ИСТИНА

Hippocampal place cells are a well-known object of plenty of studies about learning and memory. Their firing patterns, or place fields, constitute cognitive maps that support animal's navigation in space. However, most of the place cell studies used rewarded models, where animals were pre-trained to run back and forth for food or water reward. Thereby a question is still unclear about the speed of formation of such cognitive maps in a novel context, i.e., which time is required for place cells to tune on a special piece of the environment. To address this question and to exclude the influence of explicitly reinforced behavior, we developed a reward-free model where animals can explore a novel environment (custom made circular O-shaped track with visual cues on surrounding curtains, see Fig. 1А). Then we recorded the neural activity from the hippocampal field CA1 of mice with a head-mounted Inscopix nVista HD miniature microscope (see Fig. 1B), while the animals were placed in the track, which was a completely novel environment for them. We aligned these data with trajectories of mice and determined place cells as neurons that have a preferential activation zone (putative place field) and that activated in most of the moments the animal visited the field (i.e., more than in 50% of such cases). As expected, place fields were distributed uniformly across the track, without significant gaps and clusters which might be caused by the proximity of visual cues (Fig. 1C-1D). Then, we estimated the time which is required for each individual cell to begin firing with respect to its place field. The majority of cells got tuned within the first three minutes of context exploration, the distribution of times of first in-field activation can be seen on Fig. 1E. However, different animals demonstrate different exploration dynamics, so we re-calculated this distribution in terms of full laps along the circular track. More than 1/3 of cells got tuned within the first lap, while more than 60% of cells do that within the first three laps. Taken together, these data suggest that a cognitive map is mainly formed while an animal makes it first laps in a novel environment, which is consistent with previous findings in rats. Also, the obtained data should serve as a base for searching for possible behavior or intrinsic triggers for cognitive map formation in a novel environment.