MRS study of glutamate metabolism in cultured neurons/glia

[U-13C]Glutamate metabolism was studied in primary brain cell cultures. Cell extracts as well as redissolved lyophilized media were subjected to nuclear magnetic resonance spectroscopy in order to identify 13C labeled metabolites. Both neurons and astrocytes metabolized glutamate extensively with 13C label appearing in aspartate in all cultures. Additionally, GABA is synthesized in the GABAergic cortical neurons. Labeling of lactate and glutamine was prominent in medium from astrocytes, but not detectable in cerebral cortical neurons. Cerebellar granule neurons showed some labeling of lactate. Glutamate derived from the first turn of the tricarboxylic acid cycle (1,2,3-13C3-isotopomer) is present in all cell types analyzed. However, glutamate derived from the second turn of the cycle was only detected in granule neurons. In astrocytes, the transaminase inhibitor aminooxyacetic acid not only abolished the appearance of aspartate, but also of the 1,2,3-13C3-isotopomer of glutamate, thus showing that transamination is necessary for the conversion of 2-oxoglutarate to glutamate. The entry of glutamate into the tricarboxylic acid cycle was, however, not seriously impaired. 3-nitropropionic acid abolished the appearance of aspartate, the 1,2,3-13C3-isotopomer of glutamate and lactate in cerebellar granule neurons.