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 (ScienceNOW, 10 January:). 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.
To investigate further, neurobiologists Thomas Hnasko, Bethany Sotak, and Richard Palmiter at the University of Washington in Seattle gave morphine to dopamine-deficient mice. Once the morphine wore off, the team let the animals roam freely between two chambers: one where they had received the morphine and another one. Dopamine-deficient mice given moderate to high levels of morphine favored the morphine-associated chamber, just like normal mice did, further evidence that dopamine isn't necessary to experience the rewards of morphine, the team reports 8 December in Nature.
Another set of experiments suggested that dopamine may be a motivator rather than a pleasure-giver. When both groups of mice received low levels of morphine, the dopamine-deficient mice showed no chamber preference, while the normal mice still preferred the morphine-associated chamber. But when the researchers restored dopamine levels in the dopamine-deficient mice after the mice had received a low dose of morphine, the rodents did show a preference for the morphine-associated chamber. To put it in human terms, this indicates that while dopamine may not be responsible for the enjoyment a vice such as chocolate gives us, it may help provide the craving that sends us back to the candy aisle.
"It is an excellent and exciting study," says neuroscientist Kent Berridge of the University of Michigan in Ann Arbor. The findings tell us that dopamine is not needed to have a rewarding experience, he says. Rather "dopamine seems more important to the motivation or 'wanting' for the reward."