Sleep remains one of the most mysterious things that animals do. However, a fascinating new investigation by Indrani Ganguly-Fitzgerald and colleagues shows that sleep helps organisms process social information, and they made their discovery in a rather unexpected little creature: the fruit-fly, Drosophila melanogaster.
Most fruit-fly sleep (as defined by prolonged periods of inactivity) takes place at night, but flies can also doze off for several minutes at a time during the day, making up around 40% of their total sleep. Furthermore, if fiendish experimenters keep flies awake, the insects need to recuperate their lost sleep, just like us. Interested in the molecular connection between sleep, social experience and memory, the authors investigated whether flies living in groups slept for longer than their solitary counterparts. Keeping flies in groups and in isolation, the team found that flies kept in groups slept longer during the day than flies housed alone, and slept in bouts approximately 60 min long, compared with 15 min bouts for isolated flies. When the team removed flies from the group and isolated them, they found that this experience affected the amount of day-time sleep: it reduced compared to when the fly was in the group. But isolating crowded flies did not reduce their amount of night-time sleep, suggesting that different processes are involved in the two sleep phases.
Because an individual's requirements for day-time sleep also increased with the number of flies in its group, the authors investigated the sensory stimuli that enabled flies to detect the presence of other individuals. Blind flies and flies that could not smell showed the same low amount of day-time sleep,whether they were in groups or isolated, suggesting that visual and olfactory inputs are required for social-experience-driven sleep changes.
Next the team investigated the levels of neurotransmitters in the insects'brains to determine what kind of neurons are involved in sleep. They found that longer-sleeping, socially stimulated flies had three times as much of the neurotransmitter dopamine in their brains compared to isolated flies. When the investigators killed neurons that use this neurotransmitter, crowded flies slept the same amount when isolated, not less, suggesting that these neurons are required for socially driven sleep.
Because dopamine is also known to be involved in memory, the authors then investigated whether socially driven sleep patterns were altered in flies with gene mutations known to affect learning and memory. After studying mutations in 49 different genes, they concluded that flies with long-term memory gene mutations slept the same amount when isolated from the crowd, suggesting that they could not store social information. Some of these genes are expressed in the mushroom bodies in the fly's brain, which are involved in olfactory learning, implying that social signals are partly processed in this structure. This suggestion was reinforced when the team found that day-time sleep increased in males that had learnt not to court an unresponsive fly. This process is probably mediated by cuticular hydrocarbons, which are also processed in the mushroom bodies, implying that chemical communication may interact with visual and olfactory signals to produce crowded flies' sleep patterns.
This fascinating study of how social stimuli affect sleep shows the enormous opportunities that exist in investigating complex behaviours in the fruit-fly. It provides information about the specific structures and neural networks that underlie the fly's day-time sleep, and also gives those interested in vertebrate sleep new tools and approaches for investigating a subject that is vital to every one of us.
