| Health / Health News |
The brain may actively forget during dream sleep
Rapid eye movement, or REM, sleep is a fascinating period when most of our dreams are made. Now, in a study of mice, a team of Japanese and U.S. researchers show that it may also be a time when the brain actively forgets. Their results suggest that forgetting during sleep may be controlled by neurons found deep inside the brain that were previously known for making an appetite stimulating hormone.
A team of researchers studied how the activity of neurons found deep inside the mouse brain may control forgetting during REM sleep. Photo: Yamanaka lab, Nagoya University, Japan
“Ever wonder why we forget many of our dreams?” said Thomas Kilduff, Ph.D., director of the Center for Neuroscience at SRI International, Menlo Park, California, and a senior author of the study. “Our results suggest that the firing of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night’s sleep.”
REM is one of several sleep stages the body cycles through every night. It first occurs about 90 minutes after falling asleep and is characterized by darting eyes, raised heart rates, paralyzed limbs, awakened brain waves and dreaming.
For more than a century, scientists have explored the role of sleep in storing memories. While many have shown that sleep helps the brain store new memories, others, including Francis Crick, the co-discoverer of the DNA double helix, have raised the possibility that sleep – in particular REM sleep – may be a time when the brain actively eliminates or forgets excess information.
Moreover, recent studies in mice have shown that during sleep – including REM sleep – the brain selectively prunes synaptic connections made between neurons involved in certain types of learning. However, until this study, no one had shown how this might happen.
Dr. Kilduff’s lab and that of his collaborator, Akihiro Yamanaka, Ph.D., at Nagoya University in Japan, have spent years examining the role of a hormone called hypocretin/orexin in controlling sleep and narcolepsy.
Narcolepsy is a disorder that makes people feel excessively sleepy during the day and sometimes experience changes reminiscent of REM sleep, like loss of muscle tone in the limbs and hallucinations.
Their labs and others have helped to show how narcolepsy may be linked to the loss of hypocretin/orexin-making neurons in the hypothalamus, a peanut-sized area found deep inside the brain.
In this study, Dr. Kilduff worked with Dr. Yamanaka’s lab and Akira Terao’s, D.V.M., Ph.D., lab at Hokkaido University, Sapporo, Japan, to look at neighboring cells that produce melanin concentrating hormone (MCH), a molecule known to be involved in the control of both sleep and appetite.
In agreement with previous studies, the researchers found that a majority (52.8%) of hypothalamic MCH cells fired when mice underwent REM sleep whereas about 35% fired only when the mice were awake and about 12% fired at both times.
They also uncovered clues suggesting that these cells may play a role in learning and memory. Electrical recordings and tracing experiments showed that many of the hypothalamic MCH cells sent inhibitory messages, via long stringy axons, to the hippocampus, the brain’s memory center.
They found that “turning on” MCH cells during retention worsened memory whereas turning the cells off improved memories. For instance, activating the cells reduced the time mice spent sniffing around new objects compared to familiar ones, but turning the cells off had the opposite effect.
Further experiments suggested that MCH neurons exclusively played this role during REM sleep. Mice performed better on memory tests when MCH neurons were turned off during REM sleep. In contrast, turning off the neurons while the mice were awake or in other sleep states had no effect on memory. (National Institutes of Health)
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