Computer-assisted 3-D reconstructions of Golgi-impregnated neurons in the cortical regions of the inferior colliculus of rat.

The inferior colliculus (IC) is the main auditory nucleus in the midbrain. This auditory center is made of a central nucleus (CNIC) characterized by a distinct laminar organization that is surrounded by cortical regions. The neuronal types in the CNIC are well established but thus far, the neuronal composition and functional roles of the cortical regions are not fully appreciated. As dendritic architecture is critical for the synaptic integrative properties of neurons, a detailed analysis of the dendritic architecture of the neurons in the collicular cortical regions should shed light on our understanding of their roles in collicular function. In the present study, we have used the del Rio-Hortega Golgi procedure to impregnate individual neurons within the IC. Rat brains were embedded in resin and sectioned serially to allow quantitative 3-D analyses of single neurons or groups of neurons. Our results demonstrate that the cortical regions of the IC are made up of unique sets of neuronal types and that there is an interdigitation of dendrites at the cortical borders. This latter feature may have led to difficulty in delineating a sharp border between the CNIC and cortical regions in previous studies. The quantitative analysis further demonstrates that there are significant differences in many of the dendritic parameters tested when compared to the neurons from the CNIC. Moreover, we observed that the neuronal populations of the cortical regions vary from the laminar pattern of the CNIC and from each other. Since the main organizing principle of the CNIC is the laminar organization of 'flat' neurons, evidence that cortical IC regions lack flat neurons supports the subdivision schema presented here.