Soma Ca2+ is decoupled from daily synaptic activity and neuropeptide release in Drosophila clock neurons

Drosophila sLNv clock neurons release the co-packaged neuropeptides PDF and sNPF to regulate circadian behaviors and nighttime sleep. 1 , 2 , 3 , 4 Many studies of membrane potential and cytoplasmic Ca 2+ at the sLNv soma emphasized elevations late at night or in the very early morning, 5 , 6 , 7 , 8 , 9 although action potential activity and synaptic release were not quantified. Recently, exocytosis of neuropeptide-containing dense-core vesicles (DCVs) at sLNv terminals was found to peak hours later at midmorning. 10 To resolve the basis of the timing mismatch between somatic measurements and terminal exocytosis, recently developed probes were used to measure daily rhythms in sLNv neuron synaptic Ca 2+ and sNPF release. Remarkably, at midmorning after soma Ca 2+ has dropped, both Ca 2+ spiking and clock-dependent native neuropeptide release peak in the distal terminals of the protocerebrum. Furthermore, Ca 2+ in the soma and terminals differ in dependence on Ca 2+ influx. Finally, synaptic DCV exocytosis requires Ca 2+ spike activity at terminals that is not evident at the soma. These results lead to two striking conclusions. First, soma Ca 2+ recording, which is the focus of many circuit studies, is not indicative of presynaptic Ca 2+ and neuropeptide release in distal sLNv terminals. Second, daily clock- and activity-dependent sLNv terminal neuropeptide release occurs many hours in advance of known sLNv neuropeptide effects on nighttime sleep and morning behavior.