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Some of us feel our pain more, scientists say:

The same mechanism that let a stone-age hunter sprint away from a lion despite a sprained ankle may help 21st century scientists find better treatments for pain, researchers said on Sunday.

They described research that is aimed at nailing down genetic differences not only in the response to pain, but the actual perception of pain.

So far they have found not only that people vary greatly in how they feel pain, but that men and women seem to have different mechanisms for it.

“If I can figure out the differences, maybe we can make therapies that will work better,'' Jeffrey Mogil of the department of psychology at the University of Illinois in Urbana-Champaign told a news conference.

His studies show that 50 percent of the differences in feeling pain are genetic. Mogil's team has bred mice that are extremely sensitive to pain or that are virtually resistant to it. ``The differences are just heroic,'' he said.

“This is a naturally occurring variation that if we could figure it out, we could exploit,'' Mogil added.

He suggested that perhaps gene therapy could be used to treat pain, or that genetic tests could tell doctors in advance which patients will respond best to a particular medication.

Mogil told a meeting of the American Association for the Advancement of Science that he has confirmed, in rats and mice, the long-held belief that males and females feel pain in different ways.

“Both feel pain, but they are responding differently, by activating different circuitry in the brain,'' he said.

He said the brain cells of males use a different receptor, or chemical doorway, to allow opiate drugs to work on brain cells than the brain cells of females do.

Evolution could explain this.  “We evolved as hunter- gatherers,'' he said. “Perhaps men are more adapted to trauma, you know, lion bites and spear points.''

With rats, researchers use objective ways to measure pain. They can put a rat on a hot plate, for example, and warm it up until the rat shakes a paw.

But humans consciously control their responses to pain.

Catherine Bushnell of McGill University in Montreal described how she objectively measures pain by using functional magnetic resonance imaging (FMRI) or positron emission tomography (PET) scans to trace the areas of the brain being activated.

She confirms in people what Mogil has seen in mice -- that there is a huge variation in the way people experience pain.

Pain lights up two main areas of the brain -- the somatosensory region and the limbic region, which is linked to emotion and believed to reflect a person's emotional response to whatever pain is being felt.

And the brain can re-wire itself to make sure pain gets felt.

Bushnell said researchers who have tried to surgically block pain by cutting into pain circuits have been foiled when the brain simply redirects the pain signal, and the body's ability to develop resistance to even the highest doses of opiates is well-documented.

But Bushnell says very early research suggests that if pain itself cannot be blocked, perhaps the emotional trauma that it causes can be.

“When you do something as simple as using distraction to change a person's perception of pain ... there is a reduction in pain,'' she said. The MRI images show a correlating reduction.

Her team recruited volunteers and hooked them up to a mechanism that delivered a mild burning sensation similar to picking up a too-hot cup of coffee, and at the same time played them tones through headsets.

The volunteers were told either to pick out differences in the tones, or to say which of several burns felt worse.

“They rate the pain as more intense when they are paying attention to it,'' Bushnell said.

She said this could be an important message for both doctors and patients. “This doesn't make the pain disappear,'' she said -- but it does prove that people can sometimes control their own perception of the pain.

However scientists choose to attack pain in the future, they will target one organ -- the brain, said Alan Basbaum, head of the anatomy department at the University of California San Francisco.

“The pain's not in the spinal cord, the pain's not at the injury site -- the pain's in the brain,'' he said.


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