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 Ca²⁺ 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 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 many hours in advance of known sLNv neuropeptide effects on nighttime sleep and morning behavior.