The roles of GABAergic and glycinergic inhibition in the adult MNTB

描述（由申请人提供）：梯形体内侧核（MNTB）中的神经元向多个其他听觉核投射甘氨酸抑制，这些听觉核又将投射发送到其他听觉中枢。因此，MNTB 直接或间接控制大量听觉核的发射，并参与耳间强度分析、耳间时间分析和许多单耳任务等多种计算。了解这些过程需要详细了解 MNTB 中执行的尖峰序列转换，以及有关功能环境的知识，在这些功能环境下，MNTB 神经元可能保持沉默，从而暂停对其他听觉区域的抑制控制。然而，我们对 MNTB 的理解中存在一个令人惊讶的巨大差距，阻碍了这种理解水平：MNTB 主要神经元本身接收大量的抑制性输入，这些输入在强度上与兴奋性输入相匹配，并且能够抑制 MNTB 的活动。然而，对于这些抑制性输入、它们的功能特性、它们被激活的条件以及它们如何与兴奋性输入相互作用以形成 MNTB 活性，我们几乎一无所知。我们建议使用体内和体外电生理学相结合的方法来研究这些抑制输入的作用。我们的首要假设是，MNTB 的抑制性输入塑造了神经元对复杂且持续的刺激的反应。具体来说，对 MNTB 的抑制可能作为一种机制来增强时间对比度，并增强对复杂声音活动的刺激开始和瞬态成分的反应。在目标 1 中，我们建议在体外测试以下假设：当出现某些声音刺激时，特定的抑制输入会抑制 MNTB 中的尖峰。我们将确定响应简单和复杂活动模式的抑制输入的突触特性，并测试复杂和持续刺激模式处理过程中兴奋和抑制之间的相互作用。在目标 2 中，我们将再次通过体外方法关注甘氨酸作为兴奋性传递调节剂的作用，该调节剂还可以进一步增强正在进行的活动的起始反应。我们将检验以下假设：甘氨酸在刺激序列开始时增强兴奋性传递，但在持续活动期间抑制兴奋性传递，从而增强 MNTB 神经元对复杂和持续活动的起始反应。在目标 3 中，我们将通过体内方法研究 GABA 能和甘氨酸能输入对 MNTB 的作用。我们将测试以下假设：MNTB 的抑制输入被调整到高于神经元相应最佳兴奋频率的最佳频率，并以比相应激励稍长的延迟到达 MNTB 神经元。因此，抑制性输入会增强体内对声音刺激的起始反应，并另外抑制 MNTB 神经元对声音刺激中的调频成分（例如向下）的反应。所有实验都将在成年沙鼠（p40 及以上）中进行。最近几个月，我们开发了成功记录成年沙鼠制备的 MNTB 脑切片的技术。我们还开发了对年龄匹配的动物进行体内记录的技术，并使用多管电极进行体内药理操作。此外，我们还有一种新型光敏谷氨酸激动剂可供使用，它可以通过闪光来控制脑干神经元放电。该方法将用于研究相当远的抑制源对 MNTB 的影响。拟议的研究重点是了解听觉脑干神经抑制最重要来源的细胞核，因此，其结果对于开发针对因抑制不平衡而导致的医疗状况（例如耳鸣、听源性癫痫发作或老年性耳聋）的治疗方法具有重要意义。 公共卫生相关性：梯形体内侧核 (MNTB) 是听觉脑干核，通过向这些核发送抑制性投射来控制许多其他听觉中枢的信息处理。然而，我们对 MNTB 功能的理解存在很大差距：它的神经元本身接受来自其他大脑区域的强大抑制控制，而对此几乎一无所知。我们建议研究 MNTB 的这些抑制输入并确定它们的功能作用。该研究项目的结果将有助于我们了解老年人的一些病理状况和听力障碍，例如听源性癫痫、耳鸣或言语感知能力下降。

DESCRIPTION (provided by applicant): Neurons in the medial nucleus of the trapezoid body (MNTB) project glycinergic inhibition to multiple other auditory nuclei, which in turn send projections to additional auditory centers. Thus, the MNTB controls, directly or indirectly, the firing of a large number of auditory nuclei, and participates in computations as diverse as interaural intensity analysis, interaural time analysis, and a number of monaural tasks. Understanding these processes should require a detailed understanding of spike train transformations performed in the MNTB, and knowledge about functional circumstances, under which MNTB neurons may be silent and thus suspend their inhibitory control over the other auditory areas. However, one surprisingly large gap in our understanding of MNTB inhibits this level of understanding: MNTB principal neurons themselves receive substantial inhibitory inputs, which match the excitatory inputs in strength, and which are capable of suppressing activity in the MNTB. However, virtually nothing is known about these inhibitory inputs, their functional properties, the conditions under which they are activated, and how they interact with the excitatory inputs to shape MNTB activity. We propose to use a combined in-vivo and in-vitro electrophysiology approach to study the role of these inhibitory inputs. Our over-arching hypothesis is that inhibitory inputs to the MNTB shape the neuron's responses to complex and ongoing stimuli. Specifically, inhibition to MNTB may act as a mechanism to enhance temporal contrast, and sharpen responses to stimulus onset and transient components of complex sound activity. In aim 1 we propose to test in-vitro the hypothesis that specific inhibitory inputs suppress spiking in MNTB when certain sound stimuli are presented. We will determine the synaptic properties of the inhibitory inputs in response to simple and complex activity patterns, and test the interaction between excitation and inhibition during the processing of complex and ongoing stimulus patterns. In aim 2 we will focus, again with an in-vitro approach, on the role of glycine as a modulator of excitatory transmission that additionally sharpens onset responses of ongoing activity. We will test the hypothesis that glycine enhances excitatory transmission at the onset of stimulus trains, but depresses excitatory transmission during ongoing activity and thus acts to sharpen the onset response of MNTB neurons to complex and ongoing activity. In aim 3 we will investigate the role of GABAergic and glycinergic inputs to MNTB by using an in-vivo approach. We will test the hypothesis that inhibitory inputs to MNTB are tuned to best frequencies above the neuron's corresponding best excitatory frequency, and arrive at MNTB neurons with a slightly longer latency than the corresponding excitation. Thus, inhibitory inputs sharpen onset responses to sound stimuli in- vivo, and additionally suppress responses in MNTB neurons to frequency modulated components in sound stimuli, such as downward. All experiments will be performed in adult gerbils (p40 and older). In recent months we developed the techniques to successfully record from MNTB brain slices prepared from adult gerbils. We also developed the techniques to perform in-vivo recordings from age-matched animals, and perform pharmacological manipulations in-vivo by using multibarrel electrodes. Furthermore, we have a novel light-sensitive glutamate agonists at our disposal that allows for flashes of light to control brainstem neuronal firing. This method will be used to study the impact of fairly distant sources of inhibition to the MNTB. The proposed studies focus on understanding a nucleus that is the most important source for neural inhibition to the auditory brain stem, and the results will thus be significant for developing treatments for medical conditions resulting from unbalanced inhibition, such as tinnitus, audiogenic seizures, or presbycusis. PUBLIC HEALTH RELEVANCE: The medial nucleus of the trapezoid body (MNTB) is an auditory brain stem nucleus that controls information processing in many other auditory centers by sending inhibitory projections to these nuclei. However, there is a large gap in our understanding of the functioning of the MNTB: Its neurons themselves receive powerful inhibitory control from other brain areas, about which almost nothing is known. We propose to study these inhibitory inputs to MNTB and determine their functional role. The results from the research project will help our understanding of a number of pathological conditions and hearing impairments such as audiogenic seizures, tinnitus, or a decline in speech perception among the elderly.