Neurotrophin Gene Therapy in Deafened Ears with Cochlear Implants: Long-term Effects on Nerve Survival and Functional Measures

Because cochlear implants function by stimulating the auditory nerve, it is assumed that the condition of the nerve plays an important role in the efficacy of the prosthesis. Thus, considerable research has been devoted to methods of preserving the nerve following deafness. Neurotrophins have been identified as a potential contributor to neural health, but most of the research to date has been done in young animals and for short periods (less than 3 to 6 months) after the onset of treatment. The first objective of the current experiment was to examine the effects of a neurotrophin gene therapy delivery method on spiral ganglion neuron (SGN) preservation and function in the long term (5 to 14 months) in mature guinea pigs with cochlear implants. The second objective was to examine several potential non-invasive monitors of auditory nerve health following the neurotrophin gene therapy procedure. Eighteen mature adult male guinea pigs were deafened by cochlear perfusion of neomycin and then one ear was inoculated with an adeno-associated viral vector with an Nft3-gene insert (AAV.Ntf3) and implanted with a cochlear implant electrode array. Five control animals were deafened and inoculated with an empty AAV and implanted. Data from 43 other guinea pig ears from this and previous experiments were used for comparison: 24 animals implanted in a hearing ear, nine animals deafened and implanted with no inoculation, and ten normal-hearing non-implanted ears. After 4 to 21 months of psychophysical and electrophysiological testing, the animals were prepared for histological examination of SGN densities and inner hair cell (IHC) survival. Seventy-eight percent of the ears deafened and inoculated with AAV.Ntf3 showed better SGN survival than the 14 deafened-control ears. The degree of SGN preservation following the gene therapy procedure was variable across animals and across cochlear turns. Slopes of psychophysical multipulse integration (MPI) functions were predictive of SGN density, but only in animals with preserved IHCs. MPI was not affected by the AAV.Ntf3 treatment, but there was a minor improvement in temporal integration (TI). AAV.Ntf3 treatment had significant effects on ECAP and EABR amplitude growth func-tion (AGF) slopes; the reduction in slope in deafened ears was ameliorated by the AAV.Ntf3 treatment. Slopes of the ECAP and EABR AGFs were predictive of SGN density in a broad area near and just apical to the implant. The highest ensemble spontaneous activity (ESA) values were seen in animals with surviving IHCs, but AAV.Ntf3 treatment in deafened ears resulted in slightly higher ESA values compared to deafened untreated ears. Overall, a combination of the psychophysical and electrophysiological measures can be useful for monitoring the health of the implanted cochlea in guinea pigs. These measures should be applicable for assessing cochlear health in human subjects.