Real-time visualization of mRNA synthesis during memory formation in live animals

Memories are thought to be encoded in populations of neurons called memory trace or engram cells. However, little is known about the dynamics of these cells because of the difficulty in real-time monitoring of them over long periods of timein vivo. To overcome this limitation, we present a genetically-encoded RNA indicator (GERI) mouse for intravital chronic imaging of endogenousArcmRNA—a popular marker for memory trace cells. We used our GERI to identifyArc-positive neurons in real time without the delay associated with reporter protein expression in conventional approaches. We found that theArc-positive neuronal populations rapidly turned over within two days in the hippocampal CA1 region, whereas ∼4% of neurons in the retrosplenial cortex (RSC) consistently expressedArcfollowing contextual fear conditioning and repeated memory retrievals. Dual imaging of GERI and a calcium indicator in CA1 of mice navigating a virtual reality environment revealed that only the population of neurons expressingArcduring both encoding and retrieval exhibited relatively high calcium activity in a context-specific manner. Thisin vivoRNA imaging approach opens the possibility of unraveling the dynamics of the neuronal population underlying various learning and memory processes.