Dopamine acts through Cryptochrome to promote acute arousal in Drosophila.

The fruit fly, Drosophila melanogaster, is generally diurnal, but a few mutant strains, such as the circadian clock mutant Clk(Jrk), have been described as nocturnal. We report here that increased nighttime activity of Clk mutants is mediated by high levels of the circadian photoreceptor CRYPTOCHROME (CRY) in large ventral lateral neurons (l-LN(v)s). We found that CRY expression is also required for nighttime activity in mutants that have high dopamine signaling. In fact, dopamine signaling is elevated in Clk(Jrk) mutants and acts through CRY to promote the nocturnal activity of this mutant. Notably, dopamine and CRY are required for acute arousal upon sensory stimulation. Because dopamine signaling and CRY levels are typically high at night, this may explain why a chronic increase in levels of these molecules produces sustained nighttime activity. We propose that CRY has a distinct role in acute responses to sensory stimuli: (1) circadian responses to light, as previously reported, and (2) noncircadian effects on arousal, as shown here.