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the species. In addition, humans respond to more complex rewards such
as scholarly or athletic achievement and social interaction.
Drugs such as cocaine hijack the reward system. Cocaine makes a
person “high” by stimulating the transmission normally involved in feel-
ing good for other, more legitimate reasons. But the price paid for these
temporary feelings is also high, so to speak. As users consume more of the
drug, the brain adapts to the altered circumstances, reducing dopamine
transmission in an effort to return to normal. This regulation may involve
the amount of dopamine released, the number of receptors, or both, and
plays a role in the response to psychiatric medications. Regulation also
strongly influences the brain's response to repeated drug use. Users find
they have to take more of the drug to get the same effects, a phenom-
enon known as tolerance. Another result of the adaptation occurs if users
abruptly stop consuming the drug, in which case the system becomes un-
balanced and users experience strongly negative feelings—these are with-
drawal symptoms. To avoid withdrawal, users continue the habit, even
though the drug may no longer make them feel good.
Drug abuse is a vicious, downward spiral, invariably leading to
disaster. While many people manage to avoid the trap and stay away
from these drugs, some people seem disposed to use and abuse these
substances, and they have great difficulty escaping. Some people never
escape. In order to understand why this happens, researchers are study-
ing how dopamine neurotransmission influences and rewards behavior.
Rats are often used in these studies—rats, like other animals, become
addicted when exposed to drugs such as cocaine and will perform tasks
to obtain the drug even as their health deteriorates.
Jeffrey W. Dalley, at the University of Cambridge in the United
Kingdom, along with his colleagues, studied rat behavior in combi-
nation with chemical analyses and PET. (PET works with laboratory
animals as well as with humans.) Of particular interest is an area of
the brain called the nucleus accumbens, a region deep in the brain that
receives a projection from neurons that use dopamine as a neurotrans-
mitter. Drug use significantly increases this transmission.
An important question is whether the tendency to engage impulsively
in risky behavior can predict who might be more susceptible to drug use.
Dalley and his colleagues found that rats which show reckless or impul-
sive behavior—for instance, choosing the instant gratification of a minor
reward for completing a task instead of a larger but delayed reward—are
susceptible to addiction and also have fewer dopamine receptors in the
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