AMINO ACIDS IN SYNAPTIC VESICLES FROM MAMMALIAN CEREBRAL CORTEX: A REAPPRAISAL

Synaptic vesicles were prepared from rat cerebral cortex and separated by gel filtration from small molecular weight compounds contaminating this fraction. Electron microscopy of the vesicle suspension showed that vesicles were by far the most abundant morphological entities. The amino acid content of the purified vesicle fraction was examined and the two amino acids appearing in the most significant amounts were found to be taurine and glutamate. This amino acid pool was not osmotically sensitive as is the vesicular pool of ACh and remained attached to the vesicular protein after passage through Sephadex columns equilibrated in water. However, amino acids added to the vesicle fraction prior to passage through Sephadex did not become associated with this pool and this indicated that the vesicular pool was not likely to be an artifact due to the vesicular protein non-specifically adsorbing amino acids. The release of taurine from incubated synaptosome beds was studied and elevated medium K⁺ (56 mm ) was found to cause a small increase (36 per cent) in the amount of the taurine released to the medium. During the same experiments another physiologically active amino acid, glutamate, was released in more significant amounts, increasing in the medium by 186 per cent. The possible significance of the presence of taurine is discussed.