Abstract
Light profoundly affects the circadian clock and the activity levels of animals. Along with the systematic changes in intensity and spectral composition, over the 24-h day, light shows considerable irregular fluctuations (noise). Using light as the Zeitgeber for the circadian clock is, therefore, a complex task and this might explain why animals utilize multiple photoreceptors to entrain their circadian clock. The fruit fly Drosophila melanogaster possesses light-sensitive Cryptochrome and seven Rhodopsins that all contribute to light detection. We review the role of Rhodopsins in circadian entrainment, and of direct light-effects on the activity, with a special emphasis on the newly discovered Rhodopsin 7 (Rh7). We present evidence that Rhodopsin 6 in receptor cells 8 of the compound eyes, as well as in the extra retinal Hofbauer-Buchner eyelets, plays a major role in entraining the fly’s circadian clock with an appropriate phase-to-light–dark cycles. We discuss recent contradictory findings regarding Rhodopsin 7 and report original data that support its role in the compound eyes and in the brain. While Rhodopsin 7 in the brain appears to have a minor role in entrainment, in the compound eyes it seems crucial for fine-tuning light sensitivity to prevent overshooting responses to bright light.
Highlights
A circadian clock has no survival value unless biological time is adjusted to local time.To most organisms, the profound changes in the light environment provide a local time cue
We have performed additional quantitative PCR analysis, improved our immunostainings with an Rhodopsin 7 (Rh7) antibody and recorded ERGs of the wild-type flies and the Rh70 mutants to elucidate a putative role of the Rh7 in light-sensitivity of the compound eyes
Rh7 is Expressed in the Compound Eyes and in the Brain, but Not in the l-LNv
Summary
A circadian clock has no survival value unless biological time is adjusted (entrained) to local time.To most organisms, the profound changes in the light environment provide a local time cue (calledZeitgeber). Over the 24-h day, the amount of light, its spectral composition and its direction, change in a systematic way In theory, all these features could be used for entrainment, but each would be subject to considerable variation. All these features could be used for entrainment, but each would be subject to considerable variation This may be the reason why animals use multiple photoreceptors to entrain their circadian clocks [1,2]. For example use the Rhodopsins in the rods and cones of their eyes and a specialized opsin, melanopsin, in the retinal ganglion cells to entrain their daily activity rhythms to the daily light–dark cycles [3,4,5,6,7]. A further opsin, Opn, is expressed in a subset of retinal ganglion cells and was recently shown to contribute to the entrainment of the circadian clock to UV light (380 nm) [9]
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