Micro-Anatomical Mechanisms of Neuronal Circadian Timekeeping, Output, and Entrainment

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Abstract Circadian clocks orchestrate myriad molecular, physiological, and behavioral processes to insure internal temporal order and optimal daily timing. In animals, the master clock resides deep within the brain where it relies on complex neural networks to ensure a robust internal sense of time that can readily synchronize with 24-h environmental cycle. There is growing consensus that the operation of our master circadian clock under modern light and social environments contributes significantly to a troubling array of health challenges. Understanding the neural mechanisms underlying circadian timekeeping and the synchronization of the master pacemaker with environmental cycles (i.e., entrainment) is therefore critical. A significant barrier to our understanding of the central circadian clock is the complexity of its constituent neural networks, a complexity compounded by the fact that clock-containing neurons employ multiple neurochemical signals that act via distinct signaling mechanisms. Critical clock neurons in both mammals and insects express multiple transmitters – including peptide co- transmitters - some of which function as local signals across defined synapses while others act as diffusible signals that act over large distances. Peptide co-release, though a common feature of nervous systems, is not well understood. Likewise, how clock neurons employ both local and paracrine signals to mediate circadian timekeeping and entrainment remains enigmatic. Here we propose to study key peptidergic clock neurons in Drosophila as a model to examine how two neuropeptides released from the same neuron can mediate distinct behavioral and physiological functions to support robust circadian timekeeping and entrainment. Our work will not only inform our understanding of circadian timekeeping in the mammalian brain but will also be relevant to the mechanism of peptide co-release generally.