Inhibition of choline efflux results in enhanced acetylcholine synthesis and release in the guinea-pig corticocerebral synaptosomes

Synthesis and release of [³H]acetylcholine ([³H]ACh) were measured in synaptosomes from the guinea pig cerebral cortex after preloading with [³H]choline ([³H]Ch). We demonstrate here that inhibition of choline (Ch) efflux results in an increase in acetylcholine (ACh) synthesis and release. Our findings are as follows: (1) inhibition of [³H]Ch efflux by hemicholinium-3 (HC-3) (100 μM), increased the levels of both the released (116% of control) and the residing (115% of control) [³H]ACh. (2) The muscarinic agonist, McN-A-343 (100 μM), which was previously shown to inhibit Ch efflux, also increased the released (121% of control) and the residing (109% of control) [³H]ACh. (3) Omission of Na⁺ ions (which are required for Ch transport) from the incubation medium had similar effects to those observed with McN-A-343 and HC-3. These results suggest inverse relationships between Ch efflux on one hand, and ACh synthesis and release on the other hand. (4) Depolarization with 50 mM K⁺, or with the K⁺ channel blocker, 4-aminopyridine (100 μM), also increased the total level of [³H]ACh (113 and 107% of nondepolarized synaptosomes, respectively). However, whereas conditions that inhibit Ch transport such as HC-3, McN-A-343 and "no sodium" increased both the residing and the released [³H]ACh depolarization with high K⁺ or 4-aminopyridine reduced the residing (79 and 87% of control, respectively) and increased only the released [³H]ACh (182 and 148% of control, respectively). Taken as a whole, our data suggest that inhibition of Ch efflux reduces the loss of Ch from the nerve terminal and that the accumulating intrasynaptosomal Ch is incorporated into ACh, which is then released under the control of muscarinic presynaptic receptors.