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2009-01-02 10:33:51
Scientists say they have found physical evidence of brain differences which may drive "thrill-seekers" to act impulsively or dangerously.
A small study from Vanderbilt University in the US found the biggest "risk-takers" processed a brain "reward" chemical dopamine differently.
Scans spotted fewer "receptors" for the chemical on the cells which make it.
The Journal of Neuroscience study could help explain why some are vulnerable to drug abuse and other addictions.
Animal experiments have already shown that, like humans, some respond differently to novel environments - and those who explore them are more likely to self-administer cocaine when given the chance.
This behaviour is believed to be bound up in the activity of dopamine, a brain hormone which, among other things, can produce a sense of enjoyment connected with certain activities.
Dopamine-producing cells have an inbuilt self-regulating system which is supposed to stop them making too much of the hormone.
"Autoreceptors" on the surface of these cells respond to rising levels by cutting down production.
Rats which show more impulsive behaviour also have fewer of these autoreceptors, and the Vanderbilt scientists set out to see if this was also true in humans.
Free-spenders
They used PET scans to look at the level of dopamine autoreceptors in 34 healthy humans, who had also been quizzed to find out their personality type.
Just as in the animals, a propensity towards thrill-seeking, spending money freely, and spontaneity, could be linked to lower levels of autoreceptors.
Dr David Zald, who led the study, said: "We've found that the density of these dopamine autoreceptors is inversely related to an individuals interest in and desire for novel experiences.
"Our research suggests that in high novelty-seeking individuals the brain is less able to regulate dopamine, and this may lead these individuals to be particularly responsive to novel and rewarding situations that normally induce dopamine release."