Changes in the relative amounts of aspartate and glutamate released and retained in hippocampal slices during...

It has been found previously that the ratio of aspartate to glutamate released and retained by brain slices reversibly changes with changing glucose concentrations in the medium. To find out whether increased neuronal activity also results in changes in the ratio of aspartate to glutamate, in this study electrical-field stimulation was applied for 10 min to hippocampal slices in the presence of 0.2-5 mM glucose. In 5 mM glucose, the ratio of aspartate to glutamate released did not change during stimulation, but the amount of aspartate retained at the end of stimulation was reduced. In contrast, in 1 mM or less glucose, the ratio of aspartate to glutamate released increased progressively and the rate of increase was inversely proportional to the glucose content of the medium. The evoked release of aspartate and glutamate both in low and high glucose was nearly suppressed in low (0.1 mM) Ca2+ or by tetrodotoxin. In low glucose, the ratio of aspartate to glutamate contained in the slices also increased as a result of stimulation. This increase was reduced only a little in low Ca2+, but was nearly eliminated by tetrodotoxin. Results suggest that increased neuronal activity causes a shift in the ratio of aspartate to glutamate released in the presence of glucose concentrations similar to those found in the brain in normoglycemic rats. This shift, due to an increased energy demand, probably originates from terminals which release aspartate and glutamate in different proportions.