Perception of sound with cochlear implants (CIs) is currently accomplished, in most cases, by stimulating spiral ganglion neuron (SGN) bodies in Rosenthal's canal, since cochleae without hair cells typically lack auditory nerve fibers in the basilar membrane area (BMA). If auditory nerve fibers could be induced to regenerate back into the BMA, stimulation of such fibers could potentially lower the amount of current required for stimulus detection, increase dynamic range, improve temporal response properties and decrease channel interaction, thereby enhancing the perception of CI stimulation. Using recently developed methods, we generated preliminary data demonstrating the feasibility for long-term over-expression of a neurotrophin which targeted to the BMA of deaf ears, leading to robust regrowth of auditory nerve fibers into this tissue. This nerve regeneration was accomplished using non-toxic long-acting adeno- associated viral vectors that delivered into the cochlea using clinically feasible routes. Our proposed experiments will test the global hypothesis that presence of neurotrophins secreted by cells in and around the auditory epithelium will attract and maintain auditory nerve fibers, leading to improvement in measurable parameters of the functional (psychophysical and electrophysiological) responses to CI stimulation. Experiments in Aim 1 will compare the efficiency of BDNF versus NTF3 in attracting neurons to the deaf BMA and characterize the source of the neurons and their position in the tissue. Work in Aim 2 will compare detection threshold levels, dynamic ranges, temporal integration properties, spatial selectivity, and amplitude growth functions using established animal psychophysics procedures and electrically-induced auditory brainstem responses (EABR) in deaf reinnervated cochleae and deaf non-reinnervated cochleae.
Aim 3 will determine if auditory nerve fiber regeneration into the BMA improves SGN survival and central connections.
Aim 4 will determine how the combined effects of long-term chronic electrical stimulation and neurotrophin over-expression influence SGN cell bodies and projections. These experiments will set the groundwork for clinical methods to induce nerve regeneration that could enhance the outcome of cochlear implant procedures in patients with severe or profound sensorineural hearing loss.
The need to develop methods for inducing, directing and maintaining auditory nerve regeneration has been a critical barrier to the field. Our recent break-through in the application of gene transfer now provides the ability to target cells in the deaf inner ear, and insert genes into these cells for secreting a protein of choice (neurotrophin) leading to stable, long-term nerve regeneration in these ears. Auditory nerve fibers will grow into an area where they will be in close proximity to the cochlear implant electrode. Our proposed studies can, for the first time, test the long-term influence of auditory nerve regeneration on the psychophysical responses to electrical stimulation provided by the cochlear implant. We will further correlate these responses with survival and condition of the neurons and their peripheral and central processes. The outcome of the proposed work may improve communication for thousands of people who have severe or profound hearing impairments and use cochlear implants.
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