Abstract
Glutamate transporters (EAATs) are important to maintain spatial and temporal specificity of synaptic transmission. Their efficiency to uptake and transport glutamate into the intracellular space depends on several parameters including the intracellular concentrations of Na+ and glutamate, the elevations of which may slow down the cycling rate of EAATs. In astrocytes, glutamate is maintained at low concentration due to the presence of specific enzymes such as glutamine synthase (GS). GS inhibition results in cytosolic accumulation of glutamate suggesting that the conversion of glutamate by GS is important for EAATs operation. Here we recorded astrocytes from juvenile rat neocortical slices and analyzed the consequences of elevated intracellular glutamate concentrations and of GS inhibition on the time course of synaptically evoked transporter current (STC). In slices from rats treated with methionine sulfoximine (MSO), a GS inhibitor, STC evoked by short burst of high frequency stimulation (HFS; 100 Hz for 100 ms) but not by low frequency stimulation (LFS; 0.1 Hz) was twice slower than STC evoked from saline injected rats. Same results were obtained for astrocytes recorded with pipette containing 3-10 mM glutamate and compared with cells recorded with 0 or1 mM glutamate in the patch pipette. We also showed that HFS elicited significantly larger NMDAR-excitatory postsynaptic currents (EPSCs) with a stronger peri/extrasynaptic component in pyramidal cells from MSO-treated compared with saline treated rats. Taken together our data demonstrate that the conversion of glutamate by GS is fundamental to ensure an efficient clearance of glutamate by EAATs and to prevent glutamate spillover. GLIA 2017;65:401-415.
Highlights
Glutamate is the main excitatory transmitter in the CNS and plays major roles in brain development and complex physiological functions, including neurogenesis, cell migration, synapse formation, learning, memory, plasticity
Evidences that glutamine synthase (GS) inactivation affected glutamate transporters efficiency have not yet been provided. We investigated this issue by measuring the time course of synaptically activated transporter current (STC) in juvenile rat neocortical astrocytes and showed that inactivation of GS slowed the decay time constant of synaptically evoked transporter current (STC) evoked by short burst of high frequency stimulation and facilitated activation of peri/extrasynaptic NMDA receptors
Astrocytes can be visualized in slices with the fluorescent marker sulforhodamine 101 (SR-101), which allows to distinguish them from neurons (Kafitz et al, 2008)
Summary
Glutamate is the main excitatory transmitter in the CNS and plays major roles in brain development and complex physiological functions, including neurogenesis, cell migration, synapse formation, learning, memory, plasticity. The influence exerted by glutamate in the brain is determined by the spatiotemporal distribution of its concentration in the extracellular space ([Glu]o). This parameter influences the number and subtype of glutamate receptors activated by the transmitter and so the amplitude and the duration of the synaptic signal. Regulation of [Glu]o is strongly controlled by excitatory amino acid transporters (EAATs). EAATs are located at the cell surface of neurons and glial cells and they bind and transport glutamate
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