Light Activates Output From Evening Neurons And Inhibits Output From Morning Neurons In The Drosophila Circadian Clock

Living organisms have evolved circadian clocks that anticipate daily
changes in their environment. Their clockwork is fully endogenous, but can
be
reset by external cues. (Light is the most efficient cue.)

The circadian
neuronal network of the fruit fly (Drosophila) brain perceives light
through
the visual system and a dedicated photoreceptor molecule, cryptochrome.
Flies exhibit a bimodal locomotor activity pattern which peaks at dawn and
dusk in light-dark conditions. These morning and evening activity bouts
are controlled by two distinct neuronal clocks in the fly brain. By using
flies with a deficient cryptochrome pathway, François Rouyer and
colleagues have uncovered an unexpected role for light in the circadian
system.

The
study is published online this week in the open-access journal PLoS
Biology. In addition to synchronizing the two oscillators to solar time,
light
also controls their behavioral output. The morning oscillator can
periodically rouse the fly in constant darkness, but not in constant
light, while
the evening oscillator can do the same in constant light but not in
constant darkness. This suggests the existence of a light-dependent switch
between
oscillators that appears to require the visual system. Such a mechanism
likely contributes to better separate the active periods of the fly at
dawn
and dusk and may help the animal to adapt to seasonal changes in day
length.



Citation: Yang C, Czech L, Gerboth S, Kojima S, Scita G, et al. (2007)
Novel roles of formin mDia2 in lamellipodia and filopodia formation in
motile
cells. PLoS Biol 5(11): e317.doi:10.1371/journal.pbio.0050317

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