Metabolism of glutamate and glutamine in cultured mouse cerebral cortical astrocytes has been investigated using either radioactively labelled (¹⁴C) amino acids or ¹³C-labelled amino acids combined with NMR spectroscopy of cell extracts and lyophilyzed incubation media. Using [U-¹³C]glutamate it has been shown that in astrocytes exogenously supplied glutamate is primarily (70%) metabolized oxidatively through the tricarboxylic acid (TCA) cycle and to a lesser jextent (30%) directly to glutamine. Glutamate metabolized in the TCA cycle is to a large extent recovered as lactate showing that the astrocyte-specific enzyme, malic enzyme is functionally active. Incubation with [U-¹⁴C]glutamine led to a higher specific radioactivity in glutamate than in glutamine. It could also be shown that glutamate and glutamine were metabolized differently to aspartate and alanine. These results taken together strongly suggest that glutamate/glutamine metabolism in astrocytes is compartmentalized and a model with multiple cytoplasmic and mitochondrial compartments of these amino acids is proposed.

Use of 13C-labeled precursors for the neuroactive amino acids glutamate and GABA as well as [U-13C]glutamate and glutamine combined with NMR spectroscopy has allowed detailed studies to be performed on cultured neurons and astrocytes yielding new information about synthesis and metabolism of these amino acids at the cellular level. Thus, it has become clear that astrocytes metabolize glutamate extensively through the tricarboxylic acid (TCA) cycle in a rather complex manner enabling the cells to generate lactate from malate. It has been shown that astrocytes can supply neurons not only with glutamine but also with TCA cycle constituents, lactate and alanine. Hence, an extended version of the glutamate/glutamine cycle is presented. Moreover, it has been demonstrated that citrate synthesized in astrocytes and released into the extracellular space can modify neuronal activity by regulating the Zn2+ concentration and thereby modulate N-methyl-D-aspartate receptor-mediated depolarization.