Mice that kill at the flip of a switch may reveal how hunting behavior evolved hundreds of millions of years ago.
The mice became aggressive predators when two sets of neurons in the amygdala were activated with laser light, a team reported Thursday in the journal Cell.
"The animals become very efficient in hunting," says Ivan de Araujo, an associate professor of psychiatry at Yale University and an associate fellow at The John B. Pierce Laboratory in New Haven. "They pursue the prey [a live cricket] faster and they are more capable of capturing and killing it."
Activating the neurons even caused the mice to attack inanimate objects, including sticks, bottle caps and an insectlike toy. "The animals intensively bite the toy and use their forepaws in an attempt to kill it," De Araujo says.
But the aggressive behavior is reserved for prey. Mice didn't attack each other, even when both sets of neurons were activated.
The results hint at how the brain changed hundreds of millions of years ago when the first animals with jaws began to appear. This new ability to pursue and kill prey "must have influenced the way the brain is wired up in a major way," De Araujo says.
Specifically, the brain needed to develop hunting circuits that would precisely coordinate the movements of a predator's jaw and neck. "This is a very complex and demanding task," De Araujo says.
The researchers expected to find these hunting circuits in mice because many mice kill and eat insects. And one species known as the killer mouse "basically feeds on live prey, including sometimes even other mice," De Araujo says.
Sure enough, the scientists found one set of neurons in the amygdala, a structure involved in emotion and motivation, that became active when a mouse was pursuing prey. They found a second set of neurons in the amygdala that became active when the animal was biting and killing.
Then the team used a technique called optogenetics to create mice in which both sets of neurons could be controlled using light from a laser. That gave the researchers "an on-off switch for either or both of the circuits," De Araujo says.
"When we stimulate [both sets of] neurons it is as if there is a prey in front of the animal," De Araujo says. "They assume the body posture and actions usually associated with real hunting."
Researchers have found evidence of similar hunting circuits in rats and other species, including humans, whose survival once depended on their ability to hunt and kill large animals.
RACHEL MARTIN, HOST:
If you're scared of mice, this next story is not going to help you get over that fear. We're going to hear about how lab mice can be turned into predatory killers with just the flip of a switch. NPR's Jon Hamilton reports on an experiment that let researchers take control of brain circuits that tell animals to hunt.
JON HAMILTON, BYLINE: Mice are often portrayed as meek creatures nibbling on crumbs, but Ivan de Araujo of Yale University says they have a dark side.
IVAN DE ARAUJO: There is, for example, one species of mouse that is known as the killer mouse that basically feeds on live prey including sometimes even other mice.
HAMILTON: And many mice kill and eat insects when they get the chance. De Araujo and a team of scientists found evidence that this hunting behavior was controlled by neurons in a part of the brain called the amygdala. De Araujo says the team used a technique called optogenetics to control the brain cells. The cells were triggered by light from a laser.
DE ARAUJO: When we stimulate these neurons, it is as if we were telling these neurons that there is a prey in front of the animal.
HAMILTON: The scientists put mice in a cage with a live cricket. Even without stimulation, the mice would eventually hunt and kill the insect. But De Araujo says when the laser went on, the rodents got serious.
DE ARAUJO: The animals become very efficient in hunting, so they pursue the prey faster, and they are more capable of capturing and killing the insects.
HAMILTON: Even more dramatic, though, is what happened when the scientists put an insect-like toy in the cage. When the laser was off, the mice would move away from the toy.
DE ARAUJO: But when the circuit is activated, then the animals intensively bite it and use their forepaws in an attempt to kill it.
HAMILTON: This behavior was just about hunting, the mice never attacked each other. De Araujo says the research helps explain how the brain evolved to hunt. The team located the hunting circuit in two sets of neurons - one that controlled muscles involved in pursuing prey, the other controlled jaw muscles. He says these circuits probably began evolving hundreds of millions of years ago, when the first animals with jaws started to appear.
DE ARAUJO: It must have influenced the way the brain is wired up in a major way.
HAMILTON: And now, De Araujo says, these hunting circuits exist in a wide range of animals including humans. The research appears in the journal Cell. Jon Hamilton, NPR News.
(SOUNDBITE OF KID LOCO'S "FLYIN' ON 747") Transcript provided by NPR, Copyright NPR.