The Purpose of Sleep? To Forget, Scientists Say

["Dave Farber" <dave () farber net>]

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From: Dewayne Hendricks <dewayne () warpspeed com>
Date: Fri, Feb 3, 2017 at 2:18 PM
Subject: [Dewayne-Net] The Purpose of Sleep? To Forget, Scientists Say
To: Multiple recipients of Dewayne-Net <dewayne-net () warpspeed com>


The Purpose of Sleep? To Forget, Scientists Say
By Carl Zimmer
Feb 2 2017
<
https://www.nytimes.com/2017/02/02/science/sleep-memory-brain-forgetting.html
>

Over the years, scientists have come up with a lot of ideas about why we
sleep.

Some have argued that it’s a way to save energy. Others have suggested that
slumber provides an opportunity to clear away the brain’s cellular waste.
Still others have proposed that sleep simply forces animals to lie still,
letting them hide from predators.

A pair of papers published on Thursday in the journal Science offer
evidence for another notion: We sleep to forget some of the things we learn
each day.

In order to learn, we have to grow connections, or synapses, between the
neurons in our brains. These connections enable neurons to send signals to
one another quickly and efficiently. We store new memories in these
networks.

In 2003, Giulio Tononi and Chiara Cirelli, biologists at the University of
Wisconsin-Madison, proposed that synapses grew so exuberantly during the
day that our brain circuits got “noisy.” When we sleep, the scientists
argued, our brains pare back the connections to lift the signal over the
noise.

In the years since, Dr. Tononi and Dr. Cirelli, along with other
researchers, have found a great deal of indirect evidence to support the
so-called synaptic homeostasis hypothesis.

It turns out, for example, that neurons can prune their synapses — at least
in a dish. In laboratory experiments on clumps of neurons, scientists can
give them a drug that spurs them to grow extra synapses. Afterward, the
neurons pare back some of the growth.

Other evidence comes from the electric waves released by the brain. During
deep sleep, the waves slow down. Dr. Tononi and Dr. Cirelli have argued
that shrinking synapses produce this change.

Four years ago, Dr. Tononi and Dr. Cirelli got a chance to test their
theory by looking at the synapses themselves. They acquired a kind of deli
slicer for brain tissue, which they used to shave ultrathin sheets from a
mouse’s brain.

Luisa de Vivo, an assistant scientist working in their lab, led a
painstaking survey of tissue taken from mice, some awake and others asleep.
She and her colleagues determined the size and shape of 6,920 synapses in
total.

The synapses in the brains of sleeping mice, they found, were 18 percent
smaller than in awake ones. “That there’s such a big change over all is
surprising,” Dr. Tononi said.

The second study was led by Graham H. Diering, a postdoctoral researcher at
Johns Hopkins University. Dr. Diering and his colleagues set out to explore
the synaptic homeostasis hypothesis by studying the proteins in mouse
brains. “I’m really coming at it from this nuts-and-bolts place,” Dr.
Diering said.

In one experiment, Dr. Diering and his colleagues created a tiny window
through which they could peer into mouse brains. Then he and his colleagues
added a chemical that lit up a surface protein on brain synapses.

Looking through the window, they found that the number of surface proteins
dropped during sleep. That decline is what you would expect if the synapses
were shrinking.

Dr. Diering and his colleagues then searched for the molecular trigger for
this change. They found that hundreds of proteins increase or decrease
inside of synapses during the night. But one protein in particular, called
Homer1A, stood out.

In earlier experiments on neurons in a dish, Homer1A proved to be important
for paring back synapses. Dr. Diering wondered if it was important in
sleep, too.

[snip]

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