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Am J Physiol Regul Integr Comp Physiol 283: R1314-R1320, 2002. First published August 22, 2002; doi:10.1152/ajpregu.00362.2002
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Vol. 283, Issue 6, R1314-R1320, December 2002

Constant darkness restores entrainment to phase-delayed Siberian hamsters

Norman F. Ruby, Nirav Joshi, and H. Craig Heller

Department of Biological Sciences, Stanford University, Stanford, California 94305

Over 90% of Siberian hamsters (Phodopus sungorus) fail to reentrain to a 5-h phase delay of a 16:8-h photocycle. Because constant darkness (DD) restores rhythms disrupted by constant light, we tested whether DD could also restore entrainment. DD began 0, 5, or 14 days after a 5-h phase delay, and the light-dark cycle was reinstated 14 days later. All hamsters exposed to DD on day 0 reentrained, whereas 42% reentrained irrespective of whether DD began 5 or 14 days later. For these latter two groups, tau (tau ) and alpha (alpha ) in DD predicted reentrainment; animals that reentrained had a mean tau  and alpha  of 24.1 and 8.9 h, respectively, whereas those that failed to reentrain maintained a mean tau  and alpha  of 25.0 and of 7.1 h, respectively. Restoration of entrainment by DD is somewhat paradoxical because it suggests that reentrainment to the photocycle was prevented by continued exposure to that same photocycle. The dichotomy of circadian responses to DD suggests "entrainment" phenotypes that are similar to those of photoperiodic responders and nonresponders.

circadian; phase shift; light-dark cycle; activity rhythm


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