Cytology of large neurons in the guinea pig dorsal cochlear nucleus contacting the inferior colliculus

Large neurons in the dorsal cochlear nucleus of the guinea pig which project to the inferior colliculus were identified after injections of the neural tracer WGA-HRP. Retrograde labelled cells (pyramidal and giant neurons) in the dorsal cochlear nucleus were glycine and GABA immunonegative and showed a similar ultrastructure. Between 30 and 60% of their perimeter was covered by axo-somatic boutons, most of which (>50%) contained pleomorphic synaptic vesicles. Other boutons (about 40% of total) contained flat vesicles and few (5-6%) contained round vesicles, a characteristic of the excitatory cells innervating the inferior colliculus. Immunogold-cytochemistry, coupled to silver intensification, showed that more than 50% of axo-somatic pleomorphic boutons and over 90% of boutons containing flat and pleomorphic vesicles store glycine. Rare WGA-HRP labelled axo-somatic boutons containing flat-pleomorphic vesicles were seen on pyramidal and giant neurons. This suggests that a few inhibitory collicular terminals contact the excitatory large neurons in the dorsal cochlear nucleus.