Glutamate uptake in mammalian retinal glia is voltage- and potassium-dependent

Astrocytic modulation of neuronal signalling

Neuronal signalling is a key element in neuronal communication and is essential for the proper functioning of the CNS. Astrocytes, the most prominent glia in the brain play a key role in modulating neuronal signalling at the molecular, synaptic, cellular, and network levels. Over the past few decades, our knowledge about astrocytes and their functioning has evolved from considering them as merely a brain glue that provides structural support to neurons, to key communication elements. Astrocytes can regulate the activity of neurons by controlling the concentrations of ions and neurotransmitters in the extracellular milieu, as well as releasing chemicals and gliotransmitters that modulate neuronal activity. The aim of this review is to summarise the main processes through which astrocytes are modulating brain function. We will systematically distinguish between direct and indirect pathways in which astrocytes affect neuronal signalling at all levels. Lastly, we will summarize pathological conditions that arise once these signalling pathways are impaired focusing on neurodegeneration.

Destabilizing COXIV in Müller Glia Increases Retinal Glycolysis and Alters Scotopic Electroretinogram

Müller glia (MG), the principal glial cell of the retina, have a metabolism that defies categorization into glycolytic versus oxidative. We showed that MG mount a strong hypoxia response to ocular hypertension, raising the question of their relative reliance on mitochondria for function. To explore the role of oxidative phosphorylation (OXPHOS) in MG energy production in vivo, we generated and characterized adult mice in which MG have impaired cytochrome c oxidase (COXIV) activity through knockout of the COXIV constituent COX10. Histochemistry and protein analysis showed that COXIV protein levels were significantly lower in knockout mouse retina compared to control. Loss of COXIV activity in MG did not induce structural abnormalities, though oxidative stress was increased. Electroretinography assessment showed that knocking out COX10 significantly impaired scotopic a- and b-wave responses. Inhibiting mitochondrial respiration in MG also altered the retinal glycolytic profile. However, blocking OXPHOS in MG did not significantly exacerbate retinal ganglion cell (RGC) loss or photopic negative response after ocular hypertension (OHT). These results suggest that MG were able to compensate for reduced COXIV stability by maintaining fundamental processes, but changes in retinal physiology and metabolism-associated proteins indicate subtle changes in MG function.

Intrinsic Expression of Coagulation Factors and Protease Activated Receptor 1 (PAR1) in Photoreceptors and Inner Retinal Layers

The aim of this study was to characterize the distribution of the thrombin receptor, protease activated receptor 1 (PAR1), in the neuroretina. Neuroretina samples of wild-type C57BL/6J and PAR1^−/− mice were processed for indirect immunofluorescence and Western blot analysis. Reverse transcription quantitative real-time PCR (RT-qPCR) was used to determine mRNA expression of coagulation Factor X (FX), prothrombin (PT), and PAR1 in the isolated neuroretina. Thrombin activity following KCl depolarization was assessed in mouse neuroretinas ex vivo. PAR1 staining was observed in the retinal ganglion cells, inner nuclear layer cells, and photoreceptors in mouse retinal cross sections by indirect immunofluorescence. PAR1 co-localized with rhodopsin in rod outer segments but was not expressed in cone outer segments. Western blot analysis confirmed PAR1 expression in the neuroretina. Factor X, prothrombin, and PAR1 mRNA expression was detected in isolated neuroretinas. Thrombin activity was elevated by nearly four-fold in mouse neuroretinas following KCl depolarization (0.012 vs. 0.044 mu/mL, p = 0.0497). The intrinsic expression of coagulation factors in the isolated neuroretina together with a functional increase in thrombin activity following KCl depolarization may suggest a role for the PAR1/thrombin pathway in retinal function.

Potential Mechanism of Cellular Uptake of the Excitotoxin Quinolinic Acid in Primary Human Neurons

In Alzheimer's disease (AD), excessive amounts of quinolinic acid (QUIN) accumulate within the brain parenchyma and dystrophic neurons. QUIN also regulates glutamate uptake into neurons, which may be due to modulation of Na⁺-dependent excitatory amino acid transporters (EAATs). To determine the biological relationships between QUIN and glutamate dysfunction, we first quantified the functionality and kinetics of [³H]QUIN uptake in primary human neurons using liquid scintillation. We then measured changes in the protein expression of the glutamate transporter EAAT3 and EAAT1b in primary neurons treated with QUIN and the EAAT inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (2,4-PDC) using western blotting and immunohistochemistry. Immunohistochemistry was further used to elucidate intracellular transport of exogenous QUIN and the lysosomal-associated membrane protein 2 (LAMP2). Structural insights into the binding between QUIN and EAAT3 were further investigated using molecular docking techniques. We report significant temperature-dependent high-affinity transport leading to neuronal uptake of [³H]QUIN with a Km of 42.2 μM, and a V_max of 9.492 pmol/2 min/mg protein, comparable with the uptake of glutamate. We also found that QUIN increases expression of the EAAT3 monomer while decreasing the functional trimer. QUIN uptake into primary neurons was shown to involve EAAT3 as uptake was significantly attenuated following EAAT inhibition. We also demonstrated that QUIN increases the expression of aberrant EAAT1b protein in neurons further implicating QUIN-induced glutamate dysfunction. Furthermore, we demonstrated that QUIN is metabolised exclusively in lysosomes. The involvement of EAAT3 as a modulator for QUIN uptake was further confirmed using molecular docking. This study is the first to characterise a mechanism for QUIN uptake into primary human neurons involving EAAT3, opening potential targets to attenuate QUIN-induced excitotoxicity in neuroinflammatory diseases.

Suppression of SNARE-dependent exocytosis in retinal glial cells and its effect on ischemia-induced neurodegeneration

Nervous tissue is characterized by a tight structural association between glial cells and neurons. It is well known that glial cells support neuronal functions, but their role under pathologic conditions is less well understood. Here, we addressed this question in vivo using an experimental model of retinal ischemia and transgenic mice for glia‐specific inhibition of soluble N‐ethylmaleimide‐sensitive factor attachment protein receptor (SNARE)‐dependent exocytosis. Transgene expression reduced glutamate, but not ATP release from single Müller cells, impaired glial volume regulation under normal conditions and reduced neuronal dysfunction and death in the inner retina during the early stages of ischemia. Our study reveals that the SNARE‐dependent exocytosis in glial cells contributes to neurotoxicity during ischemia in vivo and suggests glial exocytosis as a target for therapeutic approaches.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Spreading depolarizations mediate excitotoxicity in the development of acute cortical lesions

Spreading depolarizations (SD) are mass depolarizations of neurons and astrocytes that occur spontaneously in acute brain injury and mediate time-dependent lesion growth. Glutamate excitotoxicity has also been extensively studied as a mechanism of neuronal injury, although its relevance to in vivo pathology remains unclear. Here we hypothesized that excitotoxicity in acute lesion development occurs only as a consequence of SD. Using glutamate-sensitive microelectrodes, we found that SD induced by KCl in normal rat cortex elicits increases in extracellular glutamate (11.6±1.3μM) that are synchronous with the onset, sustainment, and resolution of the extracellular direct-current shift of SD. Inhibition of glutamate uptake with d,l-threo-β-benzyloxyaspartate (TBOA, 0.5 and 1mM) significantly prolonged the duration of the direct-current shift (148% and 426%, respectively) and the glutamate increase (167% and 374%, respectively) in a dose-dependent manner (P<0.05). These prolonged events produced significant cortical lesions as indicated by Fluoro-Jade staining (P<0.05), while no lesions were observed after SD in control conditions or after cortical injection of 1mM glutamate (extracellular increase: 243±50.8μM) or 0.5mM TBOA (glutamate increase: 8.5±1.6μM) without SD. We then used an embolic focal ischemia model to determine whether glutamate elevations occur independent of SD in the natural evolution of a cortical lesion. In both the ischemic core and penumbra, glutamate increased only in synchrony with anoxic terminal SD (6.1±1.1μM) and transient SDs (11.8±2.4μM), and not otherwise. Delayed terminal SDs were also observed in two animals at 98 and 150min after ischemic onset and induced similar glutamate elevations. Durations of SDs and glutamate increases were significantly correlated in both normal and ischemic animals (P<0.05). These data suggest that pathologically prolonged SDs are a required mechanism of acute cortical lesion development and that glutamate elevations and the mass electrochemical changes of SD and are merely different facets of the same pathophysiologic process.

Group I mGluR-mediated inhibition of Kir channels contributes to retinal Müller cell gliosis in a rat chronic ocular hypertension model

Müller cell gliosis, which is characterized by upregulated expression of glial fibrillary acidic protein (GFAP), is a universal response in many retinal pathological conditions. Whether down-regulation of inward rectifying K+ (Kir) channels, which commonly accompanies the enhanced GFAP expression, could contribute to Müller cell gliosis is poorly understood. We investigated changes of Kir currents, GFAP and Kir4.1 protein expression in Müller cells in a rat chronic ocular hypertension (COH) model, and explored the mechanisms underlying Müller cell gliosis. We show that Kir currents and Kir4.1 protein expression in Müller cells were reduced significantly, while GFAP expression was increased in COH rats, and these changes were eliminated by MPEP, a group I metabotropic glutamate receptors (mGluR I) subtype mGluR5 antagonist. In normal isolated Müller cells, the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) suppressed the Kir currents and the suppression was blocked by MPEP. The DHPG effect was mediated by the intracellular Ca2+ -dependent PLC/IP3-ryanodine/PKC signaling pathway, but the cAMP-PKA pathway was not involved. Moreover, intravitreal injection of DHPG in normal rats induced changes in Müller cells, similar to those observed in COH rats. The DHPG-induced increase of GFAP expression in Müller cells was obstructed by Ba2+, suggesting the involvement of Kir channels. We conclude that overactivation of mGluR5 by excessive extracellular glutamate in COH rats could contribute to Müller cell gliosis by suppressing Kir channels.

Müller glial cells in retinal disease

Virtually all pathogenic stimuli activate Müller cells. Reactive Müller cells exert protective and toxic effects on photoreceptors and neurons. They contribute to oxidative stress and glutamate toxicity due to malfunctions of glutamate uptake and glutathione synthesis. Downregulation of potassium conductance disrupts transcellular potassium and water transport, resulting in neuronal hyperexcitability and edema. Protective effects of reactive Müller cells include upregulation of adenosine 5'-triphosphate (ATP)-degrading ectoenzymes, which enhances the extracellular availability of the neuroprotectant adenosine, abrogation of the osmotic release of ATP, which might protect retinal ganglion cells from apoptosis, and the release of antioxidants and neurotrophic factors. The dedifferentiation of reactive Müller cells to progenitor-like cells might have an impact on future therapeutic approaches. A better understanding of the gliotic mechanisms will be helpful in developing efficient therapeutic strategies aiming at increased protective and regenerative properties and decreased toxicity of reactive Müller cells.

A diminished response to formalin stimulation reveals a role for the glutamate transporters in the altered pain sensitivity of mice with experimental autoimmune encephalomyelitis (EAE)

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) in which neuropathic pain is now recognized as a major symptom. To date, few studies have examined the underlying mechanisms of neuropathic pain in MS. Recently we showed that in a chronic-relapsing animal model of MS, experimental autoimmune encephalomyelitis (EAE), characteristic neuropathic behaviours develop. However, responses to persistent noxious stimuli in EAE remain unexplored. We, therefore set out to characterize the changes in pain sensitivity in our EAE model to subcutaneous injection of formalin. We show here that female C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG(35-55)) display a significant decrease in elicited pain behaviours in response to formalin injection. These effects were found to involve dysregulation of the glutamatergic system in EAE. We show here that these effects are mediated by decreased glutamate transporter expression associated with EAE. Our findings demonstrate that dysregulation of glutamate transporter function in EAE mice is an important mechanism underlying the abnormal pain sensitivity in response to persistent noxious stimulation of mice with EAE and also sheds light on a potential mechanism underlying neuropathic pain behaviours in this model.

Neuroprotective agents: is effective on toxicity in glial cells?

1. Glial cells are the most abundant cell population in the central nervous system. The aim of this study was to examine the effects of melatonin, 7-nitroindazole, and riluzole on glutamate toxicity in primary glial cell culture. 2. Glutamate toxicity was induced by addition of 100 microM glutamate to the cultures, and then 100 microM melatonin, 500 microM 7-nitroindazole, and 10 (M riluzole were administered in different groups. Lactate Dehydrogenase activity and nitrite levels were determined at the 1st, 6th, and 24th h. 3. Melatonin, 7-nitroindazole, and riluzole decrease Lactate Dehydrogenase activity at the 1st, 6th, and 24th h (at all hours, p<0.05). Nitrite levels were decreased by melatonin and riluzole at the 1st, 6th, and 24th h. 4. In this study, we observed that melatonin, 7-nitroindazole, and riluzole are effective as protective agents on glutamate toxicity in mixed glial cells.

Atypical gliosis in Müller cells of the slowly degenerating rds mutant mouse retina

Retinal Müller glial cells are known to undergo reactive changes (gliosis) in various retinal diseases. In virtually all cases studied, an upregulation of glial fibrillary acidic protein (GFAP) and a hypertrophy can be observed. Physiological alterations, such as a strong downregulation of inwardly rectifying K+ (Kir) currents, were found after retinal detachment (man, rabbit) and after ischemia/reperfusion (rat) but not in more slowly progressing retinal degenerations (Borna Disease Virus-infected rats, RCS rats). This led us to hypothesize that Müller cells respond with 'typical' reactive gliosis only to rapid but not to slow retinal degeneration. To test this hypothesis, we studied Müller cells from rds mutant mice (PrphRd2), which show a retinal degeneration of early onset and slow progression, resulting in a complete loss of photoreceptors after 9-12 months. In Müller cells of rds mice, we found immunoreactivity for GFAP, a marker of gliosis in Müller cells, from postnatal day 21 on, accompanied by a moderately increased membrane capacitance (taken as an indicator of hypertrophy), whereas no change in the expression of the Kir4.1 protein occurred in adult rds mice. We failed to observe significant changes in the membrane resistance and the membrane potential of cells from rds mice from first week after birth until 1 year of age. Current densities were decreased in cells from 3- and 5-week old rds mice. Furthermore, as in control cells from wildtype animals, these cells displayed dominant Kir currents, voltage-dependent Na+ currents, and glutamate uptake currents. These data support the idea that in mice as well as previously shown in rats, slow retinal degeneration induces an atypical gliosis of Müller cells.

Amyloid beta peptide causes chronic glial cell activation and neuro-degeneration after intravitreal injection

We have previously demonstrated that amyloid beta (Abeta) peptide is acutely toxic to retinal neurones in vivo and that this toxicity is mediated by an indirect mechanism. We have now extended these studies to look at the chronic effect of intravitreal injection of Abeta peptides on retinal ganglion cells (RGC), the projection neurones of the retina and the glial cell response. 5 months after injection of Abeta1-42 or Abeta42-1 there was no significant reduction in RGC densities but there was a significant reduction in the retinal surface area after both peptides. Phosphate-buffered saline (PBS) injection had no effect on retinal size or RGC density. There was a pronounced reduction in the number of large RGCs with a concomitant significant increase in medium and small RGCs. There was no change in cell sizes 5 months after injection with PBS. At 5 months after injection of both peptides, there was marked activation of Muller glial cells and microglia. There was also expression of the major histocompatibility complex (MHC) class II molecule on some of the microglial cells but we saw no evidence of T-cell infiltration into the injected retinas. In order to elucidate potential toxic mechanisms, we have looked at levels of glutamine synthetase and nitric oxide synthase. As early as 2 days after injection we noted that activation of Muller glia was associated with a decrease in glutamine synthetase immuno-reactivity but there was no detectable expression of inducible nitric oxide synthase in any retinal cells. These results suggest that chronic activation of glial cells induced by Abeta peptides may result in chronic atrophy of projection neurones in the rat retina.

Role of spinal cord glutamate transporter during normal sensory transmission and pathological pain states

Glutamate is a neurotransmitter critical for spinal excitatory synaptic transmission and for generation and maintenance of spinal states of pain hypersensitivity via activation of glutamate receptors. Understanding the regulation of synaptically and non-synaptically released glutamate associated with pathological pain is important in exploring novel molecular mechanisms and developing therapeutic strategies of pathological pain. The glutamate transporter system is the primary mechanism for the inactivation of synaptically released glutamate and the maintenance of glutamate homeostasis. Recent studies demonstrated that spinal glutamate transporter inhibition relieved pathological pain, suggesting that the spinal glutamate transporter might serve as a therapeutic target for treatment of pathological pain. However, the exact function of glutamate transporter in pathological pain is not completely understood. This report will review the evidence for the role of the spinal glutamate transporter during normal sensory transmission and pathological pain conditions and discuss potential mechanisms by which spinal glutamate transporter is involved in pathological pain.

Neuronal versus glial cell swelling in the ischaemic retina

Under normal conditions, the pigment epithelium dehydrates the outer retina while Müller glial cells mediate the rapid water transport within the inner retina. Gliotic alterations of Müller cells may be implicated in the development of oedema in the post-ischaemic retina. Here, we suggest a mechanism of Müller cell-supported neuronal cell swelling and apoptosis in the ischaemic retina. During ischaemia, over-excitation of ionotropic glutamate receptors leads to neuronal cell depolarization that causes excess Ca(2+) influx into the cells, and to activation of the apoptosis machinery. The ion fluxes into the retinal neurons are associated with water movements that are mediated by aquaporin-4 water channels expressed by Müller cells and result in neuronal cell swelling. After reperfusion, the glial cells may swell due to the down-regulation of their K(+) conductance, which results in intracellular K(+) overload and water movements from the blood and vitreous into the cells. An inhibition of the glial cell-mediated water movements during ischaemic episodes should reduce the ion shifts at the neuronal synapses, resulting in decreased neuronal cell swelling and apoptosis. An inhibition of the water movements in the post-ischaemic phase may prevent cytotoxic Müller cell swelling but may impair the fluid clearance from retinal tissue in the presence of vasogenic oedema. Thus, pharmacological modification of the ion and fluid clearance functions of Müller cells may become a novel way to resolve both cytotoxic and vasogenic oedema in the retina.

Physiological properties of retinal Muller glial cells from the cynomolgus monkey, Macaca fascicularis--a comparison to human Muller cells

Retinae from rabbits and laboratory rodents are often used as 'models' of the human retina, although there are anatomical differences. To test whether monkey eyes provide a better model, a physiological study of Muller glial cells was performed comparing isolated cells and retinal wholemounts from the cynomolgus monkey, Macaca fascicularis and from man. The membrane conductance of Muller cells from both species was dominated by inward and outward K(+) currents. Cells displayed glutamate uptake currents and responded to nucleotides by intracellular Ca(2+) increases. However, there were also species differences, such as a lack of GABA(A) receptors and of Ca(2+)-dependent K(+) currents in monkey cells. Thus, the use of Muller cells from cynomolgus monkeys may be advantageous for investigating a few specific properties; in general, monkey cells are no more similar to human cells than those from standard laboratory animals.

Ocular inflammation alters swelling and membrane characteristics of rat Müller glial cells

Ocular inflammation is a common cause of retinal edema that may involve swelling of Müller glial cells. In order to investigate whether endotoxin-induced ocular inflammation in rats alters the swelling and membrane characteristics of Müller cells, lipopolysaccharide (LPS; 0.5%) was intravitreally injected. At 3 and 7 days after treatment, hypotonic challenge induced swelling of Müller cell somata that was not observed in non-treated control eyes. Müller cells of LPS-treated eyes displayed a downregulation of inward K(+) currents and upregulation of A-type K(+) currents that was associated with a decreased expression of Kir4.1 protein in retinal slices. The data suggest that ocular inflammation induces alterations of both the swelling characteristics and the K(+) channel expression of Müller cells.

Altered membrane physiology in Müller glial cells after transient ischemia of the rat retina

Inwardly rectifying K+ (Kir) channels have been implicated in the mediation of retinal K+ homeostasis by Muller glial cells. To assess possible involvement of altered glial K+ channel expression in ischemia-reperfusion injury, transient retinal ischemia was induced in rat eyes. Acutely isolated Muller cells from postischemic retinae displayed a fast downregulation of their Kir currents, which began within 1 day and reached a maximum at 3 days of reperfusion, with a peak decrease to 20% as compared with control. This strong decrease of Kir currents was accompanied by an increase of the incidence of cells which displayed depolarization-evoked fast transient (A-type) K+ currents. While no cell from untreated control rats expressed A-type K+ currents, all cells investigated from 3- and 7-day postischemic retinae displayed such currents. An increased incidence of cells displaying fast transient Na+ currents was observed at 7 days after ischemia. These results suggest a role of altered glial Kir channel expression in postischemic neuronal degeneration via disturbance of retinal K+ siphoning.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Modulation of glutamatergic transmission by bergmann glial cells in rat cerebellum in situ

We obtained patch-clamp recordings from neuron-glial cell pairs in cerebellar brain slices to examine the contribution of glutamate (Glu) uptake by Bergmann glial cells to shaping excitatory postsynaptic currents (EPSCs) at the parallel fiber to Purkinje cell synapse. We show that electrical stimulation of parallel fibers not only activates EPSCs in Purkinje cells but also activates inward currents in antigenically identified Bergmann glial cells that invest Purkinje cell synapse with their processes. The inward current is partially due to 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX)- and 2-amino-5-phosphonopentanoic acid (AP5)-sensitive ionotropic Glu receptors, but >/=70% of the current was mediated by D,L-threo-beta-hydroxyaspartate (THA)-sensitive Glu transporters. Glu inward currents were completely and reversibly inhibited by depolarization of Bergmann glial cells to positive membrane potentials allowing biophysical inhibition of Glu uptake into a single glial cell. Inhibition of Glu transport into Bergmann glial cells by voltage-clamping the cell to depolarized potentials caused a reversible increase in spontaneous EPSC frequency in the Purkinje cell. This increase could also be achieved by pharmacological inhibition of Glu transport with the Glu transport inhibitor THA, suggesting that inhibition of Glu uptake into Bergmann glial cells is responsible for the modulation of postsynaptic EPSCs. THA modulation of spontaneous EPSCs could only be observed in the absence of TTX, suggesting primarily a presynaptic effect. Taken together these data suggest that glial Glu uptake can profoundly affect excitatory transmission in the cerebellum, most likely by regulating presynaptic glutamate release.

Characterization of l-2-Amino-4-Phosphonobutanoate Action Following Sensitization by Quisqualate in Rat Hippocampal Slice Cultures

An excitatory action of l-2-amino-4-phosphonobutanoate (l-AP4), a glutamate analogue, is observed following pre-exposure of tissue to quisqualate. We have studied the mechanism of sensitization of l-AP4 responses by quisqualate in voltage-clamped CA3 pyramidal cells in rat hippocampal slice cultures in the presence of tetrodotoxin. Prior to quisqualate addition, CA3 cells did not respond to l-AP4 (50 - 1000 microM). Following brief application of quisqualate (500 nM for 30 s), l-AP4 (50 - 200 microM) induced a complex excitatory response which could be obtained for >1 h. l-AP4 caused an ionotropic inward current associated with a conductance increase. This response was in part sensitive to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and in part sensitive to d-2-amino-5-phosphonovalerate (d-AP5) and Mg2+ ions. At depolarizing potentials, in the presence of CNQX and d-AP5, l-AP4 caused excitation by depressing K+ currents, mimicking the metabotropic action of glutamate. This indicates that the action of l-AP4 is mediated by three different receptor types: N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors, and glutamatergic metabotropic receptors. The l-AP4 response persisted in solutions containing low Ca2+ and high Mg2+ concentrations or 100 - 200 microM Cd2+, suggesting that it is independent of extracellular Ca2+. We were unable to identify any substance other than quisqualate capable of sensitizing the l-AP4 action. This effect also occurred when quisqualate was applied in Ca2+-free solution or in solutions containing low concentrations of Na+ or Cl-. Sensitization of l-AP4 responses by quisqualate was not observed in acutely dissociated pyramidal cells recorded by means of the whole-cell recording mode, although ionotropic quisqualate responses were present. Sensitization was readily reversed by short applications of the endogenous excitatory amino acids glutamate, aspartate and homocysteate at concentrations of 10 - 100 microM. Our data are consistent with the hypothesis that the excitatory action of l-AP4 results from a Ca2+-independent release of endogenous excitatory amino acids from some presynaptic neuronal or glial site.

Amyloid-beta peptide is toxic to neurons in vivo via indirect mechanisms

We have studied the neurotoxicity of amyloid-beta (Abeta) after a single unilateral intravitreal injection. Within the retina apoptotic cells were seen throughout the photoreceptor layer and the inner nuclear layer but not in the ganglion cell layer at 48 h after injection of Abeta(1-42) compared to vehicle control and control peptide. At 5 months, there was a significant reduction in total cell numbers in the ganglion cell layer in Nissl stained retinas. There was glial cell dysfunction with upregulation of glial fibrillary acidic protein and a reduction in the expression of Müller cell associated proteins in the injected retinas. These results suggest an indirect cytotoxic effect of Abeta on retinal neurons and an important role for dysfunction of Müller glia in mediating Abeta neurotoxicity.

Electrophysiological characterization of retinal Müller glial cells from mouse during postnatal development: comparison with rabbit cells

The electrophysiology of murine Müller cells, and of their precursors, during postnatal development was investigated by using the whole-cell patch-clamp technique. Membrane potential, membrane capacitance, and expression of voltage-gated Na+ currents increased during the first 3 postnatal weeks. During the same period, the membrane resistance decreased due to an upregulation of both inward and outward K+ currents. Glutamate transporter-mediated currents increased during the first postnatal weeks, as well. In Müller (precursor) cells from rabbits, these transporter currents already achieved adult levels at postnatal day 6, the earliest stage studied. Together with the developmental time course of K+ currents, this indicates a delay in the maturation of murine Müller cells, as compared with cells from rabbit.

Cell death in retinitis pigmentosa: gap junctions and the 'bystander' effect

Retinitis pigmentosa (RP) constitutes a group of genetically mediated, degenerative retinal diseases that display a broad range of phenotypes. There is appreciable heterogeneity in the pathogenetic mechanisms that underlie the various forms of RP, but a substantial percentage of the known cases arise as a consequence of mutations in rhodopsin or other rod-specific proteins. However, despite the fact that the genetic defect is expressed solely in the rod photoreceptors, otherwise healthy cone photoreceptors invariably die, resulting in severe visual impairment. In this paper, the author proposes a mechanism that may be responsible, at least in part, for this unfortunate circumstance. The basic premise of the hypothesis is that the spread of the disease from dying rods to genetically normal cones is a form of 'bystander' effect, mediated by the gap junctions that exist between these photoreceptor subtypes. On this view, agents that trigger the apoptotic process permeate the intercellular gap-junctional channels to carry the disease from rods to neighboring cones. If permeation of noxious substances through gap junctions is a significant factor in the non-cell-autonomous spread of photoreceptor degeneration, blocking transmission through these channels may provide a means for therapeutic intervention. Many substances are known to block gap-junctional communication, but if the rod-cone channel is to be targeted, it will be essential to identify the connexins that form the gap junctions between the two types of photoreceptor, and to develop drugs that selectively affect their junctional properties. Clearly, passage of toxic agents through gap junctions may not be the only form of cell-cell interaction by which dying rods could cause cone cell death, and in this brief account, the author considers other avenues that are currently being explored to explain this phenomenon.

Activation of P2Y receptors stimulates potassium and cation currents in acutely isolated human Müller (glial) cells

The ability of various neurotransmitters/neuroactive substances to induce fast, transient rises of Ca(2+)-activated K(+) currents (I(BK)) caused by release of Ca(2+) from intracellular stores was investigated in Müller glial cells of the human retina. Müller cells were enzymatically isolated from retinas of healthy donors or of patients with proliferative vitreoretinopathy, and the transmembrane ionic currents were recorded using the whole-cell and the cell-attached patch-clamp techniques. The results of the screening experiments indicate that human Müller cells express, in addition to GABA(A) and perhaps glutamatergic and cholinergic receptors, predominantly P2 receptors. ATP and other nucleotides exerted two effects on membrane currents: repetitive transient increases of the I(BK) amplitude and, in a subpopulation of cells investigated, the appearance of a transient cation conductance at negative potentials. ATP and UTP increased dose-dependently the I(BK) amplitude with half-maximal effects at 0.33 and 0.50 microM, respectively. Since several different P2 receptor agonists increased the I(BK), it is assumed that human Müller cells express a mixture of different types of P2Y receptors. In cell-attached patches, extracellular application of ATP or UTP transiently increased the open probability of single putative BK channels. The increase of I(BK) and the appearance of the cation conductance in whole-cell records were abolished when intracellular Ca(2+) was buffered by a high-EGTA pipette solution or when IP(3) was included in the pipette solution. The expression of agonist-evoked transient cation currents was found to be stronger in cells from patients as compared to cells from healthy donors. It is concluded that human Müller glial cells express P2Y receptors that, via IP(3) formation, cause intracellular Ca(2+) release. The increased intracellular Ca(2+) concentration stimulates the activity of BK channels and may induce opening of cation channels. Both the ATP-induced activity of BK channels and the increased expression of Ca(2+)-gated cation channels may be important in respect to proliferative Müller cell gliosis.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Midazolam and ketamine inhibit glutamate release via a cloned human brain glutamate transporter

PURPOSE

In cerebral ischemia/anoxia, the glutamate transporter runs in reverse and releases glutamate into the extracellular space, causing irreversible neuronal damage. Intravenous anesthetics attenuate overall glutamate release and prevent neuronal injury during anoxia/ischemia, but their effect on the glutamate transporter is variable.

METHODS

A human glial glutamate transporter (hGLT-I) cDNA was isolated by screening a human cerebral cortical library. Cloned cDNA was transfected in Chinese hamster ovary cells. The effect of the intravenous anesthetics midazolam (0.3 to 30 microM), ketamine (10 to 100 microM), thiopental (30 to 300 microM), and propofol (3 to 30 microM) on reversed uptake of L-glutamate via hGLT-I was examined by whole-cell patch-clamp.

RESULTS

Midazolam at a concentration 3 microM reduced outward currents arising from reversed L-glutamate uptake via hGLT-I in a concentration-dependent manner. While, ketamine at 100 microM attenuated the same outward currents, to 53.3+/-11.4% of those seen in controls without anesthetics (P<0.05, n=5). In contrast, neither thiopental nor propofol showed effects on outward currents mediated by reversed operation of hGLT-I.

CONCLUSIONS

These results suggest that midazolam and ketamine, but not thiopental and propofol, have a capacity to inhibit glutamate release via GLT- I directly.

Effects of mild versus deep hypothermia on a cloned human brain glutamate transporter (GLT-1) expressed in Chinese hamster ovary cells

Glutamate transporters, widely distributed in the brain and spinal cord, maintain extracellular glutamate concentrations below neurotoxic levels. In cerebral ischemia/anoxia, the glutamate transporter runs in reverse and releases glutamate into the extracellular space, causing irreversible neuronal damage. Although hypothermia reduces the elevation of extracellular glutamate concentration during cerebral ischemia/anoxia, little is known about the effect of hypothermia on the glutamate transporter. A human glial glutamate transporter (hGLT-1) cDNA was isolated by screening a human cerebral cortical library, and cloned cDNA was stably transfected in Chinese hamster ovary (CHO) cells. Effects of deep hypothermia (22 to 23 degrees C) on uptake and release of L-glutamate via hGLT-1 were investigated by whole-cell patch-clamp. The control study was performed at 34 to 35 degrees C. The hGLT-1 transporter had the capacity to take up extracellular L-glutamate under essentially physiological ionic conditions, whereas this transporter promoted release of L-glutamate under a nonphysiological condition mimicking complete ischemia. Deep hypothermia decreased a) uptake and b) release of L-glutamate via hGLT-1 to a) 4.8+/-4.8% (P < .01, n = 7) and b) 19.0+/-4.5% (P < .01, n = 15) of control values, respectively. The results suggest that deep hypothermia is a potent inhibitor of glutamate uptake by intact glial cells as well as glutamate release from glial cells under certain pathophysiological circumstances. The balance between these antagonistic effects of hypothermia may attenuate the elevation of the extracellular glutamate concentration during ischemia/anoxia.

Glutamate and gamma-aminobutyric acid content and release of synaptosomes from temporal lobe epilepsy patients

During surgical intervention in medically refractory temporal lobe epilepsy (TLE) patients, diagnosed with either mesial temporal lobe sclerosis (MTS)- or tumor (T)-associated TLE, biopsies were taken from the anterior temporal neocortex and the hippocampal region. Synaptosomes, isolated from these biopsies were used to study intrasynaptosomal Ca(2+) levels ([Ca(2+)](i)), and glutamate and gamma-aminobutyric acid (GABA) contents and release. All synaptosomal preparations demonstrated a basal [Ca(2+)](i) of about 200 nM, except neocortical synaptosomes from MTS-associated TLE patients (420 nM). K(+)-induced depolarization resulted in a robust increase of the basal [Ca(2+)](i) in all preparations. Neocortical synaptosomes from TLE patients contained 22.9 +/- 3.0 nmol glutamate and 4.6 +/- 0.5 nmol GABA per milligram synaptosomal protein, whereas rat cortical synaptosomes contained twice as much glutamate and four times as much GABA. Hippocampal synaptosomes from MTS-associated TLE patients, unlike those from T-associated TLE patients, contained about 70% less glutamate and 55% less GABA than neocortical synaptosomes. Expressed as percentage of total synaptosomal content, synaptosomes from MTS-associated TLE patients exhibited an increased basal and a reduced K(+)-induced glutamate and GABA release compared to rat cortical synaptosomes. In MTS-associated TLE patients, only GABA release from neocortical synaptosomes was partially Ca(2+)-dependent. Control experiments in rat synaptosomes demonstrated that at least part of the reduction in K(+)-induced release can be ascribed to resection-induced hypoxia in biopsies. Thus, synaptosomes from MTS-associated TLE patients exhibit a significant K(+)-induced increase in [Ca(2+)](i), but the consequent release of glutamate and GABA is severely impaired. Our data show that at least part of the differences in glutamate and GABA content and release between human biopsy material and fresh rat tissue is due to the resection time.

Acute decrease in net glutamate uptake during energy deprivation

The extracellular glutamate concentration ([glu](o)) rises during cerebral ischemia, reaching levels capable of inducing delayed neuronal death. The mechanisms underlying this glutamate accumulation remain controversial. We used N-methyl-D-aspartate receptors on CA3 pyramidal neurons as a real-time, on-site, glutamate sensor to identify the source of glutamate release in an in vitro model of ischemia. Using glutamate and L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) as substrates and DL-threo-beta-benzyloxyaspartate (TBOA) as an inhibitor of glutamate transporters, we demonstrate that energy deprivation decreases net glutamate uptake within 2-3 min and later promotes reverse glutamate transport. This process accounts for up to 50% of the glutamate accumulation during energy deprivation. Enhanced action potential-independent vesicular release also contributes to the increase in [glu](o), by approximately 50%, but only once glutamate uptake is inhibited. These results indicate that a significant rise in [glu](o) already occurs during the first minutes of energy deprivation and is the consequence of reduced uptake and increased vesicular and nonvesicular release of glutamate.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

The glutathione content of retinal Müller (glial) cells: effect of pathological conditions

Maintenance of isolated retinal Müller (glial) cells in glutamate-free solutions over 7 h causes a significant loss of their initial glutathione content; this loss is largely prevented by the blockade of glutamine synthesis using methionine sulfoximine (5 mM). Anoxia does not reduce the glutathione content of Müller cells when glucose (11 mM), glutamate and cystine (0.1 mM each) are present. In contrast, simulation of total ischemia (i.e., anoxia plus removal of glucose) decreases the glutathione levels dramatically, even in the presence of glutamate and cystine. Less severe effects are caused by high extracellular K+ (40 mM). Reactive oxygen species are generated in the retina under various conditions, such as anoxia, ischemia, and reperfusion. One of the crucial substances protecting the retina against reactive oxygen species is glutathione, a tripeptide constituted of glutamate, cysteine and glycine. It was recently shown that glutathione can be synthesized in retinal Müller glial cells and that glutamate is the rate-limiting substance. In this study, glutathione levels were determined in acutely isolated guinea-pig Müller cells using the glutathione-sensitive fluorescent dye monochlorobimane. The purpose was to find out how the glial glutathione content is affected by anoxia/ischemia and accompanying pathophysiological events such as depolarization of the cell membrane. Our results further strengthen the view that glutamate is rate-limiting for the glutathione synthesis in glial cells. During glutamate deficiency, as caused by e.g., impaired glutamate uptake, this amino acid is preferentially delivered to the glutamate-glutamine pathway, at the expense of glutathione. This mechanism may contribute to the finding that total ischemia (but not anoxia) causes a depletion of glial glutathione. In situ depletion may be accelerated by the ischemia-induced increase of extracellular K+, decreasing the driving force for glutamate uptake. The ischemia-induced lack of glutathione is particularly fatal considering the increased production of reactive oxygen species under this condition. Therefore the therapeutic application of exogenous free radical scavengers is greatly recommended.

Role of glial K(+) channels in ontogeny and gliosis: a hypothesis based upon studies on Müller cells

The electrophysiological properties of Müller cells, the principal glial cells of the retina, are determined by several types of K(+) conductances. Both the absolute and the relative activities of the individual types of K(+) channels undergo important changes in the course of ontogenetic development and during gliosis. Although immature Müller cells express inwardly rectifying K(+) (K(IR)) currents at a very low density, the membrane of normal mature Müller cells is predominated by the K(IR) conductance. The K(IR) channels mediate spatial buffering K(+) currents and maintain a stable hyperpolarized membrane potential necessary for various glial-neuronal interactions. During "conservative" (i.e., non-proliferative) reactive gliosis, the K(IR) conductance of Müller cells is moderately reduced and the cell membrane is slightly depolarized; however, when gliotic Müller cells become proliferative, their K(IR) conductances are dramatically down-regulated; this is accompanied by an increased activity of Ca(2+)-activated K(+) channels and by a conspicuous unstability of their membrane potential. The resultant variations of the membrane potential may increase the activity of depolarization-activated K(+), Na(+) and Ca(2+) channels. It is concluded that in respect to their K(+) current pattern, mature Müller cells pass through a process of dedifferentiation before proliferative activity is initiated.

Real-time monitoring of glutamate transmitter release with anoxic depolarization during anoxic insult in rat striatum

In vivo continuous real-time measurement of glutamate concentration was performed during anoxia using a dialysis electrode. By this method, the temporal resolution of the measurement of glutamate concentration was improved due to shortening of the time delay compared with the microdialysis method, and changes in the glutamate concentration were more clearly represented with greater reproducibility. After exposure to anoxia, the glutamate concentration showed biphasic changes. A relationship between the DC potential and release of glutamate was confirmed by the synchronization of anoxic depolarization (AD) with the 1st phase of glutamate release. Since the 1st phase disappeared and AD was delayed and suppressed by blocking Ca2+ influx, exocytosis is considered to play an important role in the construction of the 1st phase, which had a close relation with the occurrence of AD. Moreover, since blocking Ca2+ influx also had an effect on the glutamate release from the metabolic pool (2nd phase), reversed uptake may be involved with energy failure in the 1st phase, Ca2+ influx into the cell and rapid changes of the ionic environment associated with AD.

Reverse transport of glutamate during depolarization in immature hippocampal slices

We studied the source of extracellular glutamate released by hippocampal slices obtained from P14 or adult rats, during 50 mM K+ depolarization by using two potent inhibitors of Na+-dependent glutamate transport: l-trans-pyrrolidine-2,4-dicarboxylate (PDC), which is a relatively non-selective inhibitor of various glutamate transporter subtypes and dihydrokainic acid (DHK), a specific inhibitor of the glial transporter, GLT-1. Most depolarization-induced glutamate release was Ca2+-dependent in adults, while in P14 slices most glutamate release was Ca2+-independent. PDC decreased depolarization-induced glutamate release in P14 slices but not in adults. DHK increased glutamate release in adults but not in P14 slices. These data suggest that most depolarization-induced glutamate release in immature hippocampal slices is due to reversal of transport through a PDC-sensitive Na+-dependent glutamate transporter, presumably acting on presynaptic or cytoplasmic neuronal pools, and is not due to exocytosis from vesicular pools.

Microdomains for neuron-glia interaction: parallel fiber signaling to Bergmann glial cells

Astrocytes are considered a reticulate network of cells, through which calcium signals can spread easily. In Bergmann glia, astrocytic cells of the cerebellum, we identified subcellular compartments termed 'glial microdomains'. These elements have a complex surface consisting of thin membrane sheets, contain few mitochondria and wrap around synapses. To test for neuronal interaction with these structures, we electrically stimulated parallel fibers. This stimulation increased intracellular calcium concentration ([Ca2+]i) in small compartments within Bergmann glial cell processes similar in size to glial microdomains. Thus, a Bergmann glial cell may consist of hundreds of independent compartments capable of autonomous interactions with the particular group of synapses that they ensheath.

Glutamate uptake in Purkinje cells in rat cerebellar slices

We have described how whole-cell clamping of neurons in brain slices has allowed a characterization of postsynaptic transporters, probably a mixture of EAAC1 and EAAT4, in cerebellar Purkinje cells. Similar experiments have been carried out on transporters (mainly GLAST) in cerebellar Bergmann glia, and have revealed an uptake current occurring as these carriers remove glutamate released at the parallel fiber synapses. As more transporters are cloned and their regulation is characterized in heterologous expression systems, it will be increasingly important to use methods similar to those outlined above to investigate to what extent the behavior of the carriers is similar in situ in the nervous system.

Patch-clamp, ion-sensing, and glutamate-sensing techniques to study glutamate transport in isolated retinal glial cells

We have described how a combination of electrical, ion-sensing, and glutamate-sensing techniques has advanced our understanding of glutamate uptake into isolated salamander retinal glial cells. The next steps in understanding glutamate transport will inevitably depend strongly on molecular biological methods, as described elsewhere in this book, but will also require more detailed study of transporters in their normal environment, perhaps by using patch-clamping or imaging techniques to study cells in situ.

Free radicals and glutamate uptake in the retina

1. Glutamate (Glu) uptake in neurons and astrocytes is essential to prevent the persistence of excitotoxic levels of Glu in the synaptic cleft. 2. We investigated the effect of oxidative stress, which is also involved in ischemia-reperfusion, on the Glu transporter in isolated rat retinal cells. 3. Hydrogen peroxide (H2O2 3-300 microM) decreases the Na+-dependent Glu uptake. This effect is not related to a free radical production and is partly reversed by reducing agents, suggesting a transporter modulation by a redox-related event.

Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors

We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. Horizontal cells depolarized from about -70 to -20 mV as the exogenous glutamate concentration was increased from approximately 1 to 40 microM, so horizontal cells can serve as "glutamate electrodes" during the light response. DHK slowed the rate of hyperpolarization of the horizontal cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 microM DHK, the rate of the horizontal cell hyperpolarization was slowed to only 17 +/- 8.5% (mean +/- SD) of control. Translating this to changes in glutamate concentration using the slice dose response curve as calibration in Fig. 2, DHK reduced the rate of removal of glutamate from approximately 0.12 to 0.031 microM/s. The voltage dependence of uptake rate in the transporter alone was capable of modulating glutamate concentration: we blocked vesicular released glutamate with bathed 20 mM Mg2+ and then added 30 microM glutamate to the bath to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these conditions, a light flash elicited a 17-mV hyperpolarization in the horizontal cells. When we substituted kainate, which is not transported, for glutamate, horizontal cells were depolarized but light did not elicit any response, indicating that the transporter alone was responsible for the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics.

Protection against glutamate neurotoxicity in retinal cultures by acidic conditions

We evaluated the effects of extracellular acidic conditions on glutamate-induced death in cultured retinal neurons. Primary retinal cultures, obtained from 3- to 5-day-old Wistar rats, were estimated to be consisted of mainly amacrine cells (90%) together with a small population of horizontal (8%) and ganglion cells (2%). We examined the effects of acidic pH (pH 6.0 to 7.0) on glutamate neurotoxicity by monitoring the delayed death of retinal neurons induced by brief (10 min) exposure to 1 mM glutamate followed by a 24-hr incubation. The glutamate-induced delayed death of cultured retinal neurons was attenuated with an acidic pH between 6.0 and 7.0. Furthermore, whole-cell patch-clamp recordings were taken from retinal neurons to examine the effects of acidic pH on N-methyl-D-aspartate (NMDA) or kainate receptor-mediated currents. NMDA- and kainate-induced currents were suppressed at pH 6.0 to 7.0 and pH 6.0 to 6.5, respectively. The acidity of the medium protected the retinal neurons from glutamate-induced delayed death, probably by inhibiting NMDA and/or kainate receptor activation.

Effects of AWD 140-190 on stimulus-induced field potentials and on different patterns of epileptiform activity induced by low calcium or low magnesium in rat entorhinal cortex hippocampal slices

AWD 140-190 a potent new anticonvulsant was tested on several types of epileptiform activities in entorhinal cortex hippocampal slices. AWD 140-190 suppressed completely and in a dose-dependent manner spontaneous seizure-like events induced by lowering extracellular Ca2+. In the low magnesium model, AWD 140-190 applied with 200 microM reduced recurrent short discharges in area CA1 by 48.1 +/- 14.7%, while in the entorhinal cortex seizure-like events were not depressed. Late recurrent discharges were increased in frequency to 213.8 +/- 78.1 and reduced in amplitude by 50.1 +/- 14.4%. Responses to paired pulse stimuli with intervals ranging from 20 to 150 ms were reduced both with alvear and stratum radiatum stimulation. Decreases in [Ca2+]0 and associated slow field potentials evoked by repetitive stimulation of stratum radiatum were also depressed in a dose-dependent manner. AWD 140-190 also reduced stimulus-induced rises in [K+]0. AWD 140-190 200 microM diminished the amplitude of slow field potentials observed during high K(+)-induced spreading depression by about 17% in CA1 and 34% in entorhinal cortex without any significant effect on SD-associated rises in [K+]0. These results suggest that AWD 140-190 has an anticonvulsant effect presumably by interfering with repetitive generation of action potentials. AWD 140-190 may also possess modulatory effects on glial cells as suggested by the strong depression of SD-associated slow negative potential shifts.

Peptide growth factors but not ganglioside protect against excitotoxicity in rat retinal neurons in vitro

Glutamate is the major excitatory neurotransmitter in the retina, but excessive stimulation of its receptors leads to widespread neuronal stress and death. Both growth factors and gangliosides display important influences on responses to neuronal injury and degeneration. In this study, we have investigated the potential protective effects of two well characterized growth factors, epidermal and basic fibroblast growth factor (EGF and bFGF respectively), and the monosialoganglioside GM1, on cultured rat retinal neurons submitted to toxic levels of excitatory amino acids. Application of 1 mM glutamic acid reduced global neuronal viability by 80% when compared to control untreated cultures, whereas treatment with the glutamic acid agonist kainic acid (1 mM) led to specific, large decreases (75% reduction) in amacrine cell numbers. 24 h pretreatment with either EGF or bFGF (500 pM each) prevented the majority of excitatory amino acid-induced neuronal death, whereas similar treatment with 10(-5) M GM1 did not block neuronal degeneration. These findings demonstrate that EGF and bFGF act as neuroprotective agents against retinal excitotoxicity in vitro, whereas ganglioside GM1 is not effective in this particular paradigm.

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.

Glutamate transporter EAAC-1-deficient mice develop dicarboxylic aminoaciduria and behavioral abnormalities but no neurodegeneration

Four L-glutamate neurotransmitter transporters, the three Na(+)-dependent GLAST-1, GLT-1 and EAAC-1, and the Cl(-)-dependent EAAT-4, form a new family of structurally related integral plasma membrane proteins with different distribution in the central nervous system. They may have pivotal functions in the regulation of synaptic L-glutamate concentration during neurotransmission and are believed to prevent glutamate neurotoxicity. To investigate the specific physiological and pathophysiological role of the neuronal EAAC-1, which is also expressed in kidney and small intestine, we have generated two independent mouse lines lacking EAAC-1. eaac-1(-/-) mice develop dicarboxylic aminoaciduria. No neurodegeneration has been observed during a period of >12 months, but homozygous mutants display a significantly reduced spontaneous locomotor activity.

The rapid L- and D-aspartate uptake in cultured astrocytes

Astrocytes in primary culture possess a rapid L-aspartate saturable transport system (K(m) = 93 microM; V(max) = 81 nmol/min/mg protein), which shows certain stereospecificity since V(max) was 36% lower for D-aspartate uptake. These are values obtained at short incubation time (15 seconds), to obtain approximate initial rate conditions. Metabolic energy inhibitors, rotenone and iodoacetate very potently inhibited the L- and D-aspartate uptake processes, indicating that the transport process is an active one. However, the accumulation of L-aspartate was "enhanced" by inhibitors of L-aspartate metabolism, such as the aspartate aminotransferase inhibitor, aminooxyacetate and L-methionine sulfoximine, an inhibitor of glutamine synthetase, whereas D-aspartate (a non-metabolizable analog of L-aspartate) uptake was not affected. The accumulated levels of L-aspartate in the presence of aminooxyacetate were similar to the levels of D-aspartate. These effects of L-aspartate metabolic inhibitors, suggest that due to metabolism of the the L-aspartate, short incubation time (eg., 15 seconds) is necessary to measure the initial rate of L-aspartate uptake, in order to obtain the "true" kinetic parameters.