Histochemical localization of acetylcholinesterase in the cochlear and superior olivary nuclei. A reappraisal with emphasis on the cochlear granule cell system.

The present account reconsiders the acetylcholinesterase (AChE) staining pattern of the cochlear nuclei with special emphasis on positively stained afferents from the olivocochlear neurons to the cochlear granule cells system. The main part of the study is based on AChE- and silver-stained sections of normal and brainstem-operated cats. AChE-stained sections of normal mouse, cat and chinchilla are used for comparative purposes. The rat superior olive contains three types of AChE-positive neurons probably contributing to the olivocochlear bundle; densely stained large neurons of the periolivary region, densely stained small neurons at the margin of the lateral superior olive (LSO), and weakly stained small neurons within LSO. Largely uncrossed fibers, probably collaterals of the olivocochlear bundle, enter the cochlear nuclei via three routes, defined here as the strial, subpeduncular and ventral routes. Collectively they form a terminal fiber plexus in certain portions of the granule cell domain, but with some fibers branching in the extragranular regions of the nuclear complex as well. The individual fibers end in a fashion resembling cerebellar mossy fibers. The cochlear nuclei in addition contain conspicuous dense patches of precipitate, which, like the AChE-positive fibers, appear after short incubation and are mainly restricted to the granule cell domain. In contrast to the fibers, however, they are resistant to central deafferentation and therefore may represent intrinsic structures of the granule cell system. Moreover, there is a diffuse neuropil precipitate which grows in distribution and density with incubation time. It is present both in granular and extragranular areas of the complex and is partly dependent upon the integrity of the weakly stained trapezoid body. In spite of considerable interspecies variations with regard to the described AChE-positive elements, these are present also in mouse, cat, and chinchilla. The significance of the findings and of the interspecies differences are discussed. A simplified terminology of the superior olivary complex is proposed.