Circadian pacemaker neurons change synaptic contacts across the day

Area of Science:

• Neuroscience
• Chronobiology
• Molecular Biology

Background:

• Circadian rhythms regulate daily physiological and behavioral cycles.
• Locomotor activity in Drosophila is controlled by specific neuronal clusters, notably the small ventral lateral neurons (sLNvs).
• sLNvs exhibit circadian remodeling of axonal projections, suggesting dynamic changes in neural connectivity.

Purpose of the Study:

• To investigate the functional implications of circadian plasticity in sLNv axonal projections beyond structural changes.
• To determine if sLNv neurons alter synaptic partners throughout the day.
• To elucidate the role of dynamic synaptic connectivity in mediating circadian behaviors.

Main Methods:

• Quantitative analysis of axonal projection plasticity in sLNv neurons.
• Assessment of active zone quantity over a 24-hour cycle.
• Application of the GFP reconstitution across synaptic partners (GRASP) technique to identify real-time synaptic contacts.

Main Results:

• The extent of daily structural plasticity in sLNv projections is greater than previously described, involving fasciculation changes and axonal pruning/extension.
• The number of active zones in sLNv terminals fluctuates daily, indicating dynamic synapse formation and elimination.
• sLNv neurons form connections with distinct synaptic partners at different times of the day, revealed by GRASP.

Conclusions:

• Circadian plasticity of sLNv axonal projections involves dynamic changes in synaptic partners, not just structural remodeling.
• These daily alterations in connectivity provide a mechanism for the circadian network to orchestrate day-night physiological and behavioral differences.
• The changing synaptic pathways of sLNv neurons represent a novel layer of circadian control.