Addicted to Rewards
Brain region that helps us learn good things also may help us learn bad
THURSDAY, Sept. 6, 2001 (HealthDayNews) -- The part of the brain babies use to learn to go potty may be the same part of the brain that gets them hooked on drugs, a study in rats suggests.
The common denominator between that unlikely pair, the researchers say, is the part of the brain that uses rewards to learn. The reward process, they say, involves a primitive part of the organ, the substantia nigra, located deep within the midbrain. This part of the brain also produces dopamine, a brain chemical associated with emotional response, pleasure and movement.
The substantia nigra is best known for its role in Parkinson's disease, says study author Jeffrey Wickens, an associate professor of anatomy and structural biology at the University of Otago Medical School in Dunedin, New Zealand. Parkinson's is marked by nerve damage that increasingly shows up as shakiness and other uncontrolled movement.
"In both humans and animals, loss of these nerve cells [in the substantia nigra] causes difficulty making intentional movements," Wickens says. "It becomes difficult to get the brain to tell the body to move."
Because of this, he says, scientists assumed that it was the cells in the substantia nigra that were telling the body to move. So, they looked at the substantia nigra in monkeys and rats when they were moving and measured the cells that produce dopamine.
To their surprise, however, the cells did not fire when the animal moved.
"Instead, they fired when the animal was given a reward -- such a piece of apple or a sip of sweet liquid -- when the rewards came as a surprise," Wickens explains. The findings appear in today's issue of Nature.
Another part of the brain may play a hand in all this.
"There is a neural pathway connecting the substantia nigra to another part of the brain called the striatum, and those may be the nerve cells [in the striatum] that tell the body to move," Wickens says. "While scientists knew that other parts of the brain could tell the striatum to make the body move, no one knew how dopamine affected the nerve cells in the striatum."
To see if dopamine were interacting with the striatum, Wickens and his colleagues taught rats to press a lever. The lever was connected to an electrode in the animals' brains that excited those parts of the substantia nigra that respond when the animals expect a reward.
"And even though the rats did not get any kind of reward for learning the behavior, nevertheless they pressed the lever to stimulate their substantia nigra dopamine neurons in preference to food or drink," Wickens says.
So what does the experiment tell us?
"We have shown the chemical reward [dopamine] signal strengthened the connections between the cerebral cortex, [the part of the brain responsible for higher functions, such as thinking and language], and the striatum," Wickens explains. "This firing activity may represent learning of new responses."
The strength of the connection between the cerebral cortex and the striatum was directly related to the amount of time it took the rats to learn how to press the lever, he adds.
But what might that have to do with potty training and addiction?
"Certain types of human learning involve repeated attempts which are reinforced by positive results," Wickens says. "These results tell us that activating the dopamine nerve cells is an effective way to promote learning."
"For instance, as infants develop, they learn many things by reward-related learning," he says. "Potty training may be one example, as when a child is given praise for responding to stimuli from a full bladder in time to make it to the pot. The praise is rewarding and strengthens the association between the stimuli and the socially acceptable response."
"In addiction, a substance may activate the same reward pathways directly," Wickens says. "The direct activation of these pathways bypasses the brain's usual control mechanisms and can lead to excessive and inappropriate release of dopamine. This can strengthen the association between the situation in which the drug is taken and drug-taking activity. When the situation occurs again, it can trigger the drug-taking activity."
David Wirtshafter, a professor of biopsychology at the University of Illinois at Chicago, calls the research "one of the clearer demonstrations to date that stimulating dopamine cells can produce changes in the way individual neurons and parts of the brain communicate."
"But while it's an interesting paper, it's not coming out of nowhere," Wirtshafter says. "Other people have suggested similar results to this."
The latest study is "compatible with the view that ties learning, memory, addiction and reward mechanisms together, but it doesn't prove it," he says. "We've still got a lot of work to do."
What To Do
If you're interested in learning more about the substantia nigra and Parkinson's disease, check out information provided by Brown University.