Embryonic stem cells injected into the brain corrected the symptoms of Parkinson's disease in rats by transforming into neurons that made dopamine, a key brain chemical.

In a study appearing Tuesday in the Proceedings of the National Academy of Sciences, researchers at Harvard Medical School and McLean Hospital in Belmont, Mass., showed in tests that in rats whose brains had been chemically damaged, embryonic stem would spontaneously convert to correct the Parkinson's symptoms.

Some experts said the study was significant because it showed embryonic stem cells may be used to treat brain disorders, but they cautioned that the cells also possibly could cause tumors.

Dr. Ole Isacson, senior author of the study, said that if further experiments are successful, there could be human trials of the technique in about five years.

Federally funded research on human embryonic stem cells is limited because producing such cells requires the death of human embryos. President Bush last summer approved some such research, but limited it to cell colonies that already exist _ about 60 cell lines.

Many researchers believe that embryonic stem cells hold the promise of creating new organs or cells to replace or renew ailing hearts, livers and other organs. Some earlier laboratory studies have suggested that embryonic stem cells could be directed to transform into curative cells for such diseases as Alzheimer's, Parkinson's or diabetes.

In the current study, researchers demonstrated that the embryonic cells could spontaneously correct some symptoms of Parkinson's.

To conduct the study, scientists first caused rats to develop Parkinson's symptoms by injecting into their brains a toxin that killed neurons that typically die during the course of that disease.

The researchers then injected embryonic stem cells that had been extracted from early mouse embryos and were capable of growing into any type of cell.

About nine weeks after the injections, the embryonic stem cells converted to neurons that make dopamine, a brain chemical lacking in Parkinson's disease patients, Isacson said.

The injected stem cells, said Isacson, grew into the type of neurons that typically die in the brains of Parkinson's patients.

One of the symptoms the Parkinson's rats had was a tendency to turn aimlessly in their cages, up to 10 times a minute, after they had been injected with amphetamine. Nine weeks after the stem cell injections, Isacson said, the rats' tendency to turn was stopped.

The researchers also conducted imaging tests and found that blood flow was restored to parts of the brains that had died from the Parkinson's effect.

Dr. Arlene Y. Chiu of the National Institute of Neurological Disorders and Stroke said Isacson's study was an important advance because it showed that embryonic stem cells will grow into specific neurons in the brain.

But she noted that five of the 19 animals used in the study also developed tumors and cautioned that this was a problem that must be solved before the technique could be used on humans.

"One of the great fears about using undifferentiated stem cells is that they will develop tumors," said Chiu.

She said Isacson reduced this problem by injecting only about 1,000 stem cells into each of the test animals. In some earlier studies, researchers injected more than 100,000 cells and many test animals developed tumors.