Desynchronized Somas and Terminals in a Morning Clock Neuron: Presynaptic Ca²⁺ Spiking and Native Neuropeptide...

Drosophila sLNv clock neurons release the co-packaged neuropeptides PDF and sNPF to regulate circadian behaviors (e.g., morning anticipation) and nighttime sleep^1-3. Previous studies of membrane potential and cytoplasmic Ca²⁺ at the soma suggested that sLNv neuron activity peaks at night^4,5, 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.