Researchers have found in animal studies that the brain's reward chemical dopamine can trigger both desire and dread.
While scientists have for long known that dopamine motivates animals and people to seek positive rewards, this is the first time a study has shown that it can promote negative feelings like fear too.
Reported in The Journal of Neuroscience, the finding may help explain why dopamine dysfunction is implicated not only in drug addiction, which involves excessive desire, but in schizophrenia and some phobias, which involve excessive fear.
"This study changes our thinking about what dopamine does. There is a huge body of evidence out there to support the idea that dopamine mediates positive effects, like reward, happiness, and pleasure. This study says, it does do that, but it can also promote negative behaviors through actions in an adjacent brain area," said Dr. Howard Fields of the University of California, San Francisco, an expert unaffiliated with the study.
Led by Dr. Kent Berridge, a team of University of Michigan experts identified dopamine's dual effect on the nucleus accumbens, a brain region that motivates people and animals to seek out pleasurable rewards like food, sex, or drugs.
The researchers revealed that the same region is also involved in fear.
When the researchers inhibited dopamine's normal function, it prevented the nucleus accumbens neurons from inducing both rewarding and fearful behaviours.
Based on that observation, the researchers came to the conclusion that dopamine is important in both.
Berridge and colleagues had found in a previous study that a distance of only a few millimetres separated desire and dread functions in the nucleus accumbens, which is only about five millimetres long in humans.
Given the importance of dopamine in this brain structure, the researchers investigated its role in generating these functions in their latest study.
When dopamine was allowed to act normally, injection of a chemical to model normal signalling in the front of the nucleus accumbens caused rats to eat nearly three times as much as they normally do.
In contrast, injection of the chemical in the back of the nucleus accumbens caused rats to display fearful behaviour normally shown in response to a predator.
"It has always been assumed that discrete neurotransmitters might separate fear from desire, but this report shows that transmitters such as dopamine play a constant role and that the anatomy is providing for emotional discretion," said Dr. Peter Kalivas of the Medical University of South Carolina, who was unaffiliated with the study.
Berridge believes that disruptions of dopamine neurotransmission in one region of the nucleus accumbens may be a mechanism for pathological excesses of fear in disorders like schizophrenia, while disruptions in dopamine neurotransmission in an adjacent region may be a mechanism for excessive reward-seeking in conditions like addiction