[Influences of free intake of nicotine on several circadian rhythms in rats]

Male Wistar rats, aged 4 weeks, were kept in a temperature controlled room with a 12 hr light-dark cycle and given food and water ad libitum. The nicotine-treated group of rats was given water containing nicotine, which was estimated to be 10 mg/kg/day, for 40 days. Spontaneous motor activities, drinking activities and serum corticosterone levels showed circadian rhythms characteristic of a nocturnal animal in both the control and the nicotine-treated groups. As compared to the control group, however, the nicotine-treated group showed an increase in ambulatory activities, a decrease in drinking activities and a diminution of weight gain. In comparison with diurnal variations, serum corticosterone levels and liver nicotine oxidase activities increased in the nicotine-treated group during the dark period. However, the pattern of circadian rhythms characteristic of a nocturnal animal were not altered.