Stability of synaptic memory engrams

记忆被认为被编码为大脑中持久的物理变化。在小鼠模型中的几项研究已经发现，在记忆形成过程中，不同脑区的神经元亚群显示出神经元活动增加，并表明操纵这些细胞可以诱导人工恢复或丢失存储的记忆。这表明记忆的存储和提取是由特定的神经元群体介导的，因此被认为是细胞印记。然而，印记可以在不同的空间尺度上识别，从系统到突触水平。根据Hebbian假设，具有高度相关活动模式的神经元之间的连接被加强，而活动模式弱相关的神经元之间的连接被抑制甚至丢失。因此，共同活动神经元之间突触强度的增加增加了在编码过程中发生的相同神经活动的时空模式在稍后的提取过程中再次发生的可能性。这一假说表明，共同激活的神经元之间的突触加强形成了记忆的神经底物，或突触印记。最近，由于跨突触伙伴的绿色荧光蛋白重建(MGRASP)技术，标记突触神经元之间的突触并研究其性质成为可能。这种技术允许用GFP表达标记表达突触前和突触后mGRASP成分的神经元之间的突触。将该系统与已建立的基于即刻早期基因标记印记神经元的方法相结合，最终提供了在突触水平上可视化印记的具体可能性。在这个项目中，我建议使用纵向双光子深部脑光学成像来研究CA1区结构突触印记的稳定性。首先，我们将标记结构CA1突触印记，使用mGRASP突出CA3和CA1神经元之间的突触，并使用立即早期基因Arc的启动子将mGRASP前后的结构靶向CA3和CA1印记神经元。其次，我们将使用光学成像来跟踪CA1突触恐惧印记在基线、消退和强化下的长期稳定性。这将使我们能够第一次研究持续学习对结构突触纹章的影响。

Memories are thought to be encoded as enduring physi¬cal changes in the brain. Several studies in murine models have identified subpopulations of neurons throughout various brain regions showing increased neuronal activity during memory formation and shown that manipulation of these cells can induce either artificial retrieval or loss of stored memories. This demonstrates that memory storage and retrieval are mediated by specific populations of neurons which are thus believed to be cellular engrams. Engrams, however, can be identified at different spatial scales, ranging from the systems to the synaptic levels. According to the Hebbian postulate connections between neurons with highly correlated activity patterns are strengthened while connections between neurons whose activity patterns are weakly correlated are depressed or even lost. Increase in synaptic strength between co-active neurons thus increases the likelihood that the same spatial-temporal pattern of neural activity that occurred dur¬ing encoding will occur again at a later time, during retrieval. This hypothesis suggests that synaptic strengthening between coactivated neurons forms the neural substrate of memory, or a synaptic engram. It has recently become possible to label synapses between engram neurons and study their properties, thanks to the Green fluorescent protein Reconstitution Across Synaptic Partners (mGRASP) technique. This technique allows to label with GFP expression synapses between neurons expressing the presynaptic and the postsynaptic mGRASP components. Combining this system with established methods to label engram neurons based on Immediate-Early genes, finally offers the concrete possibility to visualize the engram at the synaptic level. In this project I propose to investigate the stability of structural synaptic engrams in the CA1 by using longitudinal two-photon deep-brain optical imaging in mice. First, We will label structural CA1 synaptic engrams by using mGRASP to highlight synapses between CA3 and CA1 neurons and by using the promoter of the immediate early gene Arc to target the pre- and post-mGRASP constructs to CA3 and CA1 engram neurons. Second, we will employ optical imaging to track the long-term stability of CA1 synaptic fear engrams under baseline, extinction and reinforcement. This will enable us to investigate for the first time the effects of continued learning on structural synaptic engrams.