Resveratrol reduces morphine tolerance by inhibiting microglial activation via AMPK signalling

Resveratrol activates autophagy and protects from UVA-induced photoaging in human skin fibroblasts and the skin of male mice by regulating the AMPK pathway

Skin photoaging is mostly caused by ultraviolet A (UVA), although active medications to effectively counteract UVA-induced photoaging have not yet been created. Resveratrol, a naturally occurring polyphenol found in the skin of grapes, has been shown to have various biological functions such as anti-inflammatory and antioxidant characteristics. However, the role of resveratrol in UVA-induced photoaging has not been clarified. We investigated the mechanism of action of resveratrol by UVA irradiation of human skin fibroblasts (HSF) and innovatively modified a mouse model of photoaging. The results demonstrated that resveratrol promoted AMP-activated protein kinase (AMPK) phosphorylation to activate autophagy, reduce reactive oxygen species (ROS) production, inhibit apoptosis, and restore normal cell cycle to alleviate UVA-induced photoaging. In addition, subcutaneous injection of resveratrol not only improved the symptoms of roughness, erythema, and increased wrinkles in the skin of UVA photodamaged mice, but also alleviated epidermal hyperkeratosis and hyperpigmentation, reduced inflammatory responses, and inhibited collagen fiber degradation. In conclusion, our studies proved that resveratrol can treat UVA-induced photoaging and elucidated the possible molecular mechanisms involved, providing a new therapeutic strategy for future anti-aging.

Small G-Protein Rheb Gates Mammalian Target of Rapamycin Signaling to Regulate Morphine Tolerance in Mice

BACKGROUND

Analgesic tolerance due to long-term use of morphine remains a challenge for pain management. Morphine acts on μ-opioid receptors and downstream of the phosphatidylinositol 3-kinase signaling pathway to activate the mammalian target of rapamycin (mTOR) pathway. Rheb is an important regulator of growth and cell-cycle progression in the central nervous system owing to its critical role in the activation of mTOR. The hypothesis was that signaling via the GTP-binding protein Rheb in the dorsal horn of the spinal cord is involved in morphine-induced tolerance.

METHODS

Male and female wild-type C57BL/6J mice or transgenic mice (6 to 8 weeks old) were injected intrathecally with saline or morphine twice daily at 12-h intervals for 5 consecutive days to establish a tolerance model. Analgesia was assessed 60 min later using the tail-flick assay. After 5 days, the spine was harvested for Western blot or immunofluorescence analysis.

RESULTS

Chronic morphine administration resulted in the upregulation of spinal Rheb by 4.27 ± 0.195-fold (P = 0.0036, n = 6), in turn activating mTOR by targeting rapamycin complex 1 (mTORC1). Genetic overexpression of Rheb impaired morphine analgesia, resulting in a tail-flick latency of 4.65 ± 1.10 s (P < 0.0001, n = 7) in Rheb knock-in mice compared to 10 s in control mice (10 ± 0 s). Additionally, Rheb overexpression in spinal excitatory neurons led to mTORC1 signaling overactivation. Genetic knockout of Rheb or inhibition of mTORC1 signaling by rapamycin potentiated morphine-induced tolerance (maximum possible effect, 52.60 ± 9.56% in the morphine + rapamycin group vs. 16.60 ± 8.54% in the morphine group; P < 0.0001). Moreover, activation of endogenous adenosine 5'-monophosphate-activated protein kinase inhibited Rheb upregulation and retarded the development of morphine-dependent tolerance (maximum possible effect, 39.51 ± 7.40% in morphine + metformin group vs. 15.58 ± 5.79% in morphine group; P < 0.0001).

CONCLUSIONS

This study suggests spinal Rheb as a key molecular factor for regulating mammalian target of rapamycin signaling.

EDITOR’S PERSPECTIVE

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Deciphering resveratrol's role in modulating pathological pain: From molecular mechanisms to clinical relevance

Pathological pain, a multifaceted and debilitating ailment originating from injury or post-injury inflammation of the somatosensory system, poses a global health challenge. Despite its ubiquity, reliable therapeutic strategies remain elusive. To solve this problem, resveratrol, a naturally occurring nonflavonoid polyphenol, has emerged as a potential beacon of hope owing to its anti-inflammatory, antioxidant, and immunomodulatory capabilities. These properties potentially position resveratrol as an efficacious candidate for the management of pathological pain. This concise review summaries current experimental and clinical findings to underscore the therapeutic potential of resveratrol in pathological pain, casting light on the complex underlying pathophysiology. Our exploration suggests that resveratrol may exert its analgesic effect by the modulating pivotal signaling pathways, including PI3K/Akt/mTOR, TNFR1/NF-κB, MAPKs, and Nrf2. Moreover, resveratrol appears to attenuate spinal microglia activation, regulate primary receptors in dorsal root sensory neurons, inhibit pertinent voltage-gated ion channels, and curb the expression of inflammatory mediators and oxidative stress responses. The objective of this review is to encapsulate the pharmacological activity of resveratrol, including its probable signaling pathways, pharmacokinetics, and toxicology pertinent to the treatment of pathological pain. Hopefully, we aim to map out promising trajectories for the development of resveratrol as a potential analgesic.

Microglial inflammation modulates opioid analgesic tolerance

As we all know, opioids are the drugs of choice for treating severe pain. However, very often, opioid use leads to tolerance, dependence, and hyperalgesia. Therefore, understanding the mechanisms underlying opioid tolerance and designing strategies for increasing the efficacy of opioids in chronic pain are important areas of research. Microglia are brain macrophages that remove debris and dead cells from the brain and participate in immune defense of the central nervous system during an insult or injury. However, recent studies indicate that microglial activation and generation of proinflammatory molecules (e.g., cytokines, nitric oxide, eicosanoids, etc.) in the brain may contribute to opioid tolerance and other side effects of opioid use. In this review, we will summarize the evidence and possible mechanisms by which proinflammatory molecules produced by activated microglia may antagonize the analgesic effect induced by opioids, and thus, lead to opioid tolerance. We will also delineate specific examples of studies that suggest therapeutic targets to counteract the development of tolerance clinically using suppressors of microglial inflammation.

Dietary polyphenols drive dose-dependent behavioral and molecular alterations to repeated morphine

Opioid Use Disorder (OUD) is associated with tremendous morbidity and mortality. Despite this burden, current pharmacotherapies for OUD are ineffective or intolerable for many patients. As such, interventions aimed at promoting resilience against OUD are of immense clinical interest. Treatment with a Bioactive Dietary Polyphenol Preparation (BDPP) promotes resilience and adaptive neuroplasticity in multiple models of neuropsychiatric disease. Here, we assessed effects of BDPP treatment on behavioral and molecular responses to repeated morphine treatment in male mice. BDPP pre-treatment alters responses for both locomotor sensitization and conditioned place preference. Most notably, polyphenol treatment consistently reduced formation of preference at low dose (5 mg/kg) morphine but enhanced it at high dose (15 mg/kg). In parallel, we performed transcriptomic profiling of the nucleus accumbens, which again showed a dose × polyphenol interaction. We also profiled microbiome composition and function, as polyphenols are metabolized by the microbiome and can act as prebiotics. The profile revealed polyphenol treatment markedly altered microbiome composition and function. Finally, we investigated involvement of the SIRT1 deacetylase, and the role of polyphenol metabolites in behavioral responses. These results demonstrate polyphenols have robust dose-dependent effects on behavioral and physiological responses to morphine and lay the foundation for future translational work.

Teneligliptin Co-Infusion Alleviates Morphine Tolerance by Inhibition of Spinal Microglial Cell Activation in Streptozotocin-Induced Diabetic Rats

Morphine (MOR) is a commonly prescribed drug for the treatment of moderate to severe diabetic neuropathic pain (DNP). However, long-term MOR treatment is limited by morphine analgesic tolerance (MAT). The activation of microglial cells and the release of glia-derived proinflammatory cytokines are known to play an important role in the development of MAT. In this study, we aimed to investigate the effects of the dipeptidyl peptidase-4 inhibitor (DPP-4i) teneligliptin (TEN) on MOR-induced microglial cell activation and MAT in DNP rats. DNP was induced in four groups of male Wistar rats through a single intraperitoneal injection of streptozotocin (STZ) (50 mg/kg, freshly dissolved in 5 mmol/L citrate buffer, pH 4.5). Sham rats were administered with the vehicle. Seven days after STZ injection, all rats were implanted with an intrathecal (i.t) catheter connected to a mini-osmotic pump, divided into five groups, and infused with the following combinations: sham + saline (1 µL/h, i.t), DNP + saline (1 µL/h, i.t), DNP + MOR (15 µg/h, i.t), DNP + TEN (2 µg/h, i.t), and DNP + MOR (15 µg/h, i.t) + TEN (2 µg/h, i.t) for 7 days at a rate of 1 μL/h. The MAT was confirmed through the measurement of mechanical paw withdrawal threshold and tail-flick tests. The mRNA expression of neuroprotective proteins nuclear factor erythroid 2-related factor (Nrf2) and heme oxygenase-1 (HO-1) in the dorsal horn was evaluated by quantitative PCR (qPCR). Microglial cell activation and mononucleate cell infiltration in the spinal cord dorsal horn were assessed by immunofluorescence assay (IFA) and Western blotting (WB). The results showed that co-infusion of TEN with MOR significantly attenuated MAT in DNP rats through the restoration of neuroprotective proteins Nrf2 and HO-1 and suppression of microglial cell activation in the dorsal horn. Though TEN at a dose of 2 μg has mild antinociceptive effects, it is highly effective in limiting MAT.

HSP90 inhibition in the mouse spinal cord enhances opioid signaling by suppressing an AMPK-mediated negative feedback loop

Opioids and other agonists of the μ-opioid receptor are effective at managing acute pain, but their chronic use can lead to tolerance that limits their efficacy. We previously reported that inhibiting the chaperone protein HSP90 in the spinal cords of mice promotes the antinociceptive effects of opioids in a manner that involved increased activation of the kinase ERK. Here, we found that the underlying mechanism involves the relief of a negative feedback loop mediated by the kinase AMPK. Intrathecal treatment of male and female mice with the HSP90 inhibitor 17-AAG decreased the abundance of the β1 subunit of AMPK in the spinal cord. The antinociceptive effects of 17-AAG with morphine were suppressed by intrathecal administration of AMPK activators and enhanced by an AMPK inhibitor. Opioid treatment increased the abundance of phosphorylated AMPK in the dorsal horn of the spinal cord, where it colocalized with a neuronal marker and the neuropeptide CGRP. Knocking down AMPK in CGRP-positive neurons enhanced the antinociceptive effects of morphine and demonstrated that AMPK mediated the signal transduction between HSP90 inhibition and ERK activation. These data suggest that AMPK mediates an opioid-induced negative feedback loop in CGRP neurons of the spinal cord and that this loop can be disabled by HSP90 inhibition to enhance the efficacy of opioids.

Metformin attenuates depressive-like behaviour of methamphetamine withdrawal in mice: A mechanistic approach

OBJECTIVES

Methamphetamine (METH) as a potent psychostimulant drug with a high potency of dependence rate that results in neurotoxicity has become a major drug of abuse in many parts of the world. Unfortunately, there is limited evidence regarding treatment of METH withdrawal syndrome. Therefore, we aimed to investigate whether metformin mitigate the methamphetamine (METH) withdrawal syndrome in male mice. Based on the literature, depression and anxiety are the major METH withdrawal symptoms.

METHODS

Here, METH (2 mg/kg) was administered to mice twice a day for 14 constitutive days to induce animal model of METH-induced withdrawal syndrome. To do this, mice in control group and those with METH withdrawal syndrome were divided into treatment (receiving metformin in 3 doses of 50, 100 and 200 mg/kg for 10 days) and non-treatment sub-groups. Following the behavioural test, the animals were sacrificed; their hippocampus was dissected to measure oxidative stress parameters and expression of cellular energy homeostasis and immune-inflammatory genes.

RESULTS

Our data revealed that metformin provoked antidepressant effects in behavioural tests through AMPK overexpression as an important mitochondrial energetic sensor and inhibition of Tlr4 overexpression in the immune system gene expression. In addition, metformin was able to improve oxidative stress biomarkers and neuronal damage in the hippocampus and restore cellular energy homeostasis and immune system gene expression.

CONCLUSIONS

The data suggested that metformin can influence the hippocampus through targeting mitochondria and their performance, and consequently, neuroinflammation responses and brain metabolic changes. It is supposed to be a new therapeutic option in clinical trials of depression and anxiety following METH withdrawal treatment.

Research progress of p38 as a new therapeutic target against morphine tolerance and the current status of therapy of morphine tolerance

With the development of the medical industry, new painkillers continue to appear in people's field of vision, but so far no painkiller can replace morphine. While morphine has a strong analgesic effect, it is also easy to produce pain sensitivity and tolerance. Due to the great inter-individual differences in patient responses, there are few clear instructions on how to optimise morphine administration regimens, which complicates clinicians' treatment strategies and limits the effectiveness of morphine in long-term pain therapy. P38MAPK is a key member of the MAPK family. Across recent years, it has been discovered that p38MAPK rises dramatically in a wide range of morphine tolerance animal models. Morphine tolerance can be reduced or reversed by inhibiting p38MAPK. However, the role and specific mechanism of p38MAPK are not clear. In this review, we synthesise the relevant findings, highlight the function and potential mechanism of p38MAPK in morphine tolerance, as well as the present status and efficacy of morphine tolerance therapy, and underline the future promise of p38MAPK targeted morphine tolerance treatment.

Resveratrol Inhibits Activation of Microglia after Stroke through Triggering Translocation of Smo to Primary Cilia

Activated microglia act as a double-edged sword for stroke. In the acute phase of stroke, activated microglia might deteriorate neurological function. Therefore, it is of great clinical transforming potential to explore drugs or methods that can inhibit abnormal activation of microglia in the acute phase of stroke to improve neurological function after stroke. Resveratrol has a potential effect of regulating microglial activation and anti-inflammation. However, the molecular mechanism of resveratrol-inhibiting microglial activation has not been fully clarified. Smoothened (Smo) belongs to the Hedgehog (Hh) signaling pathway. Smo activation is the critical step that transmits the Hh signal across the primary cilia to the cytoplasm. Moreover, activated Smo can improve neurological function via regulating oxidative stress, inflammation, apoptosis, neurogenesis, oligodendrogenesis, axonal remodeling, and so on. More studies have indicated that resveratrol can activate Smo. However, it is currently unknown whether resveratrol inhibits microglial activation via Smo. Therefore, in this study, N9 microglia in vitro and mice in vivo were used to investigate whether resveratrol inhibited microglial activation after oxygen-glucose deprivation/reoxygenation (OGD/R) or middle cerebral artery occlusion/reperfusion (MCAO/R) injury and improved functional outcome via triggering translocation of Smo in primary cilia. We definitively found that microglia had primary cilia; resveratrol partially inhibited activation and inflammation of microglia, improved functional outcome after OGD/R and MCAO/R injury, and triggered translocation of Smo to primary cilia. On the contrary, Smo antagonist cyclopamine canceled the above effects of resveratrol. The study suggested that Smo receptor might be a therapeutic target of resveratrol for contributing to inhibit microglial activation in the acute phase of stroke.

Attenuation of Morphine Dependence by Ganoderma lucidum Extract in Mice

Background: Opioids are the principal drugs of choice for managing acute severe pain; however, physical dependence is still reported as one of the main limiting factors in the clinical application of these drugs. In the present study, the effect of Ganoderma lucidum (G. lucidum) was assessed on morphine dependence in mice. Methods: A 19-day administration schedule was applied to induce morphine dependence in male adult NMRI mice. The mice were given intraperitoneal (i.p.) morphine sulfate once daily in an increasing dose of 10, 20, and 40 mg/kg. Then, G. lucidum hydroalcoholic extract (12.5, 25, and 50 mg/kg, i.p.) was given to the mice from days 10 to 18. Another group of mice received single doses of the extract (50, 100, and 200 mg/kg, i.p.) only on the 19th day. Naloxone (3 mg/kg, i.p.) was used to precipitate withdrawal syndrome. Normal saline and diazepam (0.25 mg/kg) were used as the negative and positive controls, respectively. Results: The administration of single doses of G. lucidum extract (100 and 200 mg/kg, i.p.) significantly decreased the number of jumps, leanings, and diarrhea in mice subjected to morphine dependence. The repeated administration of G. lucidum extract (25 and 50 mg/kg for nine days) significantly attenuated the number of jumps, leanings, and diarrhea in morphine-dependent mice. Conclusions: Overall, G. lucidum extract attenuates induced morphine dependence and inhibits withdrawal syndrome symptoms in mice.

Sestrin2 overexpression attenuates osteoarthritis pain via induction of AMPK/PGC-1α-mediated mitochondrial biogenesis and suppression of neuroinflammation

BACKGROUND

Our previous study indicated that reactive oxygen species (ROS) are critically involved in chronic pain. Sestrin2 (Sesn2), a novel stress-inducible protein, is evidenced to reduce the generation of ROS. The study examined the role of Sesn2 in osteoarthritis (OA) pain and delineated the underlying molecular mechanisms.

METHODS

In the present study, we investigated the impact of Sesn2 on mitochondrial biogenesis in a rat model of OA pain. After adeno-associated viral (AAV)-Sesn2EGFP was injected for 14 days, OA was induced by intra-articular injection of monosodium iodoacetate (MIA). We assessed pain behaviors (weight-bearing asymmetry and paw withdrawal threshold) and explored possible mechanisms in the L4-6 spinal cord.

RESULTS

Our results showed that overexpression of Sesn2 in the spinal cord alleviated pain behaviors in OA rats. Moreover, overexpression of Sesn2 increased the activity of AMP-activated protein kinase (AMPK) signaling and significantly restored mitochondrial biogenesis. Besides, Sesn2 overexpression inhibited the activation of astrocytes and microglia, and decreased the production of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in the spinal cord of the OA pain rats. These effects were significantly reversed by an AMPK inhibitor.

CONCLUSIONS

Collectively, these results suggest that Sesn2 overexpression ameliorates mechanical allodynia and weight-bearing asymmetry in OA rats via activation of AMPK/PGC-1α-mediated mitochondrial biogenesis in the spinal cord. Moreover, Sesn2 overexpression attenuates OA-induced neuroinflammation at least partly by activating AMPK signaling. Sesn2 may become an encouraging therapeutic strategy for OA pain relief and other disorders.

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.

Resveratrol Inhibits NLRP3 Inflammasome-Induced Pyroptosis and miR-155 Expression in Microglia Through Sirt1/AMPK Pathway

Resveratrol is a natural polyphenolic compound with a wide range of biological activities such as antioxidant, anti-carcinogenic, anti-obesity, anti-aging, anti-inflammatory, immunomodulatory properties. Accumulating evidence suggests that resveratrol has pharmacological benefits in life-threatening diseases, including cardiovascular disease, cancer, diabetes, and neurodegenerative diseases. Resveratrol is widely known for its anti-inflammatory properties; however, signaling mechanisms of anti-inflammatory action are still elusive. Studies have illustrated that resveratrol can control different regulatory pathways by altering the expression and consequently regulatory effects of microRNAs. Our study aims to clarify the regulatory mechanisms of resveratrol in its anti-inflammatory features in the N9 microglial cell line. Our results demonstrated that resveratrol inhibits LPS- and ATP-activated NLRP3 inflammasome and protects microglial cells upon oxidative stress, proinflammatory cytokine production, and pyroptotic cell death resulting from inflammasome activation. Additionally, resveratrol inhibits nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling and activates AMPK/Sirt1 pathways. Furthermore, our results indicated that resveratrol downregulated inflammasome-induced miR-155 expression. Then, inhibition of AMPK and Sirt1 pathways has significantly reversed protective effect of resveratrol on miR-155 expression. To sum up, our results suggest that resveratrol suppresses the NLRP3 inflammasome and miR-155 expression through AMPK and Sirt1 pathways in microglia.

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

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

Effects of resveratrol in the signaling of neuropathic pain involving P2X3 in the dorsal root ganglion of rats

Neuropathic pain is a major public health problem because it has a considerable impact on life quality of patients. Neuropathic pain caused by a lesion or disease of the somatosensory nervous system, which causes unpleasant and abnormal sensation (dysesthesia), an increased response to painful stimuli (hyperalgesia), and pain in response to a stimulus that does not normally provoke pain (allodynia). P2X receptors from dorsal root ganglion (DRG) play a crucial role in facilitating pain transmission at peripheral and spinal sites. Resveratrol (Res) has neuroprotective effects and improves the pathological and behavioral outcomes of various types of nerve injury. The present study examined the effects of Res on neuropathic pain. Neuropathic pain animal model was created by partial sciatic nerve ligation (pSNL) surgery. We found that consecutive intraperitoneal administration of Res for 21 days reduced the mechanical and thermal nociceptive responses induced by pSNL in a dose-dependent manner. Moreover, Res administration reversed P2X3 expression and phosphorylation of ERK in DRG neurons after peripheral nerve injury. Our results suggested that Res may ameliorate neuropathic pain by suppressing P2X3 up-regulation and ERK phosphorylation in DRG of neuropathic pain rats. Therefore, we concluded that Res has a significant analgesic effect on alleviating neuropathic pain, and thus may serve as a therapeutic approach for neuropathic pain.

Pharmacological Manipulation of Translation as a Therapeutic Target for Chronic Pain

Dysfunction in regulation of mRNA translation is an increasingly recognized characteristic of many diseases and disorders, including cancer, diabetes, autoimmunity, neurodegeneration, and chronic pain. Approximately 50 million adults in the United States experience chronic pain. This economic burden is greater than annual costs associated with heart disease, cancer, and diabetes combined. Treatment options for chronic pain are inadequately efficacious and riddled with adverse side effects. There is thus an urgent unmet need for novel approaches to treating chronic pain. Sensitization of neurons along the nociceptive pathway causes chronic pain states driving symptoms that include spontaneous pain and mechanical and thermal hypersensitivity. More than a decade of preclinical research demonstrates that translational mechanisms regulate the changes in gene expression that are required for ongoing sensitization of nociceptive sensory neurons. This review will describe how key translation regulation signaling pathways, including the integrated stress response, mammalian target of rapamycin, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase-interacting kinases, impact the translation of different subsets of mRNAs. We then place these mechanisms of translation regulation in the context of chronic pain states, evaluate currently available therapies, and examine the potential for developing novel drugs. Considering the large body of evidence now published in this area, we propose that pharmacologically manipulating specific aspects of the translational machinery may reverse key neuronal phenotypic changes causing different chronic pain conditions. Therapeutics targeting these pathways could eventually be first-line drugs used to treat chronic pain disorders. SIGNIFICANCE STATEMENT: Translational mechanisms regulating protein synthesis underlie phenotypic changes in the sensory nervous system that drive chronic pain states. This review highlights regulatory mechanisms that control translation initiation and how to exploit them in treating persistent pain conditions. We explore the role of mammalian/mechanistic target of rapamycin and mitogen-activated protein kinase-interacting kinase inhibitors and AMPK activators in alleviating pain hypersensitivity. Modulation of eukaryotic initiation factor 2α phosphorylation is also discussed as a potential therapy. Targeting specific translation regulation mechanisms may reverse changes in neuronal hyperexcitability associated with painful conditions.

The involvement of ventral hippocampal microglial cells, but not cannabinoid CB1 receptors, in morphine-induced analgesia in rats

It is well known that glial cells are involved in pain processing. The purpose of the present study was to investigate the possible involvement of the ventral hippocampal (VH) glial cells in morphine-induced analgesia. A tail-flick apparatus was used to measure pain sensitivity in male Wistar rats that were bilaterally cannulated in the VH by stereotaxic surgery. The results showed that intraperitoneal (i.p.) administration of morphine (2.5-7.5 mg/kg) induced analgesia in a time-dependent manner. The blockade of the VH glial cell activation by bilateral microinjection of a glial inhibitor, minocycline (5-15 µg/rat) into the VH with an ineffective dose of morphine (2.5 mg/kg, i.p) significantly increased morphine analgesia. Considering that the endocannabinoid system via CB1 receptors play a crucial role in pain modulation, we also assessed the possible role of the VH cannabinoid CB1 receptors in the functional interaction between minocycline and morphine in acute pain. Our results indicated that intra-VH injection of the cannabinoid CB1 receptor agonist, arachidonylcyclopropylamide (ACPA; 4-12 ng/rat) had no effect on minocycline-induced potentiation of morphine analgesia. It should be considered that intra-VH microinjection of minocycline or ACPA by itself had no effect on tail-flick latency. Our findings suggest that the activation of the VH microglial cells may be involved in mediating pain sensation, because the inhibition of these cells by intra-VH injection of minocycline could potentiate morphine-induced analgesia. Although endocannabinoids have a regulatory role in glia function, the activation of CB1 receptors could not affect the potentiative effect of minocycline on morphine analgesia.

Baicalin prevents the up-regulation of TRPV1 in dorsal root ganglion and attenuates chronic neuropathic pain

Background

Neuropathic pain is a major public health problem because it has a considerable impact on life quality of patients. TRP channels from dorsal root ganglion (DRG) play a crucial role in facilitating pain transmission at peripheral and spinal sites. Baicalin has neuroprotective effects and improves the pathological and behavioural outcomes of various types of nerve injury. The present study aims to examine the analgesic effects of baicalin on chronic neuropathic pain.

Methods

Neuropathic pain animal model was created by chronic constriction injury of the sciatic nerve (CCI). Behavioural tests were performed by von Frey and hot plate tests. mRNA and protein expression levels were examined by quantitative RT‐PCR and western blot.

Results

Consecutive intraperitoneal administration of baicalin for 16 days reduced the mechanical and thermal nociceptive responses induced by CCI surgery in a dose‐dependent manner. The mRNA expression levels of Trpv1 and Trpa1 were significantly increased in the DRG of CCI rats. Moreover baicalin administration reversed TRPV1 expression and phosphorylation of ERK in DRG neurons after peripheral nerve injury.

Conclusions

Our results suggested that baicalin may ameliorate neuropathic pain by suppressing TRPV1 up‐regulation and ERK phosphorylation in DRG of neuropathic pain rats. Baicalin has a significant analgesic effect on alleviating neuropathic pain and thus may serve as a therapeutic approach for neuropathic pain.

Resveratrol alleviates diabetic mechanical allodynia in rats by downregulating P2X3R

Mechanical allodynia, which develops in patients of diabetes mellitus as a neuropathic manifestation, remains without an effective treatment. The aim of the present study was to investigate the effects and potential mechanisms underlying resveratrol (RES) in a rat model of streptozocin (STZ)‑induced diabetic mechanical allodynia (DMA). The rat model of DMA was established by the administration of an intraperitoneal injection of STZ. From day 8 post‑STZ injection, rats were administered with an intragastric injection of various doses of RES for 14 consecutive days. The von Frey filaments were applied to detect the paw withdrawal threshold and evaluate the analgesic effects of RES. Based on the dose‑effect curve, the ED50 of RES was calculated. Immunofluorescence staining and western blotting were performed to detect the expression of purinergic receptor P2X3 (P2X3R) in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH) following RESED50 treatment. The results indicated that RES significantly alleviated mechanical allodynia in DMA model rats in a dose‑dependent manner. Compared with the control group, the expression of P2X3R in DRG neurons and SDH terminals was markedly decreased following the administration of RESED50 (P<0.05). Collectively, the results indicated that RES displayed a dose‑dependent analgesic effect on DMA model rats. Furthermore, P2X3R expression downregulation in the DRG and SDH may be a mechanism underlying the analgesic effects of RES on DMA‑related behaviors.

Evaluation of the effects of metformin administration on morphine tolerance in mice

Opioids are used in clinical practice to relieve moderate to severe pain. Prolonged use of opioids can lead to a situation of analgesic tolerance and dependence. Several mechanisms are involved in the tolerance to analgesic opioids, including desensitization or internalization of the opioid receptor, elevation of cAMP levels, microglial activation and neuroinflammation, elevation of spinal mTOR activity and change in the expression of some proteins involved in tolerance, such as nNOS. Activation of the AMPK pathway inhibits mTOR and p38 MAPK ameliorating neuroinflammation and tolerance induced by morphine. Metformin, a potent antidiabetic agent, can also activate AMPK. Morphine tolerance was induced in mice by intraperitoneal administration three times daily at 08:00, 11.00 and 16.00 h of 50, 50 and 75 mg/kg morphine, respectively during four days. On the fifth day mice received a single injection of morphine 50 mg/kg. To evaluate the effects of metformin in development of morphine-induced analgesic tolerance a group of mice received metformin (10 mg/kg) 45 min before each morphine administration. Tail flick and hot plate tests were performed to estimate analgesic latency on days 1, 3 and 5. At five days, the animals were sacrificed, the brain dissected and nitrite levels determined. Chronic metformin administration significantly increased the analgesic latency on days 3 and 5 compared to the morphine group in hot plate test and in tail flick test. Chronic and acute metformin administration significantly decreased nitric oxide level compare to morphine group. The present results revealed that metformin attenuated analgesic tolerance induced by repeated intraperitoneal injections of morphine in mice.

Natural Inhibitors on Over-Activation of Microglia from Herbals

Neuroinflammation manifested by over-activation of microglial cells plays an essential role in neurodegenerative diseases. Short-term activation of microglia can be beneficial, but chronically activated microglia can aggravate neuronal dysfunction possibly by secreting potentially cytotoxic substances such as tumor necrosis factor-alpha (TNF-α) and nitric oxide (NO), which can result in dysfunction and death of neurons. Therefore inhibiting over-activation of microglia and the production of cytotoxic intermediates may become an effective therapeutic approach for neuroinflammation. In this paper, we review our continuous research on natural inhibitors of over-activated microglia from traditional herbals, including flavonoids, lignans, sesquiterpene coumarins, and stilbenes.

Targeting of oxidative stress and inflammation through ROS/NF-kappaB pathway in phosphine-induced hepatotoxicity mitigation

AIMS

Poisoning with aluminium phosphide (AlP) commonly has a high rate of mortality and morbidities. Phosphine gas is the main cause of AlP poisoning that has deleterious effect on multi-organs especially heart, kidney, and liver. Furthermore, several studies reported that resveratrol has cytoprotective effects through its pleiotropic property. The purpose of this study was to estimate the dose-dependent role of resveratrol on phosphine induced acute hepatic toxicity in rat model.

MAIN METHODS

The rats have been exposed to LD₅₀ of AlP (12 mg/kg) by gavage, and resveratrol doses (20, 40, and 80 mg/kg) were injected 30 min after intoxication. After 24 h, the serum and liver tissue were collected for present study.

KEY FINDINGS

The results indicated that phosphine causes an alteration in oxidative stress markers including elevation of ROS, and GSH level, MPO activity, reduction in SOD, catalase and G6PD activity as well as reduction in SOD1 and catalase expression. Furthermore, phosphine significantly induced phosphorylation of IkappaB, NF-kappaB and up-regulation of TNF-α, IL-1β, IL-6, and ICAM-1 expression. Also, phosphine induces markedly reduced hepatocytes lives cell and elevated apoptosis and necrosis. Co-treatment of resveratrol in a dose-dependent manner reversed aforementioned alterations. All in all, histological analysis indicated a deleterious effect of phosphine on the liver, which is mitigated by resveratrol administration.

SIGNIFICANCE

The results of the present study suggest targeting ROS/NF-kappaB signalling pathway by resveratrol may have a significant effect on the improvement of hepatic injury induced by phosphine. It also may be a possible candidate for the treatment of phosphine-poisoning.

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

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

The Novel Neuroprotective Compound KMS99220 Has an Early Anti-neuroinflammatory Effect via AMPK and HO-1, Independent of Nrf2

We have previously reported a novel synthetic compound KMS99220 that prevented degeneration of the nigral dopaminergic neurons and the associated motor deficits, suggesting a neuroprotective therapeutic utility for Parkinson's disease. Microglia are closely associated with neuroinflammation, which plays a key role in the pathogenesis of neurodegenerative diseases. In this study, we investigated the effects of KMS99220 on the signaling involving AMP-activated protein kinase (AMPK) and heme oxygenase-1 (HO-1), the enzymes thought to regulate inflammation. KMS99220 was shown to elevate the enzyme activity of purified AMPK, and phosphorylation of cellular AMPK in BV2 microglia. It increased the level of HO-1, and this was attenuated by AMPK inhibitors. KMS99220 lowered phosphorylation of IκB, nuclear translocation of NFκB, induction of inducible nitric oxide synthase, and generation of nitric oxide in BV2 cells that had been challenged with lipopolysaccharide. This anti-inflammatory response involved HO-1, because both its pharmacological inhibition and knockdown of its expression abolished the response. The AMPK inhibitors also reversed the anti-inflammatory effects of KMS99220. The induction of HO-1 by KMS99220 occurred within 1 h, and this appeared not to involve the transcription factor Nrf2, because Nrf2 knockdown did not affect the compound's HO-1 inducing- and anti-inflammatory effects in this time window. These findings indicated that KMS99220 leads to AMPK-induced HO-1 expression in microglia, which in turn plays an important role in early anti-inflammatory signaling. Together with its neuroprotective property, KMS99220 may serve as a feasible therapeutic agent against neuroinflammation and neurodegeneration.

Remifentanil suppresses increase in interleukin-6 mRNA in the brain by inhibiting cyclic AMP synthesis

PURPOSE

Neuronal inflammation is caused by systemic inflammation and induces cognitive dysfunction. IL-6 plays a crucial role in therapies for neuronal inflammation and cognitive dysfunction. Remifentanil, an ultra-short-acting opioid, controls inflammatory reactions in the periphery, but not in the brain. Therefore, the anti-inflammatory effects of remifentanil in neuronal tissue and the involvement of cAMP in these effects were investigated in the present study.

METHODS

Mice were divided into 4 groups: control, remifentanil, LPS, and LPS + remifentanil. Brain levels of pro-inflammatory cytokine mRNA, and serum levels of corticosterone, catecholamine and IL-6 were measured in the 4 groups. The co-localization of IL-6 and astrocytes in the mouse brain after the LPS injection was validated by immunostaining. LPS and/or remifentanil-induced changes in intracellular cAMP levels in cultured glial cells were measured, and the effects of cAMP on LPS-induced IL-6 mRNA expression levels were evaluated.

RESULTS

Remifentanil suppressed increase in IL-6 mRNA levels in the mouse brain, and also inhibited the responses of plasma IL-6, corticosterone, and noradrenaline in an inflammatory state. In the hypothalamus, IL-6 was localized in the median eminence, at which GFAP immunoreactivity was specifically detected. In cultured cells, remifentanil suppressed increase in IL-6 mRNA levels and intracellular cAMP levels after the administration of LPS, and this enhanced IL-6 mRNA expression in response to LPS.

CONCLUSION

Remifentanil suppressed increase in IL-6 mRNA levels in the brain in an inflammatory state, and this effect may be attributed to its direct action on neuronal cells through the inhibition of intracellular cAMP rather than corticosterone.

A deep convolutional neural network approach for astrocyte detection

Astrocytes are involved in various brain pathologies including trauma, stroke, neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases, or chronic pain. Determining cell density in a complex tissue environment in microscopy images and elucidating the temporal characteristics of morphological and biochemical changes is essential to understand the role of astrocytes in physiological and pathological conditions. Nowadays, manual stereological cell counting or semi-automatic segmentation techniques are widely used for the quantitative analysis of microscopy images. Detecting astrocytes automatically is a highly challenging computational task, for which we currently lack efficient image analysis tools. We have developed a fast and fully automated software that assesses the number of astrocytes using Deep Convolutional Neural Networks (DCNN). The method highly outperforms state-of-the-art image analysis and machine learning methods and provides precision comparable to those of human experts. Additionally, the runtime of cell detection is significantly less than that of other three computational methods analysed, and it is faster than human observers by orders of magnitude. We applied our DCNN-based method to examine the number of astrocytes in different brain regions of rats with opioid-induced hyperalgesia/tolerance (OIH/OIT), as morphine tolerance is believed to activate glia. We have demonstrated a strong positive correlation between manual and DCNN-based quantification of astrocytes in rat brain.

In vitro evaluation of effects of metformin on morphine and methadone tolerance through mammalian target of rapamycin signaling pathway

The chronic use of opioids leads to tolerance, psychological, and physical dependence that limits their use as an effective long-term pain control. Several studies have shown that mammalian target of rapamycin (mTOR) plays a crucial role in the development of opioid tolerance. Metformin activates 5' adenosine monophosphate-activated protein kinase (AMPK) which directly suppresses the mTOR complex 1 signaling pathway. On the other hand, metformin can also inhibit mTOR directly and in an AMPK-independent manner. Thus, in the current study, we aimed to investigate the effects of metformin on the development of morphine and/or methadone-induced tolerance in human glioblastoma (T98G) cell line. We examined the effects of chronic treatment of morphine and/or methadone in the presence or absence of metformin with or without AMPK inhibitor (dorsomorphin hydrochloride) on levels of nitric oxide and cyclic adenosine monophosphate (cAMP), phosphorylated and dephosphorylated ribosomal protein S6 kinase β-1 (S6K1) and 4E-binding protein 1 (4E-BP1) in T98G cells. Pretreatment of cells with metformin (40 µM) with or without AMPK inhibitor (dorsomorphin hydrochloride; 1 µM) before adding of morphine (2.5 µM) or methadone (1 µM) revealed a protective effects on the development of opioid tolerance. Prior administration of metformin reversed the elevation of nitric oxide levels induced by morphine (p < 0.001) and methadone (p < 0.001) and also prevented the raise of cAMP levels induced by morphine in T98G cells (p < 0.05). Contribution of mTOR signaling pathway in metformin-induced effect was shown by the inhibition of phosphorylation of S6K1 and 4E-BP1, the downstream targets of mTOR. mTOR activation suppresses opioid-induced antinociception, and its activity has also been increased during opioid tolerance.

Recent development in antihyperalgesic effect of phytochemicals: anti-inflammatory and neuro-modulatory actions

INTRODUCTION

Pain is an unpleasant sensation triggered by noxious stimulation. It is one of the most prevalent conditions, limiting productivity and diminishing quality of life. Non steroidal anti inflammatory drugs (NSAIDs) are widely used as pain relievers in present day practice as pain is mostly initiated due to inflammation. However, due to potentially serious side effects, long term use of these antihyperalgesic drugs raises concern. Therefore there is a demand to search novel medicines with least side effects. Herbal products have been used for centuries to reduce pain and inflammation, and phytochemicals are known to cause fewer side effects. However, identification of active phytochemicals of herbal medicines and clear understanding of the molecular mechanism of their action is needed for clinical acceptance.

MATERIALS AND METHODS

In this review, we have briefly discussed the cellular and molecular changes during hyperalgesia via inflammatory mediators and neuro-modulatory action involved therein. The review includes 54 recently reported phytochemicals with antihyperalgesic action, as per the literature available with PubMed, Google Scholar and Scopus.

CONCLUSION

Compounds of high interest as potential antihyperalgesic agents are: curcumin, resveratrol, capsaicin, quercetin, eugenol, naringenin and epigallocatechin gallate (EGCG). Current knowledge about molecular targets of pain and their regulation by these phytochemicals is elaborated and the scope of further research is discussed.

Blocking ATP-sensitive potassium channel alleviates morphine tolerance by inhibiting HSP70-TLR4-NLRP3-mediated neuroinflammation

BACKGROUND

Long-term use of morphine induces analgesic tolerance, which limits its clinical efficacy. Evidence indicated morphine-evoked neuroinflammation mediated by toll-like receptor 4 (TLR4) - NOD-like receptor protein 3 (NLRP3) inflammasome was important for morphine tolerance. In our study, we investigated whether other existing alternative pathways caused morphine-induced activation of TLR4 in microglia. We focused on heat shock protein 70 (HSP70), a damage-associated molecular pattern (DAMP), which was released from various cells upon stimulations under the control of K_ATP channel and bound with TLR4-inducing inflammation. Glibenclamide, a classic K_ATP channel blocker, can improve neuroinflammation by inhibiting the activation of NLRP3 inflammasome. Our present study investigated the effect and possible mechanism of glibenclamide in improving morphine tolerance via its specific inhibition on the release of HSP70 and activation of NLRP3 inflammasome induced by morphine.

METHODS

CD-1 mice were used for tail-flick test to evaluate morphine tolerance. The microglial cell line BV-2 and neural cell line SH-SY5Y were used to investigate the pharmacological effects and the mechanism of glibenclamide on morphine-induced neuroinflammation. The activation of microglia was accessed by immunofluorescence staining. Neuroinflammation-related cytokines were measured by western blot and real-time PCR. The level of HSP70 and related signaling pathway were evaluated by western blot and immunofluorescence staining.

RESULTS

Morphine induced the release of HSP70 from neurons. The released HSP70 activated microglia and triggered TLR4-mediated inflammatory response, leading to the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) p65 and the activation of NLRP3 inflammasome. Moreover, anti-HSP70 neutralizing antibody partly attenuated chronic morphine tolerance. The secretion of HSP70 was under the control of MOR/AKT/K_ATP/ERK signal pathway. Glibenclamide as a classic K_ATP channel blocker markedly inhibited the release of HSP70 induced by morphine and suppressed HSP70-TLR4-NLRP3 inflammasome-mediated neuroinflammation, which consequently attenuated morphine tolerance.

CONCLUSIONS

Our study indicated that morphine-induced extracellular HSP70 was an alternative way for the activation of TLR4-NLRP3 in analgesic tolerance. The release of HSP70 was regulated by MOR/AKT/K_ATP/ERK pathway. Our study suggested a promising target, K_ATP channel and a new leading compound, glibenclamide, for treating morphine tolerance.

Lidocaine alleviates morphine tolerance via AMPK-SOCS3-dependent neuroinflammation suppression in the spinal cord

Background

Morphine tolerance is a clinical challenge, and its pathogenesis is closely related to the neuroinflammation mediated by Toll-like receptor 4 (TLR4). In Chinese pain clinic, lidocaine is combined with morphine to treat chronic pain. We found that lidocaine sufficiently inhibited neuroinflammation induced by morphine and improved analgesic tolerance on the basis of non-affecting pain threshold.

Methods

CD-1 mice were utilized for tail-flick test to evaluate morphine tolerance. The microglial cell line BV-2 was utilized to investigate the mechanism of lidocaine. Neuroinflammation-related cytokines were measured by western blotting and real-time PCR. The level of suppressor of cytokine signaling 3 (SOCS3) and adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK)-related signaling pathway was evaluated by western blotting, real-time PCR, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence staining.

Results

Lidocaine potentiated an anti-nociceptive effect of morphine and attenuated the chronic analgesic tolerance. Lidocaine suppressed morphine-induced activation of microglia and downregulated inflammatory cytokines, interleukin-1β (IL-1β), and tumor necrosis factor-alpha (TNF-α) via upregulating SOCS3 by activating AMPK. Lidocaine enhanced AMPK phosphorylation in a calcium-dependent protein kinase kinase β (CaMKKβ)-dependent manner. Furthermore, lidocaine decreased the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and inhibited the nuclear factor-κB (NF-κB) in accordance with the inhibitory effects to TLR4.

Conclusions

Lidocaine as a prevalent local anesthetic suppresses morphine tolerance efficiently. AMPK-dependent upregulation of SOCS3 by lidocaine plays a crucial role in the improvement of analgesic tolerance.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-0983-6) contains supplementary material, which is available to authorized users.

AMPK activation: Role in the signaling pathways of neuroinflammation and neurodegeneration

Adenosine monophosphate-activated protein kinase (AMPK) is an evolutionarily conserved sensor of cellular energy status and has been reported to be involved in chronic inflammatory disorders. AMPK is expressed in immune cells, such as dendritic cells, macrophages, lymphocytes and neutrophils, and is an important regulator of inflammatory responses through the regulation of complex signaling networks in part by inhibiting downstream cascade pathways, such as nuclear factor kB, which is a key regulator of innate immunity and inflammation, as well as acting as a negative regulator of toll-like receptors. Recent data suggest that AMPK dysregulation may participate in neurodegenerative diseases, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and neuropathies. However, there are conflicting reports on the benefits or detrimental effects of AMPK in distinct pathological conditions. This paper offers a review of the recent literature on the pharmacological modulation of the AMPK system as a potential molecular target in the management of neurodegenerative diseases.

AMPK activation by peri-sciatic nerve administration of ozone attenuates CCI-induced neuropathic pain in rats

Neuropathic pain is a debilitating clinical condition with few efficacious treatments, warranting development of novel therapeutics. Ozone is widely used as an alternative therapy for many different pain conditions, with exact mechanisms still elusive. In this study, we found that a single peri-sciatic nerve injection of ozone decreased mechanical allodynia and thermal hyperalgesia, and normalized the phosphorylation of protein kinase C γ, N-methyl-D-aspartate receptor, and extracellular signal-regulated kinase in a chronic constriction injury (CCI) model in rat sciatic nerve. Meanwhile, ozone significantly suppressed CCI-induced activation of spinal microglia. More importantly, the anti-nociceptive effect of ozone depended on the activation of 5'-adenosine monophosphate (AMP)-activated protein kinase (AMPK), which was proved by the fact that the phosphorylated AMPK level increased during the ozone therapy and AMPK antagonist abolished the effect of ozone in vivo and in vitro. In addition, direct injection of AMPK agonist could replicate the anti-nociceptive effect of ozone in CCI rats. In conclusion, our observations indicate that peri-sciatic nerve injection of ozone activates AMPK to attenuate CCI-induced neuropathic pain.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Blockade of neuronal dopamine D2 receptor attenuates morphine tolerance in mice spinal cord

Tolerance induced by morphine remains a major unresolved problem and significantly limits its clinical use. Recent evidences have indicated that dopamine D2 receptor (D2DR) is likely to be involved in morphine-induced antinociceptive tolerance. However, its exact effect and molecular mechanism remain unknown. In this study we examined the effect of D2DR on morphine antinociceptive tolerance in mice spinal cord. Chronic morphine treatment significantly increased levels of D2DR in mice spinal dorsal horn. And the immunoreactivity of D2DR was newly expressed in neurons rather than astrocytes or microglia both in vivo and in vitro. Blockade of D2DR with its antagonist (sulpiride and L-741,626, i.t.) attenuated morphine antinociceptive tolerance without affecting basal pain perception. Sulpiride (i.t.) also down-regulated the expression of phosphorylation of NR1, PKC, MAPKs and suppressed the activation of astrocytes and microglia induced by chronic morphine administration. Particularly, D2DR was found to interact with μ opioid receptor (MOR) in neurons, and chronic morphine treatment enhanced the MOR/D2DR interactions. Sulpiride (i.t.) could disrupt the MOR/D2DR interactions and attenuate morphine tolerance, indicating that neuronal D2DR in the spinal cord may be involved in morphine tolerance possibly by interacting with MOR. These results may present new opportunities for the treatment and management of morphine-induced antinociceptive tolerance which often observed in clinic.

Metformin reduces morphine tolerance by inhibiting microglial-mediated neuroinflammation

Background

Tolerance seriously impedes the application of morphine in clinical medicine. Thus, it is necessary to investigate the exact mechanisms and efficient treatment. Microglial activation and neuroinflammation in the spinal cord are thought to play pivotal roles on the genesis and maintaining of morphine tolerance. Activation of adenosine monophosphate-activated kinase (AMPK) has been associated with the inhibition of inflammatory nociception. Metformin, a biguanide class of antidiabetic drugs and activator of AMPK, has a potential anti-inflammatory effect. The present study evaluated the effects and potential mechanisms of metformin in inhibiting microglial activation and alleviating the antinociceptive tolerance of morphine.

Methods

The microglial cell line BV-2 cells and mouse brain-derived endothelial cell line bEnd3 cells were used. Cytokine expression was measured using quantitative polymerase chain reaction. Cell signaling was assayed by western blot and immunohistochemistry. The antinociception and morphine tolerance were assessed in CD-1 mice using tail-flick tests.

Results

We found that morphine-activated BV-2 cells, including the upregulation of p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation, pro-inflammatory cytokines, and Toll-like receptor-4 (TLR-4) mRNA expression, which was inhibited by metformin. Metformin suppressed morphine-induced BV-2 cells activation through increasing AMPK phosphorylation, which was reversed by the AMPK inhibitor compound C. Additionally, in BV-2 cells, morphine did not affect the cell viability and the mRNA expression of anti-inflammatory cytokines. In bEnd3 cells, morphine did not affect the mRNA expression of interleukin-1β (IL-1β), but increased IL-6 and tumor necrosis factor-α (TNF-α) mRNA expression; the effect was inhibited by metformin. Morphine also did not affect the mRNA expression of TLR-4 and chemokine ligand 2 (CCL2). Furthermore, systemic administration of metformin significantly blocked morphine-induced microglial activation in the spinal cord and then attenuated the development of chronic morphine tolerance in mice.

Conclusions

Metformin significantly attenuated morphine antinociceptive tolerance by suppressing morphine-induced microglial activation through increasing AMPK phosphorylation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0754-9) contains supplementary material, which is available to authorized users.

Morphine dependence is attenuated by red ginseng extract and ginsenosides Rh2, Rg3, and compound K

BACKGROUND

Red ginseng and ginsenosides have shown plethoric effects against various ailments. However, little is known regarding the effect of red ginseng on morphine-induced dependence and tolerance. We therefore investigated the effect of red ginseng extract (RGE) and biotransformed ginsenosides Rh2, Rg3, and compound K on morphine-induced dependence in mice and rats.

METHODS

While mice were pretreated with RGE and then morphine was injected intraperitoneally, rats were infused with ginsenosides and morphine intracranially for 7 days. Naloxone-induced morphine withdrawal syndrome was estimated and conditioned place preference test was performed for physical and psychological dependence, respectively. Western blotting was used to measure protein expressions.

RESULTS

Whereas RGE inhibited the number of naloxone-precipitated jumps and reduced conditioned place preference score, it restored the level of glutathione in mice. Likewise, ginsenosides Rh2, Rg3, and compound K attenuated morphine-dependent behavioral patterns such as teeth chattering, grooming, wet-dog shake, and escape behavior in rats. Moreover, activated N-methyl-D-aspartate acid receptor subunit 1 and extracellular signal-regulated kinase in the frontal cortex of rats, and cultured cortical neurons from mice were downregulated by ginsenosides Rh2, Rg3, and compound K despite their differential effects.

CONCLUSION

RGE and biotransformed ginsenosides could be considered as potential therapeutic agents against morphine-induced dependence.

Downregulation of miR-219 enhances brain-derived neurotrophic factor production in mouse dorsal root ganglia to mediate morphine analgesic tolerance by upregulating CaMKIIγ

BACKGROUND

Increasing evidence suggests that microRNAs are functionally involved in the initiation and maintenance of pain hypersensitivity, including chronic morphine analgesic tolerance, through the posttranscriptional regulation of pain-related genes. We have previously demonstrated that miR-219 regulates inflammatory pain in the spinal cord by targeting calcium/calmodulin-dependent protein kinase II gamma (CaMKIIγ). However, whether miR-219 regulates CaMKIIγ expression in the dorsal root ganglia to mediate morphine tolerance remains unclear.

RESULTS

MiR-219 expression was downregulated and CaMKIIγ expression was upregulated in mouse dorsal root ganglia following chronic morphine treatment. The changes in miR-219 and CaMKIIγ expression closely correlated with the development of morphine tolerance, which was measured using the reduction of percentage of maximum potential efficiency to thermal stimuli. Morphine tolerance was markedly delayed by upregulating miR-219 expression using miR-219 mimics or downregulating CaMKIIγ expression using CaMKIIγ small interfering RNA. The protein and mRNA expression of brain-derived neurotrophic factor were also induced in dorsal root ganglia by prolonged morphine exposure in a time-dependent manner, which were transcriptionally regulated by miR-219 and CaMKIIγ. Scavenging brain-derived neurotrophic factor via tyrosine receptor kinase B-Fc partially attenuated morphine tolerance. Moreover, functional inhibition of miR-219 via miR-219-sponge in naive mice elicited thermal hyperalgesia and spinal neuronal sensitization, which were both suppressed by CaMKIIγ small interfering RNA or tyrosine receptor kinase B-Fc.

CONCLUSIONS

These results demonstrate that miR-219 contributes to the development of chronic tolerance to morphine analgesia in mouse dorsal root ganglia by targeting CaMKIIγ and enhancing CaMKIIγ-dependent brain-derived neurotrophic factor expression.

Novel compound from Polygonum multiflorum inhibits inflammatory response in LPS-stimulated microglia by upregulating AMPK/Nrf2 pathways

Polygonum multiflorum extracts are known to improve memory and learning ability, and have neuroprotective and anti-aging activity. However, its function and the underlying mechanisms in neuroinflammation-mediated neurodegenerative disease remain poorly understood. In the present study, we investigated the anti-neuroinflammatory effects of several compounds from P. multiflorum, and found a novel compound, CRPE55IB. The CRPE55IB-induced suppression of NO and PGE₂ production correlated with inhibition of iNOS and COX-2 protein expression and promoter activity in lipopolysaccharide (LPS)-stimulated microglia. CRPE55IB also reduced the production of pro-inflammatory cytokines (TNF-α and IL-6) induced by LPS. Furthermore, investigation of the molecular mechanism indicated that CRPE55IB inhibited LPS-induced NF-κB activation by inactivating phosphorylation of IKKα/β, and phosphorylation and degradation of IκBα. We further found that CRPE55IB inhibited the phosphorylation of ERK and JNK at a lower concentration than that for p38 MAPK. Further experiments revealed that CRPE55IB treatment considerably increased the activation of Nrf2/ARE, and the expression of its target genes, including HO-1 and NQO1. Moreover, the Knockdown of Nrf2, HO-1, and NQO1 by siRNA abrogated the inhibitory effect of CRPE55IB on iNOS and COX-2 promoter activity. CRPE55IB also induced phosphorylation of AMPK/LKB/CaMKII in microglia. Analysis using a specific inhibitor of AMPK demonstrated that AMPK activation was involved in CRPE55IB-induced HO-1 and NQO1 expression. In addition, the CRPE55IB-induced anti-neuroinflammatory effect was abrogated by a specific inhibitor of AMPK, indicating the important role of AMPK in CRPE55IB-induced anti-neuroinflammation. Collectively, these results demonstrate that CRPE55IB exerts anti-neuroinflammatory effects against LPS via the Nrf2/AMPK signaling pathways.

Ghrelin-AMPK Signaling Mediates the Neuroprotective Effects of Calorie Restriction in Parkinson's Disease

Calorie restriction (CR) is neuroprotective in Parkinson's disease (PD) although the mechanisms are unknown. In this study we hypothesized that elevated ghrelin, a gut hormone with neuroprotective properties, during CR prevents neurodegeneration in an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD. CR attenuated the MPTP-induced loss of substantia nigra (SN) dopamine neurons and striatal dopamine turnover in ghrelin WT but not KO mice, demonstrating that ghrelin mediates CR's neuroprotective effect. CR elevated phosphorylated AMPK and ACC levels in the striatum of WT but not KO mice suggesting that AMPK is a target for ghrelin-induced neuroprotection. Indeed, exogenous ghrelin significantly increased pAMPK in the SN. Genetic deletion of AMPKβ1 and 2 subunits only in dopamine neurons prevented ghrelin-induced AMPK phosphorylation and neuroprotection. Hence, ghrelin signaling through AMPK in SN dopamine neurons mediates CR's neuroprotective effects. We consider targeting AMPK in dopamine neurons may recapitulate neuroprotective effects of CR without requiring dietary intervention.

Activation of AMPK attenuates LPS-induced acute lung injury by upregulation of PGC1α and SOD1

Evidence suggests that an imbalance between oxidation and antioxidation is involved in the pathogenesis of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Activation of AMP-activated protein kinase (AMPK) has been shown to inhibit the occurrence of ALI/ARDS. However, it is unknown whether activation of AMPK benefits ALI/ARDS by restoration of the oxidant and antioxidant balance, and which mechanisms are responsible for this process. The present study aimed to address these issues. Lipopolysaccharide (LPS) induced pronounced pathological changes of ALI in mice; these were accompanied by elevated production of malondialdehyde (MDA) and decreased activity of superoxide dismutase (SOD) compared with control mice. Prior treatment of mice with the AMPK agonist metformin significantly suppressed the LPS-induced development of ALI, reduced the elevation of MDA and increased the activity of SOD. Further analysis indicated that activation of AMPK also stimulated the protein expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) and superoxide dismutase 1 (SOD1). This study suggests that activation of AMPK by metformin inhibits oxidative stress by upregulation of PGC1α and SOD1, thereby suppressing the development of ALI/ARDS, and has potential value in the clinical treatment of such conditions.

Effects of metformin on inflammation and short-term memory in streptozotocin-induced diabetic mice

The aim of the present study was to analyze the action of metformin on short-term memory, glial cell activation and neuroinflammation caused by experimental diabetic encephalopathy in C57BL/6 mice. Diabetes was induced by the intraperitoneal injection of a dose of 90mg/kg of streptozotocin on two successive days. Mice with blood glucose levels ≥200dl/ml were considered diabetic and were given metformin hydrochloride at doses of 100mg/kg and 200mg/kg (by gavage, twice daily) for 21 days. On the final day of treatment, the mice underwent a T-maze test. On the 22nd day of treatment all the animals were anesthetized and euthanized. Diabetic animals treated with metformin had a higher spatial memory score. The hippocampus of the diabetic animals presented reactive gliosis, neuronal loss, NF-kB signaling activation, and high levels of IL-1 and VEGF. In addition, the T-maze test scores of these animals were low. Treatment with metformin reduced the expression of GFAP, Iba-1 (astrocyte and microglial markers) and the inflammation markers (p-IKB, IL-1 and VEGF), while enhancing p-AMPK and eNOS levels and increasing neuronal survival (Fox-1 and NeuN). Treatment with metformin also improved the spatial memory scores of diabetic animals. In conclusion, the present study showed that metformin can significantly reduce neuroinflammation and can decrease the loss of neurons in the hippocampus of diabetic animals, which can subsequently promote improvements in spatial memory.

Resveratrol suppresses glial activation and alleviates trigeminal neuralgia via activation of AMPK

Background

Glial activation and neuroinflammation in the spinal trigeminal nucleus (STN) play a pivotal role in the genesis and maintenance of trigeminal neuralgia (TN). Resveratrol, a natural compound from grape and red wine, has a potential anti-inflammatory effect. We hypothesized that resveratrol could significantly suppress neuroinflammation in the STN mediated by glial activation and further relieve TN. In this study, we evaluated whether resveratrol could alleviate trigeminal allodynia and explore the mechanism underlying the antinociceptive effect of resveratrol.

Methods

Animals were orally injected with resveratrol after chronic constriction injury (CCI) of the infraorbital nerve. Mechanical thresholds of the affected whisker pad were measured to assess nociceptive behaviors. The STN was harvested to quantify the changing levels of p-NR1, p-PKC, TNF-α, and IL1-β by western blotting and detect the expression of calcitonin gene-related peptide (CGRP) and c-Fos by immunofluorescence. Glial activation was observed by immunofluorescence and western blotting. Mitogen-activated protein kinase (MAPK) phosphorylation in vivo and in vitro was examined by western blotting.

Results

We found that resveratrol significantly attenuated trigeminal allodynia dose-dependently and decreased the increased expression of CGRP and c-Fos in the STN. Additionally, resveratrol showed an inhibitory effect on CCI-evoked astrocyte and microglia activation and reduced production of pro-inflammatory cytokines in the STN. Furthermore, the antinociceptive effect of resveratrol was partially mediated by reduced phosphorylation of MAP kinases via adenosine monophosphate-activated protein kinase (AMPK) activation.

Conclusions

AMPK activation in the STN glia via resveratrol has utility in the treatment of CCI-induced neuroinflammation and further implicates AMPK as a novel target for the attenuation of trigeminal neuralgia.

Resveratrol attenuates inflammatory hyperalgesia by inhibiting glial activation in mice spinal cords

The present study aimed to investigate the effect of resveratrol on inflammatory pain. Mice were injected intraperitoneally with lipopolysaccharide (LPS) for 5 consecutive days to induce subacute systemic inflammation. Acetic acid‑induced writhing tests and tail‑flick tests were performed following the final LPS injection. Glial fibrillary acidic protein (GFAP; an astrocyte‑specific activation marker), ionized calcium binding adapter molecule 1 (Iba‑1; a microglia‑specific activation marker) and sirtuin 1 (SIRT1) protein expression levels were detected using immunohistochemistry analysis or western blotting. Following administration of LPS for 5 days, the number of writhes increased and the tail‑flick latency decreased. Resveratrol (10 or 20 mg/kg) partly inhibited LPS‑induced hyperalgesia and prevented the increase in tumor necrosis factor‑α and interleukin 6 levels induced by LPS. LPS injection reduced the SIRT1 protein expression and increased the number of GFAP‑positive and Iba‑1‑positive cells in the spinal cord. Resveratrol increased the SIRT1 protein expression levels and decreased the number of GFAP‑positive and Iba‑1‑positive cells in LPS‑treated mice. The protective effect of resveratrol was partly blocked by a selective SIRT1 inhibitor, EX‑257. Results from the present study suggest that subacute treatment with LPS induced the activation of glial cells and hyperalgesia. Resveratrol was demonstrated to inhibit the activation of glial cells and attenuate inflammatory hyperalgesia in a SIRT1‑dependent manner.

Procyanidins alleviates morphine tolerance by inhibiting activation of NLRP3 inflammasome in microglia

Background

The development of antinociceptive tolerance following repetitive administration of opioid analgesics significantly hinders their clinical use. Evidence has accumulated indicating that microglia within the spinal cord plays a critical role in morphine tolerance. The inhibitor of microglia is effective to attenuate the tolerance; however, the mechanism is not fully understood. Our present study investigated the effects and possible mechanism of a natural product procyanidins in improving morphine tolerance via its specific inhibition on NOD-like receptor protein3 (NLRP3) inflammasome in microglia.

Methods

CD-1 mice were used for tail-flick test to evaluate the degree of pain. The microglial cell line BV-2 was used to investigate the effects and the mechanism of procyanidins. Reactive oxygen species (ROS) produced from BV-2 cells was evaluated by flow cytometry. Cell signaling was measured by western blot assay and immunofluorescence assay.

Results

Co-administration of procyanidins with morphine potentiated its antinociception effect and attenuated the development of acute and chronic morphine tolerance. Procyanidins also inhibited morphine-induced increase of interleukin-1β and activation of NOD-like receptor protein3 (NLRP3) inflammasome. Furthermore, procyanidins decreased the phosphorylation of p38 mitogen-activated protein kinase, inhibited the translocation of nuclear factor-κB (NF-κB), and suppressed the level of reactive oxygen species in microglia.

Conclusions

Procyanidins suppresses morphine-induced activation of NLRP3 inflammasome and inflammatory responses in microglia, and thus resulting in significant attenuation of morphine antinociceptive tolerance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0520-z) contains supplementary material, which is available to authorized users.