Oct. 26, 1999 Where does the mind go when we sleep? As dreamers, we have long suspected this mysteriously sealed condition leads a purposeful life of its own. Science, however, has only lately supported a specific role for brain activity during sleep: cementing the memories we acquire while awake. In the October issue of Learning & Memory, Sidarta Ribeiro, Constantine Pavlides, and colleagues (Rockefeller University) show that exposure to a "memorable" environment causes the brain to turn on a gene called zif-268 during subsequent sleep. Because activation of zif-268 can alter nerve cell behavior, this discovery offers an intriguing glimpse of how the sleeping brain could consolidate recently formed memories.
In rats, certain brain cells that activate during daytime exploration tend to reactivate during sleep. Scientists speculate the sleeping brain reenacts waking activity in order to lay down lasting memories, but the way it might do this is unknown. Ribeiro and colleagues focused on the contribution of zif-268, which turns on after heightened brain activity and is associated with strengthened communication between nerve cells.
The researchers exposed a group of rats to novel, enriched environments (labyrinths with toys) and another group of rats to their normal home cages. Afterwards, the rats went to sleep, passing through successive stages known as slow wave and rapid eye movement (REM) sleep. During slow wave sleep, zif-268 turned off in all rats, regardless of which environment they had experienced. During REM sleep, however, zif-268 turned on in rats that had explored the labyrinths and stayed off in rats that had not. This retrieval of zif-268 activity during REM sleep may couple with other reactivated brain mechanisms to "process" memories of novel experiences. Such processing may in turn prove important for memory consolidation. So sleep well: Nature's sweet restorer has a job to do.
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