A few years ago, scientists found that the common fruit fly, Drosophila melanogaster, sleeps in a remarkably human-like way. Now a new study on fruit fly brains shows that a specific brain region—previously linked to the fly's memories of smell—is also vital to sleep. These results support the idea that memory consolidation may play a big part in fly sleep, and, likely, human sleep. "The link between sleep and learning a lot of people can identify with – particularly college students who pull all-nighters," says lead researcher Jena Pitman of Northwestern University, "But that link is pretty universal." Might a sleeping fruit fly be digesting the memory of a rotten banana it ate for dinner?
Fruit flies and humans share many of what scientists call the "essential features" of sleep. Both species sleep for many hours at night, for instance, says Ravi Allada, one of the other neuroscientists involved in the study. With flies, too, as Allada explains, "the longer they're asleep, the harder you have to poke them to get them to wake up." If you deprive them of sleep, flies will try to catch up on sleep the next day. And fly sleep patterns respond to some drugs in the same way we do: antihistamines make them drowsy and caffeine keeps them awake. This all suggests that "the mechanism of sleep is very similar" in fruit flies and humans, says Allada.
Though the similarities between fly sleep and human sleep were well-established a few years ago, no one had studied the specific fly brain regions involved in sleeping until now. The latest Northwestern study, published in Nature on June 7, sought to figure out which part of the brain—if any—could be isolated for sleep studies. To do this, the researchers used an engineered gene, known as shi, that when expressed in different brain regions would block communication between neurons. For most of the brain regions tested, this blocking did nothing to the flies' normal sleeping patterns. But when regions called the mushroom bodies (there's one on each side of the brain) were blocked, the flies' sleeping periods were much shorter. The mushroom bodies, each with about 2,500 neurons, are a "small but significant" part of the 100,000 neurons that make up the fly brain, says Allada.
The researchers were surprised that blocking neuron communication affected sleeping patterns only when done in the mushroom bodies. But perhaps most intriguing is that in previous research, the mushroom bodies had been associated with the retrieval and consolidation of memories of smell, suggesting that sleep and memory are linked in the fly brain. Pitman says that when the study began four years ago, they had no idea the mushroom bodies would be involved in sleep, though to find that a region involved in sleep is also involved in memory "makes a lot of sense."
Allada says isolating the mushroom bodies is an important step in understanding why we sleep. The next step, he says, is to focus specifically on the mushroom bodies when looking for genes that are expressed during sleep. "I think over the next several years we'll be able to identify genes important to sleep. And I think the genes we find in the fly may turn out to be relevant in humans—that's the most exciting part to us."