ACETYLCHOLINESTERASE AT THE MYONEURAL JUNCTION: CYTOCHEMICAL ULTRASTRUCTURE AND SOME BIOCHEMICAL CONSIDERATIONS

Abstract

Recent studies of acetylcholinesterase (AChE) and related enzyme systems at the myoneural junction are reviewed primarily from the ultrastructural cytochemical point of view. The four most prevalent methods used to determine localization of AChE ultrastructurally are: (a) a hexazotized pararosaniline diazo coupling method, (b) a thiolacetic acid method and modifications, (c) a thiocholine method and modifications and (d) osmiophilic thiolester methods. After an initial discussion of key questions germaine to the subject matter and a short summary to introduce ultrastructural considerations, the principles upon which the above four groups of methods are based are described with a critical evaluation of their advantages and disadvantages. Such characteristics as specificity, localization, penetration, ascertaining bound and unbound enzymes, dangers of nonenzymatic reactions and contrast in the electron microscope from the primary reaction product are detailed and compared for each method. A limited number of biochemically related topics are also considered which may have a bearing on cytochemical ultrastructural results. Of immediate interest are recent studies of isolation, purification and assay of AChE along with quantitation and a comparison of findings in animals and man. Other enzyme systems (other than AChE) at the motor end plate are also surveyed. The characteristics of the acetylcholine system and other neurohumoral transmitter systems are noted in some detail. Other less closely related topics included in this review but still of great interest are: localization in neural and nonneural tissue; the use of inhibitors, antibiotics and radiochemical methods; localization and isolation of receptor sites; use of brain synaptosomes; and two general theories linking neurohumoral transmission concepts and the metabolism of phosphorus compounds to the localization of AChE. No attempt has been made to be complete with these biochemical considerations. Rather, the reader is led into the literature as a first step in relating the cytochemical findings to biochemical and physiologic mechanisms.