Postsynaptic Activity of Type II Cochlear Afferents

描述(申请人提供)：耳蜗含有两种类型的初级传入神经元，即I型和II型螺旋神经节神经元。95%的螺旋神经节由编码声刺激的I型神经元组成。其余5%的螺旋神经节，即II型神经元，广泛分布于外毛细胞(OHC)之间。人们对它们的电学性质知之甚少，因为只有几个神经元进行了记录。这种功能信息的缺乏排除了关于类型II传入在听觉刺激知觉中的作用的假设。我们已经开发出一种技术，可以在突触输入附近的II型传入树突中进行千兆海豹细胞内记录。使用神经元示踪剂可以根据与文献中描述的已知树突形态的比较来识别神经元。我们建议证明从毛细胞到II型传入的突触输入是有功能的，并描述这些输入到II型神经元的强度。将突触输入引起的激活与本建议目标中确定的纤维激活阈值进行比较，可以估计可能是诱导II型传入超阈值激活所必需的OHC的刺激水平。II型树突的电生理参数将被合并到神经元的电缆模型中。这可以用来确定刺激将产生足够的树突场的激活以产生动作电位的可能性，然后动作电位可能被集中传输，以结合到我们对听觉刺激的感知中。嘌呤刺激II型传入，既有直接的，也有通过诱发毛细胞的突触输入的(初步数据)。虽然三磷酸腺苷有多种用途，但它也可以从受损的细胞中释放出来，并可能参与细胞损伤的信号传递，以响应响亮的声音。其他与细胞损伤有关的化合物将被测试其激活II型树突的能力，以进一步探索这一假说。此外，我们将开始描述神经元被敏化的过程。UTP是代谢性P2Y受体的激动剂，它的激活刺激了II型树突状细胞。因此，我们将使用药理学工具来操纵可能位于P2Y激活下游的第二信使通路，以确定它们在II型树突兴奋性中的作用。我们还将测试其他可能类似地启动第二信使级联的化合物，如P物质，以了解它们对II型传入树突的激活和敏化的作用。在为这一提议提供初步数据之前，对仅有的两类初级听觉神经元中的一种对刺激的反应几乎一无所知。因此，分析II型传入神经元的功能对于更广泛地理解声音知觉是至关重要的。 与公共健康相关：耳蜗包含两种类型的神经元，将感觉信息传递到中枢神经系统，第一类神经元携带声音定时、强度和频率的声学信息，第二类神经元知之甚少，人们对其知之甚少。这项研究计划旨在开发技术来测量II型神经元内的电活动，这可能有助于确定它们的功能角色。对这两类听觉神经元的反应的理解对于完全理解声音的感知是必要的。

DESCRIPTION (provided by applicant): The cochlea contains two types of primary afferent neurons, the Type I and Type II spiral ganglion neurons. 95% of the spiral ganglion population consists of the Type I neurons, which encode acoustic stimuli. The remaining 5% of the spiral ganglion population, the Type II neurons, ramify extensively among outer hair cells (OHCs). Little is known about their electrical properties as only a few recordings have been performed from the neurons. This lack of functional information precludes hypotheses as to the role of the Type II afferents in perception of auditory stimuli. We have developed a technique that allows gigaohm-seal intracellular recordings in Type II afferent dendrites near their synaptic inputs. Use of a neuronal tracer allows identification of the neurons based upon comparison to the known dendritic morphology well described in the literature. We propose to demonstrate that synaptic inputs to the Type II afferents from OHCs are functional, and to describe the strength of these inputs to Type II neurons. Comparison of the activation induced by synaptic inputs to the threshold for fiber activation as determined within the aims of this proposal will allow estimation of the level of stimulation of OHCs that may be necessary to induce supra-threshold activation of the Type II afferent. Electrophysiological parameters of the Type II dendrite will be incorporated into a cable model of the neuron. This can be used to determine the likelihood that stimuli will produce enough activation of the dendritic field to generate action potentials that may then be transmitted centrally to be incorporated into our perception of auditory stimuli. Purines stimulate the Type II afferents, both directly and by eliciting synaptic input from OHCs (preliminary data). While ATP serves many purposes, it can also be released from damaged cells and may be involved in signaling of cell damage in response to loud sounds. Other compounds implicated in cellular damage will be tested for their ability to activate Type II dendrites to further probe this hypothesis. Further, we will begin to describe processes by which the neurons are sensitized. Activation of UTP, an agonist at metabotropic P2Y receptors, stimulates Type II dendrites. Therefore, we will use pharmacological tools to manipulate second messenger pathways that may be downstream of P2Y activation in order to determine their role in Type II dendrite excitability. We will also test other compounds that may similarly initiate these second messenger cascades, such as Substance P, for their actions on activation and sensitization of Type II afferent dendrites. Prior to preliminary data presented for this proposal, nearly nothing was known regarding the activity in response to stimuli of one of the only two classes of primary auditory neurons. Therefore, analysis of the functions of Type II afferent neurons is crucial to a broader understanding of perception of sound. PUBLIC HEALTH RELEVANCE: The cochlea contains two types of neurons that transmit sensory information to the central nervous system, the Type I neurons which carry the acoustic information of sound timing, intensity and frequency, and the poorly understood Type II neurons about which little is known. This research proposal aims to develop techniques to measure electrical activity within the Type II neurons, which may enable a determination of their functional roles. An understanding of responses of both classes of auditory neurons is necessary for a complete understanding of perception of sound.