Caffeine reduces amphetamine-induced activity in asymmetrical interaction

Differences in the effect of chronic and acute caffeine on self-administration of cocaine in mice

We have compared the ability of an acute injection of caffeine (3 mg/kg, i.p.) and long-term peroral caffeine consumption for 10 days ( approximately 150 mg/kg/day in tap water) to affect cocaine self-administration in mice. The peak plasma and brain levels of caffeine and its metabolites were similar in the two experimental set-ups. Moreover, the levels reached are close to those obtained in humans upon coffee ingestion. Neither type of caffeine administration affected the reinforcing effect of cocaine, defined as a selective increase in nose-poke responses in mice self-administering cocaine compared to their yoked controls. However, caffeine injection increased the amount of cocaine self-administered whereas caffeine drinking reduced it. A low dose of cocaine, by itself essentially ineffective, produced an increase in c-fos and NGFI-A mRNA in the cerebral cortex in mice that had been drinking caffeine. An acute caffeine injection also enhanced the immediate early gene response to cocaine, but to a lesser degree. Cocaine and caffeine also synergistically increased NGFI-A expression in caudate-putamen. Thus, regular caffeine drinking decreased the cocaine intake without significantly affecting its reinforcing properties, perhaps because it enhanced the activation of the predominantly inhibitory frontal cortical areas produced by low doses of cocaine.

Aversive conditioning properties of caffeine in rats

Four experiments tested the conditioning effects of caffeine. Flavor and place cues were paired with IP caffeine injections and followed by tests for cue preference. In Experiment 1A, saccharin was paired with 1.25, 5 or 20 mg/kg of caffeine. In Experiment 1B, caffeine was delivered 30 min before, 5 min before, or 30 min after saccharin. Dose- and time-dependent conditioned taste aversions were produced. In Experiment 2, a place and taste cue were paired simultaneously with 5 or 20 mg/kg of caffeine. Conditioned place and taste aversions developed at 20, but not at 5 mg/kg. In Experiment 3, a place cue alone was paired with 0, 5, or 20 mg/kg of caffeine; dose-dependent conditioned place aversions developed. In Experiment 4, place and taste cues were paired with control treatments: pH-buffered caffeine, purine or vehicle. Caffeine produced taste aversions whereas the purine and vehicle did not. These aversive conditioning effects of caffeine across a variety of situations, doses and temporal arrangements stand in contrast to results obtained with other psychoactive drugs, such as amphetamine and alcohol.