Melanopsin has been implicated as an important photoreceptive molecule connecting the circadian clock in the hypothalamic suprachiasmatic nucleus to the retinal photoreceptors. Two papers in the December 13 Science confirm that in mice melanopsin acts as a signal molecule (but not the only one) transmitting light information from the eye to the part of the brain that controls the internal clock.

Norman F. Ruby and colleagues at Stanford University, California, USA measured behavioral responses to light in wild type and melanopsin knockout mice. They observed that the magnitude of these behavioral responses in knockout mice was 40% lower than in wild-type mice (Science 298:2211-2213, December 13, 2002).

In the second paper, Satchidananda Panda and colleagues at the Genomics Institute of the Novartis Research Foundation, San Diego, California, USA, show that melanopsin knockout mice display severely attenuated phase resetting in response to brief pulses of monochromatic light, but...

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