Abstract
Neuropeptide release by Drosophila sLNv clock neurons controls circadian behaviors and sleep. Remarkably, neuropeptide content in sLNv terminals is rhythmic with late-night accumulation occurring while the axon arbor is expanding in preparation for midmorning synaptic exocytosis of neuropeptide-containing dense-core vesicles (DCVs). Past studies showed increased synaptic neuropeptide content can be produced by delivery of more neuropeptide to terminals or activity-dependent capture of circulating DCVs. To distinguish between these mechanisms, neuropeptide-containing DCVs were imaged in the living brain explant. First, post-exocytosis DCV axonal transport and synaptic neuropeptide accumulation following retrograde transport inhibition show that sLNv DCVs circulate. Furthermore, anterograde transport to terminals is constant throughout the day demonstrating there is no increase in DCV delivery. Rather, capture of circulating DCVs produces the daily boost in terminal neuropeptide content. Remarkably, this capture occurs before the daily increase in Ca (2+) spike activity and is also independent of concurrent IP (3) signaling and daily axon arbor expansion. Finally, a per clock gene mutation inhibits rhythmic DCV capture. Thus, rather than participating in axonal plasticity or responding to Ca (2+) signaling, capture of circulating DCVs in sLNv presynapses is increased by the molecular clock in anticipation of activity-induced release hours later.