Light-Mediated Circuit Switching in the Drosophila Neuronal Clock Network

Oct 1, 2019Current biology : CB

Light-driven changes in the fruit fly's brain clock network

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Abstract

A single photoreceptor subtype is essential for long-day adaptation in circadian rhythms.

Light signals are crucial for the circadian clock's adjustment to environmental changes.
Key circadian neurons, the large ventral-lateral neurons (lLNvs), are activated by this photoreceptor subtype.
The neuropeptide pigment-dispersing factor (PDF) released by lLNvs is necessary for delaying evening activity in long-day conditions.
PDF from lLNvs contrasts with that from small ventral-lateral neurons (sLNvs), which is important for maintaining rhythmicity in constant darkness.
The mechanism identified is specific for long-day adaptation and does not appear to function during equinox or constant darkness.
External light cues may reorganize neuronal interactions, contributing to behavioral flexibility.

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The circadian clock is a timekeeper but also helps adapt physiology to the outside world. This is because an essential feature of clocks is their ability to adjust (entrain) to the environment, with light being the most important signal. Whereas cryptochrome-mediated entrainment is well understood in Drosophila, integration of light information via the visual system lacks a neuronal or molecular mechanism. Here, we show that a single photoreceptor subtype is essential for long-day adaptation. These cells activate key circadian neurons, namely the large ventral-lateral neurons (lLNvs), which release the neuropeptide pigment-dispersing factor (PDF). RNAi and rescue experiments show that PDF from these cells is necessary and sufficient for delaying the timing of the evening (E) activity in long-day conditions. This contrasts to PDF that derives from the small ventral-lateral neurons (sLNvs), which are essential for constant darkness (DD) rhythmicity. Using a cell-specific CRISPR/Cas9 assay, we show that lLNv-derived PDF directly interacts with neurons important for E activity timing. Interestingly, this pathway is specific for long-day adaptation and appears to be dispensable in equinox or DD conditions. The results therefore indicate that external cues cause a rearrangement of neuronal hierarchy, which contributes to behavioral plasticity.

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Light-Mediated Circuit Switching in the Drosophila Neuronal Clock Network

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