Researchers looking for treatments for Parkinson's disease, a movement disorder affecting millions, say they've found that transplanting neurons derived from human embryonic stem cells into rats in the laboratory can restore motor function, a finding that could lead to cell replacement therapy in human clinical trials.

Parkinson's disease is the result of the gradual loss of neurons in the human brain that produce dopamine, a neurologic chemical that, along with other functions, helps regulate and control movement.

There is no cure for the disease, and while drugs have been developed that can reduce the symptoms, they cannot slow the progression of the disease.

Human embryonic stem cells have been the focus of research as a possible source of new dopamine-producing cells, but have proven difficult to turn to the purpose.

Now scientists in Sweden say they've managed to push human embryonic stem cells into producing a new generation of dopamine-producing cells that, when transplanted into the brains of laboratory rats, behave like native dopamine cells.

"The study shows that the cells that we generate from stem cells, they function equally as well as the cells that we find in the brain," says Malin Parmar of the University of Lund's Department of Medicine, leader of a study published in the journal Cell Stem Cell.

Since the newly created cells display all the functions and properties of the dopamine neurons typically lost in Parkinson's disease, and stem cells represent a potentially unlimited supply, there is every reason to consider human clinical trials and applications, the researchers say.

In the rat brains, in which the existing dopamine-producing cells had been destroyed, the new cells created from stem cells were found to survive over the long term, and restored dopamine production in the animal's brains, with levels returning to normal within 5 months.

The dopamine cells also showed the ability to produce the required links to correct regions of the brain, and the resulting axons they create "meet the requirements for use in humans," the researcher report.

"This has been our goal for some time, and the next step is to produce the same cells under the necessary regulations for human use," Parmar says, noting that the study consitutes "strong preclinical support" for the use of dopamine-producing cells created by human embryonic stem cells.

Human trials for testing dopamine cells created from human stem cells could be in place within about 3 years, the researchers say.