GABA-like and glycine-like immunoreactivities of the cochlear root nucleus in rat

The cochlear root nucleus is part of the cochlear nuclear complex in small rodents. Its cells, the large root neurons, have a superficial resemblance to the globular neurons of the ventral cochlear nucleus. It has been a matter of debate, therefore, whether the root neurons and globular neurons represent the same or different types of cell. In the present study the two cell types with adjacent neuropil structures were compared by light microscopic, postembedding immunocytochemistry. Pairs of 0.5 microns sections of resin-embedded, glutaraldehyde-fixed material were treated with purified antisera raised against GABA- and glycine-glutaraldehyde-protein conjugates, respectively. Both types of cell were found to be immunonegative. Striking differences, however, occurred in what was interpreted as afferent nerve terminals. The globular cells appeared to receive numerous afferents with GABA- or glycine-like immunoreactivity on their somata. Immunoreactive terminals on the root neurons, on the contrary, were mostly GABA-positive and located on the dendrites. Although of unknown origin, the immunoreactive afferents were clearly different from the primary fibres as demonstrated both by the immunonegativity of the latter and by the different size and distribution of the terminals labelled anterogradely after horseradish peroxidase injections into the spiral ganglion.