Glutamate and GABA metabolism in transient and permanent middle cerebral artery occlusion in rat: Importance of astrocytes for neuronal survival

The aim of the present study was to identify the distinguishing metabolic characteristics of brain tissue salvaged by reperfusion following focal cerebral ischemia. Rats were subjected to 120 min of middle cerebral artery occlusion followed by 120 min of reperfusion. The rats received an intravenous bolus injection of [1-13C]glucose plus [1,2-13C]acetate. Subsequently two brain regions considered to represent penumbra and ischemic core, i.e. the frontoparietal cortex and the lateral caudoputamen plus lower parietal cortex, respectively, were analyzed with 13C NMRS and HPLC. The results demonstrated four metabolic events that distinguished the reperfused penumbra from the ischemic core. (1) Improved astrocytic metabolism demonstrated by increased amounts of [4,5-13C]glutamine and improved acetate oxidation. (2) Neuronal mitochondrial activity was better preserved although the flux of glucose via pyruvate dehydrogenase into the tricarboxylic acid (TCA) cycle in glutamatergic and GABAergic neurons was halved. However, NAA content was at control level. (3) Glutamatergic and GABAergic neurons used relatively more astrocytic metabolites derived from the pyruvate carboxylase pathway. (4) Lactate synthesis was not increased despite decreased glucose metabolism in the TCA cycle via pyruvate dehydrogenase. In the ischemic core both neuronal and astrocytic TCA cycle activity declined significantly despite reperfusion. The utilization of astrocytic precursors originating from the pyruvate carboxylase pathway was markedly reduced compared the pyruvate dehydrogenase pathway in glutamate, and completely stopped in GABA. The NAA level fell significantly and lactate accumulated. The results demonstrate that preservation of astrocytic metabolism is essential for neuronal survival and a predictor for recovery.