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Researchers from Washington University School of Medicine in St. Louis found evidence that sleep, a promoter of long-term memories additionally helps to make room in the brain for new learning.

Neurologists have asked themselves how many synapses, or junctures where nerve cells communicate with each other are actually modified by sleep? They believe creation of new synapses is one key way the brain encodes memories and learning. However, the brain may not do all the work on its own and this may be where sleep comes in.

There are a few reasons why the brain can't indefinitely add synapses, including the finite spatial constraints of the skull. The researchers were able to track the creation of new synapses in fruit flies during learning experiences, and to show that sleep pushed that number back down. However, they don't yet know how the synapses are eliminated. According to theory, only the less important connections are trimmed back, while connections encoding important memories are maintained.

Many aspects of fly sleep are similar to human sleep. Both species when deprived of sleep will one day try to make up for the loss by sleeping more the next day. Because the human brain is much more complex, the researchers used the flies as models for answering questions about sleep and memory.

Sleep is a recognized promoter of learning, but three years ago research senior author Paul Shaw, Ph.D., assistant professor of neurobiology at Washington University School of Medicine in St. Louis turned that association around and revealed that learning increases the need for sleep in the fruit fly. In his 2006 research, he and his collegues found that two separate scenarios, each of which gave the fruit fly's brain a workout, increased the need for sleep.

The first scenario was inspired by human research linking an enriched environment to improved memory and other brain functions. Scientists found that flies raised in an enhanced social environment - a test tube full of other flies - slept approximately 2-3 hours longer than flies raised in isolation. Researchers also gave male fruit flies their first exposure to female fruit flies, but with a catch - the females were either already mated or were actually male flies altered to emit female pheromones. Either fly rebuffed the test fly's attempts to mate. The test flies were then kept in isolation for two days and exposed to receptive female flies. Test flies that remembered their prior failures didn't try to mate again; they also slept more. Researchers concluded that these flies had encoded memories of their prior experience, more directly proving the connection between sleep and new memories.

Scientists repeated these tests for the new study, but this time they used flies genetically altered to make it possible to track the development of new synapses, the junctures at which brain cells communicate.

Out of 200,000 fly brain cells, only 16 were required for the formation of new memories. These sixteen are lateral ventral neurons, which are part of the circadian circuitry that let the fly brain perform certain behaviors at particular times of day.
When flies slept, the number of new synapses formed during social enrichment decreased. When researchers deprived them of their sleep, the decline did not occur.

Jeffrey Donlea , the first study author identified three genes essential to the links between learning and increased need for sleep: rutabaga, period and blistered. Flies lacking any of those genes did not have increased need for sleep after social enrichment or the mating test.

Blistered is the fruit fly equivalent to a human gene known as serum response factor (SRF). Scientists have previously linked SRF to plasticity, a term for brain change that includes both learning and memory and the general ability of the brain to rewire itself to adapt to injury or changing needs. The new study shows that SRF could offer an important advantage for scientists hoping to study plasticity: unlike other genes connected to plasticity, it's not also associated with cell survival.

The researchers plan further investigations of the connections between memory and sleep, including the question of how increased synapses induce the need for sleep.

The newest data suggest that the best thing you can do to make sure you stay sharp is to get enough sleep instead of spending sleepless nights worrying about problems.

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How can you tell if a fruit fly is sleeping? Does it lose its grip on the surface that it is holding onto and then roll over? How do people actually examine individual brain cells of fruit flies -scanning EM, transmission EM? This sounds like an awfully technical study.
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