Characterization of termination patterns of layers 5 and 6 cortical neurons in the mouse inferior colliculus.

Top-down modulation is thought to be one mechanism by which the brain can disambiguate sensory signals, and descending projections are believed to be the substrate through which this modulation occurs. One of such projections - the corticocollicular (CC) projection - recently garnered much attention due to its potential to alter response properties in the inferior colliculus (IC). However, the anatomical organization of this pathway remains poorly understood. Earlier we demonstrated significant heterogeneity in the distributions of layer 5 and 6 CC neurons in the cortex, with layer 6 cells spanning across a broader area of the cortex than layer 5. However, the comparison of the termination patterns from layer 5 and layer 6 neurons is yet to be made. In the present study, we used two viral approaches in Rbp4-cre mouse line to differentially express fluorescent markers in layer 5 and layer 6 corticocollicular neurons in the same animal, and later compare the distributions of their axons in all subdivisions of the IC. Using confocal microscopy, we found that both layer 5 and layer 6 CC neurons heavily projected to lateral (LC) and dorsal cortices (DC) in the ipsilateral IC, with some terminals found in the contralateral DC and ipsilateral central nucleus (CN). Additionally, layer 6 CC axons and terminals tended to be thinner and smaller on average than axons and terminals from layer 5 CC cells. These findings further suggest different functions for layer 5 and 6 CC neurons and provide a foundation for further differential optogenetic manipulations of layer 5 and 6 corticocollicular projections.NIDCD R01 DC013073 02Ope