Ultrastructural and immunocytochemical characterization of commissural neurons in the ventral cochlear nucleus of the rat

Medium to large-giant multipolar neurons in the rat ventral cochlear nucleus were retrograde labelled after injection of the tracer Wheat Germ Agglutinin conjugated to Horse Radish Peroxidase into the contralateral cochlear nucleus. Light microscopy immunocytochemistry showed that 42.45% of these retrograde labelled neurons, generally strongly labelled with the tracer, were markedly glycine immunopositive, and that 57.55%, usually weakly retrograde labelled neurons, were immunonegative or weakly positive for glycine. These commissural neurons were generally GABA negative and variably immunopositive for glutamate. About 1/3rd of the commissural neurons had variably developed a rough endoplasmic reticulum whilst axe-somatic boutons covered 20-40% of the cell body. These cells were recognized as multipolar neurons of type I. Most of them were weakly glycine positive or even negative and a few appeared glycinergic. A little less than the remaining 2/3rds of the whole commissural population in the postero-ventral cochlear nucleus presented a surface which was 65-85% covered with synaptic boutons, among which some also appeared labelled. These cells were recognized as multipolar neurons of type II. Many microtubules and neurofilaments were present, free ribosomes being more numerous around Nissl bodies with short cisternae. A few low retrograde labelled type II were weakly or non glycinergic. A small number of large to giant neurons type II, strongly retrograde labelled, appeared to be glycine positive, consistently GABA negative and variably glutamate positive. A very small proportion of retrograde labelled neurons appeared having the characteristics of globular bushy neurons. Their weak labelling, however, suggests that they project by collaterals or thin axons to the contralateral cochlear nucleus. Spherical bushy cells in the rat anteroventral cochlear nucleus lack the nuclear capping of rough endoplasmic reticulum observed in the cat, and none was labelled after injection into the contralateral cochlear nucleus. Globular and spherical neurons were variably glutamate positive but glycine and GABA negative. In conclusion, the present study suggests that commissural neurons include a small number of strongly labelled large to giant glycinergic and presumably inhibitory type II and, less frequently type I. A larger group of less heavily labelled commissural neurons of type I and II contain low levels or no glycine, which is probably used for metabolic purposes rather than as a neurotransmitter. This suggests that these neurons are presumably excitatory.