Synaptic and circuit mechanisms of hippocampal place-cell sequences

描述(申请人提供)：我们的目标是研究海马区单个单位反应水平的神经活动如何调节突触可塑性和海马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电路机制感兴趣，这些机制负责以精确的序列激活大鼠的海马区单位，这些序列描绘了过去和未来的行为轨迹。(1)开创性的工作表明，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，然而，位置场的经验依赖性变化往往是相当微妙和具有挑战性的。相比之下，只需很少的经验就可以检测到轨迹特定的位置细胞序列。我们将检验这种可能的学习效应对与突触可塑性相关的机制的依赖性，特别注意有趣的反假设，即序列可能先于经验存在。初步数据表明，经验过程中需要突触可塑性来编码重放记忆，而如果潜在记忆已经形成，则重放的产生本身不需要突触可塑性。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在重播产生中的核心作用是一个长期但未经检验的预测。以前使用遗传沉默CA3输入的研究发现，CA1中保留了涟漪模式，但这些研究可能已经允许由于长时间的抑制过程而产生补偿效应。我们将寻求一种用于瞬时抑制的光遗传方法。初步数据显示，涟漪和与涟漪相关的尖峰实际上取决于本地记录的CA1区的CA3输入。我们还将实时在线解码重放序列，以选择性地扰乱序列和测序子组件。我们将剖析CA3对重播的发起、指导、传播和终止的贡献。(3)在theta探索状态中，CA1单位由两个主要输入驱动：CA3和内嗅皮层(EC)，最近的报道表明，这两个输入可能以复杂的方式相互作用，包括在不同的theta周期内和跨不同的theta周期。这些假设的相互作用还没有直接测试，所以我们将利用我们的光遗传学方法来检测CA1单位在这种状态下的活动。初步数据显示，与涟漪相反，当CA3输入在theta期间被抑制时，CA1尖峰只有部分减少，从而揭示了EC的贡献。我们将研究位场、位相进动、theta序列，以及CA1低/高伽马与CA3/EC的同步，无论是否有CA3输入。综上所述，这些特定的目标代表了一种独特的方法，它利用超高密度单位记录的力量，结合药物和光遗传操作，提供对学习和记忆的神经基础的见解。我们的结果将对理解那些损害海马体学习和记忆的疾病产生重大影响，例如阿尔茨海默病、癫痫、中风和正常衰老。

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.