One moment, a mouse nonchalantly shares a cage with a cricket; the next, the rodent leaps on the insect and rips its head off—all because a scientist flipped a switch. For the first time, researchers have hacked into the part of the brain that makes animals hunt, using lasers to target specific neurons. What’s more, they’ve found this hunting center in a surprising place: the region of the brain responsible for fear.
“This is really exciting stuff and it’s sort of out of the blue,” says Kay Tye, a neuroscientist at Massachusetts Institute of Technology in Cambridge who was not involved with the study. “How does this relate to fear or avoidance? It’s almost the opposite of hunting.”
Ivan de Araujo wasn’t initially interested in turning mice into maniacs. A neurobiologist at Yale University, he usually studies rodent feeding behavior in his lab. But a few years ago he came across a 2005 study that suggested the amygdala—a small, almond-shaped part of the brain linked to fear and anxiety—was active during hunting and feeding in rats. That seemed odd, because most research about the amygdala is focused on defensive or submissive emotions.
To further explore the connection, De Araujo and his team turned to optogentics, which stimulates neurons with laser light. In the past, researchers have used the technique in mice to do everything from altering their memories to making them feel thirsty, and De Araujo and colleagues wondered whether they could use it to make mice mimic specific hunting and eating behaviors. They didn’t necessarily expect that the rodent would perform an entire hunting sequence from start to finish: rear its neck, spot its prey, chase it down, grab it, sink its teeth into it, and deliver a lethal bite. But that’s exactly what happened.
Two pathways work in tandem to execute a hunt, the team found. One controls prey pursuit (PAG), and the other controls bite accuracy (PCRt). Targeting PAG with the laser made the mouse move faster or slower, and targeting PCRt made its bite weaker or stronger. When the scientists stimulated both at the same time, the mouse stopped in its tracks and hunted down almost anything it could find—crickets, woodchips, even bottle caps—and bit into it, the team reports today in Cell. “The central amygdala seems to be a center for organizing motor behavior … it has not been conceptualized this way before,” De Araujo says.
Still, activating PAG and PCRt doesn’t turn mice into unchecked killers. The rodents only went after small objects, not other mice. This suggests that other parts of the brain may be keeping the amygdala in check, De Araujo says.
As for why the fear and hunting center would be located in the same part of the brain, Tye suspects it may be because the two behaviors are related in the wild. When a mouse leaves its burrow to hunt, it also has to be concerned about predators, she says. “Like every good scientific discovery, this one raises a lot of questions. It’s raising a lot of questions about the amygdala, but also, how the brain works, really.”