Glutamate, Glutamine, and GABA as Substrates for the Neuronal and Glial Compartments After Focal Cerebral Ischemia in Rats

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Abstract
Background and Purpose —Even though the utilization of substrates alternative to glucose may play an important role in the survival of brain cells under ischemic conditions, evidence on changes in substrate selection by the adult brain in vivo during ischemic episodes remains very limited. This study investigates the utilization of glutamate, glutamine, and GABA as fuel by the neuronal and glial tricarboxylic acid cycles of both cerebral hemispheres after partially reversible focal cerebral ischemia (FCI). Methods —Right hemisphere infarct was induced in adult Long-Evans rats by permanent occlusion of the right middle cerebral artery and transitory occlusion of both common carotid arteries. (1,2- 13 C 2 ) acetate was infused for 60 minutes in the right carotid artery immediately after carotid recirculation had been re-established (1-hour group) or 23 hours later (24-hour group). Extracts from both cerebral hemispheres were prepared and analyzed separately by 13 C nuclear magnetic resonance and computer-assisted metabolic modeling. Results —FCI decreased the oxidative metabolism of glucose in the brain in a time-dependent manner. Reduced glucose oxidation was compensated for by increased oxidations of ( 13 C) glutamate and ( 13 C) GABA in the astrocytes of the ipsilateral hemispheres of both groups. Increased oxidative metabolism of ( 13 C) glutamine in the neurons was favored by increased activity of the neuronal pyruvate recycling system in the 24–hour group. Conclusions —Data were obtained consistent with time-dependent changes in the utilization of glutamate and GABA or glutamine as metabolic substrates for the glial or neuronal compartments of rat brain after FCI.

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