PAIN PERCEPTION CAN VARY ENORMOUSLY AMONG INDIVIDUALS: One person's sore arm is another's debilitating anguish. But don't assume that sufferers should just "put up and shut up." Their pain sensitivity could be genetic.

Researchers at Johns Hopkins University and the National Institute on Drug Abuse say they have discovered variations in a single gene responsible for the molecule that binds with the body's own painkilling chemicals. They report in the July 16 issue of Proceedings of the National Academy of Sciences that those variations are probably responsible for differences in pain sensitivity. And because the molecule also binds with morphine, the gene variations could explain why the level of relief from the same dose of morphine differs so much from patient to patient.

The researchers studied the so-called mu opiate receptor gene in different strains of mice and discovered differences in the number of opiate receptors. Furthermore, the quantity of receptors predicted how the mice would respond to a mildly painful stimulus: The fewer the receptors, the more pain the lab animals felt. Also, the mice with sparse receptors required more morphine.

After the mouse studies, the scientists turned to human subjects. They found that the number of mu receptors can differ dramatically among individuals as well. "People have long been skeptical that pain has a genetic basis," says Johns Hopkins neurologist Dr. George R. Uhl. The new finding, he says, could result in painkillers that are tailored to genetic sensitivities.

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