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The Brain’s Alarm Clock

Neurons in the brain that boost the morning response of the circadian rhythm are themselves directly responsive to light. Circadian rhythms are linked to the external light-dark cycle through inputs from photoreceptors that signal into networks that regulate neural and physiological function. One of the key photoreceptors is CRYPTOCHROME, which is sensitive to blue-light wavelengths. Fogle et al. (see the Perspective by Im and Taghert) now find that CRYPTOCHROME has an unexpectedly direct effect on circadian physiology in fruit flies. A small group of neurons that are part of the circadian circuit and that are usually more active in the morning express CRYPTOCHROME. These neurons normally receive plenty of input from the circadian circuit that perceives cycles and drives responses. However, when those inputs are blocked, it seems that these neurons are able to respond directly to blue light. K. J. Fogle, K. G. Parson, N. A. Dahm, T. C. Holmes, CRYPTOCHROME is a blue-light sensor that regulates neuronal firing rate. Science 331, 1409–1413 (2011). [Abstract] [Full Text] S. H. Im, P. H. Taghert, A CRY to rise. Science 331, 1394–1395 (2011). [Abstract] [Full Text]