Origins and targets of commissural connections between the cochlear nuclei in guinea pigs.

Our objective was to identify the origins and targets of axons that project from one cochlear nucleus to the other. First, retrograde tracers were injected into one cochlear nucleus to label commissural cells in the opposite nucleus. In the dorsal cochlear nucleus, a few cells in the deep layers were labeled; they were not further classified according to type. In the ventral cochlear nucleus, all commissural cells that could be classified were multipolar cells. Second, an anterograde tracer was injected into one cochlear nucleus, and the distribution of boutons in the opposite cochlear nucleus was examined. Labeled boutons were present throughout the ventral cochlear nucleus, where they appeared to contact multipolar cells, spherical and globular bushy cells, and octopus cells. In the dorsal cochlear nucleus, labeled boutons were present in the fusiform cell and deep layers and appeared to contact fusiform cells and cells of unknown type. Many labeled terminals were also present in the granule cell regions. Injections into regions associated with high or low frequencies labeled boutons in corresponding regions in the contralateral ventral cochlear nucleus. Third, multiple tracers were used to determine whether cells that project to the inferior colliculus are contacted by commissural axons. Boutons labeled by anterograde transport of one tracer placed in the cochlear nucleus were frequently observed to be apposed to cells that were labeled by retrograde transport of a different tracer placed in the contralateral inferior colliculus. We conclude that commissural projections originate from multipolar cells throughout the ventral cochlear nucleus (and from a small number of cells in the dorsal cochlear nucleus) and make contact with all major cell types of the cochlear nuclei, including at least some of those that project to the inferior colliculus.