Researchers in Japan have discovered that capillary blood flow in the brain is increased in mice during the dream-active REM phase of sleep, possibly preventing a buildup of waste products.
Scientists have long wondered why almost all animals sleep, despite the disadvantages to survival of being unconscious. Now, researchers led by a team from the University of Tsukuba have found new evidence of brain refreshing that takes place during a specific phase of sleep: rapid eye movement (REM) sleep, where dreaming occurs.
Previous studies have seen conflicting results when measuring differences in blood flow in the brain between REM sleep, non-REM sleep, and wakefulness using various methods. For this study, the investigators used a technique to directly visualise red blood cell movement in the brain capillaries of mice during awake and asleep states.
“We used a dye to make the brain blood vessels visible under fluorescent light, using a technique known as two-photon microscopy,” explained the senior study author, Professor Yu Hayashi. “In this way, we could directly observe the red blood cells in capillaries of the neocortex in non-anaesthetised mice.”
The researchers also measured electrical activity in the brain to identify REM sleep, non-REM sleep, and wakefulness, and looked for differences in blood flow between these phases.
“We were surprised by the results,” said Professor Hayashi. “There was a massive flow of red blood cells through the brain capillaries during REM sleep, but no difference between non-REM sleep and the awake state, showing that REM sleep is a unique state”
The research team then disrupted the mice’s sleep, resulting in ‘rebound’ REM sleep, which is a stronger form of REM sleep to compensate for the earlier disruption. During rebound REM sleep, blood flow was increased even further, suggesting an association between blood flow and REM sleep strength. However, when the researchers repeated the same experiments in mice without adenosine A2a receptors (blocking these receptors makes you feel more awake after a coffee), there was less of an increase in blood flow during REM sleep, even during rebound REM sleep.
“These results suggest that adenosine A2a receptors may be responsible for at least some of the changes in blood flow in the brain during REM sleep,” said Professor Hayashi.
Given that reduced blood flow in the brain and decreased REM sleep are correlated with the development of Alzheimer’s disease, in which waste products are seen to build up in the brain, this increased blood flow in the brain capillaries during REM sleep could be important for waste removal from the brain. This study highlights the role of adenosine A2a receptors in this process, perhaps leading to the development of new treatments for Alzheimer’s disease and other conditions.
Source: University of Tsukuba
