Researchers have long known that the body can activate its own form of pain relief in response to painful stimuli. Now, UC San Francisco investigators have determined that, in rats, this long-lasting relief is produced by the brain's "reward" pathway -- the neural circuitry activated by drugs of abuse. In their study, published in the August 15 issue of Journal of Neuroscience, the investigators determined that, at its maximum, the pain relief was as potent as a high dose of morphine.
While various individual structures in the brain have been known to produce analgesia, or pain relief, when electrically stimulated or exposed to narcotic painkillers, the finding provides the first physiological evidence that pain itself elicits analgesia.
It also provides a surprising twist on the perceived workings of the neural circuitry associated with gratification, said the lead author of the study, Robert W. Gear, PhD, assistant clinical professor of Oral and Maxillofacial Surgery in the NIH Pain Center at UCSF.
"We're showing that something aversive -- exposure to a painful stimulus -- as well as exposure to drugs of abuse, stimulate the same reward circuit," said Gear, whose lab is directed by senior author Jon D. Levine, MD, PhD, a professor of Oral and Maxillofacial Surgery and Medicine and director of the NIH Pain Center.
"Our result casts new light on how to look at the key structure in the reward
pathway, the nucleus accumbens, and the role it plays in affirming certain
behaviors and thus motivating individuals to act in particular ways," said Gear.
The reward pathway is a neural network in the middle of the brain that prompts
good feelings in response to certain behaviors, such as relieving hunger,
quenching thirst or having sex, and it thereby reinforces these evolutionarily
important drives. However, the circuit also responds to drugs of abuse, such as
heroin, cocaine, amphetamine and nicotine, which seem to hijack the circuitry,
Contact: Jennifer O'Brien
University of California - San Francisco