A study of the cytochemical localization of acetylcholiriesterase activity, combining histochemistry with electron microscopy, showed that the final product of the reaction, which was deposited at or near enzyme sites, occurred at four places in the myoneural junction. These included: plasma membrane of the muscle covering the junctional folds, the primary and secondary synaptic clefts, parts of the plasma membrane covering the axon terminal, and vesicular structures in the terminal axoplasm. No reaction occurred in the presence of 10-4 eserine or DFP, whereas 10-5 DFP inhibited the reaction at all sites except in the vesicles of the terminal axon. These findings are discussed with reference to the histochemical method used and to the occurrence of esterolytic activity in the vesicles, as well as to some of the current hypotheses concerning the relationship of the site of acetylcholinesterase and synaptic transmission.

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Copyright, 1962, by The Rockefeller Institute Press
1962