The researchers found that those who took the dopamine-boosting drug were better at predicting which choice would win more money, according to results to be presented at the American Academy of Neurology meeting in Boston next week. However, when they lost, those with the dopamine boost did not behave any differently than those taking a placebo. "Dopamine teaches you to get what you want but not to avoid what you fear," says Mathias Pessiglione, a neuroscientist now at the Salpetriere Hospital, in Paris, who led the study. "One can imagine that excessive levels of dompanine could create an imbalance between experiencing reward and experiencing punishment. That could explain the compulsive behavior observed with dopamine replacement therapy."

In this case, dopamine likely acts by boosting the cellular changes that underlie learning, but only in the positive condition. "Dopamine acts in [the] brain by reinforcing synapses that lead to rewarding behavior," says Pessiglione.

Further studies that will specifically examine Parkinson's patients are now under way or in the planning stages.For example, Weintraub intends to use brain-imaging technologies to look specifically at dopamine levels in the brains of Parkinson's patients with compulsive disorders, both while the patients take their medication and after they stop. Pessiglioneand his collaborators are now running similar tests on people with Parkinson's disease, as well as Tourette's syndrome and obsessive-compulsive disorders, both of which have been linked to dopamine dysfunction.

Scientists hope that such studies will explain why some patients are more at risk of developing these behaviors than others are, and that the studies will ultimately help clinicians when prescribing drugs for their patients.