Electron-microscope cytochemistry of acetylcholine-like cation by means of low-temperature "ionic fixation".

A fresh preparation of frog neuromuscle was fixed at low temperatures (0 degree-4 degrees C) by means of an "ionic-fixation" procedure which is based on the precipitation of quaternary ammonium cations, such as choline and acetylcholine, with molybdic or tungstic heteropolyanions. A low temperature was used to slow down drastically the biological processus of vesicular exocytosis so that ionic fixation, the speed of which is only slightly influenced by temperature variation, could be performed efficiently. In addition to the conventional point-like precipitate in the synaptic vesicle which is considered to be vesicular acetylcholine, numerous spot-like precipitates were observed in the synaptic cleft. Most of these were contiguous to the active zone, and some were in a paired form and corresponded to the double rows of the synaptic vesicles in contact with active zones. It is concluded that these spot-like precipitates were acetylcholine-like cations of the synaptic vesicles which had been discharged into the synaptic cleft by exocytosis and captured by the ionic fixation procedure. The results are discussed in relation to the vesicular and non-vesicular hypothesis of acetylcholine release.