Effects of nicotine, caffeine, and their combination on locomotor activity in rats.

The interactive effect of caffeine and nicotine on spontaneous locomotor activity in a tunnel maze was determined in nicotine-naive and nicotine-tolerant rats. Rats were daily injected subcutaneously for 12 days with nicotine (0.4 mg/kg) to induce nicotine tolerance. Nicotine-naive rats were injected with saline. During the next two days, they were exposed to a tunnel maze for two 6-min trials. On the third day, locomotor activity was measured (30-min trial) in the tunnel maze 15 minutes after subcutaneous injection of saline, nicotine (0.2 mg/kg), caffeine (8 mg/kg), or nicotine (0.2 mg/kg) and caffeine (8 mg/kg) in combination. Acute exposure to nicotine decreased locomotor activity in nicotine-naive rats. This decrease was antagonized by simultaneous injection of caffeine. Chronic nicotine exposure induced the development of tolerance to the acute behavioral depressive effects of nicotine. In nicotine-tolerant rats, caffeine and nicotine in combination significantly increased locomotor activity above saline level, whereas given alone they had no significant stimulant effect. Neither chronic nicotine treatment nor acute drug treatments affected exploratory efficiency of rats.