It is one of life's greatest unsolved puzzles: why do we sleep? Now, researchers have designed a tool to help answer that question – one that allows scientists to induce sleep in fruit flies, and thus study the positive effects of snoozing, rather than simply depriving a subject of sleep to study the negative effects, as most sleep studies do.
Using the method, scientists at Washington University in St. Louis found that sleep facilitates the formation of long-term memories in flies. The results, reported this week in Science, unite two seemingly disparate theories in sleep research experimentally for the first time.
"This is the future," said Jan Born, a leading sleep researcher at the University of Lubeck in Germany who was not involved in the research. "We've needed a method, an approach, or a tool to experimentally induce sleep."
"Now we can ask questions we couldn’t ask before," said neurobiologist Paul Shaw, lead investigator of the study at Washington University. "I think a lot of people are going to use this tool in really amazing ways."
The most common approach to studying sleep -- depriving a subject of its Zs, then trying to infer sleep’s function, "is not a logical approach," said Born. Lack of sleep causes a stress response, which can mask or blur results, he explained. "The ability to induce sleep or enhance sleep… [is] much more meaningful for the field and more significant at producing significant results."
To induce sleep in a fly, Shaw and his team worked with a strain of Drosophila that express a bacterial sodium channel in a part of the brain called the dorsal fan-shaped body. When open, the channel makes neurons in the region more excitable, which sends a signal to the rest of the brain to go to sleep. To make the flies sleep on cue, the team engineered the channel to be temperature-sensitive, so it only opened at 31ºC -- 6º higher than normal fly room temperature. Thus, the team could put the flies to sleep by simply bumping up the heat. They could also wake the flies on demand, using an apparatus to jostle the insects and force them to walk.
Flies on the right express a transgene that causes them to fall asleep when the temperature is raised. Control flies shown on the left. Courtesy of Blake Sakran, Kevin Li, and Paul Shaw. From thescientistllc on Vimeo.
To determine what effects sleep had on the flies, the team designed experiments to test two reigning theories: synaptic homeostasis, the idea that neuronal connections increase during wakefulness but are downscaled during sleep, pruning synapses created during the day so only the strongest connections remain intact; and memory consolidation, which proposes the purpose of sleep is to replay and consolidate memories from the previous day, reactivating certain connections to make them more powerful.
To test the synaptic homeostasis model, flies were placed in social enrichment environments, with lots of other flies. Afterward, the flies were either induced to sleep or not, then taught a courtship ritual. Flies that slept shortly after social enrichment developed long-term memories of the ritual; flies that did not sleep did not retain the memory. The results support the model, showing that sleep is necessary to renormalize synapses after being awake and stimulated, pruning them down to making space for new learning. A second paper published in Science this week also provides additional evidence for the synaptic homeostasis model.
Shaw's team then tested the memory consolidation theory by training flies with a protocol designed to give them short-term memories. Surprisingly, when sleep was induced immediately after the training, the flies retained a memory lasting days instead of just hours -- supporting the memory consolidation model that sleep strengthens synaptic connections and memories.
"This is the first experimental demonstration I know of that both theories are supported by the data," said Born. "Both processes can take place simultaneously…one relieving the brain from the pressure of too much connectivity and the other maintaining and enhancing long-term memories."
J. Donlea, et al., “Inducing sleep by remote control facilitates memory consolidation in Drosophila,” Science, 332:1571-6, 2011.