chemokine/chemokine receptor axis, might be the right compromise for a favorable risk/benefit ratio in NP management.

New pharmacological perspectives and therapeutic options for opioids: Differences matter

Opioids remain the major drug class for the treatment of acute, chronic and cancer pain, but have major harmful effects such as dependence and opioid-induced ventilatory impairment. Although no new typical opioids have come onto the market in the past almost 50 years, a plethora of new innovative formulations has been developed to meet the clinical need. This review is intended to shed light on new understanding of the molecular pharmacology of opioids, which has arisen largely due to the genomic revolution, and what new drugs may become available in the coming years. Atypical opioids have and are being developed which not only target the mu opioid receptor but other targets in the pain pathway. Biased mu agonists have been developed but remain 'unbiased' clinically. The contribution of drugs targeting non-mu opioid receptors either alone or as heterodimers shows potential promise but remains understudied. That gene splice variants of the mu opioid receptor produce multiple receptor isoforms in different brain regions, and may change with pain chronicity and phenotype, presents new challenges but also opportunities for precision pain medicine. Finally, that opioids also have pro-inflammatory effects not aligned with mu opioid receptor binding affinity implicates a fresh understanding of their role in chronic pain, whether cancer or non-cancer. Hopefully, a new understanding of opioid analgesic drug action may lead to new drug development and better precision medicine in acute and chronic pain relief with less patient harm.

AMPK-autophagy-mediated inhibition of microRNA-30a-5p alleviates morphine tolerance via SOCS3-dependent neuroinflammation suppression

BACKGROUND

The development of morphine tolerance is a clinical challenge for managing severe pain. Studies have shown that neuroinflammation is a critical aspect for the development of analgesic tolerance. We found that AMPK-autophagy activation could suppress neuroinflammation and improve morphine tolerance via the upregulation of suppressor of cytokine signaling 3 (SOCS3) by inhibiting the processing and maturation of microRNA-30a-5p.

METHODS

CD-1 mice were utilized for the tail-flick test to evaluate morphine tolerance. The microglial cell line BV-2 was utilized to investigate the mechanism of AMPK-autophagy-mediated posttranscriptional regulation of SOCS3. Proinflammatory cytokines were measured by western blotting and real-time PCR. The levels of SOCS3 and miRNA-processing enzymes were evaluated by western blotting, real-time PCR and immunofluorescence staining.

RESULTS

Based on experimental verification, miRNA-30a-5p could negatively regulate SOCS3. The AMPK activators AICAR, resveratrol and metformin downregulated miRNA-30a-5p. We found that AMPK activators specifically inhibited the processing and maturation of miRNA-30a-5p in microglia by degrading DICER and AGO2 via autophagy. Furthermore, a miRNA-30a-5p inhibitor significantly improved morphine tolerance via upregulation of SCOS3 in mice. It markedly increased the level of SOCS3 in the spinal cord of mice and subsequently inhibited morphine-induced phosphorylation of NF-κB p65. In addition, a miRNA-30a-5p inhibitor decreased the levels of IL-1β and TNF-α caused by morphine in microglia.

CONCLUSION

AMPK-autophagy activation suppresses neuroinflammation and improves morphine tolerance via the upregulation of SOCS3 by inhibiting miRNA-30a-5p.

Neuroimmunological complications arising from chemotherapy-induced gut toxicity and opioid exposure in female dark agouti rats

Cancer patients may experience symptom clusters, including chemotherapy-induced (CI) gut toxicity (CIGT) and cognitive impairment. Analgesic selection for pain associated with CIGT is difficult as opioids induce glial reactivity and unwanted side effects. This study quantified central glial reactivity and proinflammatory effects in rats with CIGT using three mechanistically different analgesics. Regional adaptations were indicative of immune-to-brain signaling routes. Utilizing a 5-fluorouracil-induced GT (5IGT) rat model and analgesic intervention (carprofen (CAR), buprenorphine (BUP), and tramadol (TRAM)), spinal and brain neuroimmune modulation was examined via microglial, astrocyte, and proinflammatory (cluster of differentiation molecule 11b; CD11b, glial fibrillary associated protein; GFAP, and interleukin-1 beta; IL1β) reactivity marker expression changes by western blot analysis. 5IGT significantly increased thoracic GFAP (p < 0.05) and IL-1β (p < 0.0001) expression, CAR and BUP ameliorated these effects. BUP and TRAM with 5-FU synergistically increased hippocampal GFAP expression. CAR administered with 5IGT significantly elevated hippocampal and thoracic CD11b expression levels (p < 0.05). The neuroimmune responses observed in this study suggest activation of peripheral-to-central immune signaling pathways. We speculate that the opioid-induced hippocampal changes inferred a humorally mediated mechanism, whereas thoracic neuroimmune modifications indicated activation of an indirect neural route. Although TRAM ameliorated 5IGT-intestinal inflammation, this opioid presents complications relating to bodyweight and regional glial dysregulation (neuroinflammation) and may not be optimal in the management of pain associated with 5IGT. The chemotherapy-induced gut-derived neuroimmune consequences observed suggest a potential mechanistic contribution to central components of the cancer symptom cluster experience, while the opioid-related glial changes have implications for optimal pain management in this setting warranting further investigation.

Chemokine receptor antagonists enhance morphine's antinociceptive effect but not respiratory depression

AIMS

We have shown that chemokines injected into the periaqueductal gray region of the brain blocks opioid-induced analgesia in the rat cold-water tail flick test (CWTF). The present experiments tested whether chemokine receptor antagonists (CRAs), in combination with sub-analgesic doses of morphine, would provide maximal analgesia in the CWTF test and the mouse formalin pain assay. The effect of CRAs on respiratory depression was also evaluated.

MAIN METHODS

One, two or four CRAs (AMD3100/CXCR4, maraviroc/CCR5, RS504393/CCR2 orAZD8797/CX3CR1) were used in combination with sub-analgesic doses of morphine, all given systemically. Pain was assessed using the rat CWTF test or formalin injection into the paw of mice scored by licking. Respiration and oxygen saturation were measured in rats using a MouseOX® Plus - pulse oximeter.

KEY FINDINGS

In the CWTF test, a sub-maximal dose of morphine in combination with maraviroc alone, maraviroc plus AMD3100, or with the four chemokine receptor antagonists, produced synergistic increases in antinociception. In the formalin test, the combination of four CRAs plus a sub-maximal dose of morphine resulted in increased antinociception in both male and female mice. AMD3100 had an additive effect with morphine in both sexes. Coadministration of CRAs with morphine did not potentiate the opioid respiratory depressive effect.

SIGNIFICANCE

These results support the conclusion that combinations of CRAs can increase the potency of sub-analgesic doses of morphine analgesia without increasing respiratory depression. The results support an "opioid sparing" strategy for alleviation of pain using reduced doses of opioids in combination with CRAs to achieve maximal analgesia.

Intrathecal implantation surgical considerations in rodents; a review

Intrathecal access in humans is a routine clinical intervention. However, intrathecal access is limited to drug delivery purposes in rodents, and intrathecal implantation is not a common surgical practice. Preclinically, we have successfully adopted different intrathecal implantation surgical methods for different implant materials in rodents. However, employing the appropriate intrathecal implantation method is a challenging process for surgeons, which includes several steps such as preoperative evaluations and postoperative care. The aim of this review is to define and compare the major documented surgical approaches applicable for intrathecal implantation in rodents along with the associated side effects, as well as highlighting the critical preoperative and postoperative considerations. Overall, this review will provide surgeons with the principles of intrathecal implantation approaches applicable for different implant materials.

The Histamine H4 Receptor Participates in the Anti-Neuropathic Effect of the Adenosine A3 Receptor Agonist IB-MECA: Role of CD4+ T Cells

A₃ adenosine receptor (A₃AR) agonists have emerged as potent relievers of neuropathic pain by a T cell-mediated production of IL-10. The H₄ histamine receptor (H₄R), also implicated in pain modulation, is expressed on T cells playing a preeminent role in its activation and release of IL-10. To improve the therapeutic opportunities, this study aimed to verify the hypothesis of a possible cross-talk between A₃AR and H₄R in the resolution of neuropathic pain. In the mouse model of Chronic Constriction Injury (CCI), the acute intraperitoneal co-administration of the A₃AR agonist IB-MECA (0.5 mg/kg) and the H₄R agonist VUF 8430 (10 mg/kg), were additive in counteracting mechano-allodynia increasing IL-10 plasma levels. In H₄R^−/− mice, IB-MECA activity was reduced, lower pain relief and lower modulation of plasma IL-1β, TNF-α, IL-6 and IL-10 were shown. The complete anti-allodynia effect of IB-MECA in H₄R^−/− mice was restored after intravenous administration of CD4⁺ T cells obtained from naïve wild type mice. In conclusion, a role of the histaminergic system in the mechanism of A₃AR-mediated neuropathic pain relief was suggested highlighting the driving force evoked by CD4⁺ T cells throughout IL-10 up-regulation.

Aging and miR-155 in mice influence survival and neuropathic pain after spinal cord injury

Spinal cord injury (SCI) elicits chronic pain in 65% of individuals. In addition, SCI afflicts an increasing number of aged individuals, and those with SCI are predisposed to shorter lifespan. Our group previously identified that deletion of the microRNA miR-155 reduced neuroinflammation and locomotor deficits after SCI. Here, we hypothesized that aged mice would be more susceptible to pain symptoms and death soon after SCI, and that miR-155 deletion would reduce pain symptoms in adult and aged mice and improve survival. Adult (2 month-old) and aged (20 month-old) female wildtype (WT) and miR-155 knockout (KO) mice received T9 contusion SCI. Aged WT mice displayed reduced survival and increased autotomy - a symptom of spontaneous pain. In contrast, aged miR-155 KO mice after SCI were less susceptible to death or spontaneous pain. Evoked pain symptoms were tested using heat (Hargreaves test) and mechanical (von Frey) stimuli. At baseline, aged mice showed heightened heat sensitivity. After SCI, adult and aged WT and miR-155 KO mice all exhibited heat and mechanical hypersensitivity at all timepoints. miR-155 deletion in adult (but not aged) mice reduced mechanical hypersensitivity at 7 and 14 d post-SCI. Therefore, aging predisposes mice to SCI-elicited spontaneous pain and expedited mortality. miR-155 deletion in adult mice reduces evoked pain symptoms, and miR-155 deletion in aged mice reduces spontaneous pain and expedited mortality post-SCI. This study highlights the importance of studying geriatric models of SCI, and that inflammatory mediators such as miR-155 are promising targets after SCI for improving pain relief and longevity.

Morphine promotes microglial activation by upregulating the EGFR/ERK signaling pathway

Although they represent the cornerstone of analgesic therapy, opioids, such as morphine, are limited in efficacy by drug tolerance, hyperalgesia and other side effects. Activation of microglia and the consequent production of proinflammatory cytokines play a key pathogenic role in morphine tolerance, but the exact mechanisms are not well understood. This study aimed to investigate the regulatory mechanism of epidermal growth factor receptor (EGFR) on microglial activation induced by morphine in mouse microglial BV-2 cells. In this research, BV-2 cells were stimulated with morphine or pretreated with AG1478 (an inhibitor of EGFR). Expression levels of cluster of differentiation molecule 11b (CD11b), EGFR, and phospho-EGFR were detected by immunofluorescence staining. Cell signaling was assayed by Western blot. The migration ability of BV-2 cells was tested by Transwell assay. The production of interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in the cell supernatant was determined by ELISA. We observed that the expression of CD11b induced by morphine was increased in a dose- and time- dependent manner in BV-2 cells. Phosphorylation levels of EGFR and ERK1/2, migration of BV-2 cells, and production of IL-1β and TNFα were markedly enhanced by morphine treatment. The activation, migration, and production of proinflammatory cytokines in BV-2 cells were inhibited by blocking the EGFR signaling pathway with AG1478. The present study demonstrated that the EGFR/ERK signaling pathway may represent a novel pharmacological strategy to suppress morphine tolerance through attenuation of microglial activation.

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.

Molecular mechanisms of opioid tolerance: From opioid receptors to inflammatory mediators (Review)

Opioids are considered the most effective analgesics for the treatment of both acute and chronic pain. However, prolonged opioid use can induce a certain level of tolerance to its analgesic effects, leading to a reduction in its effectiveness, addiction and abuse. A better understanding of the mechanisms underlying opioid tolerance may provide insights into this phenomenon and aid in the development of novel methods to combat the side effects of opioid tolerance. The present review focused on two major contributors to tolerance, opioid receptors and inflammatory mediators. The molecular mechanisms involved in the desensitization of the opioid receptors were briefly described, including their phosphorylation, internalisation and recycling. Subsequently, the effects of Toll like receptor 4/NOD-like receptor family pyrin domain containing 3-mediated proinflammatory responses in opioid tolerance were discussed, aiming in supporting the identification of novel therapeutic targets.

Mitochondrial biogenesis factor PGC-1α suppresses spinal morphine tolerance by reducing mitochondrial superoxide

Opioid use disorders (OUDs) have reached an epidemic level in the United States. The opioid epidemic involves illicit opioid use, prescription opioids for analgesia, counterfeit opioids, new psychoactive substances, and diverted opioids. Opioids remain the last option for the treatment of intractable clinical pain, but chronic use of opioids are limited in part due to antinociceptive/analgesic tolerance. Peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha (PGC-1α), a mitochondrial biogenesis factor can reduce toxic reactive oxygen species (ROS) that play a role in morphine tolerance (MT). Decreased PGC-1α expression has been shown to contribute to various metabolic disorders or neurodegeneration diseases through increasing ROS. We examined the relationship of PGC-1α and ROS in MT. To induce MT, adult Sprague-Dawley rats received intrathecal morphine for 7 days. Mechanical threshold was measured using the von Frey test and thermal latency was examined using the heat plate test. Expression of PGC-1α in the spinal cord dorsal horn (SCDH) was examined using RT-PCR and western blots. Mitochondrial superoxide was detected using MitoSox Red, a mitochondrial superoxide indicator. The antinociceptive effect of recombinant PGC-1α (rPGC-1α) or Mito-Tempol (a mitochondria-targeted superoxide scavenger) was determined using the von Frey test and hot plate test. Furthermore, we examined the effect of rPGC-1α on mitochondrial superoxide using cultured neurons. Our findings include that: (i) spinal MT decreased the expression of spinal PGC-1α in the SCDH neurons; (ii) rPGC-1α increased mechanical threshold and thermal latency in MT animals; (iii) Mito-Tempol reduced MT behavioral response; (iv) rPGC-1α reduced MT-induced mitochondria-targeted superoxide; and (v) cultured neuronal cells treated with TNFα increased mitochondria-targeted superoxide that can be inhibited by rPGC-1α. The present findings suggest that spinal PGC-1α reduce MT through decreasing mitochondria-targeted superoxide in the SCDH.

Microglial TLR4-induced TAK1 phosphorylation and NLRP3 activation mediates neuroinflammation and contributes to chronic morphine-induced antinociceptive tolerance

BACKGROUND AND PURPOSE

The aim of this work was to investigate the role and signal transduction of toll-like receptor 4 (TLR4), TGF-β-activated kinase 1 (TAK1) and nod-like receptor protein 3 (NLRP3) in microglial in the development of morphine-induced antinociceptive tolerance.

METHODS

TLR4 and NLRP3 knockout mice and 5Z-7-oxozeaeno (a selective inhibitor against TAK1 activity) were used to observe their effect on the development of morphine tolerance. Intrathecal injections of morphine (0.75 mg/kg once daily for 7 days) were used to establish anti-nociceptive tolerance, which was measured by the tail-flick test. Spinal TLR4, TAK1, and NLRP3 expression levels and phosphorylation of TAK1 were evaluated by Western blotting and immunofluorescence.

RESULTS

Repeated treatment with morphine increased total expression of spinal TLR4, TAK1, and NLRP3 and phosphorylation of TAK1 in wild-type mice. TLR4 knockout attenuated morphine-induced tolerance and inhibited the chronic morphine-induced increase in NLRP3 and phosphorylation of TAK1. Compared with controls, mice that received 5Z-7-oxozeaenol showed decreased development of morphine tolerance and inhibition on repeated morphine-induced increase of NLRP3 but not TLR4. NLRP3 knockout mice showed resistance to morphine-induced analgesic tolerance with no effect on chronic morphine-induced expression of TLR4 and TAK1. TLR4, TAK1, and NLRP3 were collectively co-localized together and with the microglia marker Iba1.

CONCLUSIONS

Microglial TLR4 regulates TAK1 expression and phosphorylation and results in NLRP3 activation contributes to the development of morphine tolerance through regulating neuroinflammation. Targeting TLR4-TAK1-NLRP3 signaling to regulate neuro-inflammation will be alternative therapeutics and strategies for chronic morphine-induced antinociceptive tolerance.

Thiazolidine Derivatives Attenuate Carrageenan-Induced Inflammatory Pain in Mice

BACKGROUND

Peripheral inflammation leads to the development of persistent thermal hyperalgesia and mechanical allodynia associated with increased expression of interleukin-1β (IL-1β) in the spinal cord. The aim of the present study was to investigate the effects of thiazolidine derivatives, 1b ([2-(2-hydroxyphenyl)-1,3-thiazolidin-4-yl](morpholin-4-yl)methanone) and 1d (2-hydroxy-4-{[2-(2-hydroxyphenyl)-1,3-thiazolidine-4-carbonyl]amino}benzoic acid), on thermal hyperalgesia, mechanical allodynia and on IL-1β expression during carrageenan-induced inflammation in the spinal cord in mice. Inflammatory pain was induced by injecting 1% carrageenan into the right hind paw of the mice.

METHODS

The animals were administered thiazolidine derivatives, 1b and 1d (1 mg/kg, 3 mg/kg, or 10 mg/kg), intraperitoneally 30 minutes before carrageenan administration. The animals' behavior was evaluated by measuring thermal hyperalgesia, mechanical allodynia, and motor coordination. The IL-1β expression was measured by enzyme-linked immunosorbent assay. Acute and sub-acute toxicity studies were conducted to evaluate the toxicity profile of compounds.

RESULTS

Treatment with the thiazolidine derivative, 1b and 1d, attenuated carrageenan-induced thermal hyperalgesia and mechanical allodynia at doses of 1 mg/kg, 3 mg/kg, and 10 mg/kg. No motor coordination deficits were observed in animals. The compounds also reduced IL-1β expression in the spinal cord of mice. Acute and sub-acute toxicity studies revealed that both compounds were safe.

CONCLUSION

The compounds exhibit promising activity against inflammatory pain due to their ability to produce anti-hyperalgesic and anti-allodynic effects and to inhibit IL-1β expression in the spinal cord.

HPLC-DAD-UV analysis, anti-inflammatory and anti-neuropathic effects of methanolic extract of Sideritis bilgeriana (lamiaceae) by NF-κB, TNF-α, IL-1β and IL-6 involvement

Medicinal plants remain an invaluable source for therapeutics of diseases that affect humanity. Sideritis bilgeriana (Lamiaceae) is medicinal plant used in Turkey folk medicine to reduce inflammation and pain, but few studies scientific corroborates its medicinal use so creating a gap between popular use and scientific evidence. Thus, we aimed to evaluate the pharmacological effects of the methanolic extract of S. bilgeriana (MESB) in rodents nociception models and also performed its phytochemical analysis. Firstly, a screening was carried out that enabled the identification of the presence of phenolic compounds and flavonoids. In view of this, a chromatographic method by HPLC-DAD-UV was developed that made it possible to identify chlorogenic acid and its quantification in MESB. MESB-treated mice (MESB 50, 100 and 200 mg/kg, p.o.) reduced mechanical hyperalgesia and myeloperoxidase activity (p < 0.01), and also showed a reduced pain behavior in capsaicin test. In the carrageenan-induced pleurisy test, MESB (100 mg/kg p.o.) significantly reduced the leukocyte (polymorphonuclear) count in the pleural cavity and equally decreased the TNF-α and IL-1β levels (p < 0.001). In the PSNL model, mechanical hyperalgesia was reduced on the first evaluation day and during the 7 days of evaluation compared to the vehicle group (p < 0.001). Thermal hyperalgesia was also reduced 1 h after treatment compared to the vehicle group (p < 0.001) and reversed the loss of force initially displayed by the animals, thus inferring an analgesic effect in the muscle strength test. Analysis of the marrow of these animals showed a decrease in the level of pro-inflammatory cytokine IL-6 (p < 0.001) and factor NF-κB, in relation to the control group (p < 0.05). Moreover, the MESB treatment produced no noticeable side effects, no disturb in motor performance and no signs of gastric or hepatic injury. Together, the results suggests that MESB could be useful to management of inflammation and neuropathic pain mainly by the management of pro-inflammatory mediators (NF-κB, TNF-α, IL-1β and IL-6), so reinforcing its use in popular medicine and corroborating the need for further chemical and pharmacological studies for the species.

Cynandione A Alleviates Neuropathic Pain Through α7-nAChR-Dependent IL-10/β-Endorphin Signaling Complexes

Cynandione A, an acetophenone isolated from Cynanchum Wilfordii Radix, exhibits antineuropathic pain effect. This study further explored the target molecule and signaling mechanisms underlying cynandione-A-induced antineuropathic pain. Intrathecal injection of cynandione A significantly attenuated mechanical allodynia in neuropathic rats and substantially increased spinal expression of IL-10 and β-endorphin but not dynorphin A. Cynandione A treatment also enhanced expression of IL-10 and β-endorphin but not α7 nicotinic acetylcholine receptors (nAChRs) in cultured microglia. The IL-10 antibody attenuated cynandione-A-induced spinal or microglial gene expression of β-endorphin and mechanical allodynia, whereas the β-endorphin antiserum blocked cynandione-A-induced mechanical antiallodynia but not spinal or microglial IL-10 gene expression. The α7 nAChR antagonist methyllycaconitine significantly reduced cynandione-A-induced mechanical antiallodynia and spinal or microglial expression of IL-10 and β-endorphin. Furthermore, cynandione A stimulated microglial phosphorylation of PKA, p38, and CREB in an α7-nAChR-dependent manner, and treatment with their inhibitors attenuated cynandione-A-induced mechanical antiallodynia and spinal or microglial expression of IL-10 and β-endorphin. In addition, cynandione A stimulated spinal phosphorylation of the transcription factor STAT3, which was inhibited by methyllycaconitine, the PKA activation inhibitor or IL-10 antibody. The STAT3 inhibitor NSC74859 also abolished cynandione-A-induced mechanical antiallodynia and spinal expression of β-endorphin. These findings suggest that cynandione A suppresses neuropathic pain through α7-nAChR-dependent IL-10/β-endorphin signaling pathway in spinal microglia.

Effect of captopril, an angiotensin-converting enzyme inhibitor, on morphine analgesia and tolerance in rats, and elucidating the inflammation and endoplasmic reticulum stress pathway in this effect

The purpose of current study was to examine the possible involvement of captopril, an angiotensin-converting enzyme inhibitor, on nociception, morphine analgesia and morphine tolerance development involving inflammation and ER-stress pathways in rats. In this study, thirty-six male Wistar rats were used. Animals were divided into six groups: Saline, 50 mg/kg captopril, 5 mg/kg morphine, morphine + captopril, morphine tolerance and morphine tolerance + captopril. The resulting analgesic effect was measured with hot plate and tail flick analgesia tests. The dorsal root ganglions (DRG) tissues were collected for inflammation parameters, endoplasmic reticulum (ER) stress and apoptosis proteins by using ELISA. Captopril showed anti-nociceptive effect when given alone (p < 0.05 to p < 0.01). In addition, captopril increased the analgesic effect of morphine (p < 0.05 to p < 0.001) and also decreased the tolerance to morphine at a significant level (p < 0.05 to p < 0.001). However, it decreased inflammation and ER-stress when applied with single-dose morphine and tolerance induction (p < 0.001). Moreover, captopril decreased apoptosis proteins after tolerance development (p < 0.001). In conclusion, captopril has antinociceptive properties, increasing analgesic effect of morphine, and preventing tolerance development. These effects may occur by suppressing inflammation and ER-stress pathways.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

Anakinra, an interleukin-1 receptor antagonist, increases the morphine analgesic effect and decreases morphine tolerance development by modulating oxidative stress and endoplasmic reticulum stress in rats

Background/aim

Recent studies have shown that inflammation plays a role in morphine analgesia and tolerance development. Anakinra is a competitive inhibitor of IL-1 receptors and an antiinflammatory protein regulating IL-1β’s biological activity by avoiding signal transduction. In this study, we aimed to examine the effects of anakinra on morphine analgesia and tolerance.

Materials and methods

In this study, 36 Wistar Albino (230–250 g) male rats were used. Animals were divided into 6 groups: saline (S), 100 mg/kg anakinra (A), 5mg/kg morphine (M), M+A, morphine tolerance (MT), and MT+A. The resulting analgesic effect was measured with hot plate and tail-flick analgesia tests. After the analgesia tests, the dorsal root ganglions (DRG) tissues were removed. Oxidative stress parameters [total antioxidant status (TAS), total oxidant status (TOS)], endoplasmic reticulum (ER) stress, and apoptosis proteins [E74-like factor 2 (elF-2α), activating transcription factor 4 (ATF-4), C/EBP homologous protein (CHOP), caspase-3, and bcl-2-associated X protein (bax)] were measured in DRG tissues.

Results

Anakinra showed an antinociceptive effect when given alone (P < 0.001). In addition, anakinra increased the analgesic effect of morphine (P < 0.05 to P < 0.001), and also decreased the tolerance to morphine at a significant level (P < 0.05 to P < 0.001). Moreover, it decreased oxidative stress and ER-stress when given as a single-dose morphine and tolerance induction (P < 0.01 to P < 0.001). Furthermore, anakinra decreased apoptosis proteins after tolerance development (P < 0.001).

Conclusion

Anakinra has antinociceptive properties, and it increases the analgesic effect of morphine and also prevents tolerance development. These effects probably occur by the modulation of oxidative stress and ER-stress pathways.

Impact of anesthesia and analgesia techniques on glioblastoma progression. A narrative review

Glioblastoma (GBM) is an aggressive malignant CNS tumor with a median survival of 15 months after diagnosis. Standard therapy for GBM includes surgical resection, radiation, and temozolomide. Recently, anesthetics and analgesics have received attention for their potential involvement in mediating tumor growth. This narrative review investigated whether various members of the 2 aforementioned classes of drugs have a definitive impact on GBM progression by summarizing pertinent in vitro, in vivo, and clinical studies. Recent publications regarding general anesthetics have been inconsistent, showing that they can be pro-tumoral or antitumoral depending on the experimental context. The local anesthetic lidocaine has shown consistent antitumoral effects in vitro. Clinical studies looking at anesthetics have not concluded that their use improves patient outcomes. In vitro and in vivo studies looking at opioid involvement in GBM have demonstrated inconsistent findings regarding whether these drugs are pro-tumoral or antitumoral. Nonsteroidal anti-inflammatory drugs, and specifically COX-2 inhibitors, have shown inconsistent findings across multiple studies looking at whether they are beneficial in halting GBM progression. Until multiple repeatable studies show that anesthetics and analgesics can suppress GBM growth, there is no strong evidence to recommend changes in the anesthetic care of these patients.

The role of the M1/M2 microglia in the process from cancer pain to morphine tolerance

Cancer pain, especially bone cancer pain, is a pain state often caused by inflammation or dysfunctional nerves. Moreover, in the management of cancer pain, opioid especially morphine is widely used, however, it also brings severe side effects such as morphine tolerance to the patient (Deandrea et al., 2008). A growing body of literatures demonstrated that neuroinflammation is mediated by microglia. As the macrophages like immune cells, microglia play an important role in the pathogenesis of cancer pain and morphine tolerance. Microglia acquire different activation states to regulate the function of these cells. As to M1 phenotype, microglia release pro-inflammatory cytokines and neurotoxic molecules that promote inflammation and cytotoxic reactions. Conversely, when microglia represent M2 phenotypes secreting anti-inflammatory cytokines and nutrient factors that promote the function of repair, regeneration and restore homeostasis. A better understanding of microglia activation in cancer pain and morphine tolerance is crucial for the development of hypothesized neuroprotective drugs. Targeting microglia different polarization states by the inhibition of their deleterious pro-inflammatory neurotoxicity and/or enhancing their beneficial anti-inflammatory protective function seems to be an effective treatment for cancer pain and morphine tolerance.

Sphingosine-1-phosphate receptor subtype 1 activation in the central nervous system contributes to morphine withdrawal in rodents

Opioid therapies for chronic pain are undermined by many adverse side effects that reduce their efficacy and lead to dependence, abuse, reduced quality of life, and even death. We have recently reported that sphingosine-1-phosphate (S1P) 1 receptor (S1PR1) antagonists block the development of morphine-induced hyperalgesia and analgesic tolerance. However, the impact of S1PR1 antagonists on other undesirable side effects of opioids, such as opioid-induced dependence, remains unknown. Here, we demonstrate that naloxone-precipitated morphine withdrawal in mice altered de novo sphingolipid metabolism in the dorsal horn of the spinal cord and increased S1P that accompanied the manifestation of several withdrawal behaviors. Blocking de novo sphingolipid metabolism with intrathecal administration of myriocin, an inhibitor of serine palmitoyltransferase, blocked naloxone-precipitated withdrawal. Noteworthy, we found that competitive (NIBR-15) and functional (FTY720) S1PR1 antagonists attenuated withdrawal behaviors in mice. Mechanistically, at the level of the spinal cord, naloxone-precipitated withdrawal was associated with increased glial activity and formation of the potent inflammatory/neuroexcitatory cytokine interleukin-1β (IL-1β); these events were attenuated by S1PR1 antagonists. These results provide the first molecular insight for the role of the S1P/S1PR1 axis during opioid withdrawal. Our data identify S1PR1 antagonists as potential therapeutics to mitigate opioid-induced dependence and support repurposing the S1PR1 functional antagonist FTY720, which is FDA-approved for multiple sclerosis, as an opioid adjunct.

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.