Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

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.

Differential gene expression and chromatin accessibility in the medial prefrontal cortex associated with individual differences in rat behavioral models of opioid use disorder

Opioid use disorder (OUD) is a neuropsychological disease that has a devastating impact on public health. Substantial individual differences in vulnerability exist, the neurobiological substrates of which remain unclear. To address this question, we investigated genome-wide gene transcription (RNA-seq) and chromatin accessibility (ATAC-seq) in the medial prefrontal cortex (mPFC) of male and female rats exhibiting differential vulnerability in behavioral paradigms modeling different phases of OUD: Withdrawal-Induced Anhedonia (WIA), Demand, and Reinstatement. Ingenuity Pathway Analysis (IPA) of RNA-seq revealed greater changes in canonical pathways in Resilient (vs. Saline) rats in comparison to Vulnerable (vs. Saline) rats across 3 paradigms, suggesting brain adaptations that might contribute to resilience to OUD across its trajectory. Analyses of gene networks and upstream regulators implicated processes involved in oligodendrocyte maturation and myelination in WIA, neuroinflammation in Demand, and metabolism in Reinstatement. Motif analysis of ATAC-seq showed changes in chromatin accessibility to a small set of transcription factor (TF) binding sites as a function either of opioid exposure (i.e., morphine versus saline) generally or of individual vulnerability specifically. Some of these were shared across the 3 paradigms and others were unique to each. In conclusion, we have identified changes in biological pathways, TFs, and their binding motifs that vary with paradigm and OUD vulnerability. These findings point to the involvement of distinct transcriptional and epigenetic mechanisms in response to opioid exposure, vulnerability to OUD, and different stages of the disorder.

Prevalence and services for the treatment of chronic pain at residential substance treatment centers

Introduction: Studies suggest that a large proportion of patients with substance use disorders (SUDs) also have underlying chronic pain. There is limited data on prevalence of chronic pain treatment as a component of residential substance use treatment. This initiative sought to investigate the prevalence and type of chronic pain services offered at these residential programs.Methods: This study was a retrospective review of information obtained from residential substance use treatment facility websites contained in SAMHSA's treatment navigator. Nine hundred-fifty out of 2952 websites were randomly selected for analysis. The primary outcome was prevalence of facilities that had chronic pain programs. Services offered were specified as available. Descriptive statistics were used to summarize data.Results: Nine-hundred nine websites (95.7%, [94,97]) were accessible. Twenty-six facilities (2.9%,[1.9,4.2]) had a chronic pain program and of these 22 (84.6%, [64.3,95.0]) specified services offered. Common services included physical therapy (6, 27.3%), massage (12, 54.6%), and acupuncture (10, 45.5%). Of the remaining sites, 630 (69.3%, [66.2,72.3]) specified services offered, including yoga (122, 19.4%) and exercise (199, 31.6%).Conclusion: Our study demonstrated that despite most facilities offering adjunctive services, few had chronic pain programs. This suggests that there is a possible need for better updating of facility websites or possibly an area for improvement in residential substance use treatment settings.

The Inflammation/NF-κB and BDNF/TrkB/CREB Pathways in the Cerebellum Are Implicated in the Changes in Spatial Working Memory After Both Morphine Dependence and Withdrawal in Rat

We aimed to explore the impact of the cerebellum on the decline in spatial working memory following morphine dependence and withdrawal. Two groups of male Wistar rats received intraperitoneal injections of either saline (1 ml/kg) or morphine (10 mg/kg) twice daily for 10 days, serving as the control and dependent groups. Additionally, a withdrawal group underwent a 30-day withdrawal period after the dependence phase. Spatial working memory was assessed using a Y maze test. ELISA and western blot were used to assess protein levels in the cerebellum. On day 1, morphine impaired spatial working memory, deteriorated further after 10 days of morphine use, and nearly returned to its initial level following a 30-day withdrawal period. On day 10, significant increases in TNF-α, IL-1β, and CXCL12 and a notable decrease in IL-10 levels were detected in the morphine-dependent group, which did not completely restore in the withdrawal group. The protein levels of CXCR4, TLR4, P2X7R, and NF-κB sharply increased in the morphine-dependent group. However, these levels almost returned to normal after withdrawal. In the morphine-dependent group, BDNF decreased, while TrkB and CREB1 increased noticeably. Nevertheless, after withdrawal, TrkB and CREB1 but not BDNF levels returned to normal. In the morphine-dependent group, both CACNA1 and KCNMA1 decreased significantly and after withdrawal, only KCNMA1 showed partial restoration, while CACNA1 did not. It can be concluded that inflammation/NF-κB and BDNF/TrkB/CREB pathways play key roles in neural adaptation within the cerebellum, contributing to the decline in spatial working memory after both morphine dependence and withdrawal.

Impact of Morphine withdrawal on genes related to the TLR2 pathway expressed in the Prefrontal Cortex

Millions of people suffer from opioid use disorder, because of the ongoing opioid epidemic. The aversive symptoms of withdrawal are a leading factor for drug relapses, yet there are limited therapeutic options to minimize or prevent withdrawal symptoms. The mechanism behind opioid withdrawal is still not fully understood, thus preventing the development of new therapeutics. This study is an extension of our previously proposed mechanism of a toll-like receptor 2 (TLR2) mediated withdrawal response as a result of morphine induced microbial change that occurs during morphine withdrawal. Transcriptome analysis of the pre-frontal cortex indicated that there was increased expression of genes related to TLR2 signaling in morphine withdrawal treated animals compared to placebo controls. Antibiotic treatment further altered TLR2 related genes, recovering some of the morphine induced effect and leading to additional suppression of some genes related to the TLR2 pathway. Morphine withdrawal induced gene expression was attenuated in a whole body TLR2 knockout model. These results provide more support that TLR2 plays an integral role in morphine withdrawal mechanisms and could be a potential therapeutic target to minimize opioid withdrawal associated co-morbidities.

Tsc22d3 promotes morphine tolerance in mice through the GPX4 ferroptosis pathway

BACKGROUND

Morphine tolerance refers to gradual reduction in response to drug with continuous or repeated use of morphine, requiring higher doses to achieve same effect.

METHODS

The morphine tolerance dataset GSE7762 profiles, obtained from gene expression omnibus (GEO) database, were used to identify differentially expressed genes (DEGs). Weighted Gene Co-expression Network Analysis (WGCNA) was applied to explore core modules of DEGs related to morphine tolerance. Core genes were input into Comparative Toxicogenomics Database (CTD). Animal experiments were performed to validate role of Tsc22d3 in morphine tolerance and its relationship with ferroptosis-related pathway.

RESULTS

500 DEGs were identified. DEGs were primarily enriched in negative regulation of brain development, neuronal apoptosis processes, and neurosystem development. Core gene was identified as Tsc22d3. Tsc22d3 gene-associated miRNAs were mmu-miR-196b-5p and mmu-miR-196a-5p. Compared to Non-morphine tolerant group, Tsc22d3 expression was significantly upregulated in Morphine tolerant group. Tsc22d3 expression was upregulated in Morphine tolerant+Tsc22d3_OE, expression of HIF-1alpha, GSH, GPX4 in GPX4 ferroptosis-related pathway showed a more pronounced decrease. As Tsc22d3 expression was downregulated in Morphine tolerant+Tsc22d3_KO, expression of HIF-1alpha, GSH, GPX4 in GPX4 ferroptosis-related pathway exhibited a more pronounced increase. Upregulation of Tsc22d3 in Morphine tolerant+Tsc22d3_OE led to a more pronounced increase in expression of apoptosis proteins (P53, Caspase-3, Bax, SMAC, FAS). The expression of inflammatory factors (IL6, TNF-alpha, CXCL1, CXCL2) showed a more pronounced increase with upregulated Tsc22d3 expression in Morphine tolerant+Tsc22d3_OE.

CONCLUSIONS

Tsc22d3 is highly expressed in brain tissue of morphine-tolerant mice, activating ferroptosis pathway, enhancing apoptosis, promoting inflammatory responses in brain cells.

The effect of cannabinoid type 2 receptor agonist on morphine tolerance

Pain highly impacts the quality of life of patients. Morphine is used for pain treatment; however, its side effects, especially morphine tolerance, limit its use in the clinic. The problem of morphine tolerance has plagued health workers and patients for years. Unfortunately, the exact mechanism of morphine tolerance has not been fully clarified. The mechanisms of morphine tolerance that are currently being studied may include μ-opioid receptor (MOR) desensitization and internalization, mitogen-activated protein kinase (MAPK) pathway activation and crosstalk, the effects of microglia and the increase in inflammatory factors. Morphine tolerance can be alleviated by improving the pathophysiological changes that lead to morphine tolerance. Previous studies have shown that a cannabinoid type 2 (CB2) receptor agonist could attenuate morphine tolerance in a variety of animal models. Many studies have shown an interaction between the cannabinoid system and the opioid system. The CB2 receptor may modulate the effect of morphine through a pathway that is common to the MOR, since both receptors are G protein-coupled receptors (GPCRs). This study introduces the potential mechanism of morphine tolerance and the effect of CB2 receptor agonists on reducing morphine tolerance, which can provide new ideas for researchers studying morphine and provide beneficial effects for patients suffering from morphine tolerance.

Microglia in neuroimmunopharmacology and drug addiction

Drug addiction is a chronic and debilitating disease that is considered a global health problem. Various cell types in the brain are involved in the progression of drug addiction. Recently, the xenobiotic hypothesis has been proposed, which frames substances of abuse as exogenous molecules that are responded to by the immune system as foreign "invaders", thus triggering protective inflammatory responses. An emerging body of literature reveals that microglia, the primary resident immune cells in the brain, play an important role in the progression of addiction. Repeated cycles of drug administration cause a progressive, persistent induction of neuroinflammation by releasing microglial proinflammatory cytokines and their metabolic products. This contributes to drug addiction via modulation of neuronal function. In this review, we focus on the role of microglia in the etiology of drug addiction. Then, we discuss the dynamic states of microglia and the correlative and causal evidence linking microglia to drug addiction. Finally, possible mechanisms of how microglia sense drug-related stimuli and modulate the addiction state and how microglia-targeted anti-inflammation therapies affect addiction are reviewed. Understanding the role of microglia in drug addiction may help develop new treatment strategies to fight this devastating societal challenge.

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.

Assessing effects of tamoxifen on tolerance, dependence, and glutamate and glutamine levels in frontal cortex and hippocampus in chronic morphine treatment

Tamoxifen has been shown to reduce glutamate release from presynaptic glutamatergic nerves and reverse tolerance to morphine-induced respiratory depression. Changes in glutamatergic neurotransmission in the central nervous system contribute to morphine tolerance, dependence, and withdrawal. This study, therefore, evaluated effects of tamoxifen on development of analgesic tolerance and dependence, and brain glutamate and glutamine levels in chronic morphine administration. Mice implanted with placebo or morphine pellets were injected with tamoxifen (0.6-2 mg/kg) or vehicle twice daily for 3 days. Nociceptive response was evaluated in the hot plate and tail immersion tests, 4, 48 and 72 h post-implant, and following a challenge dose of morphine (10 mg/kg). Withdrawal signs were determined after naloxone (1 mg/kg) administration. Morphine increased nociceptive threshold which declined over time. At 72 h, acute morphine elicited tolerance to the analgesic effect in the hot plate test in vehicle or tamoxifen administered animals. In the tail immersion test, however, tolerance to morphine analgesia was observed in tamoxifen, but not vehicle, co-administration. Tamoxifen did not reduce withdrawal signs. In contrast to previous reports, glutamate and glutamine levels in the hippocampus and frontal cortex did not change in the morphine-vehicle group. Confirming previous findings, tamoxifen (2 mg/kg) decreased glutamate and glutamine concentrations in the hippocampus in animals with placebo pellets. Both doses of tamoxifen significantly changed glutamate and/or glutamine concentrations in both regions in morphine pellet implanted animals. These results suggest that tamoxifen has no effect on dependence but may facilitate tolerance development to the antinociception, possibly mediated at the spinal level, in chronic morphine administration.

Multimodal Acute Pain Management in the Parturient with Opioid Use Disorder: A Review

The opioid epidemic in the United States has led to an increasing number of pregnant patients with opioid use disorder (OUD) presenting to obstetric units. Caring for this complex patient population requires an interdisciplinary approach involving obstetricians, anesthesiologists, addiction medicine physicians, psychiatrists, and social workers. The management of acute pain in the parturient with OUD can be challenging due to several factors, including respiratory depression, opioid tolerance, and opioid-induced hyperalgesia. Patients with a history of OUD can present in one of three categories: 1) those with untreated OUD; 2) those who are currently abstinent from opioids; 3) those being treated with medications to prevent withdrawal. A patient-centered, multimodal approach is essential for optimal peripartum pain relief and prevention of adverse maternal and neonatal outcomes. Medications for opioid use disorder (MOUD), previously referred to as medication-assisted therapy (MAT), include opioids like methadone, buprenorphine, and naltrexone. These are prescribed for pregnant patients with OUD, but appropriate dosing and administration of these medications are critical to avoid withdrawal in the mother. Non-opioid analgesics such as acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) can be used in a stepwise approach, and regional techniques like neuraxial anesthesia and truncal blocks offer opioid-sparing options. Other medications like ketamine, clonidine, dexmedetomidine, nitrous oxide, and gabapentinoids show promise for pain management but require further research. Overall, a comprehensive pain management strategy is essential to ensure the well-being of both the mother and the fetus in pregnant patients with OUD.

Genetically supported causality between gut microbiota, immune cells and morphine tolerance: a two-sample Mendelian randomization study

Background

Previous researches have suggested a significant connection between the gut microbiota/immune cells and morphine tolerance (MT), but there is still uncertainty regarding their causal relationship. Hence, our objective is to inverstigate this causal association and reveal the impact of gut microbiota/immune cells on the risk of developing MT using a two-sample Mendelian randomization (MR) study.

Methods

We conducted a comprehensive analysis using genome-wide association study (GWAS) summary statistics for gut microbiota, immune cells, and MT. The main approach employed was the inverse variance-weighted (IVW) method in MR. To assess horizontal pleiotropy and remove outlier single-nucleotide polymorphisms (SNPs), we utilized the Mendelian randomization pleiotropy residual sum and outlier (MR-PRESSO) technique as well as MR-Egger regression. Heterogeneity detection was performed using Cochran's Q-test. Additionally, leave-one-out analysis was carried out to determine if any single SNP drove the causal association signals. Finally, we conducted a reverse MR to evaluate the potential of reverse causation.

Results

We discovered that 6 gut microbial taxa and 16 immune cells were causally related to MT (p < 0.05). Among them, 2 bacterial features and 9 immunophenotypes retained a strong causal relationship with lower risk of MT: genus. Lachnospiraceae NK4A136group (OR: 0.962, 95% CI: 0.940-0.987, p = 0.030), genus. RuminococcaceaeUCG011 (OR: 0.960, 95% CI: 0.946-0.976, p = 0.003), BAFF-R on B cell (OR: 0.972, 95% CI: 0.947-0.998, p = 0.013). Furthermore, 4 bacterial features and 7 immunophenotypes were identified to be significantly associated with MT risk: genus. Flavonifractor (OR: 1.044, 95% CI: 1.017-1.069, p = 0.029), genus. Prevotella9 (OR: 1.054, 95% CI: 1.020-1.090, p = 0.037), B cell % CD3-lymphocyte (OR: 1.976, 95% CI: 1.027-1.129, p = 0.026). The Cochrane's Q test revealed no heterogeneity (p > 0.05). Furthermore, the MR-Egger and MR-PRESSO analyses reveal no instances of horizontal pleiotropy (p > 0.05). Besides, leave-one-out analysis confirmed the robustness of MR results. After adding BMI to the multivariate MR analysis, the gut microbial taxa and immune cells exposure-outcome effect were attenuated.

Conclusion

Our research confirm the potential link between gut microbiota and immune cells with MT, shedding light on the mechanism by which gut microbiota and immune cells may contribute to MT. These findings lay the groundwork for future investigations into targeted prevention strategies.

The Effect of Prenatal and Neonatal Fluoride Exposure to Morphine-Induced Neuroinflammation

Physical dependence is associated with the formation of neuroadaptive changes in the central nervous system (CNS), both at the molecular and cellular levels. Various studies have demonstrated the immunomodulatory and proinflammatory properties of morphine. The resulting neuroinflammation in drug dependence exacerbates substance abuse-related behaviors and increases morphine tolerance. Studies prove that fluoride exposure may also contribute to the development of neuroinflammation and neurodegenerative changes. Morphine addiction is a major social problem. Neuroinflammation increases tolerance to morphine, and neurodegenerative effects caused by fluoride in structures related to the development of dependence may impair the functioning of neuronal pathways, change the concentration of neurotransmitters, and cause memory and learning disorders, which implies this element influences the development of dependence. Therefore, our study aimed to evaluate the inflammatory state of selected brain structures in morphine-dependent rats pre-exposed to fluoride, including changes in cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) expression as well as microglial and astroglial activity via the evaluation of Iba1 and GFAP expression. We provide evidence that both morphine administration and fluoride exposure have an impact on the inflammatory response by altering the expression of COX-1, COX-2, ionized calcium-binding adapter molecule (Iba1), and glial fibrillary acidic protein (GFAP) in brain structures involved in dependence development, such as the prefrontal cortex, striatum, hippocampus, and cerebellum. We observed that the expression of COX-1 and COX-2 in morphine-dependent rats is influenced by prior fluoride exposure, and these changes vary depending on the specific brain region. Additionally, we observed active astrogliosis, as indicated by increased GFAP expression, in all brain structures of morphine-dependent rats, regardless of fluoride exposure. Furthermore, the effect of morphine on Iba1 expression varied across different brain regions, and fluoride pre-exposure may influence microglial activation. However, it remains unclear whether these changes are a result of the direct or indirect actions of morphine and fluoride on the factors analyzed.

Ultramicronized N-palmitoylethanolamine Contributes to Morphine Efficacy Against Neuropathic Pain: Implication of Mast Cells and Glia

BACKGROUND

In the current management of neuropathic pain, in addition to antidepressants and anticonvulsants, the use of opioids is wide, despite their related and well-known issues.

OBJECTIVE

N-palmitoylethanolamine (PEA), a natural fatty-acid ethanolamide whose anti-inflammatory, neuroprotective, immune-modulating and anti-hyperalgesic activities are known, represents a promising candidate to modulate and/or potentiate the action of opioids.

METHODS

This study was designed to evaluate if the preemptive and morphine concomitant administration of ultramicronized PEA, according to fixed or increasing doses of both compounds, delays the onset of morphine tolerance and improves its analgesic efficacy in the chronic constriction injury (CCI) model of neuropathic pain in rats.

RESULTS

Behavioral experiments showed that the preemptive and co-administration of ultramicronized PEA significantly decreased the effective dose of morphine and delayed the onset of morphine tolerance. The activation of spinal microglia and astrocytes, commonly occurring both on opioid treatment and neuropathic pain, was investigated through GFAP and Iba-1 immunofluorescence. Both biomarkers were found to be increased in CCI untreated or morphine treated animals in a PEA-sensitive manner. The increased density of endoneural mast cells within the sciatic nerve of morphine-treated and untreated CCI rats was significantly reduced by ultramicronized PEA. The decrease of mast cell degranulation, evaluated in terms of reduced plasma levels of histamine and N-methyl-histamine metabolite, was mainly observed at intermediate-high doses of ultramicronized PEA, with or without morphine.

CONCLUSION

Overall, these results show that the administration of ultramicronized PEA in CCI rats according to the study design fully fulfilled the hypotheses of this study.

Substance Use and Addiction

Efforts to reveal the molecular, cellular, and circuit mechanisms of addiction have largely focused on neurons. Yet accumulating data regarding the ability of glial cells to impact synaptic function, circuit activity, and behavior demands that we explore how these nonneuronal cells contribute to substance use disorders and addiction. Important work has shown that glial cells, including microglia, exhibit changes in phenotype following exposure to drugs of abuse and that modification of glial responses can impact behaviors related to drug seeking and drug taking. While these are critical first steps to understanding how microglia can impact addiction, there are still substantial gaps in knowledge that need to be addressed. This chapter reviews some of the key studies that have shown how microglia are affected by and can contribute to addiction. It also discusses areas where more knowledge is urgently needed to reveal new therapeutic and preventative approaches.

Direct inhibition of microglial activation by a μ receptor selective agonist alleviates inflammatory-induced pain hypersensitivity

Opioids are widely used in the treatment of moderate and severe pain. Nociceptive stimulation has been reported to potentially promote microglial activation and neuroinflammation, which also causes chronic pain sensitization. The aim of this study was to demonstrate whether the novel μ receptor agonist MEL-0614 could inhibit activated microglia directly and the associated signaling pathway. Mice were administered lipopolysaccharide and formalin to induce allodynia. Von Frey test was used to detect the anti-allodynia effect of MEL-0614 before and after LPS and formalin injection. In the spinal cord, the levels of proinflammatory cytokines and microglial activation were determined after MEL-0614 administration. BV2 and primary microglia were cultured to further explore the effect of MEL-0614 on LPS-induced microglial activation and key signaling pathways involved. MEL-0614 partially prevented and reversed allodynia induced by LPS and formalin in vivo, which was not inhibited by the μ receptor antagonist CTAP. Minocycline was effective in reversing the established allodynia. MEL-0614 also downregulated the activation of microglia and related proinflammatory cytokines in the spinal cord. Additionally, in BV2 and primary microglia, MEL-0614 inhibited the LPS-induced upregulation of proinflammatory factors, which was unaffected by CTAP. The NLR family pyrin domain containing 3 (NLRP3) related signaling pathway may be involved in the interaction between MEL-0614 and microglia. The opioid agonist MEL-0614 inhibited the activation of microglia and the subsequent upregulation of proinflammatory factors both in vivo and in vitro. Notably, this effect is partially mediated by the μ receptor.

Morphine aggravates inflammatory, behavioral, and hippocampal structural deficits in septic rats

Although pain and sepsis are comorbidities of intensive care units, reported data on whether pain control by opioid analgesics could alter inflammatory and end-organ damage caused by sepsis remain inconclusive. Here, we tested the hypothesis that morphine, the gold standard narcotic analgesic, modifies behavioral and hippocampal structural defects induced by sepsis in male rats. Sepsis was induced with cecal ligation and puncture (CLP) and behavioral studies were undertaken 24 h later in septic and/or morphine-treated animals. The induction of sepsis or exposure to morphine (7 mg/kg) elicited similar: (i) falls in systolic blood pressure, (ii) alterations in spatial memory and learning tested by the Morris water maze, and (iii) depression of exploratory behavior measured by the new object recognition test. These hemodynamic and cognitive defects were significantly exaggerated in septic rats treated with morphine compared with individual interventions. Similar patterns of amplified inflammatory (IL-1β) and histopathological signs of hippocampal damage were noted in morphine-treated septic rats. Additionally, the presence of intact opioid receptors is mandatory for the induction of behavioral and hemodynamic effects of morphine because no such effects were observed when the receptors were blocked by naloxone. That said, our findings suggest that morphine provokes sepsis manifestations of inflammation and interrelated hemodynamic, behavioral, and hippocampal deficits.

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.

The bidirectional relationship between opioids and the gut microbiome: Implications for opioid tolerance and clinical interventions

Opioids are widely used in treating patients with acute and chronic pain; however, this class of drugs is also commonly abused. Opioid use disorder and associated overdoses are becoming more prevalent as the opioid crisis continues. Chronic opioid use is associated with tolerance, which decreases the efficacy of opioids over time, but also puts individuals at risk of fatal overdoses. Therefore, it is essential to identify strategies to reduce opioid tolerance in those that use these agents. The gut microbiome has been found to play a critical role in opioid tolerance, with opioids causing dysbiosis of the gut, and changes in the gut microbiome impacting opioid tolerance. These changes in turn have a detrimental effect on the gut microbiome, creating a positive feedback cycle. We review the bidirectional relationship between the gut microbiome and opioid tolerance, discuss the role of modulation of the gut microbiome as a potential therapeutic option in opioid-induced gut dysbiosis, and suggest opportunities for further research and clinical interventions.

Obesity-mediated Lipoinflammation Modulates Food Reward Responses

Accumulation of white adipose tissue (WAT) during obesity is associated with the development of chronic low-grade inflammation, a biological process known as lipoinflammation. Systemic and central lipoinflammation accumulates pro-inflammatory cytokines including IL-6, IL-1β and TNF-α in plasma and also in brain, disrupting neurometabolism and cognitive behavior. Obesity-mediated lipoinflammation has been reported in brain regions of the mesocorticolimbic reward circuit leading to alterations in the perception and consumption of ultra-processed foods. While still under investigation, lipoinflammation targets two major outcomes of the mesocorticolimbic circuit during food reward: perception and motivation ("Wanting") and the pleasurable feeling of feeding ("Liking"). This review will provide experimental and clinical evidence supporting the contribution of obesity- or overnutrition-related lipoinflammation affecting the mesocorticolimbic reward circuit and enhancing food reward responses. We will also address neuroanatomical targets of inflammatory profiles that modulate food reward responses during obesity and describe potential cellular and molecular mechanisms of overnutrition linked to addiction-like behavior favored by brain lipoinflammation.

Targeting Opioid Receptors in Addiction and Drug Withdrawal: Where Are We Going?

This review article offers an outlook on the use of opioids as therapeutics for treating several diseases, including cancer and non-cancer pain, and focuses the analysis on the opportunity to target opioid receptors for treating opioid use disorder (OUD), drug withdrawal, and addiction. Unfortunately, as has been well established, the use of opioids presents a plethora of side effects, such as tolerance and physical and physiological dependence. Accordingly, considering the great pharmacological potential in targeting opioid receptors, the identification of opioid receptor ligands devoid of most of the adverse effects exhibited by current therapeutic agents is highly necessary. To this end, herein, we analyze some interesting molecules that could potentially be useful for treating OUD, with an in-depth analysis regarding in vivo studies and clinical trials.

PAG neuronal NMDARs activation mediated morphine-induced hyperalgesia by HMGB1-TLR4 dependent microglial inflammation

Morphine is one of the most effective and widely used analgesic drugs. However, chronic morphine use caused opioid-induced hyperalgesia (OIH). The development of OIH limits the use of morphine. The mechanisms of OIH are not fully understood. Toll-like receptor4 (TLR4) and glutamate receptors in the periaqueductal gray (PAG) are critical in OIH, however, the association between TLR4 and N-methyl-D-aspartate Receptors (NMDARs) activation in PAG remains unclear. Microglia activation, increased TLR4/p65 nuclear factor-kappa B (p65 NF-κB) and proinflammatory cytokines in microglia, and phosphorylation of NMDAR1 subunit (NR1) and NMDAR2B subunit (NR2B) in neurons were observed in PAG of OIH mice. Up-regulations of TLR4/p65 NF-κB and proinflammatory cytokines (IL-1β, IL-6, TNF-α) in BV2 cells were prevented by inhibiting and knocking down TLR4. By inhibiting myeloid differentiation factor 2 (MD2) and knocking down the High-mobility group box 1 (HMGB1), we found that morphine activated TLR4 by HMGB1 but not MD2. We co-cultured Neuro-2a (N2A) with BV2 microglial cell line and found that instead of directly phosphorylating NMDAR subunits, morphine increased the phosphorylation of NR1 and NR2B by inducing TLR4-mediated microglia inflammation. Knocking TLR4 out of PAG by Lentivirus-GFP-TLR4 shRNA reversed these changes and relieved OIH. Our findings suggested that the secretion of HMGB1 induced by morphine-activated TLR4 in microglia, and the proinflammatory factors released by activated microglia phosphorylated NR1 and NR2B of adjacent neurons, induced increased neuronal excitability. In conclusion, TLR4/NMDARs in PAG were involved in the development and maintenance of OIH and supported novel strategies for OIH treatment.

Dexmedetomidine, an α2 agonist, increases the morphine analgesic effect and decreases morphine tolerance development by suppressing oxidative stress and TNF/IL-1 signalling pathway in rats

BACKGROUND

The aim of the present study is to examine the possible effect de dexmedetomidine on the development of morphine tolerance in rats including nociception, morphine analgesia, apoptosis, oxidative stress, and tumour necrosis factor (TNF)/ interleukin-1 (IL-1) pathways.

MATERIALS AND METHODS

In this study, 36 Wistar Albino (225-245 g) rats were used. Animals were divided into 6 groups: saline (S), 20 mcg/kg dexmedetomidine (D), 5 mg/kg morphine (M), M + D, morphine tolerance (MT), and MT + D. The analgesic effect was measured with hot plate and tail-flick analgesia tests. After the analgesia tests, the dorsal root ganglia (DRG) tissues were excised. Oxidative stress parameters [total antioxidant status (TAS), total oxidant status (TOS)], TNF, IL-1 and apoptosis enzymes (Caspase-3, Caspase-9), were measured in DRG tissues.

RESULTS

Dexmedetomidine showed an antinociceptive effect when given alone (p < 0.05 to p < 0.001). In addition, dexmedetomidine increased the analgesic effect of morphine (p < 0.001), and also decreased the tolerance to morphine at a significant level (p < 0.01 to p < 0.001). Moreover, it decreased oxidative stress (p < 0.001) and TNF/IL-1 levels when given as an additional drug of single-dose morphine and morphine tolerance group (p < 0.001). Furthermore, dexmedetomidine decreased Caspase-3 and Caspase-9 levels after tolerance development (p < 0.001).

CONCLUSION

Dexmedetomidine 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, inflammation and apoptosis.

Cerebrolysin Attenuates Exacerbation of Neuropathic Pain, Blood-spinal Cord Barrier Breakdown and Cord Pathology Following Chronic Intoxication of Engineered Ag, Cu or Al (50-60 nm) Nanoparticles

Neuropathic pain is associated with abnormal sensations and/or pain induced by non-painful stimuli, i.e., allodynia causing burning or cold sensation, pinching of pins and needles like feeling, numbness, aching or itching. However, no suitable therapy exists to treat these pain syndromes. Our laboratory explored novel potential therapeutic strategies using a suitable composition of neurotrophic factors and active peptide fragments-Cerebrolysin (Ever Neuro Pharma, Austria) in alleviating neuropathic pain induced spinal cord pathology in a rat model. Neuropathic pain was produced by constrictions of L-5 spinal sensory nerves for 2-10 weeks period. In one group of rats cerebrolysin (2.5 or 5 ml/kg, i.v.) was administered once daily after 2 weeks until sacrifice (4, 8 and 10 weeks). Ag, Cu and Al NPs (50 mg/kg, i.p.) were delivered once daily for 1 week. Pain assessment using mechanical (Von Frey) or thermal (Hot-Plate) nociceptive showed hyperalgesia from 2 weeks until 10 weeks progressively that was exacerbated following Ag, Cu and Al NPs intoxication in nerve lesioned groups. Leakage of Evans blue and radioiodine across the blood-spinal cord barrier (BSCB) is seen from 4 to 10 weeks in the rostral and caudal cord segments associated with edema formation and cell injury. Immunohistochemistry of albumin and GFAP exhibited a close parallelism with BSCB leakage that was aggravated by NPs following nerve lesion. Light microscopy using Nissl stain exhibited profound neuronal damages in the cord. Transmission electron microcopy (TEM) show myelin vesiculation and synaptic damages in the cord that were exacerbated following NPs intoxication. Using ELISA spinal tissue exhibited increased albumin, glial fibrillary acidic protein (GFAP), myelin basic protein (MBP) and heat shock protein (HSP 72kD) upregulation together with cytokines TNF-α, IL-4, IL-6, IL-10 levels in nerve lesion that was exacerbated following NPs intoxication. Cerebrolysin treatment significantly reduced hyperalgesia and attenuated BSCB disruption, edema formation and cellular changes in nerve lesioned group. The levels of cytokines were also restored near normal levels with cerebrolysin treatment. Albumin, GFAP, MABP and HSP were also reduced in cerebrolysin treated group and thwarted neuronal damages, myelin vesiculation and cell injuries. These neuroprotective effects of cerebrolysin with higher doses were also effective in nerve lesioned rats with NPs intoxication. These observations suggest that cerebrolysin actively protects spinal cord pathology and hyperalgesia following nerve lesion and its exacerbation with metal NPs, not reported earlier.

Allostatic load in opioid use disorder: a scoping review protocol

INTRODUCTION

Opioid use disorder affects 2.1 million individuals in the USA, causing more than 100 000 overdose-related deaths annually. While the neurobiological model of addiction is well described and accepted, there is a lack of morbidity and mortality prognosticators for patients struggling with opioid use disorder. Allostatic load index is a promising candidate for the basis of a prognostication tool. Previous studies show that allostatic load predicts both morbidity and mortality in a variety of cohorts. This scoping review protocol provides the rationale and steps for summarising and presenting existing evidence surrounding allostatic load in the context of opioid use disorder. Identification of current knowledge gaps will pave the way for subsequent prospective studies.

METHODS AND ANALYSIS

This scoping review protocol will follow the five-step method designed by Arksey and O'Malley. All studies written in English on allostatic load in the context of opioid use disorder, as defined in our inclusion criteria, will be included. There will be no limit on the year of publication. We will search PubMed, Embase, CINAHL, PsycINFO and Google Scholar. We will hand-review reference lists of included articles, and we will hand search grey literature. We will then group, analyse and present the data in narrative, tabular and diagrammatic format according to themes identified in the scoping review.

ETHICS AND DISSEMINATION

Ethics approval is not necessary, as data are gathered from publicly accessible sources. The results will be disseminated through a peer-reviewed journal and reported at conferences related to addiction medicine.

TRIAL REGISTRATION NUMBER

10.17605/OSF.IO/4J6DQ.

Repeated Use of Morphine Induces Anxiety by Affecting a Proinflammatory Cytokine Signaling Pathway in the Prefrontal Cortex in Rats

We examined the role of toll-like receptors (TLRs) and proinflammatory cytokine signaling pathways in the prefrontal cortex (PFC) in anxiety-like behaviors after repeated use of morphine. Morphine (10 mg/kg) was used twice daily for 8 days to induce morphine dependence in male Wistar rats. On day 8, opioid dependence was confirmed by measuring naloxone-precipitated withdrawal signs. On days 1 and 8, anxiety-like behaviors were evaluated using a light/dark box test. Expression of TLR1 and 4, proinflammatory cytokines, and some of the downstream signaling molecules was also evaluated in the bilateral PFC at mRNA and protein levels following morphine dependence. The results revealed that morphine caused anxiolytic-like effects on day 1 while induced anxiety following 8 days of repeated injection. On day 8, a significant decrease in TLR1 expression was detected in the PFC in morphine-dependent rats, but TLR4 remained unaffected. Repeated morphine injection significantly increased IL1-β, TNFα, and IL6 expression, but decreased IL1R and TNFR at mRNA and protein levels except for IL6R at the protein level in the PFC. The p38α mitogen-activated protein (MAP) kinase expression significantly increased but the JNK3 expression decreased in the PFC in morphine-dependent rats. Repeated injection of morphine also significantly increased the NF-κB expression in the PFC. Further, significant increases in Let-7c, mir-133b, and mir-365 were detected in the PFC in morphine-dependent rats. We conclude that TLR1 and proinflammatory cytokines signaling pathways in the PFC are associated with the anxiogenic-like effects of morphine following its chronic use in rats via a MAP kinase/NF-κB pathway.

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.

Effects of treadmill exercise and chronic stress on anxiety-like behavior, neuronal activity, and oxidative stress in basolateral amygdala in morphine-treated rats

The basolateral amygdala (BLA), which is sensitive to stress, is necessary for reward-seeking behavior and addiction. Regular exercise can produce various positive effects by affecting the BLA. Therefore, we aimed to investigate the effects of chronic stress and treadmill running (TR) on anxiety-like behavior, neuronal activity, lipid peroxidation (measured by malondialdehyde (MDA) levels, a marker for oxidative stress), and total thiol in BLA, in morphine-treated rats. Male Wistar rats were restricted in restraint stress and/or ran on the treadmill and treated with morphine (5 mg/kg) for 21 days. Anxiety-like behavior was evaluated using an elevated plus maze (EPM) and open field tests (OFTs), on day 22. On day 23, neuronal activity in BLA was assessed via single-unit recording. Finally, MDA and total thiol were assessed in BLA. Our results showed that chronic administration of morphine (5 mg/kg) did not affect anxiety-like behavior. However, the morphine-treated rats, subjected to chronic stress and exercise, showed fewer anxiety-like behaviors. Morphine increased BLA's MDA levels but it was prevented by TR. Glutamatergic and GABAergic basal neuronal activities were low in morphine-treated rats but after acute morphine application, there was a significant decrease in GABAergic neuronal activities in the morphine-exercise-stress (Mor-Exe-St) group. The results of this study showed that in morphine-treated rats, stress and exercise or their combination could have either co-directional or opposite effects to the chronic effects of morphine. These results indicate the existence of common pathways similar to endogenous opioids.

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.

Plasma Brain-Derived Neurotrophic Factor and Opioid Therapy: Results of Pilot Cross-Sectional Study

Objective: The neurotoxic effect of opioid has not been thoroughly described. No studies have been conducted to explain the effect of opioids in chronic non-cancer pain therapy on the neurotrophic factors level. Due to the ability to cross the blood-brain barrier, it seems the determination of serum Brain-derived neurotrophic factor (BDNF) concentration is a reliable presentation of the concentration in the central nervous system. The aim of the study was to explore the changes of plasma BDNF concentration during long-term opioid therapy.Methods: The study group included 28 patients with chronic low back pain treated with opioid therapy buprenorphine (n=10), tramadol (n=8), oxycodone (n=6), morphine (n=3), fentanyl (n=1). The control group included 11 patients. Measurements of plasma BDNF concentrations were performed, and information about opioid therapy were recorded (age, sex, opioid substance type, daily dose and the duration of opioid therapy). Data were analyzed using nonparametric tests.Results: The median BDNF level in the study group was significantly lower (2.73 ng/mL) than that in the control group (5.04 ng/mL, P<0.05). BDNF levels did not differ among groups based on the type of opioid substance used, but the lowest median value was observed for tramadol (2.62 ng/mL), and the highest median value was observed for buprenorphine (2.73 ng/mL). The widest minimum-maximum ranges of BDNF for oxycodone were noted, minimum 1.23 ng/mL and maximum 4.57 ng/mL, respectively. BDNF concentrations were correlated with age in the tramadol group and with the duration of opioid therapy in the buprenorphine group.Conclusion: Chronic opioid therapy for noncancer pain induces specific changes in the BDNF concentration. Tramadol and buprenorphine exerted an important effect on BDNF levels in the examined patients. The BDNF level depends on duration of opioid therapy with buprenorphine, and age in tramadol therapy.

Effects of Different Opioid Drugs on Oxidative Status and Proteasome Activity in SH-SY5Y Cells

Opioids are the most effective drugs used for the management of moderate to severe pain; however, their chronic use is often associated with numerous adverse effects. Some results indicate the involvement of oxidative stress as well as of proteasome function in the development of some opioid-related side effects including analgesic tolerance, opioid-induced hyperalgesia (OIH) and dependence. Based on the evidence, this study investigated the impact of morphine, buprenorphine or tapentadol on intracellular reactive oxygen species levels (ROS), superoxide dismutase activity/gene expression, as well as β2 and β5 subunit proteasome activity/biosynthesis in SH-SY5Y cells. Results showed that tested opioids differently altered ROS production and SOD activity/biosynthesis. Indeed, the increase in ROS production and the reduction in SOD function elicited by morphine were not shared by the other opioids. Moreover, tested drugs produced distinct changes in β2(trypsin-like) and β5(chymotrypsin-like) proteasome activity and biosynthesis. In fact, while prolonged morphine exposure significantly increased the proteolytic activity of both subunits and β5 mRNA levels, buprenorphine and tapentadol either reduced or did not alter these parameters. These results, showing different actions of the selected opioid drugs on the investigated parameters, suggest that a low µ receptor intrinsic efficacy could be related to a smaller oxidative stress and proteasome activation and could be useful to shed more light on the role of the investigated cellular processes in the occurrence of these opioid drug side effects.

Immune System Acts on Orthodontic Tooth Movement: Cellular and Molecular Mechanisms

Orthodontic tooth movement (OTM) is a tissue remodeling process based on orthodontic force loading. Compressed periodontal tissues have a complicated aseptic inflammatory cascade, which are considered the initial factor of alveolar bone remodeling. Since skeletal and immune systems shared a wide variety of molecules, osteoimmunology has been generally accepted as an interdisciplinary field to investigate their interactions. Unsurprisingly, OTM is considered a good mirror of osteoimmunology since it involves immune reaction and bone remolding. In fact, besides bone remodeling, OTM involves cementum resorption, soft tissue remodeling, orthodontic pain, and relapse, all correlated with immune cells and/or immunologically active substance. The aim of this paper is to review the interaction of immune system with orthodontic tooth movement, which helps gain insights into mechanisms of OTM and search novel method to short treatment period and control complications.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Angiotensin II receptor type 1 blocker candesartan improves morphine tolerance by reducing morphine‑induced inflammatory response and cellular activation of BV2 cells via the PPARγ/AMPK signaling pathway

Morphine is the most common drug of choice in clinical pain management; however, morphine tolerance presents a significant clinical challenge. The pathogenesis of morphine tolerance is known to be closely associated with angiotensin II receptor type 1 (AT1R) in microglia. As an AT1R antagonist, candesartan may serve an important role in regulating morphine tolerance. Therefore, the present study aimed to investigate the role of candesartan in morphine tolerance, and to explore the underlying mechanism. To meet this aim, BV2 microglial cells were treated with morphine or candesartan alone, or as a combination, and the expression levels of AT1R in BV2 cells were detected by reverse transcription‑quantitative PCR (RT‑qPCR) and western blotting. The levels of the inflammatory cytokines tumor necrosis factor‑α, interleukin (IL)‑1β and IL‑6 were subsequently detected by ELISA and western blotting. In addition, immunofluorescence analysis, western blotting and RT‑qPCR were used to detect the expression levels of the BV2 cell activation marker, ionized calcium‑binding adaptor molecule 1 (IBA‑1). Western blotting was also used to detect the expression levels of peroxisome proliferator‑activated receptor‑γ/AMP‑activated protein kinase (PPARγ/AMPK) signaling pathway‑associated proteins. Finally, the cells were treated with the PPARγ antagonist GW9662 and the AMPK inhibitor compound C to further explore the mechanism underlying the effects of candesartan on improving morphine tolerance. The expression levels of AT1R were revealed to be significantly increased following morphine induction; however, candesartan treatment inhibited the expression levels of AT1R, the levels of inflammatory cytokines and the protein expression levels of IBA‑1 in morphine‑induced BV2 cells in a dose‑dependent manner. These processes may be associated with activation of the PPARγ/AMPK signaling pathway. Taken together, the present study revealed that treatment with candesartan reduced morphine‑induced inflammatory response and cellular activation of BV2 cells via PPARγ/AMPK signaling.

The potential interplay between Opioid and the Toll-Like Receptor 4 (TLR-4)

CONTEXT

Opioids are available for the management of severe and chronic pain. However, long-term use of high-dose opioids could lead to physiologic tolerance, hyperalgesia, gastrointestinal immobility, addiction, respiratory depression, tumor progression, and inhibition of the immune system. It seems some of these adverse effects of opioids might be induced by TLR-4 signaling.

OBJECTIVE

The review aims to investigate the potential interplay between opioids and TLR-4 in CNS, gastrointestinal, cancer, and immune system.

METHODS

The search of PubMed, Embase, Scopus, web of sciences, and Google scholar was performed for all relevant studies published. From a total of 513 papers obtained at the initial database search, publications including in silico, in vitro, and in vivo studies were selected for the review.

RESULTS

A comprehensive review of studies indicated that using opioids for the reduction of pain might induce adverse effects such as analgesic tolerance, hyperalgesia, cancer progression, and suppression of the immune system. Some studies have indicated these effects may be due to a change in the level of expression and signaling pathway of TLR-4. The generalizability of the results was limited due to the inconsistency of findings.

CONCLUSIONS

More studies are needed to clarify TLR-4-mediated opioid effects on the biology or stages of the disease as well as the role of different types of opioids, appropriate dosage, and exposure in various contexts. Designing the drug candidate and doing many formulation studies for different diseases and various stages of disease could be associated with effective treatment and pain management.

Tyrosine 7.43 is important for mu-opioid receptor downstream signaling pathways activated by fentanyl

G protein–coupled receptors can signal through both G proteins and ß-arrestin2. For the µ-opioid receptor (MOR), early experimental evidence from a single study suggested that G protein signaling mediates analgesia and sedation, whereas ß-arrestin signaling mediates respiratory depression and constipation. Then, receptor mutations were used to clarify which residues interact with ligands to selectively regulate signals in a ligand-specific manner. However, there is no systematic study on how to determine these residues and clarify the molecular mechanism of their influence on signal pathways. We have therefore used molecular docking to predict the amino acid sites that affect the binding of ligands and MOR. Then, the corresponding sites were mutated to determine the effect of the structural determinant of MOR on G_i/o protein and ß-arrestin pathways. The pharmacological and animal behavioral experiments in combination with molecular dynamics simulations were used to elucidate the molecular mechanism of key residues governing the signaling. Without affecting ligand binding to MOR, MOR^Y7.43A attenuated the activation of both G_i/o protein and ß-arrestin signaling pathways stimulated by fentanyl, whereas it did not change these two pathways stimulated by morphine. Likewise, the activation peak time of extracellular regulated protein kinases was significantly prolonged at MOR^Y7.43A compared with that at MOR^wildtype stimulated by fentanyl, but there was no difference stimulated by morphine. In addition, MOR^Y7.43A significantly enhanced analgesia by fentanyl but not by morphine in the mice behavioral experiment. Furthermore, the molecular dynamics simulations showed that H6 moves toward the cellular membrane. H6 of the fentanyl–Y7.43A system moved outward more than that in the morphine–Y7.43A system. Y7.43 mutation disrupted hydrophobic interactions between W6.48 and Y7.43 in the fentanyl–Y7.43A system but not in the morphine–Y7.43A system. Our results have disclosed novel mechanisms of Y7.43 mutation affecting MOR signaling pathways. Y7.43 mutation reduced the activation of the G_i/o protein pathway and blocked the ß-arrestin2 recruitment, increased the H6 outward movement of MOR, and disrupted hydrophobic interactions. This may be responsible for the enhanced fentanyl analgesia. These findings are conducive to designing new drugs from the perspective of ligand and receptor binding, and Y7.43 is also expected to be a key site to structure optimization of synthesized compounds.

Examining the interrelationships between mindfulness-based interventions, depression, inflammation, and cancer survival

Depression is highly prevalent in those diagnosed with cancer and is also associated with poorer prognostic outcomes. Mindfulness-based interventions are effective in reducing depressive symptoms and improving quality of life in patients with cancer. The objective of this review was to investigate whether mindfulness practices can improve survival and, if so, what mechanisms of action may contribute to these outcomes. Although no long-term studies have investigated this hypothesis, the current literature supports an inflammatory basis for depression, implicating proinflammatory cytokines and hypothalamic-pituitary-adrenal axis dysfunction as contributing factors. Markers of inflammation, such as interleukin-6, tumor necrosis factor-α, and cortisol, are all found at elevated concentrations in many depressed individuals. These exact mechanisms are associated with higher mortality in patients with cancer. Mindfulness has been studied for its effects on cytokine and cortisol levels, and there are promising data to support that the intervention can measurably decrease inflammation. Therefore, it is conceivable that mindfulness programs can affect survival in this population. There are limited data on the long-term effects of mindfulness on depression and inflammatory markers in patients with cancer, and there are potential barriers to the implementation of mindfulness-based interventions as part of a comprehensive treatment plan. Therefore, it is necessary to further explore these questions through longitudinal studies to establish a survival correlation. CA Cancer J Clin. 2022;72:490-502.

Dysregulation of Vesicular Glutamate Transporter VGluT2 via BDNF/TrkB Pathway Contributes to Morphine Tolerance in Mice

Morphine is widely used in the treatment of moderate to severe pain. Long-term use of morphine leads to various adverse effects, such as tolerance and hyperalgesia. Vesicular glutamate transporter 2 (VGluT2) accumulates glutamate into synaptic vesicles and plays multiple roles in the central nervous system. However, the specific role of VGluT2 in morphine tolerance has not been fully elucidated. Here, we investigated the regulatory role of VGluT2 in morphine tolerance and assessed the potential role of the brain-derived neurotrophic factor (BDNF)/tyrosine kinase B (TrkB) pathway in VGluT2 mediated morphine antinociceptive tolerance in mice. In the present study, we found that VGluT2 is upregulated in the spinal cord after the development of morphine tolerance. Furthermore, inhibition of VGluT2 with its antagonist (Chicago sky blue 6 B, CSB6B) or knockdown of VGluT2 by lentivirus restored the analgesic effect of morphine, suppressed the activation of astrocytes and microglia, and decreased glial-derived pro-inflammatory cytokines. Overexpression of VGluT2 by lentivirus facilitated morphine tolerance and mechanical hyperalgesia. In addition, we found the expression of BDNF is correlated with VGluT2 expression in the spinal cord after chronic morphine administration. Intrathecal injection of the BDNF/TrkB pathway antagonist K252a attenuated the development of morphine tolerance and decreased the expression of VGluT2 in the spinal cord, which suggested the BDNF/TrkB pathway participates in the regulation of VGluT2 in morphine tolerance. This study elucidates the functional capability of VGluT2 in modulating morphine tolerance and identifies a novel mechanism and promising therapeutic target for morphine tolerance.

Clinical Observation of the Effects of Oral Opioid on Inflammatory Cytokines and Gut Microbiota in Patients with Moderate to Severe Cancer Pain: A Retrospective Cohort Study

Introduction

Methods

Results

Conclusion

Ketamine Improves Desensitization of µ-Opioid Receptors Induced by Repeated Treatment with Fentanyl but Not with Morphine

The issue of tolerance to continuous or repeated administration of opioids should be addressed. The ability of ketamine to improve opioid tolerance has been reported in clinical studies, and its mechanism of tolerance may involve improved desensitization of μ-opioid receptors (MORs). We measured changes in MOR activity and intracellular signaling induced by repeated fentanyl and morphine administration and investigated the effects of ketamine on these changes with human embryonic kidney 293 cells expressing MOR using the CellKey™, cADDis cyclic adenosine monophosphate, and PathHunter® β-arrestin recruitment assays. Repeated administration of fentanyl or morphine suppressed the second MOR responses. Administration of ketamine before a second application of opioids within clinical concentrations improved acute desensitization and enhanced β-arrestin recruitment elicited by fentanyl but not by morphine. The effects of ketamine on fentanyl were suppressed by co-treatment with an inhibitor of G-protein-coupled receptor kinase (GRK). Ketamine may potentially reduce fentanyl tolerance but not that of morphine through modulation of GRK-mediated pathways, possibly changing the conformational changes of β-arrestin to MOR.

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

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

Differential expression of H19, BC1, MIAT1, and MALAT1 long non-coding RNAs within key brain reward regions after repeated morphine treatment

Morphine-induced analgesic tolerance and dependence are significant limits of pain control; however, the exact molecular mechanisms underlying morphine tolerance and dependence have remained unclear. The role of long non-coding RNAs (lncRNAs) in morphine tolerance and dependence is yet to be determined. We aimed to explore the association of specific lncRNAs expression in key brain reward regions after repeated injection of morphine. Male Wistar rats received subcutaneous injections of twice-daily morphine (10 mg/kg) or saline (1 mL/kg) for eight days. On day 8 of the repeated injections, induction of morphine analgesic tolerance and dependence was confirmed through a hotplate test and a naloxone-precipitated withdrawal analysis, respectively. Expression of H19, BC1, MIAT1, and MALAT1 lncRNAs was determined from the midbrain, striatum, hypothalamus, prefrontal cortex (PFC), and hippocampus by real-time PCR on day 8 of the repeated injections. The H19 expression was significantly different between morphine-treated and control saline-treated rats in all investigated areas except for the hippocampus. The BC1 expression significantly altered in the midbrain, hypothalamus, and hippocampus, but not in the striatum and PFC after repeated morphine treatment. The MIAT1 and MALAT1 expression site-specifically altered in the midbrain, hypothalamus, and striatum; however, no significant changes were detected in their expression in the PFC and hippocampus after repeated morphine treatment. We conclude that alterations in the expression of these lncRNAs in the brain reward regions especially in the midbrain, striatum and hypothalamus may have critical roles in the development of morphine dependence and tolerance, which need to be considered in future researches.

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.

Genome-Wide Correlation of DNA Methylation and Gene Expression in Postmortem Brain Tissues of Opioid Use Disorder Patients

BACKGROUND

Opioid use disorder (OUD) affects millions of people, causing nearly 50 000 deaths annually in the United States. While opioid exposure and OUD are known to cause widespread transcriptomic and epigenetic changes, few studies in human samples have been conducted. Understanding how OUD affects the brain at the molecular level could help decipher disease pathogenesis and shed light on OUD treatment.

METHODS

We generated genome-wide transcriptomic and DNA methylation profiles of 22 OUD subjects and 19 non-psychiatric controls. We applied weighted gene co-expression network analysis to identify genetic markers consistently associated with OUD at both transcriptomic and methylomic levels. We then performed functional enrichment for biological interpretation. We employed cross-omics analysis to uncover OUD-specific regulatory networks.

RESULTS

We found 6 OUD-associated co-expression gene modules and 6 co-methylation modules (false discovery rate <0.1). Genes in these modules are involved in astrocyte and glial cell differentiation, gliogenesis, response to organic substance, and response to cytokine (false discovery rate <0.05). Cross-omics analysis revealed immune-related transcription regulators, suggesting the role of transcription factor-targeted regulatory networks in OUD pathogenesis.

CONCLUSIONS

Our integrative analysis of multi-omics data in OUD postmortem brain samples suggested complex gene regulatory mechanisms involved in OUD-associated expression patterns. Candidate genes and their upstream regulators revealed in astrocyte, and glial cells could provide new insights into OUD treatment development.

Examination of neuroinflammatory cytokine interleukin-1 beta expression in the medial prefrontal cortex, amygdala, and hippocampus for the paradoxical effects of reward and aversion induced by morphine

A growing body of evidence has shown that abused drugs could simultaneously induce the paradoxical effect-reward and aversion. Moreover, the medial prefrontal cortex (mPFC), amygdala, and hippocampus were involved in this paradoxical effect by abused drugs. However, no research examined whether neuroinflammatory changes in the mPFC [including cingulate cortex area 1 (Cg1); prelimbic cortex (PrL); infralimbic cortex (IL)], basolateral amygdala, and hippocampus [e.g., CA1, CA2, CA3, and dentate gyrus (DG)] after morphine-induced reward in conditioned place preference (CPP) and aversion in conditioned taste aversion (CTA). The results showed that after morphine administration, the consumption of a 0.1% saccharin solution decreased; the mean time spent in the morphine-paired side compartment of the CPP box increased, indicating that morphine simultaneously induced the paradoxical effects of reward and aversion. The PrL and IL of the mPFC, the BLA of the amygdala, the CA1, CA2, CA3, and DG of the hippocampus but not the Cg1 presented hyperactive IL-1β expression in response to morphine's aversion and reward. The mPFC, amygdala, and hippocampus may appear neuroinflammation activity following morphine-induced paradoxical effect-reward in CPP and aversion in CTA. The present data may provide a better understanding of the relationship between neuroinflammation and morphine addiction.

The impact of adherence to a guideline for minimizing opioid use for treatment of pain in an urban emergency department

INTRODUCTION

The opioid epidemic has significantly evolved over the last three decades. The initiation and continuation of prescription opioids for pain control were one of the primary contributors, across different medical settings. The emergency department (ED) is typically the first place patients go to for management of acute pain, and often where opioid naïve patients first become exposed to opioids. In 2018, the ED of University Hospital located in Newark, NJ implemented a pain guideline to ensure that patients are not unnecessarily exposed to opioids. The goal of our study was to determine whether provider adherence was successful in reducing opioid administration.

METHODS

We conducted a retrospective review of pharmacy records of patients treated for pain in the ED within the time frame January 1, 2017 and December 31, 2019. We analyzed the change in our practice by comparing the amount of opioid and non-opioid medications administered and the number of patients administered each type, as well as the change in our utilization of specific medications. The t-test or the χ² test were used as applicable.

RESULTS

There were decreases in the mean number of opioid doses administered in 2017 (1273) compared to 2019 (498; p = 0.027). There was an increase in non-opioid analgesics administered, (mean 2017: 1817, mean 2019: 2432.5, p = 0.018). There was also an increase in the proportion of patients given non-opioid analgesics (mean 2017: 22%, mean 2019: 28%, p < 0.0001). There were increases in administrations of acetaminophen (40% to 52%) and ibuprofen (30% to 35.1%), and decreases in administrations of hydromorphone (2.5% to 0.03%), morphine (11.5% to 5.6%), oxycodone (10.6% to 5.3%), and tramadol (5.7% to 1.9%) (all p < 0.0001).

DISCUSSION

A guideline that emphasizes the use of non-opioid analgesics first line treatment for acute pain can be effective for reducing opioid administration in the ED. Through the use of our guideline, we reduced the number of patients who have received opioid analgesics and, at the same time, increased non-opioid analgesic administration. Future studies should explore readmission rates, duration of pain relief in patients managed with non-opioid versus opioid analgesics, and perception of relief through the use of satisfaction scores.

The self-serving benefits of being a good host: A role for our micro-inhabitants in shaping opioids' function

Opioids are highly efficacious in their ability to relieve pain, but they are liable for abuse, dependence, and addiction. Risk factors to develop opioid use disorders (OUD) include chronic stress, socio-environment, and preexisting major depressive disorders (MDD) and posttraumatic stress disorders (PTSD). Additionally, opioids reduce gut motility, induce loss of gut barrier function, and alter the composition of the trillions of microbes hosted in the gastrointestinal tract, known as the gut microbiota. The microbiota are significant contributors to the reciprocal communication between the central nervous system (CNS) and the gut, termed the gut-brain axis. They have strong influences on their host behaviors, including the ability to cope with stress, sociability, affect, mood, and anxiety. Thus, they are implicated in the etiology of MDD and PTSD. Here we review the latest studies demonstrating that intestinal flora can, directly and indirectly, by affecting sociability levels, responses to stress, and mental state, alter the responses to opioids. It suggests that microbiota can potentially be used to increase the resilience to develop analgesic tolerance and OUD.