Daily ultrastructural remodeling of clock neurons

Area of Science:

• Neuroscience
• Chronobiology
• Cellular Biology

Background:

• Circadian rhythms are regulated by a network of approximately 250 clock neurons in the Drosophila brain.
• Small Lateral ventral Neurons (s-LNvs) are crucial for synchronizing this network via the neuropeptide Pigment-Dispersing Factor (PDF).
• The neurites of s-LNvs undergo daily structural changes, including variations in morphology, PDF levels, and synaptic connectivity, a process termed circadian structural plasticity.

Purpose of the Study:

• To investigate the subcellular mechanisms underlying circadian structural plasticity in s-LNv terminals.
• To visualize and analyze the 3D structure of s-LNv terminals at key circadian time points.

Main Methods:

• Serial Block-face Scanning Electron Microscopy (SBEM) was used to generate high-resolution 3D volumes of s-LNv terminals.
• Analysis was performed at three critical time points: two hours before lights-ON, two hours after lights-ON, and two hours after lights-OFF.

Main Results:

• A reduction in the number of neuronal varicosities was observed during the night.
• In the morning, an increase in presynaptic sites, dense core vesicle accumulation, and release was detected.
• Periodic changes in mitochondrial structure indicated daily modulation of neuronal activity.

Conclusions:

• Circadian structural plasticity in s-LNv terminals involves daily remodeling of presynaptic varicosities.
• These structural rhythms likely modulate the s-LNvs' influence on the circadian clock network.
• Mitochondrial dynamics are implicated in the daily regulation of s-LNv function.