Animals partition their daily activity rhythms through their internal circadian clocks, which are synchronized by oscillating day-night cycles of light. The fruitfly Drosophila melanogaster senses day-night cycles in part through rhodopsin-dependent light reception in the compound eye and photoreceptor cells in the Hofbauer-Buchner eyelet. A more noteworthy light entrainment pathway is mediated by central pacemaker neurons in the brain. The Drosophila circadian clock is extremely sensitive to light. However, the only known light sensor in pacemaker neurons, the flavoprotein cryptochrome (Cry), responds only to high levels of light in vitro. These observations indicate that there is an additional light-sensing pathway in fly pacemaker neurons. Here we describe a previously uncharacterized rhodopsin, Rh7, which contributes to circadian light entrainment by circadian pacemaker neurons in the brain. The pacemaker neurons respond to violet light, and this response depends on Rh7. Loss of either cry or rh7 caused minor defects in photoentrainment, whereas loss of both caused profound impairment. The circadian photoresponse to constant light was impaired in rh7 mutant flies, especially under dim light. The demonstration that Rh7 functions in circadian pacemaker neurons represents, to our knowledge, the first role for an opsin in the central brain.

Drosophila CRYPTOCHROME (dCRY) mediates electrophysiological depolarization and circadian clock resetting in response to blue or ultraviolet (UV) light. These light-evoked biological responses operate at different timescales and possibly through different mechanisms. Whether electron transfer down a conserved chain of tryptophan residues underlies biological responses following dCRY light activation has been controversial. To examine these issues in in vivo and in ex vivo whole-brain preparations, we generated transgenic flies expressing tryptophan mutant dCRYs in the conserved electron transfer chain and then measured neuronal electrophysiological phototransduction and behavioral responses to light. Electrophysiological-evoked potential analysis shows that dCRY mediates UV and blue-light-evoked depolarizations that are long lasting, persisting for nearly a minute. Surprisingly, dCRY appears to mediate red-light-evoked depolarization in wild-type flies, absent in both cry-null flies, and following acute treatment with the flavin-specific inhibitor diphenyleneiodonium in wild-type flies. This suggests a previously unsuspected functional signaling role for a neutral semiquinone flavin state (FADH^•) for dCRY. The W420 tryptophan residue located closest to the FAD-dCRY interaction site is critical for blue- and UV-light-evoked electrophysiological responses, while other tryptophan residues within electron transfer distance to W420 do not appear to be required for light-evoked electrophysiological responses. Mutation of the dCRY tryptophan residue W342, more distant from the FAD interaction site, mimics the cry-null behavioral light response to constant light exposure. These data indicate that light-evoked dCRY electrical depolarization and clock resetting are mediated by distinct mechanisms.

Mosquitoes pose widespread threats to humans and other animals as disease vectors [1]. Day- versus night-biting mosquitoes occupy distinct time-of-day niches [2, 3]. Here, we explore day- versus night-biting female and male mosquitoes' innate temporal attraction/avoidance behavioral responses to light and their regulation by circadian circuit and molecular mechanisms. Day-biting mosquitoes Aedes aegypti, particularly females, are attracted to light during the day regardless of spectra. In contrast, night-biting mosquitoes, Anopheles coluzzii, specifically avoid ultraviolet (UV) and blue light during the day. Behavioral attraction to/avoidance of light in both species change with time of day and show distinct sex and circadian neural circuit differences. Males of both diurnal and nocturnal mosquito species show reduced UV light avoidance in anticipation of evening onset relative to females. The circadian neural circuits of diurnal/day- and nocturnal/night-biting mosquitoes based on PERIOD (PER) and pigment-dispersing factor (PDF) expression show similar but distinct circuit organizations between species. The basis of diurnal versus nocturnal behaviors is driven by molecular clock timing, which cycles in anti-phase between day- versus night-biting mosquitoes. Observed differences at the neural circuit and protein levels provide insight into the fundamental basis underlying diurnality versus nocturnality. Molecular disruption of the circadian clock severely interferes with light-evoked attraction/avoidance behaviors in mosquitoes. In summary, attraction/avoidance behaviors show marked differences between day- versus night-biting mosquitoes, but both classes of mosquitoes are circadian and light regulated, which may be applied toward species-specific control of harmful mosquitoes.

Drosophila melanogaster CRYPTOCHROME (CRY) mediates behavioral and electrophysiological responses to blue light coded by circadian and arousal neurons. However, spectroscopic and biochemical assays of heterologously expressed CRY suggest that CRY may mediate functional responses to UV-A (ultraviolet A) light as well. To determine the relative contributions of distinct phototransduction systems, we tested mutants lacking CRY and mutants with disrupted opsin-based phototransduction for behavioral and electrophysiological responses to UV light. CRY and opsin-based external photoreceptor systems cooperate for UV light-evoked acute responses. CRY mediates behavioral avoidance responses related to executive choice, consistent with its expression in central brain neurons.

Blue light activation of the photoreceptor CRYPTOCHROME (CRY) evokes rapid depolarization and increased action potential firing in a subset of circadian and arousal neurons in Drosophila melanogaster. Here we show that acute arousal behavioral responses to blue light significantly differ in mutants lacking CRY, as well as mutants with disrupted opsin-based phototransduction. Light-activated CRY couples to membrane depolarization via a well conserved redox sensor of the voltage-gated potassium (K(+)) channel β-subunit (Kvβ) Hyperkinetic (Hk). The neuronal light response is almost completely absent in hk(-/-) mutants, but is functionally rescued by genetically targeted neuronal expression of WT Hk, but not by Hk point mutations that disable Hk redox sensor function. Multiple K(+) channel α-subunits that coassemble with Hk, including Shaker, Ether-a-go-go, and Ether-a-go-go-related gene, are ion conducting channels for CRY/Hk-coupled light response. Light activation of CRY is transduced to membrane depolarization, increased firing rate, and acute behavioral responses by the Kvβ subunit redox sensor.