Circadian variation in susceptibility to the ambulation-increasing effect of scopolamine in mice.

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Abstract
Circadian variation in susceptibility to the ambulation-increasing effect of scopolamine in mice was investigated. The mice housed under a 12 h light-dark situation (light period: 0600-1800 h; dark period: 1800-0600 h) for 5 wk were administered scopolamine hydrobromide at 0.5 or 2 mg/kg s.c. at 1 of 6 times of the day (0300, 0700, 1100, 1500, 1900 and 2300 h), and the ambulatory activity was measured for 2 h. Scopolamine induced a dose-dependent increase in the ambulatory activity. There was a clear circadian variation in the susceptibility to the ambulation-increasing effect of scopolamine. The highest and the lowest overall activity counts during the 2 h observation period were found when the drug was administered at 0300 and 1500 h, respectively. The susceptibility to scopolamine in mice is apparently dependent on the time of day at which the drug is administered. The reasons which may induce this circadian variation in susceptibility to scopolamine are discussed.