Abstract
Our experiments address two long-standing models for the function of the Drosophila brain circadian network: a dual oscillator model, which emphasizes the primacy of PDF-containing neurons, and a cell-autonomous model for circadian phase adjustment. We identify five different circadian (E) neurons that are a major source of rhythmicity and locomotor activity. Brief firing of PDF cells at different times of day generates a phase response curve (PRC), which mimics a light-mediated PRC and requires PDF receptor expression in the five E neurons. Firing also resembles light by causing TIM degradation in downstream neurons. Unlike light however, firing-mediated phase-shifting is CRY-independent and exploits the E3 ligase component CUL-3 in the early night to degrade TIM. Our results suggest that PDF neurons integrate light information and then modulate the phase of E cell oscillations and behavioral rhythms. The results also explain how fly brain rhythms persist in constant darkness and without CRY.
Highlights
Animals use endogenous circadian pacemakers to control their physiology and behavior with roughly 24-hr periodicity (Bass and Takahashi, 2010; Thut et al, 2012)
Diurnal behavior in Drosophila is currently best explained by a dual-oscillator model, which emphasizes the M cells as master pacemakers and the E cells as secondary slave oscillators
The 5 Dv-E cells are responsible for the evening anticipation peak and its timing, but they contribute to the morning peak, free running rhythms and total activity. These cells appear to play a key role in all aspects of circadian rhythms and locomotor activity
Summary
Animals use endogenous circadian pacemakers to control their physiology and behavior with roughly 24-hr periodicity (Bass and Takahashi, 2010; Thut et al, 2012). Intracellular timekeeping mechanisms include transcriptional feedback loops, which involve many key genes in Drosophila They include period (per), timeless (tim), clock (clk), cycle (cyc), and doubletime (dbt). Biochemical oscillations in the head occur in part through a direct interaction of PER and TIM with the positive transcription factor CLK: CYC (Dubruille and Emery, 2008; Menet and Rosbash, 2011). They require the photoreceptor cryptochrome (CRY) as well as a cycling light:dark (LD) environment, that is, RNA and protein oscillations damp rapidly in constant darkness (Stanewsky et al, 1998). The central brain is probably different as its molecular and behavioral rhythms persist in constant darkness and is CRY-independent (Stanewsky et al, 1998)
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