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REM sleep helps the brain forget memories: Japan, US researchers

In this Feb. 21, 2017 file photo, a child is seen sleeping in Matsudo, Chiba Prefecture. (Mainichi)

TOKYO -- A type of neuron that becomes active in the brain during rapid eye movement (REM) sleep, when vivid dreams tends to occur, plays a role in eliminating memories, an investigative team including researchers at Nagoya University found in a study of mice.

Though most of our dreams are made during REM sleep, people often forget them soon after waking up. The team of Japanese and U.S. researchers says the recent finding will likely be a breakthrough in determining why people tend to forget dreams.

It is believed that memories are stored or eliminated during sleep, but there has been little understanding of its mechanism. Akihiro Yamanaka, professor of neurophysiology at Nagoya University, and others focused on neurons that produce melanin-concentrating hormone (MCH) in the hypothalamus -- a small region found deep inside the brain that controls instincts such as sleep and hunger.

In a study of mice, researchers found that the activation of the end of axons connecting hypothalamic MCH neurons to the hippocampus, the brain's memory center, caused inhibition of hippocampal neurons.

Researchers then experimented with genetically modified mice that allowed them to turn on and off MCH neuron activities by using light and chemical substances. Several memory tests were conducted including ones that used toys. Activation of MCH neurons during REM sleep worsened the animals' memory whereas turning them off improved memory.

Further experiments showed that mice performed better on memory tests only when the neurons were turned off during REM sleep. In contrast, it did not have an effect when the animals were awake or in deep sleep.

The team concluded that MCH neurons eliminate memories by causing inhibition of hippocampal neurons. Researchers think the human brain works in the same way. They published the results of the study in the electronic version of the American scientific journal "Science," dated Sept. 19.

Yamanaka stated, "We tend to forget our dreams, which might be caused by MCH neurons that become active during REM sleep. Our finding may lead to the development of a treatment method for eliminating memories causing post-traumatic stress disorder."

(Japanese original by Momoko Suda, Science & Environment News Department)

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