The ultrastructure of the central nucleus of the inferior colliculus of the Sprague-Dawley rat

Previous studies from this laboratory have indicated that increased numbers of GABAergic neurons, as well as total neurons, occur in the central nucleus of the inferior colliculus (IC) of genetically epilepsy-prone rats (GEPRs) as compared to non-seizuring Sprague-Dawley rats. Since electron microscopic studies of the IC have not been reported for rats, we wanted to determine the ultrastructure of neurons and their processes in this brain region to serve as a basis for future studies on neuronal circuitry in the GEPRs. Both disc-shaped and stellate types were found for each of three size categories: large, medium and small. Thus, six types of neuron were distinguished by differences in somatic size, shape, organelles and dendritic orientation. Large neurons (longest diameter greater than 25 micron), which are the least frequent cell type, contained vast perikaryal cytoplasm, eccentrically located nuclei and abundant granular endoplasmic reticulum (GER) adjacent to the nucleus as well as clustered in the cytoplasm; many axosomatic, symmetric synapses were present. Medium-sized neuronal somata (15-25 micron in diameter) had smooth as well as infolded nuclear membranes and clusters of GER in their cytoplasm but no GER adjacent to the nucleus; synapses were sparse along the surface of their somata. Small neurons (10-15 micron in diameter), which are the most frequent cell type, had scant perikaryal cytoplasm, usually infolded nuclei, frequently two nucleoli, and few or no stacked cisternae of GER in the perikaryal cytoplasm; only infrequent axosomatic synapses were found. Based on previous retrograde and immunocytochemical studies, most large disc-shaped and stellate cells project to the medial geniculate body and are probably excitatory, but some large stellate neurons have been shown to be GABAergic and it is doubtful that such neurons participate in this projection. A dense plexus of terminals that form symmetric synapses covers the soma and proximal dendrites of large neurons, and may provide a strong GABAergic inhibition of this type of projection neuron. Small and medium-sized disc-shaped cells also project to the thalamus but they lack this dense axosomatic plexus. The stellate cells from these same two size categories probably do not project to the thalamus and may be GABAergic local circuit neurons. Other ultrastructural features of IC neurons that were analysed include dendrites, dendritic spines, axon hillocks, initial segments and terminals, as well as the laminae of myelinated axons. Dendrites were either beaded or smooth and few spines were observed.(ABSTRACT TRUNCATED AT 400 WORDS)