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Dreaming May Help the Brain Forget Excess Memories, a Study of Mice Shows

Researchers identify neurons linked with memory retention, performance in mice

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smithsonian.com

In a study of mice, scientists have pinpointed neurons that helping the brain forget.

As researchers led by Akihiro Yamanaka of Japan’s Nagoya University report in the journal Science, special cells called melanin-concentrating hormone, or M.C.H., neurons, release electrical signals during R.E.M. sleep—a sleep phase marked by rapid eye movement, heightened heart rate and intense dreams. This process, in turn, enables the brain to filter out unneeded information and create room for new memories.

“We’re inundated with so much information every day, and much of that information is turned into memories in the brain,” Ronald Davis, a neurobiologist at the Scripps Research Institute who was not involved in the new research, tells the New York Times’ Knvul Sheikh. “We simply cannot deal with all of it.”

According to Sheikh, Yamanaka and his colleagues realized M.C.H. neurons’ significance while studying sleep patterns in mice. Spurred by the realization that these cells interfere with the hippocampus, a brain region needed to consolidate memories, the team decided to conduct a series of tests.

Per a press release, the researchers found that the majority, or 52.8 percent, of mice’s M.C.H. neurons were active during R.E.M. sleep. Comparatively, just 35 percent fired when the mice were awake.

Thomas Kilduff, director of S.R.I. International’s Center for Neuroscience and a senior author of the study, says the results aligned with previous research on the special neurons. “We already knew that M.C.H. cells were active during R.E.M. sleep,” he explains in the statement. “After discovering this new circuit, we thought these cells might help the brain store memories.”

To test this theory, the scientists presented mice with two toys—a plastic banana and a wooden object—then either artificially activated or suppressed their M.C.H. neurons. Next, the team substituted a new toy for one of these items and stepped back to see how the animals would respond.

As the Times’ Sheikh reports, subjects with active M.C.H. cells approached both toys with the same frequency, suggesting they had no memory of their previous interaction with the older object. Those with inhibited neurons, meanwhile, were more interested in exploring the new toy. Crucially, these behavioral changes only occurred when the researchers manipulated cells during REM sleep rather than while the animals were awake or in a different sleep phase.

“These results suggest that hypothalamic M.C.H. neurons help the brain actively forget new information that is not important,” Yamanaka says to the Times. “... The neurons may be clearing up memory resources for the next day.”

Writing for the Conversation, Giuliana Mazzoni, a psychologist at England’s University of Hull who was not involved in the research, describes the findings as “a major breakthrough in understanding a fundamental memory mechanism.” Still, she notes, several questions remain unanswered: namely, do these results apply to humans as well as mice? How can scientists determine whether M.C.H. cells selectively prune out excess information or simply impair memory overall?

Kilduff of S.R.I. International tells Newsweek’s Kashmira Gander the study could have implications for the treatment of Alzheimer’s disease and similar memory disorders. As he concludes, previous research has shown that disrupted sleep may encourage the build-up of beta-amyloid, a protein linked to the condition. Consequently, “enhancing sleep may slow this process.”

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