The hedonic effects of amphetamine and pentobarbital in goldfish.

Goldfish were confined in a distinctive chamber while drugged with amphetamine in Experiment A or pentobarbital in Experiment P. During a later test, the goldfish in Experiment A showed a preference for the chamber associated with amphetamine, whereas those in Experiment P showed an aversion to the chamber associated with pentobarbital. Thus, amphetamine produced a rewarding effect while pentobarbital was aversive. The mechanism of pentobarbital's aversive effect is unknown. However, there is convincing evidence that amphetamine produces a rewarding effect in rats, monkeys and humans by increasing the synaptic concentration of dopamine in the central reward system. Since the goldfish brain has cells containing dopamine, the same mechanism is likely to be responsible for amphetamine's rewarding effect in goldfish. This similarity suggests that the central reward systems of such diverse species as goldfish, rats, monkeys, and humans have a common evolutionary origin.