Behavioral interactions between nicotine and caffeine.

Nicotine (0.01-1.0 mg/kg) was administered alone and together with three doses of caffeine (3.0, 10.0 and 30.0 mg/kg) to rats responding on a fixed-interval 2-min schedule of food reinforcement. The effects on overall response rate depended on dose: with the 3.0 mg/kg dose of caffeine response rate was increased by an amount approximately equal to the effect of the caffeine alone, while 10.0 mg/kg of caffeine reduced and 30.0 mg/kg completely abolished the increases in response rate produced by nicotine. The within-interval pattern of responding was affected in a similar, dose-related manner by both nicotine and caffeine alone. These effects added to produce large changes in the pattern when high doses of the two drugs were co-administered. The changes comprised increases in lower response rates in the early and middle parts of the interval and proportionally smaller increases or decreases in the higher rates occurring later in the interval. The interaction between nicotine and caffeine cannot be characterized simply, but depends on the particular aspect of behavior under examination.