Effect of protein kinase C activation on the release of [H-3]Acetylcholine in the presence of vesamicol

The present work tested whether pharmacological activation of protein kinase C (PKC) influences the release of [H-3]acetylcholine ([H-3]ACh) synthesized in the presence of vesamicol, an inhibitor of the vesicular acetylcholine transporter (VAChT). Newly synthesized [H-3]ACh was released from hippocampal slices by field stimulation (15 Hz) in the absence of vesamicol, but as expected [H-3]ACh synthesized during exposure to vesamicol was not released significantly by stimulation. Treatment of slices with the PKC activator phorbol myristate acetate (PMA) decreased the inhibitory effect of vesamicol on [H-3]ACh release. The effect of PMA was dose-dependent, was sensitive to calphostin C, a PKC-selective inhibitor, and could not be mimicked by alpha-PMA, an inactive phorbol ester. PMA did not alter the release of [H-3]ACh in the absence of vesamicol, suggesting that the site of PKC action could be related to the VAChT. In agreement with this observation, immunoprecipitation of VAChT from P-32-labeled synaptosomes showed that phosphorylation occurs and that incorporation of P-32 in the VAChT protein increases in the presence of PMA. We suggest that PKC alters the output of [H-3]ACh formed in the presence of vesamicol and also provide circumstantial evidence for a role of phosphorylation of VAChT in this process.