THE SUBCELLTJLAR DISTRIBUTION OF FREE H³-GLUTAMIC ACID IN RAT CEREBRAL CORTICAL SLICES

¹ Departments of Pharmacology and Experimental Therapeutics and Psychiotriy and the Behavioral Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland

The subcellular localization of free H³-glutamic acid, other free H³-amino acids and C¹⁴- norepinephrine in rat cerebral cortex has been examined. Slices of rat cerebral cortex were incubated with H³-amino acids, homogenized and subjected to subcellular fractionation. Some putative transmitters (glutamic acid, glycine and norepinephrine), the basic amino acids (arginine, lysine, ornithine and histidine) and the small neutral amino acids (serine, alanine and threonine) were localized in particulate fractions more than others. Differential centrifugation showed that several of these amino acids were in a particulate fraction (P2) enriched with pinched-off nerve endings ("synaptosomes") and mitochondria, and were in osmotically sensitive compartments. When P2 fractions were centrifuged on continuous density gradients, H³-glutamic acid and H³-glycine were localized to the same fractions as C¹⁴-norepinephrine, a synaptosomal marker. However, the synaptosomal peak for H³- glutamic acid and H³-glycine was broader than for C¹⁴-norepinephrine. In contrast to the synaptosomal localization of H³-glutamic acid, endogenous free glutamic acid was primarily localized in the soluble supernatant fraction. However, the pattern of distribution of exogenous and endogenous glutamic acid in continuous sucrose gradients was identical, implying a mixing of exogenous with endogenous glutamic acid. All of the H³-amino acids examined were found to be mostly unmetabolized in the tissue slices, although the metabolites of H³-glutamic acid accumulated in the incubation medium.

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