Synaptic and peptidergic connectomes of theDrosophilacircadian clock

Abstract

The circadian clock of animals regulates various physiological and behavioral processes in anticipation of, and adaptation to, daily environmental fluctuations. Consequently, the circadian clock and its output pathways play a pivotal role in maintaining homeostasis and optimizing daily functioning. To obtain novel insights into how diverse rhythmic physiology and behaviors are orchestrated, we have generated the first comprehensive connectivity map of an animal circadian clock using theDrosophilaFlyWire brain connectome. We reveal hitherto unknown extensive contralateral synaptic connectivity between the clock neurons, which might contribute to the robustness of the clock by synchronizing clock neurons across the two hemispheres. In addition, we discover novel direct and indirect light input pathways to the clock neurons that could facilitate clock entrainment. Intriguingly, we observe sparse monosynaptic connectivity between clock neurons and downstream higher-order brain centers and neurosecretory cells known to regulate several behaviors and physiology. Therefore, we integrated single-cell transcriptomic analysis and receptor mapping to additionally decipher paracrine peptidergic signaling between clock neurons and with neurosecretory cells. Our analyses identified additional novel neuropeptides expressed in clock neurons and suggest that peptidergic signaling greatly enriches the interconnectivity within the clock network. Neuropeptides also form the basis of output pathways which regulate rhythmic hormonal signaling. TheDrosophilacircadian clock and neurosecretory center connectomes provide the framework to understand more complex clock and hormonal networks, respectively, as well as the rhythmic processes regulated by them.