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Thursday, April 3, 2014

Why are drugs like Heroin so addictive?

The death of actor Philip Seymour Hoffman has raised many queries about drug addiction. Over the past 20 years, researcher has identified several chemical and physical changes to the brain brought on by addictive substances.

There’s a wad of nerve cells in the central part of your brain, measuring about half an inch across, called the nucleus accumbens. When you eat a doughnut, have sex or do something else your brain associates with anointing and breeding, this region is inundated with dopamine, a neurotransmitter.

This chemical transaction is partly responsible for the experience of pleasure you get from these activities.

Drugs such as Heroin also trigger this response, but the dopamine surge from drugs is faster and long-lasting.

When a person repeatedly subjects his nucleus accumbens to this narcotic-induced flood, the nerve cells that dopamine acts upon become exhausted. The brain reacts by dampening its response -- not just to Heroin or Cocaine, but probably to all forms of pleasure. In addition, some of the receptors themselves take place to die off.

As a result, hyper-stimulating drugs become the only way to trigger a palpable dopamine response. Drug addicts seek larger hits to achieve an ever-diminishing pleasure experience, and have unease feeling pleasure from the things healthy people enjoy.

Behavioral conditioning also plays a role. Once your brain becomes used to the idea that eating a doughnut or having sex will give pleasure, just seeing a doughnut or an attractive potential mate triggers the dopamine cascade.

That's part of the reason it is so difficult to recover drug addicts to stay clean. Sights, sounds and smells associated with the high -- needles, for example, or friends with whom they used to get high -- prime this dopamine response, and the inspiration to seek a drug hit builds.

Research suggests that the connection between these cues and the inspiration to seek a high strengthens over time in the brain of hardened addicts.
Peter Kalivas, a neuroscientist at the Medical University of South Carolina, has a laboratory full of rats addicted to heroin, cocaine, nicotine and other drugs. When he sounds a tone and flicks on a light, the rats know their next hit will soon be attainable. The more times the rat experiences the routine, the more efficiently a chemical signal is dispatched in the brain, solidifying the neural pathway between the cue and the desire for drugs.

While the drug-seeking pathway strengthens in the brain of addicted animals, their ability to make alternative pathways diminishes. Researchers refer to this as a loss of plasticity.

"Cues that are not coding directly for the drug cannot produce good plasticity in the brain of an addict," says Kalivas. "The system can't learn."

People long addicted to drugs accumulate a large number of cues that lead them to seek a high. Eventually, so much of their life becomes associated with getting high that it becomes nearly impossible for them to resist.

Some pharmaceuticals may help demote transmission along the neural pathway that leads from the cue to the craving for drugs. But until there is a medical solution, it helps to replace the negative voice in an addict's head with the supportive voices of friends and family. The plasticity of an addicted brain is diminished -- not eliminated.

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