Researchers have discovered that during rapid eye movement or REM sleep, the brain actively forgets. Their research suggests that this forgetting may be controlled by certain neurons which are found deep within the brain, in the hypothalamus, that were previously known to make a hormone that stimulates the appetite.
Experiments were conducted by researchers from the US and Japan and funded by the National Institute of Neurological Disorders and Stroke (NINDS), which is a part of the National Institutes of Health (NIH) which is headquartered in Bethesda, Maryland. the NIH is part of the US Department of Health and Human Services.
For over a hundred years scientists have studied the role sleep in the storing memories and found that indeed sleep does help the brain to store new memories. But other scientists believe that REM may also be a time when the brain actively gets rid of or forgets excess information. Recent studies have shown that synaptic connections that are made between neurons involved in certain types of learning are cut of by the brain during REM, but until now scientists did not know how this happens.
Why do scientists want to know the role of sleep in forgetting? Because it will help them to understand the wide range of memory-related illnesses like PTSD, post-traumatic stress disorder as well ad Alzheimer’s says, Janet He, Ph.D., who is the program program director at the NINDS.
So researchers tested mice and examined the role of a hormone called hypocretin/orexin which is active in controlling sleep and narcolepsy. Narcolepsy is a condition where a person sleeps excessively during the day experiencing REM sleep. In narcolepsy, scientists found that the hypocretin/orexin-making neurons are not present in the hypothalamus.
Researchers found that cells surrounding the hypothalamus produced melanin concentrating hormone (MCH) which has been known to control both sleep and memory. The majority of these cells (52.8%) fired when the mice went into REM sleep and scientists felt that it was during this time that the cells helped the brain store memories.
To test this, a number of genetic tools were used to turn the MCH neurons on and off while the mice underwent memory tests, specifically for retention. They found, to their surprise, that turning the MCH cells off improved memory and that memory worsened when they were turned on.
Researchers concluded that during REM sleep when the MCH cells are turned on the content of the dream is kept from being stored in the hippocampus and so the memory is lost.
Future studies are being planned to explore what part this activity has in sleep and memory disorders.