WASHINGTON, Sept. 21 (Xinhua) -- A team of Japanese and U.S. researchers found that forgetting during sleep may be controlled by neurons known for making an appetite stimulating hormone, which explained why one cannot remember things happening in dreams.
The study published in this week's Science magazine showed that about 53 percent of hypothalamic cells that produce melanin concentrating hormone (MCH), a molecule known to be involved in the control of appetite, fired when mice underwent rapid eye movement (REM) 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.
About 35 percent of those cells fired when the mice were awake and about 12 percent fired at both times, according to 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," said the paper's senior author Thomas Kilduff, director of the Center for Neuroscience at SRI International.
The researchers also uncovered clues suggesting that those 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, through long stringy axons, to the hippocampus, the brain's memory center, according to the study.
"From previous studies done in other labs, we already knew that MCH cells were active during REM sleep. After discovering this new circuit, we thought these cells might help the brain store memories," said Kilduff.
The researchers used a variety of genetic tools to turn on and off MCH neurons in mice during memory tests, examining the role that MCH cells played in retention, the period after learning something new but before the new knowledge is stored, or consolidated, into long term memory.
They found that "turning on" MCH cells during retention worsened memory whereas turning the cells off improved memories. Mice performed better on memory tests when MCH neurons were turned off during REM sleep, according to the study.
"Since dreams are thought to primarily occur during REM sleep, the sleep stage when the MCH cells turn on, activation of these cells may prevent the content of a dream from being stored in the hippocampus - consequently, the dream is quickly forgotten," said Kilduff.