Circadian Rhythms in the Incidence of Apoptotic Cells and Number of Clonogenic Cells in Intestinal Crypts after Radiation Using Normal and Reversed Light Conditions

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Abstract
Variations in the number of radiation-induced morphologically dead or dying cells (apoptotic cells) in the crypts in the small intestine of the mouse have been studied throughout a 24-h period under a normal light regimen (light on, 07.00–19.00 h; light off, 19.00–07.00 h). A clear circadian rhythm was displayed in the apoptotic incidence 3 or 6 h after irradiation for each gamma-ray dose studied (range 0·14–9·0 Gy). The most prominent circadian rhythm was obtained after 0·5 Gy. The peak time of day for inducing apoptosis was 06.00–09.00 h, and the trough occurred at 18.00–21.00 h. Some mice were also transferred to a room with the light cycle reversed, and were irradiated on different days after the transfer. The apoptosis induced by 0·5 Gy or 9·0 Gy, or the number of surviving crypts (microcolonies) after 11·0 Gy or 13·0 Gy was examined. The transition point for reversal (i.e. the switch time from the normal-light pattern to the reversed-light pattern) of the circadian rhythm in apoptosis (after 0·5 Gy) occurred 7 days after the transfer and the rhythm was reversed by 14 days. The rhythm for crypt survival (i.e. for clonogenic cell radiosensitivity) was disturbed on 1 day and the transition point for reversal occurred 3 days after the transfer. The rhythm became reversed by 7 days. These observations are discussed in relation to the identity of clonogenic cells, (functional) stem cells, proliferating transit cells and the cells sensitive to small doses of radiation (i.e. hypersensitive cells) in the crypt.