Dopamine is a chemical messenger in the brain long known to play a role in feelings of desire. But strangely, dopamine also produces fear, according to new research. Reporter Eric Libby has details. 

Hiding is an antipredator response, produced by dread

The difference between fear and desire in rats is two millimeters.

That's because both of these feelings are generated in the same part of the brain, called the nucleus accumbens. Biopsychologist Kent Berridge explains that dopamine's ability to produce such dramatically different emotions is like a child at play. Although children may have a favorite playmate, they play differently depending on whether they are at school or at home. "And that's what dopamine is doing," Berridge says. "It's playing with glutamate signals, but it's playing differently to generate desire in the front of the nucleus accumbens and generating fear in the back."

Berridge and his colleagues at the University of Michigan altered levels of those 'playmates' – dopamine and glutamate – in the brains of rats. In their experiment, the rats received painless microinjections of a chemical that lowered glutamate signals either in the front or the back of the nucleus accumbens. Dopamine then acted as an on-off switch for emotion. If the researchers blocked it, there was no change in the rat's behavior; however, if dopamine was present, they observed either fear or desire.

But how do you observe feelings like desire in rats?

For one thing, Berridge says, the rats eat in a frenzy. "They throw themselves on food and they eat three to four times what they normally would." They also will repeatedly return to the place where they got the microinjection.

There are also specific observable behaviors when the microinjections provoke fear. The rats refuse to eat and show the same anti-predator responses reserved for scorpions and snakes. "They kick sand at them if there is sand to kick. They'll throw sand in the face and throw sand at us with these kicking movements if we're in the room." The rats also squeal when touched. The behaviors are, therefore, easy to distinguish.

Findings Have Implications for Humans 

The nucleus accumbens is a primitive area of the brain common to many different species, including humans. So these findings in rats should apply to us and a variety of our psychological disorders, as well.

Berridge observes, "It helps us understand how dopamine could promote really intense desires like the kind of desires that come up in drug addiction that become very compulsive and powerful, and yet also, in other people, produce very fearful and intense terrors." He adds that his work may also explain why people can flip between these intense feelings after taking a high dose of an addictive drug.

Berridge's work suggests that the brain uses the same basic ingredients to produce desire and fear, meaning these two powerful and very different human emotions may spring from the same biochemical well.