REGULATION OF HEMICHOLINIUM BINDING-SITES IN ISOLATED NERVE-TERMINALS

High-affinity uptake of choline, the rate-limiting, regulatory step for the synthesis of acetylcholine is regulated via presynaptic auto- and heteroreceptors. Binding studies using tritiated hemicholinium-3 ([H-3]HCh-3) as the specific ligand for the choline carrier resealed that the number of hemicholinium binding sites in nerve terminals isolated from insect brain changes corresponding to the activity of synaptosomal kinase A and kinase C. Activation of kinase A apparently increases the total number of hemicholinium binding sites by recruiting additional occult carriers, Whereas the effect of kinase C activity is Most appropriately explained by presenting a down-regulation of carrier proteins. The kinase-mediated regulation of choline transporters is obviously due to a phosphorylation of the carrier protein itself.