Through these measurements they conclude that extracellular glutamate concentrations in hippocampal slices are between 23 and 89nM (Cavelier and Attwell,2005; Herman and Jahr,2007). different technologies. Keywords:glutamate, synapse, glia, glutamate uptake, cystineglutamate exchange, mGluR The concentration of glutamate Biapenem varies dramatically depending upon the biological compartment being measured. For example, plasma glutamate levels are estimated in the range of 150 M, cerebrospinal fluid levels around 10 M, and intracellular glutamate concentrations in the brain are approximately 10 mM (Danbolt,2001; Featherstone and Shippy,2008). The concentration of glutamate in the synaptic cleft following action potential mediated release exceeds 1 mM for <10 ms, and rapidly returns to <20 nM between release events due high affinity glutamate uptake by neurons and glia (Dzubay and Jahr,1999). In contrast to the Biapenem agreed upon glutamate concentrations in these biological compartments, estimates of the tonic basal concentration of glutamate within the extracellular space outside of the synaptic cleft varies over three orders of magnitude, ranging from 0.02 to 30 M (Herman and Jahr,2007; Chefer et al.,2009). The variance in estimated extracellular glutamate concentration results from electrophysiological estimates madein vitrofrom Biapenem tissue slices (0.020.1 M) versusin vivomeasurements using microdialysis or voltammetry (130 M). Discerning the reasons behind these discrepancies and ascertaining the correct level of extracellular glutamate outside the synaptic cleft is critical to understanding the physiology of how glutamate receptors, transporters, and antiporters regulate synaptic and glial glutamate release. Also, the concentration of extracellular glutamate will determine its role in metabolic processes such as cellular redox potential and neurometabolic coupling between synaptic activity, glial metabolism, and blood flow (Aoyama et al.,2008; Magistretti,2009). Knowledge of how these glial and neuronal glutamatergic processes homeostatically regulate extracellular glutamate concentrations is central to understanding the physiology of excitatory neuroplasticity and its emerging role in neuropsychiatric diseases. For example, extracellular levels outside the synaptic cleft in the low micromolar range place glutamate in the range of theKdfor glutamate binding to extrasynaptic metabotropic glutamate receptors (mGluR), glutamate transporters, and extrasynaptic NMDA ionotropic glutamate receptors, while levels in low nanomolar range would not significantly occupy these binding sites (Conn and Pin,1997; Danbolt,2001; Herman and Jahr,2007). Here we explore possible reasons why thein vitroelectrophysiology andin vivoexperiments deliver such disparate values. Further, we propose that both measurements are correctly assessing extracellular subcompartments that differentially contain nanomolar and micromolar extracellular glutamate, and that the concentration difference arises from the patterned expression of release and uptake sites on the membrane surface. The multicompartment proposal is illustrated in Figure1, where we outline a working definition of extracellular glutamate into three subcompartments, including GNAS synaptic, perisynaptic, and nonsynaptic subcompartments. == Figure 1. == Tonic levels of extracellular glutamate measured in the synaptic, perisynaptic, and nonsynaptic compartments of the extracellular space. The levels of glutamate shown are approximated for resting conditions (i.e., in the absence of action potential mediated synaptic release). It is proposed that extracellular glutamate in compartmentalized morphologically by close appositions between neuronal and glial processes (note that the distances between glianeuron distances are exaggerated for illustrative purposes), and by the patterned distribution of glutamate uptake sites and neuronal or glial release sites. This creates 2 regions of nanomolar glutamate high in iGluR, uptake sites and either synaptic (1) or glial (2) glutamate release sites. A second region surrounding the nanomolar region contains low micromolar levels of glutamate that are maintained largely from glial release and relatively lower glutamate uptake via high affinity transporters. This latter compartment is in part populated by mGluR that receive tonic glutamate stimulation and thereby modulate synaptic (and probably glial; DAscenzo et al.,2007) release and neuroplasticity. == In vivo(M) Versusin vitro(nM) Measures of Extracellular Glutamate in Brain == Estimates of thein vivoconcentration of extracellular glutamate using microdialysis have been made in a variety of mammalian species and in many brain regions (Chefer et al.,2009). The most accepted approach is the no-net-flux method where increasing concentrations of exogenous glutamate are passed through the probe and extracellular glutamate is inferred as the point wherein vivoglutamate and exogenously applied glutamate are equal (i.e., no-net-flux of glutamate across the dialysis membrane). These studies measure 130 M extracellular glutamate in different brain regions between different mammalian species, with the majority of studies finding levels between 1 and 5 M (Jacobson et al.,1985; Lerma et al.,1986; Miele et al.,1996; Baker et al.,2003; Galvan et al.,2003; Szumlinski et al.,2004,2005; Melendez et al.,2005a; Miller et al.,2008; Berglind et al.,2009). Microdialysis typically provides measurements at 1 min intervals, with most samples obtained in the range of 1030 min Biapenem intervals, and the measurement surface.