CELLULAR & BIOCHEMICAL MECHANISMS UNDERLYING MOSSY FIBER LONG TERM POTENTIATION

大脑中的大多数兴奋性突触都会释放谷氨酸。这些突触表现出显著的可塑性，在这些突触中，短暂的重复激活会导致长时程增强或LTP。在大多数突触，LTP需要激活突触后的NMDA受体。然而，海马苔藓纤维突触表现出一种不依赖于NMDA受体激活的LTP形式。我们在过去十年中的实验强烈表明苔藓纤维 LTP(MfLTP)是一种完全的突触前可塑性形式。这笔赠款将探索与mfLTP基础机制有关的三个独立但重叠的问题。1)利用KAR基因敲除小鼠和一种新的KAR拮抗剂，探讨KAR在mfLTP中的作用。初步结果表明，KARS可以控制mfLTP的诱导阈值。2)我们最近发现，超极化激活的非选择性阳离子电流L-h介导了mfLTP的表达。在我们的模型中，我们认为L-h的cAMP依赖性增强导致苔藓纤维的去极化，从而促进递质释放，继而导致棘波加宽。我们将用一系列实验来检验这个模型，a)如果mfLTP与终末的去极化有关，那么K+对突触传递的增强应该被mfLTP所阻断，b)我们将从苔藓纤维中记录下来，以直接确定去极化是否导致棘波扩大，以及mfLTP是否与去极化有关，c)我们的模型预测，所有由齿状颗粒细胞形成的突触都应该表现出类似于苔藓纤维到锥体细胞突触的LTP。然而，其他人报告说，由苔藓纤维连接到灵芝中间神经元的突触不显示LTP。因此，我们将分析这个突触的属性，以了解它无法生成LTP的原因。3)我们有初步的结果表明，周围的细胞外腺苷 对苔藓纤维递质的释放产生深刻的紧张性抑制作用，确实是苔藓纤维表达短期和长期可塑性所必需的。我们将分析这种紧张性作用对苔藓纤维突触特性的作用，以及为什么腺苷对苔藓纤维有如此明显的选择性作用。这些实验将有助于阐明控制递质释放的机制，更具体地说，这种控制是如何参与短期和长期突触可塑性的。我们的研究结果应该能让我们深入了解学习和记忆背后的细胞机制。

Most excitatory synapses in the brain release glutamate. These synapses show a remarkable degree of plasticity, in which brief repetitive activation results in a long term potentiation or LTP. At most synapses LTP requires the activation of postsynaptic NMDA receptors. However, hippocampal mossy fiber synapses, exhibit a form of LTP that is independent of NMDA receptor activation. Our experiments during the past decade strongly suggest that mossy fiber LTP (mfLTP) is an entirely presynaptic form of plasticity. This grant will explore three separate, but overlapping, issues concerning the mechanisms underlying mfLTP. 1) The role of kainate receptors (KARs) in mfLTP will be explored, using KAR knockout mice and a novel KAR antagonist. Preliminary results suggest that KARs can control the threshold for the induction of mfLTP. 2) We have recently found that the hyperpolarization-activated nonselective cation current l-h mediates the expression of mfLTP. In our model we propose that the cAMP-dependent enhancement of l-h causes a depolarization of the mossy fibers which enhances transmitter release secondary to spike broadening. We will test this model with a number of experiments, a) If mfLTP is associated with a depolarization of the terminals, then the enhancement of synaptic transmission by K+ should occlude with mfLTP, b) We will record from mossy fiber boutons to directly determine if depolarization causes spike broadening and whether mfLTP is associated with a depolarization, c) Our model predicts that all synapses formed by dentate granule cells should show an LTP similar to that found at the mossy fiber to pyramidal cell synapse. However, others have reported that the synapses made by mossy fibers onto interneurons in s. lucidum do not show LTP. Thus we will analyze the properties of this synapse to understand why it fails to generate LTP. 3) We have preliminary results indicating that ambient extracellular adenosine exerts a profound tonic inhibition of mossy fiber transmitter release and, indeed, is necessary for mossy fibers to express both short-term and long-term plasticity. We will analyze the role that this tonic action has on the properties of mossy fiber synapses and why adenosine has such a pronounced and selective action on mossy fibers. These experiments will help elucidate the mechanisms involved in controlling transmitter release and more specifically how this control is involved in short term and long term synaptic plasticity. Our results should provide insight into the cellular mechanisms underlying learning and memory.