Nov. 18, 1999 (Indianapolis) -- Current treatments for chronic pain may be ineffective or have unwanted side effects. Research in rats, published in the Nov. 19 issue of the journal Science, shows that pain can be relieved by the use of "smart bombs" that target and destroy only those nerve cells that send pain messages to the brain.
Chronic pain appears to be the result of signals generated by a small group of nerve cells, or neurons, in the spinal cord. They communicate with each other using the chemical messenger known as Substance P (SP). When SP binds with the receptor, the whole molecule is "swallowed" and taken into the neuron. The researchers took advantage of this by linking a potent chemical that kills neurons, a neurotoxin, to SP and having it "hitchhike" into the cell.
"Current methods of treatment for chronic pain have major side effects such as the drowsiness brought about by medicines or the loss of feeling if nerves are cut," author Patrick W. Mantyh, PhD, of the University of Minnesota and the Minneapolis Veterans Affairs Medical Center tells WebMD in an interview. "Using SP to introduce a neurotoxin, allows us to specifically target those neurons that are causing the pain while still leaving sensation, touch, and other neurons intact. This means that the animal can still feel mild pain, touch and the other sensory inputs that serve as warnings of dangers in the environment," he says.
A combination of SP and the toxin was infused into the spinal cord of rats. Thirty days later, there was damage only to the neurons that had the SP receptor (SPR). To find out if this combination worked to block pain behaviors in the animals, they were given capsaicin, the active ingredient in pepper sprays used in self-defense. Thirty days after getting the SP/toxin infusion, there was a reduction in pain-induced behaviors, such as chewing on the painful area. The researchers noted that this treatment did not interfere with the action of morphine and other medications of the opioid class, meaning that they were still available to doctors treating minor pains.
"If further research proves that this concept works in humans, it would probably be reserved at first for very severe pain such as with cancers," says Mantyh. "The next step in research would be to find substances that can be combined with SP, but do not kill the neurons. If you could block the pain without the long-term effects seen with SP-SAP [SP/toxin], it would allow the treatment of many more kinds of pain."
Rollin Gallagher, MD, director of pain medicine at MCP/Hahnemann University in Philadelphia, thinks that this could potentially be a major advance in the treatment of chronic pain states. It suggests that you can be very specific in lessening pain without influencing normal responsiveness to possibly injury as other treatments can.
"This might give us a method to get rid of the chronic pain without interfering with the kind of pain receptors that let you know something is on your foot, or your hand is too close to the fire," says Gallagher, who was not involved in the study. "One of the promises is that you can use opioids off and on for breakthrough pain. So, it not only takes care of the chronic pain, but actually lets doctors use the other medications more effectively when needed."