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There is now a considerable amount of data to support the hypothesis that cholinergic transmission occurs at certain synapses in the central nervous system (Feldberg, '45, '50, '51; Koelle and Gilman, '49). One line of evidence is based upon the parallelism found between known cholinergic neurons of the peripheral nervous system and certain regions of the central nervous system with respect to their high concentrations of acetylcholine ( ACh), choline acetylase and specific (true, aceto-, or acetyl-) cholinesterase (ChE). Burgen and Chipman ( '51) recently demonstrated for various regions of the dog's brain relatively close correlations between their own figures for specific ChE activity and those of Feldberg and Vogt ('48) for choline acetylase and of MacIntosh ('41) for ACh. It has been pointed out frequently that conventional determinations of ChE activity employing homogenates, while providing quantitative results, leave much to be desired as far as details of localization are concerned. Thus, when a region of the brain is found to have high activity it.is impossible to be certain whether it is attributable to enzyme associated with the entering axons, with neurons originating in

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By means of the gold-thiocholine (AuThCh) and gold-thiolacetic acid (AuThAc) methods, it has been demonstrated electron microscopically that acetylcholinesterase (AChE) is located at the prejunctional axoplasmic membrane and the postjunctional sarcoplasmic membrane, including the full lengths of its invaginations, at the motor end plate of mouse intercostal muscle. Nonspecific cholinesterase (ChE) is present in relatively low concentrations at the same sites, and in greater concentrations in the teloglial Schwann sheath cells.

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The localization of acetylcholinesterase and non-specific cholinesterase was studied in the cone retinas of the pigeon and ground squirrel, and in the predominantly rod retinas of the rabbit, rat and cat. The enzymes were localized histochemically using the copper or gold thiocholine method. In the cone retinas, acetylcholinesterase was found in discrete bands in the inner plexiform layer. Cells corresponding in position and morphological detail to amacrine cells were stained for acetylcholinesterase. Cells of the ganglion cell layer that resembled displaced amacrine cells were also stained for acetylcholinesterase, as well as other cells that appeared to be ganglion cells. No stained processes could be followed from these latter cells, however. The horizontal cells were stained for non-specific cholinesterase; these cells were localized to specific areas of the retina, which differed in the two species. In the predominantly rod retinas, there was no staining for non-specific cholinesterase, but there was diffuse staining for acetylcholinesterase throughout the inner plexiform layer. In the rabbit, this staining pattern was not changed by chronic section of the optic nerve. No stained amacrine cells could be seen distinctly in any of the three species with predominantly rod retinas. After an intravenous dose of diisopropyl phosphorofluoridate sufficient to inactivate all the retinal acetylcholinesterase, however, synthesis of new enzyme could be followed in the amacrine cells and their processes. In all five species examined, therefore, the amacrine cells appeared to be the major or sole source of acetylcholinesterase for the inner plexiform layer of the retina. The possible function of this enzyme with regard to synaptic transmission in the retina is discussed.