From Science:
What’s behind the satisfaction we get from a tasty dessert? Researchers have long assumed it has something to do with the neurotransmitter dopamine. But a new study suggests that this so-called “pleasure molecule” isn’t necessary for us to enjoy that piece of cake after all.
Without the pleasurable kick dopamine is thought to provide, researchers assumed that people would be less inclined to get hooked on gambling or drugs. But recent evidence has called this theory into question. For example, mice lacking a type of receptor for dopamine still seek out morphine, suggesting that they find the drug rewarding even without dopamine signaling.
More here.