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

Abstract

Drosophila sLNv clock neurons release the co-packaged neuropeptides PDF and sNPF to regulate circadian behaviors (e.g., morning anticipation) and nighttime sleep1-3. Previous studies of membrane potential and cytoplasmic Ca2+ at the soma suggested that sLNv neuron activity peaks at night4,5, but exocytosis of neuropeptide-containing dense-core vesicles (DCVs) at their terminals peaks hours later at midmorning6. To resolve the basis of the timing mismatch between somatic physiology and terminal exocytosis, recently developed probes were used to measure daily rhythms in sLNv neuron synaptic Ca2+ and sNPF release. Remarkably, at midmorning after soma Ca2+ has dropped, both Ca2+ spiking and clock-dependent native neuropeptide release peak in the distal terminals of the protocerebrum. Furthermore, Ca2+ in the soma and terminals differ in dependence on Ca2+ influx. Finally, synaptic DCV exocytosis requires Ca2+ spike activity at terminals that is not evident at the soma. These results lead to two striking conclusions. First, soma Ca2+ recording, which is the focus of many circuit studies, is not indicative of presynaptic Ca2+ and neuropeptide release in distal sLNv terminals. Second, daily clock- and activity-dependent sLNv terminal neuropeptide release occurs [~]9-18 hours in advance of known sLNv neuropeptide effects on nighttime sleep and morning behavior.