By Sharon Begley

NEW YORK (Reuters) - When patients with Parkinson's disease received an injection described as an effective drug costing $1,500 per dose, their motor function improved significantly more than when they got one supposedly costing $100, scientists reported on Wednesday.

Underlining the power of expectations, the motor improvements, measured by a standard Parkinson's assessment, occurred even though both injections contained only saline and no active ingredients.

The research, said an editorial in the journal Neurology, which published it, "takes the study of placebo effect to a new dimension."

More and more studies have documented the power of placebos, in which patients experience an improvement in symptoms despite receiving sugar pills, sham surgery, or other intervention with no intrinsic therapeutic value. Placebo responses have been shown to alleviate pain, depression, and osteoarthritis, among other conditions.

That has posed challenges for drugmakers, since clinical trials typically pit an experimental compound against a placebo. When placebos are powerful, actual drugs often fail to best them.

Earlier studies have shown that patients' expectations can lead to improvements in Parkinson's, a progressive motor disease in which the brain's production of dopamine plummets.

As it happens, dopamine release is increased by belief, novelty, and the expectation of reward - mental states that underlie placebo effects, said neurologist Alberto Espay of the University of Cincinnati, who led the new study.

He and colleagues told 12 Parkinson's patients they would receive shots of two formulations of equal effectiveness of the same drug, getting the second after the first wore off. One cost $100 to manufacture, they were told; the other, $1,500.

In reality, both were saline.

When patients received the "$1,500" drug first, their motor function improved two-fold compared to cheap placebo, and 28 percent over baseline, but less than with the Parkinson's drug levodopa.

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Because the study deceived the 12 volunteers, usually a bioethics no-no, it received extra scrutiny from the review board that okays human research.

When the volunteers were debriefed, Espay said, eight said they expected the "expensive" drug to be more effective; they experienced the greatest improvement relative to the "cheap" injection. Four said they had no expectation of greater benefits, "and they showed little overall changes" in motor function, Espay said.

Ordinarily, a study with only a dozen people would not be published in a top journal. But because earlier studies, going back more than a decade, have also shown a placebo effect in Parkinson's, the research has greater credibility, experts said.

(Reporting by Sharon Begley; Editing by Nick Zieminski)