Light is the major stimulus for the synchronization of circadian clocks with day-night cycles. The light-driven entrainment of the clock that controls rest-activity rhythms in Drosophila relies on different photoreceptive molecules. Cryptochrome (CRY) is expressed in most brain clock neurons, whereas six different rhodopsins (RH) are present in the light-sensing organs. The compound eye includes outer photoreceptors that express RH1 and inner photoreceptors that each express one of the four rhodopsins RH3-RH6. RH6 is also expressed in the extraretinal Hofbauer-Buchner eyelet, whereas RH2 is only found in the ocelli. In low light, the synchronization of behavioral rhythms relies on either CRY or the canonical rhodopsin phototransduction pathway, which requires the phospholipase C-β encoded by norpA (no receptor potential A). We used norpA(P24) cry(02) double mutants that are circadianly blind in low light and restored NORPA function in each of the six types of photoreceptors, defined as expressing a particular rhodopsin. We first show that the NORPA pathway is less efficient than CRY for synchronizing rest-activity rhythms with delayed light-dark cycles but is important for proper phasing, whereas the two light-sensing pathways can mediate efficient adjustments to phase advances. Four of the six rhodopsin-expressing photoreceptors can mediate circadian entrainment, and all are more efficient for advancing than for delaying the behavioral clock. In contrast, neither RH5-expressing retinal photoreceptors nor RH2-expressing ocellar photoreceptors are sufficient to mediate synchronization through the NORPA pathway. Our results thus reveal different contributions of rhodopsin-expressing photoreceptors and suggest the existence of several circuits for rhodopsin-dependent circadian entrainment. J. Comp. Neurol. 524:2828-2844, 2016. © 2016 Wiley Periodicals, Inc.

In Drosophila, the clock that controls rest-activity rhythms synchronizes with light-dark cycles through either the blue-light sensitive cryptochrome (Cry) located in most clock neurons, or rhodopsin-expressing histaminergic photoreceptors. Here we show that, in the absence of Cry, each of the two histamine receptors Ort and HisCl1 contribute to entrain the clock whereas no entrainment occurs in the absence of the two receptors. In contrast to Ort, HisCl1 does not restore entrainment when expressed in the optic lobe interneurons. Indeed, HisCl1 is expressed in wild-type photoreceptors and entrainment is strongly impaired in flies with photoreceptors mutant for HisCl1. Rescuing HisCl1 expression in the Rh6-expressing photoreceptors restores entrainment but it does not in other photoreceptors, which send histaminergic inputs to Rh6-expressing photoreceptors. Our results thus show that Rh6-expressing neurons contribute to circadian entrainment as both photoreceptors and interneurons, recalling the dual function of melanopsin-expressing ganglion cells in the mammalian retina.