Anticipatory Capture of Circulating Peptidergic Vesicles in a Clock Neuron

Neuropeptide release by <i>Drosophila</i> sLNv clock neurons controls circadian behaviors and sleep. Strikingly, 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 <i>ex vivo</i> brain explant preparation. First, post-exocytosis DCV axonal transport and presynaptic 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 Ca²⁺ spike activity and is independent of concurrent IP₃ signaling and axon arbor expansion. Finally, a <i>per</i> clock gene mutation inhibits rhythmic DCV capture. Thus, rather than responding to Ca²⁺ signaling or axonal plasticity, capture of circulating DCVs in sLNv presynapses is increased by the molecular clock in anticipation of activity-induced release hours later. [Media: see text].