Activity-dependent capture of neuropeptide vesicles prepares clock neuron synapses for daily release.

Drosophila brain sLNv clock neurons release the neuropeptide PDF to control circadian rhythms. Strikingly, PDF content in sLNv terminals is rhythmic with a peak in the morning. Peak content drops because of activity-dependent release from dense-core vesicles (DCVs), but the mechanism for the daily increase in presynaptic PDF in the hours prior to release is unknown. Although transport from the soma was proposed to drive the daily increase in presynaptic PDF, live imaging in sLNv neurons shows that anterograde axonal DCV transport is constant throughout the day. Instead, capture of circulating DCVs, indicated by decreased retrograde axonal transport, rhythmically boosts presynaptic neuropeptide content. Genetic manipulations demonstrate that the late night increase in capture requires electrical activity but is independent of daily morphological changes. These results suggest that each day, during the hours of ongoing electrical activity, a toggle switches from inducing vesicle capture to triggering exocytosis, thereby maximizing daily rhythmic bursts of synaptic neuropeptide release by clock neurons.