A peripheral pacemaker drives the circadian rhythm of synaptic boutons in Drosophila independently of synaptic...

Circadian rhythms in the morphology of neurons have been demonstrated in the fly Drosophila melanogaster. One such rhythm is characterized by changes in the size of synaptic boutons of an identified flight motor neuron, with larger boutons during the day compared with those at night. A more detailed temporal resolution of this rhythm shows here that boutons grow at a time of increased locomotor activity during the morning but become gradually smaller during the day and second period of increased locomotor activity in the evening. We have experimentally manipulated the synaptic activity of the fly during short periods of the day to investigate whether changes in bouton size might be a consequence of the different levels of synaptic activity associated with the locomotion rhythm of the fly. In the late night and early morning, when the flies normally have an intense period of locomotion, the boutons grow independently of whether the flies are active or completely paralyzed. Bouton size is not affected by sleep-deprivation during the early night. The cycle in bouton size persists for 2 days even in decapitated flies, which do not move, reinforcing the notion that it is largely independent of synaptic activity, and showing that a pacemaker other than the main biological clock can drive it.