Integrating and storing visuo-spatial cues in the retrosplenial cortex

记忆在大脑中编码和存储的位置和方式是神经科学的圣杯之一。大约100年前，理查德·西蒙（Richard Semon）假设，外部刺激会产生一种“永久的记录”。“写或刻在易激物质上”，即大脑，并为这种记忆痕迹创造了术语“痕迹”。但到目前为止，只有少数研究能够识别特定情况下的记忆痕迹，主要涉及海马体和杏仁核等大脑区域的恐惧记忆。在我们的初步研究中，我们第一次证明了在被称为压后皮质（RSC）的大脑区域中的神经元活动的特定模式与小鼠在所谓的径向臂迷宫中的空间记忆任务中的表现直接相关。RSC已经成为参与人类情景和地形记忆以及啮齿动物空间记忆的关键区域。dysgranular部分（Rdg）接收来自视觉区和海马复合体（海马旁区和下托）的密集输入，并且它包含空间响应细胞，使其成为整合导航信息的主要候选者。我们的初步研究并没有告诉我们这些RSC痕迹实际上代表什么：它们可以反映环境的代表，环境中奖励的代表，记忆的位置，或者动物在环境中的运动和导航的代表。这个问题与海马相对于视觉输入对RSC印迹形成的相对重要性有关。为了解决这个问题，我们将切断海马体或视觉皮层，并对迷宫和老鼠记住正确位置的奖励进行大量操作。然后，我们将比较动物的表现与RSC痕迹的稳定性。我们要解决的第二个问题是，对Rdg的视觉输入是否是空间记忆所必需的，以及这种作用是否是有时间限制的，以便其他输入，如触觉或自我运动线索，可以替代视觉输入。我们将通过在正常照明的环境中训练小鼠，然后依次去除视觉线索，最后在记忆测试阶段将它们转移到暗室中，或者通过在黑暗中训练和测试动物，从而最大化跨模态可塑性的潜力，并在去除触觉线索后重新测试它们，只留下自我运动（路径整合）线索。第三个问题是关于视觉和运动输入是如何整合到Rdg中的。我们将再次使用荧光活性指示剂来测试作为空间记忆印迹的组成部分激活的细胞（<所有Rdg细胞的10%）是否也对视觉刺激和/或在运动期间响应。我们将在线性跑步机上训练头部固定的小鼠进行虚拟走廊任务，并分析单个神经元与任务表现的相关性。通过去除视觉刺激或使视觉输入沉默或通过在视觉刺激和路径长度之间产生失配，我们将确定参与空间记忆记忆痕迹的Rdg神经元主要依赖于视觉输入还是路径整合。最后，我们将研究涉及海马、压后皮质和视觉皮质的空间记忆网络如何随着时间的推移相互作用，以及在多大程度上记忆印记的形成和/或检索依赖于它们。我们将观察海马或视觉皮层，并评估对记忆印记形成或表达的影响。此外，我们亦会同时记录压后皮质、海马及视皮质的电活动，并分析在空间记忆获得及提取的不同阶段，三个脑区之间的信息流向。

Where and how memory is encoded and stored in the brain is one of the 'holy grails' of neuroscience. Nearly 100 years ago Richard Semon hypothesised that external stimuli produce a "permanent record,... written or engraved on the irritable substance," i.e. the brain, and coined the term 'engram' for this memory trace. But only a handful of studies have so far been able to identify engrams for specific situations, mostly involving fear memory in brain areas such as the hippocampus and amygdala. In our pilot study we were the first to demonstrate that a specific pattern of neuronal activity in an area of the brain known as the retrosplenial cortex (RSC) is directly correlated with the performance of mice in a spatial memory task in a so-called radial arm maze. The RSC has emerged as a key area involved in episodic and topographical memory in humans as well as spatial memory in rodents. The dysgranular portion (Rdg) receives dense inputs from both visual areas and the hippocampal complex (parahippocampal region and subiculum), and it contains spatially-responsive cells, making it a prime candidate for integrating navigational information. Our pilot study did not tell us what those RSC engrams actually represent: they could reflect a representation of the environment, a representation of rewarded, remembered locations in the environment, or a representation of the animal's movement and navigation within the environment. This question is linked to the relative importance of hippocampal vs. visual inputs for RSC engram formation. In order to tackle it we will pharmacologically inactivate either the hippocampus or the visual cortex and carry out a number of manipulations on the maze and the reward the mice receive for remembering the correct locations. We will then compare the performance of the animals with stability of the RSC engrams.The second question we will tackle is whether visual input to the Rdg is necessary for spatial memory and if this role is time-limited such that other inputs, such as tactile or self-motion cues, can substitute for visual input. We will achieve this by either training mice in a normally lit environment and then successively remove visual cues, finally transferring them into a dark room during the memory testing phase, or by training and testing animals in darkness, thereby maximising the potential for cross-modal plasticity, and re-testing them after removal of tactile cues, leaving only self-motion (path integration) cues. The third question concerns how visual and locomotion inputs are integrated in the Rdg. We will again use fluorescent activity indicators to test whether cells activated as constituents of a spatial memory engram (<10% of all Rdg cells) also respond to visual stimuli and/or during locomotion. We will train head-fixed mice on a virtual corridor task on a linear treadmill and analyse the correlation of individual neurons with task performance. By removing visual stimuli or pharmacologically silencing visual inputs to Rdg or by creating a mismatch between visual stimuli and path length, we will establish whether Rdg neurons involved in spatial memory engrams depend primarily on visual input or path integration.Finally, we will investigate how the spatial memory network, involving the hippocampus, retrosplenial cortex and visual cortex, interacts over time, and to what extent engram formation and/or retrieval depends on them. We will pharmacologically inactivate either the hippocampus or the visual cortex and assess the effects on engram formation or expression. In addition, we will record electrical activity in retrosplenial cortex, hippocampus and visual cortex at the same time, and we will analyse the direction of information flow between the three brain areas at different stages of the acquisition and retrieval of spatial memory.