AFFERENT SYNAPTIC TRANSMISSION IN THE MAMMALIAN COCHLEA

描述（由申请人提供）：本提案旨在研究哺乳动物耳蜗内毛细胞传入突触的突触传递机制。在内耳，声音信号被转换成毛细胞受体电位，随后在传入突触被翻译成听觉神经纤维的放电速率。声音编码在很大程度上取决于听觉神经纤维的不同放电特性。听觉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“速率水平功能”，描述了不同声压水平下放电率的变化。耳蜗传递中的三个主要“部位”被认为可以决定听神经纤维的特性：1)基底膜力学；2) IHC受体电位；3) IHC传入突触传递。有令人信服的论据表明，具有不同特性的听觉神经纤维（自发率高低；激活阈值高低）支配着相同的ihc。然而，接触同一IHC的所有10-20个传入纤维“感觉”相同的受体电位，因此传入纤维特性的差异很可能是在单个突触上产生的。本研究的目的是询问在IHC传入突触中出现的听神经纤维的哪些特性，以及突触前和/或突触后成分如何特异性地决定这些特性。从2-4周龄的啮齿动物身上切除的耳蜗组织将被用于同时进行ihc和传入纤维终端直接接触ihc的全细胞记录。通过控制IHC膜电位和监测传入端突触后活动，可以直接测量IHC传入突触的传递功能。细胞外记录将用于监测传入端动作电位的速率。这种方法的关键在于，当听觉神经纤维的各种特性已经发育成熟时，它允许在听力发作后进行录音。传入纤维的自发速率、阈值和传递函数将被确定，传入纤维的适应性将被测量。同时记录成对的传入纤维将提供一种直接的测试，以确定具有不同性质的纤维是否支配单个IHC。基于体内实验的假设表明，特定位点是特定听觉神经纤维特性的起源，这些假设将被重新评估。具体的突触前或突触后机制负责特定的放电模式将进一步研究。更好地了解不同听觉神经纤维放电特性产生的机制将为人工耳蜗设计的改进提供基础。本提案中概述的研究旨在了解听觉通路中第一突触突触传递的机制，即毛细胞和听觉神经纤维之间的突触。将毛细胞受体电位转化为听觉神经纤维的放电速率是将声音信号编码并传输到大脑的重要步骤。我们的结果将支持旨在模拟听觉神经活动如何产生的研究。这些方法可以为将来更好的人工耳蜗设计提供依据。

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.