The research group led by Lecturer Tomomi Tsunematsu of the Faculty of Science at Hokkaido University in collaboration with Associate Professor Shuzo Sakata of the University of Strathclyde in the United Kingdom performed a study using mice to investigate the relationship between two waves, the pontine (P)-wave in the brainstem and a wave in the hippocampus that is essential for memory consolidation, and they found that the P-waves oppositely work in non-REM (NREM) and REM sleep. The same P-waves may play opposite roles in memory consolidation during NREM and REM sleep. The results are expected to help uncover the mechanisms of memory consolidation and erasure and were published in the international academic journal SLEEP.
There are two types of sleep: REM sleep, during which we are dreaming; and NREM sleep, which is important for resting the brain. Characteristic brain waves exist in both of the sleep stages. P-waves are generated in the pons of the brainstem, and theta waves (7-10 Hz) are generated in the hippocampus during REM sleep. Sharp ripple waves (80-250 Hz) are generated in the hippocampus during NREM sleep. All wave types are important for memory consolidation. P-waves were historically considered nonexistent in mice, but in 2020, a research group successfully recorded murine P-waves for the first time.
In this study, the researchers additionally examined the relationship between P-waves and hippocampal waves and found that the former cooperated with hippocampal waves during REM sleep but suppressed hippocampal waves during NREM sleep. The same was observed in rhesus monkeys.
The researchers inserted a multi-electrode probe into the mouse brain, which repeatedly sleeps and wakes, recorded P-waves and many neural activities, and then performed various analyses.
Using spike sorting, the researchers clustered neural activities and classified them into individual neurons. Next, they investigated the electrophysiological relationship between brain waves and neural activity. As a result, although P-waves have been long recognized as being characteristic of REM sleep, they also observed P-waves in NREM sleep, although at a low frequency. Conversely, P-waves tended to occur in clusters during REM sleep versus singly during NREM sleep.
Examination of the relationship between P-waves in REM sleep and hippocampal theta waves revealed that the former acted in cooperation with the latter and may play an important role in memory consolidation. The researchers further investigated the relationship between the P-waves during NREM and the sharp wave ripples in the hippocampus and found antagonistic effects.
Their findings suggested that NREM P-waves suppress sharp ripples that are important for memory. P-waves during NREM and REM sleep may play opposite roles in memory consolidation.
P-waves have long been proposed as a dream-generating mechanism, and further research on P-waves may enable the closer examination of the neural circuit of dreaming and the physiological significance of dreams. Dreaming during REM sleep is peculiar and narrative, while dreaming during NREM sleep is short and contemplative. The difference in the dream content between the two sleeping stages may be attributable to the difference in the physiological role of P-waves.
Title: Pontine Waves Accompanied by Short Hippocampal Sharp Wave-Ripples During Non-rapid Eye Movement Sleep
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