Organization of inhibitory feed-forward synapses from the dorsal to the ventral cochlear nucleus in the cat: a quantitative analysis of endings by vesicle morphology.

The main ascending, excitatory pathway from the cochlea undergoes synaptic interruption in the dorsal and ventral cochlear nuclei. The dorsal cochlear nucleus also forms a feed-forward circuit, which receives cochlear input and projects to the ventral cochlear nucleus by a tuberculo-ventral tract. This circuit may provide an inhibitory fringe (side bands) surrounding the center bands of the main ascending pathway. Biotinylated dextran injections into the dorsal cochlear nucleus anterogradely labeled the tuberculo-ventral tract and its endings in the anteroventral cochlear nucleus but also retrogradely filled cochlear nerve fibers and their terminals in the same regions. To distinguish tuberculo-ventral from cochlear nerve terminals, we used electron microscopy of the immunolabeled endings. Images were digitized and filter-enhanced, and the sizes and shapes of synaptic vesicles were used to construct quantitative profiles of the terminal types. The cochlear nerve endbulbs mapped to the same iso-frequency band of the injection site (main band). Flanking the main band were smaller labeled endings. About 45% of labeled terminals were pleomorphic and equally represented in the main band and side bands. Therefore, if there is an inhibitory fringe in the main projection pathway, it was not selective for tuberculo-ventral tract endings. Surprisingly, an excitatory category of round vesicles of intermediate size was labeled in the main band but not in the side bands. These intermediate endings may balance the feed-forward inhibition from the tuberculo-ventral tract. The quantitative method devised for classification of ending types by their vesicle profiles should be a generally useful tool for analysis.