Emory Report

June 22, 1998

 Volume 50, No. 34

Researcher Neill examines
dual brain reward systems

"Most people think that there's a single reward system in the brain, but I think there are at least two," said Darryl Neill, chairman of the psychology department. A recent recipient of a University Research Committee grant that facilitated data collection for a grant from the National Science Foundation, Neill is conducting intracranial self-stimulation (ICSS) experiments with rats to test his hypothesis.

ICSS is a phenomenon in which an animal performs some response, such as pressing a bar, to deliver a brief burst of electrical stimulation to a brain site. Neill's investigations are based on findings dating from the 1950s that rats and other animals, including humans, like to self-stimulate certain sites in their brains.

In ICSS experiments, rats often deliver as many as 6,000 bar presses per hour. "The rats will go for about 18 hours straight before they fall over from exhaustion," he said. "There's no satiation or loss of pleasure."

Research on the ICSS phenomenon since its discovery has led to the idea of so-called reward systems in the brain, Neill said. "The idea is that just as there are brain systems for vision and for movement and so forth, there are brain systems for aspects of emotion, which would include pleasure."

The idea of reward systems in the brain has been extended to common human problems, he said. "For instance, why do some people eat more than others? Maybe it's because food is much more pleasurable to them. Why do some people get hung up on drugs? A major hypothesis now is that drugs of abuse work by activating brain reward systems."

Neill's research is designed to show that a consummatory reward system in the brain is separate from an appetitive reward system. He believes that these two reward systems evolved for controlling different aspects of motivation.

Appetitive behaviors involve desire, while consummatory behaviors involve immediate gratification, Neill explained. He used the example of "rat courtship" to explain the difference between appetitive and consummatory behaviors.

"The female rats wiggle their ears and do some jumping from side to side, and this gets male rats all excited. Male rats, on the other hand, have to pursue the female rats," he said, noting that this would be appetitive behavior. "Once mating actually starts, that would be consummatory behavior."

According to Neill's research, consummatory behaviors are controlled by the hypothalamus, while appetitive behaviors are controlled by the ventral tegmental area (VTA) of the brain. The VTA contains nerve cells that release the neurotransmitter dopamine, a chemical believed to be the basis of the pleasure in many abused drugs.

Neill's research may apply to the study of depression as well as drug abuse. When described in behavioral terms, depression is often characterized as a motivational dysfunction. One symptom of severe depression is "anhedonia," which means a loss of the appreciation of pleasure. "Understanding of these two functionally different brain reward systems may have relevance to understanding subtypes of depression," Neill said.

"My interest is in the functional organization of mammalian brains-how brains are put together to produce behavior," Neill said, noting that his major interest is these reward systems. While this particular project concerns brain stimulation reward, for the last few years he's also been studying where cocaine works in the brain.

Neill noted that rats like all the drugs that get people into trouble, with one major exception: "Rats don't care for hallucinogens," he said. But "as far as mood-changing drugs, rats like all the ones people do."

-Linda Klein

Return to June 22, 1998 Contents Page