Fast removal of synaptic glutamate by postsynaptic transporters

Glutamatergic Neurotransmission in Aging and Neurodegenerative Diseases: A Potential Target to Improve Cognitive Impairment in Aging

Aging is characterized by the decline in many of the individual's capabilities. It has been recognized that the brain undergoes structural and functional changes during aging that are occasionally associated with the development of neurodegenerative diseases. In this sense, altered glutamatergic neurotransmission, which involves the release, binding, reuptake, and degradation of glutamate (Glu) in the brain, has been widely studied in physiological and pathophysiological aging. In particular, changes in glutamatergic neurotransmission are exacerbated during neurodegenerative diseases and are associated with cognitive impairment, characterized by difficulties in memory, learning, concentration, and decision-making. Thus, in the present manuscript, we aim to highlight the relevance of glutamatergic neurotransmission during cognitive impairment to develop novel strategies to prevent, ameliorate, or delay cognitive decline. To achieve this goal, we provide a comprehensive review of the changes reported in glutamatergic neurotransmission components, such as Glu transporters and receptors during physiological aging and in the most studied neurodegenerative diseases. Finally, we describe the current therapeutic strategies developed to target glutamatergic neurotransmission.

Single-Molecule-Resolution Approaches in Synaptic Biology

Synapses between neurons are the primary loci for information transfer and storage in the brain. An individual neuron, alone, can make over 10000 synaptic contacts. It is, however, not easy to investigate what goes on locally within a synapse because many synaptic compartments are only a few hundred nanometers wide in size─close to the diffraction limit of light. To observe the biomolecular machinery and processes within synapses, in situ single-molecule techniques are emerging as powerful tools. Guided by important biological questions, this Perspective will highlight recent advances in using these techniques to obtain in situ measurements of synaptic molecules in three aspects: the cell-biological machinery within synapses, the synaptic architecture, and the synaptic neurotransmitter receptors. These advances showcase the increasing importance of single-molecule-resolution techniques for accessing subcellular biophysical and biomolecular information related to the brain.

Glutamate-Transporter Unbinding in Probabilistic Synaptic Environment Facilitates Activation of Distant NMDA Receptors

Once outside the synaptic cleft, the excitatory neurotransmitter glutamate is rapidly bound by its high-affinity transporters, which are expressed in abundance on the surface of perisynaptic astroglia. While this binding and the subsequent uptake of glutamate constrain excitatory transmission mainly within individual synapses, there is growing evidence for the physiologically important extrasynaptic actions of glutamate. However, the mechanistic explanation and the scope of such actions remain obscure. Furthermore, a significant proportion of glutamate molecules initially bound by transporters could be released back into the extracellular space before being translocated into astrocytes. To understand the implications of such effects, we simulated the release, diffusion, and transporter and receptor interactions of glutamate molecules in the synaptic environment. The latter was represented via trial-by-trial stochastic generation of astroglial and neuronal elements in the brain neuropil (overlapping spheroids of varied sizes), rather than using the 'average' morphology, thus reflecting the probabilistic nature of neuropil architectonics. Our simulations predict significant activation of high-affinity receptors, such as receptors of the NMDA type, at distances beyond half-micron from the glutamate release site, with glutamate-transporter unbinding playing an important role. These theoretical predictions are consistent with recent glutamate imaging data, thus lending support to the concept of significant volume-transmitted actions of glutamate in the brain.

2, 3, 5, 4'-tetrahydroxystilbene-2-O-beta-D-glucoside protects against neuronal cell death and traumatic brain injury-induced pathophysiology

Traumatic brain injury (TBI) is a global health issue that affects at least 10 million people per year. Neuronal cell death and brain injury after TBI, including apoptosis, inflammation, and excitotoxicity, have led to detrimental effects in TBI. 2, 3, 5, 4’-tetrahydroxystilbene-2-O-beta-D-glucoside (THSG), a water-soluble compound extracted from the Chinese herb Polygonum multiflorum, has been shown to exert various biological functions. However, the effects of THSG on TBI is still poorly understood. THSG reduced L-glutamate-induced DNA fragmentation and protected glial and neuronal cell death after L-glutamate stimulation. Our results also showed that TBI caused significant behavioral deficits in the performance of beam walking, mNSS, and Morris water maze tasks in a mouse model. Importantly, daily administration of THSG (60 mg/kg/day) after TBI for 21 days attenuated the injury severity score, promoted motor coordination, and improved cognitive performance post-TBI. Moreover, administration of THSG also dramatically decreased the brain lesion volume. THSG reduced TBI-induced neuronal apoptosis in the brain cortex 24 h after TBI. Furthermore, THSG increased the number of immature neurons in the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus. Our results demonstrate that THSG exerts neuroprotective effects on glutamate-induced excitotoxicity and glial and neuronal cell death. The present study also demonstrated that THSG effectively protects against TBI-associated motor and cognitive impairment, at least in part, by inhibiting TBI-induced apoptosis and promoting neurogenesis.

Real-time imaging of glutamate transients in the extracellular space of acute human brain slices using a single-wavelength glutamate fluorescence nanosensor

Glutamate is the most important excitatory neurotransmitter in the brain. The ability to assess glutamate release and re-uptake with high spatial and temporal resolution is crucial to understand the involvement of this primary excitatory neurotransmitter in both normal brain function and different neurological disorders. Real-time imaging of glutamate transients by fluorescent nanosensors has been accomplished in rat brain slices. We performed for the first time single-wavelength glutamate nanosensor imaging in human cortical brain slices obtained from patients who underwent epilepsy surgery. The glutamate fluorescence nanosensor signals of the electrically stimulated human cortical brain slices showed steep intensity increase followed by an exponential decrease. The spatial distribution and the time course of the signal were in good agreement with the position of the stimulation electrode and the dynamics of the electrical stimulation, respectively. Pharmacological manipulation of glutamate release and reuptake was associated with corresponding changes in the glutamate fluorescence nanosensor signals. We demonstrated that the recently developed fluorescent nanosensors for glutamate allow to detect neuronal activity in acute human cortical brain slices with high spatiotemporal precision. Future application to tissue samples from different pathologies may provide new insights into pathophysiology without the limitations of an animal model.

A subset of synaptic transmission events is coupled to acetyl coenzyme A production

Biological principles sustain the inference that synaptic function is coupled to neural metabolism, but the precise relationship between these two activities is not known. For example, it is unclear whether all synaptic transmission events are uniformly dependent on metabolic flux. Most synapses use glutamate, and the principal metabolic function of the brain is glucose oxidation, which starts with glycolysis. Thus, we asked how glutamatergic synaptic currents are modified by partial deficiency of the main glycolytic enzyme pyruvate dehydrogenase (PDH), which generates the intermediary metabolism product acetyl coenzyme A (acetyl-CoA). Using brain slices obtained from mice that were genetically modified to harbor a behaviorally relevant degree of PDH suppression, we also asked whether such impact is indeed metabolic via the bypassing of PDH with a glycolysis-independent acetyl-CoA substrate. We analyzed spontaneous synaptic currents under recording conditions that minimize artificial metabolic augmentation. Principal component analysis identified synaptic charge transfer as the major difference between a subset of wild-type and PDH-deficiency (PDHD) postsynaptic currents. This was due to reduced charge transfer as well as diminished current rise and decay times. The alternate acetyl-CoA source acetate rapidly restored these features but only for events of large amplitude as revealed by correlational and kernel density analyses. Application of tetrodotoxin to block large-amplitude events evoked by action potentials removed synaptic event charge transfer and decay-time differences between wild-type and PDHD neurons. These results suggest that glucose metabolic flux and excitatory transmission are intimately coupled for synaptic events characterized by large current amplitude.NEW & NOTEWORTHY In all tissues, metabolism and excitation are coupled but the details of this relationship remain elusive. Using a brain-targeted genetic approach in mice, reduction of pyruvate dehydrogenase, a major gateway in glucose metabolism, leads to changes that affect the synaptic event charge associated primarily with large excitatory (i.e., glutamate mediated) synaptic potentials. This can be modified in the direction of normal using the alternative fuel acetate, indicating that this phenomenon depends on rapid metabolic flux.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Microcircuit Rules Governing Impact of Single Interneurons on Purkinje Cell Output In Vivo

The functional impact of single interneurons on neuronal output in vivo and how interneurons are recruited by physiological activity patterns remain poorly understood. In the cerebellar cortex, molecular layer interneurons and their targets, Purkinje cells, receive excitatory inputs from granule cells and climbing fibers. Using dual patch-clamp recordings from interneurons and Purkinje cells in vivo, we probe the spatiotemporal interactions between these circuit elements. We show that single interneuron spikes can potently inhibit Purkinje cell output, depending on interneuron location. Climbing fiber input activates many interneurons via glutamate spillover but results in inhibition of those interneurons that inhibit the same Purkinje cell receiving the climbing fiber input, forming a disinhibitory motif. These interneuron circuits are engaged during sensory processing, creating diverse pathway-specific response functions. These findings demonstrate how the powerful effect of single interneurons on Purkinje cell output can be sculpted by various interneuron circuit motifs to diversify cerebellar computations.

α2δ-2 Protein Controls Structure and Function at the Cerebellar Climbing Fiber Synapse

α2δ proteins (Cacna2d1-4) are auxiliary subunits of voltage-dependent calcium channels that also drive synapse formation and maturation. Because cerebellar Purkinje cells (PCs) predominantly, if not exclusively, express one isoform of this family, α2δ-2 (Cacna2d2), we used PCs as a model system to examine roles of α2δ in excitatory synaptic function in male and female Cacna2d2 knock-out (KO) mice. Whole-cell recordings of PCs from acute cerebellar slices revealed altered climbing fiber (CF)-evoked complex spike generation, as well as increased amplitude and faster decay of CF-evoked EPSCs. CF terminals in the KO were localized more proximally on PC dendrites, as indicated by VGLUT2⁺ immunoreactive puncta, and computational modeling demonstrated that the increased EPSC amplitude can be partly attributed to the more proximal location of CF terminals. In addition, CFs in KO mice exhibited increased multivesicular transmission, corresponding to greater sustained responses during repetitive stimulation, despite a reduction in the measured probability of release. Electron microscopy demonstrated that mutant CF terminals had twice as many vesicle release sites, providing a morphologic explanation for the enhanced glutamate release. Though KO CFs evoked larger amplitude EPSCs, the charge transfer was the same as wild-type as a result of increased glutamate reuptake, producing faster decay kinetics. Together, the larger, faster EPSCs in the KO explain the altered complex spike responses, which degrade information transfer from PCs and likely contribute to ataxia in Cacna2d2 KO mice. Our results also illustrate the multidimensional synaptic roles of α2δ proteins.SIGNIFICANCE STATEMENT α2δ proteins (Cacna2d1-4) regulate synaptic transmission and synaptogenesis, but coexpression of multiple α2δ isoforms has obscured a clear understanding of how various α2δ proteins control synaptic function. We focused on roles of the α2δ-2 protein (Cacna2d2), the deletion of which causes cerebellar ataxia and epilepsy in mice and humans. Because cerebellar Purkinje cells (PCs) only express this single isoform, we studied excitatory climbing fiber synaptic function onto PCs in Cacna2d2 KO mice. Using optical and electrophysiological analysis, we provide a detailed description of the changes in PCs lacking α2δ-2, and provide a comprehensive mechanistic explanation for how functional synaptic phenotypes contribute to the altered cerebellar output.

Investigation of the allosteric coupling mechanism in a glutamate transporter homolog via unnatural amino acid mutagenesis

Glutamate transporters harness the ionic gradients across cell membranes for the concentrative uptake of glutamate. The sodium-coupled Asp symporter, Glt_Ph is an archaeal homolog of glutamate transporters and has been extensively used to understand the transport mechanism. A critical aspect of the transport cycle in Glt_Ph is the coupled binding of sodium and aspartate. Previous studies have suggested a major role for hairpin-2 (HP2), which functions as the extracellular gate for the aspartate binding site, in the coupled binding of sodium and aspartate to Glt_Ph In this study, we develop a fluorescence assay for monitoring HP2 movement by incorporating tryptophan and the unnatural amino acid, p-cyanophenylalanine into Glt_Ph We use the HP2 assays to show that HP2 opening with Na⁺ follows an induced-fit mechanism. We also determine how residues in the substrate binding site affect the opening and closing of HP2. Our data, combined with previous studies, provide the molecular sequence of events in the coupled binding of sodium and aspartate to Glt_Ph.

How to Build a Fast and Highly Sensitive Sound Detector That Remains Robust to Temperature Shifts

Frogs must have sharp hearing abilities during the warm summer months to successfully find mating partners. This study aims to understand how frog hair cell ribbon-type synapses preserve both sensitivity and temporal precision during temperature changes. Under room (∼24°C) and high (∼32°C) temperature, we performed in vitro patch-clamp recordings of hair cells and their afferent fibers in amphibian papillae of either male or female bullfrogs. Afferent fibers exhibited a wide heterogeneity in membrane input resistance (R_in) from 100 mΩ to 1000 mΩ, which may contribute to variations in spike threshold and firing frequency. At higher temperatures, most fibers increased their frequency of spike firing due to an increase in spontaneous EPSC frequencies. Hair cell resting membrane potential (V_rest) remained surprisingly stable during temperature increases, because Ca²⁺ influx and K⁺ outflux increased simultaneously. This increase in Ca²⁺ current likely enhanced spontaneous EPSC frequencies. These larger "leak currents" at V_rest also lowered R_in and produced higher electrical resonant frequencies. Lowering R_in will reduce the hair cells receptor potential and presumably moderate the systems sensitivity. Using membrane capacitance measurements, we suggest that hair cells can partially compensate for this reduced sensitivity by increasing exocytosis efficiency and the size of the readily releasable pool of synaptic vesicles. Furthermore, paired recordings of hair cells and their afferent fibers showed that synaptic delays shortened and multivesicular release becomes more synchronous at higher temperatures, which should improve temporal precision. Together, our results explain many previous in vivo observations on the temperature dependence of spikes in auditory nerves.SIGNIFICANCE STATEMENT The vertebrate inner ear detects and transmits auditory information over a broad dynamic range of sound frequency and intensity. It achieves remarkable sensitivity to soft sounds and precise frequency selectivity. How does the ear of cold-blooded vertebrates maintain its performance level as temperature changes? More specifically, how does the hair cell to afferent fiber synapse in bullfrog amphibian papilla adjust to a wide range of physiological temperatures without losing its sensitivity and temporal fidelity to sound signals? This study uses in vitro experiments to reveal the biophysical mechanisms that explain many observations made from in vivo auditory nerve fiber recordings. We find that higher temperature facilitates vesicle exocytosis and electrical tuning to higher sound frequencies, which benefits sensitivity and selectivity.

Photolysis of a Caged, Fast-Equilibrating Glutamate Receptor Antagonist, MNI-Caged γ-D-Glutamyl-Glycine, to Investigate Transmitter Dynamics and Receptor Properties at Glutamatergic Synapses

Fast uncaging of low affinity competitive receptor antagonists can in principle measure the timing and concentration dependence of transmitter action at receptors during synaptic transmission. Here, we describe the development, synthesis and characterization of MNI-caged γ-D-glutamyl-glycine (γ-DGG), which combines the fast photolysis and hydrolytic stability of nitroindoline cages with the well-characterized fast-equilibrating competitive glutamate receptor antagonist γ-DGG. At climbing fiber-Purkinje cell (CF-PC) synapses MNI-caged-γ-DGG was applied at concentrations up to 5 mM without affecting CF-PC transmission, permitting release of up to 1.5 mM γ-DGG in 1 ms in wide-field flashlamp photolysis. In steady-state conditions, photoreleased γ-DGG at 0.55–1.7 mM inhibited the CF first and second paired EPSCs by on average 30% and 60%, respectively, similar to reported values for bath applied γ-DGG. Photolysis of the L-isomer MNI-caged γ-L-glutamyl-glycine was ineffective. The time-course of receptor activation by synaptically released glutamate was investigated by timed photolysis of MNI-caged-γ-DGG at defined intervals following CF stimulation in the second EPSCs. Photorelease of γ-DGG prior to the stimulus and up to 3 ms after showed strong inhibition similar to steady-state inhibition; in contrast γ-DGG applied by a flash at 3–4 ms post-stimulus produced weaker and variable block, suggesting transmitter-receptor interaction occurs mainly in this time window. The data also show a small and lasting component of inhibition when γ-DGG was released at 4–7 ms post stimulus, near the peak of the CF-PC EPSC, or at 10–11 ms. This indicates that competition for binding and activation of AMPA receptors occurs also during the late phase of the EPSC, due to either delayed transmitter release or persistence of glutamate in the synaptic region. The results presented here first show that MNI-caged-γ-DGG has properties suitable for use as a synaptic probe at high concentration and that its photolysis can resolve timing and extent of transmitter activation of receptors in glutamatergic transmission.

Loss of cerebellar glutamate transporters EAAT4 and GLAST differentially affects the spontaneous firing pattern and survival of Purkinje cells

Loss of excitatory amino acid transporters (EAATs) has been implicated in a number of human diseases including spinocerebellar ataxias, Alzhiemer's disease and motor neuron disease. EAAT4 and GLAST/EAAT1 are the two predominant EAATs responsible for maintaining low extracellular glutamate levels and preventing neurotoxicity in the cerebellum, the brain region essential for motor control. Here using genetically modified mice we identify new critical roles for EAAT4 and GLAST/EAAT1 as modulators of Purkinje cell (PC) spontaneous firing patterns. We show high EAAT4 levels, by limiting mGluR1 signalling, are essential in constraining inherently heterogeneous firing of zebrin-positive PCs. Moreover mGluR1 antagonists were found to restore regular spontaneous PC activity and motor behaviour in EAAT4 knockout mice. In contrast, GLAST/EAAT1 expression is required to sustain normal spontaneous simple spike activity in low EAAT4 expressing (zebrin-negative) PCs by restricting NMDA receptor activation. Blockade of NMDA receptor activity restores spontaneous activity in zebrin-negative PCs of GLAST knockout mice and furthermore alleviates motor deficits. In addition both transporters have differential effects on PC survival, with zebrin-negative PCs more vulnerable to loss of GLAST/EAAT1 and zebrin-positive PCs more vulnerable to loss of EAAT4. These findings reveal that glutamate transporter dysfunction through elevated extracellular glutamate and the aberrant activation of extrasynaptic receptors can disrupt cerebellar output by altering spontaneous PC firing. This expands our understanding of disease mechanisms in cerebellar ataxias and establishes EAATs as targets for restoring homeostasis in a variety of neurological diseases where altered cerebellar output is now thought to play a key role in pathogenesis.

A Computational Model of Major Depression: the Role of Glutamate Dysfunction on Cingulo-Frontal Network Dynamics

Major depression disease (MDD) is associated with the dysfunction of multinode brain networks. However, converging evidence implicates the reciprocal interaction between midline limbic regions (typified by the ventral anterior cingulate cortex, vACC) and the dorso-lateral prefrontal cortex (dlPFC), reflecting interactions between emotions and cognition. Furthermore, growing evidence suggests a role for abnormal glutamate metabolism in the vACC, while serotonergic treatments (selective serotonin reuptake inhibitor, SSRI) effective for many patients implicate the serotonin system. Currently, no mechanistic framework describes how network dynamics, glutamate, and serotonin interact to explain MDD symptoms and treatments. Here, we built a biophysical computational model of 2 areas (vACC and dlPFC) that can switch between emotional and cognitive processing. MDD networks were simulated by slowing glutamate decay in vACC and demonstrated sustained vACC activation. This hyperactivity was not suppressed by concurrent dlPFC activation and interfered with expected dlPFC responses to cognitive signals, mimicking cognitive dysfunction seen in MDD. Simulation of clinical treatments (SSRI or deep brain stimulation) counteracted this aberrant vACC activity. Theta and beta/gamma oscillations correlated with network function, representing markers of switch-like operation in the network. The model shows how glutamate dysregulation can cause aberrant brain dynamics, respond to treatments, and be reflected in EEG rhythms as biomarkers of MDD.

Downregulation of Glutamate Transporter EAAT4 by Conditional Knockout of Rheb1 in Cerebellar Purkinje Cells

Excitatory amino acid transporter 4 (EAAT4) is believed to be critical to the synaptic activity of cerebellar Purkinje cells by limiting extracellular glutamate concentrations and facilitating the induction of long-term depression. However, the modulation of EAAT4 expression has not been elucidated. It has been shown that Ras homolog enriched in brain (Rheb)/mammalian target of rapamycin (mTOR) signaling plays essential roles in the regulation of protein translation, cell size, and cell growth. In addition, we previously found that a cascade including mTOR suppression and Akt activation induces increased expression of EAAT2 in astrocytes. In the present work, we explored whether Rheb/mTOR signaling is involved in the regulation of EAAT4 expression using conditional Rheb1 knockout mice. Our results demonstrated that Rheb1 deficiency resulted in the downregulation of EAAT4 expression, as well as decreased activity of mTOR and increased activity of Akt. The downregulation of EAAT4 was also confirmed by reduced EAAT4 currents and slowed kinetics of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor-mediated currents. On the other hand, conditional knockout of Rheb1 did not alter the morphology of Purkinje cell layer and the number of Purkinje cells. Overall, our findings suggest that small GTPase Rheb1 is a modulator in the expression of EAAT4 in Purkinje cells.

Ethanol Modulates the Spontaneous Complex Spike Waveform of Cerebellar Purkinje Cells Recorded in vivo in Mice

Cerebellar Purkinje cells (PCs) are sensitive to ethanol, but the effect of ethanol on spontaneous complex spike (CS) activity in these cells in vivo is currently unknown. Here, we investigated the effect of ethanol on spontaneous CS activity in PCs in urethane-anesthetized mice using in vivo patch-clamp recordings and pharmacological manipulation. Ethanol (300 mM) induced a decrease in the CS-evoked pause in simple spike (SS) firing and in the amplitude of the afterhyperpolarization (AHP) under current clamp conditions. Under voltage-clamp conditions, ethanol significantly decreased the area under the curve (AUC) and the number of CS spikelets, without changing the spontaneous frequency of the CSs or the instantaneous frequency of the CS spikelets. Ethanol-induced a decrease in the AUC of spontaneous CSs was concentration dependent. The EC₅₀ of ethanol for decreasing the AUC of spontaneous CSs was 168.5 mM. Blocking N-methyl-D-aspartate receptors (NMDARs) failed to prevent the ethanol-induced decreases in the CS waveform parameters. However, blockade of cannabinoid receptor 1 (CB1) significantly suppressed the ethanol-induced effects on the CS-evoked pause in SS firing, amplitude of the AHP, spikelet number and the AUC of CSs. Moreover, a CB1 receptor agonist not only reduced the number of spikelets and the AUC of CSs, but also prevented the ethanol-induced inhibition of CS activity. Our results indicate that ethanol inhibits CS activity via activation of the CB1 receptor in vivo in mice, suggesting that excessive ethanol intake inhibits climbing fiber (CF)–PC synaptic transmission by modulating CB1 receptors in the cerebellar cortex.

Neuron-glia cross talk revealed in reverberating networks by simultaneous extracellular recording of spikes and astrocytes' glutamate transporter and K+ currents

In neocortex networks, we simultaneously captured spikes and the slower astrocytes' K⁺and glutamate transporter (GluT) currents with the use of individual MEA electrodes. Inward and outward K⁺currents in different regions of the glial syncytium suggested that spatial buffering was operant. Moreover, in organotypic slices from ventral tegmental area and prefrontal cortex, the large GluT current amplitudes allowed to measure transporter currents with a single electrode. Our method allows direct study of the dynamic interplay of different cell types in excitable and nonexcitable tissue.

Astrocytes uptake synaptically released glutamate with electrogenic transporters (GluT) and buffer the spike-dependent extracellular K⁺ excess with background K⁺ channels. We studied neuronal spikes and the slower astrocytic signals on reverberating neocortical cultures and organotypic slices from mouse brains. Spike trains and glial responses were simultaneously captured from individual sites of multielectrode arrays (MEA) by splitting the recorded traces into appropriate filters and reconstructing the original signal by deconvolution. GluT currents were identified by using dl-threo-β-benzyloxyaspartate (TBOA). K⁺ currents were blocked by 30 μM Ba²⁺, suggesting a major contribution of inwardly rectifying K⁺ currents. Both types of current were tightly correlated with the spike rate, and their astrocytic origin was tested in primary cultures by blocking glial proliferation with cytosine β-d-arabinofuranoside (AraC). The spike-related, time-locked inward and outward K⁺ currents in different regions of the astrocyte syncytium were consistent with the assumptions of the spatial K⁺ buffering model. In organotypic slices from ventral tegmental area and prefrontal cortex, the GluT current amplitudes exceeded those observed in primary cultures by several orders of magnitude, which allowed to directly measure transporter currents with a single electrode. Simultaneously measuring cell signals displaying widely different amplitudes and kinetics will help clarify the neuron-glia interplay and make it possible to follow the cross talk between different cell types in excitable as well as nonexcitable tissue.

Autocrine glutamatergic transmission for the regulation of embryonal carcinoma stem cells

Glutamate behaves as the principal excitatory neurotransmitter in the vertebrate central nervous system and recently demonstrates intercellular signaling activities in periphery cancer cells. How the glutamatergic transmission is organized and operated in cancer stem cells remains undefined. We have identified a glutamatergic transmission circuit in embryonal carcinoma stem cells. The circuit is organized and operated in an autocrine mechanism and suppresses the cell proliferation and motility. Biological analyses determined a repertoire of glutamatergic transmission components, glutaminase, vesicular glutamate transporter, glutamate NMDA receptor, and cell membrane excitatory amino-acid transporter, for glutamate biosynthesis, package for secretion, reaction, and reuptake in mouse and human embryonal carcinoma stem cells. The glutamatergic components were also identified in mouse transplanted teratocarcinoma and in human primary teratocarcinoma tissues. Released glutamate acting as the signal was directly quantified by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Genetic and pharmacological abolishment of the endogenously released glutamate-induced tonic activation of the NMDA receptors increased the cell proliferation and motility. The finding suggests that embryonal carcinoma stem cells can be actively regulated by establishing a glutamatergic autocrine/paracrine niche via releasing and responding to the transmitter.

N-methyl-D-Aspartate Receptors Contribute to Complex Spike Signaling in Cerebellar Purkinje Cells: An In vivo Study in Mice

N-methyl-D-aspartate receptors (NMDARs) are post-synaptically expressed at climbing fiber-Purkinje cell (CF-PC) synapses in cerebellar cortex in adult mice and contributed to CF-PC synaptic transmission under in vitro conditions. In this study, we investigated the role of NMDARs at CF-PC synapses during the spontaneous complex spike (CS) activity in cerebellar cortex in urethane-anesthetized mice, by in vivo whole-cell recording technique and pharmacological methods. Under current-clamp conditions, cerebellar surface application of NMDA (50 μM) induced an increase in the CS-evoked pause of simple spike (SS) firing accompanied with a decrease in the SS firing rate. Under voltage-clamp conditions, application of NMDA enhanced the waveform of CS-evoked inward currents, which expressed increases in the area under curve (AUC) and spikelet number of spontaneous CS. NMDA increased the AUC of spontaneous CS in a concentration-dependent manner. The EC₅₀ of NMDA for increasing AUC of spontaneous CS was 33.4 μM. Moreover, NMDA significantly increased the amplitude, half-width and decay time of CS-evoked after-hyperpolarization (AHP) currents. Blockade of NMDARs with D-(-)-2-amino-5-phosphonopentanoic acid (D-APV, 250 μM) decreased the AUC, spikelet number, and amplitude of AHP currents. In addition, the NMDA-induced enhancement of CS activity could not be observed after α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors were blocked. The results indicated that NMDARs of CF-PC synapses contributed to the spontaneous CS activity by enhancing CS-evoked inward currents and AHP currents.

Allosteric Mechanisms of Molecular Machines at the Membrane: Transport by Sodium-Coupled Symporters

Solute transport across cell membranes is ubiquitous in biology as an essential physiological process. Secondary active transporters couple the unfavorable process of solute transport against its concentration gradient to the energetically favorable transport of one or several ions. The study of such transporters over several decades indicates that their function involves complex allosteric mechanisms that are progressively being revealed in atomistic detail. We focus on two well-characterized sodium-coupled symporters: the bacterial amino acid transporter LeuT, which is the prototype for the "gated pore" mechanism in the mammalian synaptic monoamine transporters, and the archaeal GltPh, which is the prototype for the "elevator" mechanism in the mammalian excitatory amino acid transporters. We present the evidence for the role of allostery in the context of a quantitative formalism that can reconcile biochemical and biophysical data and thereby connects directly to recent insights into the molecular structure and dynamics of these proteins. We demonstrate that, while the structures and mechanisms of these transporters are very different, the available data suggest a common role of specific models of allostery in their functions. We argue that such allosteric mechanisms appear essential not only for sodium-coupled symport in general but also for the function of other types of molecular machines in the membrane.

Motor Learning and the Cerebellum

Although our ability to store semantic declarative information can nowadays be readily surpassed by that of simple personal computers, our ability to learn and express procedural memories still outperforms that of supercomputers controlling the most advanced robots. To a large extent, our procedural memories are formed in the cerebellum, which embodies more than two-thirds of all neurons in our brain. In this review, we will focus on the emerging view that different modules of the cerebellum use different encoding schemes to form and express their respective memories. More specifically, zebrin-positive zones in the cerebellum, such as those controlling adaptation of the vestibulo-ocular reflex, appear to predominantly form their memories by potentiation mechanisms and express their memories via rate coding, whereas zebrin-negative zones, such as those controlling eyeblink conditioning, appear to predominantly form their memories by suppression mechanisms and express their memories in part by temporal coding using rebound bursting. Together, the different types of modules offer a rich repertoire to acquire and control sensorimotor processes with specific challenges in the spatiotemporal domain.

Fatty acids in energy metabolism of the central nervous system

In this review, we analyze the current hypotheses regarding energy metabolism in the neurons and astroglia. Recently, it was shown that up to 20% of the total brain's energy is provided by mitochondrial oxidation of fatty acids. However, the existing hypotheses consider glucose, or its derivative lactate, as the only main energy substrate for the brain. Astroglia metabolically supports the neurons by providing lactate as a substrate for neuronal mitochondria. In addition, a significant amount of neuromediators, glutamate and GABA, is transported into neurons and also serves as substrates for mitochondria. Thus, neuronal mitochondria may simultaneously oxidize several substrates. Astrocytes have to replenish the pool of neuromediators by synthesis de novo, which requires large amounts of energy. In this review, we made an attempt to reconcile β-oxidation of fatty acids by astrocytic mitochondria with the existing hypothesis on regulation of aerobic glycolysis. We suggest that, under condition of neuronal excitation, both metabolic pathways may exist simultaneously. We provide experimental evidence that isolated neuronal mitochondria may oxidize palmitoyl carnitine in the presence of other mitochondrial substrates. We also suggest that variations in the brain mitochondrial metabolic phenotype may be associated with different mtDNA haplogroups.

Disulfide cross-linking of transport and trimerization domains of a neuronal glutamate transporter restricts the role of the substrate to the gating of the anion conductance

Excitatory amino acid transporters remove synaptically released glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. A structure of an archeal homologue, which reflects an early intermediate on the proposed substrate translocation path, has been suggested to be similar to an anion conducting conformation. To probe this idea by functional studies, we have introduced two cysteine residues in the neuronal glutamate transporter EAAC1 at positions predicted to be close enough to form a disulfide bond only in outward-facing and early intermediate conformations of the homologue. Upon treatment of Xenopus laevis oocytes expressing the W441C/K269C double mutant with dithiothreitol, radioactive transport was stimulated >2-fold but potently inhibited by low micromolar concentrations of the oxidizing reagent copper(II)(1,10-phenanthroline)3. The substrate-induced currents by the untreated double mutant, reversed at approximately -20 mV, close to the reversal potential of chloride, but treatment with dithiothreitol resulted in transport currents with the same voltage dependence as the wild type. It appears therefore that in the oocyte expression system the introduced cysteine residues in many of the mutant transporters are already cross-linked and are only capable of mediating the substrate-gated anion conductance. Reduction of the disulfide bond now allows these transporters to execute the full transport cycle. Our functional data support the idea that the anion conducting conformation of the neuronal glutamate transporter is associated with an early step of the transport cycle.

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

Decreased expression of GLT-1 in the R6/2 model of Huntington's disease does not worsen disease progression

Excitotoxicity is thought to be important in the pathogenesis of Huntington's disease (HD). Glutamate is the predominant excitatory neurotransmitter in the brain, and excess activation of glutamate receptors can cause neuronal dysfunction and death. Glutamate transporters regulate the extracellular concentration of glutamate. GLT-1 is the most abundant glutamate transporter, and accounts for most of the glutamate transport in the brain. Administration of ceftriaxone, an antibiotic that increases the functional expression of GLT-1, can improve the behavioral phenotype of the R6/2 mouse model of HD. To test the hypothesis that GLT-1 expression critically affects the HD disease process, we generated a novel mouse model that is heterozygous for the null allele of GLT-1 and carries the R6/2 transgene (double mutation). We demonstrated that the protein expression of total GLT-1, as well as two of its isoforms, is decreased within the cortex and striatum of 12-week-old R6/2 mice, and that the expression of EAAC1 was decreased in the striatum. Protein expression of GLT-1 was further decreased in the cortex and striatum of the double mutation mice compared with the R6/2 mice at 11 weeks. However, the effects of the R6/2 transgene on weight loss, accelerating rotarod, climbing and paw-clasping were not exacerbated in these double mutants. Na(+) -dependent glutamate uptake into synapatosomes isolated from the striatum and cortex of 11-week-old R6/2 mice was unchanged compared with controls. These results suggest that changes in GLT-1 expression or function per se are unlikely to potentiate or ameliorate the progression of HD.

A computational model to investigate astrocytic glutamate uptake influence on synaptic transmission and neuronal spiking

Over the past decades, our view of astrocytes has switched from passive support cells to active processing elements in the brain. The current view is that astrocytes shape neuronal communication and also play an important role in many neurodegenerative diseases. Despite the growing awareness of the importance of astrocytes, the exact mechanisms underlying neuron-astrocyte communication and the physiological consequences of astrocytic-neuronal interactions remain largely unclear. In this work, we define a modeling framework that will permit to address unanswered questions regarding the role of astrocytes. Our computational model of a detailed glutamatergic synapse facilitates the analysis of neural system responses to various stimuli and conditions that are otherwise difficult to obtain experimentally, in particular the readouts at the sub-cellular level. In this paper, we extend a detailed glutamatergic synaptic model, to include astrocytic glutamate transporters. We demonstrate how these glial transporters, responsible for the majority of glutamate uptake, modulate synaptic transmission mediated by ionotropic AMPA and NMDA receptors at glutamatergic synapses. Furthermore, we investigate how these local signaling effects at the synaptic level are translated into varying spatio-temporal patterns of neuron firing. Paired pulse stimulation results reveal that the effect of astrocytic glutamate uptake is more apparent when the input inter-spike interval is sufficiently long to allow the receptors to recover from desensitization. These results suggest an important functional role of astrocytes in spike timing dependent processes and demand further investigation of the molecular basis of certain neurological diseases specifically related to alterations in astrocytic glutamate uptake, such as epilepsy.

Charge compensation mechanism of a Na+-coupled, secondary active glutamate transporter

Forward glutamate transport by the excitatory amino acid carrier EAAC1 is coupled to the inward movement of three Na(+) and one proton and the subsequent outward movement of one K(+) in a separate step. Based on indirect evidence, it was speculated that the cation binding sites bear a negative charge. However, little is known about the electrostatics of the transport process. Valences calculated using the Poisson-Boltzmann equation indicate that negative charge is transferred across the membrane when only one cation is bound. Consistently, transient currents were observed in response to voltage jumps when K(+) was the only cation on both sides of the membrane. Furthermore, rapid extracellular K(+) application to EAAC1 under single turnover conditions (K(+) inside) resulted in outward transient current. We propose a charge compensation mechanism, in which the C-terminal transport domain bears an overall negative charge of -1.23. Charge compensation, together with distribution of charge movement over many steps in the transport cycle, as well as defocusing of the membrane electric field, may be combined strategies used by Na(+)-coupled transporters to avoid prohibitive activation barriers for charge translocation.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.

Glutamate spillover between mammalian cone photoreceptors

Cone photoreceptors transmit signals at high temporal frequencies and mediate fine spatial vision. High-frequency transmission requires a high rate of glutamate release, which could promote spillover to neighboring cells, whereas spatial vision requires that cones within a tightly packed array signal light to postsynaptic bipolar cells with minimal crosstalk. Glutamate spread from the cone terminal is thought to be limited by presynaptic transporters and nearby glial processes. In addition, there is no ultrastructural evidence for chemical synapses between mammalian cones, although such synapses have been described in lower vertebrate retinas. We tested for cone-cone glutamate diffusion by recording from adjacent cone pairs in the ground squirrel retina, and instead found that the glutamate released by one cone during electrical stimulation activates glutamate transporter Cl(-) conductances on neighboring cones. Unlike in other systems, where crosstalk is diminished by increasing the temperature and by moving to a more intact preparation, glutamate spread persisted at physiological temperatures (37°C) and in retinal flat mounts. The glutamate-gated anion conductance in cones has a reversal potential of ∼-30 mV compared with a cone resting potential of ∼-50 mV; thus, crosstalk should have a depolarizing effect on the cone network. Cone-cone glutamate spread is regulated by the physiological stimulus, light, and under physiological conditions can produce a response of ∼2 mV, equivalent to 13-20% of a cone's light response. We conclude that in the absence of discrete chemical synapses, glutamate flows between cones during a light response and may mediate a spatially distributed positive feedback.

Identification of the third Na+ site and the sequence of extracellular binding events in the glutamate transporter

The transport cycle in the glutamate transporter (GlT) is catalyzed by the cotransport of three Na(+) ions. However, the positions of only two of these ions (Na1 and Na2 sites) along with the substrate have been captured in the crystal structures reported for both the outward-facing and the inward-facing states of Glt(ph). Characterizing the third ion binding site (Na3) is necessary for structure-function studies attempting to investigate the mechanism of transport in GlTs at an atomic level, particularly for the determination of the sequence of the binding events during the transport cycle. In this study, we report a series of molecular dynamics simulations performed on various bound states of Glt(ph) (the apo state, as well as in the presence of Na(+), the substrate, or both), which have been used to identify a putative Na3 site. The calculated trajectories have been used to determine the water accessibility of potential ion-binding residues in the protein, as a prerequisite for their ion binding. Combined with conformational analysis of the key regions in the protein in different bound states and several additional independent simulations in which a Na(+) ion was randomly introduced to the interior of the transporter, we have been able to characterize a putative Na3 site and propose a plausible binding sequence for the substrate and the three Na(+) ions to the transporter during the extracellular half of the transport cycle. The proposed Na3 site is formed by a set of highly conserved residues, namely, Asp(312), Thr(92), and Asn(310), along with a water molecule. Simulation of a fully bound state, including the substrate and the three Na(+) ions, reveals a stable structure--showing closer agreement to the crystal structure when compared to previous models lacking an ion in the putative Na3 site. The proposed sequence of binding events is in agreement with recent experimental models suggesting that two Na(+) ions bind before the substrate, and one after that. Our results, however, provide additional information about the sites involved in these binding events.

Zones of enhanced glutamate release from climbing fibers in the mammalian cerebellum

Purkinje cells in the mammalian cerebellum are remarkably homogeneous in shape and orientation, yet they exhibit regional differences in gene expression. Purkinje cells that express high levels of zebrin II (aldolase C) and the glutamate transporter EAAT4 cluster in parasagittal zones that receive input from distinct groups of climbing fibers (CFs); however, the physiological properties of CFs that target these molecularly distinct Purkinje cells have not been determined. Here we report that CFs that innervate Purkinje cells in zebrin II-immunoreactive (Z(+)) zones release more glutamate per action potential than CFs in Z(-) zones. CF terminals in Z(+) zones had larger pools of release-ready vesicles, exhibited enhanced multivesicular release, and produced larger synaptic glutamate transients. As a result, CF-mediated EPSCs in Purkinje cells decayed more slowly in Z(+) zones, which triggered longer-duration complex spikes containing a greater number of spikelets. The differences in the duration of CF EPSCs between Z(+) and Z(-) zones persisted in EAAT4 knock-out mice, indicating that EAAT4 is not required for maintaining this aspect of CF function. These results indicate that the organization of the cerebellum into discrete longitudinal zones is defined not only by molecular phenotype of Purkinje cells within zones, but also by the physiological properties of CFs that project to these distinct regions. The enhanced release of glutamate from CFs in Z(+) zones may alter the threshold for synaptic plasticity and prolong inhibition of cerebellar output neurons in deep cerebellar nuclei.

Glutamate induces neurotrophic factor production from microglia via protein kinase C pathway

Microglia are intrinsic immune cells in the central nervous system and play key roles in the pathogenesis of various central nervous system disorders. Microglia have been shown to attack damaged neurons by secreting a variety of neurotoxic factors including inflammatory cytokines, reactive oxygen species and glutamate. On the other hand, they can produce neurotrophic factors (NTFs) which support neuronal survival and growth. However, the precise mechanism that regulates microglial NTF production is not fully understood, and the relation between glutamate and NTFs remains unclear. In the present study, we show that glutamate significantly induces microglial NTF production by the activation of N-methyl-d-aspartate (NMDA) receptors, group III metabotropic glutamate receptors, and glutamate transporters. Activation of NMDA receptors and group III metabotropic glutamate receptors induces intracellular Ca(2+) release from the endoplasmic reticulum. Further, stimulation of glutamate transporters leads to influx of extracellular Ca(2+) in a Na(+)-dependent manner. This intracellular Ca(2+) elevation activates the protein kinase C pathway which induces microglial NTF expression and production. These results suggest that microglia play a neuroprotective role during the excitotoxic state in neurodegenerative diseases.

Functions of glutamate transporters in cerebellar Purkinje cell synapses

Glutamate transporters play a critical role in the maintenance of low extracellular concentrations of glutamate, which prevents the overactivation of post-synaptic glutamate receptors. Four distinct glutamate transporters, GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3 and EAAT4, are distributed in the molecular layer of the cerebellum, especially near glutamatergic synapses in Purkinje cells (PCs). This review summarizes the current knowledge about the differential roles of these transporters at excitatory synapses of PCs. Data come predominantly from electrophysiological experiments in mutant mice that are deficient in each of these transporter genes. GLAST expressed in Bergmann glia contributes to the clearing of the majority of glutamate that floods out of the synaptic cleft immediately after transmitter release from the climbing fibre (CF) and parallel fibre (PF) terminals. It is indispensable to maintain a one-to-one relationship in synaptic transmission at the CF synapses by preventing transcellular glutamate spillover. GLT-1 plays a similar but minor role in the uptake of glutamate as GLAST. Although the loss of neither GLAST nor GLT-1 affects cerebellar morphology, the deletion of both GLAST and GLT-1 genes causes the death of the mutant animal and hinders the folium formation of the cerebellum. EAAT4 removes the low concentrations of glutamate that escape from uptake by glial transporters, preventing the transmitter from spilling over into neighbouring synapses. It also regulates the activation of metabotropic glutamate receptor 1 (mGluR1) in perisynaptic regions at PF synapses, which in turn affects mGluR1-mediated events including slow EPSCs and long-term depression. No change in synaptic function is detected in mice that are deficient in EAAC1.

The neuromediator glutamate, through specific substrate interactions, enhances mitochondrial ATP production and reactive oxygen species generation in nonsynaptic brain mitochondria

The finding that upon neuronal activation glutamate is transported postsynaptically from synaptic clefts and increased lactate availability for neurons suggest that brain mitochondria (BM) utilize a mixture of substrates, namely pyruvate, glutamate, and the tricarboxylic acid cycle metabolites. We studied how glutamate affected oxidative phosphorylation and reactive oxygen species (ROS) production in rat BM oxidizing pyruvate + malate or succinate. Simultaneous oxidation of glutamate + pyruvate + malate increased state 3 and uncoupled respiration by 52 and 71%, respectively. The state 4 ROS generation increased 100% over BM oxidizing pyruvate + malate and 900% over that of BM oxidizing glutamate + malate. Up to 70% of ROS generation was associated with reverse electron transport. These effects of pyruvate + glutamate + malate were observed only with BM and not with liver or heart mitochondria. The effects of glutamate + pyruvate on succinate-supported respiration and ROS generation were not organ-specific and depended only on whether mitochondria were isolated with or without bovine serum albumin. With the non-bovine serum albumin brain and heart mitochondria oxidizing succinate, the addition of pyruvate and glutamate abrogated inhibition of Complex II by oxaloacetate. We conclude that (i) during neuronal activation, simultaneous oxidation of glutamate + pyruvate temporarily enhances neuronal mitochondrial ATP production, and (ii) intrinsic inhibition of Complex II by oxaloacetate is an inherent mechanism that protects against ROS generation during reverse electron transport.

Glutamate forward and reverse transport: from molecular mechanism to transporter-mediated release after ischemia

Glutamate transporters remove the excitatory neurotransmitter glutamate from the extracellular space after neurotransmission is complete, by taking glutamate up into neurons and glia cells. As thermodynamic machines, these transporters can also run in reverse, releasing glutamate into the extracellular space. Because glutamate is excitotoxic, this transporter-mediated release is detrimental to the health of neurons and axons, and it, thus, contributes to the brain damage that typically follows a stroke. This review highlights current ideas about the molecular mechanisms underlying glutamate uptake and glutamate reverse transport. It also discusses the implications of transporter-mediated glutamate release for cellular function under physiological and patho-physiological conditions.

Interaction between the glutamate transporter GLT1b and the synaptic PDZ domain protein PICK1

Synaptic plasticity is implemented by the interaction of glutamate receptors with PDZ domain proteins. Glutamate transporters provide the only known mechanism of clearance of glutamate from excitatory synapses, and GLT1 is the major glutamate transporter. We show here that GLT1 interacts with the PDZ domain protein PICK1, which plays a critical role in regulating the expression of glutamate receptors at excitatory synapses. A yeast two-hybrid screen of a neuronal library using the carboxyl tail of GLT1b yielded clones expressing PICK1. The GLT1b C-terminal peptide bound to PICK1 with high affinity (K(i) = 6.5 +/- 0.4 microM) in an in vitro fluorescence polarization assay. We also tested peptides based on other variants of GLT1 and other glutamate transporters. GLT1b co-immunoprecipitated with PICK1 from rat brain lysates and COS7 cell lysates derived from cells transfected with plasmids expressing PICK1 and GLT1b. In addition, expression of GLT1b in COS7 cells changed the distribution of PICK1, bringing it to the surface. GLT1b and PICK1 co-localized with each other and with synaptic markers in hippocampal neurons in culture. Phorbol ester, an activator of protein kinase C (PKC), a known PICK1 interactor, had no effect on glutamate transport in rat forebrain neurons in culture. However, we found that exposure of neurons to a myristolated decoy peptide with sequence identical to the C-terminal sequence of GLT1b designed to block the PICK1-GLT1b interaction rendered glutamate transport into neurons responsive to phorbol ester. These results suggest that the PICK1-GLT1b interaction regulates the modulation of GLT1 function by PKC.

Rapid increase of glial glutamate uptake via blockade of the protein kinase A pathway

Glutamate is the main excitatory neurotransmitter in the vertebrate central nervous system. Removal of this transmitter from the synaptic cleft by glial and neuronal transporter systems plays an important role in terminating glutamatergic neurotransmission. The effects of different activators and blockers of PKA and PKC on glutamate uptake were studied in primary glial cells cultivated from the rat cortex using the patch-clamp recording technique and immunocytochemical methods. GF 109203X enhances glutamate-induced membrane currents in a concentration- and time-dependent manner. After pre-application for 40 s the maximal transport capacity was increased by 30-80%. The estimated Km-value of the transport system did not change after drug application and the enhanced glutamate uptake was reversible within a few minutes upon washout. Activators and blockers of the PKC pathway did not affect glutamate uptake, whereas H89, a selective blocker of PKA, mimicked the effects of GF 109203X, indicating involvement of the protein kinase A pathway. The GF 109203X-induced increase in transport capacity is likely to be mediated by GLAST since the GLT-1 selective blocker dihydrokainate was unable to block basal or stimulated glutamate uptake. Furthermore, the increase in transport activity may well be based on an increase in cell surface expression of the transporter protein since preincubation with cytochalasin-B, a protein that blocks actin polymerization, almost completely abolished the effect of GF 109203X and H89. These results indicate that GF 109203X and H89 enhance glial glutamate uptake via blockade of the PKA. The described effect may affect glutamatergic neurotransmission by reducing the glutamate concentration in the synaptic cleft.

Glutamate transporters: confining runaway excitation by shaping synaptic transmission

Traditionally, glutamate transporters have been viewed as membrane proteins that harness the electrochemical gradient to slowly transport glutamate from the extracellular space into glial cells. However, recent studies have shown that glutamate transporters on glial and neuronal membranes also rapidly bind released glutamate to shape synaptic transmission. In this Review, we summarize the properties of glutamate transporters that influence synaptic transmission and are subject to regulation and plasticity. We highlight how the diversity of glutamate-transporter function relates to transporter location, density and affinity.

Brain metabolite abnormalities in the white matter of elderly schizophrenic subjects: implication for glial dysfunction

BACKGROUND

Abnormalities in the white matter of the brain may occur in individuals with schizophrenia as well as with normal aging. Therefore, elderly schizophrenic patients may suffer further cognitive decline as they age. This study determined whether elderly schizophrenia participants, especially those with declined cognitive function (Clinical Dementia Rating score > 1), show white matter metabolite abnormalities on proton magnetic resonance spectroscopy and whether there are group differences in age-dependent changes in these brain metabolites.

METHOD

Twenty-three elderly schizophrenia and twenty-two comparison participants fulfilling study criteria were enrolled. Localized, short echo-time (1)H MRS at 4 Tesla was used to assess neurometabolite concentrations in several white matter regions.

RESULTS

Compared with healthy subjects, schizophrenia participants had lower N-acetyl compounds (-12.6%, p = .0008), lower myo-inositol (-16.4%, p = .026), and higher glutamate + glutamine (+28.7%, p = .0016) concentrations across brain regions. Schizophrenia participants with Clinical Dementia Rating >/= 1 showed the lowest NA in the frontal and temporal regions compared with control subjects. Interactions between age and schizophrenia status on total creatine and choline-containing compounds were observed; only schizophrenia participants showed age-related decreases of these metabolites in the right frontal region.

CONCLUSIONS

Decreased NA in these white matter brain regions likely reflects reduced neuronal content associated with decreased synapses and neuronal cell volumes. The elevated glutamate + glutamine, if reflecting elevated glutamate, could result from excess neuronal glutamate release or glial dysfunction in glutamate reuptake. The decreased myo-inositol in participants with schizophrenia suggests decreased glial content or dysfunctional glia, which might result from glutamate-mediated toxicity.

NMDA receptor contribution to the climbing fiber response in the adult mouse Purkinje cell

Among integrative neurons displaying long-term synaptic plasticity, adult Purkinje cells seemed to be an exception by lacking functional NMDA receptors (NMDA-Rs). Although numerous anatomical studies have shown both NR1 and NR2 NMDA-R subunits in adult Purkinje cells, patch-clamp studies failed to detect any NMDA currents. Using more recent pharmacological and immunodetection tools, we demonstrate here that Purkinje cells from adult mice respond to exogenous NMDA application and that postsynaptic NMDA-Rs carry part of the climbing fiber-mediated EPSC (CF-EPSC), with undetectable contribution from presynaptic or polysynaptic NMDA currents. We also detect NR2-A/B subunits in adult Purkinje cells by immunohistochemistry. The NMDA-mediated CF-EPSC is barely detectable before 3 weeks postnatal. From the end of the third week, the number of cells displaying the NMDA-mediated CF-EPSC rapidly increases. Soon, this EPSC becomes detectable in all the Purkinje cells but is still very small. Its amplitude continues to increase until 12 weeks after birth. In mature Purkinje cells, we show that the NMDA-Rs contribute to the depolarizing plateau of complex spikes and increase their number of spikelets. Together, these observations demonstrate that mature Purkinje cells express functional NMDA receptors that become detectable in CF-EPSCs at approximately 21 d after birth and control the complex spike waveform.

Imaging of glutamate in brain slices using FRET sensors

The neurotransmitter glutamate is the mediator of excitatory neurotransmission in the brain. Release of this signaling molecule is carefully controlled by multiple mechanisms, yet the methods available to measure released glutamate have been limited in spatial and/or temporal domains. We have developed a novel technique to visualize glutamate release in brain slices using three purified fluorescence (Forster) energy resonance transfer (FRET)-based glutamate sensor proteins. Using a simple loading protocol, the FRET sensor proteins diffuse deeply into the extracellular space and remain functional for many tens of minutes. This allows imaging of glutamate release in brain slices with simultaneous electrophysiological recordings and provides temporal and spatial resolution not previously possible. Using this glutamate FRET sensor loading and imaging protocol, we show that changes in network excitability and glutamate re-uptake alter evoked glutamate transients and produce correlated changes in evoked-cortical field potentials. Given the sophisticated advantages of brain slices for electrophysiological and imaging protocols, the ability to perform real-time imaging of glutamate in slices should lead to key insights in brain function relevant to plasticity, development and pathology. This technique also provides a unique assay of network activity that compliments alternative techniques such as voltage-sensitive dyes and multi-electrode arrays.

The immune-mediated alteration of serotonin and glutamate: towards an integrated view of depression

Beside the well-known deficiency in serotonergic neurotransmission as pathophysiological correlate of major depression (MD), recent evidence points to a pivotal role of increased glutamate receptor activation as well. However, cause and interaction of these neurotransmitter alterations are not understood. In this review, we present a hypothesis integrating current concepts of neurotransmission and hypothalamus-pituitary-adrenal (HPA) axis dysregulation with findings on immunological alterations and alterations in brain morphology in MD. An immune activation including increased production of proinflammatory cytokines has repeatedly been described in MD. Proinflammatory cytokines such as interleukin-2, interferon-gamma, or tumor necrosis factor-alpha activate the tryptophan- and serotonin-degrading enzyme indoleamine 2,3-dioxygenase (IDO). Depressive states during inflammatory somatic disorders are also associated with increased proinflammatory cytokines and increased consumption of tryptophan via activation of IDO. An enhanced consumption of serotonin and its precursor tryptophan through IDO activation could well explain the reduced availability of serotonergic neurotransmission in MD. An increased activation of IDO and its subsequent enzyme kynurenine monooxygenase by proinflammatory cytokines, moreover, leads to an enhanced production of quinolinic acid, a strong agonist of the glutamatergic N-methyl-D-aspartate receptor. In inflammatory states of the central nervous system, IDO is mainly activated in microglial cells, which preferentially metabolize tryptophan to the NMDA receptor agonist quinolinic acid, whereas astrocytes - counteracting this metabolism due to the lack of an enzyme of this metabolism - have been observed to be reduced in MD. Therefore the type 1/type 2 immune response imbalance, associated with an astrocyte/microglia imbalance, leads to serotonergic deficiency and glutamatergic overproduction. Astrocytes are further strongly involved in re-uptake and metabolic conversion of glutamate. The reduced number of astrocytes could contribute to both, a diminished counterregulation of IDO activity in microglia and an altered glutamatergic neurotransmission. Further search for antidepressant agents should take into account anti-inflammatory drugs, for example, cyclooxygenase-2 inhibitors, might exert antidepressant effects by acting on serotonergic deficiency, glutamatergic hyperfunction and antagonizing neurotoxic effects of quinolinic acid.