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Am J Physiol Regul Integr Comp Physiol (August 22, 2002). doi:10.1152/ajpregu.00362.2002
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Articles in PresS, published online ahead of print August 22, 2002
Am J Physiol Regu Physiol, 10.1152/ajpregu.00362.2002
Submitted on June 18, 2002
Accepted on August 14, 2002

Constant darkness restores entrainment to phase-delayed Siberian hamsters

Norman F Ruby1*, Nirav Joshi1, and H. Craig Heller1

1 Department of Biological Sciences, Stanford University, Stanford, CA, USA

* To whom correspondence should be addressed. E-mail: ruby{at}stanford.edu.

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 LD 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, while 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.




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