[3H]Acetylcholine and [3H]5-hydroxytryptamine release from rat midbrain slices and the effects of calcium and phenobarbital.

The K-stimulated release of [3H]ACh from rat midbrain slices prelabeled by incubation with [3H]choline was dependent on extracellular Ca. Phenobarbital inhibited the K-stimulated [3H]ACh release and the IC50 was equal to that found for K-stimulated endogenous ACh release. These results support the suggestion that barbiturates primarily inhibit the Ca-dependent stimulated release of ACh and affect ACh synthesis only indirectly. K-Stimulated release of [3H]5-HT was also inhibited by removing Ca from the medium or by adding phenobarbital which further supports the effects of barbiturates on the depolarization-induced release process. Fluoxetine, an inhibitor of 5-HT uptake, increased the amount of [3H]5-HT found in the medium but did not fully block the uptake of [3H]5-HT in this slice preparation.