When we sleep, our brains go through four stages, we already know that. The first three are stages of non-rapid eye movement (non-REM).
The first stage includes the transition from wakefulness to sleep, when the body begins to slow down with respect to the daytime rhythm.
The second stage, also of non-REM sleep, involves light sleep.
The third stage of sleep is deeper and provides the kind of rest we need to feel rested in the morning.
Finally, the fourth stage arrives: the moment when our brain generates dreams, the REM sleep stage. But why do we forget our dreams most of the time? And when is the memory of the dream erased?
New research in mice suggests that REM sleep also contains a period of “active forgetting .” This is most likely to happen to avoid information overload, according to the new study led by Thomas Kilduff, director of the Center for Neuroscience at the SRI International research institute in Menlo Park, USA, in collaboration with Akihiro Yamanaka of the Nagoya University (Japan) and publishes the journal Science. L as neurons responsible for this neglect are also neurons that help control appetite.
Key neurons for sleep
Previous studies by the same researchers had focused on a hormone involved in regulating sleep in narcolepsy, a medical condition that can cause a person to fall asleep involuntarily at inopportune times during the day.
The hormone is called orexin / hypocretin , and loss of the neurons that produce it in the hippocampus may be what triggers narcolepsy, Kilduff and Yamanaka have shown.
For their new work, the scientists set out to examine a group of neighboring neurons in the hippocampus. They produce melanin concentrating hormone (MCH), a molecule that helps regulate both sleep and appetite .
Scientists already knew from previous research that these MCH-producing neurons would be active during REM sleep, but electrical recordings of sleep activity in mice and neuronal tracking experiments have revealed that these neurons also send inhibitory messages to the hippocampus.
Since the hippocampus is key to learning and memory, the scientists wondered if these neurons had “something to say” to preserve memories.
To find out, the scientists used thegenetic ablation in miceand they found that “turning off” these neurons improved the rodents’ memory. Specifically, the researchers used standard memory tests that examined the rodents’ ability to retain new information; that is, they tested the memory retention phase, which occurs just after learning new information, but before the information is stored in long-term memory.
During the memory retention phase, mice with MCH-producing neurons turned off performed better on screening tests.
Other memory tests showed that MCH-producing neuronsThey only affected memory in this way when they had been disturbed during REM sleep. That is, the mice performed better on memory tests when the researchers had turned off their MCH-producing neurons during REM sleep.Turning off these neurons at any other stage of sleep or during wakefulness did not appear to affect the rodents’ memory.
“Our results suggest that the activation of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night’s sleep,” says Kilduff. “These results suggest that MCH neurons help the brain to actively forget new, possibly unimportant information.”
“Since dreams are believed to occur primarily during REM sleep, the stage of sleep when MCH cells are activated, the activation of these cells can prevent the content of a dream from being stored in the hippocampus, thereforethe dream is quickly forgotten “, concludes the expert.
Referencia:.. REM sleep-active MCH neurons are involved in forgetting hippocampus-dependent memories Shuntaro Izawa Srikanta Chowdhury Toh Miyazaki Yasutaka Mukai Daisuke Ono Ryo Inoue Yu Ohmura Hiroyuki Mizoguchi Kazuhiro Kimura Mitsuhiro Yoshioka Akira Terao Thomas S. Kilduff Akihiro Yamanaka Science 20 Sep 2019 : Vol. 365, Issue 6459, pp. 1308-1313
DOI: 10.1126 / science.aax9238
