EN
Using 13C labeled compounds and 13C magnetic resonance spectroscopy (MRS) it is possible to monitor cellular metabolism and astrocyte-neuronal interactions. Various 13C labeled substrates are used to unravel different aspects of cerebral metabolism. This presentation will focus on [1-13C]glucose, [U-13C] glucose, [2-13C]glucose and [3-13C]glucose metabolism in cerebellar and cerebro-cortical neurons and astrocytes in culture. [1-13C]Glucose is metabolized by both astrocytes and neurons and labeling of metabolites from this isotopomer of glucose will not be affected by the pentose phosphate pathway (PPP). Using [U-13C]glucose and 3-nitropropionic acid it could be confirmed that pyruvate carboxylation takes place in cortical astrocytes but not neurons. This carboxylation leads to the formation of oxaloacetate, which condenses with acetyl coenzyme A to form citrate. However, oxaloacetate may also be converted to malate and fumarate before being regenerated. This redundant pathway is termed the oxaloacetate-fumarate-flux, or backflux and has been shown to be extensive using [2-13C]- and [3-13C]glucose in cultured cerebral cortical and cerebellar cultures. It could also be calculated to be present in vivo. [2-13C]- and [3-13C]glucose can also be used to probe the PPP in neurons where pyruvate carboxylation is not present. Indeed, the PPP contributed to labeling of glutamate and other metabolites.