Effects of Animal Care Procedures on Plasma Corticosterone Levels in Group-Housed Mice during the Nocturnal Active Phase

C57BL/6 mice were housed five per cage on a 12:12 h light/dark cycle. All animal care, including bed cleaning, was carried out during the nonactive phase. After 2 weeks, mean plasma corticosterone levels, collected during the nonactive (ZT6) and active (ZT18) phases, were 66.0 and 270.9 ng/ml, respectively. The values at ZT18 gradually increased in the order of the mice used for blood collection, but not at ZT6. When animal care was carried out at ZT18, the increasing pattern of plasma corticosterone levels previously observed at ZT18 was less pronounced after 2 weeks of acclimatization, and was not observed after 4 weeks. Therefore, animal care should be carried out in the active phase for at least 4 weeks before experiments involving stress responses in the active phase.