Veratridine-induced release of acetylcholine from mouse forebrain minces: Dependence on the hydrolysis of cytoplasmic acetylcholine for a source of choline

The importance of depolarization induced hydrolysis of cytoplasmic acetylcholine (ACh) in providing choline for the veratridine- and high K⁺-induced release of acetylcholine was studied in mouse forebrain minces. Results indicated that a loss of hydrolyzable cytoplasmic ACh prior to depolarization reduced the amount of ACh released by veratridine but not the amount released by high K⁺. The reduction in the veratridine-induced release of ACh did not occur during the first 5 min of incubation. Loss of vesicular ACh prior to depolarization reduced both the veratridine- and K⁺-induced release of ACh during the first 5 min of incubation. Blockade of extra-cellular choline transport by hemicholinium (HC-3) did not affect the veratridine-induced release of ACh during a 10 min incubation period unless the cytoplasmic pool of ACh had first been depleted and was unavailable as a source of choline. In contrast, HC-3 reduced the K⁺-induced release of ACh from brain tissue with normal stores of cytoplasmic ACh. These results indicate that both depolarizing agents primarily stimulate the release of preformed ACh from a vesicular fraction during the first 5 min of mince incubation. Thereafter, they both stimulate the release of newly synthesized ACh, however, they differ in one important respect. The principal source of choline for the veratridine-induced release of newly synthesized ACh appears to be the cytoplasmic pool of ACh, whereas the major source of choline for the K⁺-induced release of newly synthesized ACh appears to be extracellular choline.