Displacement of endogenous glutamate with D-aspartate: an effective strategy for reducing the calcium-independent component of glutamate release from synaptosomes.

D-aspartate was used in the present study to partially deplete the cytosolic pool of glutamate, which is released independent of extracellular Ca2+, prior to measuring the K(+)-evoked release of this endogenous acidic amino acid from rat hippocampal mossy fiber synaptosomes. This pretreatment is known to be an effective method for substantially reducing the Ca(2+)-independent component of glutamate release. The rate of glutamate efflux is dependent on the concentration of sodium ions in the external medium and can be stimulated by exposure of hippocampal mossy fiber synaptosomes to external D-aspartate (50 microM). Following the partial displacement of this cytosolic pool of glutamate with D-aspartate, the K(+)-evoked release of the residual, presumably vesicular, pool of endogenous glutamate has a strict requirement for external calcium and is highly dependent on the extent to which depolarization elevates the level of free cytosolic calcium. It is concluded that the protocol described in this study for the displacement of cytosolic glutamate with D-aspartate provides a useful alternative method of controlling for the Ca(2+)-independent component of glutamate release in synaptosomal preparations.