Precursor dependence of acetylcholine release from rat brain in vitro.

These experiments were designed to test the extent to which the concentration of extracellular choline affects the synthesis and subsequent release of acetylcholine (ACh) by rat cortex in vitro. We found that the rate of potassium-depolarized ACh release from rat cortical minces was significantly accelerated when choline chloride was added to the incubation medium at concentrations of either 60 or 100 microM. The ACh content of the cortical minces was reduced by prolonged depolarization; this depletion was prevented by incubating minces with choline (100 microM). Raising the extracellular choline concentration of the incubation medium did not elevate the amount of ACh released spontaneously (4.7 mM K+) and had no effect on the accumulation of transmitter that occurs when cortical minces are incubated in physiologic buffer. A single dose of choline chloride administered orally to rats (20 mmol/kg) was without effect on the subsequent release of ACh and choline from cortical minces in vitro. The ACh and choline concentrations of rat cortex in vitro were similarly unaffected by in vivo choline administration. These results indicate that ACh release from rat cortex, in vitro, depends upon the direct availability of extracellular choline under conditions of prolonged neuronal depolarization.