Scientists have long wondered why, despite the dangers of being unconscious, almost all animals sleep. Now, a team of researchers led by the University of Tsukuba has discovered new evidence of brain refreshing that occurs during a specific stage of sleep called rapid eye movement (REM) sleep, which is when you dream a lot.
Previous research has used various methods to measure differences in blood flow in the brain between REM sleep, non-REM sleep, and wakefulness, with mixed results. The Tsukuba-led team used a technique to directly visualize red blood cell movement in mouse brain capillaries (where nutrients and waste products are exchanged between brain cells and blood) during awake and asleep states.
Professor Yu Hayashi, the study’s senior author, says, “We used a dye to make the brain blood vessels visible under fluorescent light, using a technique known as two-photon microscopy.” “We were able to directly observe red blood cells in neocortex capillaries in non-anesthetized mice in this way.”
The researchers also examined for changes in blood flow between REM sleep, non-REM sleep, and awake by measuring electrical activity in the brain.
Professor Hayashi explains, “We were surprised by the results.” “During REM sleep, there was a massive flow of red blood cells through the brain capillaries, but no difference between non-REM sleep and the awake state, demonstrating that REM sleep is a distinct state.”
The mice’s sleep was then disrupted, resulting in “rebound” REM sleep, which is a stronger form of REM sleep that compensates for the earlier disruption. During rebound REM sleep, blood flow in the brain increased even more, implying a link between blood flow and REM sleep strength. When the same experiments were repeated in mice lacking adenosine A2a receptors (the receptors whose blockade makes you feel more awake after drinking coffee), blood flow increased less during REM sleep, even during rebound REM sleep.
Professor Hayashi says, “These findings suggest that adenosine A2a receptors may be responsible for at least some of the changes in blood flow in the brain during REM sleep.”
Given that decreased blood flow in the brain and decreased REM sleep are linked to the development of Alzheimer’s disease, which is characterized by the accumulation of waste products in the brain, it may be worthwhile to investigate whether increased blood flow in the brain capillaries during REM sleep is important for waste removal. This research paves the way for further research into the role of adenosine A2a receptors in this process, which could lead to the development of new treatments for conditions like Alzheimer’s disease.