Ultrastructural demonstration of acetylcholinesterase activity of motor endplates via osmiophilic diazothioethers

Summary

The ultrastructural chemical localization of acetylcholinesterase of motor endplates of rat intercostal muscle has been studied with three new esterase substrates. These substrates, although not specific for acetylcholinesterase, have differential affinities for various types of esterases; two of them (NTA and TAB) are hydrolyzed preferentially by acetyl esterase enzymes, and the third (TPB) is a propionic acid ester and is hydrolyzed preferentially by pseudocholinesterase and other esterases. The end-product of the enzymatic reaction is converted to a diazothioether (droplet form) and upon osmication this is converted to a coordination polymer of osmium which has ideal properties for electron microscopy.

Although this study supports previous observations that enzymatic activity can be found primarily on the post-synaptic membranes of the motor endplate, no enzymatic activity was noted on the pre-synaptic membrane, within the synaptic cleft, or on the basement membrane unless incubation was prolonged, resulting in overstaining. Neither was enzyme activity seen on membrane-free ribosomes and the ribosome-studded sarcoplasmic reticulum. Axonal vesicles also failed to exhibit enzymatic activity which had been noted with the method using thiolacetic acid and lead. A correlation of esterase activity with ultrastructural localization, using the substrate TPB, suggests that a “buffer” zone of nonspecific esterase activity is present beneath the subneural apparatus which limits the aberrant, accidental, or abnormal distribution of acetylcholine within a clearly defined area of sarcoplasm in the vicinity of the motor endplate of the muscle fiber.