AFFERENT SYNAPTIC TRANSMISSION IN THE MAMMALIAN COCHLEA

描述(申请人提供)：本提案旨在研究哺乳动物耳蜗内毛细胞(IHC)传入突触的突触传递机制。在内耳，声音信号被转化为毛细胞受体电位，随后在传入突触处转化为听神经纤维的放电率。声音的编码在很大程度上取决于听神经纤维的不同放电特性。听神经纤维的重要特征是它们的自发放电率、它们的激活阈值和它们的“率水平函数”，它描述了不同声压水平下放电率的变化。耳蜗神经传递的三个主要“部位”决定了听神经纤维的特性：1)基底膜力学；2)IHC受体电位；3)IHC传入突触传递。令人信服的论点是，具有不同特性(低自发频率和高自发频率；高激活阈值和低激活阈值)的听神经纤维支配相同的IHC。然而，与一个IHC接触的所有10-20个传入纤维都“感觉”到相同的受体电位，因此很可能传入纤维特性的差异必然出现在单个突触上。本研究的目的是探讨听神经纤维的哪些特性出现在IHC传入突触，以及突触前和/或突触后成分如何具体决定这些特性。取2-4周龄啮齿类动物的耳蜗骨组织，同时进行IHC和直接接触IHC的传入纤维终末的全细胞记录。通过控制IHC膜电位和监测传入终末的突触后活动，可以直接测量IHC传入突触的传递函数。细胞外记录将被用来监测传入终末的动作电位率。这种方法的关键是，当听神经纤维的各种特性发展起来时，它允许在听力开始后进行录音。将确定传入纤维的自发频率、阈值和传递函数，并测量传入纤维的适应性。来自成对传入纤维的同时记录将提供具有不同性质的纤维是否确实支配单个IHC的直接测试。基于活体实验的假说将被重新评估，这些假说表明特定的位置是特定的听觉神经纤维特性的来源。负责特定放电模式的特定突触前或突触后机制将被进一步研究。更好地理解产生不同听觉神经纤维激发特性的机制将为改进人工耳蜗的设计提供基础。这项建议中概述的研究试图了解在听觉通路的第一个突触-毛细胞和听神经纤维之间的突触-突触传递背后的机制。将毛细胞感受器电位转换为听神经纤维的放电率是对声音信号进行编码以传输到大脑的重要步骤。我们的结果将支持旨在模拟听神经活动是如何产生的研究。这些方法可以为更好的人工耳蜗设计提供未来的基础。

DESCRIPTION (provided by applicant): This proposal seeks to investigate mechanisms of synaptic transmission at the inner hair cell (IHC) afferent synapse in the mammalian cochlea. In the inner ear, sound signals are converted into hair cell receptor potentials, and subsequently are translated at the afferent synapse into firing rates in auditory nerve fibers. Coding of sound critically depends on the diverse firing properties of auditory nerve fibers. Important features of auditory nerve fibers are their spontaneous firing rates, their thresholds of activation and their `rate level functions', describing the changes in firing rates in response to different sound pressure levels. Three main `sites' in cochlear transmission have been suggested to determine auditory nerve fiber properties: 1) basilar membrane mechanics; 2) IHC receptor potential; 3) IHC afferent synaptic transmission. Convincing arguments have been made that auditory nerve fibers with distinct properties (low versus high spontaneous rates; high versus low thresholds of activation) innervate the same IHCs. However, all 10-20 afferent fibers contacting one IHC `sense' the same receptor potential and therefore it is likely that differences in afferent fiber properties must arise at single synapses. The aim of this study is to ask which properties of auditory nerve fibers arise at the IHC afferent synapse, and how pre- and/or postsynaptic components specifically determine these properties. Excised cochlear tissue from 2-4 week old rodents will be used to perform simultaneous whole cell recordings from IHCs and afferent fiber terminals directly where they contact the IHCs. The transfer function at the IHC afferent synapse will be measured directly by controlling the IHC membrane potential and monitoring postsynaptic activity in the afferent terminal. Extracellular recordings will be used to monitor the rate of action potentials at the afferent terminal. Key to this approach is that it allows recordings after hearing onset, when the diverse properties of auditory nerve fibers have developed. Afferent fiber spontaneous rates, thresholds and transfer functions will be determined and afferent fiber adaptation will be measured. Simultaneous recordings from pairs of afferent fibers will provide a direct test of whether fibers with different properties do innervate a single IHC. Hypotheses based on in vivo experiments that have suggested specific loci as the origin for specific auditory nerve fiber properties will be reevaluated. The specific pre-or postsynaptic mechanisms responsible for specific firing patterns will be further investigated. A better understanding of the mechanisms that underlie the generation of diverse auditory nerve fiber firing properties will provide the basis for an improvement in cochlear implant design. The studies outlined in this proposal seek to understand the mechanisms that underlie synaptic transmission at the first synapse in the auditory pathway, the synapse between hair cells and auditory nerve fibers. The conversion of the hair cell receptor potential into a firing rate in the auditory nerve fibers is an important step in coding the sound signal for transmission to the brain. Our results will support studies that aim to model how auditory nerve activity is generated. These approaches can provide a future basis for better cochlear implant design.