Pharmacological characterization of the glutamate receptor in cultured astrocytes

Ca2+ waves in astrocytes: computational modeling and experimental data

This paper examines different computational models for Calcium wave propagation in astrocytes. Through a comparative analysis of models by Goldbeter, De Young-Keizer, Atri, Li-Rinzel, and De Pittà and of experimental data, the study highlights the model contributions for the understanding of Calcium dynamics. Tracing the evolution from simple to complex models, this work emphasizes the importance of integrating experimental data in order to further refine these models. The results allow to improve our understanding of the physiological functions of astrocytes, suggesting the importance of more accurate astrocyte models.

Differential changes in Wnt7 and Dkk1 levels in astrocytes exposed to glutamate or TNFα

Wnt signaling plays an important role in adult brain function, and its dysregulation has been implicated in the loss of neuronal homeostasis. Despite the existence of many studies on the participation of the Wnt pathway in adult neurons, its regulation in astrocytes has been scarcely explored. Several reports point to the presence of Wnt ligands in astrocytes and their possible impact on neuronal plasticity or neuronal death. We aimed to analyze the effect of the neurotransmitter glutamate and the inflammatory cytokine TNFα on the mRNA and protein levels of the canonical Wnt agonist Wnt7a and the antagonist Dkk1 in cultured astrocytes. Primary astrocyte cultures from rat cerebral cortices were exposed to glutamate or TNFα. Wnt7a and Dkk1 expression was analyzed by RT-qPCR and its protein abundance and distribution was assessed by immunofluorescence. We found high basal expression and protein levels of Wnt7a and Dkk1 in unstimulated astrocytes and overproduction of Dkk1 mRNA induced by the two stimuli. These results reveal the astrocytic source of the canonical Wnt ligands Wnt7a and Dkk1, whose levels are differentially regulated by glutamate and TNFα. Astrocytes are a significant source of Wnt ligands, the production of which can be differentially regulated under excitatory or proinflammatory conditions, thereby impacting neuronal function.

Neuron-Glia Interactions and Brain Circuits

Recent evidence suggests that glial cells take an active role in a number of brain functions that were previously attributed solely to neurons. For example, astrocytes, one type of glial cells, have been shown to promote coordinated activation of neuronal networks, modulate sensory-evoked neuronal network activity, and influence brain state transitions during development. This reinforces the idea that astrocytes not only provide the "housekeeping" for the neurons, but that they also play a vital role in supporting and expanding the functions of brain circuits and networks. Despite this accumulated knowledge, the field of computational neuroscience has mostly focused on modeling neuronal functions, ignoring the glial cells and the interactions they have with the neurons. In this chapter, we introduce the biology of neuron-glia interactions, summarize the existing computational models and tools, and emphasize the glial properties that may be important in modeling brain functions in the future.

Hemisynapse Formation Between Target Astrocytes and Cortical Neuron Axons in vitro

One of the most fundamental organizing principles in the mammalian brain is that neurons do not establish synapses with the other major cell type, the astrocytes. However, induced synapse formation between neurons and astrocytes appears conceivable, because astrocytes are well known to express functional ionotropic glutamate receptors. Here, we attempted to trigger synapse formation between co-cultured neurons and astrocytes by overexpressing the strongly synaptogenic adhesion protein LRRTM2 in astrocytes physically contacted by cortical axons. Interestingly, control experiments with immature cortical astrocytes without any overexpression resulted in the induction of synaptic vesicle clustering in contacting axons (hemisynapse formation). This synaptogenic activity correlated with the endogenous expression of the synaptogenic protein Neuroligin1. Hemisynapse formation was further enhanced upon overexpression of LRRTM2 in cortical astrocytes. In contrast, cerebellar astrocytes required overexpression of LRRTM2 for induction of synaptic vesicle clustering in contacting axons. We further addressed, whether hemisynapse formation was accompanied by the appearance of fully functional glutamatergic synapses. We therefore attempted to record AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) in innervated astrocytes using the whole-cell patch-clamp technique. Despite the endogenous expression of the AMPA receptor subunits GluA2 and to a lesser extent GluA1, we did not reliably observe spontaneous AMPA mEPSCs. In conclusion, overexpression of the synaptogenic protein LRRTM2 induced hemisynapse formation between co-cultured neurons and astrocytes. However, the formation of fully functional synapses appeared to require additional factors critical for nano-alignment of presynaptic vesicles and postsynaptic receptors.

NMDA Receptors in Astrocytes: In Search for Roles in Neurotransmission and Astrocytic Homeostasis

Studies of the last two decades have demonstrated the presence in astrocytic cell membranes of N-methyl-d-aspartate (NMDA) receptors (NMDARs), albeit their apparently low abundance makes demonstration of their presence and function more difficult than of other glutamate (Glu) receptor classes residing in astrocytes. Activation of astrocytic NMDARs directly in brain slices and in acutely isolated or cultured astrocytes evokes intracellular calcium increase, by mutually unexclusive ionotropic and metabotropic mechanisms. However, other than one report on the contribution of astrocyte-located NMDARs to astrocyte-dependent modulation of presynaptic strength in the hippocampus, there is no sound evidence for the significant role of astrocytic NMDARs in astrocytic-neuronal interaction in neurotransmission, as yet. Durable exposure of astrocytic and neuronal co-cultures to NMDA has been reported to upregulate astrocytic synthesis of glutathione, and in this way to increase the antioxidative capacity of neurons. On the other hand, overexposure to NMDA decreases, by an as yet unknown mechanism, the ability of cultured astrocytes to express glutamine synthetase (GS), aquaporin-4 (AQP4), and the inward rectifying potassium channel Kir4.1, the three astroglia-specific proteins critical for homeostatic function of astrocytes. The beneficial or detrimental effects of astrocytic NMDAR stimulation revealed in the in vitro studies remain to be proven in the in vivo setting.

Computational Models for Calcium-Mediated Astrocyte Functions

The computational neuroscience field has heavily concentrated on the modeling of neuronal functions, largely ignoring other brain cells, including one type of glial cell, the astrocytes. Despite the short history of modeling astrocytic functions, we were delighted about the hundreds of models developed so far to study the role of astrocytes, most often in calcium dynamics, synchronization, information transfer, and plasticity in vitro, but also in vascular events, hyperexcitability, and homeostasis. Our goal here is to present the state-of-the-art in computational modeling of astrocytes in order to facilitate better understanding of the functions and dynamics of astrocytes in the brain. Due to the large number of models, we concentrated on a hundred models that include biophysical descriptions for calcium signaling and dynamics in astrocytes. We categorized the models into four groups: single astrocyte models, astrocyte network models, neuron-astrocyte synapse models, and neuron-astrocyte network models to ease their use in future modeling projects. We characterized the models based on which earlier models were used for building the models and which type of biological entities were described in the astrocyte models. Features of the models were compared and contrasted so that similarities and differences were more readily apparent. We discovered that most of the models were basically generated from a small set of previously published models with small variations. However, neither citations to all the previous models with similar core structure nor explanations of what was built on top of the previous models were provided, which made it possible, in some cases, to have the same models published several times without an explicit intention to make new predictions about the roles of astrocytes in brain functions. Furthermore, only a few of the models are available online which makes it difficult to reproduce the simulation results and further develop the models. Thus, we would like to emphasize that only via reproducible research are we able to build better computational models for astrocytes, which truly advance science. Our study is the first to characterize in detail the biophysical and biochemical mechanisms that have been modeled for astrocytes.

A Cytotoxic, Co-operative Interaction Between Energy Deprivation and Glutamate Release From System xc- Mediates Aglycemic Neuronal Cell Death

The astrocyte cystine/glutamate antiporter (system xc(-)) contributes substantially to the excitotoxic neuronal cell death facilitated by glucose deprivation. The purpose of this study was to determine the mechanism by which this occurred. Using pure astrocyte cultures, as well as, mixed cortical cell cultures containing both neurons and astrocytes, we found that neither an enhancement in system xc(-) expression nor activity underlies the excitotoxic effects of aglycemia. In addition, using three separate bioassays, we demonstrate no change in the ability of glucose-deprived astrocytes--either cultured alone or with neurons--to remove glutamate from the extracellular space. Instead, we demonstrate that glucose-deprived cultures are 2 to 3 times more sensitive to the killing effects of glutamate or N-methyl-D-aspartate when compared with their glucose-containing controls. Hence, our results are consistent with the weak excitotoxic hypothesis such that a bioenergetic deficiency, which is measureable in our mixed but not astrocyte cultures, allows normally innocuous concentrations of glutamate to become excitotoxic. Adding to the burgeoning literature detailing the contribution of astrocytes to neuronal injury, we conclude that under our experimental paradigm, a cytotoxic, co-operative interaction between energy deprivation and glutamate release from astrocyte system xc(-) mediates aglycemic neuronal cell death.

Astrocyte elevated gene-1 is a novel modulator of HIV-1-associated neuroinflammation via regulation of nuclear factor-κB signaling and excitatory amino acid transporter-2 repression

Astrocyte elevated gene-1 (AEG-1), a novel human immunodeficiency virus (HIV)-1 and tumor necrosis factor (TNF)-α-inducible oncogene, has generated significant interest in the field of cancer research as a therapeutic target for many metastatic aggressive tumors. However, little is known about its role in astrocyte responses during HIV-1 central nervous system (CNS) infection and whether it contributes toward the development of HIV-associated neurocognitive disorders (HAND). Therefore, in this study, we investigated changes in AEG-1 CNS expression in HIV-1-infected brain tissues and elucidated a potential mechanism of AEG-1-mediated regulation of HAND. Immunoblotting and immunohistochemical analyses of HIV-1 seropositive and HIV-1 encephalitic human brain tissues revealed significantly elevated levels of AEG-1 protein. Immunohistochemical analyses of HIV-1 Tat transgenic mouse brain tissues also showed a marked increase in AEG-1 staining. Similar to in vivo observations, cultured astrocytes expressing HIV-1 Tat also revealed AEG-1 and cytokine up-regulation. Astrocytes treated with HAND-relevant stimuli, TNF-α, interleukin (IL)-1β, and HIV-1, also significantly induced AEG-1 expression and nuclear translocation via activation of the nuclear factor (NF)-κB pathway. Co-immunoprecipitation studies demonstrated IL-1β- or TNF-α-induced AEG-1 interaction with NF-κB p65 subunit. AEG-1 knockdown decreased NF-κB activation, nuclear translocation, and transcriptional output in TNF-α-treated astrocytes. Moreover, IL-1β treatment of AEG-1-overexpressing astrocytes significantly lowered expression of excitatory amino acid transporter 2, increased expression of excitatory amino acid transporter 2 repressor ying yang 1, and reduced glutamate clearance, a major transducer of excitotoxic neuronal damage. Findings from this study identify a novel transcriptional co-factor function of AEG-1 and further implicate AEG-1 in HAND-associated neuroinflammation.

NMDA Receptors in Glial Cells: Pending Questions

Glutamate receptors of the N-methyl-D-aspartate (NMDA) type are involved in many cognitive processes, including behavior, learning and synaptic plasticity. For a long time NMDA receptors were thought to be the privileged domain of neurons; however, discoveries of the last 25 years have demonstrated their active role in glial cells as well. Despite the large number of studies in the field, there are many unresolved questions connected with NMDA receptors in glia that are still a matter of debate. The main objective of this review is to shed light on these controversies by summarizing results from all relevant works concerning astrocytes, oligodendrocytes and polydendrocytes (also known as NG2 glial cells) in experimental animals, further extended by studies performed on human glia. The results are divided according to the study approach to enable a better comparison of how findings obtained at the mRNA level correspond with protein expression or functionality. Furthermore, special attention is focused on the NMDA receptor subunits present in the particular glial cell types, which give them special characteristics different from those of neurons – for example, the absence of Mg²⁺ block and decreased Ca²⁺ permeability. Since glial cells are implicated in important physiological and pathophysiological roles in the central nervous system (CNS), the last part of this review provides an overview of glial NMDA receptors with respect to ischemic brain injury.

Non-cell autonomous influence of the astrocyte system xc- on hypoglycaemic neuronal cell death

Despite longstanding evidence that hypoglycaemic neuronal injury is mediated by glutamate excitotoxicity, the cellular and molecular mechanisms involved remain incompletely defined. Here, we demonstrate that the excitotoxic neuronal death that follows GD (glucose deprivation) is initiated by glutamate extruded from astrocytes via system xc---an amino acid transporter that imports L-cystine and exports L-glutamate. Specifically, we find that depriving mixed cortical cell cultures of glucose for up to 8 h injures neurons, but not astrocytes. Neuronal death is prevented by ionotropic glutamate receptor antagonism and is partially sensitive to tetanus toxin. Removal of amino acids during the deprivation period prevents--whereas addition of L-cystine restores--GD-induced neuronal death, implicating the cystine/glutamate antiporter, system xc-. Indeed, drugs known to inhibit system xc- ameliorate GD-induced neuronal death. Further, a dramatic reduction in neuronal death is observed in chimaeric cultures consisting of neurons derived from WT (wild-type) mice plated on top of astrocytes derived from sut mice, which harbour a naturally occurring null mutation in the gene (Slc7a11) that encodes the substrate-specific light chain of system xc- (xCT). Finally, enhancement of astrocytic system xc- expression and function via IL-1β (interleukin-1β) exposure potentiates hypoglycaemic neuronal death, the process of which is prevented by removal of l-cystine and/or addition of system xc- inhibitors. Thus, under the conditions of GD, our studies demonstrate that astrocytes, via system xc-, have a direct, non-cell autonomous effect on cortical neuron survival.

Regulation of system x(c)(-)activity and expression in astrocytes by interleukin-1β: implications for hypoxic neuronal injury

We recently demonstrated that interleukin-1β (IL-1β) increases system x(c)(-) (cystine/glutamate antiporter) activity in mixed cortical cell cultures, resulting in an increase in hypoxic neuronal injury when glutamate clearance is impaired. Herein, we demonstrate that neurons, astrocytes, and microglia all express system x(c)(-) subunits (xCT, 4F2hc, RBAT) and are capable of cystine import. However, IL-1β stimulation increases mRNA for xCT--the light chain that confers substrate specificity--in astrocytes only; an effect blocked by the transcriptional inhibitor actinomycin D. Additionally, only astrocytes show an increase in cystine uptake following IL-1β exposure; an effect associated with a change in xCT protein. The increase in cystine uptake that follows IL-1β is lacking in astrocytes derived from mice harboring a mutation in Slc7a11 (sut gene), which encodes for xCT, and in wild-type astrocytes treated with the protein synthesis inhibitor cycloheximide. IL-1β does not regulate the light chain of the amino acid transporter, LAT2, or the expression and function of astrocytic excitatory amino acid transporters (EAATs), demonstrating some target selectivity. Finally, the enhanced neuronal vulnerability to hypoxia that followed IL-1β treatment in our mixed culture system was not observed in chimeric cultures consisting of wild-type neurons plated on top of sut astrocytes. Nor was it observed in wild-type cultures treated with a system x(c)(-) inhibitor or an NMDA receptor antagonist. Overall, our data demonstrate that IL-1β selectively regulates system x(c)(-) activity in astrocytes and that this change is specifically responsible for the deleterious, excitotoxic effects of IL-1β found under hypoxic conditions.

Cytosolic phospholipase A2 alpha inhibition prevents neuronal NMDA receptor-stimulated arachidonic acid mobilization and prostaglandin production but not subsequent cell death

Phospholipase A(2) (PLA(2)) enzymes encompass a superfamily of at least 13 extracellular and intracellular esterases that hydrolyze the sn-2 fatty acyl bonds of phospholipids to yield fatty acids and lysophospholipids. The purpose of this study was to characterize which phospholipase paralog regulates NMDA receptor-mediated arachidonic acid (AA) release. Using mixed cortical cell cultures containing both neurons and astrocytes, we found that [(3)H]-AA released into the extracellular medium following NMDA receptor stimulation (100 microM) increased with time and was completely prevented by the addition of the NMDA receptor antagonist MK-801 (10 microM) or by removal of extracellular Ca(2+). Neither diacylglycerol lipase inhibition (RHC-80267; 10 microM) nor selective inhibition of Ca(2+)-independent PLA(2) [bromoenol lactone (BEL); 10 microM] alone had an effect on NMDA receptor-stimulated release of [(3)H]-AA. Release was prevented by methyl arachidonyl fluorophosphonate (MAFP) (5 microM) and AACOCF(3) (1 microM), inhibitors of both cytosolic PLA(2) (cPLA(2)) and Ca(2+)-independent PLA(2) isozymes. This inhibition effectively translated to block of NMDA-induced prostaglandin (PG) production. An inhibitor of p38MAPK, SB 203580 (7.5 microM), also significantly reduced NMDA-induced PG production providing suggestive evidence for the role of cPLA(2)alpha. Its involvement in release was confirmed using cultures derived from mice deficient in cPLA(2)alpha, which failed to produce PGs in response to NMDA receptor stimulation. Interestingly, neither MAFP, AACOCF(3) nor cultures derived from cPLA(2)alpha null mutant animals showed any protection against NMDA-mediated neurotoxicity, indicating that inhibition of this enzyme may not be a viable protective strategy in disorders of the cortex involving over-activation of the NMDA receptor.

Changes in secondary glutamate release underlie the developmental regulation of excitotoxic neuronal cell death

Vulnerability to excitotoxicity increases during development in vivo and in vitro. To determine whether the mere presence of mature N-methyl-D-aspartate (NMDA) receptors coincides with the emergence of excitotoxicity or whether post-receptor signaling processes may also contribute, we examined the temporal relationship of NMDA receptor expression, function and toxicity using cortical cell cultures. Surface expression of all NMDA receptor subunits increased with time in culture. This correlated with NMDA receptor function, assessed both biochemically and electrophysiologically, but not with the appearance of excitotoxicity. Specifically, cells at day in vitro (DIV) 10 were less susceptible to NMDA receptor-induced neurotoxicity than those cultured for 14 days, even though receptor expression/function was identical. In addition, cell-attached single channel recordings revealed that NMDA receptor conductance, open probability, and frequency of channel openings were not significantly different between the two days. Intriguingly, depolarization-induced release of glutamate from cultures grown for 10 days was significantly lower than that released from cultures grown for 14 days. Further, exogenous addition of glutamate receptor agonists immediately after removal of NMDA rendered cultures at DIV 10 susceptible to excitotoxicity, while toxicity was significantly reduced by addition of an NMDA receptor antagonist immediately after exposure to NMDA at DIV 14. These data are the first to demonstrate that the subsequent, secondary release of glutamate plays an equal, if not more important, role than NMDA receptor development per se, in mediating the enhanced vulnerability of neurons to excitotoxicity that occurs with age.

Electrogenic glutamate transporters in the CNS: molecular mechanism, pre-steady-state kinetics, and their impact on synaptic signaling

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The spatiotemporal profile of the glutamate concentration in the synapse is critical for excitatory synaptic signalling. The control of this spatiotemporal concentration profile requires the presence of large numbers of synaptically localized glutamate transporters that remove pre-synaptically released glutamate by uptake into neurons and adjacent glia cells. These glutamate transporters are electrogenic and utilize energy stored in the transmembrane potential and the Na+/K+-ion concentration gradients to accumulate glutamate in the cell. This review focuses on the kinetic and electrogenic properties of glutamate transporters, as well as on the molecular mechanism of transport. Recent results are discussed that demonstrate the multistep nature of the transporter reaction cycle. Results from pre-steady-state kinetic experiments suggest that at least four of the individual transporter reaction steps are electrogenic, including reactions associated with the glutamate-dependent transporter halfcycle. Furthermore, the kinetic similarities and differences between some of the glutamate transporter subtypes and splice variants are discussed. A molecular mechanism of glutamate transport is presented that accounts for most of the available kinetic data. Finally, we discuss how synaptic glutamate transporters impact on glutamate receptor activity and how transporters may shape excitatory synaptic transmission.

Interleukin-1β potentiates neuronal injury in a variety of injury models involving energy deprivation

The purpose of this study was to develop a suitable in vitro model system to study the biochemical pathway(s) by which interleukin-1beta (IL-1beta) contributes to the pathogenesis of cerebral ischemia. Thus, the effect of IL-1beta on a number of injury paradigms associated with energy deprivation was investigated using murine mixed cortical cell cultures. While IL-1beta by itself was not neurotoxic, pre-treatment-but not concurrent or post-treatment-with this cytokine potentiated neuronal injury induced by depriving cultures of either oxygen, glucose, or both oxygen and glucose. Cytotoxicity was abolished by an IL-1beta-neutralizing antibody. Together, these results demonstrate the establishment of reliable and reproducible in vitro models that will now allow detailed investigation of the cellular and molecular mechanisms relating to IL-1beta-mediated neuronal cell death.

Localization of kainate receptor subunit GluR5-immunoreactive cells in the rat hypothalamus

Glutamate is the major excitatory neurotransmitter in the hypothalamus, which exerts its effects by activating ion channel-forming (ionotropic) or G-protein-coupled (metabotropic) receptors. Kainate-preferring glutamate receptor subunits (GluR5, GluR6, GluR7, KA1, and KA2) form one of the three ionotropic receptor families. In the present study, we analyzed the distribution of GluR5 subunit protein in the rat hypothalamus with immunohistochemistry. GluR5 immunoreactivity was observed in perikarya and processes of many hypothalamic cells some of which, based upon their morphological differentiation by size and structure, appeared to be neurons and others glial cells. Analyses revealed that higher number of glial cells were GluR5 positive when compared to the moderate number of GluR5-labeled neurons in the anteroventral periventricular nucleus. Numerous GluR5-expressing neurons and similar number of glia were detected in the suprachiasmatic nucleus. In the arcuate nucleus more glial cells were identified with GluR5 immunoreactivity than the number of labeled neurons. Scattered GluR5-positive cells were present in the periventricular nucleus. Specific immunostaining was not seen in the ventromedial nucleus or dorsomedial nucleus. In conclusion, it is suggested that the GluR5 subunits participate in the glutamatergic regulation of several neuroendocrine systems, such as the tubero-infundibular systems as well as in the control of circadian output through neuron-to-neuron and/or neuron-to-glia interactions.

Enhanced release of synaptic glutamate underlies the potentiation of oxygen-glucose deprivation-induced neuronal injury after induction of NOS-2

Reactive nitrogen oxide species (RNOS) may contribute to the progression/enhancement of ischemic injury by augmentation of glutamate release, reduction of glutamate uptake, or a combination of both. Consistent with this, induction of nitric oxide synthase (NOS-2) in murine neocortical cell cultures potentiated neuronal cell death caused by combined oxygen-glucose deprivation in association with a net increase in extracellular glutamate accumulation. However, uptake of glutamate via high affinity, sodium-dependent glutamate transporters was unimpaired by induction of NOS-2 under either aerobic or anaerobic conditions. Further, blocking possible routes of extra-synaptic glutamate release with NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a volume-sensitive organic anion channel blocker, or TBOA (d,l-threo-beta-benzyloxyaspartate), an inhibitor of glutamate transport, exacerbated rather than ameliorated injury. Finally, treatment with riluzole or tetanus toxin attenuated the enhancement in both glutamate accumulation and oxygen-glucose deprivation-induced neuronal injury supporting the idea that increased synaptic release of glutamate underlies, at least in part, the potentiation of neuronal injury by RNOS after NOS-2 induction.

The role of glial membrane ion channels in seizures and epileptogenesis

Epilepsy is one of the most common neurological disorders, but the cellular basis of human epilepsy remains largely a mystery, and about 30% of all epilepsies remain uncontrolled. The vast bulk of epilepsy research has focused on neuronal and synaptic mechanisms, but the hypersynchronous firing that is the hallmark of epilepsy could also result from the abnormal function of glial cells by virtue of their critical role in the homeostasis of the brain's extracellular milieu. Therefore, increasing our understanding of glial pro-epileptic and epileptogenic mechanisms holds promise for the development of improved pharmacological treatments for epilepsy. Reactive astrocytes, a prominent feature of the human epileptic brain, undergo changes in their membrane properties and electrophysiology, in particular in the expression of membrane K(+) and Na(+) channels, which result in pro-epileptic changes in their homeostatic control of the extracellular space. Nonetheless, a causal role for reactive astrocytosis in epilepsy has been difficult to determine because glial reactivity can be induced by a wide range of central nervous system insults, including epileptic seizures themselves. A complicating factor is that different insults to the central nervous system result in reactive astrocytes with different membrane properties. Therefore, most animal models of epilepsy preselect the properties of the reactive glia studied. Finally, a causal role for reactive glia in epilepsy cannot be firmly established by examining human epileptic tissue because of its chronic and pharmacoresistant pathological condition that warranted the surgical intervention. Therefore, the development of clinically relevant models of reactive astrocytosis, and of symptomatic epileptogenesis, is needed to investigate the issue. A recently developed model of post-traumatic epileptogenesis in the rat, where chronic spontaneous recurrent seizures develop after a single event of a clinically relevant form of closed head injury, the fluid percussion injury, offers hope to help understand the role of reactive glia in seizures and epileptogenesis and lead to the development of improved therapies.

Electrical and pharmacological properties of the slow depolarization induced by Schaffer collateral stimulation in astrocytes of the rat hippocampal CA1 region

Intracellular recordings were made from astrocytes of the hippocampal CA1 region in rat slice preparations. A single stimulus at the Schaffer collaterals with high intensities (>/=10 V), but not low intensities (<10 V), induced a slow depolarization similar to the evoked postsynaptic potentials observed in hippocampal CA1 neurons. The slow depolarization was almost abolished in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM) and N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonopentanoic acid (100 microM). Tetrodotoxin (0.5 microM) or Co(2+) (2 mM) also abolished this slow depolarization. The slow depolarization reversed its polarity at -8 mV in the presence of tetraethylammonium (20 mM). These results suggest that the synaptically released glutamate (Glu) induces depolarization, which is mainly mediated by ionotropic Glu receptors, in astrocytes in the hippocampal CA1 region.

Oligodendrocytes as providers of growth factors

Glial cells recently are being appreciated as supporters of brain neurons. This review addresses their role as growth factor providers. While the function of astrocytes in this capacity is known, new data indicate that oligodendrocytes, the myelinating cells of the brain, exhibit similar abilities. Oligodendrocytes provide trophic signals to nearby neurons and synthesize defined growth factors. Expression of growth factors is influenced by neural signals. The review summarizes these roles and their implications in brain function.

SIN-1-induced cytotoxicity in mixed cortical cell culture: peroxynitrite-dependent and -independent induction of excitotoxic cell death

3-Morpholinosyndnomine (SIN-1) has been reported to be a peroxynitrite (OONO(-)) donor because it produces both nitric oxide (NO) and superoxide (O(2)(-).) upon decomposition in aqueous solution. However, SIN-1 can decompose to primarily NO in the presence of electron acceptors, including those found in biological tissues, making it necessary to determine the release product(s) formed in any given biological system. In a mixed cortical cell culture system, SIN-1 caused a concentration-dependent increase in cortical cell injury with a parallel increase in the release of cellular proteins containing 3-nitrotyrosine into the culture medium. The increase in 3-nitrotyrosine immunoreactivity, a footprint of OONO(-) production, was specific for SIN-1 as exposure to neurotoxic concentrations of an NO donor (Z)-1-[2-aminoethyl)-N-(2-ammonioethyl) aminodiazen-1-ium-1,2-diolate (DETA/NO), or NMDA did not result in the nitration of protein tyrosine residues. Both SIN-1-induced injury and 3-nitrotyrosine staining were prevented by the addition of either 5,10,15,20-Tetrakis (4-sulfonatophenyl) prophyrinato iron (III) [FeTPPS], an OONO(-) decomposition catalyst, or uric acid, an OONO(-) scavenger. Removal of NO alone was sufficient to inhibit the formation of OONO(-) from SIN-1 as well as its cytotoxicity. Removal of O(2)(-). and the subsequently formed H(2)O(2) by superoxide dismutase (SOD) plus catalase likewise prevented the nitration of protein-bound tyrosine but actually enhanced the cytotoxicity of SIN-1, indicating that cortical cells can cope with the oxidative but not the nitrosative stress generated. Finally, neural injury induced by SIN-1 in unadulterated cortical cells was prevented by antagonism of AMPA/kainate receptors, while blockade of the NMDA receptor was without effect. In contrast, activation of both NMDA and non-NMDA receptors contributed to the SIN-1-mediated neurotoxicity when cultures were exposed in the presence of SOD plus catalase. Thus, whether SIN-1 initiates neural cell death in an OONO(-)-dependent or -independent manner is determined by the antioxidant status of the cells. Further, the mode of excitotoxicity by which injury progresses is determined by the NO-related species generated.

Differential mechanisms of Ca2+ responses in glial cells evoked by exogenous and endogenous glutamate in rat hippocampus

The mechanisms of Ca2+ responses evoked in hippocampal glial cells in situ, by local application of glutamate and by synaptic activation, were studied in slices from juvenile rats using the membrane permeant fluorescent Ca2+ indicator fluo-3AM and confocal microscopy. Ca2+ responses induced by local application of glutamate were unaffected by the sodium channel blocker tetrodotoxin and were therefore due to direct actions on glial cells. Glutamate-evoked responses were significantly reduced by the L-type Ca2+ channel blocker nimodipine, the group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), and the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)2-amino-5-phosphonopentanoic acid (APV). However, glutamate-induced Ca2+ responses were not significantly reduced by the non-NMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). These results indicate that local application of glutamate increases intracellular Ca2+ levels in glial cells via the activation of L-type Ca2+ channels, NMDA receptors, and metabotropic glutamate receptors. Brief (1 s) tetanization of Schaffer collaterals produced increases in intracellular Ca2+ levels in glial cells that were dependent on the frequency of stimulation (> or =50 Hz) and on synaptic transmission (abolished by tetrodotoxin). These Ca2+ responses were also antagonized by the L-type Ca2+ channel blocker nimodipine and the metabotropic glutamate receptor antagonist MCPG. However, the non-NMDA receptor antagonist CNQX significantly reduced the Schaffer collateral-evoked Ca2+ responses, while the NMDA antagonist APV did not. Thus, these synaptically mediated Ca2+ responses in glial cells involve the activation of L-type Ca2+ channels, group I/II metabotropic glutamate receptors, and non-NMDA receptors. These findings indicate that increases in intracellular Ca2+ levels induced in glial cells by local glutamate application and by synaptic activity share similar mechanisms (activation of L-type Ca2+ channels and group I/II metabotropic glutamate receptors) but also have distinct components (NMDA vs. non-NMDA receptor activation, respectively). Therefore, neuron-glia interactions in rat hippocampus in situ involve multiple, complex Ca2+-mediated processes that may not be mimicked by local glutamate application.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

Freshly isolated astrocyte (FIA) preparations: a useful single cell system for studying astrocyte properties

Astrocytes are cell constituents of the mammalian CNS whose intricate relationships with neurons, blood vessels and meninges in situ are well documented. These relationships and their complex morphologies imply numerous functions. Over the past quarter century or so, however, the main experimental basis for determining which roles are likely have been derived from studies on primary astrocyte cultures, usually prepared from neonatal rodent brains. We list a number of examples where these cultures have shown quantitative and qualitative differences from the properties exhibited by astrocytes in situ. The absence of an adequate reliable database makes proposals of likely hypotheses of astrocyte function difficult to formulate. In this article we describe representative studies from our laboratory showing that freshly isolated astrocytes (FIAs), can be used to determine the properties of astrocytes that seem more in concordance with the properties exhibited in situ. Although the cells are most easily isolated from < or =15 day old rat hippocampi they can be isolated from up to 30 day old rats. The examples we describe are that several different types of K(+) currents can be determined by patch clamp electrophysiology, of all the mGluRs only mGluR3 and 5 were detected by single cell RT-PCR, and that single cell Ca(2+) imaging shows that the mGluR5 receptor is functional. It was found that the frequency of cells expressing mGluR5 declines with the age of the animal with the mGluR5b type splice variant replacing the mGluR5a type, as occurs in the intact brain. It is concluded that FIAs can be used to determine the individual characteristics of astrocytes and their properties without the problems of indirect effects inherent in a heterogeneous system such as the slice, and without the problem of cultures unpredictably reflecting the in situ state. The FIAs obviously cannot be used to study interactions of astrocytes with the other CNS components but we propose that they will provide a good database on which hypotheses regarding such interactions can be tested in slices. FIAs can also be isolated from brain slices or intact brain after various pharmacological or electrophysiological perturbations to determine the changes in astrocyte properties that correlate with the perturbations.

Blocking the glycine-binding site of NMDA receptors prevents the progression of ischemic pathology induced by bilateral carotid artery occlusion in spontaneously hypertensive rats

This study was designed to investigate the participation of N-methyl-D-aspartate (NMDA) receptors in the progression of the pathology induced by bilateral carotid artery occlusion (BCAo) in spontaneously hypertensive rats (SHRs). We examined the effects of the selective NMDA receptor glycine-binding site antagonist SM-18400 on the mortality rate, deterioration of neurological signs, and formation of brain edema in the SHR-BCAo model. SM-18400 (15 or 30 mg/kg) was administered via the tail vein immediately and 2 h after BCAo. Neurological signs were monitored continuously for 8 h after BCAo, and the mortality rates were followed for 5 days. All SM-18400-treated animals were still alive 5 h after BCAo, whereas 38% of the animals died in the vehicle-treated group. The mortality rates of the SM-18400-treated groups were still lower than those of the vehicle-treated group 5 days after BCAo. In addition, SM-18400 markedly prevented the deterioration of neurological signs. The water content of the telencephalon and diencephalon/mesencephalon in the vehicle-treated group, measured 3 h after BCAo, was significantly higher than in the sham-operated group. SM-18400 significantly inhibited the increase in water content in both regions in a dose-dependent manner. These findings suggest that NMDA receptors participate in the increase in the mortality rate, deterioration of neurological signs, and formation of brain edema following ischemic brain damage in the SHR-BCAo model, and that SM-18400 can prevent ischemic insults.

Quantitative immunochemistry on neuronal loss, reactive gliosis and BBB damage in cortex/striatum and hippocampus/amygdala after systemic kainic acid administration

Cell specific markers were quantified in the hippocampus, the amygdala/pyriform cortex, the frontal cerebral cortex and the striatum of the rat brain after systemic administration of kainic acid. Neuron specific enolase (NSE) reflects loss of neurons, glial fibrillary acidic protein (GFAP) reflects reactive gliosis, and brain levels of serum proteins measures blood-brain-barrier permeability. While the concentration of NSE remained unaffected in the frontal cerebral cortex and the striatum, their GFAP content increased during the first three days. In the hippocampus and amygdala, NSE levels decreased significantly. GFAP levels in the hippocampus were unaffected after one day and decreased in the amygdala/pyriform cortex. After that, GFAP increased strikingly until day 9 or, in the case of amygdala/pyriform cortex, even longer. This biphasic time course for GFAP was accompanied by a decrease of S-100 during days 1-9 followed by a significant increase at day 27 above the initial level. The regional differences in GFAP and S-100 could result from the degree of neuronal degeneration, the astrocytic receptor set-up and/or effects on the blood-brain barrier.

Association between cell swelling and glycogen content in cultured astrocytes

Treatment of cultured rat astrocytes with hypotonic media or with 1 mM glutamate for 90 min caused cell swelling and a significant increase in glycogen content. Conversely, treatment with hypertonic media caused cell shrinkage with a corresponding decrease in astrocyte glycogen, which was proportional to the increasing osmolality of the hypertonic media. The glutamate receptor antagonist, MK-801, lowered both the glutamate-induced swelling and glycogen increase. These findings demonstrate a correlation between changes in cell volume and astrocyte glycogen content. This may explain the increased astrocytic glycogen observed in many neuropathological conditions where astrocyte swelling occurs. Because glycogen represents the largest energy reserve in the central nervous system, a swelling-induced disturbance in glycogen metabolism may lead to abnormal glial-neuronal interactions resulting in impaired brain bioenergetics.

Astroglial dysfunction in hepatic encephalopathy

While the pathogenesis of hepatic encephalopathy (HE) remains elusive, there is considerable evidence pointing to a key role of ammonia-induced dysfunction of astrocytes in this condition. Deficits in the ability of astrocytes to take up glutamate from the extracellular space may lead to abnormal glutamatergic neurotransmission. Furthermore, excessive stimulation of neuronal and glial glutamate receptors by elevated extracellular levels of glutamate may lead to excitotoxicity and greater glial dysfunction. Ammonia also causes upregulation of astroglial peripheral-type benzodiazepine receptors (PBRs) which is associated with increased production of neurosteroids. These neurosteroids have potent positive modulatory effects on the neuronal GABA(A) receptor which, combined with an ammonia-induced astroglial defect in GABA uptake, may result in enhanced GABAergic tone. Brain edema, associated with fulminant hepatic failure, may also result from astroglial abnormalities as the edema appears to be principally caused by swelling of these cells. Increased amounts of glutamine in astrocytes resulting from elevated brain ammonia levels may be a factor in this swelling. Other osmolytes such as glutathione may also be involved. Glial swelling may also result from NH4+ - and K+ -mediated membrane depolarization as well as by the actions of PBR agonists and neurosteroids. These findings show that an ammonia-induced gliopathy is a major factor in the pathogenesis of HE.

N-acetylaspartylglutamate activates cyclic AMP-coupled metabotropic glutamate receptors in cerebellar astrocytes

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.

Pharmacological characterization of Na+ influx via voltage-gated Na+ channels in spinal cord astrocytes

Spinal cord astrocytes display a high density of voltage-gated Na+ channels. To study the contribution of Na+ influx via these channels to Na+ homeostasis in cultured spinal cord astrocytes, we measured intracellular Na+ concentration ([Na+]i) with the fluorescent dye sodium-binding benzofuran isophthalate. Stellate and nonstellate astrocytes, which display Na+ currents with different properties, were differentiated. Baseline [Na+]i was 8.5 mM in these cells and was not altered by 100 microM tetrodotoxin (TTX). Inhibition of Na+ channel inactivation by veratridine (100 microM) evoked a [Na+]i increase of 47.1 mM in 44% of stellate and 9.7 mM in 64% of nonstellate astrocytes. About 30% of cells reacted to veratridine with a [Na+]i decrease of approximately 2 mM. Qualitatively similar [Na+]i changes were caused by aconitine. The effects of veratridine were blocked by TTX, amplified by (alpha-)scorpion toxin and usually were readily reversible. Veratridine-induced [Na+]i increases were reduced upon membrane depolarization with elevated extracellular [K+]. Recovery to baseline [Na+]i was unaltered during blocking of K+ channels with 4-aminopyridine. [Na+]i increases evoked by the ionotropic non-N-methyl--aspartate receptor agonist kainate were not altered by TTX. Our results indicate that influx of Na+ via voltage- gated Na+ channels is not a prerequisite for glial Na+,K+-ATPase activity in spinal cord astrocytes at rest nor does it seem to be involved in [Na+]i increases evoked by kainate. During pharmacological inhibition of Na+ channel inactivation, however, Na+ channels can serve as prominent pathways of Na+ influx and mediate large perturbations in [Na+]i, suggesting that Na+ channel inactivation plays an important functional role in these cells.

Glutamate-induced release of the nitric oxide precursor, arginine, from glial cells

Arginine, the nitric oxide precursor, is predominantly localized in glial cells, whereas the constitutive nitric oxide synthase is mainly found in neurons. Therefore, a transfer of arginine from glial cells to neurons is needed to replenish the neuronal precursor pool. This is further supported by the finding that arginine is released upon selective pathway stimulation both in vitro and in vivo. We investigated the mechanism underlying this glial-neuronal interaction by analysing the effect of glutamate receptor agonists on the extracellular [3H]arginine level in cerebellar and cortical slices and in cultures of either cortical astroglial cells or neurons. We present data indicating that arginine is released from cerebellar and cortical slices and astroglial cell cultures upon activation of ionotropic non-NMDA glutamate receptors. Glutamate had no effect on the extracellular [3H]arginine level in neuronal cultures. Moreover, the effect of glutamate in cerebellar slices was tetrodotoxin-insensitive, and the calcium ionophore A23187 evoked the release of [3H]arginine from astroglial cell cultures. Thus, nitric oxide synthesis and nitric oxide transmission may be based on the glial-neuronal transfer of arginine which is induced by activation of excitatory amino acid receptors on glial cells.

Intracellular calcium transients and potassium current oscillations evoked by glutamate in cultured rat astrocytes

Glutamate responses in cultured rat astrocytes from cerebella of neonatal rats were investigated using the perforated-patch configuration to record membrane currents without rundown of intracellular messenger cascades, and microfluorometric measurements to measure the intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) with fura-2 AM and 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxy methylester respectively. In the perforated-patch mode, glutamate evoked single or multiple outward current transients in 82% of the cells, which disappeared when the recording technique was converted into a conventional whole-cell mode. The outward current transients were accompanied by [Ca2+]i transients, whereas pHi fell monophasically, without any sign of oscillation. Pharmacological analysis of the glutamate-induced responses indicated that ionotropic receptor activation evoked an inward current but no outward current transients, and metabotropic receptor activation (of the mGluR1/5 type) elicited outward current transients but no inward current. The outward current transients were reduced in frequency, or even abolished, after depletion of the intracellular Ca2+-stores by the Ca2+-ATPase inhibitor cyclopiaconic acid (10 microM). They reversed near -85 mV and were reduced by tetraethylammonium (10 mM), suggesting that they were caused by K+ channel activation. It is concluded that glutamate evoked these K+ outward current transients by oscillatory Ca2+ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K+ uptake capacity and other voltage-dependent processes.

Metabotropic glutamate receptor expression in cultured rat astrocytes and human gliomas

In order to confirm the existence of metabotropic glutamate receptors in astroglial cultures and to provide information on different receptor subtypes, the expression of different mGluRs was analysed in cultures highly enriched in rat astroglial cells. mRNA levels for mGluR1, 2, 3, 4, 7 were undetectable by Northern blot analysis in primary type-1 astroglial cultures derived from total cerebral hemispheres, cerebral cortex and striatum. Interestingly, these cultures expressed a low, but detectable, level of mGluR5 mRNA. The more sensitive technique Reverse Transcription-Polymerase Chain Reaction (RT-PCR) confirmed the presence of mGluR5 transcript in cultured astrocytes and, in addition, revealed the presence of mGluR3 mRNA. The lack of expression of mGluR5 in CG-4 cells, a rat cell line able to differentiate in type-2 astrocytes or oligodendrocytes depending on the culture conditions, suggested that the presence of mGluR5 was not a general feature of cells of glial origin. Moreover, all the examined mGluR transcript were undetectable by RT-PCR in CG4 cells. In order to confirm the possible expression of mGluR5 in cell of glial origin we examined the mRNA levels for this receptor in tissue samples from human gliomas obtained after surgical resection of the tumors: only 1 sample (grade II astrocytoma), out of 8 examined, showed the presence of mGluR5 mRNA. In conclusion our data show that the only cloned metabotropic receptor linked to phosphoinositide hydrolysis, whose expression is detectable in cultured type-1 astrocytes, in mGluR5. It remains to be established if the low level of expression of mGluR3 could be responsible for the group II metabotropic glutamate receptor activity previously observed in cultured astroglial cells.

Activation of p42 mitogen-activated protein kinase by glutamate in cultured radial glia

The effect of L-glutamate (Glu) and its structural analogs N-methyl-D-aspartate (NMDA), kainate (KA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), on the activation of p42 mitogen activated protein kinase (MAPK) was examined in cultured chick radial glia cells, namely retinal Muller cells and cerebellar Bergmann cells. Glu, NMDA, AMPA and KA evoked a dose and time dependent increase in MAPK activity. AMPA and KA responses were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) whereas NMDA responses were sensitive to 3-[(RS)-2-carboxypiperazin-4-yl)]-propyl-1-phosphonate (CPP) indicating that the increase in MAPK activity is mediated by AMPA/low affinity KA and NMDA subtypes of Glu receptors. The present findings open the possibility of a MAPK cascade involvement in the regulation of Glu-induced gene expression in radial glia.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Mechanisms of H+ and Na+ changes induced by glutamate, kainate, and D-aspartate in rat hippocampal astrocytes

The excitatory transmitter glutamate (Glu), and its analogs kainate (KA), and D-aspartate (D-Asp) produce significant pH changes in glial cells. Transmitter-induced pH changes in glial cells, generating changes in extracellular pH, may represent a special form of neuronal-glial interaction. We investigated the mechanisms underlying these changes in intracellular H+ concentration ([H+]i) in cultured rat hippocampal astrocytes and studied their correlation with increases in intracellular Na+ concentration ([Na+]i), using fluorescence ratio imaging with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) or sodium-binding benzofuran isophthalate (SBFI). Glu, KA, or D-Asp evoked increases in [Na+]i; Glu or D-Asp produced parallel acidifications. KA, in contrast, evoked biphasic changes in [H+]i, alkaline followed by acid shifts, which were unaltered after Ca2+ removal and persisted in 0 CI(-)-saline, but were greatly reduced in CO2/HCO3(-)-free or Na(+)-free saline, or during 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) application. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked KA-evoked changes in [H+]i and [Na+]i, indicating that they were receptor-ionophore mediated. In contrast, CNQX increased the [H+]i change and decreased the [Na+]i change induced by Glu. D-Asp, which is transported but does not act at Glu receptors, induced [H+]i and [Na+]i changes that were virtually unaltered by CNQX. Our study indicates that [Na+]i increases are not primarily responsible for Glu- or KA-induced acidifications in astrocytes. Instead, intracellular acidifications evoked by Glu or D-Asp are mainly caused by transmembrane movement of acid equivalents associated with Glu/Asp-uptake into astrocytes. KA-evoked biphasic [H+]i changes, in contrast, are probably attributable to transmembrane ion movements mediated by inward, followed by outward, electrogenic Na+/HCO3- cotransport, reflecting KA-induced biphasic membrane potential changes.

MK-801 reduces uptake and stimulates efflux of excitatory amino acids via membrane depolarization

MK-801 and related compounds reduce excitotoxic neuronal injury by blocking N-methyl-D-aspartate (NMDA) receptorgated ion channels. These agents also cause neuronal vacuolization and block glutamate-induced astrocyte swelling, effects that may be unrelated to actions at the NMDA receptor. In the present study, high concentrations of MK-801 (100-1,000 microM) caused uncompetitive inhibition of glutamate uptake in astrocyte and neuronal cultures and stimulated D-aspartate efflux from astrocytes. MK-801 (500 microM) reduced the maximal velocity for glutamate uptake in astrocytes from 31 to 17 nmol.mg protein-1.min-1, whereas competitive NMDA receptor antagonists did not affect glutamate uptake. MK-801 also inhibited uptake of gamma-aminobutyric acid (GABA). Because both GABA uptake and glutamate uptake are electrogenic, one mechanism by which MK-801 could inhibit uptake is by membrane depolarization. Whole cell patch-clamp recording confirmed that MK-801 in the range of 100-1,000 microM caused dose-dependent and reversible depolarization. These concentrations are far higher than necessary to block NMDA receptors, and the findings suggest that actions at sites other than NMDA receptors could contribute to the effects of high doses of MK-801 in some experimental and clinical settings.

Striatal and cortical NMDA receptors are altered by a neurotoxic regimen of methamphetamine

Methamphetamine (m-AMPH) treatment produces long-lasting damage to striatal and cortical monoaminergic terminals and may also injure nonmonoaminergic cortical neurons. Evidence suggests that both dopamine (DA) and glutamate (GLU) play crucial roles in producing this damage. We used quantitative autoradiography to examine [3H]mazindol ([3H]MAZ) binding to striatal DA transporters and [3H]GLU binding to N-methyl-D-aspartate (NMDA) receptors in the striatum and cortex 1 week and 1 month after a neurotoxic regimen of m-AMPH. Rats received m-AMPH (4 mg/kg) or saline (SAL) (1 ml/kg) in four s.c. injections separated by 2 h intervals. One week after m-AMPH, the ventral and lateral sectors of the striatum showed the greatest decreases in both [3H]MAZ and [3H]GLU binding, while the nucleus accumbens (NA) showed no significant decreases. One month after m-AMPH, striatal [3H]MAZ binding was still significantly decreased, while NMDA receptor binding had recovered. Surprisingly, the parietal cortex showed a m-AMPH-induced increase in NMDA receptor binding in layers II/III and IV 1 week after m-AMPH and only in layers II/III 1 month after m-AMPH. The prefrontal cortex showed no m-AMPH-induced changes in NMDA receptor binding at either time point. This is the first demonstration that a regimen of m-AMPH that results in long-lasting damage to DA terminals can alter forebrain NMDA receptor binding. Thus, repeated m-AMPH treatments may produce changes in glutamatergic transmission in selected striatal and cortical regions.

Editing for an AMPA receptor subunit RNA in prefrontal cortex and striatum in Alzheimer's disease, Huntington's disease and schizophrenia

Animal studies and cell culture experiments demonstrated that posttranscriptional editing of the transcript of the GluR-2 gene, resulting in substitution of an arginine for glutamine in the second transmembrane region (TM II) of the expressed protein, is associated with a reduction in Ca2+ permeability of the receptor channel. Thus, disturbances in GluR-2 RNA editing with alteration of intracellular Ca2+ homeostasis could lead to neuronal dysfunction and even neuronal degeneration. The present study determined the proportions of edited and unedited GluR-2 RNA in the prefrontal cortex of brains from patients with Alzheimer's disease, in the striatum of brains from patients with Huntington's disease, and in the same areas of brains from age-matched schizophrenics and controls, by using reverse transcriptase-polymerase chain reaction, restriction endonuclease digestion, gel electrophoresis and scintillation radiometry. In the prefrontal cortex of controls, < 0.1% of all GluR-2 RNA molecules were unedited and > 99.9% were edited; in the prefrontal cortex both of schizophrenics and of Alzheimer's patients approximately 1.0% of all GluR-2 RNA molecules were unedited and 99% were edited. In the striatum of controls and of schizophrenics, approximately 0.5% of GluR-2 RNA molecules were unedited and 99.5% were edited; in the striatum of Huntington's patients nearly 5.0% of GluR-2 RNA was unedited. In the prefrontal white matter of controls, approximately 7.0% of GluR-2 RNA was unedited. In the normal human prefrontal cortex and striatum, the large majority of GluR-2 RNA molecules contains a CGG codon for arginine in the TMII coding region; this implies that the corresponding AMPA receptors have a low Ca2+ permeability, as previously demonstrated for the rat brain. The process of GluR-2 RNA editing is compromised in a region-specific manner in schizophrenia, in Alzheimer's disease and Huntington's Chorea although in each of these disorders there is still a large excess of edited GluR-2 RNA molecules. Disturbances of GluR-2 RNA editing leading to excessive Ca2+ permeability, may contribute to neuronal dysfunction in schizophrenia and to neuronal death in Alzheimer's disease and Huntington's disease.

Excitatory amino acid receptors in glia: different subtypes for distinct functions?

It is now well established that expression of voltage- and ligand-gated ionic channels, as well as G protein-coupled receptors, is not a property unique to neurons, but is also shared by macroglial cells (astrocytes and oligodendrocytes). These glial cells can receive a variety of signals from neurons at different stages of their development. Activation of membrane receptors may affect glial cell activity, proliferation, maturation, and survival through a complex cascade of intracellular events leading to long-term changes in glial cell phenotype and functional organization. Here we review the experimental evidence for glutamate receptor expression in glial cells in culture and in situ, and the molecular and functional properties of these receptors. We also describe some experimental models that identify possible functions of glutamate receptors in glia. Now that the existence of glutamate receptors in glia has been unambiguously demonstrated, future research will have to 1) determine which receptor subtypes are expressed in macroglial cells in vivo; 2) analyze, in adequate experimental models, the short- and long-term changes produced by glutamate receptor activation in glia; and 3) establish whether these receptors play a role in neuron-glia communication in the brain.

In vivo studies on NMDA-evoked release of amino acids in the rat spinal cord

In the present study, spontaneous and evoked release of selected amino acids in the rat spinal cord was studied using in vivo microdialysis. Perfusion of the microdialysis probe with 100 K+ evoked a 2-4-fold increase in release of the putative neurotransmitters aspartate, glutamate and taurine while glutamine was decreased. K(+)-evoked release of glutamate was almost completely Ca(2+)-dependent while that of aspartate was partially Ca(2+)-dependent. Taurine release was not affected by substituting Ca2+ with Co2+. Perfusion with 5 mM N-methyl-D-aspartate (NMDA) evoked 3-9-fold release of glutamate, glycine and taurine and a small increase in extracellular beta-alanine. No significant changes in glutamine and serine were found. 5 mM of the competitive NMDA antagonist 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) reduced NMDA-evoked release of glutamate and taurine by approx. 50%. 5 mM 3-amino-1-hydroxypyrrolid-2-one (HA-966), an agonist at the glycine site of the NMDA receptor with very low efficacy, completely inhibited NMDA-evoked release of taurine and reduced the levels of released glutamate below baseline, similar to the effect of 1 mM CPP alone. The present results show that in situations of excessive release of excitatory amino acids such as spinal ischemia and trauma. NMDA receptor-evoked release of glutamate may amplify the deleterious process and spread the damage.

Intracellular acidification and Ca2+ transients in cultured rat cerebellar astrocytes evoked by glutamate agonists and noradrenaline

The effect of different neurotransmitters on the intracellular pH (pHi) and intracellular calcium (Ca2+i) was studied in cultured astrocytes from neonatal rat cerebellum, using the fluorescent dyes 2,7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) and Fura-2. Application of glutamate or kainate (100 microM) in a HEPES-buffered, CO2/HCO3(-) -free saline induced a decrease in pHi and an increase in Ca2+i. Amplitude and time course of the pHi and Ca2+i transients were different. Glutamate and kainate evoked a mean acidification of 0.22 +/- 0.05 (n = 29) and 0.20 +/- 0.09 (n = 12) pH units, respectively. The changes in pHi and Ca2+i induced by kainate, but not by glutamate, were inhibited by 6-cyano-7-dinitroquinozalin-2,3-dion (CNQX; 50 microM). In order to elucidate the mechanism of the agonist-induced acidification, whether the pHi changes were secondary to the Ca2+ rises was tested. In the absence of extracellular Ca2+, the kainate-induced Ca2+i transient was suppressed, while the intracellular acidification was only reduced by 13%. Removal of extracellular Ca2+ reduced the glutamate-induced pHi change by 8%, while the second component of the Ca2+i transient was abolished. Application of trans-( +/- )-1-amino-(1S,3R)-cyclopentadicarboxylic acid (t-ACPD, 100 microM), a metabotropic glutamate receptor agonist, and of noradrenaline (20 microM) evoked a Ca2+i increase, but no change of pHi. D-aspartate, which has a low affinity to glutamate receptors, but is known to be transported by the glutamate uptake system in some astrocytes, evoked an intracellular acidification, similar to that induced by glutamate, but no Ca2+i transient. The results suggest that the kainate-induced acidification is only partly due to the concomitant Ca2+i rise, while the glutamate/aspartate-induced acidification is mainly due to the activation of the glutamate uptake system.

Excitatory amino acid-induced AP-1 DNA binding activity in Müller glia

The effect of L-glutamate (L-Glu) and its structural analogs N-methyl-D-aspartate (NMDA), quisqualate (QA), and kainate (KA) on the DNA binding activity of the Activator Protein 1 (AP-1) and the Ca2+/cAMP Responsive Element Binding Protein (CREB) families of transcription factors was examined in cultured chick retinal Müller glia cells. L-Glu, NMDA, and KA evoked a dose and time dependent increase in AP-1 DNA binding activity and had no effect on CREB binding. The order of potency for stimulating AP-1 DNA binding was NMDA > or = Glu > KA >> QA. L-Glu responses were partially blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by 3-[RS)-2-carboxypiperazin-4-yl)]-propyl-1-phosphonate (CPP) indicating that the increase in DNA binding is mediated both by an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/low affinity KA and a NMDA subtypes of L-Glu receptors. Since Müller glia L-Glu receptors are probably mediators of the efficacy of the excitatory transmission in the retina, the present findings suggest that a stimulus-transcription coupling triggered by L-Glu in the glial cells might have a role in the long-term modulation of these synapses.

Nordihydroguaiaretic acid and RHC 80267 potentiate astroglial injury during combined glucose-oxygen deprivation

Membrane phospholipid degradation has been proposed to play a key role in hypoxic-ischemic brain injury. We tested the hypotheses that both nordihydroguaiaretic acid, a phospholipase A2 and lipoxygenase inhibitor, and RHC 80267, a diacylglycerol lipase inhibitor, would decrease the release of [3H]arachidonic acid metabolites from prelabeled cultures of astroglia subjected to combined glucose-oxygen deprivation and that these inhibitors would also decrease astroglial injury during combined glucose-oxygen deprivation. Both nordihydroguaiaretic acid and RHC 80267 significantly inhibited the release of [3H]arachidonic acid metabolites during combined glucose-oxygen deprivation. This suggests that two separate enzymic pathways, the phospholipase A2 pathway and the phospholipase C/diacylglycerol lipase pathway, contribute to the release of astroglial [3H]arachidonic acid metabolites during combined glucose-oxygen deprivation. However, both of these lipase inhibitors increased astroglial cell death during combined glucose-oxygen deprivation, probably due to inhibition of arachidonic acid release. We speculate that arachidonic acid release may be a mechanism of astroglial self-preservation during combined glucose-oxygen deprivation.

Glutamate-activated ionic currents in cultured astrocytes from trout: evidence for the occurrence of non-N-methyl-D-aspartate receptors

Glutamate-induced currents were recorded from cultured trout astrocytes with the whole-cell variation of the patch-clamp technique. Ninety percent of the tested cells were directly depolarized by the amino acid neurotransmitter in a concentration-dependent manner. The depolarizing effect was due to an inward current that reversed near 0 mV and was accompanied by a noise increase, indicating the opening of an ion channel. Ion substitution experiments revealed that the glutamate-induced current was mainly carried by sodium ions but not chloride or calcium ions. The glutamate-induced response could be mimicked by the neuronal glutamate receptor subtype agonists kainate and quisqualate, while N-methyl-D-aspartate was without detectable effect.

GFAP-positive hippocampal astrocytes in situ respond to glutamatergic neuroligands with increases in [Ca2+]i

It is becoming increasingly clear that astrocytes play very dynamic and interactive roles that are important for the normal functioning of the central nervous system. In culture, astrocytes express many receptors coupled to increases in intracellular calcium ([Ca2+]i). In vivo, it is likely that these receptors are important for the modulation of astrocytic functions such as the uptake of neurotransmitters and ions. Currently, however, very little is known about the expression or stimulation of such astrocytic receptors in vivo. To address this issue, confocal microscopy and calcium-sensitive fluorescent dyes were used to examine the dynamic changes in astrocytic [Ca2+]i within acutely isolated hippocampal slices. Astrocytes were subsequently identified by immunocytochemistry for glial fibrillary acidic protein. In this paper, we present data indicating that hippocampal astrocytes in situ respond to glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), 1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), N-methyl-D-aspartate (NMDA), and depolarization with increases in [Ca2+]i. The increases in [Ca2+]i occurred in both the astrocytic cell bodies and the processes. Temporally the changes in [Ca2+]i were very dynamic, and various patterns ranging from sustained elevations to oscillations of [Ca2+]i were observed. Individual astrocytes responded to neuroligands selective for both ionotropic and metabotropic glutamate receptors with increases in [Ca2+]i. These findings indicate that astrocytes in vivo contain glutamatergic receptors coupled to increases in [Ca2+]i and are able to respond to neuronally released neurotransmitters.

Receptors on astrocytes--what possible functions?

Receptors for transmitters, as varied as those expressed by neurons, have been described on primary astrocyte cultures prepared from new-born rats and mice. A variety of functional effects and considerable cell-to-cell and regional heterogeneity have been observed for such receptors in vitro. The various systems available for studying the presence and properties of receptors on astrocytes in situ, and the results from these studies, are discussed. Much fewer studies using these more difficult systems have been done. So far, some resemblances and differences between in situ and in vitro work have been observed. More of these in situ studies, to supplement the ongoing in vitro work, are needed to enable us to determine unequivocally which receptors are present on astrocytes, and their functions in vivo. If there is cell-to-cell and CNS regional heterogeneity in vivo comparable to that seen in vitro, these analyses will be very complex. To illustrate the importance and variety of receptor-linked functions, a number of suggestions are made in this commentary, based on current proposals for the roles of astrocytes. However, it is argued that we need to have a more complete understanding of astrocyte functions in vivo, before we can really understand the functional significance of astrocyte receptors.