The conversion of glutamate by glutamine synthase in neocortical astrocytes from juvenile rat is important to limit glutamate spillover and peri/extrasynaptic activation of NMDA receptors

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.

Inhibition of the Mitochondrial Glutamate Carrier SLC25A22 in Astrocytes Leads to Intracellular Glutamate Accumulation

“…The finding that the global concentration of ATP after glutamate stimulation is lower in astrocytes inactivated for GC1 compared to control cells suggests that processes requiring ATP could be altered in the absence of GC1. The conversion of glutamate into glutamine by GS is an ATP-dependent reaction (Sonnewald et al, 1997; Schousboe and Waagepetersen, 2003) and its absence or inhibition results in a rapid decrease of the glutamine level (Laake et al, 1995; He et al, 2010; Trabelsi et al, 2017). The observation that the glutamine level was maintained in the absence of GC1 shows that GS is functional and suggests that ATP processes are still operational, at least under our conditions.…”

“…Interestingly, GS deficiency caused glutamate accumulation in vivo (Laake et al, 1995; Perez et al, 2012) and was associated with mesial temporal lobe epilepsy (Eid et al, 2004) and EEE (Häberle et al, 2005, 2011). A recent study showed that glutamate conversion into glutamine via GS is important to limit the extracellular glutamate spill-over and the activation of the peri/extrasynaptic NMDA receptors (Trabelsi et al, 2017). The authors recorded astrocytes from juvenile rat neocortical slices and showed that, after GS inhibition with L-MSO, synaptically transporter current (STC) evoked by high frequency stimulation was twice slower than STC evoked from saline injected rats, and that NMDAR-excitatory postsynaptic currents were larger with a strong peri/extrasynaptic component in pyramidal cells (Trabelsi et al, 2017).…”

“…A recent study showed that glutamate conversion into glutamine via GS is important to limit the extracellular glutamate spill-over and the activation of the peri/extrasynaptic NMDA receptors (Trabelsi et al, 2017). The authors recorded astrocytes from juvenile rat neocortical slices and showed that, after GS inhibition with L-MSO, synaptically transporter current (STC) evoked by high frequency stimulation was twice slower than STC evoked from saline injected rats, and that NMDAR-excitatory postsynaptic currents were larger with a strong peri/extrasynaptic component in pyramidal cells (Trabelsi et al, 2017). These results suggest that a rapid clearance of extracellular glutamate is compromised when glutamate catabolism in astrocytes is altered.…”

Oligodendrocytes Support Neuronal Glutamatergic Transmission via Expression of Glutamine Synthetase

“…Following synaptic release, glutamate uptake and degradation are tightly regulated to achieve temporal and spatial signaling specificity and prevent cellular excitotoxicity (Kim et al, 2011; Sattler and Rothstein, 2006; Sheldon and Robinson, 2007). Currently, astrocytes are considered the sole glial cell type that contributes to glutamate uptake and degradation in the CNS (Jayakumar and Norenberg, 2016; Liang et al, 2006; Ortinski et al, 2010; Papageorgiou et al, 2018; Schousboe et al, 2013; Schousboe, 2019; Sun et al, 2017; Tani et al, 2014; Trabelsi et al, 2017; Yuan et al, 2017), as they express high levels of glutamate transporters and glutamine synthetase (GS), an enzyme that converts glutamate into glutamine. In keeping with this view, GS is frequently used as an astrocyte-specific marker (Armbruster et al, 2016; Habbas et al, 2015; Okuda et al, 2014; Papageorgiou et al, 2018; Theofilas et al, 2017; Tong et al, 2014).…”

Persistent increase in ventral hippocampal long‐term potentiation by juvenile stress: A role for astrocytic glutamine synthetase

“…The glutamate–glutamine cycle in astrocytes appears to be adjusted depending on neuronal activity and sets the threshold for regional excitability and plasticity (Bonansco et al, ; Rose et al, ; Tani et al, ; Trabelsi, Amri, Becq, Molinari, & Aniksztejn, ). In the current study, we set out to investigate the contribution of the glutamate–glutamine cycle in astrocytes to alter LTP as a consequence of juvenile stress exposure.…”