Synaptic and circuit mechanisms of hippocampal place-cell sequences

描述（由申请人提供）：我们的目标是研究海马体单单位反应水平的神经活动如何介导突触可塑性和海马体神经回路在学习和记忆中的已知作用。特别是，我们对突触可塑性和回路机制感兴趣，这些机制负责大鼠海马单位的激活，以精确的顺序描绘过去和未来的行为轨迹。(1)前人的研究表明，位置场反应可以反映记忆的多个方面，并表现出对突触可塑性机制的复杂依赖，但对位置场的经验依赖变化往往相当微妙，难以检测。相比之下，轨迹特异性位置细胞序列可以在很少的经验后检测到。我们将研究这种可能的学习效应对突触可塑性相关机制的依赖性，特别关注有趣的反假设，即序列可能先于经验存在。初步数据表明，为了编码重放记忆，在经历过程中需要突触可塑性，而如果基础记忆已经形成，重放本身的产生不需要突触可塑性。我们将继续进行这些实验，以了解突触可塑性的多种机制如何塑造位置细胞序列。(2) CA3在重放产生中的核心作用是一个长期存在但未经验证的预测。先前的研究使用CA3输入的基因沉默发现CA1中保留了波纹模式，但这些研究可能由于长时间的抑制过程而允许补偿效应。我们将寻求一种瞬时抑制的光遗传学方法。初步数据表明，波纹和波纹相关的尖峰实际上依赖于本地记录的CA1区域的CA3输入。我们还将实时在线解码回放序列，选择性地破坏序列和序列子组件。我们将剖析CA3对重播起始、方向、传播和终止的贡献。(3)在θ波探索状态中，CA1单元由两个主要输入驱动：CA3和内嗅皮层（EC），最近的报道表明，它们可能以复杂的方式相互作用，在不同的θ波周期内和之间。这些假设的相互作用尚未被直接测试，因此我们将利用光遗传学方法来检查这种状态下CA1单元的活性。初步数据表明，与波纹相反，当CA3输入在θ波期间被抑制时，CA1尖峰仅部分减少，从而揭示了EC的贡献。我们将研究位置场、相位进动、θ序列以及CA1低/高γ与CA3/EC的同步，无论是否有CA3输入。总的来说，这些特定的目标代表了一种独特的方法，利用超高密度单元记录的力量以及药理学和光遗传学操作，来深入了解学习和记忆的神经基础。我们的研究结果将对理解那些损害海马学习和记忆的疾病，如阿尔茨海默病、癫痫、中风和正常衰老，产生重大影响。

DESCRIPTION (provided by applicant): We aim to study how neural activity at the level of single unit responses in the hippocampus mediates the known roles of synaptic plasticity and hippocampal neural circuitry in learning and memory. In particular, we are interested in the synaptic plasticity and circuit mechanisms responsible for the activation of hippocampal units in the rat in precise sequences that depict past and future behavioral trajectories. (1) Pioneering work has established that place field responses can reflect multiple aspects of memory and exhibit a complex dependence upon mechanisms of synaptic plasticity, and yet experience-dependent changes to place fields tend to be rather subtle and challenging to detect. In contrast, trajectory-specific place-cell sequences can be detected after very little experience. We will examine the dependence of this possible learning effect on mechanisms associated with synaptic plasticity, with particular attention to the interesting counter-hypothesis that the sequences may exist prior to experience. Preliminary data show that synaptic plasticity is required during experience in order to encode replay memory, whereas the generation of replay per se does not require synaptic plasticity if the underlying memories have already been formed. We will pursue these experiments to understand how multiple mechanisms of synaptic plasticity shape place-cell sequences. (2) A central role for CA3 in the generation of replay is a longstanding but untested prediction. Previous studies using genetic silencing of CA3 input found preserved ripple patterns in CA1, but these studies may have allowed compensatory effects due to the long time course of suppression. We will pursue an optogenetic approach for instantaneous suppression. Preliminary data show that ripples and ripple-associated spiking are in fact dependent upon CA3 input to the locally recorded CA1 region. We will also online decode replay sequences in real-time, to selectively disrupt sequences and also sequence subcomponents. We will dissect the contribution of CA3 to replay initiation, direction, propagation and termination. (3) During the theta exploratory state, CA1 units are driven by two major inputs: CA3 and entorhinal cortex (EC), which recent reports show may interact in complex ways, both within and across different theta cycles. These hypothesized interactions have not been tested directly, and so we will utilize our optogenetic approach to examine CA1 unit activity during this state. Preliminary data show that in contrast to ripples, CA1 spiking is only partially reduced when CA3 input is suppressed during theta, unmasking the EC contribution. We will examine place fields, phase precession, theta sequences, and the synchronization of CA1 low/high gamma with CA3/EC, either with or without CA3 input. Taken together, these specific aims represent a unique approach that utilizes the power of ultra-high density unit recording together with pharmacological and optogenetic manipulation, to deliver insights into the neural basis of learning and memory. Our results will have a major impact on understanding those diseases that impair hippocampal learning and memory such as Alzheimer's disease, epilepsy, stroke and normal aging.