Drug Combo Blocks Pain Without Impairing Movement

Chili pepper compound plus lidocaine derivative anesthetizes without numbness in rat study.

By Steven Reinberg
HealthDay Reporter
Wednesday, October 3, 2007; 12:00 AM

Copyright © 2007 ScoutNews, LLC. All rights reserved.

WEDNESDAY, Oct. 3 (HealthDay News) -- A visit to the dentist could one day involve no pain and no numbness, Harvard researchers report.

In experiments with rats, the scientists combined capsaicin -- the ingredient that makes chili peppers hot -- and a drug called QX-314. This combination blocked pain-sensing neurons without the side effects of numbness or paralysis that typically accompany current pain relievers.

"We found a way to target local anesthetics to block only pain fibers," said lead researcher Dr. Clifford Woolf, the Richard Kitz Chair of Anesthesia Research at Harvard Medical School. "At present, any standard local anesthetic blocks pain fibers and fibers that produce numbness and paralysis. We have managed to block pain fibers without the numbness or paralysis."

QX-314 is a derivative of the lidocaine, a common local anesthetic. But QX-314 alone isn't able to get into cells to block electrical activity, and thus pain. When capsaicin is added, it opens the cell membranes of pain-sensing neurons and lets QX-314 into the cell.

One expert thinks the discovery could one day be used to treat some types of chronic pain.

Ways of suppressing localized pain haven't improved much since the advent of aspirin, explained Edwin W. McCleskey, scientific officer at the Howard Hughes Medical Institute and author of an accompanying journal editorial.

"It's basically aspirin and morphine," he said. "That's why a new strategy is big news."

"This has only been done in rats, so it is unclear that it is going to work on humans," McCleskey noted.

In the study, Woolf's team found that the combination of capsaicin and QX-314 blocked pain-sensing neurons without affecting other nerve cells.

In additional experiments, the researchers injected the drugs into the paws of rats and found that the animals could tolerate more heat than usual. They also tried the drug combination on the nerve that runs down the rat's hind leg. These rats did not show any signs of pain, and five of the six moved and behaved normally. This showed that the drugs could block pain without affecting the nerves that control movement.