Electron microscopical autoradiography of [3H]choline fixed by phosphomolybdic acid in the motor nerve terminal.

The 'ionic fixation' of soluble quaternary ammonium compounds such as choline and acetylcholine by phosphomolybdic acid was used to fix [3H]choline taken up in the frog motor nerve terminal. The rapid 'ionic fixation' allowed autoradiography of [3H]choline directly at the electron microscopic level without considerable diffusion artifacts. Most of the radioactivity was localized in the motor nerve terminal and to a lesser degree, in Schwann cell processes and muscle fibers. These findings are in favor of our previous hypothesis of acetylcholine fixation by molybdic or tungstic heteropolyanions in the motor nerve terminal.