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SUBCELLULAR LOCALIZATION OF NEWLY‐FORMED [3H]ACETYLCHOLINE IN RAT CEREBRAL CORTEX IN VITRO

—Slices from rat brain cortex were incubated for either 5 or 60 min in a medium containing [3H]choline and 4·7 or 25 mm‐KCl. Bioassayable ACh and labelled ACh were determined in the incubation medium, in the total tissue homogenate and in subcellular fractions. Raising the KCl concentration from 4·7 to 25 mm stimulated the release and synthesis of total and of labelled ACh. In medium containing 25 mm‐KCl the amounts of ACh decreased in the tissue and in the nerve ending cytoplasm, but remained constant in the synaptic vesicles. After incubation in 25 mm‐KCl medium the ACh in the vesicles was labelled to the same extent as the cytoplasmic ACh but after incubation in 4·7 mm‐KCl medium vesicular ACh was labelled less than cytoplasmic ACh. During 5 min incubation in medium containing 25 mm‐KCl the ratio of labelled to total ACh was much higher in the medium than in the homogenate, the vesicles or the cytoplasm. During the last 15 min of the 60 min incubation the ratio of labelled to total ACh in the medium was still higher than that in the tissue fractions, but less so than during the 5 min incubation. It is concluded that the vesicular and cytoplasmic fractions are not identical with the store in the tissue from which newly‐synthesized ACh is preferentially released.