Close Coupling between Astrocytic and Neuronal Metabolisms to Fulfill Anaplerotic and Energy Needs in the Rat Brain

Carbon metabolism in the rat brain was studied in animals anesthetized with a light dose of pentobarbital and in awake animals under morphine, which were infused with either [1-13C]glucose+acetate or glucose+[2-13C]acetate for various periods of time. Brain amino-acid enrichments in tissue extracts were determined by nuclear magnetic resonance (NMR) spectroscopy and their time evolution was analyzed by automatic fitting. Acetyl-coenzyme A C2 enrichment and ratio between pyruvate carboxylase and pyruvate dehydrogenase activity (PC/PDH) were determined from glutamate and glutamine labeling. The following results were obtained: (i) amino-acid enrichment patterns implied metabolic compartmentation and occurrence of the glutamate-glutamine cycle; (ii) kinetics of aspartate, GABA, and glutamate labeling from [1-13C]glucose and of glutamine labeling from [2-13C]acetate indicated a twofold higher metabolic activity in awake than in anesthetized rat brain; (iii) evaluation of the contributions of the astrocytic and neuronal metabolisms to glutamine synthesis in both groups of rats indicated a coupling between neuronal tricarboxylic acid (TCA) cycle, glutamate-glutamine cycle and glial TCA cycle; and (iv) analyzing the extrapolations back to time zero and the steady-state values of PC/PDH indicated a close coupling between PC activity and both astrocytic and neuronal TCA cycles. All these results suggest a cooperative-like behavior of astrocytic and neuronal metabolisms to fulfill the anabolic and energy needs linked to brain activation.