Acetylcholine in neurons and paraneurons: a histochemical study.

Several years ago we proposed a method for the localization of acetylcholine in the cholinergic nerve terminals. The method is based on a rapid precipitation of quarternary ammonium cations (such as acetylcholine or choline) by molybdic or tungstic heteropolyanions (such as phosphomolybdic acid, phosphotungstic acid and silicotungstic acid). The insoluble salts formed can be directly visualized under the electron microscope. In the synaptic vesicles, acetylcholine was localized as point-like precipitates, while the membranes were well preserved. Since these fixations are based on a rapid ionic interaction, the term "ionic fixation" was proposed. The ionic fixation performed on motor end-plates in various physiological conditions, provided different forms of cytochemical precipitates of acetylcholine (point-like, spot-like, diffuse or laminar). In contrast with the numerous physiological and neurochemical investigations into the acetylcholine mechanism of neurons, very little is known about paraneurons. The historical evolution of paraneuron research, more or less bound to that of the sympathetic paraganglion and of the APUD cells may explain this lack of results. In certain physiological conditions, acetylcholine can be revealed in paraneurons and seems to reflect a plasticity of the cells, as already observed in sympathetic ganglion cells. This paper further overviews the cholinergic mechanisms of exocrine secretory cells. The presence of a high cholinesterase activity exist in most of these exocrine cells, as far as we could verify. In addition to cholinesterase activity, the presence of acetylcholine, probably related to exocrine cell metabolism, was investigated with our histochemical method.