Connectomic Analysis of the Drosophila Lateral Neuron Clock Cells Reveals the Synaptic Basis of Functional Pacemaker Classes

Abstract The circadian clock orchestrates daily changes in physiology and behavior to ensure internal temporal order and optimal timing across the day. In animals, a central brain clock orchestrates circadian rhythms throughout the body and is characterized by a remarkable resilience that depends on synaptic connections between constituent neurons. The clock neuron network of Drosophila, which is built of many fewer neurons yet shares network motifs with clock networks in the mammalian brain, offers a powerful model for understanding the network properties of circadian timekeeping. Here we report an assessment of synaptic connectivity within a clock network, focusing on the critical LN clock neuron classes. Our results reveal that previously identified anatomical and functional subclasses of LNs represent distinct connectomic types with distinct synaptic output pathways. Moreover, we identify a small number of clock cell subtypes representing highly synaptically coupled nodes within the fly’s clock neuron network, and suggest that neurons lacking molecular timekeeping likely play integral roles within the circadian timekeeping network. To our knowledge, this represents the first comprehensive connectomic analysis of a neural network capable of driving endogenous circadian rhythms. The organizational principles uncovered in this study provide remarkable insights into clock network organization.