Pubdate: Tue, 18 Feb 2014
Source: Washington Post (DC)
Copyright: 2014 The Washington Post Company
Contact: http://mapinc.org/url/mUgeOPdZ
Website: http://www.washingtonpost.com/
Details: http://www.mapinc.org/media/491
Author: Brian Palmer
Page: E4

WHY AN ADDICT CRAVES MORE AND MORE

The death of Philip Seymour Hoffman this month has raised many
questions about drug addiction, among them: What do drugs such as
heroin do to the brain to make them so addictive? Can these chemical
changes be undone?

Over the past 20 years, research into drug addiction has identified
several chemical and physical changes to the brain brought on by
addictive substances.

There is 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 that your brain
associates with survival 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 from stimulation.
The brain reacts by dampening its dopamine response - not just to
heroin or cocaine, but probably to all forms of pleasurable behavior.
In addition, some of the receptors themselves appear to die off. As a
result, hyper-stimulating drugs become the only way to trigger a
palpable dopamine response. Drug addicts seek larger and larger hits
to achieve an ever-diminishing pleasure experience, and they have
trouble feeling satisfaction from the things that healthy people enjoy.

Behavioral conditioning also plays a role. Once your brain becomes
accustomed to the idea that eating a doughnut or having sex will
provide pleasure, just seeing a doughnut or an attractive potential
mate triggers the dopamine cascade into the nucleus accumbens. That's
part of the reason it is so difficult for recovering drug addicts to
stay clean over the long term. Sights, sounds and smells associated
with the drug high - needles, for example, or the friends with whom
they used to get high - prime this dopamine response, and the
motivation to seek the big reward of a drug hit builds.

Recent research suggests that the connection between these cues and
the motivation to seek a high strengthens over time in the brain of a
hardened addict, and this effect has been measured in the form of
structural changes in the brains of laboratory animals.

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 that their next hit will soon become available.

This cue sets off a series of events in the rats' brains. An
electrical signal travels along a neuron, then uses a chemical
transmitter to make the jump to the next neuron. The more times the
rat experiences the routine - tone and light, followed by a hit of
drugs - the more efficiently the chemical signal is transmitted, thus
solidifying the neural pathway between the cue and the desire to seek
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 who are addicted to drugs for years accumulate a large number
of cues that lead them to seek out a high. Eventually, so much of
their life becomes associated with getting high that it becomes nearly
impossible for them to resist the urge. Going to work makes them think
of getting high. Watching television makes them think of getting high.
Finishing a meal makes them think of getting high.

The situation is not necessarily hopeless. Some pharmaceuticals are
being studied that may help degrade 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,
redirecting him from the desire to seek drugs. The plasticity of an
addicted brain is diminished, not eliminated.
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