Mechanisms of GABA release from human astrocytes

Abstract

We have previously demonstrated that human astrocytes are GABAergic cells. Throughout the adult human brain, they express the GABA synthesizing enzyme GAD 67, the GABA metabolizing enzyme GABA-T, and the GABA(A) and GABA(B) receptors. GABA modulates the actions of microglia, indicating an important role for astrocytes beyond that of influencing neurotransmitter function. Here we report on the mechanisms by which astrocytes release GABA. Astrocytes were found to express the mRNA and protein for multiple GABA transporters, and multiple receptors for glutamate, GABA, and glycine. In culture, untreated human astrocytes maintained an intracellular GABA level of 2.32 mM. They exported GABA into the culture medium so that an intracellular-extracellular gradient of 3.64 fold was reached. Inhibitors of the GABA transporters GAT1, GAT2, and GAT3, significantly reduced this export in a Ca(2+)-independent fashion. Intracellular GABA levels were enhanced by treatment with the GABA-T inhibitors gabaculine or vigabatrin. Treatment with glutamate increased GABA release in a concentration-dependent fashion. This was partially inhibited by blockers of N-methyl-D-aspartate and kainate receptors. Conversely, glycine and D-serine, co-agonists of NMDA receptors, enhanced the GABA release. GABA release was accompanied by an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and was reduced by adding the Ca(2+) chelator, BAPTA-AM to the medium. These data indicate that astrocytes continuously synthesize GABA and that there are multiple mechanisms which can mediate its release. Each of these may play a role in the physiological functioning of astrocytes.

ONO‐2506 inhibits spike–wave discharges in a genetic animal model without affecting traditional convulsive tests via gliotransmission regulation

“…Various transmitters, such as L-glutamate, D-serine, GABA and kynurenic acid, are released from astrocytes (Hamilton and Attwell, 2010;Parpura and Zorec, 2010;Potter et al, 2010;Lee et al, 2011). Our previous study demonstrated that basal and 100 mM AMPA-evoked releases of L-glutamate and D-serine from astrocytes were inhibited by the glial toxin, FLC and the synaptobrevin inhibitor, TeNT, but such release was insensitive to the voltage-gated sodium channel inhibitor, TTX (Tanahashi et al, 2012), similar to the present results.…”

Human breast cancer metastases to the brain display GABAergic properties in the neural niche

“…Because of the abundant expression of GABA receptors and GABA in the brain microenvironment (27), we investigated whether BBM cells had any GABAergic properties by comparing the expression of their genes and proteins to the highly passaged primary breast cancer cell lines, SkBr3 and MDA-MB-231. All GABA receptor mRNA isoforms were highly up-regulated in HER2…”

Localization of SUCLA2 and SUCLG2 subunits of succinyl CoA ligase within the cerebral cortex suggests the absence of matrix substrate-level phosphorylation in glial cells of the human brain

“…The GABA transporter GAT-3 is exclusively expressed in astrocytes (Minelli et al 1996); GAT-1 and -2 are found in astrocytic processes and other neuronal-and nonneuronal elements (Conti et al 1998(Conti et al , 1999. As a result of the concerted action of these enzymes and transporters, cultured human astrocytes maintain an intracellular GABA level of 2.32 mM (Lee et al 2011b). On the same line, cultured human microglia express the mRNA and protein for GABA-T, in Fig.…”