Fig. 3.

Histological results for three representative animals differing in the treatment and implantation procedures. All three animals received an eight-electrode Nucleus animal cochlear implant (Cochlear Corp., Lane Cove, Australia) in their left ear. The implants were inserted through a cochleostomy made approximately 0.7 mm apical to the round window. The electrodes were spaced at approximately 0.75 mm intervals and were labeled A through H with A being the most apical. The “Hearing” animal received the cochlear implant in an ear that had normal hearing prior to implantation. After implantation while pulse rate data in Fig. 6 were collected, thresholds for 16 kHz pure tones were elevated by 8.1 dB relative to the pre-implant thresholds. The “Deaf + AAV.NTF-3” animal was deafened prior to implantation by systemic administration of kanamycin (400 mg/kg) and ethacrynic acid (35 mg/kg). Seven days later, the left ear was inoculated with AAV.NTF-3 (5 μL) injected into the scala tympani through the cochleostomy and then, 20 minutes after the inoculation, the cochlear implant was inserted. The “Deaf” animal was deafened in the left ear by local injection of neomycin sulfate (60 μl, 10% w/v) through the round window prior to implantation. Histological data were obtained from peri-midmodiolar sections that were centered at the location of the primary electrode used for psychophysical and electrophysiological data collection (Electrode B, which was located in the first half of the basal turn approximately 3.8 mm on average from the round window). Five 2.5 μm sections were analyzed and spaced at intervals of 25 μm. Means of results for these five sections are shown. Inner hair cells (IHC) were counted only if a nucleus was present. Percent normal (left ordinate) is shown where normal equals one hair cell per section. The outer hair cells (OHC) were counted if any part of the cell was visible and normal equals 3 hair cells per section. Individual peripheral processes could not be counted in the peri-midmodiolar sections so density of peripheral processes was estimated on a scale of 0 to 3, with 3 being normal. Spiral ganglion cell packing density (right ordinate) was estimated by counting the number of cells with a nucleus in the cross section of Rosenthal's canal and dividing by the area of that cross section. Histological results for the hearing animal and those for the deaf animal are similar to those reported by Kang and colleagues (2010).