Restoration of self-sustained circadian rhythmicity by the mutant Clock allele in mice in constant illumination

Mice mutant for the Clock gene display abnormal circadian behavior characterized by long circadian periods and a tendency to become rapidly arrhythmic in constant darkness (DID). To investigate whether this result is contingent on the absence of light, the authors studied the circadian behavior of homozygous Clock mutant mice under conditions of both constant light and DD, Fourteen of 15 Clock/Clock mice stayed rhythmic in constant light of 70 to 170 lux, where 10 of 15 wild-type mice became arrhythmic. In contrast, only 5 of 15 Clock/Clock mice and 15 of 15 wild-type mice remained rhythmic after 60 cycles when released in DID (dim red light of <1.5 lux) after 8 days of entrainment. The restoration of self-sustained rhythmicity by the Clock allele cannot be attributed to reduced sensitivity of the system to light. It underscores the fact that self-sustainment is not a secure guide to functional organization.