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It’s a gamble: dopamine levels tied to uncertainty of rewards

Posted on Friday, May. 7, 2004 — 11:17 AM

It’s a gamble: dopamine levels tied to uncertainty of rewards

NASHVILLE, Tenn. – Researchers, using a new combination of
techniques, have discovered that dopamine levels in our brains vary the
most in situations where we are unsure if we are going to be rewarded,
such as when we are gambling or playing the lottery.

The research results, "Dopamine Transmission in the Human Striatum
during Monetary Reward Tasks," were published online April 28 in the
Journal of Neuroscience.

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

"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, assistant professor of psychology at
Vanderbilt University.

Zald believes the primary significance of the study is the
possibilities it raises for future research on measuring what causes us
to experience reward from a variety of sources and what happens in our
brains when we are disappointed in our quest for those rewards. 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.

"We’re moving to a point where we can measure what’s happening to
people’s neurotransmitter systems in a way that we haven’t been able to
do before," he said.

Zald and his colleagues used positron emission topography (PET
scanners) to view brain activity in nine human research 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 to the receptors is 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 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.

Zald and his team found that over the course of the experiment,
dopamine transmission increased more in one part of the brain 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."

The data was collected in Montreal and analyzed in collaboration
with Gabriel Dichter at Vanderbilt; Isabelle Boileau and Alain Dagher
at McGill University, Montreal; Wael El-Dearedy at Liverpool John
Moores University, United Kingdom; Roger Gunn at Glaxo SmithKline,
Greenford, United Kingdom; and Francis McGlone, Unilever Research,
Wirral, United Kingdom.

The research was supported by grants from Unilever Research and the
National Science Foundation. Zald is a member of the Vanderbilt Kennedy
Center for Research on Human Development.

Media contact: Melanie Catania, (615) 322-NEWS

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