隐性听力下降小鼠耳蜗内毛细胞突触的病理改变

 目的 探讨噪声暴露后出现暂时性阈移的小鼠在听觉反应阈值完全恢复正常后耳蜗内毛细胞突触的病理改变。方法 将小鼠分为正常对照组和实验组,实验组小鼠进行强度98 dB SPL、时长2 h的噪声暴露,建立暂时性阈移模型。将对照组和暂时性阈移模型小鼠进行全基底膜铺片并行免疫荧光染色,共聚焦显微镜拍照成像并观察细胞形态。利用耳蜗长度计算耳蜗所处的频率位置,最后用图形处理软件对突触结构进行三维重建,观察对照组小鼠和实验组小鼠的耳蜗内毛细胞突触前、后结构的空间分布规律及其病理改变特点。结果 对照组小鼠耳蜗内毛细胞的突触复合体有明显的分布特点,每个内毛细胞通过5~30个突触与耳蜗神经纤维形成突触连接,靠近蜗轴侧的突触前结构体积较大,与之相匹配的突触后结构体积较小。靠近柱细胞侧的突触前结构体积偏小,与之相匹配的突触后结构体积偏大。实验组小鼠在高强度噪声暴露后第2天听觉测试结果显示,听性脑干反应(auditory brain-stem response, ABR)和畸变产物耳声发射(distortion product otoacoustic emission, DPOAE)阈值升高30~40 dB,2周后再次测试显示阈值恢复正常,但是ABR的Ⅰ波波幅下降46.1%,且耳蜗内毛细胞的突触复合体结构数量也减少了41.3%。 而内毛细胞和螺旋神经节细胞没有明显丢失。结论 听觉阈值功能测试对判断小鼠耳蜗内毛细胞突触的损害和丢失不敏感;而听觉阈上功能测试对评价小鼠耳蜗内毛细胞突触的损害和丢失相对敏感。

The pathological change of synapses in cochlear inner hair cell of hidden hearing loss mice

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.