The pathological change of synapses in cochlear inner hair cell of hidden hearing loss mice / 复旦学报（医学版）

ABSTRACT

Objective To investigate the synaptopathy of hidden hearing loss mice,and to observe the synapses of the cochlear inner hair cell after temporary threshold shift of noise exposure.Methods Mice were divided into normal control group and experiment group,the latter was exposed under noise of 98 dB SPL for 2 h to establish the model of temporary threshold shift.Mice cochleae of the two groups were dissected and prepared with whole mount and immunostaining.Cellular morphology was observed under confocal laser scanning microscope.Cochlear lengths were measured through cochlear frequency map to localize hair cells in different frequency regions.Then,3-D morphometry of synapses was constructed by Amira software to observe pre-synaptic ribbons,post-synaptic receptors and its pathological changes.Results In control group,each cochlear nerve fiber contacted a single inner hair cell by a single synapse,each inner hair cell had 5-30 synapses contacting cochlear nerve fibers.The larger ribbons patched smaller receptors located in the modiolar side,and the smaller ribbons patched larger receptors located in the pillar side.While in experiment group,noise overexposures caused moderate or completely reversible thresholds shift,i,e.,distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) thresholds increased 30-40 dB.Although returned to normal after 2 weeks,ABR wave Ⅰ amplitudes recovered to only 46.1 ％ of pre-exposure amplitudes.There was 41.3％ synapses loss of inner hair cell,but there was no loss of inner hair cells and spiral ganglion neurons.Conclusions Threshold test is not sensitive to degeneration and loss of synapse in mice inner hair cells,while super threshold test is sensitive to it.