Calcium activity of hippocampal CA1 neurons during memory formation and retrieval in young and old mice
Authors: Rogozhnikova O. S. , Ivashkina О. I. , Toropova K. А. , Sotskov V. P. , Plusnin V. V. , Anokhin K. V. 
Published: May 2024

It is known that in the process of memory formation for new experiences, neurons in many regions of the brain are activated. In particular, neurons of the CA1 area of the hippocampus are activated during memory formation when animal first observe a new context [1]. However, it is not quite clear how the activity of CA1 neurons changes during memory formation and retrieval. Moreover, the question remains what changes in neuronal activity are observed in old animals during memory formation and retrieval.
In our work, we investigated changes in the calcium activity of hippocampal CA1 neurons during the formation and retrieval of associative memory of the context model of context preexposure facilitation effect in young and aged mice.
Calcium activity of individual neurons was recorded using a miniature microscope (miniscope), which allows optical detection of active neurons through the fluorescence of the calcium sensor. For this purpose, the mice underwent stereotactic surgery in which the calcium fluorescent sensor NCaMP7 was injected into the CA1 area of the hippocampus [2]. Then, a 0.5-mm-diameter GRIN lens was implanted into the studied area, and miniscope mounts were placed on the mouse head. The experiment was performed one week after the surgery. On the first day, the procedure of pretraining was performed: the mice were placed in a new context for 5 minutes free exploration, as a result of which the mice formed a spatial perception of the context. Three days later, the mice were briefly placed in the same context and immediately received footshock for 2s (1.5mA). Thus, the previously formed perception of the context was associated with the animal's state of fear. We tested associative memory three days after shock application: mice were placed in the same context for 5 minutes.
A measure of formed associative memory was their level of freezing behaviour in the context. Young mice showed a low level of freezing on the first visit of the context. We also observed a low level of freezing in older mice on the first day of the experiment. At the same time, the retrieval of a previously formed memory significantly increased the level of the freezing in older mice, compared with the first day, indicating that this animal had formed an associative memory of the context. To analyze changes in calcium activity during memory formation and retrieval, we assessed changes in the level of neuronal activity in each individual animal freezing act. Calcium activity of individual CA1 area neurons was recorded during the first visit to the environment and during memory retrieval. In each mouse, about 20 neurons were recorded in the two groups under study in two sessions of the experiment. However, we did not find any significant changes in the number of active neurons in young and old animals at the moments of their fading into the environment.
The results indicate that the processes of associative memory formation and retrieval do not manifest themselves in changes in the number of active cells at the moment of the animal freezing behaviour. Possibly, the studied processes of memory formation and retrieval are reflected in other forms of brain activity, such as cognitive maps of the context, which represent a network of cognitively specialized neurons (place fields).