Putative commissural and collicular axo-somatic terminals on neurons of the rat ventral cochlear nucleus

The type of synaptic terminals from the cochlear nucleus and inferior colliculus that terminate in the contralateral ventral cochlear nucleus are not known. These terminals were studied with the electron microscope and immunogold after injection of wheat germ agglutinin conjugated to horseradish peroxidase into the inferior colliculus or into the cochlear nucleus. The tracer anterogradely labelled boutons onto the main neurons of the contralateral ventral cochlear nucleus. Most of these cells (95%) were glycine immuno-negative and represent excitatory neurons. After injection of the tracer into the contralateral inferior colliculus few anterogradely labelled boutons were seen on spherical and multipolar cells of type II in the anteroventral cochlear nucleus. Rare labelled boutons were present on multipolar cells of type I and II, globular neurons and octopus cells in the posteroventral cochlear nucleus. After injection into the contralateral dorsal and ventral cochlear nucleus labelled boutons were seen more frequently than after injection into the inferior colliculus. These terminals contacted most of large neurons, especially multipolar cells of type II and less frequently of type I. Also globular and spherical cells were contacted by commissural terminals. Octopus cells received less frequently putative commissural terminals. Most boutons contained pleomorphic vesicles and stored GABA. A lower number of boutons with pleomorphic and flat vesicles contained glycine and sometimes GABA, both inhibitory neurotransmitters. Few boutons containing round vesicles were immuno-negative for both glycine and GABA, and were considered putative commissural excitatory terminals. The latter often contacted glycinergic neurons of type II so that also these terminals might elicit an inhibition with at least a disynaptic mechanism after contralateral stimulation.