UPTAKE OF ACETYLCHOLINE AND CHOLINE INTO RAT BRAIN CORTICAL SLICES AND SYNAPTOSOMES AS RELATED TO 32Pi, INCORPORATION INTO THEIR PHOSPHOLIPIDS

—The role of ACh-stimulated ³²P_i incorporation into the phospholipids of rat cerebral cortex slices and isolated nerve endings (synaptosomes) has been studied. ACh stimulation is not connected with any carrier-mediated uptake of ACh. Such uptake may occur in slices in the presence of the anticholinesterase Sarin but barely in the presence of eserine. Regardless of the nature of the anticholinesterase used, rat cerebral cortex synaptosomes that respire and show high and low affinity choline uptake do not accumulate ACh against a concentration gradient. At exogenous ACh concentrations of 10^–5m and above, some ACh enters the synaptosomes by diffusion and significantly stimulates ³²P_i incorporation into phosphatidic acid. It is discussed whether, in isolated nerve endings, an increase in cytoplasmic ACh concentration due to diffusion may induce vesicle turnover to keep a balance between ‘free’ and bound ACh or if a presynaptic ACh receptor is responsible for the observed changes in phosphatidic acid. The distribution of accumulated radioactivity derived from exogenous choline and ACh respectively between ACh, choline, phosphorylcholine and betaine has been studied in slices and isolated nerve endings.