24 Jun 2011

With the invention of a genetic snooze button for fruit flies, scientists will be able to precisely investigate the link between sleep and memory in aging flies as well as flies modeling disease. In today’s Science, researchers report that activating a temperature-sensitive channel in neurons connected to a fly sleep center caused the animals to drop off to sleep. Senior author Paul Shaw of Washington University, St. Louis, Missouri, led the study with first author Jeffrey Donlea, who has since moved to Oxford University, U.K. Putting the new tool to the test, the authors found that a solid nap allowed the flies to remember a courtship experience they normally would forget.

Waking animals up is easy, but putting them to bed is hard, noted John Cirrito, who is also at Washington University but did not participate in the study. Normally, scientists study sleep with sleep-deprived animals; such research has made it clear that lack of sleep interferes with memory. By taking a complementary approach, Cirrito said, the current work shows for the first time that sleep can consolidate memory.

As people and flies age, they sleep less; insomnia is also associated with Alzheimer’s and Parkinson’s diseases. With those sleepless nights comes loss of memory (though insomnia is not thought to cause Alzheimer’s). Unfortunately, Shaw says, regular use of sleeping pills can have side effects. For example, people can become dependent on the medication. Shaw hopes that the new fly model will help prove that sleep can be a good memory tonic as well as reveal chemical pathways that drug makers could tap to induce more natural sleep.

Shaw planned to study aging, not sleep per se. But in the process of activating different nerve groups, he and Donlea discovered that a dorsal, fan-shaped body in the fly brain acted as a sleep center. Although they have not defined the precise circuitry, the researchers found that when they inserted a bacterial sodium channel transgene into the neurons leading to the fan-shaped body, the flies spent more time snoozing. Aside from the extra shuteye they got, the transgenic flies slept normally. They napped in normal one-hour increments; a tap, light, or caffeine woke them; and they were active when awake. In fact, they were more active than other flies: “They have to rush to get things done before they go back to sleep,” Shaw suggested.

The researchers then set out to make flies sleep on demand. Donlea tried using optogenetics to selectively activate neurons leading to the fan-shaped body, but the blue light that activates the neurons also woke up the flies. Instead, the researchers selectively expressed a temperature-sensitive transient receptor potential cation channel in neurons projecting to the fan-shaped body. Upon warming, the channel opens, activating the neurons to fire (Hamada et al., 2008). A toasty 31 degrees Celsius was like a sleeping pill for these flies. “And we can make them take it,” Shaw said (see video at the end of the article).

Wild-type flies (left) are unaffected by temperature, but transgenic flies (right) fall asleep when the temperature goes up. Image credit: Blake Sakran, Kevin Li, Paul Shaw

Because the fan-shaped body and the neighboring ellipsoid body are associated with memory (Liu et al., 2006; Wang et al., 2008), the researchers studied how well their remote-control sleepers remembered. To do that, they used courtship training. Donlea put sexually naïve male flies together with male flies that, unfortunately for both parties, produced female pheromones. The naïve male chased its unreceptive partner around for three hours in vain.

Normally, naïve flies do not recall this incident; two days later, they will chase a female-scented male just as doggedly. But when Donlea put the naïve males to sleep for four hours right after their experience, they retained a memory of it. These well-rested males didn’t bother trying to mate with scented males the second time around. Flies only remembered their humbling experience when they actually slept—activating the fan body but keeping the flies awake produced no memory. Activating the ellipsoid body also made no difference to memory.

The researchers have not yet figured out the mechanism by which activating the fan body causes sleep. Still, their flies should be useful in future studies, researchers said. “One thing that is still missing from the flies is the ability to record, in vivo, from neurons,” said Chiara Cirelli of the University of Wisconsin-Madison. Researchers would like to be able to perform live imaging and electrophysiology on the brain during sleep. But fly brains and neurons are so minute that even the tiniest electrode is quite invasive, and “not conducive to deep sleep,” Cirelli noted. But she and Shaw suspect that being able to activate sleep with temperature will help flies sleep even with an electrode in their brains.—Amber Dance