Sleep makes room for memories

WASHINGTON — Sleep not only refreshes the body, it may also push the reset button on the brain, helping the brain stay flexible and ready to learn, new research shows.

Whether it is slow-wave sleep or rapid eye movement (REM), sleep changes the biochemistry of the brain, and the change is necessary to continue learning new things, suggests research presented November 18 at the annual meeting of the Society for Neuroscience.

Hundreds of genes behave differently when an animal is asleep rather than awake, says Chiara Cirelli of the University of Wisconsin–Madison. Cirelli and her colleagues are trying to settle a long-standing debate about why sleep is necessary. One theory is that sleep helps solidify memories by replaying information learned during the day. Another idea holds that sleep is for energy restoration.

Cirelli and other researchers presented evidence at the neuroscience meeting suggesting that sleep may perform both functions.

In a study in rats, Cirelli and her colleagues discovered that a molecule that works with the brain chemical glutamate becomes more and more abundant the longer rats are awake. The molecule, the glutamate receptor GluR1, helps forge connections, called synapses, between neurons. When rats are awake, the amount of GluR1 in the brain may climb up to 40 percent higher than levels found when the animal has been asleep for a few hours.

A new study in fruit flies showed that all areas of the brain have much higher levels of molecules found at synapses. Normally, strengthening a synapse is a good thing. It is one of the steps thought to be important in memory formation. But brains can’t continue to build up existing connections forever, Cirelli says.

“We cannot afford to keep growing our synapses one day after another, because very soon they would become unsustainable,” she says. “Stronger synapses come at a very high price.”

It takes a lot of energy, cellular supplies and other resources to maintain the connections. And if a neuron puts all of its energy into continually strengthening old synapses, it will never form new ones, making it impossible to learn new things.

Cirelli’s group found that sleep breaks down the molecules that form synapses. In particular, slow-wave sleep was important for reducing the amount of the synapse-forming molecules in the brain. The group also showed in a new study of people that disrupting slow-wave sleep by playing a quiet sound while people were sleeping impaired performance on one type of learning task.

Disrupting slow-wave sleep may also disrupt REM sleep, says Gina Poe, a sleep researcher at the University of Michigan in Ann Arbor, so more research is needed to show that slow-wave sleep is the critical stage needed to clear the mind. But it is clear that sleep is critical for clearing out old memories to make way for new information, she says.

“Sleep is not only for building things, it’s for tearing them down,” says Poe.

Memories associating names and faces, for instance, are forged first in the hippocampus, a seahorse-shaped part of the brain. But the hippocampus is used only for short-term memories. Memories are stored in other parts of the brain and the hippocampus must be cleared to make new memories.

Poe and her colleagues found that REM sleep turns off the brain chemicals norepinephrine and serotonin, both used for stabilizing synapses. Injecting serotonin into the brains of rats during sleep disrupted the rats’ ability to form certain kinds of memories, suggesting that the ability to remove old connections during sleep is important for making new memories.