Researchers may be one step closer to understanding dopamine’s roles in the striatum for perceptions of reward or disappointment associated with tasks involving a monetary reward such as gambling, according to an article published online April 28th by the Journal of Neuroscience.

Dopamine has long been known to play an important role in how people experience rewards from a variety of natural sources, including food and sex, as well as from drugs such as cocaine and heroin, but it has been difficult to pin down the precise conditions that cause neurotransmitter release.

“Using a combination of techniques, we were actually able to measure release of the dopamine neurotransmitter under natural conditions using monetary reward,” said David Zald, MD, lead author of the study.

The research lays a foundation for a better understanding of what happens in the brain during unpredictable reward situations such as gambling and offers promise for exploring the chemical foundation of problems such as gambling addiction.

In the current study, American researchers used positron emission topography to view brain activity in 9 human subjects who had been injected with a chemical that binds to dopamine receptors in the brain but is less able to bind when the brain is releasing dopamine. A decrease in binding of the chemical agent to the receptors is thus associated with an increase in dopamine release, while an increase in binding indicates reduced release of dopamine. This technique allows researchers to study the strength and location of dopamine release more precisely than has previously been possible.

The team studied the subjects under three different scenarios. Under the first scenario, the subject selected one of four cards and knew a monetary reward of $1.00 was possible but did not know when it would occur. During the second scenario, subjects knew they would receive a reward with every fourth card they selected. Under the third scenario, subjects chose cards but did not receive or expect any rewards.

Investigators found that over the course of the experiment, dopamine transmission increased more in the left medial caudate nucleus in the unpredictable first scenario, while showing decreases in neighboring regions. In contrast, the receipt of a reward under the predictable second scenario did not result in either significant increases or decreases in dopamine transmission.

“It’s probably not just the receipt of money, but the conditions under which it occurs which makes a difference,” Zald explained.

The increase and suppression were localized to specific, separate regions of the brain, illustrating that variable reward scenarios, like gambling, have a complex effect on the brain.

“The most interesting thing we found is that there were areas that showed increased dopamine release during the unpredictable condition, and there were also other areas showing decreased dopamine release,” Zald said.
“So other than just dopamine as reward, there is a more complicated action occurring.”