Synaptic connections of clock neurons sending signals to the thinking areas of fruit flies

The neuropeptide pigment-dispersing factor (PDF) is an important output signal from clock neurons in Drosophila melanogaster.

• PDF-positive fibers from small ventral lateral neurons (s-LN(v)s) project to the dorsal protocerebrum, suggesting a key role in circadian signaling.
• These fibers contain large dense-core vesicles (DCVs) and numerous small clear vesicles, which form divergent output synapses onto other neurons.
• DCVs appear to release their contents at nonsynaptic sites, indicating a potential mechanism for paracrine signaling.
• Input synapses onto PDF-positive fibers are present, but occur less frequently than output sites, suggesting a complex interplay of signaling.
• The findings point to a dual role for PDF-positive clock neurons, both receiving inputs and providing outputs to unidentified target neurons.

Simplified