Specificity of Exogenous Acetate and Glutamate as Astrocyte Substrates Examined in Acute Brain Slices from Female Mice Using Methionine Sulfoximine (MSO) to Inhibit Glutamine Synthesis

Removal of endogenously released glutamate is mediated primarily by astrocytes and exogenous C-13-labeled glutamate has been applied to study glutamate metabolism in astrocytes. Likewise, studies have clearly established the relevance of C-13-labeled acetate as an astrocyte specific metabolic substrate. Recent studies have, however, challenged the arguments used to anchor this astrocyte specificity of acetate and glutamate. The aimof the current study was to evaluate the specificity of acetate and glutamate as astrocyte substrates in brain slices. Acutely isolated hippocampal and cerebral cortical slices from female NMRI mice were incubated in media containing [1,2-C-13] acetate or [U-C-13] glutamate, with or without methionine sulfoximine (MSO) to inhibit glutamine synthetase (GS). Tissue extracts were analyzed by gas chromatography-mass spectrometry. Blocking GS abolished the majority of glutamine C-13-labeling from [1,2-C-13] acetate as intended. However, C-13-labeling of GABA was only 40-50% reduced by MSO, suggesting considerable neuronal uptake of acetate. Moreover, labeling of glutamate from [1,2-C-13] acetate in the presence of MSO exceeded the level probable from exclusive labeling of the astrocytic pool, which likewise suggests neuronal acetate metabolism. Approximately 50% of glutamate was uniformly labeled in slices incubated with [U-C-13] glutamate in the presence of MSO, suggesting that neurons exhibit substantial uptake of exogenously provided glutamate. (C) 2017 Wiley Periodicals, Inc.