Concise description of the neurobiology of addiction

In this Baltimore Sun article Dr. David Linden of Johns Hopkins University gives an unusually succinct description of the underlying neurobiology of the development of addiction. Note the interplay between genetics and intoxicant exposure: if you're brain isn't genetically vulnerable, you are not likely to become addicted to a particular drug. Note also that this vulnerability is substance-specific, not a generalized "addictive personality."
MW

Baltimore Sun

Johns Hopkins neuroscientist David Linden explains the biology of pleasure
With his new book, he seeks to find out why vices -- and even virtues -- can hold such sway over our lives

By Mary Carole McCauley, The Baltimore Sun

April 14, 2011


Not all addictions live up to their advance press.

In the past decade, it's become common to casually and humorously describe a favorite activity in the parlance of chemical dependency. People speak of being "addicted" to chocolate or high-fat foods, playing video games, buying expensive designer shoes, watching weekly episodes of "American Idol" to sleeping on high-thread-count sheets. But according to "The Compass of Pleasure," a new book by Johns Hopkins neuroscientist David Linden that is being released today by The Viking Press, just two of those pursuits -- eating fatty foods and shopping -- can become genuine addictions for some people. Watching television, playing World of Warcraft and swaddling yourself in Egyptian cotton probably cannot.

Who knew that feeling good could be so complicated?

"Addiction is defined by the changes that certain activities can make in the brain," Linden says.

"Basically, some activities have been shown to short-circuit the medial forebrain pleasure circuit. The process is the same, whether someone is taking crack cocaine or gambling or having risky sex or shopping. These morphological changes bring about a gradual transition from liking to wanting, and the result is compulsive behavior."

The vast majority of people who pursue habit-forming activities will never become compulsive. Our genes play a major role in determining who will become addicted, how badly and to what substance. It also generally takes repeated exposure for a harmful habit to develop.

For example, Linden points out that for even a drug as highly addictive as heroin, two of every three people who inject the narcotic directly into their veins don't become junkies.

In addition, some pursuits that may technically qualify as addictive carry positive benefits for individuals and society that vastly outweigh the potential harm, such as running a marathon, or an extended bout of contemplative prayer.

"Pleasure is our compass, no matter what we do," Linden says.

"But a philosophical question arises from these findings. If we catch a pleasure buzz from our noblest instincts, does that make them less noble?"

Conversely, just because a substance doesn't create a physical dependency doesn't mean it's safe. LSD might not activate the medial forebrain, but someone who drops acid and jumps off the roof because he thinks he can fly is likely to get hurt.

"There's a whole bunch of risk-taking behaviors that aren't addictive," Linden says. "We're motivated by reasons other than seeking pleasure."

Addiction, he says, is just one form of learning. The changes that occur inside the brain when someone studies calculus are nearly identical to the changes that occurs when he smokes crack.

As Linden explains it, when human beings bliss out by watching a sunset, getting a back rub or drinking a glass of red wine, the ventral tegmental area in our brains is releasing a neurotransmitter called dopamine that is getting picked up by a nearby bundle of neurons called the nucleus accumbens.

Under normal circumstances, pleasure flows and ebbs; dopamine gets released and then reabsorbed for later use. But some activities hijack the pleasure circuit, either by increasing the amount of dopamine flooding into our systems or by blocking the portals through which the chemical messenger goes back into storage.

The result is a jolt of pleasure so intense that most people will do anything to feel it again.

"Addiction is a super-potent experience," Linden says. And, just like Pavlov's dog, "we learn to associate it with sensory cues," he says, "because these associations allow us to predict how to behave so we can repeat the experience."

When we make a connection between, say, our sweet tooth and a chocolate store located in the Inner Harbor, our brains get rewired. After repeated exposure, structures on our neurons called "dendrites" grow new spines. (The same thing happens when we learn our multiplication tables or memorize a new route to work -- both experiences that humans experience as pleasurable, though to a lesser degree and by a more indirect pathway.)

In the future, every time our car turns onto Pratt Street, we may feel a sudden urge from out of the blue to stop at the chocolatier we know is just down the block.

"If you look at the neurons under a microscope," Linden says, "you will see that the receiving ends of their dendrites have turned into a shaggily bush of spines.

"This is how people develop cravings. If you're an addict, the more times you take a drug, the more spines you'll have on your dendrites and the harder it will be to stay clean, because everything you do will trigger those associations."

There's also another problem. As any chocoholic will confirm, no mouthful will ever deliver a burst of flavor as sublime as the very first taste. In scientific jargon, we've become "habituated."

The gradual draining away of pleasure is the process that Linden is talking about when he describes "liking turning into wanting."

"We imagine that addicts experience more pleasure from their drug of choice than others, and that this motivates their compulsive drug-seeking," he says.

"But the research shows that addicts get less pleasure from their drug than other people. The chemical contact between the neurons gets less efficient over time. It gets worn out. As a result, addicts need higher and higher doses to get as much pleasure as they received the first time they took the drug."

Linden acknowledges that one of his motivations for writing this book was political.

"The Compass of Pleasure" argues that as our laws ignore the biological basis of addiction and instead treat the problem as simply a matter of insufficient willpower that can be resolved by punishment.

"Once we understand how the biology of pleasure works," Linden says, "the only reasonable conclusion that we can make is that addiction is a disease. What we're left with is a compassion model, not a model that says that addicts are losers and should be locked up in jail."

He concludes that if addictions of all types rewire the brain, the eventual solution may lie in devising medicines that can bring about permanent changes on a molecular and cellular level that can undo the damage caused by too many doughnuts, cigarettes or trips to the casino.

There's no question that addicts attempting to kick their habit face an uphill battle -- but Linden isn't letting them off the hook.

"The development of an addiction is not the addict's fault," he says.

"But believing that addiction is a disease does not absolve addicts from responsibility for their own recovery. It's not a free ride."