Dr. Ronald Wiley and his research team are working to develop toxins that will halt pain signals from the brain. (photo by Dana Johnson)
Investigator eyes new ways to block pain
Imagine being able to zap chronic pain with a single injection.
Scientists at Vanderbilt University Medical Center are working to turn this idea into reality by developing toxins that kill only selected neurons — the ones that carry pain signals from the spinal cord to the brain.
The strategy is what Dr. Ronald G. Wiley, professor of Neurology and Pharmacology, calls "molecular neurosurgery." Rather than using surgical tools to cut neuronal highways, a neurologist could use these toxins to "cut" only the lane for pain signals. Without a way to get to the brain, pain signals would stall out in the spinal cord and eliminate the sensation of pain.
"We have a terrible time treating patients with chronic pain. It can be nearly impossible to provide relief without side effects that sacrifice quality of life," said Wiley, who is also chief of Neurology at the Nashville VA Medical Center.
Wiley recently participated in a symposium featuring advances in pain research and treatment at the Society for Neuroscience's annual meeting. The symposium was sponsored by the National Institute on Drug Abuse.
Wiley and his colleagues didn't start out looking for pain treatments. They were searching for a way to kill specific neurons, and they realized that the key was getting a toxin inside only the neurons they wanted to kill.
To make the first targeted toxin, they attached the plant toxin saporin to an antibody (192 IgG) that binds to a receptor on the surface of only certain neurons. The antibody acts as both guidance system and toxin smuggler: it seeks out neurons with the receptor, gains entry to the cell, and releases its harmful cargo inside. Saporin kills by bringing protein synthesis to a screeching halt.
"It was one of those rare moments in research when you design a tool and it really works," Wiley said. "The 192 IgG-saporin selectively destroys cells of the cholinergic basal forebrain. Since degeneration of these cells is a prominent feature of Alzheimer's disease, this immunotoxin has been a very useful research tool."
Wiley was looking for other gateways into neurons when a group of researchers reported that the molecule substance P and its receptor are internalized, or taken inside the cell. He jumped at the chance to smuggle toxin inside a new set of neurons.
Substance P is released in the spinal cord by neurons carrying incoming pain signals. It binds to receptors on the surfaces of the next neurons in the pain pathway, those that send the message onward to the brain. To kill these neurons, Wiley and Douglas A. Lappi, Ph.D., of Advanced Targeting Systems in San Diego, created substance P-saporin.
When it is injected into the spinal cord of rats, substance P-saporin kills pain signaling neurons and alters pain responsiveness. Toxin-treated rats respond normally to the mild pain experienced on a heated surface. But they do not respond at all to efforts to cause increased pain sensitivity (hyperalgesia).
"The substance P-saporin ablates the ability to develop hyperalgesia, which we think is the basis of the most clinically difficult kinds of pain, like the pain from nerve injuries and diseases of the nervous system," Wiley said. "It's probably also the pain that's so difficult with persistent sources of pain like arthritis and cancer."
Wiley and colleagues have recently developed pain sensitivity tests that they believe more accurately reflect clinical pain.
"Pain research to this point has relied upon animal models that primarily use innate reflex behaviors; behaviors that don't necessarily require a brain," Wiley said. "If you work only with reflex measures, you run the risk of not getting information really pertinent to the clinical problem."
With Charles J. Vierck, Ph.D. at the University of Florida, Wiley developed what is called an operant/learned behavior test of pain sensitivity. The test utilizes a two-sided chamber: one side is dark with a heated floor; the other side is brightly lit and room temperature. To begin the experiment, a rat is placed in the dark side of the chamber.
As the temperature of the floor climbs, the rat has the option of staying in the dark (which rats prefer) and tolerating the heated floor, or of escaping to the bright side of the chamber.
"The rats learn to pick and choose where they spend their time, and it's always a conscious choice: how uncomfortable is the dark part of the chamber that they'd rather put up with the bright light on the other side," Wiley said.
When the floor is heated to low temperatures, rats tolerate it for several minutes before escaping to the bright side. Both toxin-treated and control animals move after similar lengths of time. When hyperalgesia is induced, control animals escape much more quickly — they are more sensitive to the heated floor. Toxin-treated rats do not develop hyperalgesia, and they show no change in escape behavior.
Wiley is pleased that the operant behavior test confirmed the earlier results.
"I have a lot more confidence that in the escape test we are actually measuring the unpleasantness of the experience, and the toxin is making it less unpleasant, less painful," Wiley said.
The idea that substance P-saporin might combat chronic pain in human beings will be tested in primates before moving into clinical trials with humans. And substance P-saporin is just the beginning.
Wiley and colleagues recently developed dermorphin-saporin to kill neurons with the mu opiate receptor, the site of action for the widely used painkiller morphine. After killing these neurons, they will study how morphine works to relieve pain and assess whether this toxin might also be a treatment option for severe and chronic pain.
Wiley is excited about where things are headed.
"These toxins have novel, previously never available, therapeutic potential, and as tools, they are a big step forward in our ability to study the neurobiology of pain. In the next few years, they will extend our understanding of how the pain system works," Wiley said.
"In terms of this overall approach, I think we're only seeing the tip of the iceberg. This is a targeting strategy. I don't think we have to limit ourselves to delivering toxins. Who knows, perhaps this approach can be used to deliver genes or other molecules into specific cells."