A rhodopsin in the brain functions in circadian photoentrainment in Drosophila

Abstract

Animals partition their daily activity rhythms through their internal circadian clocks, which are synchronized by oscillating day-night cycles of light. The fruit fly, Drosophila melanogaster, senses day/night cycles in part through rhodopsin-dependent light reception in the compound eye, and photoreceptor cells in the Hofbauer-Buchner (H-B) eyelet ¹ . However, a more significant light entrainment pathway is mediated in central pacemaker neurons in the brain. The Drosophila circadian clock is extremely light sensitive. However, the only known light sensor in pacemaker neurons, the flavoprotein, cryptochrome (Cry) ^{2, 3} , responds only to high levels of light in vitro ⁴ . These observations indicate the existence of an additional light-sensing pathway in fly pacemaker neurons ⁵ . Here, we identified an uncharacterized rhodopsin, Rh7, which functions in circadian light entrainment through circadian pacemaker neurons in the brain. The pacemaker neurons respond to violet light, which was dependent on Rh7. While loss of either cry or rh7 caused minor affects on photoentrainment, the defects in the double mutant were profound. The circadian photoresponse to constant light was impaired in the rh7 mutant, especially under dim light. The demonstration that Rh7 functions in circadian pacemaker neurons represents the first role for an opsin in the central brain.