ИСТИНА

The ability of hippocampal CA1 cells to form and maintain stable spatial codes of the environment across days is well described. However, much less is known about the short-term properties of such place codes at the first episodes of exploratory behavior in a novel environment. To address this, we have imaged CA1 place cells with head-mounted Nvista HD miniscopes in mice exploring a novel environment in three consequent 15-minute sessions 24 h apart. The environment had a form of circular O-shaped track with proximal and distal cues, where mice were able to arbitrarily choose and change the direction of moving (clockwise or counterclockwise) without any reinforcement. To estimate within-session stability of place fields, we constructed a conservative criterion based on selectivity score which was calculated for each attendance of a field as a rate of place-specific calcium events. Place fields were considered stable at the moment when their selectivity score reached the threshold value and held it for at least 3 times. We observed that a significant (25%) part of place fields became stable at the first moment the animal attended them, while the average latency of place field stabilization equaled 247s at the first session. This latency decreased in the next session in the same environment, however, the average selectivity score itself demonstrated no cumulation between sessions, starting with similar levels at each session. Importantly, these results were shown not depending on the retention of place codes on the second session. To verify these results we performed a population analysis: dimensionality reduction based on Laplacian eigenmaps revealed latent variables which corresponded to trajectories of mice, thus allowing us to decode the position of animals. Moreover, within-session dynamics of the precision of such decoding was found consistent with the average place field selectivity score dynamics in these animals. Our data thus reveal the fast emergence of place codes in a novel environment and may serve as a basis for further studies. This study was supported by the Russian Ministry of Science and Higher Education Project № 075-15-2020-801. The analysis of miniscope calcium activity recordings from the mouse hippocampal neurons during new memory acquisition was supported by the Russian Science Foundation Project № 20-15-00283.