Rapid changes in expression of glutamate transporters after spinal cord injury

Recent insights on astrocyte mechanisms in CNS homeostasis, pathology, and repair

Astrocytes play essential roles in development, homeostasis, injury, and repair of the central nervous system (CNS). Their development is tightly regulated by distinct spatial and temporal cues during embryogenesis and into adulthood throughout the CNS. Astrocytes have several important responsibilities such as regulating blood flow and permeability of the blood-CNS barrier, glucose metabolism and storage, synapse formation and function, and axon myelination. In CNS pathologies, astrocytes also play critical parts in both injury and repair mechanisms. Upon injury, they undergo a robust phenotypic shift known as "reactive astrogliosis," which results in both constructive and deleterious outcomes. Astrocyte activation and migration at the site of injury provides an early defense mechanism to minimize the extent of injury by enveloping the lesion area. However, astrogliosis also contributes to the inhibitory microenvironment of CNS injury and potentiate secondary injury mechanisms, such as inflammation, oxidative stress, and glutamate excitotoxicity, which facilitate neurodegeneration in CNS pathologies. Intriguingly, reactive astrocytes are increasingly a focus in current therapeutic strategies as their activation can be modulated toward a neuroprotective and reparative phenotype. This review will discuss recent advancements in knowledge regarding the development and role of astrocytes in the healthy and pathological CNS. We will also review how astrocytes have been genetically modified to optimize their reparative potential after injury, and how they may be transdifferentiated into neurons and oligodendrocytes to promote repair after CNS injury and neurodegeneration.

Sensory satellite glial Gq-GPCR activation alleviates inflammatory pain via peripheral adenosine 1 receptor activation

Glial fibrillary acidic protein expressing (GFAP+) glia modulate nociceptive neuronal activity in both the peripheral nervous system (PNS) and the central nervous system (CNS). Resident GFAP+ glia in dorsal root ganglia (DRG) known as satellite glial cells (SGCs) potentiate neuronal activity by releasing pro-inflammatory cytokines and neuroactive compounds. In this study, we tested the hypothesis that SGC Gq-coupled receptor (Gq-GPCR) signaling modulates pain sensitivity in vivo using Gfap-hM3Dq mice. Complete Freund's adjuvant (CFA) was used to induce inflammatory pain, and mechanical sensitivity and thermal sensitivity were used to assess the neuromodulatory effect of glial Gq-GPCR activation in awake mice. Pharmacogenetic activation of Gq-GPCR signaling in sensory SGCs decreased heat-induced nociceptive responses and reversed inflammation-induced mechanical allodynia via peripheral adenosine A1 receptor activation. These data reveal a previously unexplored role of sensory SGCs in decreasing afferent excitability. The identified molecular mechanism underlying the analgesic role of SGCs offers new approaches for reversing peripheral nociceptive sensitization.

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'.

A toll-like receptor 9 antagonist restores below-level glial glutamate transporter expression in the dorsal horn following spinal cord injury

Spinal cord (SC) trauma elicits pathological changes at the primary lesion and in regions distant from the injury epicenter. Therapeutic agents that target mechanisms at the injury site are likely to exert additional effects in these remote regions. We previously reported that a toll-like receptor 9 (TLR9) antagonist, oligodeoxynucleotide 2088 (ODN 2088), improves functional deficits and modulates the milieu at the epicenter in mice sustaining a mid-thoracic contusion. The present investigations use the same paradigm to assess ODN 2088-elicited alterations in the lumbar dorsal horn (LDH), a region remote from the injury site where SCI-induced molecular alterations have been well defined. We report that ODN 2088 counteracts the SCI-elicited decrease in glial glutamate aspartate transporter (GLAST) and glutamate transporter 1 (GLT1) levels, whereas the levels of the neuronal glutamate transporter excitatory amino acid carrier 1 (EAAC1) and astroglial GABA transporter 3 (GAT3) were unaffected. The restoration of GLAST and GLT1 was neither paralleled by a global effect on astrocyte and microglia activation nor by changes in the expression of cytokines and growth factors reported to regulate these transporters. We conclude that the effects of intrathecal ODN 2088 treatment extend to loci beyond the epicenter by selectively targeting glial glutamate transporters.

Neuronal-Glial Interactions Maintain Chronic Neuropathic Pain after Spinal Cord Injury

The hyperactive state of sensory neurons in the spinal cord enhances pain transmission. Spinal glial cells have also been implicated in enhanced excitability of spinal dorsal horn neurons, resulting in pain amplification and distortions. Traumatic injuries of the neural system such as spinal cord injury (SCI) induce neuronal hyperactivity and glial activation, causing maladaptive synaptic plasticity in the spinal cord. Recent studies demonstrate that SCI causes persistent glial activation with concomitant neuronal hyperactivity, thus providing the substrate for central neuropathic pain. Hyperactive sensory neurons and activated glial cells increase intracellular and extracellular glutamate, neuropeptides, adenosine triphosphates, proinflammatory cytokines, and reactive oxygen species concentrations, all of which enhance pain transmission. In addition, hyperactive sensory neurons and glial cells overexpress receptors and ion channels that maintain this enhanced pain transmission. Therefore, post-SCI neuronal-glial interactions create maladaptive synaptic circuits and activate intracellular signaling events that permanently contribute to enhanced neuropathic pain. In this review, we describe how hyperactivity of sensory neurons contributes to the maintenance of chronic neuropathic pain via neuronal-glial interactions following SCI.

Understanding the Mechanisms of Recovery and/or Compensation following Injury

Injury due to stroke and traumatic brain injury result in significant long-term effects upon behavioral functioning. One central question to rehabilitation research is whether the nature of behavioral improvement observed is due to recovery or the development of compensatory mechanisms. The nature of functional improvement can be viewed from the perspective of behavioral changes or changes in neuroanatomical plasticity that follows. Research suggests that these changes correspond to each other in a bidirectional manner. Mechanisms surrounding phenomena like neural plasticity may offer an opportunity to explain how variables such as experience can impact improvement and influence the definition of recovery. What is more, the intensity of the rehabilitative experiences may influence the ability to recover function and support functional improvement of behavior. All of this impacts how researchers, clinicians, and medical professionals utilize rehabilitation.

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.

"Targeting astrocytes in CNS injury and disease: A translational research approach"

Astrocytes are a major constituent of the central nervous system. These glia play a major role in regulating blood-brain barrier function, the formation and maintenance of synapses, glutamate uptake, and trophic support for surrounding neurons and glia. Therefore, maintaining the proper functioning of these cells is crucial to survival. Astrocyte defects are associated with a wide variety of neuropathological insults, ranging from neurodegenerative diseases to gliomas. Additionally, injury to the CNS causes drastic changes to astrocytes, often leading to a phenomenon known as reactive astrogliosis. This process is important for protecting the surrounding healthy tissue from the spread of injury, while it also inhibits axonal regeneration and plasticity. Here, we discuss the important roles of astrocytes after injury and in disease, as well as potential therapeutic approaches to restore proper astrocyte functioning.

GLT1 overexpression reverses established neuropathic pain-related behavior and attenuates chronic dorsal horn neuron activation following cervical spinal cord injury

Development of neuropathic pain occurs in a major portion of traumatic spinal cord injury (SCI) patients, resulting in debilitating and often long-term physical and psychological burdens. Following SCI, chronic dysregulation of extracellular glutamate homeostasis has been shown to play a key role in persistent central hyperexcitability of superficial dorsal horn neurons that mediate pain neurotransmission, leading to various forms of neuropathic pain. Astrocytes express the major CNS glutamate transporter, GLT1, which is responsible for the vast majority of functional glutamate uptake, particularly in the spinal cord. In our unilateral cervical contusion model of mouse SCI that is associated with ipsilateral forepaw heat hypersensitivity (a form of chronic at-level neuropathic pain-related behavior), we previously reported significant and long-lasting reductions in GLT1 expression and functional GLT1-mediated glutamate uptake in cervical spinal cord dorsal horn. To therapeutically address GLT1 dysfunction following cervical contusion SCI, we injected an adeno-associated virus type 8 (AAV8)-Gfa2 vector into the superficial dorsal horn to increase GLT1 expression selectively in astrocytes. Compared to both contusion-only animals and injured mice that received AAV8-eGFP control injection, AAV8-GLT1 delivery increased GLT1 protein expression in astrocytes of the injured cervical spinal cord dorsal horn, resulting in a significant and persistent reversal of already-established heat hypersensitivity. Furthermore, AAV8-GLT1 injection significantly reduced expression of the transcription factor and marker of persistently increased neuronal activation, ΔFosB, in superficial dorsal horn neurons. These results demonstrate that focal restoration of GLT1 expression in the superficial dorsal horn is a promising target for treating chronic neuropathic pain following SCI.

Persistent at-level thermal hyperalgesia and tactile allodynia accompany chronic neuronal and astrocyte activation in superficial dorsal horn following mouse cervical contusion spinal cord injury

In humans, sensory abnormalities, including neuropathic pain, often result from traumatic spinal cord injury (SCI). SCI can induce cellular changes in the CNS, termed central sensitization, that alter excitability of spinal cord neurons, including those in the dorsal horn involved in pain transmission. Persistently elevated levels of neuronal activity, glial activation, and glutamatergic transmission are thought to contribute to the hyperexcitability of these dorsal horn neurons, which can lead to maladaptive circuitry, aberrant pain processing and, ultimately, chronic neuropathic pain. Here we present a mouse model of SCI-induced neuropathic pain that exhibits a persistent pain phenotype accompanied by chronic neuronal hyperexcitability and glial activation in the spinal cord dorsal horn. We generated a unilateral cervical contusion injury at the C5 or C6 level of the adult mouse spinal cord. Following injury, an increase in the number of neurons expressing ΔFosB (a marker of chronic neuronal activation), persistent astrocyte activation and proliferation (as measured by GFAP and Ki67 expression), and a decrease in the expression of the astrocyte glutamate transporter GLT1 are observed in the ipsilateral superficial dorsal horn of cervical spinal cord. These changes have previously been associated with neuronal hyperexcitability and may contribute to altered pain transmission and chronic neuropathic pain. In our model, they are accompanied by robust at-level hyperaglesia in the ipsilateral forepaw and allodynia in both forepaws that are evident within two weeks following injury and persist for at least six weeks. Furthermore, the pain phenotype occurs in the absence of alterations in forelimb grip strength, suggesting that it represents sensory and not motor abnormalities. Given the importance of transgenic mouse technology, this clinically-relevant model provides a resource that can be used to study the molecular mechanisms contributing to neuropathic pain following SCI and to identify potential therapeutic targets for the treatment of chronic pathological pain.

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.

Chronic at-level thermal hyperalgesia following rat cervical contusion spinal cord injury is accompanied by neuronal and astrocyte activation and loss of the astrocyte glutamate transporter, GLT1, in superficial dorsal horn

Neuropathic pain is a form of pathological nociception that occurs in a significant portion of traumatic spinal cord injury (SCI) patients, resulting in debilitating and often long-term physical and psychological burdens. While many peripheral and central mechanisms have been implicated in neuropathic pain, central sensitization of dorsal horn spinothalamic tract (STT) neurons is a major underlying substrate. Furthermore, dysregulation of extracellular glutamate homeostasis and chronic astrocyte activation play important underlying roles in persistent hyperexcitability of these superficial dorsal horn neurons. To date, central sensitization and astrocyte changes have not been characterized in cervical SCI-induced neuropathic pain models, despite the fact that a major portion of SCI patients suffer contusion trauma to cervical spinal cord. In this study, we have characterized 2 rat models of unilateral cervical contusion SCI that behaviorally result in chronic persistence of thermal hyperalgesia in the ipsilateral forepaw. In addition, we find that STT neurons are chronically activated in both models when compared to laminectomy-only uninjured rats. Finally, persistent astrocyte activation and significantly reduced expression of the major CNS glutamate transporter, GLT1, in superficial dorsal horn astrocytes are associated with both excitability changes in STT neurons and the neuropathic pain behavioral phenotype. In conclusion, we have characterized clinically-relevant rodent models of cervical contusion-induced neuropathic pain that result in chronic activation of both STT neurons and astrocytes, as well as compromise in astrocyte glutamate transporter expression. These models can be used as important tools to further study mechanisms underlying neuropathic pain post-SCI and to test potential therapeutic interventions.

Opioid administration following spinal cord injury: implications for pain and locomotor recovery

Approximately one-third of people with a spinal cord injury (SCI) will experience persistent neuropathic pain following injury. This pain negatively affects quality of life and is difficult to treat. Opioids are among the most effective drug treatments, and are commonly prescribed, but experimental evidence suggests that opioid treatment in the acute phase of injury can attenuate recovery of locomotor function. In fact, spinal cord injury and opioid administration share several common features (e.g. central sensitization, excitotoxicity, aberrant glial activation) that have been linked to impaired recovery of function, as well as the development of pain. Despite these effects, the interactions between opioid use and spinal cord injury have not been fully explored. A review of the literature, described here, suggests that caution is warranted when administering opioids after SCI. Opioid administration may synergistically contribute to the pathology of SCI to increase the development of pain, decrease locomotor recovery, and leave individuals at risk for infection. Considering these negative implications, it is important that guidelines are established for the use of opioids following spinal cord and other central nervous system injuries.

Transient ischemia-induced changes of excitatory amino acid carrier 1 in the ventral horn of the lumbar spinal cord in rabbits

OBJECTIVES

To examine temporal changes of EAAC1 immunoreactivity and its protein level in the spinal ventral horn after transient ischemia in the rabbit to investigate the correlation between neuronal cell death and EAAC1 in the ventral horn of spinal cord.

METHODS

White rabbits weighing 2.5-3.0 kg were anesthetized with a mixture of 2.5% isoflurane in 30% oxygen and 70% nitrous oxide, and the abdominal aortic artery below the left renal artery was occluded for 15 minutes. At designated times after reperfusion, the immunohistochemical and Western blot analysis for EAAC1 was conducted using tissues of the seventh lumbar spinal segment.

RESULTS

EAAC1 immunoreactivity was detected in the neurons of the normal spinal cord. EAAC1 immunoreactivity and protein level reduced significantly 30 minutes after ischemia/reperfusion, but EAAC1 immunoreactivity and protein level again increased by 80% versus sham 3 hours after ischemia. At this time point, neurological defect in hindlimb was also detected. Thereafter, EAAC1 immunoreactivity and protein levels remained to be attenuated in the ventral horn of spinal cord until 48 hours after ischemia.

CONCLUSION

The significant change in EAAC1 expression and motor defects at early time after transient spinal cord ischemia relates to the acute events following ischemia/reperfusion. These results indicate that EAAC1 has an important role in the modulation of glutamate homeostasis in ischemic neurons in the spinal ventral horn.

Human astrocytes derived from glial restricted progenitors support regeneration of the injured spinal cord

Cellular transplantation using neural stem cells and progenitors is a promising therapeutic strategy that has the potential to replace lost cells, modulate the injury environment, and create a permissive environment for the regeneration of injured host axons. Our research has focused on the use of human glial restricted progenitors (hGRP) and derived astrocytes. In the current study, we examined the morphological and phenotypic properties of hGRP prepared from the fetal central nervous system by clinically-approved protocols, compared with astrocytes derived from hGRP prepared by treatment with ciliary neurotrophic factor or bone morphogenetic protein 4. These differentiation protocols generated astrocytes that showed morphological differences and could be classified along an immature to mature spectrum, respectively. Despite these differences, the cells retained morphological and phenotypic plasticity upon a challenge with an alternate differentiation protocol. Importantly, when hGRP and derived astrocytes were transplanted acutely into a cervical dorsal column lesion, they survived and promoted regeneration of long ascending host sensory axons into the graft/lesion site, with no differences among the groups. Further, hGRP taken directly from frozen stocks behaved similarly and also supported regeneration of host axons into the lesion. Our results underscore the dynamic and permissive properties of human fetal astrocytes to promote axonal regeneration. They also suggest that a time-consuming process of pre-differentiation may not be necessary for therapeutic efficacy, and that the banking of large quantities of readily available hGRP can be an appropriate source of permissive cells for transplantation.

Metabolite profiles correlate closely with neurobehavioral function in experimental spinal cord injury in rats

Traumatic spinal cord injury (SCI) results in direct physical damage and the generation of local factors contributing to secondary pathogenesis. Untargeted metabolomic profiling was used to uncover metabolic changes and to identify relationships between metabolites and neurobehavioral functions in the spinal cord after injury in rats. In the early metabolic phase, neuronal signaling, stress, and inflammation-associated metabolites were strongly altered. A dynamic inflammatory response consisting of elevated levels of prostaglandin E2 and palmitoyl ethanolamide as well as pro- and anti-inflammatory polyunsaturated fatty acids was observed. N-acetyl-aspartyl-glutamate (NAAG) and N-acetyl-aspartate (NAA) were significantly decreased possibly reflecting neuronal cell death. A second metabolic phase was also seen, consistent with membrane remodeling and antioxidant defense response. These metabolomic changes were consistent with the pathology and progression of SCI. Several metabolites, including NAA, NAAG, and the ω-3 fatty acids docosapentaenoate and docosahexaenoate correlated greatly with the established Basso, Beattie and Bresnahan locomotive score (BBB score). Our findings suggest the possibility of a biochemical basis for BBB score and illustrate that metabolites may correlate with neurobehavior. In particular the NAA level in the spinal cord might provide a meaningful biomarker that could help to determine the degree of injury severity and prognosticate neurologic recovery.

Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

The cervical facet joint and its capsule are a common source of neck pain from whiplash. Mechanical hyperalgesia elicited by painful facet joint distraction is associated with spinal neuronal hyperexcitability that can be induced by transmitter/receptor systems that potentiate the synaptic activation of neurons. This study investigated the temporal response of a glutamate receptor and transporters in the dorsal root ganglia (DRG) and spinal cord. Bilateral C6/C7 facet joint distractions were imposed in the rat either to produce behavioral sensitivity or without inducing any sensitivity. Neuronal metabotropic glutamate receptor-5 (mGluR5) and protein kinase C-epsilon (PKCε) expression in the DRG and spinal cord were evaluated on days 1 and 7. Spinal expression of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1), was also quantified at both time points. Painful distraction produced immediate behavioral hypersensitivity that was sustained for 7 days. Increased expression of mGluR5 and PKCε in the DRG was not evident until day 7 and only following painful distraction; this increase was observed in small-diameter neurons. Only painful facet joint distraction produced a significant increase (p<0.001) in neuronal mGluR5 over time, and this increase also was significantly elevated (p≤0.05) over responses in the other groups at day 7. However, there were no differences in spinal PKCε expression on either day or between groups. Spinal EAAC1 expression was significantly increased (p<0.03) only in the nonpainful groups on day 7. Results from this study suggest that spinal glutamatergic plasticity is selectively modulated in association with facet-mediated pain.

Spatial and temporal activation of spinal glial cells: role of gliopathy in central neuropathic pain following spinal cord injury in rats

In the spinal cord, neuron and glial cells actively interact and contribute to neurofunction. Surprisingly, both cell types have similar receptors, transporters and ion channels and also produce similar neurotransmitters and cytokines. The neuroanatomical and neurochemical similarities work synergistically to maintain physiological homeostasis in the normal spinal cord. However, in trauma or disease states, spinal glia become activated, dorsal horn neurons become hyperexcitable contributing to sensitized neuronal-glial circuits. The maladaptive spinal circuits directly affect synaptic excitability, including activation of intracellular downstream cascades that result in enhanced evoked and spontaneous activity in dorsal horn neurons with the result that abnormal pain syndromes develop. Recent literature reported that spinal cord injury produces glial activation in the dorsal horn; however, the majority of glial activation studies after SCI have focused on transient and/or acute time points, from a few hours to 1 month, and peri-lesion sites, a few millimeters rostral and caudal to the lesion site. In addition, thoracic spinal cord injury produces activation of astrocytes and microglia that contributes to dorsal horn neuronal hyperexcitability and central neuropathic pain in above-level, at-level and below-level segments remote from the lesion in the spinal cord. The cellular and molecular events of glial activation are not simple events, rather they are the consequence of a combination of several neurochemical and neurophysiological changes following SCI. The ionic imbalances, neuroinflammation and alterations of cell cycle proteins after SCI are predominant components for neuroanatomical and neurochemical changes that result in glial activation. More importantly, SCI induced release of glutamate, proinflammatory cytokines, ATP, reactive oxygen species (ROS) and neurotrophic factors trigger activation of postsynaptic neuron and glial cells via their own receptors and channels that, in turn, contribute to neuronal-neuronal and neuronal-glial interaction as well as microglia-astrocytic interactions. However, a systematic review of temporal and spatial glial activation following SCI has not been done. In this review, we describe time and regional dependence of glial activation and describe activation mechanisms in various SCI models in rats. These data are placed in the broader context of glial activation mechanisms and chronic pain states. Our work in the context of work by others in SCI models demonstrates that dysfunctional glia, a condition called "gliopathy", is a key contributor in the underlying cellular mechanisms contributing to neuropathic pain.

Sex differences in effects of excitotoxic spinal injury on below-level pain sensitivity

Effects of excitotoxic injury to the thoracic gray matter on sensitivity to below-level nociceptive stimulation were evaluated for female and male Long-Evans rats. Operant escape and lick/guard (L/G) reflex responses to thermal stimulation were evaluated before and for 13-15 weeks after: 1) injections of quisqualic acid (QUIS) into the thoracic gray matter (T8-9), 2) laminectomy and spinal exposure and penetration without injection (sham) or 3) no surgical procedure (control). L/G responding to heat stimulation (44 °C) was unaffected for females and males following thoracic QUIS injections. Similarly, male escape performance was not significantly altered for 44 °C or 10 °C stimulation after QUIS injections or sham surgery. However, escape testing following QUIS and sham injections revealed increased heat sensitivity (44 °C) and decreased cold sensitivity (10 °C) for females. This selective effect is indicative of altered sympathetic activation by the thoracic injections. The effect of sham surgery suggests that female rats are vulnerable to ischemic injury during exposure and manipulation of the spinal cord. Escape from nociceptive heat and cold sensitivity of control males and females was unchanged over 13-15 weeks of testing.

Reduction in expression of the astrocyte glutamate transporter, GLT1, worsens functional and histological outcomes following traumatic spinal cord injury

The astrocyte glutamate transporter, GLT1, is responsible for the vast majority of glutamate uptake in the adult central nervous system (CNS), thereby regulating extracellular glutamate homeostasis and preventing excitotoxicity. Glutamate dysregulation plays a central role in outcome following traumatic spinal cord injury (SCI). To determine the role of GLT1 in secondary cell loss following SCI, mice heterozygous for the GLT1 astrocyte glutamate transporter (GLT1+/-) and wild-type mice received thoracic crush SCI. Compared with wild-type controls, GLT1+/- mice had an attenuated recovery in hindlimb motor function, increased lesion size, and decreased tissue sparing. GLT1+/- mice showed a decrease in intraspinal GLT1 protein and functional glutamate uptake compared with wild-type mice, accompanied by increased apoptosis and neuronal loss following crush injury. These results suggest that astrocyte GLT1 plays a role in limiting secondary cell death following SCI, and also show that compromise of key astrocyte functions has significant effects on outcome following traumatic CNS injury. These findings also suggest that increasing intraspinal GLT1 expression may represent a therapeutically relevant target for SCI treatment.

Spatial and temporal changes in promoter activity of the astrocyte glutamate transporter GLT1 following traumatic spinal cord injury

After traumatic spinal cord injury (SCI), there is an opportunity for preserving function by attenuating secondary cell loss. Astrocytes play crucial roles in the adult CNS and are responsible for the vast majority of glutamate buffering, potentially preventing excitotoxic loss of neurons and oligodendrocytes. We examined spatial and temporal changes in gene expression of the major astrocyte glutamate transporter GLT1 following moderate thoracic contusion SCI using transgenic BAC-GLT1-eGFP promoter reporter mice. In dorsal column white matter, total intensity of GLT1-eGFP expression per region was significantly reduced following SCI at both lesion epicenter and at rostral and caudal areas where no tissue loss occurred. This regional decrease in GLT1 expression was due to significant loss of GLT1-eGFP(+) cells, partially accounted for by apoptosis of eGFP(+) /GFAP(+) astrocytes in both white and gray matter. There were also decreased numbers of GLT1-eGFP-expressing cells in multiple gray matter regions following injury; nevertheless, there was sustained or even increased regional GLT1-eGFP expression in gray matter as a result of up-regulation in astrocytes that continued to express GLT1-eGFP. Although there were increased numbers of GFAP(+) cells both at the lesion site and in surrounding intact spinal cord following SCI, the majority of proliferating Ki67(+) /GFAP(+) astrocytes did not express GLT1-eGFP. These findings demonstrate that spatial and temporal alterations in GLT1 expression observed after SCI result from both astrocyte death and gene expression changes in surviving astrocytes. Results also suggest that following SCI a significant portion of astrocytes lacks GLT1 expression, possibly compromising the important role of astrocytes in glutamate homeostasis.

Long-term changes in expressions of spinal glutamate transporters after spinal cord injury

Glutamate is a major excitatory transmitter in the central nervous system that may produce cellular injury when its concentration is abnormally increased in the synaptic cleft. Glial glutamate transporters GLAST and GLT-1, which are responsible for clearing synaptic glutamate into glial cells, play an important role in the regulation of the glutamate concentration in the synaptic cleft. However, there has been no report on long-term changes in the levels of glutamate transporters following spinal cord injury. Spinal cord injury (SCI) was induced at T12 by a New York University (NYU) impactor. Segments of the spinal cord at T9-10, L1-2, L4-5 and at the epicenter were removed after SCI, and Western blots for GLAST, GLT-1 and EAAC1 were performed. GLAST and GLT-1 were significantly decreased in the epicenter from 1day up to 8weeks after SCI. GLT-1 was significantly decreased in the spinal segments rostral to the injury site, and GLAST expression was significantly increased in the L4-5 region of the spinal cord for 8weeks. Because strategies to modulate the regulation of glutamate transporters may be applied, the present data serve as a reference for further research, although the long-term roles of glutamate transporters in pathological processes caused by SCI are not clear.

Simulated whiplash modulates expression of the glutamatergic system in the spinal cord suggesting spinal plasticity is associated with painful dynamic cervical facet loading

The cervical facet joint and its capsule have been reported to be injured during whiplash scenarios and are a common source of chronic neck pain from whiplash. Both the metabotropic glutamate receptor 5 (mGluR5) and the excitatory amino acid carrier 1 (EAAC1) have pivotal roles in chronic pain. In this study, spinal mGluR5 and EAAC1 were quantified following painful facet joint distraction in a rat model of facet-mediated painful loading and were evaluated for their correlation with the severity of capsule loading. Rats underwent either a dynamic C6/C7 joint distraction simulating loading experienced during whiplash (distraction; n = 12) or no distraction (sham; n = 6) to serve as control. The severity of capsular loading was quantified using strain metrics, and mechanical allodynia was assessed after surgery. Spinal cord tissue was harvested at day 7 and the expression of mGluR5 and EAAC1 were quantified using Western blot analysis. Mechanical allodynia following distraction was significantly (p < 0.001) higher than sham. Spinal expression of mGluR5 was also significantly (p < 0.05) greater following distraction relative to sham. However, spinal EAAC1 was significantly (p = 0.0003) reduced compared to sham. Further, spinal mGluR5 expression was significantly positively correlated to capsule strain (p < 0.02) and mechanical allodynia (p < 0.02). Spinal EAAC1 expression was significantly negatively related to one of the strain metrics (p < 0.003) and mechanical allodynia at day 7 (p = 0.03). These results suggest that the spinal glutamatergic system may potentiate the persistent behavioral hypersensitivity that is produced following dynamic whiplash-like joint loading; chronic whiplash pain may be alleviated by blocking mGluR5 expression and/or enhancing glutamate transport through the neuronal transporter EAAC1.

Impaired glial glutamate uptake induces extrasynaptic glutamate spillover in the spinal sensory synapses of neuropathic rats

Glial cell dysfunction and excessive glutamate receptor activation in spinal dorsal horn neurons are hallmark mechanisms of pathological pain. The way in which glial cell dysfunction leads to excessive glutamate receptor activation in the spinal sensory synapses remains unknown. We and others recently reported the downregulation of glial glutamate transporter (GT) protein expression in the spinal dorsal horn of neuropathic rats. In this study, we showed that excitatory postsynaptic currents originating from N-methyl-d-aspartate receptor activation (NMDA EPSCs) elicited by peripheral synaptic input in the spinal sensory synapses were enhanced in neuropathic rats with mechanical allodynia induced by partial sciatic nerve ligation. The enhanced NMDA EPSCs were accompanied by an increased proportion of NR2B receptor activation. Physically blocking the extrasynaptic glutamate with dextran or chemically scavenging the glutamate with glutamic-pyruvic transaminase ameliorated the abnormal NMDA EPSCs in neuropathic rats. Pharmacological blockade of glial GTs with dihydrokainic acid enhanced NMDA receptor activation elicited by synaptic input or puffed glutamate in normal control rats, but this effect was precluded in neuropathic rats. Thus extrasynaptic glutamate spillover and extrasynaptic NMDA receptor activation induced by deficient glial glutamate uptake in the synapses resulted in the excessive activation of NMDA receptors in neuropathic rats. It is suggested that extrasynaptic glutamate spillover may be a key synaptic mechanism related to phenotypic alterations induced by nerve injury in the spinal dorsal horn and that glial GTs are potential new targets in the development of analgesics.

Spinal cord injury causes a wide-spread, persistent loss of Kir4.1 and glutamate transporter 1: benefit of 17 beta-oestradiol treatment

During neuronal activity astrocytes function to remove extracellular increases in potassium, which are largely mediated by the inwardly-rectifying potassium channel Kir4.1, and to take up excess glutamate via glutamate transporter 1, a glial-specific glutamate transporter. Here we demonstrate that expression of both of these proteins is reduced by nearly 80% following a crush spinal cord injury in adult male rats, 7 days post-injury. This loss extended to spinal segments several millimetres rostral and caudal to the lesion epicentre, and persisted at 4 weeks post-injury. Importantly, we demonstrate that loss of these two proteins is not a direct result of astrocyte loss, as immunohistochemistry at 7 days and western blots at 4 weeks demonstrate a marked up-regulation in glial fibrillary acidic protein expression. Kir4.1 and glutamate transporter 1 expression were partially rescued by post-spinal cord injury administration of physiological levels of 17beta-oestradiol (0.08 mg/kg/day) in vivo. Utilizing an in vitro culture system we demonstrate that 17beta-oestradiol treatment (50 nM) is sufficient to increase glutamate transporter 1 protein expression in spinal cord astrocytes. This increase in glutamate transporter 1 protein expression was reversed and Kir4.1 expression reduced in the presence of an oestrogen receptor antagonist, Fulvestrant 182,780 suggesting a direct translational regulation of Kir4.1 and glutamate transporter 1 via genomic oestrogen receptors. Using whole-cell patch-clamp recordings in cultured spinal cord astrocytes, we show that changes in protein expression following oestrogen application led to functional changes in Kir4.1 mediated currents. These findings suggest that the neuroprotective benefits previously seen with 17beta-oestradiol after spinal cord injury may be in part due to increased Kir4.1 and glutamate transporter 1 expression in astrocytes leading to improved potassium and glutamate homeostasis.

Activation of p-38alpha MAPK contributes to neuronal hyperexcitability in caudal regions remote from spinal cord injury

In the present study, we examined whether activation of p-38alpha MAPK modulates mechanical allodynia and neuronal hyperexcitability, and if propentofylline (PPF, a glial modulator) modulates specifically localized activated p-38alpha MAPK expression in caudal regions remote from a low thoracic hemisection injury in rats. T13 spinal hemisection produces bilateral mechanical allodynia in hindpaws with evoked (in response to mechanical stimuli) neuronal hyperexcitability in lumbar spinal wide dynamic range (WDR) neurons compared to sham controls. The mechanical allodynia and the evoked activity of WDR neurons is attenuated by intrathecal and topical administration of SB203580, an inhibitor of p-38 MAPK activation, dose dependently (p<0.05); however, the spontaneous activity showed no significant differences compared to sham controls. After T13 spinal hemisection, significantly increased phosphorylated (activated form) p-38alpha MAPK expression was present in both superficial and deep dorsal horn neurons as well as in microglia, but not in astrocytes, in the lumbar spinal cord compared to sham controls (p<0.05). Intrathecal application of PPF significantly attenuated the expression of phosphorylated p-38alpha MAPK in superficial dorsal horn neurons (10 mM) and in microglia (1 and 10 mM) in the lumbar spinal cord compared to the hemisection group (p<0.05). In conclusion, our present data demonstrate that activated neuronal and microglial, but not astrocytic, p-38alpha MAPK contributes to the maintenance of neuronal hyperexcitability in caudal regions following spinal cord injury.

Glutamate transporters prevent excessive activation of NMDA receptors and extrasynaptic glutamate spillover in the spinal dorsal horn

Activation of N-methyl-d-aspartate (NMDA) receptors in the spinal dorsal horn neurons is a key process related to sensory transmission, neural plasticity, and pathogenesis of pain. In this study, we investigated how activation of NMDA receptors in spinal substantia gelatinosa neurons is regulated by glutamate re-uptake through glutamate transporters located in the astrocytic and neuronal plasma membranes. Using visualized whole cell patch recording techniques, NMDA excitatory postsynaptic currents evoked by graded peripheral inputs in spinal substantia gelatinosa neurons of spinal slices from young adult rats were analyzed before and after combined inhibition of glial and neuronal glutamate transporters by d-threo-beta-benzyloxyaspartate (TBOA). Blockade of glutamate transporters increased the number and duration of NMDA receptors activated by weak and by strong primary afferent inputs as well as by exogenous glutamate. The enhancement in activation of NMDA receptors induced by TBOA was greater in neurons that have weaker synaptic input at baseline. Impaired glutamate uptake increased the open probability of NMDA channels and caused glutamate spillover outside the active synapses, leading to activation of extrasynaptic NMDA receptors and/or receptors located in neighboring synapses. Finally, blockade of glutamate transporters resulted in an increased proportion of NR2B subunit activation induced by peripheral input, and this increase was further augmented by stronger afferent input. These data indicate that glutamate transporters regulate spatiotemporal and intensity coding for sensory input and prevent excessive activation of glutamate receptors in the spinal dorsal horn. It is suggested that remedying dysfunctional glutamate transporters may be a potential new avenue to prevent the pathogenesis of pain.

Mechanisms of chronic central neuropathic pain after spinal cord injury

Not all spinal contusions result in mechanical allodynia, in which non-noxious stimuli become noxious. The studies presented use the NYU impactor at 12.5 mm drop or the Infinite Horizons Impactor (150 kdyn, 1 s dwell) devices to model spinal cord injury (SCI). Both of these devices and injury parameters, if done correctly, will result in animals with above level (forelimb), at level (trunk) and below level (hindlimb) mechanical allodynia that model the changes in evoked somatosensation experienced by the majority of people with SCI. The sections are as follows: 1) Mechanisms of remote microglial activation and pain signaling in "below-level" central pain 2) Intracellular signaling mechanisms in central sensitization in "at-level" pain 3) Peripheral sensitization contributes to "above level" injury pain following spinal cord injury and 4) Role of reactive oxygen species in central sensitization in regional neuropathic pain following SCI. To summarize, differential regional mechanisms contribute to the regional chronic pain states. We propose the importance of understanding the mechanisms in the differential regional pain syndromes after SCI in the chronic condition. Targeting regional mechanisms will be of enormous benefit to the SCI population that suffer chronic pain, and will contribute to better treatment strategies for other chronic pain syndromes.

The effect of intravesical electrical stimulation on bladder function and synaptic neurotransmission in the rat spinal cord after spinal cord injury

OBJECTIVE

To investigate the effects of intravesical electrical stimulation (IVES) on bladder function and synaptic neurotransmission in the lumbosacral spinal cord in the spinalized rat, as the clinical benefits of IVES in patients with increased residual urine or reduced bladder capacity have been reported but studies on the mechanism of IVES have mainly focused on bladder A delta afferents in central nervous system-intact rats.

MATERIALS AND METHODS

In all, 30 female Sprague-Dawley rats were divided equally into three groups: normal control rats, sham-stimulated spinalized rats and IVES-treated spinalized rats. IVES was started 5 weeks after spinal cord injury (SCI) and was performed 20 min a day for 5 consecutive days. At 7 days after IVES, conscious filling cystometry was performed. Sections from the L6 and S1 spinal cord segments were examined for n-methyl-d-aspartic acid receptor 1 (NMDAR1) subunit and gamma-aminobutyric acid (GABA) immunoactivity.

RESULTS

In IVES-treated spinalized rats, the number and maximal pressure of nonvoiding detrusor contractions were significantly less than in sham-stimulated spinalized rats. The mean maximal voiding pressure was also lower in IVES-treated than in sham-stimulated spinalized rats. IVES significantly reduced the interval between voiding contractions compared with the untreated spinalized rats. There was an overall increase in NMDAR1 immunoactivity after SCI, which was significantly lower in IVES-treated spinalized rats. Immunoactivity of GABA after SCI was significantly lower than in the control group and was significantly higher in IVES-treated spinalized rats.

CONCLUSION

Our results suggest that IVES might affect voiding contractions in addition to inhibiting C-fibre activity and that IVES seems to have a more complex effect on the bladder control pathway. For synaptic neurotransmission in the spinal cord, IVES could possibly shift the balance between excitation and inhibition towards inhibition.

Effects of chronic restraint stress and estradiol replacement on glutamate release and uptake in the spinal cord from ovariectomized female rats

Glutamate is an excitatory neurotransmitter involved in neuronal plasticity and neurotoxicity. Chronic stress produces several physiological changes on the spinal cord, many of them presenting sex-specific differences, which probably involve glutamatergic system alterations. The aim of the present study was to verify possible effects of exposure to chronic restraint stress and 17beta-estradiol replacement on [3H]-glutamate release and uptake in spinal cord synaptosomes of ovariectomized (OVX) rats. Female rats were subjected to OVX, and half of the animals received estradiol replacement. Animals were subdivided in controls and chronically stressed. Restraint stress or estradiol had no effect on [3H]-glutamate release. The chronic restraint stress promoted a decrease and 17beta-estradiol induced an increase on [3H]-glutamate uptake, but the uptake observed in the restraint stress +17beta-estradiol group was similar to control. Furthermore, 17beta-estradiol treatment caused a significant increase in the immunocontent of the three glutamate transporters present in spinal cord. Restraint stress had no effect on the expression of these transporters, but prevented the 17beta-estradiol effect. We suggest that changes in the glutamatergic system are likely to take part in the mechanisms involved in spinal cord plasticity following repeated stress exposure, and that 17beta-estradiol levels may affect chronic stress effects in this structure.

Impact of alcohol abuse on protein expression of midkine and excitatory amino acid transporter 1 in the human prefrontal cortex

BACKGROUND

Alcoholism is associated with shrinkage of brain tissue and reduction in the number of neurons and dendritic arbors particularly in the prefrontal cortex. These changes correlate with the cognitive defects common in alcoholics. A recent study investigated the mRNA expression of selected genes in the prefrontal cortex and found that the levels of mRNA encoding the neurotrophic factor, midkine (MDK), and the excitatory amino acid transporter 1 (EAAT1) were significantly higher in alcoholics compared with nonalcoholic controls. This study aimed to investigate, whether the transcriptional changes observed result in alterations to protein expression. Additionally, the study aimed to expand our understanding of MDK and EAAT1 action by localizing their expression within morphologically and functionally distinct layers of this brain region.

METHODS

Quantitative changes in protein levels of MDK and EAAT1 were investigated in alcoholic and control cases using Western blots. Immunohistochemistry was utilized to localize proteins expression in formalin-fixed sagittal sections of the prefrontal cortex.

RESULTS

A marked increase was revealed in protein expression of both genes in the prefrontal cortex of chronic alcoholics. MDK-like immunofluorescence in alcoholic and control cases was present in nuclei throughout the prefrontal cortex and was particularly apparent in cell bodies of astrocytes in cortical layer II. Immunolabeling of the EAAT1 was densest in cortical layer II in control cases and induced in deeper layers in alcoholic cases.

CONCLUSION

Midkine promotes neuronal outgrowth and survival. The up-regulation of MDK protein expression may indicate the induction of reparative processes. The amino acid transporter is vital for the removal of glutamate from the synaptic cleft. At alcohol withdrawal, extracellular glutamate is thought to reach excitotoxic concentrations. Up-regulation of EAAT1 throughout the cortical layers may indicate an attempt to combat elevated glutamate concentrations. The predominant expression of the two proteins in layer II of the cortex implies a region-specific role of astrocytes.

Propentofylline attenuates allodynia, glial activation and modulates GABAergic tone after spinal cord injury in the rat

In this study, we evaluated whether propentofylline, a methylxanthine derivative, modulates spinal glial activation and GABAergic inhibitory tone by modulation of glutamic acid decarboxylase (GAD)(65), the GABA synthase enzyme, in the spinal dorsal horn following spinal cord injury (SCI). Sprague-Dawley rats (225-250 g) were given a unilateral spinal transverse injury, from dorsal to ventral, at the T13 spinal segment. Unilateral spinal injured rats developed robust bilateral hindlimb mechanical allodynia and hyperexcitability of spinal wide dynamic range (WDR) neurons in the lumbar enlargement (L4-L5) compared to sham controls, which was attenuated by intrathecal (i.t.) administration of GABA, dose-dependently (0.01, 0.1, 0.5 microg). Western blotting and immunohistochemical data demonstrated that the expression level of GAD(65) protein significantly decreased on both sides of the lumbar dorsal horn (L4/5) after SCI (p<0.05). In addition, astrocytes and microglia showed soma hypertrophy as determined by increased soma area and increased GFAP and CD11b on both sides of the lumbar dorsal horn compared to sham controls, respectively (p<0.05). Intrathecal treatment with propentofylline (PPF 10 mM) significantly attenuated the astrocytic and microglial soma hypertrophy and mechanical allodynia (p<0.05). Additionally, the Western blotting and immunohistochemistry data demonstrated that i.t. treatment of PPF significantly prevented the decrease of GAD(65) expression in both sides of the lumbar dorsal horn following SCI (p<0.05). In conclusion, our present data demonstrate that propentofylline modulates glia activation and GABAergic inhibitory tone by modulation of GAD(65) protein expression following spinal cord injury.

Glial glutamate transporter 1 regulates the spatial and temporal coding of glutamatergic synaptic transmission in spinal lamina II neurons

Glutamatergic synaptic transmission is a dynamic process determined by the amount of glutamate released by presynaptic sites, the clearance of glutamate in the synaptic cleft, and the properties of postsynaptic glutamate receptors. Clearance of glutamate in the synaptic cleft depends on passive diffusion and active uptake by glutamate transporters. In this study, we examined the role of glial glutamate transporter 1 (GLT-1) in spinal sensory processing. Excitatory postsynaptic currents (EPSCs) of substantia gelatinosa neurons recorded from spinal slices of young adult rats were analyzed before and after GLT-1 was pharmacologically blocked by dihydrokainic acid. Inhibition of GLT-1 prolonged the EPSC duration and the EPSC decay phase. The EPSC amplitudes were increased in neurons with weak synaptic input but decreased in neurons with strong synaptic input upon inhibition of GLT-1. We suggest that presynaptic inhibition, desensitization of postsynaptic AMPA receptors, and glutamate "spillover" contributed to the kinetic change of EPSCs induced by the blockade of GLT-1. Thus, GLT-1 is a key component in maintaining the spatial and temporal coding in signal transmission at the glutamatergic synapse in substantia gelatinosa neurons.

Astroglia-mediated effects of uric acid to protect spinal cord neurons from glutamate toxicity

Uric acid (UA) has been demonstrated to reduce damage to neurons elicited by oxidative stress. However, our studies utilizing cultures derived from embryonic rat spinal cord indicate that an astroglia-mediated mechanism is involved in the effects of UA to protect neurons from glutamate toxicity. The damage elicted by glutamate to neurons in a mixed culture of spinal cord cells can be reversed by UA. Furthermore, addition of UA after the termination of glutamate exposure suggests that UA plays an active role in mediating neuroprotection rather than purely binding peroxynitrite, as previously thought. Importantly, in pure neuron cultures from the same tissue, UA does not protect against glutamate toxicity. Addition of astroglia to the pure neuron cultures restores the ability of UA to protect the neurons from glutamate-induced toxicity. Our results also suggest that glia provide EAAT-1 and EAAT-2 glutamate transporters to protect neurons from glutamate, that functional EAATs may be necessary to mediate the effects of UA, and that treatment with UA results in upregulation of EAAT-1 protein. Taken together, our data strongly suggest that astroglia in mixed cultures are essential for mediating the effects of UA, revealing a novel mechanism by which UA, a naturally produced substance in the body, may act to protect neurons from damage during insults such as spinal cord injury.

Regional distribution of sodium-dependent excitatory amino acid transporters in rat spinal cord

BACKGROUND/OBJECTIVE

The excitatory amino acid transporters (EAATs), or sodium-dependent glutamate transporters, provide the primary mechanism for glutamate removal from the synaptic cleft. EAAT distribution has been determined in the rat brain, but it is only partially characterized in the spinal cord.

METHODS

The regional anatomic distribution of EAATs in spinal cord was assessed by radioligand autoradiography throughout cervical, thoracic, and lumbar cord levels in female Sprague-Dawley rats. EAAT subtype regional distribution was evaluated by inclusion of pharmacologic transport inhibitors in the autoradiography assays and by immunohistochemistry using subtype-specific polyclonal antibodies to rat GLT1 (EAAT2), GLAST (EAAT1), and EAAC1 (EAAT3) rat transporter subtypes.

RESULTS

[3H]-D-Aspartate binding was distributed throughout gray matter at the 3 spinal cord levels, with negligible binding in white matter. Inclusion of pharmacologic transport inhibitors indicates that the EAAT2/ GLT1 subtype represents 21% to 40% of binding. Both EAAT1/GLAST and EAAT3/EAAC1 contributed the remainder of binding. Immunoreactivity to subtype-specific antibodies varied, depending on cord level, and was present in both gray and white matter. All 3 subtypes displayed prominent immunoreactivity in the dorsal horn. EAAT3/EAAC1 and to a lesser extent EAAT1/GLAST immunoreactivity also occurred in a punctate pattern in the ventral horn.

CONCLUSIONS

The results indicate heterogeneity of EAAT distribution among spinal cord levels and regions. The presence of these transporters throughout rat spinal cord suggests the importance of their contributions to spinal cord function.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

The glutamate transporter GLAST is involved in spinal nociceptive processing

GLAST and GLT-1 are the most abundant glutamate transporters in the CNS and protect neurons from glutamate neurotoxicity. Here, we investigated the role of GLAST in spinal nociceptive processing. GLAST protein expression was not altered after treatment of rats with either formalin or zymosan. Surprisingly, knock-down of GLAST in the spinal cord using antisense-oligonucleotides decreased glutamate concentrations in cerebrospinal fluid (CSF) and reduced the nociceptive behaviour in the rat formalin assay. However, it did not influence thermal hyperalgesia in the zymosan-induced paw inflammation model indicating that GLAST is associated with spontaneous rather than inflammatory nociception. Mechanisms that might explain the decreased response in the formalin assay may include compensatory activation of other glutamate transporters, inhibition of glutamate release or disturbance of glutamate recycling. In conclusion, these data suggest that inhibition of GLAST expression in the spinal cord reduces excitatory synaptic activity and thereby spontaneous responses after nociceptive stimulation of the paw.

Increases in the activated forms of ERK 1/2, p38 MAPK, and CREB are correlated with the expression of at-level mechanical allodynia following spinal cord injury

Rats given moderate spinal cord injury (SCI) display increases in the expression of the activated form of the transcription factor cyclic AMP responsive element binding protein (CREB) in spinal segments of dermatomes corresponding to permanent mechanical allodynia, a model of chronic central neuropathic pain (CNP; (Crown, E.D., Ye, Z., Johnson, K.M., Xu, G.Y., McAdoo, D.J., Westlund, K.N., Hulsebosch, C.E., 2005. Upregulation of the phosphorylated form of CREB in spinothalamic tract cells following spinal cord injury: relation to central neuropathic pain. Neurosci. Lett. 384, 139-144)). Given that not all rats that receive moderate SCI develop CNP, the current study was designed to further analyze changes in persistent CREB activation and in the activation state of upstream intracellular signaling cascades (e.g., mitogen-activated protein kinases [MAPKs]) in populations of rats that receive SCI and weeks later develop CNP and rats that receive SCI but do not develop CNP. The results indicate that activated kinases such as pERK 1/2, p-p38 MAPK, but not pJNK, are upregulated in injured rats that develop CNP as compared to injured rats that fail to develop CNP. In addition, the current results replicated our previous finding that activated CREB is upregulated following SCI, however, only in SCI rats that developed CNP. Taken together, these results indicate that activation of intracellular signaling cascades traditionally associated with long-term potentiation and memory is associated with the expression of chronic CNP following SCI.

Post-translational regulation of EAAT2 function by co-expressed ubiquitin ligase Nedd4-2 is impacted by SGK kinases

The human excitatory amino acid transporter (EAAT)2 is the major glutamate carrier in the mammalian CNS. Defective expression of the transporter results in neuroexcitotoxicity that may contribute to neuronal disorders such as amyotrophic lateral sclerosis (ALS). The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in the brain and is known to interact with the ubiquitin ligase Nedd4-2 to modulate membrane transporters and ion channels. The present study aimed to investigate whether SGK isoforms and the related kinase, protein kinase B (PKB), regulate EAAT2. Expression studies in Xenopus oocytes demonstrated that glutamate-induced inward current (IGLU) was stimulated by co-expression of SGK1, SGK2, SGK3 or PKB. IGLU is virtually abolished by Nedd4-2, an effect abrogated by additional co-expression of either kinase. The kinases diminish the effect through Nedd4-2 phosphorylation without altering Nedd4-2 protein abundance. SGKs increase the transporter maximal velocity without significantly affecting substrate affinity. Similar to glutamate-induced currents, [3H] glutamate uptake and cell surface abundance of the transporter were increased by the SGK isoforms and down-regulated by the ubiquitin ligase Nedd4-2. In conclusion, all three SGK isoforms and PKB increase EAAT2 activity and plasma membrane expression and thus, may participate in the regulation of neuroexcitability.

Effect of postinjury intravenous or intrathecal methylprednisolone on spinal cord excitatory amino-acid release, nitric oxide generation, PGE2 synthesis, and myeloperoxidase content in a pig model of acute spinal cord injury

STUDY DESIGN

Prospective, randomized, in vivo acute spinal cord injury in pigs.

SETTING

Department of Anesthesiology, University of Washington, Seattle, WA, USA.

OBJECTIVES

To determine whether postinjury methylprednisolone could reduce the generation of known mediators of secondary neurological injury.

METHODS

Intrathecal microdialysis probes were used to sample cerebrospinal fluid (CSF) for measurement of PGE(2), glutamate, and citrulline (a byproduct of nitric oxide generation), before and after spinal cord injury in anesthetized pigs. The spinal cord was removed at the end of the study for measurement of myeloperoxidase and methylprednisolone concentrations. Animals were randomly allocated to receive intravenous methylprednisolone (30 mg/kg bolus then 3.4 mg/kg/h), intrathecal methylprednisolone (5 mg bolus then 5 mg/h), or saline, beginning 30 min after the spinal cord was injured by using a modification of the Allen weight drop technique.

RESULTS

Spinal cord injury significantly increased the amount of glutamate, PGE(2), myeloperoxidase, and citrulline, recovered from the CSF dialysates. However, neither intravenous nor intrathecal methylprednisolone administered after injury had any effect on the magnitude of the increase in any of the measured biochemicals. Intrathecal methylprednisolone administration produced a spinal cord methylprednisolone concentration that was eight times greater, and a plasma concentration that was 32 times less, than that achieved with intravenous administration.

CONCLUSIONS

Contrary to earlier animal studies in which methylprednisolone was administered either before or immediately after spinal cord injury, we found no effect of intravenous or intrathecal methylprednisolone on any of the parameters measured when administered 30 min postinjury.

Altered discharges of spinal wide dynamic range neurons and down-regulation of glutamate transporter expression in rats with paclitaxel-induced hyperalgesia

Changes in the signaling of wide dynamic range neurons and the expression of glutamate transporters in the lumbar spinal dorsal horn of rats with Taxol-induced hyperalgesia are detailed in this report. Deep spinal lamina neurons have significantly increased spontaneous activity and after-discharges to noxious mechanical stimuli, increased responses to both skin heating and cooling, and increased after-discharges and abnormal windup to transcutaneous electrical stimuli. The expression of glutamate transporter proteins in the dorsal horn is decreased at the time point corresponding to the physiological changes. These results suggest a state of increased excitability develops in spinal pain-signaling neurons as a consequence of decreased glutamate clearance. These changes in dorsal horn neurobiology likely in turn contribute to the hyper-responsiveness to sensory stimuli seen in animals treated with Taxol and may play a role in the pain seen in cancer patients receiving Taxol.

Inhibition of glutamate uptake in the spinal cord induces hyperalgesia and increased responses of spinal dorsal horn neurons to peripheral afferent stimulation

Glutamate is a primary excitatory neurotransmitter in the mammalian CNS. Glutamate released from presynaptic neurons is cleared from the synaptic cleft passively by diffusion and actively by glutamate transporters. In this study, the role of glutamate transporters in sensory processing in the spinal cord has been investigated in behavioral, in vivo and in vitro experiments. Intrathecal application of a non-selective glutamate transport inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid (10 microl of 100 microM solution) induced hypersensitivity to peripheral mechanical and thermal stimuli. Topical application of L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM) onto the dorsal surface of the L3-L6 spinal cord increased spontaneous activities, innocuous and noxious stimulus-evoked responses and after-discharges of wide dynamic range neurons in the L4-5 spinal segments. Whole cell recordings made from superficial dorsal horn neurons in an isolated whole spinal cord from newborn rats (2-3 weeks old) revealed that bath-applied L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM) produced partial membrane depolarization, increased spontaneous action potentials with decreased neuronal membrane resistance and time constant, but without significant changes of capacitance. Finally, the amplitude and duration of primary afferent evoked-excitatory postsynaptic currents recorded from neurons in the substantia gelatinosa in the spinal slices from young adult rats (6-8 weeks old) were increased in the presence of L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM). This study indicates that glutamate transporters regulate baseline excitability and responses of dorsal horn neurons to peripheral stimulation, and suggests that dysfunction of glutamate transporters may contribute to certain types of pathological pain.

The effect of glutamate receptor blockers on glutamate release following spinal cord injury. Lack of evidence for an ongoing feedback cascade of damage --> glutamate release --> damage --> glutamate release --> etc

It is widely hypothesized that excitotoxicity of released glutamate following a CNS insult is propagated by the cyclic cascade: glutamate release --> damage --> glutamate release --> further damage --> etc. We tested this hypothesis by determining the effects of attempting to interrupt the loop by administering glutamate receptor antagonists and Na(+)-channel blockers on glutamate release following spinal cord injury (SCI). The effects of administering the NMDA receptor blockers MK-801 and memantine, the AMPA/kainate receptor blockers NBQX and GYKI 52466, the AMPA receptor desensitization blocker cyclothiazide and the sodium channel blockers riluzole, mexiletine and QX-314 on post-SCI were determined. Agents were administered into the site of injury by direct injection, by microdialysis or systemically. None of these agents had an appreciable effect on glutamate release following SCI. Thus, it is unlikely that the above cascade produces significant secondary glutamate release and ongoing damage following SCI, although such cascades may worsen other CNS insults. We attribute our results to overwhelming effects of much greater release by direct mechanical damage and reversal of transport following SCI.