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They repeated the experiment in mice with natural, or "wild type," genomes and three strains of mutant mice with impaired circadian rhythms.

The wild-type mice and two of the mutant strains exposed to regular light-dark cycles showed no sign of the gene's expression in their livers ?it remained in the pancreas and stomach. The gene was expressed in the livers of some mice from a double-mutant strain that has absolutely no circadian response. All four genotypes of mouse kept in constant darkness had mClps expressed not only in their livers but in all peripheral tissue except the brain and kidneys.

"This is the first example of a gene that is turned on by darkness, where darkness itself is a signal," Lee said. "Twelve hours of darkness didn't do the job; it had to be at least 48 hours."

Tests showed it subsequently took five to seven hours of light exposure to inhibit the gene's expression in the liver. Taken together, these time delays pointed to the gene's expression being mediated by something in the blood. Another lab test showed elevated levels of 5'-AMP in the blood of mice exposed to constant darkness compared to those kept in the regular light-dark cycle.

To confirm the connection, the team injected 5'-AMP into mice exposed to a regular light-dark cycle. Three to four hours after injection, mClps was expressed in the livers of these mice and further tests showed expression in all tissues except the brain.

Injecting mice with 5'-AMP's more glamorous molecular cousins ?adenosine triphosphate (ATP), adenosine diphosphate (ADP), both vital to providing energy to cells, and the signaling molecule cyclic AMP -- did not produce the same effect.

The mice injected with 5'-AMP also were found to have a lower body temperature, a sign of torpor. Core body temperature measurements confirmed their lethargic state. Mice kept in constant dark a
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Source:University of Texas Health Science Center at Houston