Our result that dopamine was involved specifically in expression of ethanol reward can be interpreted in two main ways: first, that dopamine is required for retrieval of the conditioned odor-ethanol association, and second, that dopamine is involved with the conditioned motivational effects of alcohol cues without affecting cue memory retrieval. Although our results cannot directly distinguish between these hypotheses, they are consistent with the prediction-error hypothesis that supports involvement of dopamine in the motivational effects of alcohol predictive cues. In mammals, it has been proposed that dopamine is released when the actual value of a reward differs from its predicted value14,33. We found that activation of dopamine systems is required only when the conditioned odors are presented in the absence of the ethanol reward: i.e., when the flies may expect ethanol yet receive only odor cues. We therefore speculate that dopamine may be involved in the assessment of differences in actual and predicted reward in flies.
