Abstract

Sleep is a universally conserved behavior whose function still remains elusive. Our lab recently proposed a model for sleep function that links sleep need to the homeostatic regulation of synaptic strength, in a mutual bi-directional fashion. Using Drosophila as model system, we analyzed both directions of this connection.

Using two different molecular markers of synapse number (the structural proteins BRP and DLG ) and three markers of synaptic function (the components of vesicle secretion machinery: SYN , SYX, CSP ) we investigated how sleep and wakefulness affect the dynamic regulation of bona fide synaptic strength in the Drosophila brain. We found that prolonged wakefulness induces a diffuse increase in synaptic number/volume, while restorative sleep has the opposite effect. Using two different paradigms of sleep deprivation we showed that the increase is specific to and correlate with the amount of time the flies spend being awake. Conversely, sleep-induced decrease in synaptic markers is proportional to the amount of time flies spend asleep and it is independent from circadian entrainment. Using confocal imaging, we showed that the increase in the expression of synaptic markers is widespread across the entire brain and that it correlated with a volumetric increase in specific area of the brain involved in learning.

To manipulate synaptic function and strength, we ectopically expressed genes known to increase or decrease synaptic activity in vivo. We showed that a widespread increase in synaptic number in the Drosophila brain induces an increase in sleep drive and that, conversely, a decrease in synaptic number is followed by a decrease in sleep time. In particular, manipulation of synaptic activity (whether pan-neuronally or in specific area of the brain) affects length of sleep time, quality of sleep and recovery after sleep deprivation.

In conclusion, our results suggest that, in flies, sleep regulates synaptic strength dynamics and synaptic strength regulates sleep need.