The roles of GABAergic and glycinergic inhibition in the adult MNTB

描述(申请人提供)：斜方体内侧核(MNTB)的神经元将甘氨酸能抑制投射到多个其他听觉核团，这些核团又将投射发送到其他听觉中心。因此，MNTB直接或间接地控制大量听觉核团的放电，并参与各种计算，如耳间强度分析、耳间时间分析和许多单耳任务。要理解这些过程，需要详细了解MNTB中执行的棘波序列转换，并了解MNTB神经元可能保持沉默从而暂停对其他听觉区域的抑制性控制的功能环境。然而，在我们对MNTB的理解中有一个令人惊讶的巨大差距抑制了这种水平的理解：MNTB主神经元本身接受大量的抑制性输入，其强度与兴奋性输入相匹配，并且能够抑制MNTB中的活动。然而，几乎对这些抑制性输入、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塑造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性输入的作用。我们的总体假设是，MNTB的抑制性输入塑造了神经元对复杂和持续刺激的反应。具体地说，对MNTB的抑制可能作为一种机制来增强时间对比度，并增强对刺激开始和复杂声音活动的瞬时成分的反应。在目标1中，我们建议在体外测试这一假设，即当特定的声音刺激出现时，特定的抑制性输入抑制MNTB中的峰电位。我们将确定抑制输入对简单和复杂活动模式的反应的突触属性，并测试复杂和正在进行的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实验的方法，将重点放在甘氨酸作为兴奋性传递的调节器的作用上，该调节器额外地增强正在进行的活动的起始反应。我们将测试这一假设，即甘氨酸在刺激序列开始时增强兴奋性传递，但在正在进行的活动中抑制兴奋性传递，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，我们将使用体内方法研究GABA能和甘氨酸能输入到MNTB的作用。我们将测试这样的假设，即MNTB的抑制输入被调整到高于神经元相应的最佳兴奋频率的最佳频率，并到达MNTB神经元，其潜伏期略长于相应的兴奋。因此，抑制性输入在体内增强了对声音刺激的起始反应，并另外抑制了MNTB神经元对声音刺激中的频率调制成分的反应，如向下。所有实验都将在成年沙土鼠(40岁及以上)身上进行。近几个月来，我们开发了成功记录成年沙土鼠MNTB脑片的技术。我们还开发了从年龄匹配的动物身上进行体内记录的技术，并使用多管电极在体内进行药理操作。此外，我们有一种新型的光敏性谷氨酸激动剂可供我们使用，它允许闪光来控制脑干神经元的放电。这种方法将被用来研究相当遥远的抑制源对MNTB的影响。拟议的研究集中在了解一个核，它是听觉脑干神经抑制的最重要来源，因此，结果将对开发由不平衡抑制引起的医疗条件的治疗具有重要意义，如耳鸣、听源性癫痫或老年性耳聋。 公共卫生相关性：斜方体内的内侧核(MNTB)是一个听觉脑干核团，通过向这些核团发送抑制性投射来控制许多其他听觉中心的信息处理。然而，我们对MNTB功能的理解存在很大差距：它的神经元本身受到来自大脑其他区域的强大抑制控制，而这些区域几乎一无所知。我们建议研究MNTB的这些抑制输入，并确定它们的功能作用。研究项目的结果将有助于我们了解一些病理情况和听力障碍，如听源性癫痫、耳鸣或老年人言语感知能力下降。

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.