A unique synaptosomal fraction, which accumulates glutamic and aspartic acids, in brain tissue

Subcellular fractionation of rat cerebral cortical slices on sucrose density gradients provides evidence for the existence of a unique synaptosomal fraction (enriched in pinched-off nerve endings) that selectively accumulates glutamic and aspartic acids. The particles in this fraction sediment to a less dense portion of sucrose gradients than do particles that accumulate aromatic, basic, and neutral (large and small) amino acids. Particles that store gamma-aminobutyric acid are even less dense than those that contain exogenous glutamic and aspartic acids. The distribution of endogenous glutamic acid encompasses both that of exogenous glutamic acid and that of the neutral and basic amino acids. These findings provide neurochemical support for the suggestion that glutamic and/or aspartic acid has a specialized synaptic function, perhaps as a neurotransmitter, in the mammalian brain.