Adenosine Monophosphate-activated Protein Kinase Regulates Interleukin-1β Expression and Glial Glutamate Transporter Function in Rodents with Neuropathic Pain

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

Metformin inhibits paclitaxel-induced mechanical allodynia by activating opioidergic pathways and reducing cytokines production in the dorsal root ganglia and thalamus

It has been shown that AMP-activated protein kinase (AMPK) is involved in the nociceptive processing. This observation has prompted us to investigate the effects of the AMPK activator metformin on the paclitaxel-induced mechanical allodynia, a well-established model of neuropathic pain. Mechanical allodynia was induced by four intraperitoneal (i.p) injections of paclitaxel (2 mg/kg.day) in mice. Metformin was administered per os (p.o.). Naltrexoneandglibenclamide were used to investigate mechanisms mediating metformin activity. Concentrations of cytokines in the dorsal root ganglia (DRG) and thalamus were determined. After a single p.o. administration, the two highest doses of metformin (500 and 1000 mg/kg) attenuated the mechanical allodynia. This response was attenuated by all doses of metformin (250, 500 and 1000 mg/kg) when two administrations, 2 h apart, were carried out. Naltrexone (5 and 10 mg/kg, i.p.), but not glibenclamide (20 and 40 mg/kg, p.o.), attenuated metformin activity. Concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1β and CXCL-1 in the DRG were increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentrations of TNF-α, IL-1β and CXCL-1 in the DRG. Concentration of IL-6, but not TNF-α, in the thalamus was increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentration of IL-6 in the thalamus. In summary, metformin exhibits activity in the model of neuropathic pain induced by paclitaxel. This activity may be mediated by activation of opioidergic pathways and reduced production of TNF-α, IL-1β and CXCL-1 in the DRG and IL-6 in the thalamus.

Targeting the Main Sources of Reactive Oxygen Species Production: Possible Therapeutic Implications in Chronic Pain

Humans have long been combating chronic pain. In clinical practice, opioids are firstchoice analgesics, but long-term use of these drugs can lead to serious adverse reactions. Finding new, safe and effective pain relievers that are useful treatments for chronic pain is an urgent medical need. Based on accumulating evidence from numerous studies, excess reactive oxygen species (ROS) contribute to the development and maintenance of chronic pain. Some antioxidants are potentially beneficial analgesics in the clinic, but ROS-dependent pathways are completely inhibited only by scavenging ROS directly targeting cellular or subcellular sites. Unfortunately, current antioxidant treatments do not achieve this effect. Furthermore, some antioxidants interfere with physiological redox signaling pathways and fail to reverse oxidative damage. Therefore, the key upstream processes and mechanisms of ROS production that lead to chronic pain in vivo must be identified to discover potential therapeutic targets related to the pathways that control ROS production in vivo. In this review, we summarize the sites and pathways involved in analgesia based on the three main mechanisms by which ROS are generated in vivo, discuss the preclinical evidence for the therapeutic potential of targeting these pathways in chronic pain, note the shortcomings of current research and highlight possible future research directions to provide new targets and evidence for the development of clinical analgesics.

Deficient AMPK activity contributes to hyperexcitability in peripheral nociceptive sensory neurons and thermal hyperalgesia in lupus mice

Patients with systemic lupus erythematosus (SLE) often suffer from chronic pain. Little is known about the peripheral mechanisms underlying the genesis of chronic pain induced by SLE. The aim of this study was to investigate whether and how membrane properties in nociceptive neurons in the dorsal root ganglions (DRGs) are altered by SLE. We found elevation of resting membrane potentials, smaller capacitances, lower action potential thresholds and rheobases in nociceptive neurons in the DRGs from MRL/lpr mice (an SLE mouse model) with thermal hyperalgesia. DRGs from MRL/lpr mice had increased protein expressions in TNFα, IL-1β, and phosphorylated ERK but suppressed AMPK activity, and no changes in sodium channel 1.7 protein expression. We showed that intraplantar injection of Compound C (an AMPK inhibitor) induced thermal hyperalgesia in normal mice while intraplantar injection of AICAR (an AMPK activator) reduced thermal hyperalgesia in MRL/Lpr mice. Upon inhibition of AMPK membrane properties in nociceptive neurons from normal control mice could be rapidly switched to those found in SLE mice with thermal hyperalgesia. Our study indicates that increased excitability in peripheral nociceptive sensory neurons contributes to the genesis of thermal hyperalgesia in mice with SLE, and AMPK regulates membrane properties in nociceptive sensory neurons as well as thermal hyperalgesia in mice with SLE. Our study provides a basis for targeting signaling pathways regulating membrane properties of peripheral nociceptive neurons as a means for conquering chronic pain caused by SLE.

Activation of Protease-Activated Receptor-1 Causes Chronic Pain in Lupus-Prone Mice Via Suppressing Spinal Glial Glutamate Transporter Function and Enhancing Glutamatergic Synaptic Activity

Systemic lupus erythematosus (SLE) is an unpredictable autoimmune disease where the body's immune system mistakenly attacks healthy tissues in many parts of the body. Chronic pain is one of the most frequently reported symptoms among SLE patients. We previously reported that MRL lupus prone (MRL/lpr) mice develop hypersensitivity to mechanical and heat stimulation. In the present study, we found that the spinal protease-activated receptor-1(PAR1) plays an important role in the genesis of chronic pain in MRL/lpr mice. Female MRL/lpr mice with chronic pain had activation of astrocytes, over-expression of thrombin and PAR1, enhanced glutamatergic synaptic activity, as well as suppressed activity of adenosine monophosphate-activated protein kinase (AMPK) and glial glutamate transport function in the spinal cord. Intrathecal injection of either the PAR1 antagonist, or AMPK activator attenuated heat hyperalgesia and mechanical allodynia in MRL/lpr mice. Furthermore, we also identified that the enhanced glutamatergic synaptic activity and suppressed activity of glial glutamate transporters in the spinal dorsal horn of MRL/lpr mice are caused by activation of the PAR1 and suppression of AMPK signaling pathways. These findings suggest that targeting the PAR1 and AMPK signaling pathways in the spinal cord may be a useful approach for treating chronic pain caused by SLE. PERSPECTIVE: Our study provides evidence suggesting activation of PAR1 and suppression of AMPK in the spinal cord induces thermal hyperalgesia and mechanical allodynia in a lupus mouse model. Targeting signaling pathways regulating the PAR1 and AMPK could potentially provide a novel approach to the management of chronic pain caused by SLE.

Metformin inhibits spontaneous excitatory postsynaptic currents in spinal dorsal cord neurons from paclitaxel-treated rats

Introduction

Cancer patients treated with paclitaxel often develop chemotherapy-induced peripheral neuropathy, which has not been effectively treated with drugs. The anti-diabetic drug metformin is effective in the treatment of neuropathic pain. The aim of this study was to elucidate effect of metformin on paclitaxel-induced neuropathic pain and spinal synaptic transmission.

Methods

Electrophysiological experiments on rat spinal slices were performed in vitro and mechanical allodynia quantified in vitro.

Results

The present data demonstrated that intraperitoneal injection of paclitaxel produced mechanical allodynia and potentiated spinal synaptic transmission. Intrathecal injection of metformin significantly reversed the established mechanical allodynia induced by paclitaxel in rats. Either spinal or systemic administration of metformin significantly inhibited the increased frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in spinal dorsal horn neurons from paclitaxel-treated rats. We found that 1 h incubation of metformin also reduced the frequency rather than the amplitude of sEPSCs in the spinal slices from paclitaxel-treated rats.

Discussion

These results suggested that metformin was able to depress the potentiated spinal synaptic transmission, which may contribute to alleviating the paclitaxel-induced neuropathic pain.

Phytohormone abscisic acid ameliorates neuropathic pain via regulating LANCL2 protein abundance and glial activation at the spinal cord

Spinal neuroinflammation plays a critical role in the genesis of neuropathic pain. Accumulating data suggest that abscisic acid (ABA), a phytohormone, regulates inflammatory processes in mammals. In this study, we found that reduction of the LANCL2 receptor protein but not the agonist ABA in the spinal cord is associated with the genesis of neuropathic pain. Systemic or intrathecal administration of ABA ameliorates the development and pre-existence of mechanical allodynia and heat hyperalgesia in animals with partial sciatic nerve ligation (pSNL). LANCL2 is expressed only in microglia in the spinal dorsal horn. Pre-emptive treatment with ABA attenuates activation of microglia and astrocytes, ERK activity, and TNFα protein abundance in the dorsal horn in rats with pSNL. These are accompanied by restoration of spinal LANCL2 protein abundance. Spinal knockdown of LANCL2 gene with siRNA recapitulates the behavioral and spinal molecular changes induced by pSNL. Activation of spinal toll-like receptor 4 (TLR4) with lipopolysaccharide leads to activation of microglia, and over production of TNFα, which are concurrently accompanied by suppression of protein levels of LANCL2 and peroxisome proliferator activated-receptor γ. These changes are ameliorated when ABA is added with LPS. The anti-inflammatory effects induced by ABA do not requires G_i protein activity. Our study reveals that the ABA/LANCL2 system is a powerful endogenous system regulating spinal neuroinflammation and nociceptive processing, suggesting the potential utility of ABA as the management of neuropathic pain.

Astrocyte-neuron lactate transport in the ACC contributes to the occurrence of long-lasting inflammatory pain in male mice

Lactate transport is an important means of communication between astrocytes and neurons and is implicated in a variety of neurobiological processes. However, the connection between astrocyte-neuron lactate transport and nociceptive modulation has not been well established. Here, we found that Complete Freund's adjuvant (CFA)-induced inflammation pain leads to a significant increase in extracellular lactate levels in the anterior cingulate cortex (ACC). Inhibition of glycogenolysis and lactate release in the ACC disrupted the persistent, but not acute, inflammation pain induced by CFA, and this effect was reversed by exogenous L-lactate administration. Knocking down the expression of lactate transporters (MCT1, MCT4, or MCT2) also disrupted the long lasting inflammation pain induced by CFA. Moreover, glycogenolysis in the ACC is critical for the induction of molecular changes related to neuronal plasticity, including the induction of phospho- (p-) ERK, p-CREB, and Fos. Taken together, our findings indicate that astrocyte-neuron lactate transport in the ACC is critical for the occurrence of persistent inflammation pain, suggesting a novel mechanism underlying chronic 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.

Blockade of BK channels attenuates chronic visceral hypersensitivity in an IBS-like rat model

BACKGROUND

Visceral hypersensitivity in irritable bowel syndrome (IBS) is still poorly understood, despite that chronic abdominal pain is the most common symptoms in IBS patients. To study effects of BK channels on visceral hypersensitivity in IBS rats and the underlying mechanisms, IBS rats were established by colorectal distention (CRD) in postnatal rats. The expression of large-conductance calcium and voltage-dependent potassium ion channels (BK channels) of the thoracolumbar spinal cord was examined in IBS and control rats. The effects of BK channel blockade on visceral hypersensitivity were evaluated. The interaction of BK channels and N-methyl-D-aspartate acid (NMDA) receptors was explored, and synaptic transmission at superficial dorsal horn (SDH) neurons of the thoracolumbar spinal cord was recorded by whole-cell patch clamp in IBS rats.

RESULTS

The expression of the BK channels of the thoracolumbar spinal cord in IBS rats was significantly reduced. The blockade of BK channels could reduce the visceral hypersensitivity in IBS rats. There was an interaction between BK channels and NMDA receptors in the spinal cord. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in SDH neurons is significantly reduced in IBS rats. The blockade of BK channels depolarizes the inhibitory interneuron membrane and increases their excitability in IBS rats.

CONCLUSIONS

BK channels could interact with NMDA receptors in the thoracolumbar spinal cord of rats and regulate visceral hypersensitivity in IBS rats.

An Investigation of the Molecular Mechanisms Underlying the Analgesic Effect of Jakyak-Gamcho Decoction: A Network Pharmacology Study

Herbal drugs have drawn substantial interest as effective analgesic agents; however, their therapeutic mechanisms remain to be fully understood. To address this question, we performed a network pharmacology study to explore the system-level mechanisms that underlie the analgesic activity of Jakyak-Gamcho decoction (JGd; Shaoyao-Gancao-Tang in Chinese and Shakuyaku-Kanzo-To in Japanese), an herbal prescription consisting of Paeonia lactiflora Pallas and Glycyrrhiza uralensis Fischer. Based on comprehensive information regarding the pharmacological and chemical properties of the herbal constituents of JGd, we identified 57 active chemical compounds and their 70 pain-associated targets. The JGd targets were determined to be involved in the regulation of diverse biological activities as follows: calcium- and cytokine-mediated signalings, calcium ion concentration and homeostasis, cellular behaviors of muscle and neuronal cells, inflammatory response, and response to chemical, cytokine, drug, and oxidative stress. The targets were further enriched in various pain-associated signalings, including the PI3K-Akt, estrogen, ErbB, neurotrophin, neuroactive ligand-receptor interaction, HIF-1, serotonergic synapse, JAK-STAT, and cAMP pathways. Thus, these data provide a systematic basis to understand the molecular mechanisms underlying the analgesic activity of herbal drugs.

The role of resveratrol as a natural modulator in glia activation in experimental models of stroke

OBJECTIVE

Stroke is one of the most important causes of death and disability in modern and developing societies. In a stroke, both the glial cells and neurons develop apoptosis due to decreased cellular access to glucose and oxygen. Resveratrol (3, 5, 4'-trihydroxy-trans-stilbene) as a herbal compound shows neuroprotective and glioprotective effects. This article reviews how resveratrol can alleviate symptoms after stroke to help neurons to survive by modulating some signaling pathways in glia.

MATERIALS AND METHODS

Various databases such as ISI Web of Knowledge, Scopus, Medline, PubMed, and Google Scholar, were searched from 2000 to February 2020 to gather the required articles using appropriate keywords.

RESULTS

Resveratrol enhances anti-inflammatory and decreases inflammatory cytokines by affecting the signaling pathways in microglia such as AMP-activated protein kinase (5' adenosine monophosphate-activated protein kinase, AMPK), SIRT1 (sirtuin 1) and SOCS1 (suppressor of cytokine signaling 1). Furthermore, through miR-155 overexpressing in microglia, resveratrol promotes M2 phenotype polarization. Resveratrol also increases AMPK and inhibits GSK-3β (glycogen synthase kinase 3 beta) activity in astrocytes, which release energy, makes ATP available to neurons and reduces reactive oxygen species (ROS). Besides, resveratrol increases oligodendrocyte survival, which can lead to maintaining post-stroke brain homeostasis.

CONCLUSION

These results suggest that resveratrol can be considered a novel therapeutic agent for the reduction of stroke symptoms that can not only affect neuronal function but also play an important role in reducing neurotoxicity by altering glial activity and signaling.

AMP-activated protein kinase (AMPK) regulates astrocyte oxidative metabolism by balancing TCA cycle dynamics

AMP-activated protein kinase (AMPK) is an important energy sensor located in cells throughout the human body. From the periphery, AMPK is known to be a metabolic master switch controlling the use of energy fuels. The energy sensor is activated when the energy status of the cell is low, initiating energy-producing pathways and deactivating energy-consuming pathways. All brain cells are crucially dependent on energy production for survival, and the availability of energy substrates must be closely regulated. Intriguingly, the role of AMPK in the regulation of brain cell metabolism has been sparsely investigated, particularly in astrocytes. By investigating metabolism of ¹³ C-labeled energy substrates in acutely isolated hippocampal slices and cultured astrocytes, with subsequent mass spectrometry analysis, we here show that activation of AMPK increases glycolysis as well as the capacity of the TCA cycle, that is, anaplerosis, through the activity of pyruvate carboxylase (PC) in astrocytes. In addition, we demonstrate that AMPK activation leads to augmented astrocytic glutamate oxidation via pyruvate recycling (i.e., cataplerosis). This regulatory mechanism induced by AMPK activation is mediated via glutamate dehydrogenase (GDH) shown in a CNS-specific GDH knockout mouse. Collectively, these findings demonstrate that AMPK regulates TCA cycle dynamics in astrocytes via PC and GDH activity. AMPK functionality has been shown to be hampered in Alzheimer's and Parkinson's disease and our findings may therefore add to the toolbox for discovery of new metabolic drug targets.

Blockade of BDNF signalling attenuates chronic visceral hypersensitivity in an IBS-like rat model

Background

Irritable bowel syndrome (IBS) is a common functional disease characterized by chronic abdominal pain and changes in bowel movements. Effective therapy for visceral hypersensitivity in IBS patients remains challenging. This study investigated the roles of brain‐derived neurotrophic factor (BDNF) and tyrosine kinase receptor B (TrkB) and the effect of ANA‐12 (a selective antagonist of TrkB) on chronic visceral hypersensitivity in an IBS‐like rat model.

Methods

An IBS‐like rat model was established through neonatal maternal separation (NMS), and visceral hypersensitivity was assessed by electromyographic (EMG) responses of the abdominal external oblique muscles to colorectal distention (CRD). Different doses of ANA‐12 were injected intrathecally to investigate the effect of that drug on visceral hypersensitivity, and the open field test was performed to determine whether ANA‐12 had side effects on movement. Thoracolumbar spinal BDNF, TrkB receptor and Protein kinase Mζ (PKMζ) expression were measured to investigate their roles in chronic visceral hypersensitivity. Whole‐cell recordings were made from thoracolumbar superficial dorsal horn (SDH) neurons of lamina II.

Results

The expression of BDNF and TrkB was enhanced in the thoracolumbar spinal cord of the NMS animals. ANA‐12 attenuated visceral hypersensitivity without side effects on motricity in NMS rats. PKMζ expression significantly decreased after the administration of ANA‐12. The frequency of spontaneous excitatory postsynaptic currents (sEPSCs) increased in the thoracolumbar SDH neurons of lamina II in NMS rats. The amplitude and frequency of sEPSCs were reduced after perfusion with ANA‐12 in NMS rats.

Conclusions

Neonatal maternal separation caused visceral hypersensitivity and increased synaptic activity by activating BDNF‐TrkB‐PKMζ signalling in the thoracolumbar spinal cord of adult rats. PKMζ was able to potentiate AMPA receptor (AMPAR)‐mediated sEPSCs in NMS rats. ANA‐12 attenuated visceral hypersensitivity and synaptic activity by blocking BDNF/TrkB signalling in NMS rats.

Significance

ANA‐12 attenuates visceral hypersensitivity via BDNF‐TrkB‐PKMζ signalling and reduces synaptic activity through AMPARs in NMS rats. This knowledge suggests that ANA‐12 could represent an interesting novel therapeutic medicine for chronic visceral hypersensitivity.

AMP-activated protein kinase: the current landscape for drug development

Since the discovery of AMP-activated protein kinase (AMPK) as a central regulator of energy homeostasis, many exciting insights into its structure, regulation and physiological roles have been revealed. While exercise, caloric restriction, metformin and many natural products increase AMPK activity and exert a multitude of health benefits, developing direct activators of AMPK to elicit beneficial effects has been challenging. However, in recent years, direct AMPK activators have been identified and tested in preclinical models, and a small number have entered clinical trials. Despite these advances, which disease(s) represent the best indications for therapeutic AMPK activation and the long-term safety of such approaches remain to be established.

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.

Impaired AMPK‑CGRP signaling in the central nervous system contributes to enhanced neuropathic pain in high‑fat diet‑induced obese rats, with or without nerve injury

Obesity is associated with increased sensitivity to pain, including neuropathic pain, but the precise mechanisms are not fully understood. Recent evidence has revealed that AMP-activated protein kinase (AMPK) in the central nervous system (CNS) regulates the neuropeptide calcitonin gene-related peptide (CGRP), a principal neurotransmitter of the class C nerve fiber, which serves an important role in initiating and maintaining neuropathic pain. AMPK has been demonstrated to be downregulated in the CNS in obesity. The present study hypothesized that obesity may lead to increased sensitivity to neuropathic pain by downregulating AMPK and upregulating CGRP expression levels in the CNS. Sprague-Dawley rats consuming a high-fat diet (HF) for 12 weeks developed obesity; they exhibited significantly decreased levels of phospho (p)-AMPK and increased CGRP expression levels in the spinal cord (SC) and dorsal root ganglion (DRG), respectively, compared with rats consuming a low-fat (LF) diet. HF-fed rats that underwent spared nerve injury (SNI) also exhibited lower p-AMPK and higher CGRP expression levels in the SC and DRG, compared with the corresponding LF-diet rats. The 50% paw withdrawal threshold (PWT; as measured by Von Frey testing) was significantly lower in HF-fed compared with LF-fed rats, with or without SNI. Through intrathecal treatment, the AMPK activator 5-aminoimidazole-4-carboxamide riboside (AICAR) or the CGRP antagonist CGRP8-37 decreased CGRP expression levels and increased the 50% PWT; however, the AMPK inhibitor dorsomorphin augmented CGRP expression levels and further reduced the 50% PWT in HF-fed rats, but not LF-fed rats, with or without SNI. The results indicated that blocking the AMPK-CGRP pathway may enhance neuropathic pain in HF-induced obesity, with or without nerve injury. Targeting AMPK in the CNS may be a novel strategy for the prevention and treatment of obesity-associated neuropathic pain.

Downregulation of adenosine and adenosine A1 receptor contributes to neuropathic pain in resiniferatoxin neuropathy

This study demonstrates that functional prostatic acid phosphatase is required for maintaining the balance of cellular adenosine analgesia in systemic small-fiber neuropathy.

The neurochemical effects of adenosine signaling in small-fiber neuropathy leading to neuropathic pain are yet to be explored in a direct manner. This study examined this system at the level of ligand (through the ectonucleotidase activity of prostatic acid phosphatase [PAP]) and adenosine A1 receptors (A1Rs) in resiniferatoxin (RTX) neuropathy, a peripheral neurodegenerative disorder that specifically affects nociceptive nerves expressing transient receptor potential vanilloid type 1 (TRPV1). We conducted immunohistochemistry on dorsal root ganglion (DRG) neurons, high-performance liquid chromatography for functional assays, and pharmacological interventions to alter PAP and A1Rs in mice with RTX neuropathy. In DRG of RTX neuropathy, PAP(+) neurons were reduced compared with vehicle-treated mice (P = 0.002). Functionally, PAP ectonucleotidase activity was consequently reduced (ie, the content of adenosine in DRG, P = 0.012). PAP(+) neuronal density was correlated with the degree of mechanical allodynia, which was reversed by intrathecal (i.t.) lumbar puncture injection of recombinant PAP with a dose-dependent effect. Furthermore, A1Rs were downregulated (P = 0.002), and this downregulation was colocalized with the TRPV1 receptor (31.0% ± 2.8%). Mechanical allodynia was attenuated in a dose-dependent response by i.t. injection of the A1R ligand, adenosine; however, no analgesia was evident when an exogenous adenosine was blocked by A1R antagonist. This study demonstrated dual mechanisms of neuropathic pain in TRPV1-induced neuropathy, involving a reduced adenosine system at both the ligand (adenosine) and receptor (A1Rs) levels.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Translational Control Mechanisms in Persistent Pain

Persistent pain, which is poorly treated and estimated to afflict one third of the world's population, is largely mediated by the sensitization of nociceptive neurons. This sensitization involves de novo gene expression to support biochemical and structural changes required to maintain amplified pain signaling that frequently persists even after injury to tissue resolves. While transcription-dependent changes in gene expression are important, recent work demonstrates that activity-dependent regulation of mRNA translation is key to controlling the cellular proteome and the development and maintenance of persistent pain. In this review, we highlight recent advances in translational regulation of gene expression in nociceptive circuits, with a focus on key signaling pathways and mRNA targets that may be tractable for the creation of next-generation pain therapeutics.

Therapeutic opportunities for pain medicines via targeting of specific translation signaling mechanisms

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A common underlying cause of chronic pain is a phenotypic change in nociceptors in the peripheral nervous system.

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Translation regulation signaling pathways control gene expression changes that drive chronic pain.

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We focus on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics.

As the population of the world ages and as more and more people survive diseases that used to be primary causes of mortality, the incidence of severe chronic pain in most of the world has risen dramatically. This type of pain is very difficult to treat and the opioid overdose epidemic that has become a leading cause of death in the United States and other parts of the world highlights the urgent need to develop new pain therapeutics. A common underlying cause of severe chronic pain is a phenotypic change in pain-sensing neurons in the peripheral nervous system called nociceptors. These neurons play a vital role in detecting potentially injurious stimuli, but when these neurons start to detect very low levels of inflammatory meditators or become spontaneously active, they send spurious pain signals to the brain that are significant drivers of chronic pain. An important question is what drives this phenotypic shift in nociceptors from quiescence under most conditions to sensitization to a broad variety of stimuli and spontaneous activity. The goal of this review is to discuss the critical role that specific translation regulation signaling pathways play in controlling gene expression changes that drive nociceptor sensitization and may underlie the development of spontaneous activity. The focus will be on advances in technologies that allow for identification of such targets and on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics. A key advantage of pharmacological manipulation of these signaling events is that they may reverse phenotypic shifts in nociceptors that drive chronic pain thereby creating the first generation of disease modifying drugs for chronic pain.

Effects of metformin on the expression of AMPK and STAT3 in the spinal dorsal horn of rats with neuropathic pain

Neuropathic pain (NP) is a frustrating and burdensome problem. Current treatments for NP have unendurable side effects and/or questionable efficacy, and once these therapies are stopped, the symptoms often return. Thus, novel drugs are needed to enhance the effectiveness of treatments for NP. One novel target for pain treatments is adenosine monophosphate-activated protein kinase (AMPK), which regulates a variety of cellular processes, including protein translation, which is considered to be affected in NP. Metformin is a widely available drug that possesses the ability to activate AMPK. The signal transducer and activator of transcription 3 (STAT3) pathway plays an important role in neuroinflammation. The present study investigated the analgesic effect of metformin on NP induced by chronic constriction injury (CCI), and the influence of metformin on the expression of AMPK and STAT3 in the spinal dorsal horn (SDH). In CCI rats, paw withdrawal latencies in response to thermal hyperalgesia were significantly shorter, while phosphorylated (p)-AMPK was expressed at lower levels and p-STAT3 was expressed at higher levels in the SDH. Administering intraperitoneal injections of metformin (200 mg/kg) for 6 successive days activated AMPK and suppressed the expression of p-STAT3, in addition to reversing hyperalgesia. Finally, metformin inhibited the activation of microglia and astrocytes in the SDH, which may explain how it alleviates NP.

The AMPK Activator A769662 Blocks Voltage-Gated Sodium Channels: Discovery of a Novel Pharmacophore with Potential Utility for Analgesic Development

Voltage-gated sodium channels (VGSC) regulate neuronal excitability by governing action potential (AP) generation and propagation. Recent studies have revealed that AMP-activated protein kinase (AMPK) activators decrease sensory neuron excitability, potentially by preventing sodium (Na⁺) channel phosphorylation by kinases such as ERK or via modulation of translation regulation pathways. The direct positive allosteric modulator A769662 displays substantially greater efficacy than other AMPK activators in decreasing sensory neuron excitability suggesting additional mechanisms of action. Here, we show that A769662 acutely inhibits AP firing stimulated by ramp current injection in rat trigeminal ganglion (TG) neurons. PT1, a structurally dissimilar AMPK activator that reduces nerve growth factor (NGF) -induced hyperexcitability, has no influence on AP firing in TG neurons upon acute application. In voltage-clamp recordings, application of A769662 reduces VGSC current amplitudes. These findings, based on acute A769662 application, suggest a direct channel blocking effect. Indeed, A769662 dose-dependently blocks VGSC in rat TG neurons and in Na_v1.7-transfected cells with an IC₅₀ of ~ 10 μM. A769662 neither displayed use-dependent inhibition nor interacted with the local anesthetic (LA) binding site. Popliteal fossa administration of A769662 decreased noxious thermal responses with a peak effect at 5 mins demonstrating an analgesic effect. These data indicate that in addition to AMPK activation, A769662 acts as a direct blocker/modulator of VGSCs, a potential mechanism enhancing the analgesic property of this compound.

Suberoylanilide hydroxamic acid prevents downregulation of spinal glutamate transporter-1 and attenuates spinal nerve ligation-induced neuropathic pain behavior

Glutamate transporter-1 (GLT-1) reduction causes dysregulation of excitatory-inhibitory balance, contributing toward neuropathic pain development. However, the mechanisms underlying GLT-1 downregulation are still unclear. Histone acetylation plays a pivotal role in the regulation of gene expression. We sought to examine the contribution of histone acetylation on pain hypersensitivity and GLT-1 downregulation in neuropathic pain development. Histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) was intrathecally infused to rats through osmotic pumps from -5 days to 7 days after spinal nerve ligation (SNL). Behavioral tests indicated that SAHA could significantly prevent SNL-induced mechanical allodynia and thermal hyperalgesia. The effect was dose related and lasted to 10 days after SNL when the SAHA infusion was stopped on day 7. Immunohistochemistry, western blot, and real-time reverse transcription PCR analysis showed that SAHA significantly prevented SNL-induced downregulation of GLT-1 in the spinal dorsal horn. In addition, SNL-induced weakened acetylation of histone H3 (AcH3) was significantly inhibited by SAHA. Immunofluorescent histochemistry showed that both GLT-1 and AcH3 had high expressions in the dorsal horn. Double staining indicated that several GLT-1-positive cells were colocalized with AcH3. Our data provide evidence that histone deacetylation may contribute toward the loss of GLT-1 and this could be a new consideration for the development of more effective strategies for treating neuropathic pain.

Functional characterization of AMP-activated protein kinase signaling in tumorigenesis

AMP-activated protein kinase (AMPK) is a ubiquitously expressed metabolic sensor among various species. Specifically, cellular AMPK is phosphorylated and activated under certain stressful conditions, such as energy deprivation, in turn to activate diversified downstream substrates to modulate the adaptive changes and maintain metabolic homeostasis. Recently, emerging evidences have implicated the potential roles of AMPK signaling in tumor initiation and progression. Nevertheless, a comprehensive description on such topic is still in scarcity, especially in combination of its biochemical features with mouse modeling results to elucidate the physiological role of AMPK signaling in tumorigenesis. Hence, we performed this thorough review by summarizing the tumorigenic role of each component along the AMPK signaling, comprising of both its upstream and downstream effectors. Moreover, their functional interplay with the AMPK heterotrimer and exclusive efficacies in carcinogenesis were chiefly explained among genetically altered mice models. Importantly, the pharmaceutical investigations of AMPK relevant medications have also been highlighted. In summary, in this review, we not only elucidate the potential functions of AMPK signaling pathway in governing tumorigenesis, but also potentiate the future targeted strategy aiming for better treatment of aberrant metabolism-associated diseases, including cancer.

A critical time window for the analgesic effect of central histamine in the partial sciatic ligation model of neuropathic pain

Background

It is known that histamine participates in pain modulation. However, the effect of central histamine on neuropathic pain is not fully understood. Here, we report a critical time window for the analgesic effect of central histamine in the partial sciatic nerve ligation model of neuropathic pain.

Methods

Neuropathic pain was induced by partial sciatic nerve ligation (PSL) in rats, wild-type (C57BL/6J) mice and HDC^−/− (histidine decarboxylase gene knockout) and IL-1R^−/− (interleukin-1 receptor gene knockout) mice. Histidine, a precursor of histamine that can increase the central histamine levels, was administered intraperitoneally (i.p.). Histidine decarboxylase (HDC) enzyme inhibitor α-fluoromethylhistidine was administered intracerebroventricularly (i.c.v.). Histamine H₁ receptor antagonist mepyramine and H₂ receptor antagonist cimetidine were given intrathecally (i.t.) and intracisternally (i.c.). Withdrawal thresholds to tactile and heat stimuli were measured with a set of von Frey hairs and infrared laser, respectively. Immunohistochemistry and Western blot were carried out to evaluate the morphology of microglia and IL-1β production, respectively.

Results

Histidine (100 mg/kg, i.p.) administered throughout days 0–3, 0–7, or 0–14 postoperatively (PO) alleviated mechanical allodynia and thermal hyperalgesia in the hindpaw following PSL in rats. Intrathecal histamine reversed PSL-induced thermal hyperalgesia in a dose-dependent manner and intracisternal histamine alleviated both mechanical allodynia and thermal hyperalgesia. Moreover, α-fluoromethylhistidine (i.c.v.) abrogated the analgesic effect of histidine. However, histidine treatment initiated later than the first postoperative day (treatment periods included days 2–3, 4–7, and 8–14 PO) did not show an analgesic effect. In addition, histidine treatment initiated immediately, but not 3 days after PSL, inhibited microglial activation and IL-1β upregulation in the lumbar spinal cord, in parallel with its effects on behavioral hypersensitivity. Moreover, the inhibitory effects on pain hypersensitivity and spinal microglial activation were absent in HDC^−/− mice and IL-1R^−/− mice. H₁ receptor antagonist mepyramine (200 ng/rat i.t. or i.c.), but not H₂ receptor antagonist cimetidine (200, 500 ng/rat i.t. or 500 ng/rat i.c.), blocked the effects of histidine on pain behavior and spinal microglia.

Conclusions

These results demonstrate that central histamine is analgesic within a critical time window in the PSL model of neuropathic pain via histamine H₁ receptors. This effect may partly relate to the inhibition of microglial activation and IL-1β production in the spinal cord following nerve injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0637-0) contains supplementary material, which is available to authorized users.

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.

Development of a surrogate matrix for cerebral spinal fluid for liquid chromatography/mass spectrometry based analytical methods

RATIONALE

In recent years, several liquid chromatography/tandem mass spectrometry (LC/MS/MS) methods have been reported for the quantitative determination of drugs and metabolites in cerebral spinal fluid (CSF). Artificial CSF (aCSF) is often used as a surrogate for preparing calibration curves and quality control samples in these methods. However, aCSF does not accurately represent the composition of real CSF because it is missing all of the proteins and lipids, which may alter the electrospray ionization (ESI) response when performing LC/MS/MS analyses. In the current study we compared the mass spectral response of several compounds with a range of physiochemical properties in aCSF (essentially a mixture of salts and buffers), diluted plasma (ranging from 1:5 to 1:200) and real CSF to find the best surrogate for CSF in LC/MS/MS methods.

METHODS

A number of analytes from polar to non-polar, high protein binding to low protein binding, employing different sample preparation methods, were prepared in diluted plasma, actual CSF or aCSF and tested using LC/MS/MS. The analytes included cotinine and its metabolites, quetiapine, norquetiapine, chlorpromazine, efavirenz and lamivudine. The similarity of MS responses from these compounds in aCSF and diluted plasma to CSF was assessed by comparing the slopes of the calibration curves generated from using linear regression modeling.

RESULTS

For all compounds, the lowest percent difference in response ratio (0 to 17%) was observed from 1:200 diluted plasma. Our results indicated that, irrespective of the inherent physiochemical properties of the analytes or the method of sample preparation, 1:200 diluted plasma performed as the best surrogate for CSF in LC/MS/MS methods.

CONCLUSIONS

The percent difference in response ratio has been established to demonstrate how different compounds behave between CSF, aCSF and dilute plasma. Although among the compounds tested some of them showed a very similar MS response in actual and aCSF, there were analytes that demonstrated significant differences in ESI-MS signal when sprayed from these two matrices. However, even in such cases, 1:200 diluted plasma generated results with no significant difference from CSF. Therefore, we recommend that in order to develop robust and dependable bioanalytical LC/MS methods from CSF samples, it is more appropriate to prepare calibration curves and quality control samples in diluted plasma instead of aCSF. Copyright © 2016 John Wiley & Sons, Ltd.

AMPKα1 knockout enhances nociceptive behaviors and spinal glutamatergic synaptic activities via production of reactive oxygen species in the spinal dorsal horn

Emerging studies have shown that pharmacological activation of adenosine monophosphate-activated protein kinase (AMPK) produces potent analgesic effects in different animal pain models. Currently, the spinal molecular and synaptic mechanism by which AMPK regulates the pain signaling system remains unclear. To address this issue, we utilized the Cre-LoxP system to conditionally knockout the AMPKα1 gene in the nervous system of mice. We demonstrated that AMPKα1 is imperative for maintaining normal nociception, and mice deficient for AMPKα1 exhibit mechanical allodynia. This is concomitantly associated with increased glutamatergic synaptic activities in neurons located in the superficial spinal dorsal horn, which results from the increased glutamate release from presynaptic terminals and function of ligand-gated glutamate receptors at the postsynaptic neurons. Additionally, AMPKα1 knockout mice have increased activities of extracellular signal-regulated kinases (ERK) and p38 mitogen-activated protein kinases (p38), as well as elevated levels of interleukin-1β (IL-1β), reactive oxygen species (ROS), and heme oxygenase 1 (HO-1) in the spinal dorsal horn. Systemic administration of a non-specific ROS scavenger (phenyl-N-tert-butylnitrone, PBN) or a HO-1 activator (Cobalt protoporphyrin IX, CoPP) attenuated allodynia in AMPKα1 knockout mice. Bath-perfusion of the ROS scavenger or HO-1 activator effectively attenuated the increased ROS levels and glutamatergic synaptic activities in the spinal dorsal horn. Our findings suggest that ROS are the key down-stream signaling molecules mediating the behavioral hypersensitivity in AMPKα1 knockout mice. Thus, targeting AMPKα1 may represent an effective approach for the treatment of pathological pain conditions associated with neuroinflammation at the spinal dorsal horn.

Targeting AMPK for the Alleviation of Pathological Pain

Chronic pain is a major clinical problem that is poorly treated with available therapeutics. Adenosine monophosphate-activated protein kinase (AMPK) has recently emerged as a novel target for the treatment of pain with the exciting potential for disease modification. AMPK activators inhibit signaling pathways that are known to promote changes in the function and phenotype of peripheral nociceptive neurons and promote chronic pain. AMPK activators also reduce the excitability of these cells suggesting that AMPK activators may be efficacious for the treatment of chronic pain disorders, like neuropathic pain, where changes in the excitability of nociceptors is thought to be an underlying cause. In agreement with this, AMPK activators have now been shown to alleviate pain in a broad variety of preclinical pain models indicating that this mechanism might be engaged for the treatment of many types of pain in the clinic. A key feature of the effect of AMPK activators in these models is that they can lead to a long-lasting reversal of pain hypersensitivity even long after treatment cessation, indicative of disease modification. Here, we review the evidence supporting AMPK as a novel pain target pointing out opportunities for further discovery that are likely to have an impact on drug discovery efforts centered around potent and specific allosteric activators of AMPK for chronic pain treatment.

Animal Models to Study AMPK

AMPK is an evolutionary conserved energy sensor involved in the regulation of energy metabolism. Based on biochemical studies, AMPK has brought much of interest in recent years due to its potential impact on metabolic disorders. Suitable animal models are therefore essential to promote our understanding of the molecular and functional roles of AMPK but also to bring novel information for the development of novel therapeutic strategies. The organism systems include pig (Sus scrofa), mouse (Mus musculus), fly (Drosophila melanogaster), worm (Caenorhabditis elegans), and fish (Danio rerio) models. These animal models have provided reliable experimental evidence demonstrating the crucial role of AMPK in the regulation of metabolism but also of cell polarity, autophagy, and oxidative stress. In this chapter, we update the new development in the generation and application of animal models for the study of AMPK biology. We also discuss recent breakthroughs from studies in mice, flies, and worms showing how AMPK has a primary role in initiating or promoting pathological or beneficial impact on health.

Demyelination/remyelination and expression of interleukin-1β, substance P, nerve growth factor, and glial-derived neurotrophic factor during trigeminal neuropathic pain in rats

The etiology of trigeminal neuropathic pain is not clear, but there is evidence that demyelination, expression of cytokines, neuropeptides, and neurotrophic factors are crucial contributors. In order to elucidate mechanisms underlying trigeminal neuropathic pain, we evaluated the time course of morphological changes in myelinated and unmyelinated trigeminal nerve fibers, expression of cytokine IL-1β, neuropeptide substance P (SP), nerve growth factor (NGF), and glial derived neurotrophic factor (GDNF) in peripheral and ganglion tissues, using a rat model of trigeminal neuropathic pain. Chronic constriction injury (CCI) of the infraorbital nerve (IoN), or a sham surgery, was performed. Mechanical allodynia was evaluated from day 3 to day 15 post-surgery. Trigeminal nerves were divided into 2 sections - distal to CCI and ganglion - for morphological analyses, immunohistochemistry (IL-1β, SP), and protein quantification by ELISA (NGF, GDNF). At early postoperative time points, decreased mechanical responses were observed, which were associated with demyelination, glial cell proliferation, increased immunoexpression of IL-1 β and SP, and impaired GDNF production. In the late postoperative period, mechanical allodynia was present with partial recovery of myelination, glial cell proliferation, and increased immunoreactivity of IL-1β and SP. Our data show that demyelination/remyelination processes are related to the development of pain behavior. IL-1β may have effects both in ganglia and nerves, while SP may be an important mediator at the nerve endings. Additionally, low levels of GDNF may produce impaired signaling, which may be involved in generation of pain.

Blocking the GABA transporter GAT-1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life-saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel-induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT-1 but not GAT-3. In the spinal dorsal horn, GAT-1 was expressed at presynaptic terminals and astrocytes while GAT-3 was only expressed in astrocytes. In rats with paclitaxel-induced neuropathic pain, the protein expression of GAT-1 was increased while GAT-3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel-induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT-1 but not GAT-3 transporters. Paclitaxel-induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT-1 inhibitor. These findings suggest that targeting GAT-1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel-induced neuropathic pain. Patients receiving paclitaxel for cancer therapy often develop neuropathic pain and have a reduced quality of life. In this study, we demonstrated that animals treated with paclitaxel develop neuropathic pain, have enhancements of GABA transporter-1 protein expression and global GABA uptake, as well as suppression of GABAergic tonic inhibition in the spinal dorsal horn. Pharmacological inhibition of GABA transporter-1 ameliorates the paclitaxel-induced suppression of GABAergic tonic inhibition and neuropathic pain. Thus, targeting GAT-1 transporters for reversing GABAergic disinhibition in the spinal dorsal horn could be a useful approach for treating paclitaxel-induced neuropathic pain.