Fine structure and neurotransmitter cytochemistry of neurons in the rat ventral cochlear nucleus projecting to the ipsilateral dorsal cochlear nucleus

The neural tracer wheat germ agglutinin conjugated to horse radish peroxida se was injected into the rat dorsal cochlear nucleus and acoustic stria. So me labelled neurons in the ipsilateral ventral cochlear nucleus were found as a result. These neurons were studied at the ultrastructural level, and t heir axo-somatic synaptic profile and glycine immunoreactivity were determi ned. Most neurons were glycine negative and classified as type I multipolar neurons. The latter showed a different synaptic profile from that of neuro ns projecting to the contralateral inferior colliculus or cochlear nucleus. This suggests the presence of differing populations of multipolar cells ba sed on their synaptic profile. Few labelled multipolar neurons of type II w ere found, which appeared glycine negative and, rarely, glycine positive. T he latter show an ultrastructure and axo-somatic profile similar to that of glycinergic commissural neurons in the dorsal and ventral cochlear nucleus . In particular, about one-third of boutons contained round synaptic vesicl es, which are believed to contain an excitatory neurotransmitter. The ultra structural analysis of the synaptic boutons in the cochlear nucleus confirm s the presence of numerous cases of colocalization of glycine and GABA wher e flat and pleomorphic synaptic vesicles are mixed. The present study is in accordance with previous tract-tracing light microscopic studies which hav e indicated that large glycinergic neurons in the ventral cochlear nucleus act as broad-band inhibitory neurons in microcircuits of the dorsal cochlea r nucleus and contralateral cochlear nucleus.