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。近100年前，理查德·西蒙（Richard Semon）假设，外部刺激会产生“永久记录……“写在或刻在易激物质上”，即大脑上，并为这种记忆痕迹创造了“印痕”这个词。但到目前为止，只有少数研究能够识别特定情况下的印痕，主要涉及海马和杏仁核等大脑区域的恐惧记忆。在我们的初步研究中，我们首先证明了大脑中一个被称为脾后皮层（RSC）的区域的特定神经元活动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已经成为一个涉及人类情景记忆和地形记忆以及啮齿动物空间记忆的关键区域。非颗粒部分（Rdg）接收来自视觉区和海马体复合体（海马体旁区和海马体下）的密集输入，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印记实际上代表什么：它们可以反映环境的表征，环境中奖励的表征，环境中记住的位置，或者动物在环境中的运动和导航的表征。这个问题与海马体与视觉输入对RSC印痕形成的相对重要性有关。为了解决这个问题，我们将从药理学上使海马体或视觉皮层失活，并对迷宫进行一系列操作，并对小鼠记住正确位置的奖励进行奖励。然后，我们将比较动物的性能与RSC印记的稳定性。我们要解决的第二个问题是，Rdg的视觉输入是否对空间记忆是必要的，如果这种作用是有时间限制的，那么其他输入，如触觉或自我运动提示，可以代替视觉输入。我们将通过在正常光照的环境中训练小鼠，然后依次移除视觉线索，最后在记忆测试阶段将它们转移到黑暗的房间，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，并在移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如何整合到Rdg中。我们将再次使用荧光活性指示器来测试作为空间记忆印迹成分激活的细胞（<10%的Rdg细胞）是否也对视觉刺激和/或运动做出反应。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。通过移除视觉刺激或在药理学上沉默Rdg的视觉输入，或通过在视觉刺激和路径长度之间创造不匹配，我们将确定参与空间记忆印记的Rdg神经元是否主要依赖于视觉输入或路径整合。最后，我们将研究包括海马体、脾后皮层和视觉皮层在内的空间记忆网络如何随时间相互作用，以及印痕形成和/或检索在多大程度上依赖于它们。我们将在药物上灭活海马或视觉皮层，并评估对印迹形成或表达的影响。此外，我们将同时记录脾后皮层、海马体和视觉皮层的电活动，并分析在空间记忆获取和检索的不同阶段，这三个脑区之间的信息流方向。

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.