| Part1: Noise-Induced Hearing Loss ModelObjectiveTo Investigate the effects on the rats’ auditory brainstem response thresholds and rats’outer hair cells morphology of the different turn basilar membrane before and after noiseexposure.MethodsForty adult Sprague-Dawley (SD) rats were randomly divided into control and noise-exposed groups: the animals in noise-exposed group were exposed to white noise at115dB SPL for2hours a day in3consecutive days; the animals in control group were notexposed to noise. Auditory thresholds of two groups rats were assessed by auditorybrainstem response (ABR), prior to and1,3,7,14days after noise exposure. FinishedABR testing, rats in the two groups were randomly killed for cochlear sensory epitheliasurface preparation which were stained by FITC-phalloidin and observed microscopically.ResultsCompared with the control group, the ABR thresholds for all frequencies increased mostsignificantly on1st day after noise exposure. Then the change recovered gradually overtime, and stabilized in14days postexposure with a permanent threshold shift of10dB atlow-frequency and30dB at high-frequency (P<0.05). The results of stretched preparationof basilar membrane stained by FITC-phalloidin showed that outer hair cell losses in basalturn were more obvious than that in apical turn in the noise-exposure group, along withsome cilias’ disorder and fusion, while there were actually well-arranged hair cells whosecilias shaped V or W type in the same areas of the control group．There was statisticallysignificant difference of outer hair count between these two groups (P<0.05).ConclusionsIn the process of noise-induced hearing loss, there was the temporary threshold shift inearly phase, and then the thresholds stabilized in about14days after noise exposure andthe permanent threshold shift was observed at the same time. ABR results indicated thatthe redox reaction on1st day after noise exposure was most intense, and the oxidativedamage of the cochlear basilar membrane on high-frequency region was much moreserious than that on low frequency region and was also difficult to restore. Part2: Noise-induced Alteration in the ApoptosisInducing Factor (AIF) Expression of the different turnBasilar Membrane in Rats CochleaObjectiveTo Investigate the differential expression of apoptosis-inducing factor (AIF) in rats’ outerhair cells of the different turn basilar membrane before and after noise exposure, and therelationship with vulnerability of high-frequency hearing in noise-induced hearing loss.MethodsForty adult Sprague-Dawley (SD) rats were randomly divided into control andnoise-exposed groups: the animals in noise-exposed group were exposed to white noise at115dB SPL for2hours a day in3consecutive days; the animals in control group were notexposed to noise, and other conditions were same to the noise-exposed group. Within24hours after completing the noise exposure, rats in the two groups were randomly killed forcochlear sensory epithelia surface preparation and basilar membranes collection. RT-qPCR,western blot and immunofluorescence methods were used to detect the change in the AIFexpression of the different turn basilar membrane in the two group rats cochlea from thequalitative and quantitative point of views.ResultsImmunofluorescence staining showed in the control group there were AIF expressions onthe cytoplasm of the apical and basal turn outer hair cells and no expressions on thenucleus, and in the noise-exposed group the AIF fluorescence intensity of the apical andthe basal turn basilar membrane were stronger than that in control group. Moreover therewere a few AIF expressions on the nucleus. RT-qPCR results were consistent with westernblot results. The analysis revealed that the AIF expression of apical turn in the controlgroup was stronger than that of basal turn. In addition, the AIF expression of apical and basal turn in the noise-exposure group was obviously stronger than that in the controlgroup, and the change of apical turn expression was more significant (P<0.05).ConclusionsAIF is a proapoptotic factor, but it may act as a free radical scavenger in response tonoise-induced oxidant stress. Normally there was a significant difference on AIFexpression between the apical turn basilar membrane and the basal turn basilar membrane,and after noise exposure the difference was more significant. AIF played a leading role ofthe oxidoreductase in the process, and played a minor role of the apoptosis-inducingactivity. So it is likely that the significantly different ability on resisting redox reactionwhich is found between the apical and basal turn of cochlear basilar membrane may beone of the mechanisms of the high-frequency vulnerability in noise-induced hearing loss. |
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