Japanese researchers at the University of Tsukuba were able to understand how our brains work to learn and memorize during sleep. Memorization is made possible by the plasticity of the newly generated neurons in adult brains. Scientists from the International Institute for Integrative Sleep Medicine at Tsukuba University (Japan) published their results on June 4 in the journal Cell.
Neuronal Activity
Neurogenesis in the hippocampus
The key to memory during sleep is the ability of the brain cells to regenerate. “In an adult brain, cells are not replaced as they are in the liver, blood, and skin,” says Masanori Sakaguchi, the principal investigator of the study. Nonetheless, there is neurogenesis in our hippocampus, the area of the brain where memory is stored. Although this capacity for neuronal regeneration in adults is reduced, it’s still potentially beneficial.
While this process is already well known when we are awake, the researchers have now understood how it happens during sleep. To do this, they subjected mice to an anxiety conditioning test to create a new memory in them. Specifically, it involved the repetition of a moderate shock to their leg after hearing an auditory stimulus. Using a miniature microscope, the researchers recorded the activity of the new adult-born neurons.
The same process in humans as in mice
The results showed activation of these neurons immediately after the shock, but also during sleep and the next day when the researchers repeated the test. To test the memory of this experiment, the researchers inhibited the young adult-born neurons using optogenetics. This experiment showed that the mice only vaguely remembered the previous day’s experiment. It turned out that it was the new adult-born neurons that enabled learning during sleep because of their greater plasticity.
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If these experiments were to be confirmed in humans, the researchers are convinced that the same process is at work. “Since the neurogenesis of the hippocampus in the human brain has been studied quite intensively, I have no reason to doubt that the new adult-born neurons are also necessary for the consolidation of memory during REM sleep in humans,” says Masanori Sakaguchi. He believes that further studies of this process could facilitate the development of new treatments for memory disorders.
References
Kumar, D., Koyanagi, I., Carrier-Ruiz, A., Vergara, P., Srinivasan, S., Sugaya, Y., Kasuya, M., Yu, T. S., Vogt, K. E., Muratani, M., Ohnishi, T., Singh, S., Teixeira, C. M., Chérasse, Y., Naoi, T., Wang, S. H., Nondhalee, P., Osman, B. A. H., Kaneko, N., … Sakaguchi, M. (2020). Sparse activity of hippocampal adult-born neurons during REM sleep is necessary for memory consolidation. Neuron, 107(3), 552–565.e10. https://doi.org/10.1016/j.neuron.2020.05.008
