Excitability and synaptic function of type II cochlear afferents

描述（由申请人提供）：这是一项确定哺乳动物耳蜗II型传入事件功能特性的提案。虽然只占所有耳蜗传入的一小部分（5-10%），但它们独特的外毛细胞树突和听觉脑干的终止模式强烈暗示了与I型传入完全不同的功能作用。有限的数据表明，II型传入具有非常高的声学阈值，可能仅表示创伤或疼痛水平的声音。进一步类推到躯体疼痛纤维，II型传入神经被ATP激活，在耳蜗损伤时可以释放ATP，就像在受损的皮肤中一样。该项目将包括在离体耳蜗节段的II型传入的千兆欧姆密封细胞内记录，以表征II型传入的兴奋性和突触功能。细胞膜的基本性质，动作电位阈值和起始位点，以及突触输入的大小和分布将被确定。定量分析将确定外毛细胞突触强度。与记录的纤维相关的突触前和突触后结构将在突触后进行免疫标记。这些数据将被纳入解剖正确的区室模型，以获得激活II型传入所需的声刺激的估计。为了进一步探索其在耳蜗创伤中的可能作用，我们将在因大声声音和/或暴露于耳毒素而受损的耳蜗中进行II型录音。听力损失可导致听觉亢进和耳鸣幻觉。与外周敏感化和“幻肢痛”的类比促使人们将其与躯体神经病变相提并论。II型传入事件的功能作用的描述增加了耳蜗功能的重要的，长期缺失的信息，将加强听觉发病机理的理论，并可能提供新的治疗靶点。

DESCRIPTION (provided by applicant): This is a proposal to determine the functional properties of till-now-mysterious type II afferents of the mammalian cochlea. Although comprising only a small fraction (5-10%) of all cochlear afferents, their unique arborization to outer hair cells, and termination pattern in the auditory brainstem strongly imply a functional role quite distinct from that of the type I afferents. Limited data suggest that type II afferents have a very high acoustic threshold, perhaps signaling only traumatic or painful levels of sound. Further extending an analogy to somatic pain fibers, type II afferents are activated by ATP that can be released during cochlear trauma, as it is in damaged skin. This project will involve giga-ohm-seal intracellular recording from type II afferents in cochlear segments ex vivo to characterize the excitability and synaptic function of type II afferents. Basic membrane properties, action potential threshold and initiation site, and the size and distribution of synaptic inputs will be determined. Quantal analysis will determine outer hair cell synaptic strength. Pre- and postsynaptic structures associated with recorded fibers will be immunolabeled posthoc. These data will be incorporated into an anatomically-correct, compartmental model to obtain an estimate of the acoustic stimulus required to activate the type II afferent. To explore further a possible role in cochlear trauma, type II recordings will be made in cochleae that have been damaged by loud sound and/or exposure to ototoxins. Hearing loss can lead to hyperacusis and the phantom percept of tinnitus. The analogy to peripheral sensitization and 'phantom limb pain' prompts parallels with somatic neuropathy. Delineation of the functional role of type II afferents adds essential, long-missing information on cochlear function that will enhance theories of auditory pathogenesis, and may provide new therapeutic targets. PUBLIC HEALTH RELEVANCE: Hearing loss and the associated pathologies of hyperacusis and tinnitus result from loss of cochlear hair cells, and altered activity in cochlear afferent neurons. This proposal will determine the responsiveness, signaling and pharmacology of type II cochlear afferents that have until recently been entirely mysterious. Auditory pathogenesis may result from an altered balance of activity between small type II, and large type I afferents, by analogy to neuropathic pain in the somatic nervous system, thus providing the type II afferent as a novel therapeutic target.