PAIN STIMULATES OPIATE BRAIN PATHWAYS

NEW YORK, Aug 16 (Reuters Health) -- Bodily injury can sometimes trigger the
release of the body's natural opiate "pain-killers." Now, researchers
believe that the nerve pathways involved in this response are the same ones
involved in addiction to drugs such as heroin.

"We're showing that something aversive -- exposure to a painful stimulus --
as well as exposure to drugs of abuse, stimulate the same reward circuit" in
the brain, explained study lead author Dr. Robert Gear of the National
Institutes of Health's Pain Center at the University of California, San
Francisco. His team's findings are published in the August 15th issue of The
Journal of Neuroscience.

Experts have long sought to identify the brain center responsible for the
body's opiate response to painful injury. This type of response is useful on
an evolutionary level, since it allows an injured individual a brief respite
from pain so that he or she could flee to safety.

Gear and colleagues focused their research on the nucleus accumbens -- a
center for "reward systems," located in the middle of the brain. Neural
pathways located in the nucleus accumbens provide us with positive
sensations when we do "good" (survival-affirming) acts, such as eating,
relieving thirst, or having sex. These networks have also been implicated in
addictions to various drugs. Drugs such as heroin, cocaine and nicotine
appear to co-opt neural pathways in the nucleus accumbens, offering us
"good" sensations when the drugs are taken, and painful ones (withdrawal)
when they are not.

In their studies in rats, the researchers subjected anesthetized or alert
rats to mild painful stimuli such as placing the rats' paws in hot water.
Rats exhibited typical pain-response gestures, such as opening their jaws or
paw withdrawal. These responses seemed to decrease in intensity with
increasing levels of injury -- indicating a protective release of natural
painkillers in the brain.

But which part of the brain? The authors repeated their test on rats whose
nucleus accumbens had been "switched off" temporarily via drug injection.
The result? "This (analgesic) effect was blocked," according to the authors.

The drugs used to block the natural-painkiller response worked to suppress
both natural opiates and dopamine, a chemical that carries messages between
brain cells. Until now, scientists had thought that dopamine responses only
occur after positive experiences, and not negative, painful stimuli.

The study may also shed some light on the processes underlying drug
addiction, since drug withdrawal is typically characterized by an increased
sensitivity to "irritable" stimuli like noise or minor injury. Gear suggests
in a statement, that "it may be that one of the reasons people stay addicted
is to avoid going through this unpleasant state of withdrawal."

SOURCE: The Journal of Neuroscience 1999;19:7175-7181.