The effects of short light/dark cycle in mice

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The light/dark cycle is one of the most essential and important environmental element in the organism, and this determines the physiological or behavioral circadian rhythm in animals. The change of daily light/dark cycle is known to influence the circadian rhythm of various physiological functions such as secretion of hormone, cell cycle, behavior, etc. And, such circadian rhythm is now utilized for determining the timing of drug administration.In the present study, it was examined that the effects of short light/dark cycle on the DNA synthesis and tumor growth in the mouse lung and liver when mice were nursed in the 6hr light-6hr dark cycle in comparison with 12hr light-12hr dark standard cycle for long period.CD-1, A/J and C3H/He mice, which were treated with urethane or diethylnitrosamine and maintained under the short light/dark cycle (6hr light, 6hr dark), developed significantly larger sizes of lung tumors or hepatoma than the mice maintained under standard light/dark cycle (12hr light, 12hr dark), indicating that short and frequent light/dark cycle enhanced tumor growth. When 3H-labeled thymidine was injected and incorporated radioactivities in the lung tissue were measured, two peaks of DNA synthesis in the lung were observed in a day under the short light/dark cycle, and both peaks were during light periods, while only one peak was observed during the dark period under the standard light/dark cycle. Although the level of 3H-thymidine incorporation into the lung tissue in the short light/dark cycle is lower than that in the standard one, frequent or unbalanced DNA synthesis may have promoted the growth of lung tumors.