Preservation of timing in plastic auditory pathways

描述(申请人提供)：在哺乳动物听觉脑干中计算声源定位的电路中，握持的神经末梢的大花瓣是一个关键元件。从这个突触输出动作电位的精确时间被认为是这项任务的核心。然而，在高频放电过程中调节和保存动作电位时序的机制还不是很清楚。第一个要检验的假设是，神经递质(谷氨酸)的释放效率在发育过程中因释放概率的变化而变化，而释放概率受突触前动作电位波形和钙缓冲的控制。我们发现，在HOLD的花瓣成熟过程中，动作电位波形变得更快、持续时间更短。这将对突触延迟和释放概率产生重要影响。我们将使用计算机模拟和电生理学来确定突触延迟和突触交流强度如何随年龄变化。我们还将检验这一假设，即在突触成熟过程中，容易释放的突触小泡池的大小增加，以补偿随之而来的释放概率的下降。第二种假设认为，扩散在成熟突触的突触间隙快速清除谷氨酸起主要作用，而未成熟突触中的代谢性谷氨酸受体(MGluRs)作为自身受体限制谷氨酸的释放量。过多的谷氨酸对大脑是有毒的，它还可能通过脱敏离子受体而扰乱突触传递，因此我们认为mGluRs可能在发育早期发挥神经保护作用，在胶质细胞仍未成熟的时候限制谷氨酸的释放。第三种假说认为，Na+/K+ATPase在Hold花瓣对高频放电动作电位的反应能力中起主要作用。我们将确定不同亚型的Na+/K+-ATPase的位置以及它们的功能如何影响突触前和突触后的生理。听觉系统消耗的葡萄糖是人脑中最高的。大脑中的大部分能量被Na+/K+-ATPase消耗，因为它恢复和维持质膜上Na+和K*离子的离子梯度。据推测，由于其异常高的刺激率，听觉脑干和皮质消耗了相当大量的ATP，主要是通过Na+/K+-ATPase的操作。然而，由于大多数CMS神经末梢较小，突触前Na+/K*-ATPase的性质尚不清楚。对于Na+/K+-ATPase在听觉通路中的位置、亚型和功能特性也知之甚少。通过用特定的药物阻断Na*/K+-ATPase，我们将模拟缺血和中风的影响，当神经元缺乏氧气和葡萄糖时。因此，这项研究提案将产生关于大脑能量消耗的关键组成部分的基本数据和见解。

DESCRIPTION (provided by applicant): The large calyx of Held nerve terminal is a pivotal element in the circuitry that computes sound source localization in the mammalian auditory brainstem. Precise timing of action potential output from this synapse is thought to be central for this task. However, the mechanisms that modulate and preserve action potential timing during high frequency firing are not well understood. The first hypothesis to be tested is that the efficiency of neurotransmitter (glutamate) release changes during development due to changes in release probability, which is controlled by the presynaptic action potential waveform and Ca buffering. We have found that action potential waveforms become faster and shorter in duration during the maturation of the calyx of Held. This will have important consequences for synaptic delays and release probability. We will use computer simulations and electrophysiology to determine how synaptic delays and the strength of synaptic communication varies with age. We will also test the hypothesis that the size of the readily releasable pool of synaptic vesicles increases during synapse maturation in order to compensate for a concomitant decrease in release probability. The second hypothesis is that diffusion plays the major role in fast glutamate clearance from the synaptic cleft of mature calyx synapses, whereas metabotropic glutamate receptors (mGluRs) in immature synapses act as autoreceptors that limit the amount of glutamate release. Too much glutamate can be toxic for the brain, and it may also disrupt synaptic transmission by desensitizing ionotropic receptors, so we propose that mGluRs may play a neuroprotective role early during development, limiting glutamate release at a time when glia are still immature. The third hypothesis is that the Na+/K+ ATPase plays a major role in the ability of the calyx of Held to fire action potentials at high frequencies. We will determine the location of different subtypes of the Na+/K+-ATPases and how their function affects presynaptic and postsynaptic physiology. The auditory system consumes the highest amount of glucose in the human brain. Most of that energy in the brain is consumed by the Na+/K+-ATPase as it restores and maintains the ionic gradients of Na+ and K* ions across the plasma membrane. Presumably due to its unusually high rate of spiking, the auditory brainstem and cortex consume remarkably large amounts of ATP, principally via the operation of the Na+/K+-ATPase. However, the properties of presynaptic Na+/K*-ATPases are unknown due to the small size of most CMS nerve terminals. Little is also known about the location, subtype and functional properties of Na+/K+-ATPases in auditory pathways. By blocking Na*/K+-ATPases with specific pharmacological agents we will mimic the effects of ischemia and strokes, when neurons are starved for oxygen and glucose. Thus, this research proposal will generate basic data and insights on a critical component of energy consumption in the brain.