http://psychologytoday.com/articles/...000001&print=1

Since dopamine is also involved in learning, memory and motivation, the chemical helps us pay attention to the information we need to survive, act upon it, and remember it for the future. But drugs hijack that machinery, sending 5 to ten times as much dopamine surging through the nucleus accumbens and forcing the brain's motivational and attentional mechanisms to focus purely on the drug. It becomes the most interesting and important thing in the world. "In any addicted person, what's salient is the drug," says Volkow. "There's no competition."

Over time, the addict's brain adapts to the torrent of dopamine by dampening the system down. Imaging experiments show that cocaine addicts' brains don't react to the things that turn on the rest of us, whether that's romantic passion, food or cold, hard cash. Volkow's research has also shown that addicts have fewer dopamine D2 receptors, which are found in parts of the brain involved in motivation and reward behavior. With fewer receptors, the dopamine system is desensitized, and the now-understimulated addict needs more and more of the drug to feel anything at all. Meanwhile, pathways associated with other interesting stimuli are left idle and lose strength. The prefrontal cortex—the part of the brain associated with judgment and inhibitory control—also stops functioning normally. It's a neurological recipe for disaster. "You have enhanced motivation for the drug, and you have impaired prefrontal cortical systems. So you want the drugs pathologically, and you have reduced control of behavior, and what you've got is an addict," says University of Michigan, Ann Arbor psychology professor Terry Robinson, who pioneered this new way of thinking about dopamine with his University of Michigan colleague Kent Berridge.

Some people are apparently born with fewer dopamine receptors, and they are more likely to enjoy the rush of addictive drugs. In one imaging experiment, Volkow gave Ritalin, which gently lifts dopamine levels, to a group of ordinary volunteers. Some loved the feeling of the drug, but others hated it so much that they threatened to drop out of the study. Volkow was puzzled until she imaged their brains. She found that those who liked the rush from the drug had fewer dopamine D2 receptors than those who hated it. Volkow thinks that some people have a sensitive dopamine circuitry; they can't take the additional stimulation of drugs.

Obesity may involve similar malfunctions in the dopamine system. Volkow's longtime Brookhaven collaborator Gene-Jack Wang has discovered that the brains of seriously obese people seem to be tuned toward food. Even when they are lying quietly in the scanning machine, the sensory cortex of their mouth, tongue and lips is more active than it is in normal-weight people, he says: "They are putting out their antennae." Yet he also found that the dopamine circuitry of heavy people is less responsive, with fewer dopamine D2 receptors. Even among the obese, there are dopamine differences. The heaviest people in his study had fewer dopamine receptors than the lightest. Like addicts, overeaters may be compensating for a sluggish dopamine system by turning to the one thing that gets their neurons pumping.

It's a mark of changing times—and more sophisticated science—that the head of the National Institute on Drug Abuse is thinking about doughnuts as well as heroin. Just as blaming drug addiction on moral weakness was a shortsighted and unscientific way of framing a social problem, Volkow believes that focusing solely on metabolism, or blaming fat people for overindulgence and gluttony, are intellectual dead ends. "What motivates us to eat is clearly much more than hunger," she says. "We need to expand the way we think about eating." Wang and Volkow suggest that dopamine may provide a new window into weight loss: Animal studies have shown, for example, that exercise elevates dopamine release and increases dopamine D2 receptors.

Volkow and the other champions of the new view of dopamine don't deny that the chemical helps us register pleasure. But they think that pleasure is just part of a set of interconnected dopamine-related behaviors. Volkow recently found that adults with attention deficit disorder who took dopamine-boosting Ritalin before taking a math test found it easier to concentrate, in part, because the task seemed more interesting, so they felt more motivated to do the problem.

From this angle, it makes sense that the cognitive process of absorbing new information is closely tied to the brain's pleasure mechanisms. You might say that what the brain really "wants" is new information, suggests Gregory Berns, associate professor of psychiatry and behavioral sciences at Emory University in Atlanta. "Neurons really exist to process information. That's what neurons do. If you want to anthropomorphize neurons, you can say that they are happiest when they are processing information."

This urge to connect to the world and learn from it is more important than mere pleasure, says Volkow. It's part of the most basic force in behavior: the will to live. It's not automatic, she points out. Seriously ill or very depressed people can lose the will to survive. "What is the motivation we all have to be alive, to do things?" she asks. "It's not pleasure. Our lives would be so much simpler if we were motivated just for the sake of pleasure."

But dopamine sensitivity and addiction aren't genetically determined or inevitable. One experiment with monkeys showed that the dopamine system may be influenced by social interactions: Animals that lost social status also lost D2 receptors. Context is also crucial. Obviously, it's easier to get hooked if drugs are easy to get in your neighborhood, but it's not just a question of supply and demand. People who grow up in stimulating, engaging surroundings are protected against addiction, Volkow believes, even if they don't have a naturally responsive dopamine system. If you connect to the world in a meaningful way, and have more chances to get excited about natural stimuli, you're less likely to need an artificial boost.

"If you don't get excited by everyday things in life, if things look gray, and the drug makes things look extraordinary, that puts you at risk," she says. "But if you get great excitement out of a great multiplicity of things, and intensely enjoy these things—seeing a movie, or climbing a mountain—and then you try a drug, you'll think: What's the big deal?" For those lucky enough to grow up as Volkow did, surrounded by sharp minds and fascinating history, drugs are just nowhere near as interesting as everyday life.