While dopamine has no impact on liking reactions, serotonin decreases both liking and disliking reactions. When given drugs that increase serotonin levels, rats smack their lips less to good food and shake their heads less to bitter food. This is also what we would expect given the evolutionary origin of serotonin: serotonin is the satiation, things-are-okay-now, satisfaction chemical, designed to turn off valence responses.
