Toll-like receptor 4-mediated nuclear factor-κB activation in spinal cord contributes to chronic morphine-induced analgesic tolerance and hyperalgesia in rats

Network pharmacology approach to unveiling the mechanism of berberine in the amelioration of morphine tolerance

OBJECTIVE

To investigate the mechanism underlying the effect of the Huanglian decoction (, HLD) on morphine tolerance (MT), using network pharmacology, and to verify these mechanisms in vitro and in vivo.

METHODS

Available biological data on each drug in the HLD were retrieved from the Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform. The target proteins of MT were retrieved from the GeneCards, PharmGkb, Therapeutic Target Database, DrugBank, and Online Mendelian Inheritance in Man databases. Information regarding MT and the drug targets was compared to obtain overlapping elements. This information was imported into the Search Tool for the Retrieval of Interacting Genes/Proteins platform to obtain a protein-protein interaction network diagram. Then, a "component-target" network diagram was constructed using screened drug components and target information, viaCytoscape (Institute for Systems Biology, Seattle, WA, USA). The database for annotation, visualization, and integrated discovery was used for Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathways analyses. Pathway information predicted by network pharmacology was verified using animal studies and cell experiments.

RESULTS

Network pharmacology analysis identified 22 active compounds of HLD and revealed that HLD partially ameliorated MT by modulating inflammatory, apoptosis, and nuclear factor kappa B (NF-κB) signaling pathways. Berberine (BBR), one of the main components of HLD, inhibited the development of MT in mice. BBR reduced cell viability while increasing B-cell lymphoma 2 (Bcl-2) protein expression and decreasing CD86, NF-κB, Bax, and Caspase-3 protein expression in brain vascular 2 (BV2) mcroglia cells treated with morphine. Additionally, BBR contributed to a reduction in pro-inflammatory cytokine release and apoptotic cell number.

CONCLUSIONS

BBR, a key component of HLD, effectively suppressed microglial activation and neuro-inflammation by regulating the NF-κB and apoptosis signaling pathways, thereby delaying MT. This study offers a novel approach to enhance the clinical analgesic efficacy of morphine.

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.

Activation of MSK-1 exacerbates neuropathic pain through histone H3 phosphorylation in the rats' dorsal root ganglia and spinal dorsal horn

The exact mechanism underlies the development of neuropathic pain is not yet completely understood. Mitogen and stress-activated kinase 1 (MSK-1) is an important downstream kinase of the mitogen-activated protein kinase (MAPK). It has been extensively studied in the central nervous system, but whether MSK-1 is associated with the neuropathic pain remains elusive. In this experiment, Lumbar 5 spinal nerve ligation (SNL) was used to establish a neuropathic pain condition in the rats. Western blotting, qRT-PCR, immunohistochemistry, intrathecal catheterization and drugs delivery were evaluated to study the physiological responses of the animals. The results showed that SNL resulted in elevated phosphorylated MSK-1 (p-MSK-1) expression in the ipsilateral dorsal root ganglion (DRG) and the spinal dorsal horn in rats, while total MSK-1 (t-MSK-1) did not change significantly. Intrathecal injection of the MSK-1 inhibitor SB747651A partially reversed established neuropathic pain. Additionally, intrathecal administration of MSK-1 siRNA either preoperatively or 7 days postoperatively relieves the development and maintenance of pain, respectively. Meanwhile, the expression levels of p-H3S10, a downstream target of MSK-1, also displayed a significant increase after SNL. And these changes could be reversed by using MSK-1 siRNA. Collectively, the increase of MSK-1 induced by SNL participates in the development and maintenance of neuropathic pain by regulating the expression of p-H3S10 in DRG and spinal dorsal horn. Concentrating on MSK-1 may result in a novel approach to the treatment of neuropathic pain.

Activated microglia secretome and proinflammatory cytokines increase neuronal mu-opioid receptor signalling and expression

Due to its potential role in processes which rely on mu-opioid receptor function, investigating the relationship between Mu-Opioid receptors (MORs), neuroinflammation, and glial cells has gained momentum. Traditionally, MOR activation has been associated with immunosuppression, but recent findings suggest a more nuanced, bidirectional relationship with the immune system. To further investigate this relationship, herein, we investigated the role of the activated microglia secretome and proinflammatory cytokines in neuronal MOR expression and signalling. Our results show that both microglial secretome and specific cytokines increase neuronal MOR expression and enhance the [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO)-induced MOR activation. We also show that DAMGO-induced neuroinflammation increases neuronal MOR expression, activation, and regulation. Our findings suggest a feedback loop between microglial activation, cytokine release, and neuronal MOR dynamics. Future research should delve into the temporal dynamics and functional implications of this relationship, particularly concerning clinically relevant opioids like morphine and fentanyl and pain management.

Mycobacterium w - a promising immunotherapeutic intervention for diseases

Immunomodulating agents interact with the immune system and alter the outcome of specific immune processes. As our understanding of the immune system continues to evolve, there is a growing effort to identify agents with immunomodulating applications to use therapeutically to treat various diseases. Mycobacterium w (Mw), a heat-killed mycobacterium, is an atypical mycobacterial species that possesses strong immunomodulatory properties. Mw was initially evaluated as an immune-therapeutic against leprosy, but since then Mw has generated a lot of interest and been studied for therapeutic applications across a host of diseases, such as pulmonary tuberculosis, tuberculous pericarditis, sepsis, lung cancer, and more. This article summarizes a large body of work published in the past five decades, describing various aspects of Mw and its potential for further therapeutic development.

Mu-Opioid Receptor (MOR) Dependence of Pain in Chemotherapy-Induced Peripheral Neuropathy

We recently demonstrated that transient attenuation of Toll-like receptor 4 (TLR4) in dorsal root ganglion (DRG) neurons, can both prevent and reverse pain associated with chemotherapy-induced peripheral neuropathy (CIPN), a severe side effect of cancer chemotherapy, for which treatment options are limited. Given the reduced efficacy of opioid analgesics to treat neuropathic, compared with inflammatory pain, the cross talk between nociceptor TLR4 and mu-opioid receptors (MORs), and that MOR and TLR4 agonists induce hyperalgesic priming (priming), which also occurs in CIPN, we determined, using male rats, whether (1) antisense knockdown of nociceptor MOR attenuates CIPN, (2) and attenuates the priming associated with CIPN, and (3) CIPN also produces opioid-induced hyperalgesia (OIH). We found that intrathecal MOR antisense prevents and reverses hyperalgesia induced by oxaliplatin and paclitaxel, two common clinical chemotherapy agents. Oxaliplatin-induced priming was also markedly attenuated by MOR antisense. Additionally, intradermal morphine, at a dose that does not affect nociceptive threshold in controls, exacerbates mechanical hyperalgesia (OIH) in rats with CIPN, suggesting the presence of OIH. This OIH associated with CIPN is inhibited by interventions that reverse Type II priming [the combination of an inhibitor of Src and mitogen-activated protein kinase (MAPK)], an MOR antagonist, as well as a TLR4 antagonist. Our findings support a role of nociceptor MOR in oxaliplatin-induced pain and priming. We propose that priming and OIH are central to the symptom burden in CIPN, contributing to its chronicity and the limited efficacy of opioid analgesics to treat neuropathic pain.

Elucidating the molecular symphony: unweaving the transcriptional & epigenetic pathways underlying neuroplasticity in opioid dependence and withdrawal

The persistent use of opioids leads to profound changes in neuroplasticity of the brain, contributing to the emergence and persistence of addiction. However, chronic opioid use disrupts the delicate balance of the reward system in the brain, leading to neuroadaptations that underlie addiction. Chronic cocaine usage leads to synchronized alterations in gene expression, causing modifications in the Nucleus Accumbens (NAc), a vital part of the reward system of the brain. These modifications assist in the development of maladaptive behaviors that resemble addiction. Neuroplasticity in the context of addiction involves changes in synaptic connectivity, neuronal morphology, and molecular signaling pathways. Drug-evoked neuroplasticity in opioid addiction and withdrawal represents a complicated interaction between environmental, genetic, and epigenetic factors. Identifying specific transcriptional and epigenetic targets that can be modulated to restore normal neuroplasticity without disrupting essential physiological processes is a critical consideration. The discussion in this article focuses on the transcriptional aspects of drug-evoked neuroplasticity, emphasizing the role of key transcription factors, including cAMP response element-binding protein (CREB), ΔFosB, NF-kB, Myocyte-enhancing factor 2 (MEF2), Methyl-CpG binding protein 2 (MeCP2), E2F3a, and FOXO3a. These factors regulate gene expression and lead to the neuroadaptive changes observed in addiction and withdrawal. Epigenetic regulation, which involves modifying gene accessibility by controlling these structures, has been identified as a critical component of addiction development. By unraveling these complex molecular processes, this study provides valuable insights that may pave the way for future therapeutic interventions targeting the mechanisms underlying addiction and withdrawal.

YTHDF1 in periaqueductal gray inhibitory neurons contributes to morphine withdrawal responses in mice

BACKGROUND

Physical symptoms and aversion induced by opioid withdrawal strongly affect the management of opioid addiction. YTH N6-methyladenosine (m6A) RNA binding protein 1 (YTHDF1), an m6A-binding protein, from the periaqueductal gray (PAG) reportedly contributes to morphine tolerance and hyperalgesia. However, the role of YTHDF1 in morphine withdrawal remains unclear.

METHODS

A naloxone-precipitated morphine withdrawal model was established in C57/BL6 mice or transgenic mice. YTHDF1 was knocked down via adeno-associated virus transfection. Combined with the results of the single-cell RNA sequencing analysis, the changes in morphine withdrawal somatic signs and conditioned place aversion (CPA) scores were compared when YTHDF1 originating from different neurons in the ventrolateral periaqueductal gray (vlPAG) was knocked down. We further explored the role of inflammatory factors and transcription factors related to inflammatory response in morphine withdrawal.

RESULTS

Our results revealed that YTHDF1 expression was upregulated in the vlPAG of mice with morphine withdrawal and that the knockdown of vlPAG YTHDF1 attenuated morphine withdrawal-related somatic signs and aversion. The levels of NF-κB and p-NF-κB were reduced after the inhibition of YTHDF1 in the vlPAG. YTHDF1 from vlPAG inhibitory neurons, rather than excitatory neurons, facilitated morphine withdrawal responses. The inhibition of YTHDF1 in vlPAG somatostatin (Sst)-expressing neurons relieved somatic signs of morphine withdrawal and aversion, whereas the knockdown of YTHDF1 in cholecystokinin (Cck)-expressing or parvalbumin (PV)-expressing neurons did not change morphine withdrawal-induced responses. The activity of c-fos + neurons, the intensity of the calcium signal, the density of dendritic spines, and the frequency of mIPSCs in the vlPAG, which were increased in mice with morphine withdrawal, were decreased with the inhibition of YTHDF1 from vlPAG inhibitory neurons or Sst-expressing neurons. Knockdown of NF-κB in Sst-expressing neurons also alleviated morphine withdrawal-induced responses.

CONCLUSIONS

YTHDF1 originating from Sst-expressing neurons in the vlPAG is crucial for the modulation of morphine withdrawal responses, and the underlying mechanism might be related to the regulation of the expression and phosphorylation of NF-κB.

A single dose of ketamine relieves fentanyl-induced-hyperalgesia by reducing inflammation initiated by the TLR4/NF-κB pathway in rat spinal cord neurons

A large amount of clinical evidence has revealed that ketamine can relieve fentanyl-induced hyperalgesia. However, the underlying mechanism is still unclear. In the current study, a single dose of ketamine (5 mg/kg or 10 mg/kg), TAK-242 (3 mg/kg), or saline was intraperitoneally injected into rats 15 min before four subcutaneous injections of fentanyl. Results revealed that pre-administration of ketamine alleviated fentanyl-induced hyperalgesia according to hind paw-pressure and paw-withdrawal tests. High-dose ketamine can reverse the expression of toll-like receptor-dimer (d-TLR4), phospho- nuclear factor kappa-B (p-NF-κB, p-p65), cyclooxygenase-2 (COX-2), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) 1 d after fentanyl injection in the spinal cord. Moreover, fentany-linduced-hyperalgesia and changes in the expression of the aforementioned proteins can be attenuated by TAK-242, an inhibitor of TLR4, as well as ketamine. Importantly, TLR4, p-p65, COX-2, and IL-1β were expressed in neurons but not in glial cells in the spinal cord 1 d after fentanyl injection. In conclusion, results suggested that a single dose of ketamine can relieve fentanyl-induced-hyperalgesia via the TLR4/NF-κB pathway in spinal cord neurons.

Tumor necrosis factor-α-induced protein 8-like 2 alleviates morphine antinociceptive tolerance through reduction of ROS-mediated apoptosis and MAPK/NF-κB signaling pathways

Chronic morphine tolerance is a repulsive barrier to the clinical treatment of pain. Whereas the underlying molecular mechanisms of morphine tolerance remain unknown. Here, we proposed that tumor necrosis factor-α-induced protein 8-like 2 (TIPE2) is an essential control point regarding the progression of chronic morphine tolerance. We found that TIPE2 levels in the lumbar spinal cord were significantly downregulated in the morphine tolerance mouse model. Specifically, decreased TIPE2 by morphine tolerance was primarily expressed in spinal neurons, while increased expression of spinal TIPE2 distinctly attenuated the chronic morphine antinociceptive tolerance and tolerance-associated hyperalgesia. We also observed that increased expression of spinal TIPE2 significantly reduced morphine tolerance-induced neuronal ROS production and apoptosis, along with the activation of MAPKs and NF-κB signaling pathways. Moreover, the increased TIPE2 expression inhibited neuronal activation and glial reactivity in the spinal dorsal horn after chronic morphine exposure. Additionally, TIPE2 overexpression in cultured SH-SY5Y cells significantly suppressed ROS production and apoptosis in response to morphine challenge. Therefore, we can conclude that the upregulation of spinal TIPE2 may attenuate the morphine antinociceptive tolerance via TIPE2-dependent downregulation of neuronal ROS, inhibition of neuronal apoptosis, suppression of MAPKs and NF-κB activation. TIPE2 may be a potential strategy for preventing morphine tolerance in the future studies and clinical settings. Schematic diagram for the proposed mechanisms of TIPE2 regulates morphine antinociceptive tolerance. TIPE2 may alleviate morphine antinociceptive tolerance by regulating MAPK/NF-κB signaling pathways and apoptosis, which might be associated with ROS production.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

CircRalgapa1 facilitates morphine tolerance via miR-873a-5p/A20 axis in mice

Morphine tolerance (MT) caused by long-term use of morphine is a major medical problem. The underlying molecular mechanisms of morphine tolerance remain unclear. Here, we establish the morphine tolerance model in mice and verify whether a novel circRNA, circRalgapa1 is involved in morphine tolerance and its specific molecular mechanism. We show that the expression of circRalgapa1 in the spinal cord is significantly down-expressed in the spinal cord of morphine-tolerant mice. CircRalgapa1 is mainly located in the neuronal cytoplasm and co-localizes with miR-873a-5p. Mechanically, circRalgapa1 acts as competing endogenous RNAs (ceRNAs) to regulate the inhibitory of miR-873a-5p on A20 (also known as tumor necrosis factor α-induced protein 3, TNFAIP3). Functionally, overexpression of circRalgapa1 by intrathecal injection of adeno-associated virus (AAV- circRalgapa1) attenuated the formation of morphine tolerance and partially reversed the development of morphine tolerance. Moreover, overexpression of miR-873a-5p blocked the effect of AAV-circRalgapa1 on alleviating morphine tolerance in mice. In conclusion, chronic morphine administration-mediated down-regulation of circRalgapa1 in the spinal cord contributes to morphine tolerance via miR-873a-5p/A20 axis in mice. Overexpression of circRalgapa1 may be a promising RNA-based therapy for morphine tolerance.

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.

Activation of neurons and satellite glial cells in the DRG produces morphine-induced hyperalgesia

Activation of neurons and glial cells in the dorsal root ganglion is one of the key mechanisms for the development of hyperalgesia. The aim of the present study was to examine the role of neuroglial activity in the development of opioid-induced hyperalgesia. Male rats were treated with morphine daily for 3 days. The resultant phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 in the dorsal root ganglion was analyzed by immunohistochemistry and Western blotting. Pain hypersensitivity was analyzed using behavioral studies. The amount of cytokine expression in the dorsal root ganglion was also analyzed. Repeated morphine treatment induced hyperalgesia and marked induction of phosphorylated ERK1/2 in the neurons and satellite glial cells on day 3. An opioid receptor antagonist, toll like receptor-4 inhibitor, MAP/ERK kinase (MEK) inhibitor and gap junction inhibitor inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation. Morphine treatment induced alteration of cytokine expression, which was inhibited by the opioid receptor antagonist, toll like receptor-4 inhibitor, MEK inhibitor and gap junction inhibitor. Dexamethasone inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation after morphine treatment. The peripherally restricted opioid receptor antagonist, methylnaltrexone, inhibited hyperalgesia and ERK1/2 phosphorylation. Morphine activates ERK1/2 in neurons and satellite glial cells in the dorsal root ganglion via the opioid receptor and toll like receptor-4. ERK1/2 phosphorylation is gap junction-dependent and is associated with the alteration of cytokine expression. Inhibition of neuroinflammation by activation of neurons and glia might be a promising target to prevent opioid-induced hyperalgesia.

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.

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.

Elevated serum TLR4 level as a potential marker for postsurgical chronic pain in pediatric patients with different approaches to analgesia

Introduction

The perioperative period of any surgery is accompanied by immune suppression. The level of Toll-like receptor 4 (TLR4) is known to increase in inflammation and after nerve injury and contributes to the development of neuropathic pain. The interaction of TLRs in response to the effect of opioids results in paradoxical hyperalgesia. Regional anesthesia techniques are the standard of care for perioperative pain management in children.

Aim

The aim of the study was to determine and evaluate the indicators of TLR4 for different methods of pain relief in anesthetic management of hernia repair in children and their effect on pain chronification.

Materials and methods

There were examined 60 children with inguinal hernia during 2020-2022. Children were divided into 3 groups: Group I included 20 children who underwent surgery under general anesthesia using the block of the anterior abdominal wall-transversalis fascia plane block (TFPB), combined with the quadratus lumborum block (QLB-4) via a single intramuscular injection; Group II included 20 children who underwent surgery under general anesthesia using the TFPB; Group III comprised 20 children who underwent surgery under general anesthesia using opioid analgesics. The levels of TLR4 were evaluated at a discharge from the hospital, 3 and 6 months after surgery.

Results

There was no difference in age and body weight among all groups. In Group II, boys prevailed. In Group III, the length of hospital stay was the longest (3.28 ± 0.24 days, p < 0.05, t = 4.09) as compared to children of Group II and Group I (3.0 ± 0.30 (p < 0.05, t = 2.647) and 2.1 ± 0.16 days, respectively). While staying in the surgical department, children of Group III demonstrated significantly higher FLACC and VAS scores. The prevalence of chronic pain was the highest among children of Group III (35%) as compared to those in Group II and Group I (20 and 15%, respectively). The highest increase in the level of TLR4 was found in the group of opioid analgesia on the third and sixth months after surgery (68.86 + 10.31 pg/ml and 143.15 + 18.77 pg/ml (p < 0.05, t = 6.33), respectively) as compared to patients who received regional anesthesia.

Conclusions

There were confirmed the following advantages of the transversalis fascia plane block combined with the quadratus lumborum block (QLB + TFPB) via a single intramuscular injection: ease of use; adequate perioperative pain control as evidenced by the FLACC and VAS pain assessment scales; reduced perioperative use of opioid analgesics; shortening the length of hospital stay.

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5'-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G's behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

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.

Microglial ablation does not affect opioid-induced hyperalgesia in rodents

ABSTRACT

Opioids are the frontline analgesics in pain management. However, chronic use of opioid analgesics causes paradoxical pain that contributes to the decrease of their efficacy in pain control and the escalation of dose in long-term management of pain. The underling pathogenic mechanism is not well understood. Microglia have been commonly believed to play a critical role in the expression of opioid-induced hyperalgesia in animal models. We performed microglial ablation experiments using either genetic (CD11b-diphtheria toxin receptor transgenic mouse) or pharmacological (colony-stimulating factor-1 receptor inhibitor PLX5622) approaches. Surprisingly, ablating microglia using these specific and effective approaches did not cause detectable impairment in the expression of hyperalgesia induced by morphine. We confirmed this conclusion with a behavioral test of mechanical and thermal hyperalgesia, in male and female mice, and with different species (mouse and rat). These findings raise caution about the widely assumed contribution of microglia to the development of opioid-induced hyperalgesia.

Dihydroartemisinin alleviates morphine-induced neuroinflammation in BV-2 cells

Morphine tolerance poses a great challenge for clinicians, whose pathogenesis has a close connection with microglial activation and neuroinflammation. Dihydroartemisinin (DHA) that derives from artemisinin, may serve as a potential anti-inflammatory drug. In this study, the effects as well as the underlying mechanism of DHA on suppressing microglial activation and neuroinflammation were explored. The microglial cell line BV-2 cells were induced by morphine and treated with DHA or minocycline. With the application of CCK-8, the cell viability was detected. Western blot was employed to assess the expressions of Ki67, IBa-1, and TLR4 and quantitative real-time PCR (qRT-PCR) was adopted to evaluate miRNA-16 (miR-16) expression. With the adoption of ELISA kits and qRT-PCR, the release of inflammatory cytokines was evaluated. Besides, luciferase reporter assay was applied to testify the binding relationship between miR-16 and TLR4. NF-κB expression was measured by immunofluorescence. DHA reduced cell viability and decreased protein expression of Ki67 and IBa-1 in morphine-induced BV-2 cells. Additionally, DHA contributed to the declined release of pro-inflammatory cytokines. miR-16 was down-regulated by morphine but was up-regulated by DHA concentration-dependently in BV-2 cells. The inhibition of miR-16 partly abolished the inhibitory effects of DHA on morphine-induced microglial activation and neuroinflammation. Moreover, TLR4 was found to be bound to miR-16, and the inhibitory effect of DHA on TLR4/NF-κB was partly reversed by miR-16 inhibition. In conclusion, DHA remarkably suppressed microglial activation and neuroinflammation through regulating miR-16-mediated TLR4/NF-κB signaling. This study may provide a new solution to improve clinical analgesic efficacy of morphine.

The different mechanisms of peripheral and central TLR4 on chronic postsurgical pain in rats

Chronic postsurgical pain (CPSP) is a common complication after surgery; however, the underlying mechanisms of CPSP are poorly understood. As one of the most important inflammatory pathways, the Toll-like receptor 4/nuclear factor-kappa B (TLR4/NF-κB) signaling pathway plays an important role in chronic pain. However, the precise role of the TLR4/NF-κB signaling pathway in CPSP remains unclear. In the present study, we established a rat model of CPSP induced by skin/muscle incision and retraction (SMIR) and verified the effects and mechanisms of central and peripheral TLR4 and NF-κB on hyperalgesia in SMIR rats. The results showed that TLR4 expression was increased in both the spinal dorsal horn and dorsal root ganglia (DRGs) of SMIR rats. However, the TLR4 expression pattern in the spinal cord was different from that in DRGs. In the spinal cord, TLR4 was expressed in both neurons and microglia, whereas it was expressed in neurons but not in satellite glial cells in DRGs. Further results demonstrate that the central and peripheral TLR4/NF-κB signaling pathway is involved in the SMIR-induced CPSP by different mechanisms. In the peripheral nervous system, we revealed that the TLR4/NF-κB signaling pathway induced upregulation of voltage-gated sodium channel 1.7 (Nav1.7) in DRGs, triggering peripheral hyperalgesia in SMIR-induced CPSP. In the central nervous system, the TLR4/NF-κB signaling pathway participated in SMIR-induced CPSP by activating microglia in the spinal cord. Ultimately, our findings demonstrated that activation of the peripheral and central TLR4/NF-κB signaling pathway involved in the development of SMIR-induced CPSP.

Involvement of TCF7L2 in generation of morphine-induced antinociceptive tolerance and hyperalgesia by modulating TLR4/ NF-κB/NLRP3 in microglia

Morphine is an opioid agonist and a nonselective mu, kappa and delta receptor agonist. It is a commonly used analgesic drug for the treatment of acute and chronic pain as well as cancer pain. Morphine is particularly important to address the problem of morphine tolerance. Tcf7l2, known as a risk gene for schizophrenia and autism, encodes a member of the LEF1/TCF transcription factor family. TCF7L2 is an important transcription factor that is upregulated in neuropathic pain models. However, the relationship between TCF7L2 and morphine tolerance has not been reported. In this study, we found that morphine tolerance led to the upregulation of TCF7L2 in the spinal cord, and also led to the upregulation of TCF7L2 expression in glial cells, which promoted inflammation related signal, and activated TLR4 / NF-κB/NLRP3 pathway. In addition, TCF7L2 regulated microglial cell activation induced by chronic morphine treatment. Mechanically, we found that TCF7L2 transcriptionally regulated TLR4 expression, and the depletion of TCF7L2 alleviated morphine tolerance induced by chronic morphine treatment, and further alleviated pain hypersensitivity induced by chronic morphine treatment. We therefore suggested that TCF7L2 regulates the activation of TLR4/ NF-κB/NLRP3 pathway in microglia, and is involved in the formation of morphine tolerance. Our results provide a new idea for the regulation mechanism of morphine tolerance.

Spinal and Peripheral Mechanisms Individually Lead to the Development of Remifentanil-induced Hyperalgesia

The present study was performed to determine neuronal loci and individual molecular mechanisms responsible for remifentanil-induced hyperalgesia. The effect of methylnaltrexone (MNX) on remifentanil-induced behavioral hyperalgesia was assessed to distinguish contributions of the peripheral and/or central nervous system to remifentanil-induced hyperalgesia. Phosphorylation of p38 mitogen-activated protein kinase (p38MAPK) in the dorsal root ganglion (DRG) neurons after remifentanil infusion, and the effect of a p38MAPK inhibitor on remifentanil-induced hyperalgesia were analyzed to investigate involvement of p38MAPK in the peripheral mechanisms of remifentanil-induced hyperalgesia. Spinal levels of prodynorphin mRNA after remifentanil infusion, and the effect of the BK2 bradykinin receptor antagonist on remifentanil-induced hyperalgesia were investigated to assess potential spinal mechanisms. The effects of MNX and BK2 antagonists on remifentanil-induced exacerbation of post-incisional hyperalgesia were also investigated using behavioral analysis. Remifentanil infusion induced hyperalgesia in the early (4 h to 2 days) and late (8-14 days) post-infusion periods. MNX inhibited hyperalgesia only during the early post-infusion period. p38MAPK phosphorylation was observed in the DRG neuron, and the p38MAPK inhibitor inhibited hyperalgesia during the early post-infusion period. Prodynorphin expression increased in the spinal cord, and a BK2 antagonist inhibited hyperalgesia during the late post-infusion period. Remifentanil-induced exacerbation of incisional hyperalgesia was inhibited by MNX and the BK2 antagonist. The present study demonstrated that remifentanil activates peripheral and spinal neurons to promote chronologically distinctive hyperalgesia. p38MAPK phosphorylation in the DRG neuron leads to peripherally-driven hyperalgesia during the early post-infusion period, while spinal dynorphin-bradykinin signaling promotes hyperalgesia during the late post-infusion period.

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

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

Molecular Biology of Opioid Analgesia and Its Clinical Considerations

Understanding the molecular biology of opioid analgesia is essential for its proper implementation and mechanistic approach to its modulation in order to maximize analgesia and minimize undesired effects. By appreciating the molecular mechanisms intrinsic to opioid analgesia, one can manipulate a molecular target to augment or diminish a specific effect using adjuvant drugs, select an appropriate opioid for opioid rotation or define a molecular target for new opioid drug development. In this review, we present the cellular and molecular mechanisms of opioid analgesia and that of the associated phenomena of tolerance, dependence, and hyperalgesia. The specific mechanisms highlighted are those that presently can be clinically addressed.

Deep proteomics and phosphoproteomics reveal novel biological pathways perturbed by morphine, morphine-3-glucuronide and morphine-6-glucuronide in human astrocytes

Tolerance and hyperalgesia associated with chronic exposure to morphine are major limitations in the clinical management of chronic pain. At a cellular level, neuronal signaling can in part account for these undesired side effects, but unknown mechanisms mediated by central nervous system glial cells are likely also involved. Here we applied data-independent acquisition mass spectrometry to perform a deep proteome and phosphoproteome analysis of how human astrocytes responds to opioid stimulation. We unveil time- and dose-dependent effects induced by morphine and its major active metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide that converging on activation of mitogen-activated protein kinase and mammalian target of rapamycin signaling pathways. We also find that especially longer exposure to M3G leads to significant dysregulation of biological pathways linked to extracellular matrix organization, antigen presentation, cell adhesion, and glutamate homeostasis, which are crucial for neuron- and leukocyte-astrocyte interactions.

Chronic morphine-mediated upregulation of high mobility group box 1 in the spinal cord contributes to analgesic tolerance and hyperalgesia in rats

Analgesic tolerance and hyperalgesia hinder the long-term utility of opioids. We examined whether spinal high mobility group box 1 (HMGB1) is involved in morphine tolerance and its underlying mechanisms by using a model of repeated intrathecal (i.t.) injections of morphine. The results showed that chronic i.t. morphine exposure led to increased expression of HMGB1, Toll-like receptor 4 (TLR4), and receptor for advanced glycation end products (RAGE) and their mRNAs in the dorsal horn. Morphine challenge also promoted HMGB1 expression and release in cultured spinal neurons, but these effects were inhibited by TAK-242, naloxone (antagonists of TLR4), and TLR4 siRNA. Intrathecal coadministration of morphine with TAK-242 or PDTC (inhibitor of NF-κB activation) also reduced HMGB1 expression in the spinal cord. Repeated i.t. coinjections of morphine with glycyrrhizin (GL, an HMGB1 inhibitor) or HMGB1 siRNA prevented reduction of the maximal possible analgesic effect (MPAE) of morphine and alleviated morphine withdrawal-induced hyperalgesia. The established morphine tolerance and hyperalgesia were partially reversed when i.t. injections of GL or HMGB1 antibody started at day 7 of morphine injection. Repeated i.t. injections of morphine with HMGB1 siRNA inhibited the activation of NF-κB, but not that of JNK and p38. A single i.t. injection of HMGB1 in naïve rats caused pain-related hypersensitivity and reduction in MPAE. Moreover, phosphorylated NF-κB p65, TNF-α, and IL-1β levels in the dorsal horn were upregulated following this treatment, but this upregulation was prevented by coinjection with TAK-242. Together, these results suggest that morphine-mediated upregulation of spinal HMGB1 contributes to analgesic tolerance and hyperalgesia via activation of TLR4/NF-κB signaling, and the HMGB1 inhibitor might be a promising adjuvant to morphine in the treatment of intractable pain in the clinic.

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

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

Oleanolic acid administration alleviates neuropathic pain after a peripheral nerve injury by regulating microglia polarization-mediated neuroinflammation

Neuropathic pain caused by a peripheral nerve injury constitutes a great challenge in clinical treatments due to the unsatisfactory efficacy of the current strategy. Microglial activation-mediated neuroinflammation is a major characteristic of neuropathic pain. Oleanolic acid is a natural triterpenoid in food and medical plants, and fulfills pleiotropic functions in inflammatory diseases. Nevertheless, its role in neuropathic pain remains poorly elucidated. In the current study, oleanolic acid dose-dependently suppressed LPS-evoked IBA-1 expression (a microglial marker) without cytotoxicity to microglia, suggesting the inhibitory efficacy of oleanolic acid in microglial activation. Moreover, oleanolic acid incubation offset LPS-induced increases in the iNOS transcript and NO releases from microglia, concomitant with the decreases in pro-inflammatory cytokine transcripts and production including IL-6, IL-1β, and TNF-α. Simultaneously, oleanolic acid shifted the microglial polarization from the M1 phenotype to the M2 phenotype upon LPS conditions by suppressing LPS-induced M1 marker CD16, CD86 transcripts, and enhancing the M2 marker Arg-1 mRNA and anti-inflammatory IL-10 levels. In addition, the LPS-induced activation of TLR4-NF-κB signaling was suppressed in the microglia after the oleanolic acid treatment. Restoring this signaling by the TLR4 plasmid transfection overturned the suppressive effects of oleanolic acid on microglial polarization-evoked inflammation. In vivo, oleanolic acid injection alleviated allodynia and hyperalgesia in SNL-induced neuropathic pain mice. Concomitantly, oleanolic acid facilitated microglial polarization to M2, accompanied by inhibition in inflammatory cytokine levels and activation of TLR4-NF-κB signaling. Collectively, these findings confirm that oleanolic acid may ameliorate neuropathic pain by promoting microglial polarization from pro-inflammatory M1 to anti-inflammatory M2 phenotype via the TLR4-NF-κB pathway, thereby indicating its usefulness as therapeutic intervention in neuropathic pain.

Neuropathic pain caused by a peripheral nerve injury constitutes a great challenge in clinical treatments due to the unsatisfactory efficacy of the current strategy.

Drug-specific differences in the ability of opioids to manage burn pain

Burn injury pain is a significant public health problem. Burn injury treatment has improved tremendously in recent decades. However, an unintended consequence is that a larger number of patients now survive more severe injuries, and face intense pain that is very hard to treat. Although many efforts have been made to find alternative treatments, opioids remain the most effective medication available. Burn patients are frequently prescribed opioids in doses and durations that are significantly higher and longer than standard analgesic dosing guidelines. Despite this, many continue to experience unrelieved pain. They are also placed at a higher risk for developing dependence and opioid use disorder. Burn injury profoundly alters the functional state of the immune system. It also alters the expression levels of receptor, effector, and signaling molecules within the spinal cord's dorsal horn. These alterations could explain the reduced potency of opioids. However, recent studies demonstrate that different opioids signal preferentially via differential signaling pathways. This ligand-specific signaling by different opioids implies that burn injury may reduce the antinociceptive potency of opioids to different degrees, in a drug-specific manner. Indeed, recent findings hint at drug-specific differences in the ability of opioids to manage burn pain early after injury, as well as differences in their ability to prevent or treat the development of chronic and neuropathic pain. Here we review the current state of opioid treatment, as well as new findings that could potentially lead to opioid-based pain management strategies that may be significantly more effective than the current solutions.

Circular RNA expression profile in the spinal cord of morphine tolerated rats and screen of putative key circRNAs

Morphine tolerance developed after repeated or continuous morphine treatment is a global health concern hindering the control of chronic pain. In our previous research, we have reported that the expression of lncRNAs and microRNAs have been greatly modified in the spinal cord of morphine tolerated rats, and the modulating role of miR-873a-5p, miR-219-5p and miR-365 have already been confirmed. However, whether circular RNAs, another essential kind of non-coding RNA, are involved in the pathogenesis of morphine tolerance is still beyond our knowledge. In this study, we conducted microarray analysis for circRNA profile and found a large number of circRNAs changed greatly in the spinal cord by morphine treatment. Among them, we selected nine circRNAs for validation, and seven circRNAs are confirmed. Gene Ontology/Kyoto Encyclopedia of Genes and Genomes (GO/KEGG) analysis were used for functional annotation. Besides, we confirmed the modified expression of seven circRNAs after validation by real-time PCR, selected 3 most prominently modulated ones among them and predicted their downstream miRNA-mRNA network and analyzed their putative function via circRNA-miRNA-mRNA pathway. Finally, we enrolled the differentially expressed mRNAs derived from the identical spinal cord, these validated circRNAs and their putative miRNA targets for ceRNA analysis and screened a promising circRNA-miRNA-mRNA pathway in the development of morphine tolerance. This study, for the first time, provided valuable information on circRNA profile and gave clues for further study on the circRNA mechanism of morphine tolerance.

Spinal Serum- and Glucocorticoid-Regulated Kinase 1 (SGK1) Signaling Contributes to Morphine-Induced Analgesic Tolerance in Rats

Accumulating evidence indicates that phosphorylated serum- and glucocorticoid-regulated kinase 1 (SGK1) is associated with spinal nociceptive sensitization by modulating glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined whether spinal SGK1 signaling contributes to the development of morphine analgesic tolerance. Chronic morphine administration markedly induced phosphorylation of SGK1 in the spinal dorsal horn neurons. Intrathecal injection of SGK1 inhibitor GSK-650394 reduced the development of morphine tolerance with a significant leftward shift in morphine dose-effect curve. Furthermore, spinal inhibition of SGK1 suppressed morphine-induced phosphorylation of nuclear factor kappa B (NF-κB) p65 and upregulation of NMDAR NR1 and NR2B expression in the spinal dorsal horn. In contrast, intrathecal administration of NMDAR antagonist MK-801 had no effect on the phosphorylation of SGK1 in morphine-treated rats. In addition, morphine-induced upregulation of NR2B, but not NR1, was significantly abolished by intrathecal pretreatment with PDTC, a specific NF-κB activation inhibitor. Finally, spinal delivery of SGK1 small interfering RNA exhibited similar inhibitory effects on morphine-induced tolerance, phosphorylation of NF-κB p65, as well as upregulation of NR1 and NR2B expression. Our findings demonstrate that spinal SGK1 contributes to the development of morphine tolerance by enhancing NF-κB p65/NMDAR signaling. Interfering spinal SGK1 signaling pathway could be a potential strategy for prevention of morphine tolerance in chronic pain management.

Role of Nociceptor Toll-like Receptor 4 (TLR4) in Opioid-Induced Hyperalgesia and Hyperalgesic Priming

In addition to analgesia, opioids produce opioid-induced hyperalgesia (OIH) and neuroplasticity characterized by prolongation of inflammatory-mediator-induced hyperalgesia (hyperalgesic priming). We evaluated the hypothesis that hyperalgesia and priming induced by opioids are mediated by similar nociceptor mechanisms. In male rats, we first evaluated the role of nociceptor Toll-like receptor 4 (TLR4) in OIH and priming induced by systemic low-dose morphine (LDM, 0.03 mg/kg). Intrathecal oligodeoxynucleotide antisense to TLR4 mRNA (TLR4 AS-ODN) prevented OIH and prolongation of prostaglandin E₂ hyperalgesia (priming) induced by LDM. In contrast, high-dose morphine (HDM, 3 mg/kg) increased nociceptive threshold (analgesia) and induced priming, neither of which was attenuated by TLR4 AS-ODN. Protein kinase C ε (PKCε) AS-ODN also prevented LDM-induced hyperalgesia and priming, whereas analgesia and priming induced by HDM were unaffected. Treatment with isolectin B4 (IB4)-saporin or SSP-saporin (which deplete IB4⁺ and peptidergic nociceptors, respectively), or their combination, prevented systemic LDM-induced hyperalgesia, but not priming. HDM-induced priming, but not analgesia, was markedly attenuated in both saporin-treated groups. In conclusion, whereas OIH and priming induced by LDM share receptor and second messenger mechanisms in common, action at TLR4 and signaling via PKCε, HDM-induced analgesia, and priming are neither TLR4 nor PKCε dependent. OIH produced by LDM is mediated by both IB4⁺ and peptidergic nociceptors, whereas priming is not dependent on the same population. In contrast, priming induced by HDM is mediated by both IB4⁺ and peptidergic nociceptors. Implications for the use of low-dose opioids combined with nonopioid analgesics and in the treatment of opioid use disorder are discussed.SIGNIFICANCE STATEMENT Opioid-induced hyperalgesia (OIH) and priming are common side effects of opioid agonists such as morphine, which acts at μ-opioid receptors. We demonstrate that OIH and priming induced by systemic low-dose morphine (LDM) share action at Toll-like receptor 4 (TLR4) and signaling via protein kinase C ε (PKCε) in common, whereas systemic high-dose morphine (HDM)-induced analgesia and priming are neither TLR4 nor PKCε dependent. OIH produced by systemic LDM is mediated by isolectin B4-positive (IB4⁺) and peptidergic nociceptors, whereas priming is dependent on a different class of nociceptors. Priming induced by systemic HDM is, however, mediated by both IB4⁺ and peptidergic nociceptors. Our findings may provide useful information for the use of low-dose opioids combined with nonopioid analgesics to treat pain and opioid use disorders.

Innate Immune Signaling and Alcohol Use Disorders

Innate immune signaling is an important feature in the pathology of alcohol use disorders. Alcohol abuse causes persistent innate immune activation in the brain. This is seen in postmortem human alcoholic brain specimens, as well as in primate and rodent models of alcohol consumption. Further, in vitro models of alcohol exposure in neurons and glia also demonstrate innate immune activation. The activation of the innate immune system seems to be important in the development of alcohol use pathology, as anti-immune therapies reduce pathology and ethanol self-administration in rodent models. Further, innate immune activation has been identified in each of the stages of addiction: binge/intoxication, withdrawal/negative affect, and preoccupation/craving. This suggests that innate immune activation may play a role both in the development and maintenance of alcoholic pathology. In this chapter, we discuss the known contributions of innate immune signaling in the pathology of alcohol use disorders, and present potential therapeutic interventions that may be beneficial for alcohol use disorders.

Members of the same pharmacological family are not alike: Different opioids, different consequences, hope for the opioid crisis?

Pain management is the specialized medical practice of modulating pain perception and thus easing the suffering and improving the life quality of individuals suffering from painful conditions. Since this requires the modulation of the activity of endogenous systems involved in pain perception, and given the large role that the opioidergic system plays in pain perception, opioids are currently the most effective pain treatment available and are likely to remain relevant for the foreseeable future. This contributes to the rise in opioid use, misuse, and overdose death, which is currently characterized by public health officials in the United States as an epidemic. Historically, the majority of preclinical rodent studies were focused on morphine. This has resulted in our understanding of opioids in general being highly biased by our knowledge of morphine specifically. However, recent in vitro studies suggest that direct extrapolation of research findings from morphine to other opioids is likely to be flawed. Notably, these studies suggest that different opioid analgesics (opioid agonists) engage different downstream signaling effects within the cell, despite binding to and activating the same receptors. This recognition implies that, in contrast to the historical status quo, different opioids cannot be made equivalent by merely dose adjustment. Notably, even at equianalgesic doses, different opioids could result in different beneficial and risk outcomes. In order to foster further translational research regarding drug-specific differences among opioids, here we review basic research elucidating differences among opioids in pharmacokinetics, pharmacodynamics, their capacity for second messenger pathway activation, and their interactions with the immune system and the dopamine D2 receptors.

Opioid-Induced Hyperalgesia Is Associated with Dysregulation of Circadian Rhythm and Adaptive Immune Pathways in the Mouse Trigeminal Ganglia and Nucleus Accumbens

The benefits of opioid-based treatments to mitigate chronic pain can be hindered by the side effects of opioid-induced hyperalgesia (OIH) that can lead to higher consumption and risk of addiction. The present study advances the understanding of the molecular mechanisms associated with OIH by comparing mice presenting OIH symptoms in response to chronic morphine exposure (OIH treatment) relative to control mice (CON treatment). Using RNA-Seq profiles, gene networks were inferred in the trigeminal ganglia (TG), a central nervous system region associated with pain signaling, and in the nucleus accumbens (NAc), a region associated with reward dependency. The biological process of nucleic acid processing was over-represented among the 122 genes that exhibited a region-dependent treatment effect. Within the 187 genes that exhibited a region-independent treatment effect, circadian rhythm processes were enriched among the genes over-expressed in OIH relative to CON mice. This enrichment was supported by the differential expression of the period circadian clock 2 and 3 genes (Per2 and Per3). Transcriptional regulators in the PAR bZip family that are influenced by the circadian clock and that modulate neurotransmission associated with pain and drug addiction were also over-expressed in OIH relative to CON mice. Also notable was the under-expression in OIH relative to CON mice of the Toll-like receptor, nuclear factor-kappa beta, and interferon gamma genes and enrichment of the adaptive immune processes. The results from the present study offer insights to advance the effective use of opioids for pain management while minimizing hyperalgesia.

Emerging roles of microRNAs in morphine tolerance

Morphine is commonly used in clinical management to alleviate moderate-to-severe pain. However, prolonged and repeated use of morphine leads to tolerance. Morphine tolerance is a challenging clinical problem that limits its clinical application in pain treatment. The mechanisms underlying morphine tolerance are still not completely understood. MicroRNAs (miRNAs) are small noncoding RNAs containing 18~22 nucleotides that modulate gene expression in a post-transcriptional manner, and their dysregulation causes various diseases. miRNAs bind to the 3ʹ-UTR (untranslated region) of target gene mRNA, inhibiting or destabilizing translation of the transcripts. Morphine causes differential miRNA upregulation or downregulation. This review will present evidence for the contribution of miRNAs to tolerance of the antinociception effect of opioids.

miR-873a-5p Targets A20 to Facilitate Morphine Tolerance in Mice

Long-term morphine administration leads to tolerance and a gradual reduction in analgesic potency. Noncoding microRNAs (miRNAs) modulate gene expression in a posttranscriptional manner, and their dysregulation causes various diseases. Emerging evidence suggests that miRNAs play a regulatory role in the development of morphine tolerance. In the present study, we hypothesized that miR-873a-5p is a key functional small RNA that participates in the development and maintenance of morphine tolerance through the regulation of A20 (tumor necrosis factor α-induced protein 3, TNFAIP3) in mice. We measured the percentage of maximum possible effect (MPE %) to evaluate the analgesic effect of morphine. The expression of miR-873a-5p and its target gene A20 were determined after the morphine-tolerant model was successfully established. Intrathecal injection with lentivirus to intervene in the expression of A20 and the miR-873a-5p antagomir was used to explore the role of miR-873a-5p in the development of morphine tolerance. Chronic morphine administration significantly increased the expression of miR-873a-5p, which was inversely correlated with decreased A20 expression in the spinal cord of morphine-tolerant mice. Downregulation of miR-873a-5p in the spinal cord attenuated and partly reversed the development of morphine tolerance accompanied by overexpression of A20. Similarly, A20 was upregulated by a recombinant lentivirus vector, which attenuated and reversed the pathology of morphine tolerance by inhibiting the activation of nuclear factor (NF)-κB. Collectively, our results indicated that miR-873a-5p targets A20 in the spinal cord to facilitate the development of morphine tolerance in mice. Downregulating the expression of miR-873a-5p may be a potential strategy to ameliorate morphine tolerance.

Targeting toll-like receptor-4 (TLR4)-an emerging therapeutic target for persistent pain states

Toll-like receptors (TLRs) are a family of pattern recognition receptors that initiate signaling in innate and adaptive immune pathways. The highly conserved family of transmembrane proteins comprises an extracellular domain that recognizes exogenous and endogenous danger molecules and an ectodomain that activates downstream pathways in response. Recent studies suggest that continuous activation or dysregulation of TLR signaling may contribute to chronic disease states. The receptor is located not only on inflammatory cells (meningeal and peripheral macrophages) but on neuraxial glia (microglia and astrocytes), Schwann cells, fibroblasts, dorsal root ganglia, and dorsal horn neurons. Procedures blocking TLR functionality have shown pronounced effects on pain behavior otherwise observed in models of chronic inflammation and nerve injury. This review addresses the role of TLR4 as an emerging therapeutic target for the evolution of persistent pain and its role in noncanonical signaling, mediating anomalous pro-algesic actions of opiates. Accordingly, molecules targeting inhibition of this receptor have promise as disease-modifying and opioid-sparing alternatives for persistent pain states.

TLR4/NF-κB signaling activation in plantar tissue and dorsal root ganglion involves in the development of postoperative pain

Background

Severe postoperative pain remains a clinical problem that impacts patient’s rehabilitation. The present work aims to investigate the role of Toll-like receptor-4 (TLR4) activation in wounded plantar tissue and dorsal root ganglion (DRG) in the genesis of postoperative pain and its underlying mechanisms.

Results

Postoperative pain was induced by plantar incision in rat hind paw. Plantar incision led to increased expression of TLR4 in ipsilateral lumbar 4–5 (L4/L5) DRGs, which occurred at 2 h and was persistent to the third day after surgery. Similar to the change in TLR4 expression, there was also significant increase in phosphorylated nuclear factor-kappa B p65 (p-p65) in DRGs after surgery. Immunofluorescence staining revealed that the increased expressions of TLR4 and p-p65 not only in neuronal cells but also in satellite glial cells in DRG. Furthermore, the enhanced expressions of TLR4 and p-p65 were also detected in plantar tissues around the incision, which was observed starting at 2 h and lasting until the third day after surgery. Prior intrathecal (i.t.) injections of TAK-242 (a TLR4-specific antagonist) or 4',6-diamidino-2-phenylindole-dihydrochloride (PDTC, a nuclear factor-kappa B activation inhibitor) dose dependently alleviated plantar incision-induced mechanical allodynia and thermal hyperalgesia and inhibited the increased expressions of p-p65, tumor necrosis factor-alpha, and interleukin-1 beta in DRG. Prior subcutaneous (s.c.) plantar injection of TAK-242 or PDTC also ameliorated pain-related hypersensitivity following plantar incision. Moreover, the plantar s.c. injection of TAK-242 or PDTC inhibited the increased expressions of p-p65, tumor necrosis factor-alpha, and interleukin-1 beta not only in local wounded plantar tissue but also dramatically in ipsilateral lumbar 4–5 DRGs.

Conclusion

TLR4/ nuclear factor-kappa B signaling activation in local injured tissue and DRG contribute to the development of postoperative pain via regulating pro-inflammatory cytokines release. Targeting TLR4/ nuclear factor-kappa B signaling in local tissue at early stage of surgery may be an effective strategy for the treatment of postoperative pain.

CXCL12/CXCR4 signaling mediated ERK1/2 activation in spinal cord contributes to the pathogenesis of postsurgical pain in rats

Background

It has been demonstrated that upregulation of CXCL12 and CXCR4 in spinal cord involves in the pathogenesis of neuropathic, inflammatory, and cancer pain. However, whether CXCL12/CXCR4 signaling contributes to postsurgical pain remains unknown. The aim of the present study is to investigate the role of CXCL12/CXCR4 signaling in the genesis of postsurgical pain and the underlying mechanism.

Results

Plantar incision in rat hind paw resulted in increased expressions of CXCL12 and CXCR4 in spinal dorsal horn. Double immunofluorescence staining revealed that CXCL12 expressed in neurons and astrocytes, and CXCR4 exclusively co-localized with neuronal cells. Prior administration of AMD3100, a specific antagonist of CXCR4, or CXCL12 neutralizing antibody, intrathecally attenuated plantar incision-induced mechanical allodynia and thermal hyperalgesia. Plantar incision also augmented the phosphorylation of NF-κB p65 in spinal cord. Pre intrathecal (i.t.) injection of PDTC, a specific NF-κB activation inhibitor, alleviated plantar incision-induced postsurgical pain and reduced the expression of CXCL12 in spinal cord. Correlated with the upregulation of CXCL12 and CXCR4, plantar incision also resulted in an increased phosphorylation of extracellular signal-regulated kinase 1/2 and Akt in spinal cord. Prior i.t. administration of AMD3100 prevented extracellular signal-regulated kinase, but not Akt, activation in spinal cord. Rats when given a repetitive i.t. PD98059, a specific extracellular signal-regulated kinase inhibitor, started 30 min before surgery also ameliorate plantar incision-induced mechanical and thermal pain hypersensitivity.

Conclusion

Our results suggests that plantar incision-induced activation of NF-κB signaling may mediate upregulation of CXCL12 in spinal cord, and CXCL12/CXCR4 signaling via extracellular signal-regulated kinase activation contributes to the genesis of postsurgical pain.

Impact of sex on pain and opioid analgesia: a review

Chronic pain is a debilitating condition that impacts tens of millions each year, resulting in lost wages for workers and exacting considerable costs in health care and rehabilitation. A thorough understanding of the neural mechanisms underlying pain and analgesia is critical to facilitate the development of therapeutic strategies and personalized medicine. Clinical and epidemiological studies report that women experience greater levels of pain than men and have higher rates of pain-related disorders. Studies in both rodents and humans report sex differences in the anatomical and physiologic properties of the descending antinociceptive circuit, mu opioid receptor (MOR) expression and binding, morphine metabolism, and immune system activation, all of which likely contribute to the observed sex differences in pain and opioid analgesia. Although more research is needed to elucidate the underlying mechanisms, these sex differences present potential therapeutic targets to optimize pain management strategies for both sexes.

Glial cell line-derived neurotrophic factor (GDNF) attenuates the peripheral neuromuscular dysfunction without inhibiting the activation of spinal microglia/monocyte

Background

Peripheral neuromuscular dysfunctions were found in elderly individuals, and spinal microglia/monocyte plays an important role on this process. This study aims to test whether the glial cell line-derived neurotrophic factor (GDNF) could attenuate age-related neuromuscular dysfunction by inhibiting the activation of spinal microglia/monocyte.

Methods

Male Sprague-Dawley rats were divided into an adult group and an aged group. The aged rats were intrathecally injected with normal saline (NS) and GDNF. All the rats were harvested 5 days after each injection. The muscular function was tested by compound muscle action potential, and the activation of microglia/monocyte was detected by immunofluorescence staining; cytokines were assayed by enzyme-linked immunosorbent assay; the expression level of GDNF and its known receptor GFR-α in the spinal cord, the expression level of neuregulin-1 (NRG-1) in the sciatic nerve, and the expression level of γ- and α7- ε-nicotinic acetylcholine receptors in the tibialis anterior muscle were measured by western blotting.

Results

The activated microglia/monocyte was found in the aged rats compared to the adult rats. The aged rats showed a significant neuromuscular dysfunction and cytokine release as well as increased expression of γ- and α7-nAChR. The protein expression of GDNF, GFR-α, and NRG-1 in the aged rats were significantly lower than that in the adult rats. However, the exogenous injection of GDNF could alleviate the neuromuscular dysfunction but not inhibit the activation of spinal microglia/monocyte. Furthermore, the levels of GFR-α and NRG-1 also increased after GDNF treatment.

Conclusion

The GDNF could attenuate the age-related peripheral neuromuscular dysfunction without inhibiting the activation of microglia/monocyte in the spinal cord.

Identification of lncRNA expression profiles and ceRNA analysis in the spinal cord of morphine-tolerant rats

Morphine tolerance is a challenging clinical problem that limits the use of morphine in pain treatment, but the mechanisms of morphine tolerance remain unclear. Recent research indicates that long noncoding RNAs (lncRNAs) might be a novel and promising target in the pathogeneses of diseases. Therefore, we hypothesized that lncRNAs might play a role in the development of morphine tolerance. Male Sprague-Dawley rats were intrathecally injected with 10 μg morphine twice daily for 7 consecutive days. The animals were then sacrificed for lncRNA microarray tests, and the results were validated by RT-qPCR. Next, functional predictions for the differentially expressed mRNAs (DEmRNAs) were made with the Gene Ontology/Kyoto Encyclopedia of Genes and Genomes (GO/KEGG), and predictions for the differentially expressed lncRNAs (DElncRNAs) were made based on competitive endogenous RNA (ceRNA) analyses. The rats successfully developed morphine tolerance. LncRNA microarray analysis revealed that, according to the criteria of a log2 (fold change) > 1.5 and a P-value < 0.05, 136 lncRNAs and 278 mRNAs were differentially expressed in the morphine tolerance group (MT) compared with the normal saline group (NS). The functions of the DEmRNAs likely involve in the processes of the ion channel transport, pain transmission and immune response. The ceRNA analysis indicated that several possible interacting networks existed, including (MRAK150340, MRAK161211)/miR-219b/Tollip.Further annotations of the potential target mRNAs of the miRNAs according to the gene database suggested that the possible functions of these mRNAs primarily involved the regulation of ubiquitylation, G protein-linked receptors, and Toll-like receptors, which play roles in the development of morphine tolerance. Our findings revealed the profiles of differentially expressed lncRNAs in morphine tolerance conditions, and among these lncRNAs, some DElncRNAs might be new therapeutic targets for morphine tolerance.

Peripheral Leptin Signaling Mediates Formalin-Induced Nociception

Accumulating evidence suggests that obesity is associated with chronic pain. However, whether obesity is associated with acute inflammatory pain is unknown. Using a well-established obese mouse model induced by a high-fat diet, we found that: (1) the acute thermal pain sensory threshold did not change in obese mice; (2) the model obese mice had fewer nociceptive responses in formalin-induced inflammatory pain tests; restoring the obese mice to a chow diet for three weeks partly recovered their pain sensation; (3) leptin injection induced significant phosphorylation of STAT3 in control mice but not in obese mice, indicating the dysmodulation of topical leptin-leptin receptor signaling in these mice; and (4) leptin-leptin receptor signaling-deficient mice (ob/ob and db/db) or leptin-leptin receptor pathway blockade with a leptin receptor antagonist and the JAK2 inhibitor AG 490 in wild-type mice reduced their nociceptive responses in formalin tests. These results indicate that leptin plays a role in nociception induced by acute inflammation and that interference in the leptin-leptin receptor pathway could be a peripheral target against acute inflammatory pain.

In contrast to morphine, buprenorphine enhances macrophage-induced humoral immunity and, as oxycodone, slightly suppresses the effector phase of cell-mediated immune response in mice

BACKGROUND

Opioid receptors are commonly expressed on various immune cells, macrophages especially. Thus, these cells are prone to stimulation with opioids, which seems to be responsible for opioid-induced immunomodulatory effects. While morphine, fentanyl and methadone influence on mouse immune response was recently studied, little is known about the potential immunomodulatory impact of buprenorphine and oxycodone.

AIM

The current research aimed to investigate the influence of buprenorphine and oxycodone on immune responses in mice under homeostatic conditions.

METHODS AND RESULTS

Repeated administration of morphine led to intensification of CHS response in actively sensitized mice, while buprenorphine or oxycodone administration exerted the opposite effect. Further, hapten-conjugated macrophages from mice treated with morphine, when transferred into naive recipients, induced more potent CHS response. The enhanced generation of reactive oxygen intermediates and nitric oxide by macrophages from mice treated with buprenorphine, oxycodone or morphine was also shown, along with increased release of IL-6, TNFα and TGFβ. Treatment with opioids altered expression of antigen phagocytosis and presentation markers. Finally, the inhibitory effect of morphine treatment on induction of humoral immunity by macrophages was demonstrated, while oxycodone failed to influence humoral immune response and buprenorphine actually enhanced B-cell activation.

CONCLUSIONS

Current observations confirm that macrophages greatly contribute to immunomodulatory effects of opioids. Studies on immunomodulation by opioids have great importance related to the evaluation of its beneficial and adverse effects on patient condition. Our research showed that oxycodone exerts the weakest immunomodulatory properties, allowing us to assume this drug as safer than morphine during prolonged therapy.