Synaptic mechanisms underlying vestibular nerve fiber activity

描述(申请人提供)：内耳的前庭器官向大脑传递有关头部运动的信号，导致维持凝视和平衡以及对平衡和方向的感知的运动反射。因此，前庭系统功能障碍会极大地影响我们日常生活的能力。外周前庭功能障碍，如良性阵发性位置性眩晕(BPPV)和梅尼埃病，会导致眩晕和其他症状的致残性发作。为了分析这类疾病的病理生理学，了解头部运动信号在前庭周围器官中是如何处理的是至关重要的。在眉骨，半规管的感觉器官，感觉性毛细胞，对头部旋转做出反应，使其毛束偏转，激活毛细胞感受器电位。I型毛细胞几乎完全被传入前庭神经纤维的突触后末端包裹，这是前庭周围的一个独特特征，而II型毛细胞由具有更传统的突触末梢的纤维联系。这些毛细胞类型的神经支配模式相当复杂，但遵循特定的形态生理模式，并导致传入纤维的反应特性不同，例如，它们的静息放电规律，它们对外部刺激的反应特性 和传出输入。在这里，我们研究高度专门化的I型毛细胞/花冠突触的突触传递，目的是了解花冠传入纤维放电模式背后的机制。我们研制了一种从2-4周龄啮齿类动物切除的脊骨标本，用于同时记录I型毛细胞和花冠传入的电生理。使用共聚焦分析，我们还表征了花冠传入的形态特征，并使用抗体标记或荧光偶联标记的实时成像来评估特定突触蛋白的定位。在目标1中，我们描述了毛细胞膜电位与传入放电频率的关系。我们发现，谷氨酸在突触裂隙中的积累和溢出导致了缓慢的膜电位变化，并随后调制了传入放电频率。我们研究了释放特性和谷氨酸能突触传递对形成突触后反应模式的贡献。目的2研究是否存在从花冠到I型毛细胞的胆碱能反馈环，以调节传入信息的传递。在这里，我们提出了一个新的概念，包括一个花萼到毛细胞的反馈环，它可以解释体内记录的一些花瓣传入放电的反应模式。综上所述，我们研究了花萼传入放电特性的细胞机制。这些研究旨在更好地了解可能的前庭周围功能障碍，这是开发治疗此类功能障碍的先决条件。

DESCRIPTION (provided by applicant): The vestibular organs of the inner ear convey signals about head motions to the brain, resulting in motor reflexes that maintain gaze and balance as well as the perception of balance and orientation. Dysfunction of the vestibular system can therefore substantially affect the ability to lead our everyday lives. Peripheral vestibular dysfunctions, like benign paroxysmal positional vertigo (BPPV) and Meniere's disease, lead to disabling episodes of vertigo and other symptoms. To analyze the pathophysiology of such diseases, it is crucial to understand how head motion signals are processed in the vestibular peripheral organs. In the crista, the sensory organ of the semicircular canals, the sensory hair cells, respond to head rotations with a deflection of their hair bundles, activating hair cell receptor potentials. Type I hair cells are close to completely ensheathed by a postsynaptic calyx ending of the afferent vestibular nerve fiber, a unique feature of the vestibular periphery, and type II hair cells are contacted by fibers with the more conventional bouton endings. The innervation pattern of these hair cell types is quite complex, yet follows a specific morpho-physiological pattern, and results in afferent fibers with differences in their response properties for example in their regularity of resting discharge, their response properties to external stimuli and efferent inputs. Here we investigate synaptic transmission at the highly specialized type I hair cell/calyx synapse with the aim to understand the mechanisms that underlie firing patterns of the calyx afferent fibers. We have developed a preparation of excised cristae from 2-4 week old rodents to perform electrophysiological recordings from type I hair cells and calyx afferents, for some questions simultaneously. Using confocal analysis, we also characterize the morphological features of calyx afferents and assess the localization of specific synaptic proteins using antibody labeling or live imaging with fluorescently coupled markers. In Aim 1, we characterize the relation of hair cell membrane potential and afferent firing rate. We have found that glutamate accumulation and spillover in the synaptic cleft induces slow membrane potential changes and subsequent modulation of the afferent firing rate. We investigate the contribution of release properties and glutamatergic synaptic transmission to shaping the postsynaptic response pattern. Aim 2 investigates whether a cholinergic feedback loop from the calyx to the type I hair cell exists that may modulate afferent transmission. Here we put forward a new concept, including a calyx to hair cell feedback loop that may explain some of the in vivo recorded response patterns of calyx afferent firing. In Summary, we investigate the cellular mechanisms underlying calyx afferent firing properties. These studies are designed to gain a better understanding of possible vestibular peripheral dysfunctions, a prerequisite for developing treatments for such impairments.