AMPA-selective glutamate receptor subunits in glial cells of the adult bovine white matter

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Lysophosphatidic acid receptor type 2 activation contributes to secondary damage after spinal cord injury in mice

Lysophosphatidic acid (LPA) is an extracellular lipid mediator involved in many physiological functions by signaling through six known G-protein-coupled receptors (LPA₁-LPA₆). In the central nervous system (CNS), LPA mediates a wide range of effects, including neural progenitor cell physiology, astrocyte and microglia activation, neuronal cell death, axonal retraction, and contributions to pain, schizophrenia and hydrocephalus. We recently reported that LPA-LPA₁ signaling mediates functional deficits and myelin loss after spinal cord injury (SCI). Here, we provide clear evidence on the deleterious contribution of another LPA receptor, LPA₂, to myelin loss after SCI. We found that LPA₂ is constitutively expressed in the spinal cord parenchyma and its transcripts were up-regulated after contusion injury, in part, by microglial cells. We also found that the demyelinating lesion triggered by intraspinal injection of LPA into the undamaged spinal cord was markedly reduced in the lack of LPA₂. Similarly, LPA₂ deficient mice showed enhanced motor skills and myelin sparing after SCI. To gain insights into the detrimental actions of LPA₂ in spinal cord we performed cell culture studies. These experiments revealed that, similar to LPA₁, activation of microglia LPA₂ led to oligodendrocyte cell death. Moreover, we also found that the cytotoxic effects underlaying microglial LPA-LPA₂ axis were mediated by the release of purines by microglia and the activation of P₂X₇ receptor on oligodendrocytes. Overall, this study provides new mechanistic insights into how LPA contributes to SCI physiopathology, and suggest that targeting LPA₂ could be a novel therapeutic approach for the treatment of acute SCI.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

Glutamate-Induced White Matter Injury: Excitotoxicity without Synapses

White matter of the brain and spinal cord is irreversibly damaged by ischemia and trauma. Recent evidence indicates that despite the absence of synaptic elements, excitotoxic mechanisms play an important role in the pathogenesis of white matter damage. Glial cells, including astrocytes and oligodendrocytes, possess non-NMDA glutamate receptors and are injured by excessive exposure to AMPA/kainate agonists. In addition, the myelin sheath itself appears to respond directly to glutamate stimulation via AMPA receptors, which may also lead to injury of this key constituent of myelinated axons. During white matter anoxia/ischemia or trauma, endogenous glutamate is released mainly from axoplasmic pools in a nonvesicular fashion through Na+-dependent glutamate transporters, stimulated to operate in the glutamate efflux mode by collapse of transmembrane ion gradients and depolarization. It appears that parallel mechanisms are triggered by injurious stimuli, involving reverse Na+-Ca2+ exchange and voltage-gated Ca2+ channels producing Ca2+ overload of the axon cylinder, whereas glutamate release with AMPA receptor overactivation causes Ca2+-dependent damage to the ensheathing myelin and sup-porting glia. The emerging complexity of white matter injury mechanisms requires a thorough understanding of the interrelated steps to optimize therapeutic design.

N-methyl-D-aspartate receptor-mediated axonal injury in adult rat corpus callosum

Damage to white matter such as corpus callosum (CC) is a pathological characteristic in many brain disorders. Glutamate (Glut) excitotoxicity through AMPA receptors on oligodendrocyte (OL) was previously considered as a mechanism for white matter damage. Recent studies have shown that N-methyl-D-aspartate receptors (NMDARs) are expressed on myelin sheath of neonatal rat OL processes and that activation of these receptors mediated demyelization. Whether NMDARs are expressed in the adult CC and are involved in excitotoxic axonal injury remains to be determined. In this study, we demonstrate the presence of NMDARs in the adult rat CC and their distributions in myelinated nerve fibers and OL somata by means of immunocytochemical staining and Western blot. Incubation of the CC slices with Glut or NMDA induced axonal injury as revealed by analyzing amplitude of CC fiber compound action potentials (CAPs) and input-output response. Both Glut and NMDA decreased the CAP amplitude and input-output responses, suggesting an involvement of NMDARs in Glut- and NMDA-induced axonal injury. The involvement of NMDAR in Glut-induced axonal injury was further assayed by detection of β-amyloid precursor protein (β-APP) in the CC axonal fibers. Treatment of the CC slices with Glut resulted in β-APP accumulation in the CC fibers as detected by Western blot, reflecting an impairment of axonal transport function. This injurious effect of Glut on CC axonal transport was significantly blocked by MK801. Taken together, these results show that NMDARs are expressed in the adult CC and are involved in excitotoxic activity in adult CC slices in vitro.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Understanding secondary injury

Secondary injury is a term applied to the destructive and self-propagating biological changes in cells and tissues that lead to their dysfunction or death over hours to weeks after the initial insult (the "primary injury"). In most contexts, the initial injury is usually mechanical. The more destructive phase of secondary injury is, however, more responsible for cell death and functional deficits. This subject is described and reviewed differently in the literature. To biomedical researchers, systemic and tissue-level changes such as hemorrhage, edema, and ischemia usually define this subject. To cell and molecular biologists, "secondary injury" refers to a series of predominately molecular events and an increasingly restricted set of aberrant biochemical pathways and products. These biochemical and ionic changes are seen to lead to death of the initially compromised cells and "healthy" cells nearby through necrosis or apoptosis. This latter process is called "bystander damage." These viewpoints have largely dominated the recent literature, especially in studies of the central nervous system (CNS), often without attempts to place the molecular events in the context of progressive systemic and tissue-level changes. Here we provide a more comprehensive and inclusive discussion of this topic.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

Dysmyelinated axons in shiverer mice are highly vulnerable to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated toxicity

Glutamate excitotoxicity plays a role in white matter injury in many neurological diseases. Oligodendrocytes in particular are highly vulnerable to excitotoxicity, mediated through activation of AMPA/kainate receptors. Myelin may also be injured independently via NMDA (N-methyl-D-aspartic acid) receptors located on peripheral oligodendroglial processes. Central axons are susceptible to glutamate receptor activation in vivo, but it is unclear whether this is mediated directly by activation of receptors expressed on axons, or indirectly through glutamate toxicity of myelin or neighboring glial cells. We examined axonal vulnerability in mice deficient in myelin basic protein (shiverer), also expressing yellow fluorescent protein (YFP) in a subset of axons. YFP fluorescence, EM, and mouse behavior were assessed 24 h after microstereotactical injections of S-AMPA or NMDA into lumbar dorsal columns. S-AMPA injection led to impaired rotarod performance and widespread axonal degeneration and was more pronounced in shiverer mice than controls. In contrast, NMDA injection did not cause axonal injury or behavioral changes in either group. These results indicate that spinal cord axons in vivo are vulnerable to toxicity mediated by AMPA but not NMDA receptors. The presence of compact myelin is not required for excitotoxic axon damage, and its absence may increase vulnerability. Further understanding of AMPA receptor-mediated axonal toxicity may provide new targets for neuroprotective therapy in WM diseases.

Glutamatergic signaling in the brain's white matter

Glutamatergic signaling has been exceptionally well characterized in the brain's gray matter, where it underlies fast information processing, learning and memory, and also generates the neuronal damage that occurs in pathological conditions such as stroke. The role of glutamatergic signaling in the white matter, an area until recently thought to be devoid of synapses, is less well understood. Here we review what is known, and highlight what is not known, of glutamatergic signaling in the white matter. We focus on how glutamate is released, the location and properties of the receptors it acts on, the interacting molecules that may regulate trafficking or signaling of the receptors, the possible functional roles of glutamate in the white matter, and its pathological effects including the possibility of treating white matter disorders with glutamate receptor blockers.

Neurotransmitter-mediated calcium signalling in oligodendrocyte physiology and pathology

The function of oligodendrocytes is to myelinate CNS axons. Oligodendrocytes and the axons they myelinate are functional units, and neurotransmitters released by axons can influence all stages of oligodendrocyte development via calcium dependent mechanisms. Some of the clearest functional evidence is for adenosine, ATP, and glutamate, which are released by electrically active axons and regulate the migration and proliferation of oligodendrocyte progenitor cells and their differentiation into myelinating oligodendrocytes. Glutamate and ATP, released by both axons and astrocytes, continue to mediate Ca(2+) signaling in mature oligodendrocytes, acting via AMPA and NMDA glutamate receptors, and heterogeneous P2X and P2Y purinoceptors. Physiological signalling between axons, astrocytes, and oligodendrocytes is likely to play an important role in myelin maintenance throughout life. Significantly, ATP- and glutamate-mediated Ca(2+) signaling are also major components of oligodendrocyte and myelin damage in numerous pathologies, most notably ischemia, injury, periventricular leukomalacia, and multiple sclerosis. In addition, NG2-expressing glia (synantocytes) in the adult CNS are highly reactive cells that respond rapidly to any CNS insult by a characteristic gliosis, and are able to regenerate oligodendrocytes and possibly neurons. Glutamate and ATP released by neurons and astrocytes evoke Ca(2+) signaling in NG2-glia (synantocytes), and it is proposed these regulate their differentiation capacity and response to injury. In summary, clear roles have been demonstrated for neurotransmitter-mediated Ca(2+) signaling in oligodendrocyte development and pathology. A key issue for future studies is to determine the physiological roles of neurotransmitters in mature oligodendrocytes and NG2-glia (synantocytes).

Neurotransmitter receptors in the life and death of oligodendrocytes

Oligodendrocytes are crucial to the function of the mammalian brain: they increase the action potential conduction speed for a given axon diameter and thus facilitate the rapid flow of information between different brain areas. The proliferation and differentiation of developing oligodendrocytes, and their myelination of axons, are partly controlled by neurotransmitters. In addition, in models of conditions like stroke, periventricular leukomalacia leading to cerebral palsy, spinal cord injury and multiple sclerosis, oligodendrocytes are damaged by glutamate and, contrary to dogma, it has recently been discovered that this damage is mediated in part by N-methyl-D-aspartate receptors. Mutations in oligodendrocyte neurotransmitter receptors or their interacting proteins may cause defects in CNS function. Here we review the roles of neurotransmitter receptors in the normal function, and malfunction in pathological conditions, of oligodendrocytes.

Ontogeny of AMPA and NMDA receptor gene expression in the developing sheep white matter and cerebral cortex

This study examined the hypothesis that the high prevalence of white matter injury in premature infants is associated with increased expression of calcium-permeable forms of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of glutamate receptors in pre-myelinating white matter. We characterized expression of subunits of the AMPA, and for reference, the N-methyl-d-aspartate (NMDA), glutamate receptors at 0.5, 0.65, 0.85, and term gestation in the ovine fetal white matter and cerebral cortex. There was a low expression of the critical calcium-impermeable AMPA receptor GluR2 subunit in subcortical white matter both absolutely and relative to other AMPA subunits throughout gestation. In contrast, GluR2 subunit mRNA expression fell in the cerebral cortex with increasing gestation whereas protein expression increased. These findings suggest a vulnerability of subcortical white matter to AMPA receptor-mediated calcium toxicity throughout the second half of gestation. Thus, the hypothesis that AMPA receptor-mediated glutamate toxicity contributes to brain damage in premature infants needs to be revised.

Topiramate reduces AMPA-induced Ca(2+) transients and inhibits GluR1 subunit phosphorylation in astrocytes from primary cultures

Topiramate (TPM) is a structurally novel broad spectrum anticonvulsant known to have a negative modulatory effect on the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate subtypes of glutamate receptors and some types of voltage-gated Na(+) and Ca(2+) channels, and a positive modulatory effect on some types of gamma-aminobutyric acid(A) (GABA(A)) receptors and at least one type of K(+) channels in neurons. In an earlier work, we showed that the negative modulatory effect of TPM (100 mum) on AMPA/kainate receptors in neurons is dependent on TPM modulation of the phosphorylation state of these receptors. In this work, we investigated the effect of TPM on AMPA-induced intracellular calcium ([Ca(2+)](i)) responses in cultured rat cortical astrocytes, with special interest in intracellular mechanisms. Here, we report that the ability of TPM (1-100 mum) to inhibit AMPA-induced accumulation of Ca(2+) in astrocytes is inversely related to the level of protein kinase A (PKA) -mediated phosphorylation of channels activated by AMPA. The level of receptor phosphorylation was further determined with western blot using phosphorylation specific antibodies that recognize the glutamate receptor 1 (GluR1) subunit phosphorylated on Ser845. These results demonstrated that, even in cultured cortical astrocytes, TPM significantly reduced the phophorylation level of GluR1 subunits. Furthermore, it was shown that TPM binds to AMPA receptors in the dephosphorylated state and thereby exerts an allosteric modulatory effect on the ion channel.

General mechanisms of axonal damage and its prevention

Axonal degeneration is a prominent pathological feature in multiple sclerosis observed over a century ago. The gradual loss of axons is thought to underlie irreversible clinical deficits in this disease. The precise mechanisms of axonopathy are poorly understood, but likely involve excess accumulation of Ca ions. In healthy fibers, ATP-dependent pumps support homeostasis of ionic gradients. When energy supply is limited, either due to inadequate delivery (e.g., ischemia, mitochondrial dysfunction) and/or excessive utilization (e.g., conduction along demyelinated axons), ion gradients break down, unleashing a variety of aberrant cascades, ultimately leading to Ca overload. During Na pump dysfunction, Na can enter axons through non-inactivating Na channels, promoting axonal Na overload and depolarization by allowing K egress. This will gate voltage-sensitive Ca channels and stimulate reverse Na-Ca exchange, leading to further Ca entry. Energy failure will also promote Ca release from intracellular stores. Neurotransmitters such as glutamate can be released by reverse operation of Na-dependent transporters, in turn activating a variety of ionotropic and metabotropic receptors, further exacerbating overload of cellular Ca. Together, this Ca overload will inappropriately stimulate a variety of Ca-dependent enzyme systems (e.g., calpains, phospholipases), leading to structural and functional axonal injury. Pharmacological interruption at key points in these interrelated injury cascades (e.g., at voltage-gated Na channels or AMPA receptors) may confer significant neuroprotection to compromised central axons and supporting glia. Such agents may represent attractive adjuncts to currently available immunomodulatory therapies.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Increased expression and function of glutamate transporters in multiple sclerosis

Recent studies have shown that glutamate excitotoxicity may be a component in the etiology of multiple sclerosis (MS). Glutamate transporters determine the levels of extracellular glutamate and are essential to prevent excitotoxicity. We have analyzed here the expression of the glutamate transporters EAAT1, EAAT2 and EAAT3 in control and in MS optic nerve samples. We observed an overall increase in the level of the glutamate transporters EAAT1 and EAAT2 mRNA and protein. In turn, functional assays showed that glutamate uptake was also increased in MS samples. Furthermore, glutamate transporter increases were mimicked in rat optic nerves treated with excitotoxic levels of glutamate. Together, these results indicate that enhanced expression of glutamate transporters in MS constitutes a regulatory response of glial cells to toxic levels of glutamate in the CNS during inflammation and neurodegeneration.

AMPA/kainate receptors mediate axonal morphological disruption in hypoxic white matter

We used acute brain slices to investigate the hypothesis that oxygen-glucose deprivation (OGD) induced loss of axon function and neurofilament labeling are correlated to axonal morphological disruption in the corpus callosum of adult brain. Coronal brain slices including corpus callosum were prepared from adult mice. White matter immunohistochemical properties and conduction along axons remained stable over 12 h after preparation. White matter injury was assessed by recording compound action potentials (CAPs) across corpus callosum, combined with immunofluorescence for axonal neurofilaments and by bright field microscopy of myelin profiles in semi-thin sections. OGD for 30 min resulted in irreversible loss of the CAPs, formation of axon heads and bulbs, and swelling of myelin profiles in slices examined 1h after OGD. In slices followed for 9 h after OGD, there was complete loss of neurofilament labeling and myelin profiles. Because overactivation of AMPA/kainate receptors mediates axon structural and functional disruption in hypoxic corpus callosum slices, we tested whether blockade of AMPA/kainate receptors reduced OGD-induced axonal morphological disruption. NBQX (30 microM), an AMPA/kainate receptor antagonist, prevented OGD-induced formation of axon heads and bulbs, swelling of myelin profiles, loss of neurofilament staining and preserved axonal morphology. These results expand our previous findings that the AMPA/kainate receptor activation contributes to axonal morphological disruption, as well as loss of electrical function.

Astrocytosis, microglia activation, oligodendrocyte degeneration, and pyknosis following acute spinal cord injury

Glial activation and degeneration are important outcomes in the pathophysiology of acute brain and spinal cord injury (SCI). Our main goal was to investigate the pattern of glial activation and degeneration during secondary degeneration in both gray matter (GM) and white matter (WM) following SCI. Adult rats were deeply anesthetized and injected with 20 nmol of N-methyl-D-aspartate (NMDA) into the ventral horn of rat spinal cord (SC) on T7. Animals were perfused after survival times of 1, 3, and 7 days. Ten-micrometer sections were submitted to immunocytochemistry for activated macrophages/microglia, astrocytes, oligodendrocytes, and myelin. Astrocyte activation was more intense in the vacuolated white matter than in gray matter and was first noticed in this former region. Microglial activation was more intense in the gray matter and was clear by 24 h following NMDA injection. Both astrocytosis and microglial activation were more intense in the later survival times. Conspicuous WM vacuolation was present mainly at the 3-day survival time and decreased by 7 days after the primary damage. Quantitative analysis revealed an increase in the number of pyknotic bodies mainly at the 7-day survival time in both ventral and lateral white matter. These pyknotic bodies were frequently found inside white matter vacuoles like for degenerating oligodendrocytes. These results suggest a differential pattern of astrocytosis and microglia activation for white and gray matter following SCI. This phenomenon can be related to the different pathological outcomes for this two SC regions following acute injury.

Excitotoxic insults to the optic nerve alter visual evoked potentials

Excitotoxic oligodendroglial death is one of the mechanisms which has been proposed to underlie demyelinating diseases of the CNS. We describe here functional consequences of excitotoxic lesions to the rabbit optic nerve by studying the visual evoked potentials (VEPs) measured in the visual cortex. Nerves were slowly infused with the excitotoxin kainate a subcutaneously implanted osmotic pump which delivered the toxin through a cannula onto the optic nerve. Records of VEPs were obtained before pump implantation and at 1, 3 and 7 days post-implantation, and weekly evaluated thereafter for up to 4 months. We observed that the VEPs generated by light stimuli progressively changed in both amplitude and profile after the lesion as well as in comparison to those generated in control animals infused with vehicle. Histological examination of the damage caused by the excitotoxic insult showed that large areas of the optic nerve were demyelinated and their axons distorted. These observations were confirmed and extended by immunohistochemical analyses using markers to neurofilaments, myelin basic protein and the oligodendrocyte marker APC. The results of the present paper indicate that the consequences of excitotoxicity in the optic nerve share functional and morphological alterations which are found in demyelinating disorders. In addition, this experimental paradigm may be useful to evaluate the functional recovery of demyelinated optic nerves following various repair strategies.

Caspase-dependent and caspase-independent oligodendrocyte death mediated by AMPA and kainate receptors

Oligodendrocytes are vulnerable to excitotoxic signals mediated by AMPA receptors and by high- and low-affinity kainate receptors. Here we investigated the nature of the cell death triggered by activation of these receptors in primary cultures of oligodendrocytes from the rat optic nerve. Activation of AMPA receptors at both submaximal and maximal concentrations of the agonist induced massive calcium entry, mitochondrial depolarization, and a rise in the level of reactive oxygen species that correlated with a decrease in the levels of reduced glutathione. In addition, excitotoxicity initiated by submaximal, but not maximal, activation of AMPA receptors was prevented by caspase-3 blockade and by the concomitant blockade of caspases 8 and 9. In turn, maximal activation of high- or low-affinity kainate receptors induced mitochondrial events and toxicity levels similar to those observed with submaximal activation of AMPA receptors. In contrast to AMPA receptor-mediated insults, calcineurin inhibition or caspase-9 blockade was sufficient to prevent cell death triggered by both types of kainate receptors. Consistent with these results, prolonged glutamate receptor activation in freshly isolated optic nerves caused selective activation of caspase-3 and chromatin condensation in oligodendrocytes. Overall, the evidence presented here indicates that oligodendrocyte death by excitotoxicity is mediated by caspase-dependent and -independent mechanisms.

Temporal and spatial distribution of activated caspase-3 after subdural kainic acid infusions in rat spinal cord

The molecular events initiating apoptosis following traumatic spinal cord injury (SCI) remain poorly understood. Soon after injury, the spinal cord is exposed to numerous secondary insults, including elevated levels of glutamate, that contribute to cell dysfunction and death. In the present study, we attempted to mimic the actions of glutamate by subdural infusion of the selective glutamate receptor agonist, kainic acid, into the uninjured rat spinal cord. Immunohistochemical colocalization studies revealed that activated caspase-3 was present in ventral horn motor neurons at 24 hours, but not 4 hours or 96 hours, following kainic acid treatment. However, at no time point examined was there evidence of significant neuronal loss. Kainic acid resulted in caspase-3 activation in several glial cell populations at all time points examined, with the most pronounced effect occurring at 24 hours following infusion. In particular, caspase-3 activation was observed in a significant number of oligodendroglia in the dorsal and ventral funiculi, and there was a pronounced loss of oligodendroglia at 96 hours following treatment. The results of these experiments indicate a role for glutamate as a mediator of oligodendroglial apoptosis in traumatic SCI. In addition, understanding the apoptotic signaling events activated by glutamate will be important for developing therapies targeting this cell death process.

(+/-)-Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate receptor subunit expression in mouse versus rat spinal cord white matter: similarities in astrocytes but differences in oligodendrocytes

Our recent study on ionotropic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate (KA) type glutamate receptors in mouse spinal cord revealed differences compared with reports on other species. Inconsistencies in such findings may reflect real inter-species variability or differences in technique. To date, no comprehensive study has directly addressed this issue. Thus, we compared, side-by-side, AMPA/KA receptor expression in mouse and rat spinal cord white matter. Tissue from both species underwent identical processing, using the same reagents, immunofluorescence protocol, and confocal microscopy. Both mouse and rat white matter astrocytes displayed similar AMPA/KA subunit profiles (both species, GluR2/3 and GluR6/7). However, substantial inter-species differences were found for oligodendrocytes (KA2 in mouse; GluR4 and GluR2/3 in rat). Knowledge of such differences can aid in understanding the effects of variably selective AMPA/KA antagonists on glutamate excitotoxicity in white matter.

Low editing efficiency of GluR2 mRNA is associated with a low relative abundance of ADAR2 mRNA in white matter of normal human brain

The ionotropic glutamate receptor (GluR) subunits GluR2, GluR5 and GluR6 are subject to RNA editing at their Q/R sites, resulting in significant alterations in the channel properties of the receptors. RNA editing at the Q/R site of GluRs is both developmentally and regionally regulated. Here we provide the first quantitative measurements of both mRNAs of the GluR subunits and mRNAs of the RNA editing enzymes ADAR1-ADAR3 in a comparison of the efficiency of editing at the Q/R site with the expression levels of ADAR mRNA in human brain. We demonstrate that the Q/R site of GluRs in white matter is edited significantly less than in grey matter. In addition, by means of quantitative reverse transcription-polymerase chain reaction methods, we demonstrate that the relative abundance of ADAR2 mRNA to GluR2 mRNA is significantly lower in white matter than in grey matter and that the GluR2 Q/R site editing decreased only when the ratio of ADAR2 mRNA (not that of ADAR1 mRNA) to GluR2 mRNA dropped below a threshold (20 x 10(-3)). These results suggest that Q/R site of GluRs editing is regulated in a regional, and hence presumably cell-specific, manner and that the GluR2 Q/R site editing is critically regulated by ADAR2 in human brain.

Estrogen attenuates oxidative stress-induced apoptosis in C6 glial cells

We examined the mechanism of 17beta-estradiol (estrogen)-mediated inhibition of apoptosis in C6 (rat glioma) cells following exposure to hydrogen peroxide (H(2)O(2)). Cells were preincubated with 4 microM estrogen for 2 h and then exposed to 100 microM H(2)O(2) for 24 h. Exposure to H(2)O(2) caused significant increases in intracellular calcium (Ca(2+)), as determined by fura-2, which was attenuated by preincubation with estrogen. H(2)O(2) and ionomycin caused cell death in a dose-dependent manner, as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Preincubation with estrogen restored viability in cells exposed to H(2)O(2) but not in cells exposed to ionomycin. Western blot analysis showed an increase in Bax/Bcl-2 ratio, calpain activity, and caspase-3 activity following treatment with H(2)O(2), and estrogen pretreatment decreased levels of all three. Cell morphology, as evaluated by Wright staining, indicated apoptosis in cells treated with H(2)O(2), and pretreatment with estrogen reduced apoptosis. Results from MTT and Wright staining were further supported by the terminal deoxyribonucleotidyl transferase (TdT)-mediated dUTP Nick End Labeling (TUNEL) assay. These results indicate a role for estrogen in preventing apoptosis in C6 glial cells exposed to H(2)O(2). Our results suggest that estrogen may have a protective role in minimizing glial cell apoptosis in neurological diseases such as demyelinating disease or central nervous system trauma.

AMPA/kainate receptors in mouse spinal cord cell-specific display of receptor subunits by oligodendrocytes and astrocytes and at the nodes of Ranvier

Spinal cord white matter is susceptible to AMPA/kainate (KA)-type glutamate receptor-mediated excitotoxicity. To understand this vulnerability, it is important to characterize the distribution of AMPA/KA receptor subunits in this tissue. Using immunohistochemistry and laser confocal microscopy, we studied the expression sites of AMPA/KA receptor subunits in mouse spinal cord. The white matter showed consistent immunoreactivity for AMPA receptor subunit GluR2/3 and KA receptor subunits GluR6/7 and KA2. In contrast, antibodies against GluR1, GluR2, GluR4 (AMPA), and GluR5 (KA) subunits showed only weak and occasional labeling of white matter. However, gray matter neurons did express GluR1 and GluR2, as well as GluR2/3. The white matter astrocytes were GluR2/3 and GluR6/7 immunopositive, while the gray matter astrocytes displayed primarily GluR6/7. Both exclusively and abundantly, KA2 labeled oligodendrocytes and myelin, identified by CNPase expression. Interestingly, myelin basic protein, another myelin marker, showed less correlation with KA2 expression, placing KA2 at specific CNPase-containing subdomains. Focal points of dense KA2 labeling showed colocalization with limited, but distinct, axonal regions. These regions were identified as nodes of Ranvier by coexpressing the nodal marker, ankyrin G. Overall, axonal tracts showed little, if any, AMPA/KA receptor expression. The proximity of oligodendrocytic KA2 to the axonal node and the paucity of axonal AMPA/kainate receptor expression suggest that excitotoxic axonal damage may be secondary and, possibly, mediated by oligodendrocytes. Our data demonstrate differential expression of glutamate AMPA and KA receptor subunits in mouse spinal cord white matter and point to astrocytes and oligodendrocytes as potential targets for pharmacological intervention in white matter glutamate excitotoxicity.

Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Glutamate causes excitotoxicity via non-N-methyl-D-aspartate (NMDA) glutamate receptors (GluR) in oligodendrocytes. Because both oligodendrocytes and type 2 astrocytes are differentiated from oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, we investigated whether astrocytes are also vulnerable to non-NMDA GluR-mediated excitotoxicity. For these studies, oligodendrocyte-like cells (OLC) and type 2 astrocyte-like cells (2ALC) were derived from CG-4 cells, an immortalized rat O-2A progenitor cell line. About 50% of 2ALC were positive for glial fibrillary acidic protein and 90% were positive for A2B5, verifying that these cells have an type 2 astrocytic phenotype. A 24-hr exposure of OLC to 2 mM kainate, an activator of non-NMDA GluR, caused cell damage as shown by the release of lactate dehydrogenase. The extent of kainate-induced OLC damage was increased by cyclothiazide. In contrast, exposure of 2ALC to 2 mM kainate alone did not induce injury, though mild 2ALC injury was elicited by exposure to 2 mM kainate plus 100 microM cyclothiazide. Furthermore, we found that the kainate induced Ca(2+) uptake by 2ALC was 27.5% of that induced by kainate in OLC. Finally, both OLC and 2ALC expressed non-NMDA GluR subunit mRNAs, including GluR2, GluR3, GluR4, GluR6, GluR7, KA1, and KA2, but quantitative Western blot analysis revealed higher immunodetectable GluR2 and lower immunodetectable GluR3 and GluR4 in 2ALC than in OLC. Together, these results suggest that astrocytes are relatively resistant to non-NMDA GluR-mediated excitotoxicity because they have a higher expression of GluR2 and lower expression of GluR3 and GluR4.

Both MK801 and NBQX reduce the neuronal damage after impact-acceleration brain injury

To understand the pathogenesis of diffuse axonal injury, we investigated the temporal and spatial profiles of neuronal degeneration in impact-acceleration injury in rats using Fluoro-Jade (FJ) staining. Impact-acceleration injury was produced in Wistar rats by the method described by Marmarou et al. with some modifications. Animals were sacrificed 1, 2, 7, 14, or 28 days after injury. Paraffin-embedded coronal sections were stained with HE or FJ, or analyzed immunohistochemically for GFAP or amyloid precursor protein (APP). FJ-positive degenerative neurons were found primarily in the dorsal brainstem and thalamus from 1 to 2 days following injury and these were associated with GFAP expression. However, FJ-positive cells were rarely found after 7 days. In all rats, significant expression of APP was observed primarily in the cingulum, cerebral peduncle and pontomedullary junction. FJ also stained these injured axons. Intrathecal administration of both NMDA and AMPA/kinate glutamate receptor antagonists MK-801 and NBQX, respectively, reduced the neuronal injury. NBQX showed more significant effects on axonal injury than MK-801. These observations indicate that not only axonal damage, but also primary neuronal damage occurs in this impact-acceleration injury model. It is also suggested that NBQX can act both directly on neuronal cells and white matter and that NMDA could have a significant protective effect against not only neuronal, but also axonal injury.

GluR2-R4 AMPA subunit study in rat vestibular nuclei after unilateral labyrinthectomy: an in situ and immunohistochemical study

In the present investigation, we address the question of whether the expression of GluR2-R4 subunits mRNAs and GluR2 and GluR4 subunits protein of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptors are modulated in the vestibular nuclei following unilateral labyrinthectomy. Specific GluR2-R4 radioactive oligonucleotides were used to probe sections of rat vestibular nuclei according to in situ hybridization methods. The signal was detected by means of film or emulsion photography. GluR2 and GluR4 subunit expression were also measured in control and operated rats by use of specific monoclonal GluR2 and GluR4 antibodies. Animals were killed at different stages following the lesion: 1, 3 or 8 days for the in situ hybridization study and 4 and 8 days for the immunohistochemical study. In normal animals, several brainstem regions including the lateral, medial, superior and inferior vestibular nuclei expressed all the GluR2, GluR3 and GluR4 subunit mRNAs. Moreover, numerous vestibular nuclei neurons are endowed with AMPA receptors containing the GluR2 and the GluR4 subunits. In unilaterally labyrinthectomized rats, no asymmetry could be detected on autoradiographs between the two medial vestibular nuclei probed with the GluR2 and the GluR4 oligonucleotide probes regardless of the delay following the lesion. However, compared to control, a bilateral decrease (-22%) in GluR3 gene expression was observed in the medial vestibular nuclei 3 days after the lesion followed by a return to normal at day 8 post-lesion. No significant asymmetrical changes in the density of GluR2- and GluR4-immunopositive cells could be detected between the intact and deafferented sides in any part of the vestibular nuclear complex and at any times (day 4 or day 8) following the lesion. Our data show that the removal of glutamatergic vestibular input induced an absence of modulation of GluR2 and GluR4 gene and subunits expression. This demonstrates that GluR2 and GluR4 expression do not play a role in the recovery of the resting discharge of the deafferented medial vestibular nuclei neurons and consequently in the functional restoration of the static postural and oculomotor deficits. The functional role of the slight and bilateral GluR3 mRNA decrease in the vestibular nuclei remains to be elucidated.

Ampa/kainate receptor activation mediates hypoxic oligodendrocyte death and axonal injury in cerebral white matter

We developed an in situ model to investigate the hypothesis that AMPA/kainate (AMPA/KA) receptor activation contributes to hypoxic-ischemic white matter injury in the adult brain. Acute coronal brain slices, including corpus callosum, were prepared from adult mice. After exposure to transient oxygen and glucose deprivation (OGD), white matter injury was assessed by electrophysiology and immunofluorescence for oligodendrocytes and axonal neurofilaments. White matter cellular components and the stimulus-evoked compound action potential (CAP) remained stable for 12 hr after preparation. OGD for 30 min resulted in an irreversible loss of the CAP as well as structural disruption of axons and subsequent loss of neurofilament immunofluorescence. OGD also caused widespread oligodendrocyte death, demonstrated by the loss of APC labeling and the gain of pyknotic nuclear morphology and propidium iodide labeling. Blockade of AMPA/KA receptors with 30 microm NBQX or the AMPA-selective antagonist 30 microm GYKI 52466 prevented OGD-induced oligodendrocyte death. Oligodendrocytes also were preserved by the removal of Ca(2+), but not by a blockade of voltage-gated Na(+) channels. The protective action of NBQX was still present in isolated corpus callosum slices. CAP areas and axonal structure were preserved by Ca(2+) removal and partially protected by a blockade of voltage-gated Na(+) channels. NBQX prevented OGD-induced CAP loss and preserved axonal structure. These observations highlight convergent pathways leading to hypoxic-ischemic damage of cerebral white matter. In accordance with previous suggestions, the activation of voltage-gated Na(+) channels contributes to axonal damage. Overactivation of glial AMPA/KA receptors leads to oligodendrocyte death and also plays an important role in structural and functional disruption of axons.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Mechanisms of ionotropic glutamate receptor-mediated excitotoxicity in isolated spinal cord white matter

Spinal cord injury involves a component of glutamate-mediated white matter damage, but the cellular targets, receptors, and ions involved are poorly understood. Mechanisms of excitotoxicity were examined in an in vitro model of isolated spinal dorsal columns. Compound action potentials (CAPs) were irreversibly reduced to 43% of control after 3 hr of 1 mM glutamate exposure at 37 degrees C. AMPA (100 microM) and kainate (500 microM) had similar effects. Antagonists (1 mM kynurenic acid, 10 microM NBQX, 30 microM GYKI52466) were each equally protective against a glutamate challenge, improving mean CAP amplitude to approximately 80% versus approximately 40% without antagonist. Joro spider toxin (0.75 microM), a selective blocker of Ca(2+)-permeable AMPA receptors, was also protective to a similar degree. Ca(2+)-free perfusate virtually abolished glutamate-induced injury ( approximately 90% vs approximately 40%). MK-801 (10 microM) had no effect. Glutamate caused damage (assayed immunohistochemically by spectrin breakdown products) to astrocytes and oligodendrocytes consistent with the presence of GluR2/3 and GluR4 in these cells. Myelin was also damaged by glutamate likely mediated by GluR4 receptors detected in this region; however, axon cylinders were unaffected by glutamate, showing no increase in the level of spectrin breakdown. These data may guide the development of more effective treatment for acute spinal cord injury by addressing the additional excitotoxic component of spinal white matter damage.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

AMPA and kainate receptors each mediate excitotoxicity in oligodendroglial cultures

Recent studies indicate that oligodendrocytes are vulnerable to excitotoxic insults mediated by glutamate receptors. The present study was carried out to characterize the type of glutamate receptors triggering cell death in optic nerve oligodendrocyte cultures. Acute activation of either AMPA or kainate receptors was toxic to oligodendrocytes, an effect that was prevented by CNQX. However, exposure to agonists of the NMDA and metabotropic glutamate receptors did not impair cell viability. Dose-response curves showed that toxicity was mediated by three distinct populations of receptors: an AMPA-type receptor and high- and low-affinity kainate-type receptors. Expression and immunocytochemical studies suggested that the glutamate receptor subunits give rise to the native receptors in each population. In all instances, Ca(2+) entry was a major determinant of glutamate receptor excitotoxicity. However, its influence varied for each receptor subtype. These results indicate that aberrantly enhanced activation of AMPA and/or kainate receptors may be involved in demyelinating diseases.

Expression of glutamate transporters in the adult bovine corpus callosum

The presence and distribution of the glutamate transporters, EAAC1, GLAST and GLT-1, were examined in the adult bovine corpus callosum by means of Western blotting and immunohistochemistry. We found GLAST to be expressed in oligodendrocytes and in some astrocytes. GLT-1 was located throughout the processes of fibrous astrocytes. In addition, EAAC1 was present in a small population of cells that are probably adult oligodendrocyte progenitor cells.

2,3-Dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline reduces glial loss and acute white matter pathology after experimental spinal cord contusion

Focal microinjection of 2, 3-dihyro-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX), an antagonist of the AMPA/kainate subclass of glutamate receptors, reduces neurological deficits and tissue loss after spinal cord injury. Dose-dependent sparing of white matter is seen at 1 month after injury that is correlated to the dose-related reduction in chronic functional deficits. To determine whether NBQX exerts an acute effect on white matter pathology, female, adult Spague Dawley rats were subjected to a standardized weight drop contusion at T-8 (10 gm x 2.5 cm) and NBQX (15 nmol) or vehicle (VEH) solution focally injected into the injury site 15 min later. At 4 and 24 hr, tissue from the injury epicenter was processed for light and electron microscopy, and the histopathology of ventromedial white matter was compared. The axonal injury index, a quantitative representation of axoplasmic and myelinic pathologies, was significantly lower in the NBQX group at 4 hr (2.7 +/- 0.24, mean +/- SE) and 24 hr (1.4 +/- 0.19) than in VEH controls (3.8 +/- 0.33 and 2.1 +/- 0.20, respectively). Counts of glial cell nuclei indicated a loss of at least 60% at 4 and 24 hr after injury in the VEH group compared with uninjured controls. NBQX treatment reduced this glial loss by half. Immunohistochemistry revealed that the spared glia were primarily oligodendrocytes. Thus, the chronic effects of NBQX in reducing white matter loss after spinal cord injury appear to be attributable to the reduction of acute pathology and may be mediated through the protection of glia, particularly oligodendrocytes.

Characteristics of acute and chronic kainate excitotoxic damage to the optic nerve

Macroglial cells express ionotropic glutamate receptors. In the adult optic nerve, reverse transcription-PCR showed that the native receptors are formed by subunits belonging to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate classes. Because activation of AMPA and kainate receptors can be toxic to oligodendrocytes in vitro, I examined the nature of the damage caused by kainate, an agonist of both receptor classes, applied directly onto the optic nerve. Acute infusion or chronic slow delivery of the agonist caused massive nerve damage that was more extensive in the latter. Interestingly, chronic delivery also produced inflammation and demyelination in well circumscribed areas of the nerve, together with other pathological features that closely resemble those observed in multiple sclerosis patients. Acute and chronic kainate lesions were both prevented by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. However, GYKI53655, a specific AMPA receptor antagonist, did not significantly reduce the size of the lesion, suggesting that the kainate toxicity was mainly mediated through activation of kainate-preferring glutamate receptors. These observations suggest that alterations in glutamate signaling may be detrimental to oligodendrocytes and may be involved in the pathogenesis of multiple sclerosis and other demyelinating diseases.