Ultrastructural and immunocytochemical characterization of neurons in the rat ventral cochlear nucleus projecting to the inferior colliculus.

Neurons in the rat ventral cochlear nucleus which project to the inferior colliculus were identified after retrograde labelling of the neural tracer wheat germ agglutinin conjugated to horse radish peroxidase. After tracer injection into the Inferior Colliculus, electron microscopy and immunocytochemical localization of glycine, GABA and glutamate in retrograde labelled neurons were employed. In the acoustic root area and posterior ventral cochlear nucleus, most of the body surface of the neurons projecting to the inferior colliculus was 10-30% covered by axo-somatic boutons and appeared as multipolar cells of type I. These large to medium size cells with sparse stacks of ergastoplasmic cisternae organized in Nissl bodies, were heavily labelled and were the main projecting neurons to the inferior colliculus. Most of these cells were glycine and GABA negative but variably glutamate positive, suggesting that they are excitatory neurons. This result suggests the absence of an inhibitory innervation of the inferior colliculus from these cells. A few retrograde labelled, large to giant neurons showed an irregular surface, sparse short stacks of ergastoplasmic reticulum, numerous microtubules, cell bodies 60-80% covered by synaptic boutons, and they appeared as multipolar cells of type II. These cells were less strongly retrograde labelled than multipolar type I, and were occasionally glycine positive, presumably inhibitory. This suggests that at least a small caliber collateral axon stemming from these neurons can reach the inferior colliculus. Occasional glycine positive octopus cells not labelled with the tracer were also observed. The contribution of glycinergic axons to the innervation of the inferior colliculus appears therefore to by very limited. Occasional labelled cells were represented by apparently globular bushy neurons, but the weak labelling suggests that the tracer was taken up by collateral axons reaching the inferior colliculus and not by the main axon. Globular bushy cells were consistently negative for both glycine and GABA, and variably positive for glutamate. In the anteroventral cochlear nucleus, labelled multipolar type I and II showed similar immunocytological and ultrastructural characteristics to those in the posteroventral cochlear nucleus but their dimension was smaller. Cells identified as spherical bushy neurons were never retrograde labelled.