SUBCELLULAR DISTRIBUTION OF ENDOGENOUS AND [ 3 H] γ‐AMINOBUTYRIC ACID IN RAT CEREBRAL CORTEX

— Slices of rat cerebral cortex were labelled by incubation with [ 3 H]γ‐aminobutyric acid (GABA) and homogenized in isotonic sucrose. The subcellular distributions of endogenous GAB A, [ 3 H]GABA and glutamate decarboxylase (GAD) were studied by density gradient centrifugation. The subcellular distributions of the labelled and endogenous amino acid were remarkably similar, indicating that [ 3 H]GABA is taken up into the endogenous GABA pool. About 40 per cent of both endogenous and [ 3 H]GABA were recovered in particles which were tentatively identified as synaptosomes from their equilibrium density and sensitivity to osmotic shock. In slices labelled with [ 3 H]GABA and [ 14 C]α‐aminoisobutyric (AIB) acid, significantly more [ 3 H]GABA was recovered in paniculate fractions than [ 14 C]AIB. About 80 per cent of the enzyme GAD was also recovered in the same particle fractions which contained [ 3 H]GABA and endogenous GABA. Evidence is presented which suggests that a loss of particle‐bound GABA occurs during subcellular fractionation procedures.