Oral caffeine consumption by rats: The role of flavor history, concentration, concurrent food, and an adenosine agonist

Abstract

Some determinants of caffeine consumption by rats were examined using the two-bottle choice test. To describe the role of flavor history, groups of eight rats each received one of three fluids as their only source of fluid beginning at 29 days of age and continuing throughout the experiments. One group (“water”) received tapwater, a second group (“caffeine”) received 0.5 mg/ml caffeine in tapwater, and a third group (“quinine”) received 0.01 mg/ml quinine in tapwater. Two-bottle choice tests began when rats were 40 days old. In the initial tests, caffeine rats drank more caffeinated water than water rats. Quinine rats were midway between these two groups. On a second block of tests, quinine and water rats' caffeine consumption increased so that the three groups were indistinguishable. When 0.5 mg/ml caffeine was available for 24 h, about one third of the total fluid consumption was of caffeinated water for all three groups. The presence of food greatly increased both caffeine and water consumption across a range of caffeine concentrations spanning 0.125–4.0 mg/ml. Increasing caffeine concentration generally increased consumption of plain water and decreased that of caffeinated water (but not total caffeine consumed) for water rats. Caffeine rats generally drank more caffeine than water rats, largely due to a tendency toward increased consumption of the 0.5-mg/ml concentration. Consumption of caffeinated water peaked at 0.5 mg/ml and showed graded decreases at higher and lower concentrations. Caffeine consumption showed dose-related increases with presession administration of l-phenylisopropyl adenosine. The series of experiments characterize some of the determinants of caffeine consumption in rats. Overall, a history of forced exposure to caffeine, the presence of food, caffeine concentration, and preadministration of an adenosine agonist all increase the consumption of caffeine. The present experiments also provide some guidelines as to what concentrations are consumed by rats and the maximum dose level likely to be achieved in tests of this kind. There is some evidence from the present experiments that caffeine consumption is related to caffeine's pharmacological properties, although the influence of flavor has not been eliminated.