Calcium activity of hippocampal CA1 neurons during memory formation and retrieval in young and old mice
- Authors: Rogozhnikova O.S.1, Ivashkina О.I.1, Toropova K.А.1, Sotskov V.P.1, Plusnin V.V.1, Anokhin K.V.1,2
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Affiliations:
- Lomonosov Moscow State University
- Research Institute of Normal Physiology named after P.K. Anokhin
- Issue: Vol 18, No 4 (2023)
- Pages: 774-777
- Section: Conference proceedings
- URL: https://genescells.ru/2313-1829/article/view/623338
- DOI: https://doi.org/10.17816/gc623338
- ID: 623338
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Abstract
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).
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Full Text
It is widely understood that when forming memories of new experiences, neurons in multiple areas of the brain become active. Specifically, during memory formation when an animal first observes a new context, neurons within the CA1 region of the hippocampus are activated [1]. Nevertheless, it remains unclear how the activity of CA1 neurons evolves during memory formation and recall. In addition, we do not know how neuronal activity changes during memory formation and recall in aged animals.
In our study, we examined alterations in calcium activity among hippocampal CA1 neurons while forming and retrieving 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) that enables optical detection of active neurons via the fluorescence of the calcium sensor. For this purpose, the mice underwent stereotactic surgery. The calcium fluorescent sensor NCaMP7 was injected into the CA1 area of the hippocampus [2]. A 0.5-mm-diameter GRIN lens was implanted in the area of study, and miniscope mounts were affixed to the mouse’s head. One week following the surgical procedure, the experiment was conducted. Pretraining entailed a 5-minute free exploration in a novel environment on the initial day. This facilitated the development of a spatial perception of the context by the mice. Three days later, the mice were briefly placed in the same context and received a footshock of 2 s (1.5 mA) immediately. This associated the previously formed perception of the context with the animal’s state of fear. Three days after the shock application, we tested the mice’s associative memory by placing them in the same context for 5 min.
The level of freezing behavior in a given context served as a measure of associative memory formation. Young mice displayed minimal freezing on their initial visit to the context. Similarly, on the first day of the experiment, we observed low freezing levels in older mice. However, compared to the initial day, the retrieval of a previously formed memory notably increased the level of freezing in older mice, indicating the formation of an associative memory for the context. To investigate alterations in calcium activity during memory formation and retrieval, we evaluated the degree of neuronal activity during each animal’s freezing behavior. The calcium activity of individual neurons in the CA1 area was recorded during the initial exposure to the environment as well as during memory retrieval. Approximately 20 neurons were recorded in two sessions of the experiment for each mouse in both groups under investigation. However, no significant alterations were observed in the quantity of active neurons in young and old animals upon their immersion into the environment.
The findings suggest that changes in the number of active cells during animal freezing behavior are not indicative of associative memory formation and retrieval. It is possible that the processes of memory formation and retrieval studied in this experiment are reflected in other forms of brain activity, like the cognitive maps of the context, which are represented by a network of specialized neurons (place fields).
ADDITIONAL INFORMATION
Authors’ contribution. All authors made a substantial contribution to the conception of the work, acquisition, analysis, interpretation of data for the work, drafting and revising the work, final approval of the version to be published and agree to be accountable for all aspects of the work.
Funding sources. This work was supported by Russian Science Foundation grant No. 20-15-00283, by the Interdisciplinary Scientific and Educational School of Moscow University “Brain, Cognitive Systems, Artificial Intelligence”, and by the Non-commercial Foundation for Support of Science and Education “INTELLECT”.
Competing interests. The authors declare that they have no competing interests.
About the authors
O. S. Rogozhnikova
Lomonosov Moscow State University
Author for correspondence.
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow
О. I. Ivashkina
Lomonosov Moscow State University
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow
K. А. Toropova
Lomonosov Moscow State University
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow
V. P. Sotskov
Lomonosov Moscow State University
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow
V. V. Plusnin
Lomonosov Moscow State University
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow
K. V. Anokhin
Lomonosov Moscow State University; Research Institute of Normal Physiology named after P.K. Anokhin
Email: osrogozhnikova@gmail.com
Russian Federation, Moscow; Moscow
References
- Santarelli AJ, Khan AM, Poulos AM. Contextual fear retrieval-induced Fos expression across early development in the rat: an analysis using established nervous system nomenclature ontology. Neurobiol Learn Mem. 2018;155:42–49. doi: 10.1016/j.nlm.2018.0.015
- Subach OM, Sotskov VP, Plusnin VV, et al. Novel genetically encoded bright positive calcium indicator NCaMP7 based on the mNeonGreen fluorescent protein. Int J Mol Sci. 2020;21(5):1644. doi: 10.3390/ijms21051644
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