Animals use distinct sets of clock neurons to time behaviors in the morning and evening. In this article, the direct neural targets for morning neurons and the neuropeptide pigment dispersing factor are revealed in the fruit fly.
Discrete clusters of circadian clock neurons temporally organize daily behaviors such as sleep and wake. In Drosophila, a network of just 150 neurons drives two peaks of timed activity in the morning and evening. A subset of these neurons expresses the neuropeptide pigment dispersing factor (PDF), which is important for promoting morning behavior as well as maintaining robust free-running rhythmicity in constant conditions. Yet, how PDF acts on downstream circuits to mediate rhythmic behavior is unknown. Using circuit-directed rescue of PDF receptor mutants, we show that PDF targeting of just ∼30 non-PDF evening circadian neurons is sufficient to drive morning behavior. This function is not accompanied by large changes in core molecular oscillators in light-dark, indicating that PDF RECEPTOR likely regulates the output of these cells under these conditions. We find that PDF also acts on this focused set of non-PDF neurons to regulate both evening activity phase and period length, consistent with modest resetting effects on core oscillators. PDF likely acts on more distributed pacemaker neuron targets, including the PDF neurons themselves, to regulate rhythmic strength. Here we reveal defining features of the circuit-diagram for PDF peptide function in circadian behavior, revealing the direct neuronal targets of PDF as well as its behavioral functions at those sites. These studies define a key direct output circuit sufficient for multiple PDF dependent behaviors.
Animals depend on being awake at the right time of day to find food and mates and fend off predators. Circadian pacemaker neurons in the brain play a crucial role in timing of specific behaviors to the appropriate times of day. These neurons are further specialized to those primarily responsible for morning and evening behavior. We have used the fruit fly Drosophila as a simple model system to elucidate the neural circuits important for timed daily behavior. In flies, a small group of clock neurons devoted to morning behavior express a neuropeptide, PIGMENT DISPERSING FACTOR (PDF). Until now it was unclear what the direct neural targets of this peptide are and how its actions at those targets mediate timed behavior. Here we find that the so-called morning clock neurons communicate directly to other clock neurons, those responsible for evening behavior. This communication sustains high amplitude morning activity and sets the phase of evening activity as well as the period of activity rhythms in constant conditions. These studies reveal the circuit diagram for PDF function in circadian behavior.