Friday, September 5, 2008

Neurobiology of Addiction

From Monte Basgall:

The same biological system that makes the body give in to drugs is tied to a sense of bodily well being, Duke pharmacology professor Cynthia Kuhn said in a Sept 4 lunchtime talk on "the neurobiology of addiction." That means "addiction is at its core a biological phenomenon" and "dopamine gets things started," she told a group in the Sanford Institute's Rhodes Conference Room.

Dopamine is the neurotransmitter that nerve cells release to "make us like things that are life sustaining," according to Kuhn. That chemical spreads the good word to other responsive nerve cells by activating their own dopamine receptors. But drugs like cocaine, marijuana, alcohol and heroin can hihjack the process by activating those same dopamine receptors. Biologically drug-prone individuals then become adapted to that external stimulation and digress from being "dependent" -- meaning their tissues only function normally in the drug's presence -- to becoming addicted.

With addiction's onset, those compulsive drug cravings grow so powerful that users are willing to endure increasingly negative consequences to acquire the increasing amounts needed to retain any sense of pleasure. "Animals and people will self administer drugs until they die," Kuhn said. "They will skip work and not take care of their children." The prefrontal cortex, a dopamine-targeted brain area involved in prioritizing behavior, begins sending out drug-seeking orders. Tests show lab animals on drugs begin acting impulsively, for example by pulling the "reward" handle before the light they are trained to respond to comes on.

And what about genetics? While Kuhn said there is not a single, overarching "addiction gene," researchers have identified a number of candidate genes involved in the drug addiction process.

Fortunately, addiction-prone individuals are in the minority. Only about 20 percent of animals tested display addictive behavior, and those also tend to be the most impulsive, Kuhn said. Other studies have found that addicts have fewer than normal dopamine receptors. In fact, the dopamine system itself is one or several potential targets for drug treatment. Animals whose dopamine responding nerve cells have been destroyed or disabled, for example, avoid taking drugs.

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