Comfortably numb with new anesthesia method

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Comfortably numb with new anesthesia method

Approach tested in rats targets only pain-sensing cells, researchers

say

Reuters

Updated: 12:28 p.m. PT Oct 3, 2007

WASHINGTON - A new approach to anesthesia using the chemical that

gives chili peppers their kick promises an improved way to treat pain

in surgery, dentistry and childbirth, researchers said on Wednesday.

Current local anesthetics deaden all nerve cells and not just the

pain-sensing ones, causing temporary paralysis and numbness. That's

why dental patients after a root canal, for example, may leave their

dentist's office drooling, with a numb mouth and some muscles

temporarily paralyzed.

Now, researchers have found a way to target only the pain-sensing

nerve cells while avoiding the neurons responsible for muscle

movement or sensations such as touch.

They demonstrated the approach in rats and feel confident it will

also work in people.

They gave the rats injections containing capsaicin, the active

ingredient in hot peppers, and a derivative of the common local

anesthetic lidocaine. Working in concert, these chemicals targeted

pain-sensing neurons, stopping them from transmitting " ouch " signals

to the brain.

The rats were placed on an uncomfortable heat source and had their

paws pricked, but showed no signs of feeling pain and moved and

behaved normally. The injections took effect within half an hour, and

the pain relief lasted for several hours.

The first general anesthetic, ether, was introduced in 1846,

revolutionizing surgery. But not much has changed conceptually in

anesthesia in the past century or so.

Dr. Clifford Woolf of Massachusetts General Hospital, one of the

researchers in the study published in the journal Nature, said the

new approach could transform surgery as much as ether did in its day.

" I imagine it could expand to many operations, " Woolf said in a

telephone interview.

The researchers think this approach could be useful in dental

procedures like tooth extractions, knee surgery and other joint

operations, pain treatment for women during childbirth and

potentially for chronic pain.

A similar approach, they added, could stop itchiness from eczema,

poison ivy and other conditions.

" The pain sensing-neurons in rats and humans are close enough that

the same strategy should work, in principle, in humans, " added Bruce

Bean of Harvard Medical School, another of the researchers.

Woolf expressed optimism that the first tests on people could

begin " in two or three years. "

The two chemicals in the injections take advantage of a unique

characteristic of pain-sensing neurons to block their activity

without blocking signals from other nerve cells.

Lidocaine interferes with electric currents in all nerve cells. But

the lidocaine derivative used in this research, called QX-314, by

itself is unable to enter cell membranes to block their electrical

activity.

That's where the hot chili chemical came into play.

Capsaicin is capable of opening pores found only on the cell membrane

of pain-sensing nerve cells. With these pores opened by capsaicin,

the QX-314 can then enter the cell membrane and selectively block the

activity of the pain-sensing neurons while leaving alone other nerve

cells.