| ObjectivesTo understand the temporal characteristics and spatial distribution of synaptic ribbons with development,to clarify the stable time of presynaptic ribbons in C57BL/6j mice,and further to investigate the underlying mechanism for cochlear damage induced by long-term moderate-level noise exposure.Methods(1)The development and distribution of synaptic ribbons on the presynaptic membrane of the outer and outer hair cells of C57 mice were observed and recorded by confocal microscopy after the whole cochlea with immunofluorescence staining;(2)Immunoflurorescence staining,HE dye and cochlear electron microscopy scan were used to estimate the morphology of the other components of cochlea;(3)Animal model imitated environmental noise stimuli was created by continuous exposure to white noise(70dBSPL,8h/d,duration:3 months);(4)Auditory function and potential amplitudes were determined by ABR tests and wave I amplitude,respectively;(5)Tonotopic localization was used to establish the corresponding relationship between the auditory function and the whole cochlea synaptic ribbons along spiral.Results(1)The expression of Ctbp2 was observed on the cytoplasm of hair cells in C57 mice at 1th day after birth.On the 6th day after birth,the number of Ctbp2 signals reached the peak on the cytoplasm and basolateral membrane of hair cells,particularly significant on OHCs;12-60th d after birth,Ctbp2 signal number tends to be stable,and the distribution of hair cells gradually to the basolateral membrane.Functionally,detectable ABR wave appeared at 13.2th day after birth;(2)The synaptic ribbons on IHCs were enriched in the middle area of the cochlea(50%-70%from cochlear apex),reaching 18.6 per IHC,which was significantly higher than that of the synaptic ribbbons distance from cochlear apex 10%-20%(7.4/IHC)and 90%-100%(9.6/IHC)region.There was no significant difference in OHCs along cochlear spiral,1-3/OHC;(3)Moderate intensity noise exposure for 3 months,in the auditory spectrum range,the frequency band has varying degrees of hearing loss,and the 8-16kHz frequency threshold and ABR I wave amplitude decreased significantly;(4)By the cochlear tonotopic localization,the number of synaptic ribbons in the cochlear spiral was significantly reduced,and such reduction was significantly higher in the 50-70%region than the other;(5)Compared with the control group,no significant difference was found in IHCs&OHCs&SGNs counts,arrays of stereocilia and the morphology of Corti's organ in experimental group.Conclusions(1)In juvenile C57 mice,synaptic connections between hair cells and cochlear fibers have existed before birth.During 12 days after birth,the synaptic ribbons experienced dramatic changes in number and spatial distribution,and gradually became perfect and functional after 12 days.After 13.2 days of birth,the mice began to hearing-onset.Therefore,the time point of synaptic ribbons maturation was highly in coincidence with hearing-onset time,suggesting that the functional synaptic ribbons is the premise of hearing-onset;(2)The distribution of synaptic ribbons along the cochlear spiral tends to be stable in 60-day-old C57BL/6j mice;(3)Long-term moderate-level noise-exposure can cause varying degrees of hearing thresholds at multiple frequencies,and such impacts mainly involved in auditory sensitive areas(8-16kHz).The reduction of connection of IHCs and ANFs may be the underlying mechanism of noise-induced hearing loss. |
PDF Full Text Request