Li Hongzhe, Kachelmeier Allan, Furness David N, Steyger Peter S
Oregon Hearing Research Center, Oregon Health & Science University Portland, OR, USA.
School of Life Sciences, Keele University Staffordshire, UK.
Front Cell Neurosci. 2015 Apr 14;9:130. doi: 10.3389/fncel.2015.00130. eCollection 2015.
Loud sound exposure exacerbates aminoglycoside ototoxicity, increasing the risk of permanent hearing loss and degrading the quality of life in affected individuals. We previously reported that loud sound exposure induces temporary threshold shifts (TTS) and enhances uptake of aminoglycosides, like gentamicin, by cochlear outer hair cells (OHCs). Here, we explore mechanisms by which loud sound exposure and TTS could increase aminoglycoside uptake by OHCs that may underlie this form of ototoxic synergy. Mice were exposed to loud sound levels to induce TTS, and received fluorescently-tagged gentamicin (GTTR) for 30 min prior to fixation. The degree of TTS was assessed by comparing auditory brainstem responses (ABRs) before and after loud sound exposure. The number of tip links, which gate the GTTR-permeant mechanoelectrical transducer (MET) channels, was determined in OHC bundles, with or without exposure to loud sound, using scanning electron microscopy. We found wide-band noise (WBN) levels that induce TTS also enhance OHC uptake of GTTR compared to OHCs in control cochleae. In cochlear regions with TTS, the increase in OHC uptake of GTTR was significantly greater than in adjacent pillar cells. In control mice, we identified stereociliary tip links at ~50% of potential positions in OHC bundles. However, the number of OHC tip links was significantly reduced in mice that received WBN at levels capable of inducing TTS. These data suggest that GTTR uptake by OHCs during TTS occurs by increased permeation of surviving, mechanically-gated MET channels, and/or non-MET aminoglycoside-permeant channels activated following loud sound exposure. Loss of tip links would hyperpolarize hair cells and potentially increase drug uptake via aminoglycoside-permeant channels expressed by hair cells. The effect of TTS on aminoglycoside-permeant channel kinetics will shed new light on the mechanisms of loud sound-enhanced aminoglycoside uptake, and consequently on ototoxic synergy.
暴露于高强度声音会加剧氨基糖苷类药物的耳毒性，增加永久性听力损失的风险，并降低受影响个体的生活质量。我们之前报道过，暴露于高强度声音会引起暂时性阈移（TTS），并增强耳蜗外毛细胞（OHC）对庆大霉素等氨基糖苷类药物的摄取。在此，我们探讨高强度声音暴露和TTS可能增加OHC对氨基糖苷类药物摄取的机制，这可能是这种耳毒性协同作用的基础。将小鼠暴露于高强度声音水平以诱导TTS，并在固定前30分钟给予荧光标记的庆大霉素（GTTR）。通过比较高强度声音暴露前后的听觉脑干反应（ABR）来评估TTS的程度。使用扫描电子显微镜，在暴露或未暴露于高强度声音的OHC束中，确定作为GTTR渗透机械电换能器（MET）通道门控的纤毛顶端连接的数量。我们发现，与对照耳蜗中的OHC相比，诱导TTS的宽带噪声（WBN）水平也会增强OHC对GTTR的摄取。在出现TTS的耳蜗区域，OHC对GTTR摄取的增加明显大于相邻的柱细胞。在对照小鼠中，我们在OHC束中约50%的潜在位置鉴定出了静纤毛顶端连接。然而，在接受能够诱导TTS水平WBN的小鼠中，OHC顶端连接的数量显著减少。这些数据表明，TTS期间OHC对GTTR的摄取是通过存活的机械门控MET通道以及/或者高强度声音暴露后激活的非MET氨基糖苷类药物渗透通道的通透性增加而发生的。顶端连接的丧失会使毛细胞超极化，并可能通过毛细胞表达的氨基糖苷类药物渗透通道增加药物摄取。TTS对氨基糖苷类药物渗透通道动力学的影响将为高强度声音增强氨基糖苷类药物摄取的机制，进而为耳毒性协同作用的机制提供新的线索。
