Synchronous Drosophila circadian pacemakers display nonsynchronous Ca2+ rhythms in vivo

Layered versatility atop circadian clocks The circadian clock evolved to allow cells or organisms to anticipate changes in physiological requirements associated with Earth's 24-hour light/dark cycle. Some activities, however, need to occur out of phase with the core clock. Liang et al. imaged changes in intracellular concentration of Ca2+ in populations of neurons in the fruit fly brain. Although the underlying clock was synchronous, the rhythms of Ca2+ changes corresponded with distinct timing of activities associated with activity of the particular neuronal populations. Proper coordination of these distinct phases required expression of the neuropeptide pigment-dispersing factor and its receptor. Science, this issue p. 976 Neuropeptides enable synchronized circadian clocks to time events out of phase with the clock mechanism. In Drosophila, molecular clocks control circadian rhythmic behavior through a network of ~150 pacemaker neurons. To explain how the network’s neuronal properties encode time, we performed brainwide calcium imaging of groups of pacemaker neurons in vivo for 24 hours. Pacemakers exhibited daily rhythmic changes in intracellular Ca2+ that were entrained by environmental cues and timed by molecular clocks. However, these rhythms were not synchronous, as each group exhibited its own phase of activation. Ca2+ rhythms displayed by pacemaker groups that were associated with the morning or evening locomotor activities occurred ~4 hours before their respective behaviors. Loss of the receptor for the neuropeptide PDF promoted synchrony of Ca2+ waves. Thus, neuropeptide modulation is required to sequentially time outputs from a network of synchronous molecular pacemakers.