Extracellular Glutamate Concentration in Hippocampal Slice

Oregon Health and Science University, Portland, Oregon, United States
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 10/2007; 27(36):9736-41. DOI: 10.1523/JNEUROSCI.3009-07.2007
Source: PubMed

ABSTRACT Synaptic glutamate transients resulting from vesicular exocytosis are superimposed on a low baseline concentration of glutamate in the extracellular space. Reported values of baseline glutamate concentrations range up to 4 microM. If glutamate were present tonically at low micromolar concentrations, many receptors, especially the high-affinity NMDA receptors (NMDARs), would be activated or desensitized, altering neuronal excitability. Using NMDARs expressed by CA1 pyramidal cells in acute hippocampal slices to monitor extracellular glutamate, we find that its baseline concentration is much lower, near 25 nM. In addition, superfusion of low micromolar concentrations of glutamate had no effect on neurons, indicating that glutamate transport prevents access to receptors within the slice. However, equipotent concentrations of NMDA, a nontransported agonist, depolarized neurons dramatically. We suggest that ambient concentrations of glutamate in vivo are also in the nanomolar range and are too low to cause significant receptor activation.

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    • "Microgliosis, the accumulation of activated microglia, is a characteristic pathological feature in many neurologic conditions such as trauma, stroke, inflammation, epilepsy, and neurodegenerative diseases (Cagnin et al., 2001; Eikelenboom et al., 2002; McGeer and McGeer, 2002; Nelson et al., 2002; Orr et al., 2002; Bruijn et al., 2004; Pavese et al., 2006). Activated microglia release massive amounts of glutamate, at much higher levels than astrocytes and neurons (mM vs. μM), and destroy neural cells; these processes have been implicated as a major cause of neuronal damage in neurologic diseases (Piani et al., 1992; Barger and Basile, 2001; Schwartz et al., 2003; Ye et al., 2003; Kipnis et al., 2004; Takeuchi et al., 2005, 2008b; Herman and Jahr, 2007; Liang et al., 2008; Yawata et al., 2008). Therefore, blockade of glutamate signaling and inhibition of microglial activation have been explored as therapeutic candidates for several neurodegenerative diseases. "
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    ABSTRACT: Microglia are macrophage-like resident immune cells that contribute to the maintenance of homeostasis in the central nervous system (CNS). Abnormal activation of microglia can cause damage in the CNS, and accumulation of activated microglia is a characteristic pathological observation in neurologic conditions such as trauma, stroke, inflammation, epilepsy, and neurodegenerative diseases. Activated microglia secrete high levels of glutamate, which damages CNS cells and has been implicated as a major cause of neurodegeneration in these conditions. Glutamate-receptor blockers and microglia inhibitors (e.g., minocycline) have been examined as therapeutic candidates for several neurodegenerative diseases; however, these compounds exerted little therapeutic benefit because they either perturbed physiological glutamate signals or suppressed the actions of protective microglia. The ideal therapeutic approach would hamper the deleterious roles of activated microglia without diminishing their protective effects. We recently found that abnormally activated microglia secrete glutamate via gap-junction hemichannels on the cell surface. Moreover, administration of gap-junction inhibitors significantly suppressed excessive microglial glutamate release and improved disease symptoms in animal models of neurologic conditions such as stroke, multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease. Recent evidence also suggests that neuronal and glial communication via gap junctions amplifies neuroinflammation and neurodegeneration. Elucidation of the precise pathologic roles of gap junctions and hemichannels may lead to a novel therapeutic strategies that can slow and halt the progression of neurodegenerative diseases.
    Frontiers in Cellular Neuroscience 09/2014; 8(189). DOI:10.3389/fncel.2014.00189 · 4.29 Impact Factor
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    • "In addition to pharmacological block of glutamate uptake leading to increased activation of AMPA and NMDA receptors (Jabaudon et al., 1999, 2000; Cavelier and Attwell, 2005; Le Meur et al., 2007; Herman and Jahr, 2007), ischemia-induced reversed transport also leads to large increases in extracellular [Glu] and pathological receptor signaling (Rossi et al., 2000). Changes are also predicted by the probe diffusion model probe as a consequence of increases in basal glutamate release (Fig. 4B 3 ). "
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    ABSTRACT: Accurate knowledge of the ambient extracellular glutamate concentration in brain is required for understanding its potential impacts on tonic and phasic receptor signaling. Estimates of ambient glutamate based on microdialysis measurements are generally in the range of ∼2–10 μM, approximately 100-fold higher than estimates based on electrophysiological measurements of tonic NMDA receptor activity (∼25–90 nM). The latter estimates are closer to the low nanomolar estimated thermodynamic limit of glutamate transporters. The reasons for this discrepancy are not known, but it has been suggested that microdialysis measurements could overestimate ambient extracellular glutamate because of reduced glutamate transporter activity in a region of metabolically impaired neuropil adjacent to the dialysis probe. We explored this issue by measuring diffusion gradients created by varying membrane densities of glutamate transporters expressed in Xenopus oocytes. With free diffusion from a pseudo-infinite 10 μM glutamate source, the surface concentration of glutamate depended on transporter density and was reduced over 2 orders of magnitude by transporters expressed at membrane densities similar to those previously reported in hippocampus. We created a diffusion model to simulate the effect of transport impairment on microdialysis measurements with boundary conditions corresponding to a 100 μm radius probe. A gradient of metabolic disruption in a thin (∼100 μm) region of neuropil adjacent to the probe increased predicted [Glu] in the dialysate over 100-fold. The results provide support for electrophysiological estimates of submicromolar ambient extracellular [Glu] in brain and provide a possible explanation for the higher values reported using microdialysis approaches.
    Neurochemistry International 07/2014; 73(1). DOI:10.1016/j.neuint.2014.04.007 · 3.09 Impact Factor
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    • "Efficient uptake of glutamate into astrocytes via the glutamate/Na+ co-transporters depends on the transmembrane Na+ gradient generated by Na,K-ATPase [32]. The ambient glutamate concentration is around 25 nM, but can increase to levels in the range of 200 µM–1 mM following neuronal activity [25], [33]. The released glutamate needs to be rapidly cleared from the extracellular space to avoid detrimental effects of inappropriate activation of glutamate receptors. "
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    ABSTRACT: Glutamate released during neuronal activity is cleared from the synaptic space via the astrocytic glutamate/Na+ co-transporters. This transport is driven by the transmembrane Na+ gradient mediated by Na,K-ATPase. Astrocytes express two isoforms of the catalytic Na,K-ATPase α subunits; the ubiquitously expressed α1 subunit and the α2 subunit that has a more specific expression profile. In the brain α2 is predominantly expressed in astrocytes. The isoforms differ with regard to Na+ affinity, which is lower for α2. The relative roles of the α1 and α2 isoforms in astrocytes are not well understood. Here we present evidence that the presence of the α2 isoform may contribute to a more efficient restoration of glutamate triggered increases in intracellular sodium concentration [Na+]i. Studies were performed on primary astrocytes derived from E17 rat striatum expressing Na,K-ATPase α1 and α2 and the glutamate/Na+ co-transporter GLAST. Selective inhibition of α2 resulted in a modest increase of [Na+]i accompanied by a disproportionately large decrease in uptake of aspartate, an indicator of glutamate uptake. To compare the capacity of α1 and α2 to handle increases in [Na+]i triggered by glutamate, primary astrocytes overexpressing either α1 or α2 were used. Exposure to glutamate 200 µM caused a significantly larger increase in [Na+]i in α1 than in α2 overexpressing cells, and as a consequence restoration of [Na+]i, after glutamate exposure was discontinued, took longer time in α1 than in α2 overexpressing cells. Both α1 and α2 interacted with astrocyte glutamate/Na+ co-transporters via the 1st intracellular loop.
    PLoS ONE 06/2014; 9(6):e98469. DOI:10.1371/journal.pone.0098469 · 3.23 Impact Factor
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