CHICAGO, March 14 (Xinhua) -- Studying rodents, researchers at the U.S. Washington University School of Medicine have shown they can block receptors in the brain responsible for the emotional components of pain and restore the animal's motivation.
The researchers injected a substance that causes inflammation in a paw of the rats in the study. To measure the emotional effects of that pain, the researchers used a rewarding task in which the animals could work for sugar as a way to measure motivation.
After being taught to push a lever to get sugar, most rats will keep pushing. In the experiments, the animals had to push the lever progressively more each time they wanted a pellet of sucrose.
"When the animals experienced pain, they were less motivated to work to obtain the reward," said first author Nicolas Massaly, an instructor in anesthesiology. "It's often the same for people in pain who don't get as much pleasure from daily activities they usually enjoy."
But when the rats with inflamed paws were treated with a compound to block kappa opioid receptors in their brains, the animals recovered the motivation to obtain the sugar, and pushed the lever as often as those who did not have inflamed paws.
The researchers used small animal positron emission tomography (PET) imaging to assess the activity of kappa opioid receptors in the animals' brains, and were able to demonstrate that when rats were in pain, their kappa opioid receptors were very active in a part of the brain called nucleus accumbens, which is linked to emotion.
The researchers dampened this kappa opioid receptor activity by blocking the release of a natural stimulator of kappa opioid receptors called dynorphin, which is produced in the brain and is kind of like the inverse of the endorphins released by activities such as exercise.
"By blocking dynorphin release, we were able to restore motivation in the animals despite the fact that we did not completely eliminate their pain," Massaly said.
The findings could lay the groundwork for developing new, less addictive approaches to pain treatment.
The researchers acknowledge it's a long journey from rodents to people. But they already have preliminary PET data from people, suggesting it may be possible to influence kappa opioid receptors and potentially prevent the sadness and lack of motivation that can accompany physical pain.
The findings were published on Wednesday in the journal Neuron.