新托福阅读材料：DopamineDeterminesImpulsiveBehavior

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　人类冲动的行为往往会导致一些无法预料的后果，人为什么会冲动呢?最新一项研究表明这些行为与神经传递素多巴胺有关。  
$ P, k# |6 M0 v" p　　Binge-shoppers and serial daters might perpetually be living at the whim of their latest impulse, and now research is getting to the biological basis of their seemingly random behavior.

& H& N3 j1 |6 O' a, G3 v$ \　　"Individuals vary widely in their capacity to deliberate on the potential consequences of their choices before they act," note the authors of a new study on the impulsive tendency. "Highly impulsive people frequently make rash, destructive decisions."
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　　Impulsivity has long been linked to the neurotransmitter dopamine, which is involved in learning and reward. And a new model helps to illuminate the connection between the two. The work is described in a study published online July 29 in Science.

　　A team of researchers led by Joshua Buckholtz, a PhD candidate in neuroscience at Vanderbilt University, proposed that people who were more impulsive might have less active dopamine receptors in their midbrain but their brains would be more likely to fire off large quantities of the neurotransmitter when stimulated.
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　　To verify their hypothesis, the researchers used PET scans to watch the brains of 32 healthy and psychiatrically normal test subjects ages 18 to 35 (who had no history of substance abuse) while they were taking a classic test to measure impulsivity. Before the first testing round, subjects had taken a placebo pill, but before the second, they were given an oral dose of amphetamine, which can stimulate the brain's reward pathways, mobilizing dopamine.

! V; h8 i3 k4 X5 e　　People who had the higher impulsivity scores had the lowest activity in the midbrain D2/D3 autoreceptors, which are in charge of receiving dopamine. But under the influence of the amphetamine, these impulsive individuals released much more dopamine than those who were less impulsive.
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% V/ {. v; n$ R　　To see how these changes might be related to substance abuse—which has also been linked to dopamine abnormalities—the researchers polled the subjects about how much they wanted more of the amphetamine after the experiment ended.
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　　"The people who had the highest levels of dopamine release reported subjectively stronger cravings after we gave them the drug," Buckholtz says. These findings "suggest a neurobiological link between human impulsiveness and drug abuse vulnerability," the researchers noted in their paper.
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' R: O8 p( U$ ~! g( p　　But what causes these individual differences? "Our best guess is that perhaps there's some inherited or environmentally mediated predisposition to having lower midbrain dopamine autoreceptor availability," Buckholtz says.

　　The evidence for genetic inheritance is strong, and another recent study, published earlier this month in Psychological Science, found people with a certain dopamine receptor type—known as DRD4—had different drinking habits than those without it. Specifically, test subjects with this variant were more likely to drink heavily if they had seen others doing the same while those without that variant kept their drinking moderate even when surrounded by heavier boozers.
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　　The new results also suggest the potential for pharmacological interventions, Buckholtz notes. Some drugs for psychiatric conditions related to dopamine dysfunction, such as schizophrenia, work in broad strokes with "kind of a sledgehammer approach," he explains. Homing in on particular receptors and firing patterns might help develop drugs that could modulate in a "more targeted and perhaps nuanced way," he says, helping people with a broad range of dopamine-related ailments.