Cochlear neuronal tracing for frequency mapping with DiI, NeuroVue, and Golgi methods

Conclusions. Labeling experiments using NeuroVue® Red dye allowed us to demonstrate individual afferent fiber tracks in the cochlea from the synaptic region of the inner hair cell in the organ of Corti (OC) to the spiral ganglion in Rosenthal's canal. Further optimization is necessary to obtain 3-dimensional (3D) neural distribution in the apical region for frequency mapping. Objectives. We intend to develop a method by which the radial fibers of the spiral ganglion (SG) can be individually visualized and tracked in 3D from the base to the apex of the cochlea. The combined trajectories of fibers from each cochlea could then be calculated for modeling of the 3D relationship of OC and SG in cochlear implant studies to assist in the optimization of cochlear implants for music and speech perception in noise. Materials and methods. We tested three different methods to visualize cochlear nerve fibers from OC to SG. Adult rat and mouse ears were stained with DiI dye, modified Golgi-Cox method or NeuroVue dye, sectioned or whole-mounted, and viewed with confocal or standard light microscope. Results. In DiI staining, spacial resolution and the number of neurons to be stained are too low to utilize this method to create a characteristic frequency map of the cochlea. The Golgi method mainly stained efferent nerve fibers, resulting in less information on cochlear nerve distribution. NeuroVue Red dye allowed clear tracking of individual fibers when combined with DAPI counterstaining.