WASHINGTON -- A biological "smart bomb" that combines a natural protein with a toxin is able to permanently block chronic pain without the side effects common to narcotic pain drugs, researchers report.
In laboratory studies using rats, researchers at the University of Minnesota in Minneapolis report they have identified nerve cells that cause chronic pain and devised a drug that knocks out those cells without affecting other nerve signals."What we've done is developed a molecular missile for chronic pain," said Michael L. Nichols, a neurology researcher at the university. "It targets specific cells and knocks out chronic pain without the side effects that you have with opiates."
Nichols is first author of a study to be published Friday in the journal Science.
Dr. Richard Payne, director of pain and palliative care at the Memorial Sloan-Kettering Cancer Center in New York, said the study "is promising" because it suggests that chronic pain can be controlled with drugs that work only on specific neurons.
Payne said doctors have always used drugs to control pain, but for some types of pain the medications don't work and "we've always wondered why." This study begins to explain some of that by identifying specific chronic pain neurons, he said.
Although the research has been done only in rats, Nichols said it eventually could provide relief for patients suffering from the pain of cancer and other illnesses.
The Society for Neuroscience estimates that about 100 million Americans endure chronic pain. Some have a hypersensitivity condition that can cause ordinary stimuli, such as hair brushing or hot showers, to be extremely painful. A common treatment now is to dull the chronic pain with narcotics, but these drugs also blunt other senses, often putting patients into a stupor.
Nichols said he and his colleagues discovered that chronic pain originates from fewer than 2 percent of the spinal cord neurons. They reasoned that if only these neurons could be blocked it would relieve the pain without side effects.
The researchers found that the neurons communicated pain using a neurotransmitter called substance P. This substance linked up only with the chronic pain neurons and did not affect the other nerves, said Nichols.
After isolating substance P, the researchers chemically attached to it a neurotoxin called saporin.
They then tied off leg nerves in a group of lab rats to induce a condition that mimics the chronic pain suffered by cancer patients.
The substance P molecule, with its neurotoxin, was then injected into the spinal area of the rats.
Within 45 days, said Nichols, tests showed the rats were completely relieved of the chronic pain but were fully responsive to other stimulus.
Nichols said standard tests showed the rats had a normal pain response and exhibited no signs of the hypersensitivity that is a prime symptom of chronic pain. Additionally, he said the animals responded normally to injections of morphine.
Tests conducted 200 days after the injection, said Nichols, showed the chronic pain did not return, suggesting the treatment may give permanent relief.
He said the technique works by shutting down, perhaps even killing, those few neurons that carry chronic pain signals. No other neurons are affected, said Nichols.
Though the technique appears promising, Nichols cautioned that before it can be tested on humans there must be more research to ensure that it is safe. He said the same tests performed on rats must be conducted in higher animals, such as monkeys.