April 27 (HealthDay News) -- Drugs that modify levels of a key brain chemical, called dopamine, also influence how people react to success and failure, British scientists say.

They measured the effects of these drugs -- often used to fight Parkinson's disease -- on the striatum, the part of the brain that's stimulated by rewards.

Dopamine is a naturally-produced chemical that transmits signals between nerve cells.

The study, by researchers at the Wellcome Trust Centre for Neuroimaging in London, included 39 healthy people, ages 18 to 39, divided into three groups. One group received levodopa, which increases dopamine levels in the brain. The second group received a dopamine receptor blocker called haloperidol (which decreases dopamine activity), while the third group received a placebo.

The participants were shown symbols associated with winning or losing different amounts of money. In order to "win" more money, the participants had to learn through trial and error which symbols represented winning or losing.

Those who took dopamine-boosting levodopa were 95 percent more likely than those who took haloperidol to select symbols associated with higher monetary gains, the study found. As a result, the participants who took levodopa won more money but didn't lose less money.

"The results show dopamine drives us to get what we want but not avoid what we fear," study author Mathias Pessiglione said in a prepared statement.

He said the findings may help improve understanding about the side effects of dopamine-related drugs and the disorders they're use to treat, such as schizophrenia and Parkinson's disease.

"This study may explain why dopamine depletion leads to the lack of motivation often described in people with Parkinson's disease and how dopamine replacement therapy can cause compulsive behaviors, such as overeating and gambling addictions, in the same people," Pessiglione said.

The study was expected to be presented at the American Academy of Neurology's annual meeting, in Boston, April 28 to May 5.