WASHINGTON -- Researchers used embryonic stem cells to relieve symptoms of Parkinson's disease in rats, demonstrating the cells can be turned into neurons that make dopamine, a key brain chemical.
The researchers at Harvard Medical School and McLean Hospital in Belmont, Mass., showed in tests that the cells injected into rats whose brains had been chemically damaged would spontaneously convert to correct the Parkinson's symptoms.
Some experts said the study, appearing today in the Proceedings of the National Academy of Sciences, was significant because it showed embryonic stem cells could be used to treat brain disorders, but they cautioned that the cells also 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.
In the current study, researchers first developed rats that had Parkinson's disease symptoms by injecting into their brains a toxin that killed neurons.
The researchers then injected embryonic stem cells, extracted from an early mouse embryo and capable of growing into any type of cell.
About nine weeks after the stem cells were in the rat brains, they 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 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 magnetic resonance 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.