Gene transfer of GLT-1, a glial glutamate transporter, into the spinal cord by recombinant adenovirus attenuates inflammatory and neuropathic pain in rats

Astrocyte-Neuron Interactions in Spinal Cord Injury

Spinal cord injuries cause irreversible loss of sensory and motor functions. In mammals, intrinsic and extrinsic inhibitions of neuronal regeneration obstruct neural repair after spinal cord injury. Although astrocytes have been involved in a growing list of vital homeostatic functions in the nervous system, their roles after injury have fascinated and puzzled scientists for decades. Astrocytes undergo long-lasting morphological and functional changes after injury, referred to as reactive astrogliosis. Although reactive astrogliosis is required to contain spinal cord lesions and restore the blood-spinal cord barrier, reactive astrocytes have detrimental effects that inhibit neuronal repair and remyelination. Intriguingly, elevated regenerative capacity is preserved in some non-mammalian vertebrates, where astrocyte-like glial cells display exclusively pro-regenerative effects after injury. A detailed molecular and phenotypic catalog of the continuum of astrocyte reactivity states is an essential first step toward the development of glial cell manipulations for spinal cord repair.

Glutamatergic systems in neuropathic pain and emerging non-opioid therapies

Neuropathic pain, a disease of the somatosensory nervous system, afflicts many individuals and adequate management with current pharmacotherapies remains elusive. The glutamatergic system of neurons, receptors and transporters are intimately involved in pain but, to date, there have been few drugs developed that therapeutically modulate this system. Glutamate transporters, or excitatory amino acid transporters (EAATs), remove excess glutamate around pain transmitting neurons to decrease nociception suggesting that the modulation of glutamate transporters may represent a novel approach to the treatment of pain. This review highlights and summarizes (1) the physiology of the glutamatergic system in neuropathic pain, (2) the preclinical evidence for dysregulation of glutamate transport in animal pain models, and (3) emerging novel therapies that modulate glutamate transporters. Successful drug discovery requires continuous focus on basic and translational methods to fully elucidate the etiologies of this disease to enable the development of targeted therapies. Increasing the efficacy of astrocytic EAATs may serve as a new way to successfully treat those suffering from this devastating disease.

Satellite Glial Cells: Morphology, functional heterogeneity, and role in pain

Neurons in the somatic, sympathetic, and parasympathetic ganglia are surrounded by envelopes consisting of satellite glial cells (SGCs). Recently, it has become clear that SGCs are highly altered after nerve injury, which influences neuronal excitability and, consequently, the development and maintenance of pain in different animal models of chronic pain. However, the exact mechanism underlying chronic pain is not fully understood yet because it is assumed that SGCs in different ganglia share many common peculiarities, making the process complex. Here, we review recent data on morphological and functional heterogeneity and changes in SGCs in various pain conditions and their role in response to injury. More research is required to decipher the role of SGCs in diseases, such as chronic pain, neuropathology, and neurodegenerative diseases.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

Role of Microglia and Astrocytes in Spinal Cord Injury Induced Neuropathic Pain

Background:

Spinal cord injuries incite varying degrees of symptoms in patients, ranging from weakness and incoordination to paralysis. Common amongst spinal cord injury (SCI) patients, neuropathic pain (NP) is a debilitating medical condition. Unfortunately, there remain many clinical impediments in treating NP because there is a lack of understanding regarding the mechanisms behind SCI-induced NP (SCINP). Given that more than 450,000 people in the United States alone suffer from SCI, it is unsatisfactory that current treatments yield poor results in alleviating and treating NP.

Summary:

In this review, we briefly discussed the models of SCINP along with the mechanisms of NP progression. Further, current treatment modalities are herein explored for SCINP involving pharmacological interventions targeting glia cells and astrocytes.

Key message:

The studies presented in this review provide insight for new directions regarding SCINP alleviation. Given the severity and incapacitating effects of SCINP, it is imperative to study the pathways involved and find new therapeutic targets in coordination with stem cell research, and to develop a new gold-standard in SCINP treatment.

Nanotechnology for Pain Management: Current and Future Therapeutic Interventions

Pain is one of the most common medical conditions and affects more Americans than diabetes, heart disease, and cancer combined. Current pain treatments mainly rely on opioid analgesics and remain unsatisfactory. The life-threatening side effects and addictive properties of opioids demand new therapeutic approaches. Nanomedicine may be able to address these challenges as it allows for sensitive and targeted treatments without some of the burdens associated with current clinical pain therapies. This review discusses the physiology of pain, the current landscape of pain treatment, novel targets for pain treatment, and recent and ongoing efforts to effectively treat pain using nanotechnology-based approaches. We highl ight advances in nanoparticle-based drug delivery to reduce side effects, gene therapy to tackle the source of pain, and nanomaterials-based scavenging to proactively mediate pain signaling.

Reactive Astrocytes: Critical Players in the Development of Chronic Pain

Chronic pain is associated with long term plasticity of nociceptive pathways in the central nervous system. Astrocytes can profoundly affect synaptic function and increasing evidence has highlighted how altered astrocyte activity may contribute to the pathogenesis of chronic pain. In response to injury, astrocytes undergo a shift in form and function known as reactive astrogliosis, which affects their release of cytokines and gliotransmitters. These neuromodulatory substances have been implicated in driving the persistent changes in central nociceptive activity. Astrocytes also release lactate which neurons can use to produce energy during synaptic plasticity. Furthermore, recent research has provided insight into lactate's emerging role as a signaling molecule in the central nervous system, which may be involved in directly modulating neuronal and astrocytic activity. In this review, we present evidence for the involvement of astrocyte-derived tumor necrosis factor alpha in pain-associated plasticity, in addition to research suggesting the potential involvement of gliotransmitters D-serine and adenosine-5'-triphosphate. We also discuss work implicating astrocyte-neuron metabolic coupling, and the possible role of lactate, which has been sparsely studied in the context of chronic pain, in supporting pathological changes in central nociceptive activity.

MiRNA-107 contributes to inflammatory pain by down-regulating GLT-1 expression in rat spinal dorsal horn

BACKGROUND

Inflammatory pain is a severe clinical problem that affects the quality of life in patients. However, the currently available treatments for inflammatory pain have limited effect and even causes severe side effects. The aim of this study was to investigate the roles of miRNA-107 and glutamate transporter 1 (GLT-1) in the inflammatory pain of rats induced by complete Freund's adjuvant (CFA).

METHODS

Paw withdrawal threshold (PWT) of rats was measured by von Frey Filaments. The expressions of miRNA-107 and GLT-1 in the lumbar spinal dorsal horn (L4-L6) were measured with real-time quantitative PCR and western blotting analysis. Fluorescent in situ hybridization and fluorescent-immunohistochemistry were employed to detect the expression of miRNA-107, GLT-1 and co-location of miRNA-107 with GLT-1.

RESULTS

Injection of CFA significantly reduced PWT of rats. The miRNA-107 expression level was obviously up-regulated while the GLT-1 expression level was decreased in the spinal dorsal horn of CFA rats. miRNA-107 and GLT-1 were co-expressed in the same cells of the spinal dorsal horn in CFA rats. Ceftriaxone, a selective activator of GLT-1, obviously increased the PWT of CFA rats. Furthermore, antagomir of miRNA-107 reversed the down-regulation of GLT-1 and alleviated CFA-induced mechanical allodynia of CFA rats.

CONCLUSIONS

These results suggest that an increase of miR-107 contributes to inflammatory pain through downregulating GLT-1 expression, implying a promising strategy for pain therapy.

SIGNIFICANCE

The currently available treatments for inflammatory pain has limited effect even causes severe side effects. MiRNAs may have important diagnostic and therapeutic potential in inflammatory pain. In present study, we show a potential spinal mechanism of allodynia in rat inflammatory pain model induced by CFA. Increased miR-107 contribute to inflammatory pain by targeting and downregulating GLT-1 expression, implying a promising strategy for inflammatory pain.

Transport rate of EAAT2 is regulated by amino acid located at the interface between the scaffolding and substrate transport domains

Excitatory Amino Acid Transporters (EAATs) are plasma membrane proteins responsible for maintenance of low extracellular concentrations of glutamate in the CNS. Dysfunction in their activity is implicated in various neurological disorders. Glutamate transport by EAATs occurs through the movement of the central transport domain relative to the scaffold domain in the EAAT membrane protein. Previous studies suggested that residues located within the interface of these two domains in EAAT2, the main subtype of glutamate transporter in the brain, are involved in regulating transport rates. We used mutagenesis, structure-function relationship, surface protein expression and electrophysiology studies, in transfected COS-7 cells and oocytes, to examine residue glycine at position 298, which is located within this interface. Mutation G298A results in increased transport rate without changes in surface expression, suggesting a more hydrophobic and larger alanine results in facilitated transport movement. The increased transport rate does not involve changes in sodium affinity. Electrophysiological currents show that G298A increase both transport and anion currents, suggesting faster transitions through the transport cycle. This work identifies a region critically involved in setting the glutamate transport rate.

Elucidation of the Structure and Synthesis of Neuroprotective Low Molecular Mass Components of the Parawixia bistriata Spider Venom

The South American social spider Parawixia bistriata produces a venom containing complex organic compounds with intriguing biological activities. The crude venom leads to paralysis in termites and stimulates l-glutamate uptake and inhibits GABA uptake in rat brain synaptosomes. Glutamate is the major neurotransmitter at the insect neuromuscular junction and at the mammalian central nervous system, suggesting a modulation of the glutamatergic system by the venom. Parawixin1, 2, and 10 (Pwx1, 2 and 10) are HPLC fractions that demonstrate this bioactivity. Pwx1 stimulates l-glutamate uptake through the main transporter in the brain, EAAT2, and is neuroprotective in in vivo glaucoma models. Pxw2 inhibits GABA and glycine uptake in synaptosomes and inhibits seizures and neurodegeneration, and Pwx10 increases l-glutamate uptake in synaptosomes and is neuroprotective and anticonvulsant, shown in in vivo epilepsy models. Herein, we investigated the low molecular mass compounds in this venom and have found over 20 small compounds and 36 unique acylpolyamines with and without amino acid linkers. The active substances in fractions Pwx1 and Pwx2 require further investigation. We elucidated and confirmed the structure of the active acylpolyamine in Pwx10. Both fraction Pwx10 and the synthesized component enhance the activity of transporters EAAT1 and EAAT2, and, importantly, offer in vitro neuroprotection against excitotoxicity in primary cultures. These data suggest that compounds with this mechanism could be developed into therapies for disorders in which l-glutamate excitotoxicity is involved.

Novel Positive Allosteric Modulators of Glutamate Transport Have Neuroprotective Properties in an in Vitro Excitotoxic Model

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and several neurodegenerative disorders. EAAT2 is the main transporter subtype responsible for glutamate clearance in the brain, and plays a key role in regulating neurotransmission and preventing excitotoxicity. Therefore, compounds that increase the activity of EAAT2 have therapeutic potential for neuroprotection. In previous studies, we used virtual screening approaches to identify novel positive allosteric modulators (PAMs) of EAAT2. These compounds were shown to selectively increase the activity of EAAT2 and increase V_max of transport, without changing substrate affinity. In this work, our major effort was to investigate whether increasing the activity of EAAT2 by allosteric modulation would translate to neuroprotection in in vitro primary culture models of excitotoxicity. To investigate potential neuroprotective effects of one EAAT2 PAM, GT949, we subjected cultures to acute and prolonged excitotoxic insults by exogenous application of glutamate, or oxidative stress by application of hydrogen peroxide. GT949 administration did not result in neuroprotection in the oxidative stress model, likely due to damage of the glutamate transporters. However, GT949 displayed neuroprotective properties after acute and prolonged glutamate-mediated excitotoxicity. We propose that this compound prevents excess glutamate signaling by increasing the rate of glutamate clearance by EAAT2, thereby preventing excitotoxic damage and cell death. This novel class of compounds is therefore an innovative approach for neuroprotection with potential for translation in in vivo animal models of excitotoxicity.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as 'glutamosome'. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and β-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Norepinephrine-induced downregulation of GLT-1 mRNA in rat astrocytes

AIM OF THE RESEARCH

Glutamate transporter-1 (GLT-1; also known as excitatory amino acid transporter 2) plays an important role in the maintenance of glutamate homeostasis in the synaptic cleft. Downregulation of GLT-1 in the spinal cord has been reported in chronic pain models, which suggests that GLT-1 is involved in the development of chronic pain. However, the mechanism by which GLT-1 is downregulated in the spinal cord is still unknown. We hypothesized that norepinephrine is involved in the regulation of GLT-1. The aim of this study was to investigate the effect of norepinephrine on GLT-1 expression in cultured astrocytes.

METHODS

This study involved both in vivo and in vitro experiments. We first validated changes in GLT-1 mRNA expression in the spinal cord of rats with spared nerve injury (SNI) using real-time RT-PCR. Next, cultured primary astrocytes from the rat spinal cord were stimulated with norepinephrine, and GLT-1 mRNA was subsequently quantitated. RNB cells, an astrocytic cell line, were also stimulated with norepinephrine and other α-adrenoceptor agonists.

RESULTS

SNI resulted in bilateral downregulation of GLT-1 in rat spinal cord. The in vitro study showed that norepinephrine and phenylephrine dose-dependently downregulated GLT-1 in primary astrocytes and RNB cells. Furthermore, the effect of norepinephrine was reversed by an α-adrenoceptor antagonist.

CONCLUSION

Norepinephrine downregulates GLT-1 mRNA expression in astrocytes via the α₁-adrenoceptor. Our results provide new insight into the mechanisms involved in downregulation of GLT-1 in the chronic pain models.

Identification of Novel Allosteric Modulators of Glutamate Transporter EAAT2

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and neurodegenerative diseases, among others. EAAT2 is the main subtype responsible for glutamate clearance in the brain, having a key role in regulating transmission and preventing excitotoxicity. Therefore, compounds that increase the expression or activity of EAAT2 have therapeutic potential for neuroprotection. Previous studies identified molecular determinants for EAAT2 transport stimulation in a structural domain that lies at the interface of the rigid trimerization domain and the central substrate binding transport domain. In this work, a hybrid structure based approach was applied, based on this molecular domain, to create a high-resolution pharmacophore. Subsequently, virtual screening of a library of small molecules was performed, identifying 10 hit molecules that interact at the proposed domain. Among these, three compounds were determined to be activators, four were inhibitors, and three had no effect on EAAT2-mediated transport in vitro. Further characterization of the two best ranking EAAT2 activators for efficacy, potency, and selectivity for glutamate over monoamine transporters subtypes and NMDA receptors and for efficacy in cultured astrocytes is demonstrated. Mutagenesis studies suggest that the EAAT2 activators interact with residues forming the interface between the trimerization and transport domains. These compounds enhance the glutamate translocation rate, with no effect on substrate interaction, suggesting an allosteric mechanism. The identification of these novel positive allosteric modulators of EAAT2 offers an innovative approach for the development of therapies based on glutamate transport enhancement.

Reversible Induction of Pain Hypersensitivity following Optogenetic Stimulation of Spinal Astrocytes

While glial activation is an integral part of pain pathogenesis, the existence of a causal relationship between glia and pain processing has yet to be demonstrated in vivo. Here, we have investigated whether the activation of spinal astrocytes could directly evoke pain hypersensitivity in vivo via the use of optogenetic techniques. Optogenetic stimulation of channelrhopdopsin-2 (ChR)-expressing spinal astrocytes induced pain hypersensitivity in a reversible and time-dependent manner, which was accompanied by glial activation, NR1 phosphorylation, ATP release, and the production of proalgesic mediators. Photostimulation of ChR2-expressing astrocytes in culture and spinal slices recapitulated in vivo findings, demonstrating the release of proalgesic mediators and electrophysiological disinhibition of spinal projection neurons. These findings deepen our understanding of the role of astrocytes in pain pathogenesis and provide the scientific basis for an astrocyte-oriented pain treatment.

Chronic pain and impaired glial glutamate transporter function in lupus-prone mice are ameliorated by blocking macrophage colony-stimulating factor-1 receptors

Systemic lupus erythematosus (SLE) is a multi-organ disease of unknown etiology in which the normal immune responses are directed against the body's own healthy tissues. Patients with SLE often suffer from chronic pain. Currently, no animal studies have been reported about the mechanisms underlying pain in SLE. In this study, the development of chronic pain in MRL lupus-prone (MRL/lpr) mice, a well-established lupus mouse model, was characterized for the first time. We found that female MRL/lpr mice developed thermal hyperalgesia at the age of 13 weeks, and mechanical allodynia at the age of 16 weeks. MRL/lpr mice with chronic pain had activation of microglia and astrocytes, over-expression of macrophage colony-stimulating factor-1 (CSF-1) and interleukin-1 beta (IL-1β), as well as suppression of glial glutamate transport function in the spinal cord. Intrathecal injection of either the CSF-1 blocker or IL-1 inhibitor attenuated thermal hyperalgesia in MRL/lpr mice. We provide evidence that the suppressed activity of glial glutamate transporters in the spinal dorsal horn in MRL/lpr mice is caused by activation of the CSF-1 and IL-1β signaling pathways. Our findings suggest that targeting the CSF-1 and IL-1β signaling pathways or the glial glutamate transporter in the spinal cord is an effective approach for the management of chronic pain caused by SLE.

Glutamate Transport System as a Novel Therapeutic Target in Chronic Pain: Molecular Mechanisms and Pharmacology

The vast majority of peripheral neurons sensing noxious stimuli and conducting pain signals to the dorsal horn of the spinal cord utilize glutamate as a chemical transmitter of excitation. High-affinity glutamate transporter subtypes GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, and EAAT4, differentially expressed on sensory neurons, postsynaptic spinal interneurons, and neighboring glia, ensure fine modulation of glutamate neurotransmission in the spinal cord. The glutamate transport system seems to play important roles in molecular mechanisms underlying chronic pain and analgesia. Downregulation of glutamate transporters (GluTs) often precedes or occurs simultaneously with development of hypersensitivity to thermal or tactile stimuli in various models of chronic pain. Moreover, antisense knockdown or pharmacological inhibition of these membrane proteins can induce or aggravate pain. In contrast, upregulation of GluTs by positive pharmacological modulators or by viral gene transfer to the spinal cord can reverse the development of such pathological hypersensitivity. Furthermore, some multi-target drugs displaying analgesic properties (e.g., tricyclic antidepressant amitriptyline, riluzole, anticonvulsant valproate, tetracycline antibiotic minocycline, β-lactam antibiotic ceftriaxone and its structural analog devoid of antibacterial activity, clavulanic acid) can significantly increase the spinal glutamate uptake. Thus, mounting evidence points at GluTs as prospective therapeutic target for chronic pain treatment. However, design and development of new analgesics based on the modulation of glutamate uptake will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of this transport system in the spinal cord.

Impaired sensitivity to pain stimuli in plasma membrane calcium ATPase 2 (PMCA2) heterozygous mice: a possible modality- and sex-specific role for PMCA2 in nociception

Plasma membrane calcium ATPase 2 (PMCA2) is a calcium pump that plays important roles in neuronal function. Although it is expressed in pain-associated regions of the CNS, including in the dorsal horn (DH), its contribution to pain remains undefined. The present study assessed the role of PMCA2 in pain responsiveness and the link between PMCA2 and glutamate receptors, GABA receptors (GABARs), and glutamate transporters that have been implicated in pain processing in the DH of adult female and male PMCA2^+/+ and PMCA2^+/- mice. Behavioral assays evaluated mechanical and thermal pain responsiveness. Mechanical sensitivity was significantly increased by 52% and heat sensitivity was reduced by 29% in female, but not male, PMCA2^+/- mice compared with PMCA2^+/+ controls. There were female-specific changes in metabotropic glutamate receptor 1, NMDA receptor 2A, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluR1, GABA_BR1, and GABA_BR2 levels, whereas metabotropic glutamate receptor 5, NMDA receptor 2B, GluR2, and GABA_ARα2 levels were not altered. Glutamate aspartate transporter levels were higher and glial glutamate transporter 1 levels were lower in the DH of female, but not male, PMCA2^+/- mice. These findings indicate a novel role for PMCA2 in modality- and sex-dependent pain responsiveness. Female-specific molecular changes potentially account for the altered pain responses.-Khariv, V., Ni, L., Ratnayake, A., Sampath, S., Lutz, B. M., Tao, X.-X., Heary, R. F., Elkabes, S. Impaired sensitivity to pain stimuli in plasma membrane calcium ATPase 2 (PMCA2) heterozygous mice: a possible modality- and sex-specific role for PMCA2 in nociception.

Anti-Inflammatory Effects of Modified Adenoviral Vectors for Gene Therapy: A View through Animal Models Tested

The central dogma of gene therapy relies on the application of novel therapeutic genes to treat or prevent diseases. The main types of vectors used for gene transfer are adenovirus, retrovirus, lentivirus, liposome, and adeno-associated virus vectors. Gene therapy has emerged as a promising alternative for the treatment of inflammatory diseases. The main targets are cytokines, co-stimulatory molecules, and different types of cells from hematological and mesenchymal sources. In this review, we focus on molecules with anti-inflammatory effects used for in vivo gene therapy mediated by adenoviral gene transfer in the treatment of immune-mediated inflammatory diseases, with particular emphasis on autoinflammatory and autoimmune diseases.

Interactions of Opioids and HIV Infection in the Pathogenesis of Chronic Pain

Over 50% of HIV-1/AIDS patients suffer chronic pain. Currently, opioids are the cornerstone medications for treating severe pain in these patients. Ironically, emerging clinical data indicates that repeated use of opiate pain medicines might in fact heighten the chronic pain states in HIV patients. Both laboratory-based and clinical studies strongly suggest that opioids exacerbate the detrimental effects of HIV-1 infection on the nervous system, both on neurons and glia. The combination of opioids and HIV-1infection may promote the damage of neurons, including those in the pain sensory and transmission pathway, by activating both caspase-dependent and caspase-independent pro-apoptotic pathways. In addition, the opiate-HIV-1 interaction may also cause widespread disturbance of glial function and elicit glial-derived pro-inflammatory responses that dysregulate neuronal function. The deregulation of neuron-glia cross-talk that occurs with the combination of HIV-1 and opioids appears to play an important role in the development of the pathological pain state. In this article, we wish to provide an overview of the potential molecular and cellular mechanisms by which opioids may interact with HIV-1 to cause neurological problems, especially in the context of HIV-associated pathological pain. Elucidating the underlying mechanisms will help researchers and clinicians to understand how chronic use of opioids for analgesia enhances HIV-associated pain. It will also assist in optimizing therapeutic approaches to prevent or minimize this significant side effect of opiate analgesics in pain management for HIV patients.

The role of the PI3K/Akt/mTOR pathway in glial scar formation following spinal cord injury

Several studies suggest that glial scars pose as physical and chemical barriers that limit neurite regeneration after spinal cord injury (SCI). Evidences suggest that the activation of the PI3K/Akt/mTOR signaling pathway is involved in glial scar formation. Therefore, inhibition of the PI3K/Akt/mTOR pathway may beneficially attenuate glial scar formation after SCI. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates the PI3K/Akt/mTOR pathway. Therefore, we hypothesized that the overexpression of PTEN in the spinal cord will have beneficial effects after SCI. In the present study, we intrathecally injected a recombinant adenovirus carrying the pten gene (Ad-PTEN) to cause overexpression of PTEN in rats with contusion injured spinal cords. The results suggest overexpression of PTEN in spinal cord attenuated glial scar formation and led to improved locomotor function after SCI. Overexpression of PTEN following SCI attenuated gliosis, affected chondroitin sulfate proteoglycan expression, and improved axon regeneration into the lesion site. Furthermore, we suggest that the activation of the PI3K/Akt/mTOR pathway in astrocytes at 3 days after SCI may be involved in glial scar formation. Because delayed treatment with Ad-PTEN enhanced motor function recovery more significantly than immediate treatment with Ad-PTEN after SCI, the results suggest that the best strategy to attenuate glial scar formation could be to introduce 3 days after SCI. This study's findings thus have positive implications for patients who are unable to receive immediate medical attention after SCI.

The Role of Regulatory Transporters in Neuropathic Pain

Neuropathic pain arises from an injury or disease of the somatosensory nervous system rather than stimulation of pain receptors. As a result, the fine balance between excitation and inhibition is perturbed leading to hyperalgesia and allodynia. Various neuropathic pain models provide considerable evidence that changes in the glutamatergic, GABAergic, and monoaminergic systems. Neurotransmitter reuptake transporter proteins have the potential to change the temporal and spatial profile of various neurotransmitters throughout the nervous system. This, in turn, can affect the downstream effects of these neurotransmitters and hence modulate pain. This chapter explores various reuptake transporter systems and implicates their role in pain processing. Understanding the transporter systems will enhance drug discovery targeting different facets of neuropathic pain.

Tumor necrosis factor-mediated downregulation of spinal astrocytic connexin43 leads to increased glutamatergic neurotransmission and neuropathic pain in mice

Spinal cord astrocytes are critical in the maintenance of neuropathic pain. Connexin 43 (Cx43) expressed on spinal dorsal horn astrocytes modulates synaptic neurotransmission, but its role in nociceptive transduction has yet to be fully elaborated. In mice, Cx43 is mainly expressed in astrocytes, not neurons or microglia, in the spinal dorsal horn. Hind paw mechanical hypersensitivity was observed beginning 3days after partial sciatic nerve ligation (PSNL), but a persistent downregulation of astrocytic Cx43 in ipsilateral lumbar spinal dorsal horn was not observed until 7days post-PSNL, suggesting that Cx43 downregulation mediates the maintenance and not the initiation of nerve injury-induced hypersensitivity. Downregulation of Cx43 expression by intrathecal treatment with Cx43 siRNA also induced mechanical hypersensitivity. Conversely, restoring Cx43 by an adenovirus vector expressing Cx43 (Ad-Cx43) ameliorated PSNL-induced mechanical hypersensitivity. The sensitized state following PSNL is likely maintained by dysfunctional glutamatergic neurotransmission, as Cx43 siRNA-induced mechanical hypersensitivity was attenuated with intrathecal treatment of glutamate receptor antagonists MK801 and CNQX, but not neurokinin-1 receptor antagonist CP96345 or the Ca(2+) channel subunit α2δ1 blocker gabapentin. The source of this dysfunctional glutamatergic neurotransmission is likely decreased clearance of glutamate from the synapse rather than increased glutamate release into the synapse. Astrocytic expression of glutamate transporter GLT-1, but not GLAST, and activity of glutamate transport were markedly decreased in mice intrathecally injected with Cx43-targeting siRNA but not non-targeting siRNA. Glutamate release from spinal synaptosomes prepared from mice treated with either Cx43-targeting siRNA or non-targeting siRNA was unchanged. Intrathecal injection of Ad-Cx43 in PSNL mice restored astrocytic GLT-1 expression. The cytokine tumor necrosis factor (TNF) has been implicated in the induction of central sensitization, particularly through its actions on astrocytes, in the spinal cord following peripheral injury. Intrathecal injection of TNF in naïve mice induced the downregulation of both Cx43 and GLT-1 in spinal dorsal horn, as well as hind paw mechanical hypersensitivity, as observed in PSNL mice. Conversely, intrathecal treatment of PSNL mice with the TNF inhibitor etanercept prevented not only mechanical hypersensitivity but also the downregulation of Cx43 and GLT-1 expression in astrocytes. The current findings indicate that spinal astrocytic Cx43 are essential for the maintenance of neuropathic pain following peripheral nerve injury and suggest modulation of Cx43 as a novel target for developing analgesics for neuropathic pain.

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

BACKGROUND

Neuroinflammation and dysfunctional glial glutamate transporters (GTs) in the spinal dorsal horn are implicated in the genesis of neuropathic pain. The authors determined whether adenosine monophosphate-activated protein kinase (AMPK) in the spinal dorsal horn regulates these processes in rodents with neuropathic pain.

METHODS

Hind paw withdrawal responses to radiant heat and mechanical stimuli were used to assess nociceptive behaviors. Spinal markers related to neuroinflammation and glial GTs were determined by Western blotting. AMPK activities were manipulated pharmacologically and genetically. Regulation of glial GTs was determined by measuring protein expression and activities of glial GTs.

RESULTS

AMPK activities were reduced in the spinal dorsal horn of rats (n = 5) with thermal hyperalgesia induced by nerve injury, which were accompanied with the activation of astrocytes, increased production of interleukin-1β and activities of glycogen synthase kinase 3β, and suppressed protein expression of glial glutamate transporter-1. Thermal hyperalgesia was reversed by spinal activation of AMPK in neuropathic rats (n = 10) and induced by inhibiting spinal AMPK in naive rats (n = 7 to 8). Spinal AMPKα knockdown (n = 6) and AMPKα1 conditional knockout (n = 6) induced thermal hyperalgesia and mechanical allodynia. These genetic alterations mimicked the changes of molecular markers induced by nerve injury. Pharmacological activation of AMPK enhanced glial GT activity in mice with neuropathic pain (n = 8) and attenuated glial glutamate transporter-1 internalization induced by interleukin-1β (n = 4).

CONCLUSIONS

These findings suggest that enhancing spinal AMPK activities could be an effective approach for the treatment of neuropathic pain.

Current gene therapy using viral vectors for chronic pain

The complexity of chronic pain and the challenges of pharmacotherapy highlight the importance of development of new approaches to pain management. Gene therapy approaches may be complementary to pharmacotherapy for several advantages. Gene therapy strategies may target specific chronic pain mechanisms in a tissue-specific manner. The present collection of articles features distinct gene therapy approaches targeting specific mechanisms identified as important in the specific pain conditions. Dr. Fairbanks group describes commonly used gene therapeutics (herpes simplex viral vector (HSV) and adeno-associated viral vector (AAV)), and addresses biodistribution and potential neurotoxicity in pre-clinical models of vector delivery. Dr. Tao group addresses that downregulation of a voltage-gated potassium channel (Kv1.2) contributes to the maintenance of neuropathic pain. Alleviation of chronic pain through restoring Kv1.2 expression in sensory neurons is presented in this review. Drs Goins and Kinchington group describes a strategy to use the replication defective HSV vector to deliver two different gene products (enkephalin and TNF soluble receptor) for the treatment of post-herpetic neuralgia. Dr. Hao group addresses the observation that the pro-inflammatory cytokines are an important shared mechanism underlying both neuropathic pain and the development of opioid analgesic tolerance and withdrawal. The use of gene therapy strategies to enhance expression of the anti-pro-inflammatory cytokines is summarized. Development of multiple gene therapy strategies may have the benefit of targeting specific pathologies associated with distinct chronic pain conditions (by Guest Editors, Drs. C. Fairbanks and S. Hao).

Involvement of phosphatase and tensin homolog deleted from chromosome 10 in rodent model of neuropathic pain

Background

Many cancer research studies have extensively examined the phosphatase and tensin homolog deleted from chromosome 10 (PTEN) pathway. There are only few reports that suggest that PTEN might affect pain; however, there is still a lack of evidence to show the role of PTEN for modulating pain. Here, we report a role for PTEN in a rodent model of neuropathic pain.

Results

We found that chronic constriction injury (CCI) surgery in rats could elicit downregulation of spinal PTEN as well as upregulation of phosphorylated PTEN (phospho-PTEN) and phosphorylated mammalian target of rapamycin (phospho-mTOR). After examining such changes in endogenous PTEN in neuropathic rats, we explored the effects of modulating the spinal PTEN pathway on nociceptive behaviors. The normal rats exhibited mechanical allodynia after intrathecal (i.t.) injection of adenovirus-mediated PTEN antisense oligonucleotide (Ad-antisense PTEN). These data indicate the importance of downregulation of spinal PTEN for nociception. Moreover, upregulation of spinal PTEN by i.t. adenovirus-mediated PTEN (Ad-PTEN) significantly prevented CCI-induced development of nociceptive sensitization, thermal hyperalgesia, mechanical allodynia, cold allodynia, and weight-bearing deficits in neuropathic rats. Furthermore, upregulation of spinal PTEN by i.t. Ad-PTEN significantly attenuated CCI-induced microglia and astrocyte activation, upregulation of tumor necrosis factor-α (TNF-α) and phospho-mTOR, and downregulation of PTEN in neuropathic rats 14 days post injury.

Conclusions

These findings demonstrate that PTEN plays a key, beneficial role in a rodent model of neuropathic pain.

Blockade of spinal glutamate recycling produces paradoxical antinociception in rats with orofacial inflammatory pain

In our current study, we investigated the role of spinal glutamate recycling in the development of orofacial inflammatory pain. DL-threo-β-benzyloxyaspartate (TBOA) or methionine sulfoximine (MSO) was administered intracisternally to block spinal glutamate transporter and glutamine synthetase activity in astroglia. Intracisternal administration of high dose TBOA (10 μg) produced thermal hyperalgesia in naïve rats but significantly attenuated the thermal hyperalgesia in rats that had been pretreated with interleukin (IL)-1β or Complete Freund's Adjuvant (CFA). In contrast, intracisternal injection of MSO produced anti-hyperalgesic effects against thermal stimuli in CFA-treated rats only. To confirm the paradoxical antinociceptive effects of TBOA and MSO, we examined changes in c-Fos expression in the medullary dorsal horn produced by thermal stimulation in naïve, IL-1β-, or CFA-treated rats, after intracisternal injections of TBOA and MSO. Intracisternal administration of TBOA significantly increased c-Fos immunoreactivity in naïve rats. In contrast, intracisternal administration of TBOA significantly decreased the up-regulation of c-Fos immunoreactivity in the medullary dorsal horn of IL-1β- and CFA-treated rats. However, intracisternal injection of MSO blocked the up-regulation of c-Fos immunoreactivity in CFA-treated rats only. We also investigated the effects of botulinum toxin type A (BoNT-A) on TBOA-induced paradoxical antinociception in CFA-treated rats, as BoNT-A inhibits the release of neurotransmitters, including glutamate. BoNT-A treatment reversed behavioral responses produced by intracisternal administration of TBOA in CFA-treated rats. These results suggest that the paradoxical responses produced by blocking glutamate transporters under inflammatory pain conditions are mediated by the modulation of glutamate release from presynaptic terminals. Moreover, blockade of glutamate reuptake could represent a new therapeutic target for the treatment of chronic inflammatory pain conditions.

Nonneuronal central mechanisms of pain: glia and immune response

The role of central glial cells in the mechanisms underlying pain has been intensively studied in the last two decades. Most studies on glia and pain focused on the potential detrimental role of glial cells following noxious stimulus/insults manifested as an "activation" or a "reactive" state (increase in glial marker expression and production of proinflammatory/nociceptive molecules). Therefore, "activated" or "reactive" glial cells became a target for the future generation of drugs to treat chronic pain. Several glial modulators that reduce the activation of glial cells have shown great efficacy in multiple animal (rodents mostly) models of pain (acute, subacute, chronic, inflammatory, neuropathic, surgical, etc.). These encouraging findings inspired clinical trials that have been completed in the last 5 years. Unfortunately, all clinical trials with these glial modulators have failed to demonstrate efficacy for the treatment of pain. New lines of investigation and elegant experimental designs are shedding light on alternative glial functions, which demonstrate that "glial reactivity" is not necessarily deleterious in some pathological conditions. New strategies to validate findings through our current animal models are necessary to enhance the translational value of our preclinical studies. Also, more studies using human subjects would enhance our understanding of glial cells in the context of pain. This chapter explores the available literature to objectively ponder the potential role of glial cells in human pain conditions.

Glutamate transporter activators as anti-nociceptive agents

The effective management of chronic pain remains enigmatic. There is a paucity of effective mechanistically-based approaches employed. Chronic visceral pain is a particularly difficult subcategory to manage. Glutamate is the most predominant excitatory neurotransmitter and mediates many aspects of sensory function including acute and chronic pain. There is a growing literature describing the efficacy of physiologically dominant glutamate transporter GLT-1 up-regulation in attenuating chronic visceral and somatic nociception. Since glutamate is the major excitatory neurotransmitter released in the first central synapse of the pain-transmitting afferent neurons, augmentation of GLT-1 activity, which reduces extracellular levels of glutamate, may be an important target for pain management strategies. This review summarizes studies in our laboratory and others which highlight findings that GLT-1 up-regulation by transgenic, pharmacologic and viral transfection approaches attenuate a host of nociceptive responses emanating from visceral or somatic sources in animal models. The study also outlines the future work that will be required to ascertain the translational potential of this approach.

The antinociceptive effects of intracerebroventricular administration of Chicago sky blue 6B, a vesicular glutamate transporter inhibitor

Accumulating evidence suggests that vesicular glutamate transporters (VGLUTs), which control the storage and release of glutamate, may play a role in pain processing. Chicago sky blue 6B (CSB6B), which is structurally related to glutamate, is a competitive VGLUT inhibitor without affecting plasma membrane transporters. The present study was designed to investigate the antinociceptive effects of CSB6B in a number of pain models. The hot-plate test was used as an acute thermal pain test. Inflammatory pain was evaluated using acetic acid writhing, formalin, and complete Freund's adjuvant tests. Intracerebroventricular administration of CSB6B did not affect acute thermal pain responses in 50 or 55°C hot plate tests. However, CSB6B attenuated acetic acid-induced writhing in a dose-dependent and time-dependent manner. In addition, CSB6B reduced licking/biting behavior during the second phase, but not during the first phase, following an intraplantar injection of formalin. In the complete Freund's adjuvant test, a significant attenuation of thermal hyperalgesia was also observed in CSB6B-treated mice. At antinociceptive doses, CSB6B did not affect mice spontaneous locomotor activity. The present study shows that pharmacological inhibition of VGLUT activity was sufficient to attenuate experimental inflammatory pain and suggests that regulation of VGLUTs might be a novel therapeutic strategy for the treatment of pain.

Estradiol attenuates spinal cord injury-related central pain by decreasing glutamate levels in thalamic VPL nucleus in male rats

Central neuropathic pain (CNP) is a complicated medical problem that involves both the spinal and supraspinal regions of the central nervous system. Estrogen, a neuroprotective agent, has been considered a possible candidate for CNP treatment. In this study, we examined the effects of a single dose of 17β-estradiol on glutamate levels in the ventral posterolateral (VPL) nucleus of the rat thalamus. Furthermore, we determined whether there was a correlation between glutamate levels and neuropathic pain induced by unilateral electrolytic spinothalamic tract (STT) lesion. STT lesioning was performed in male Wistar rats at the T8-T9 vertebrae; rats were then administered 17β-estradiol (4 mg/kg, i.p.) 30 min after injury. Glutamate samples were collected using a microdialysis probe and quantified by high performance liquid chromatography. Mechanical allodynia (MA) and thermal hyperalgesia (TH) thresholds were measured pre-injury and 7, 14, and 28 days post-injury. We found that STT lesion significantly increased glutamate levels in the ipsilateral VPL nucleus 14 and 28 days post-injury; this was accompanied by allodynia and hyperalgesia in the hind paws of the rats. Administering 17β-estradiol to the rats decreased glutamate levels in the ipsilateral VPL nucleus and significantly increased MA and TH thresholds. These results suggest that glutamate in the VPL nucleus of the thalamus is involved in the pathology of neuropathic pain after STT injury; furthermore, 17β-estradiol may attenuate this neuropathic pain by decreasing glutamate levels.

Overexpression of the astrocyte glutamate transporter GLT1 exacerbates phrenic motor neuron degeneration, diaphragm compromise, and forelimb motor dysfunction following cervical contusion spinal cord injury

A major portion of spinal cord injury (SCI) cases affect midcervical levels, the location of the phrenic motor neuron (PhMN) pool that innervates the diaphragm. While initial trauma is uncontrollable, a valuable opportunity exists in the hours to days following SCI for preventing PhMN loss and consequent respiratory dysfunction that occurs during secondary degeneration. One of the primary causes of secondary injury is excitotoxic cell death due to dysregulation of extracellular glutamate homeostasis. GLT1, mainly expressed by astrocytes, is responsible for the vast majority of functional uptake of extracellular glutamate in the CNS, particularly in spinal cord. We found that, in bacterial artificial chromosome-GLT1-enhanced green fluorescent protein reporter mice following unilateral midcervical (C4) contusion SCI, numbers of GLT1-expressing astrocytes in ventral horn and total intraspinal GLT1 protein expression were reduced soon after injury and the decrease persisted for ≥6 weeks. We used intraspinal delivery of adeno-associated virus type 8 (AAV8)-Gfa2 vector to rat cervical spinal cord ventral horn for targeting focal astrocyte GLT1 overexpression in areas of PhMN loss. Intraspinal delivery of AAV8-Gfa2-GLT1 resulted in transduction primarily of GFAP(+) astrocytes that persisted for ≥6 weeks postinjury, as well as increased intraspinal GLT1 protein expression. Surprisingly, we found that astrocyte-targeted GLT1 overexpression increased lesion size, PhMN loss, phrenic nerve axonal degeneration, and diaphragm neuromuscular junction denervation, and resulted in reduced functional diaphragm innervation as assessed by phrenic nerve-diaphragm compound muscle action potential recordings. These results demonstrate that GLT1 overexpression via intraspinal AAV-Gfa2-GLT1 delivery exacerbates neuronal damage and increases respiratory impairment following cervical SCI.

Interleukin-1 beta enhances endocytosis of glial glutamate transporters in the spinal dorsal horn through activating protein kinase C

Excessive activation of glutamate receptors in spinal dorsal horn neurons is a key mechanism leading to abnormal neuronal activation in pathological pain conditions. Previous studies have shown that activation of glutamate receptors in the spinal dorsal horn is enhanced by impaired glial glutamate transporter functions and proinflammatory cytokines including interleukin-1 beta (IL-1β). In this study, we for the first time revealed that spinal glial glutamate transporter activities in the neuropathic animals are attenuated by endogenous IL-1β. Specifically, we demonstrated that nerve injury results in an increased expression of IL-1β and activation of PKC in the spinal dorsal horn as well as suppression of glial glutamate uptake activities. We provided evidence that the nerve-injury induced suppression of glial glutamate uptake is at least in part ascribed to endogenous IL-1β and activation of PKC in the spinal dorsal horn. IL-1β reduces glial glutamate transporter activities through enhancing the endocytosis of both GLT-1 and GLAST glial glutamate transporters. The IL-1β induced trafficking of glial glutamate transporters is through the calcium/PKC signaling pathway, and the dynamin-dependent endocytosis, which is dependent on the integrity of actin filaments. The signaling pathway regulating glial glutamate transporters revealed in this study provides novel targets to attenuate aberrant activation of glutamate receptors in the spinal dorsal horn, which could ultimately help the development of analgesics.

Therapeutic potential of targeting glutamate receptors in Parkinson's disease

Glutamate plays a complex role in many aspects of Parkinson's disease including the loss of dopaminergic neurons, the classical motor symptoms as well as associated non-motor symptoms and the treatment-related side effect, L-DOPA-induced dyskinesia. This widespread involvement opens up possibilities for glutamate-based therapies to provide a more rounded approach to treatment than is afforded by current dopamine replacement therapies. Beneficial effects of blocking postsynaptic glutamate transmission have already been noted in a range of preclinical studies using antagonists of NMDA receptors or negative allosteric modulators of metabotropic glutamate receptor 5 (mGlu5), while positive allosteric modulators of mGlu4 in particular, although at an earlier stage of investigation, also look promising. This review addresses each of the key features of Parkinson's disease in turn, summarising the contribution glutamate makes to that feature and presenting an up-to-date account of the potential for drugs acting at ionotropic or metabotropic glutamate receptors to provide relief. Whilst only a handful of these have progressed to clinical trials to date, notably NMDA and NR2B antagonists against motor symptoms and L-DOPA-induced dyskinesia, with mGlu5 negative allosteric modulators also against L-DOPA-induced dyskinesia, the mainly positive outcomes of these trials, coupled with supportive preclinical data for other strategies in animal models of Parkinson's disease and L-DOPA-induced dyskinesia, raise cautious optimism that a glutamate-based therapeutic approach will have significant impact on the treatment of Parkinson's disease.

Glia and pain: is chronic pain a gliopathy?

Activation of glial cells and neuro-glial interactions are emerging as key mechanisms underlying chronic pain. Accumulating evidence has implicated 3 types of glial cells in the development and maintenance of chronic pain: microglia and astrocytes of the central nervous system (CNS), and satellite glial cells of the dorsal root and trigeminal ganglia. Painful syndromes are associated with different glial activation states: (1) glial reaction (ie, upregulation of glial markers such as IBA1 and glial fibrillary acidic protein (GFAP) and/or morphological changes, including hypertrophy, proliferation, and modifications of glial networks); (2) phosphorylation of mitogen-activated protein kinase signaling pathways; (3) upregulation of adenosine triphosphate and chemokine receptors and hemichannels and downregulation of glutamate transporters; and (4) synthesis and release of glial mediators (eg, cytokines, chemokines, growth factors, and proteases) to the extracellular space. Although widely detected in chronic pain resulting from nerve trauma, inflammation, cancer, and chemotherapy in rodents, and more recently, human immunodeficiency virus-associated neuropathy in human beings, glial reaction (activation state 1) is not thought to mediate pain sensitivity directly. Instead, activation states 2 to 4 have been demonstrated to enhance pain sensitivity via a number of synergistic neuro-glial interactions. Glial mediators have been shown to powerfully modulate excitatory and inhibitory synaptic transmission at presynaptic, postsynaptic, and extrasynaptic sites. Glial activation also occurs in acute pain conditions, and acute opioid treatment activates peripheral glia to mask opioid analgesia. Thus, chronic pain could be a result of "gliopathy," that is, dysregulation of glial functions in the central and peripheral nervous system. In this review, we provide an update on recent advances and discuss remaining questions.

SLC1 glutamate transporters

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.

Gene-based approaches in pain research and exploration of new therapeutic targets and strategies

Large panel of gene-based techniques is used for many years specifically in the pain research field. From the first identification (cloning) of some "mythic" genes, such as those encoding opioid or capsaicin receptors allowing then the creation of first-generation knockout mice, to the today conditional (time, tissue, cell-type and even pathology-dependent) and regulatable modulation of a gene function, these approaches largely contributed to fundamental leaps forward in our understanding of the function of some proteins and of their interest as possible druggable targets. Perhaps one of the most remarkable evolution in the last years is the passage of these approaches from the bench to the patient; whether it concerns the identification of genes involved in inherited pain insensibility/susceptibility, the search for genetic markers of pain types, the individual pharmacogenomics or even the first gene therapy trials. From many possible variants of gene-grounded techniques used in pain research we focus here on gene knockouts and some recent developments, on viral vectors-based gene transfer and on transgenic models for the tracing of pain pathways. Through these selected examples we attempted to emphasize the immense potential of these approaches and their already well-recognized contribution in both the basic and clinical pain research.

Characterization of the Visceral Antinociceptive Effect of Glial Glutamate Transporter GLT-1 Upregulation by Ceftriaxone

Recent studies demonstrate that glial glutamate transporter-1 (GLT-1) upregulation attenuates visceral nociception. The present work further characterized the effect of ceftriaxone- (CTX-) mediated GLT-1 upregulation on visceral hyperalgesia. Intrathecal pretreatment with dihydrokainate, a selective GLT-1 antagonist, produced a reversal of the antinociceptive response to bladder distension produced by CTX. The hyperalgesic response to urinary bladder distension caused by intravesicular acrolein was also attenuated by CTX treatment as was the enhanced time spent licking of abdominal area due to intravesicular acrolein. Bladder inflammation via cyclophosphamide injections enhanced the nociceptive to bladder distension; cohorts administered CTX and concomitant cyclophosphamide showed reduced hyperalgesic response. Cyclophosphamide-induced bladder hyperalgesia correlated with a significant 22% increase in GluR1 AMPA receptor subunit expression in the membrane fraction of the lumbosacral spinal cord, which was attenuated by CTX coadministration. Finally, neonatal colon insult-induced hyperalgesia caused by intracolonic mustard oil (2%) administration at P9 and P11 was attenuated by CTX. These studies suggest that GLT-1 upregulation (1) attenuates the hyperalgesia caused by bladder irritation/inflammation or by neonatal colonic insult, (2) acts at a spinal site, and (3) may produce antinociceptive effects by attenuating GluR1 membrane trafficking. These findings support further consideration of this FDA-approved drug to treat chronic pelvic pain syndromes.

Peripheral nerve injury produces a sustained shift in the balance between glutamate release and uptake in the dorsal horn of the spinal cord

Peripheral nerve injury provokes heightened excitability of primary sensory afferents including nociceptors, and elicits ectopic activity in lesioned and neighboring intact nerve fibers. The major transmitter released by sensory afferents in the superficial dorsal horn of the spinal cord is glutamate. Glutamate is critically involved in nociceptive signaling and the development of neuropathic pain. We recorded miniature excitatory postsynaptic currents (mEPSCs) from neurons in lamina II of the rat dorsal horn to assess spontaneous synaptic activity after spared nerve injury (SNI), a model of chronic neuropathic pain. Following SNI, the frequency of mEPSCs doubled, indicating heightened glutamate release from primary afferents or spinal interneurons. Consistent with this finding, glutamate concentrations in the cerebrospinal fluid were elevated at 1 and 4 weeks after SNI. Transmitter uptake was insufficient to prevent the rise in extracellular glutamate as the expression of glutamate transporters remained unchanged or decreased. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride, an antagonist of metabotropic glutamate receptor 5 (mGluR5), reduced the frequency of mEPSCs to its preinjury level, suggesting a positive feedback mechanism that involves facilitation of transmitter release by mGluR5 activation in the presence of high extracellular glutamate. Treatment with the β-lactam antibiotic ceftriaxone increased the expression of glutamate transporter 1 (Glt1) in the dorsal horn after SNI, raised transmitter uptake, and lowered extracellular glutamate. Improving glutamate clearance prevented the facilitation of transmitter release by mGluR5 and attenuated neuropathic pain-like behavior. Balancing glutamate release and uptake after nerve injury should be an important target in the management of chronic neuropathic pain.