Cochlear Injury Induced By Repeated Noise Exposure And Optimization Of Cochlear Gene Therapy Pathway

Objective: A single medium-dose noise exposure can cause different levels of permanent damage to the synapse between cochlear hair cells and type I spiral ganglion neurons,and may not be accompanied by permanent threshold shift.It is unclear whether multiple noise exposures aggravate synaptic damage and whether the synaptic damage is accompanied by coding-in-noise deficits.It is known that recombinant adeno-associated virus(r AAV)carrying neurotrophic factor 3 transfected inner hair cells can partially antagonize noisy synaptic damage,but the cochlear fenestration used in previous studies will damage the structural integrity of the cochlea and accompany the risk of hearing loss.Therefore,this project further evaluates the damage of cochlear synapses caused by repeated noise exposure and optimizes the current gene therapy pathway.Methods: Normal hearing albino guinea pigs were exposed to two noises(106 d B SPL2 h,4 weeks apart).The animals were sacrificed 1 and 4 weeks after each exposure.They were sacrificed after electrophysiological testing,and the synapse density in the high frequency area was counted.Ultrasonic microbubble method was used to partially damage the round window membrane(USM-RWM),and the gelatin sponge containing r AAV-EGFP was placed.The animals were sacrificed(2 weeks after surgery)and the transfected hair cells were counted.Results: The synapse loss 1 day after the second noise exposure was significantly less than the first(18.3% vs 56.7%,p < 0.001);after 4 weeks of noise exposure,there was no significant difference in the synaptic density between single and repeated noise exposure groups.In the USM-RWM group,there was no threshold shift after surgery.The r AAV transfection efficiency of IHCs at basal turn reached 98.47%,which was not significantly different from Cochleostomy group(p = 0.376),but the transfection efficiency of OHCs(62.52%)was significantly lower than the Cochleostomy group(p<0.001).Conclusion: Synapses remaining or recovered after the initial noise exposure are resistant to noise damage.Secondary noise exposure does not increase synapse loss.There is a correlation between repeated noise exposure and hearing function defects in noisy environments.r AAV can safely and efficiently transfect cochlear hair cells via the USM-RWM pathway.