ИСТИНА

Neuronal ensembles or engrams are thought to encode memories. Although in vivo calcium imaging is an emerging technique to analyze activity of the same neurons during different behaviors, specific calcium responses of engram neurons across different memory states such as systems consolidation, recall or extinction remains unexplored. In this study we describe employment of novel Fos-Cre-GCaMP transgenic mouse strain for investigation of long-term changes in engram neurons activity. We used targeted recombination in active populations (TRAP) strategy to introduce calcium sensor into the engram neurons of conditioned fear in the mouse brain. The tamoxifen was injected one day before the training, that the neurons that expressed c-fos during fear conditioning also started to express the GCaMP3 calcium sensor. The total number of GCaMP3 positive neurons reached maximum on the fourth day after the tamoxifen injection and remain stable for at least two months. On the fifth day we started two-photon calcium imaging of TRAPed neurons in the mouse parietal cortex during presentation of the conditioned sound (CS). Three types of neuron were described: neurons that responded without CS, neurons that responded during CS and neurons that did not respond. Also, we reveal that usage of Fos-Cre-GCaMP mice allows to record calcium dynamic in dendritic spines of TRAPed neurons. Thus in vivo neuronal calcium imaging in the brain of conditioned Fos-Cre-GCaMP mice allows to investigate various forms of activity in the specific TRAPed neurons to address question about allocation, stability and dynamics of memory engram.