Miniature Microscopy of Hippocampal CA1 to Identify Engram Cells and Record Calcium Transients for Analyses of Ensemble Activities

In the brain, memories are thought to be stored by neurons termed engram cells that are activated during learning. Engram cells in the hippocampus can be targeted in c-Fos-tTA mice and have been identified as those that are activated during the acquisition and retrieval of a specific memory. Notably, the recent application of a genetically encoded calcium (Ca²⁺) indicator (GECI) combined with miniature microscopy (i.e., Ca²⁺ imaging) enables us to observe neuronal activity within deep brain regions of rodents. Here, we explain the technical details of systems for engram cell targeting and Ca²⁺ imaging with a miniature microscope specifically applied for hippocampal CA1. In addition, we introduce a method to combine engram identification and Ca²⁺ imaging, which provides the ability to analyze the ensemble activity of engram cells during memory processes.