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
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