`SMART' DRUG MAY BE ANSWER TO CHRONIC PAIN.

Byline: Paul Recer Associated Press

Chronic pain, a misery in the lives of 100 million Americans, may be controlled by a ``smart bomb'' drug that brings relief without the dulling side effects of narcotics, researchers report.

Researchers identified a specific group of neurons responsible for chronic pain and then designed a drug that shuts down those neurons without affecting other nerves.

``What we've done is developed a molecular missile for chronic pain,'' said Michael L. Nichols, a neurology researcher at the University of Minnesota in Minneapolis. Although the research has been done only in rats, he said it eventually could lead to relief for patients suffering from the pain of cancer and other illnesses.

Nichols is first author of a study to be published today 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 and potentially important 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 medications don't work for some types of pain.

``We've always wondered why,'' said Payne. ``This study begins to explain that. It is promising because it provides new information on the neurons'' that play a key role in chronic pain.

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 intractable pain without side effects.

The researchers found that the neurons communicated pain using a neurotransmitter called substance P. This substance links 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 felt normal pain, which is important for health, but 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 after 200 days, 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 to work well in rats, Nichols cautioned that before it can be tested on humans there must be more research, including tests on higher animals such as monkeys.

However, Nichols said that humans, like rats, have the same substance P neurotransmitter that relays pain signals in the nervous system.