Abstract
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.
