Improved optogenetic modification of the spiral ganglion neurons for future optical cochlear implants

Optogenetic stimulation has become a promising approach for restoring lost body function. For example, partial restoration of vision has been achieved in a blind patient and proof-of-concept has been demonstrated for optogenetic hearing restoration in rodents. In order to prepare clinical translation of hearing restoration, efficient and safe optogenetic modification of spiral ganglion neurons (SGNs) in the mature cochlea remains to be developed. Here, we established microcatheter-based administration adeno-associated virus (AAV) to scala tympani of the cochlea of Mongolian gerbils and compared it to the previously developed AAV-injection into the spiral ganglion. We probed the potential AAV-PHP.S capsid to express channelrhodopsins (ChRs) under the control of the human synapsin promotor in mature SGNs in hearing and deafened gerbils. Using the microcatheter approach, but not with the AAV-modiolus injection, we achieved reliable ChR expression in SGN enabling optogenetic stimulation of the auditory pathway in 80% of the treated animals. Yet, the efficiency of SGN transduction was modest with only ∼30% ChR-expressing SGNs. Moreover, we encountered off-target expression in hair cells in hearing gerbils in both approaches, but not ChR expression in the central nervous system using microcatheter administration. Comparing optogenetic auditory brainstem responses of gerbils with and without hair cell transduction confirmed that SGNs were the primary site of optogenetic stimulation of the pathway.