Kuribara H, Tadokoro S
Yakubutsu Seishin Kodo. 1985 Sep;5(3):279-86.
Effect of morphine (5 and 10 mg/kg sc) on ambulatory activity was investigated at different times of day (administration at 3:00, 7:00, 11:00, 15:00, 19:00 and 23:00) in mice housed under a normal light-dark condition (light period; 6:00-18:00). Morphine increased the ambulatory activity in a dose-dependent manner when it was administered to the drug-naive mice. There was no clear circadian variation in the increased ambulatory activity. The repeated administration of morphine (10 mg/kg) at intervals of 7 days produced an augmentation in the sensitivity to the ambulation-increasing effect of morphine independently of the times of day, and the mean overall ambulatory activity counts during 3 hr observation period in the 5th administration were about 2 times as high as corresponding those in the 1st administration. The mice experienced the repeated administration of morphine (10 mg/kg) for 5 times during the mid-late light, and mid dark (administration at 11:00, 15:00 and 23:00) showed an increased sensitivity to the ambulation-increasing effect of methamphetamine (1 mg/kg sc). However, the circadian variation in sensitivity to methamphetamine, which was highest and lowest during the dark and light periods, respectively, was still maintained even in the morphine-experienced mice.
