How do we calculate the right time to take a risk? And why are some people (and dogs, and fish, and rats) bigger daredevils than others? Scientists working with rats say they’ve traced the answers back to a brain region called the nucleus accumbens. They published their findings this week in the journal Nature.
Animals and risk have a curious relationship. Scientists have tested the risk-taking behaviors of many species (including humans), and nearly all of them, when taken as a whole, are more conservative than they need to be. But within each species, there are individual animals who seem to throw care to the wind, and even the most conservative individuals take risks from time to time.
“Risky behavior has its moments where it’s valuable,” psychiatrist, bioengineer, and study co-author Karl Deisseroth said in a press statement. “As a species, we wouldn’t have come as far as we have without it.”
A little risk-taking is important to keep a species, and an individual, going. But, Deisseroth notes, a predilection for dangerous choices is a liability. “I’ve seen patients whose aberrantly high-risk-seeking activity resulted in accidents, addictions and social, financial or occupational failures that exposed them to a lot of harm and blame.”
The researchers were looking at the brain’s reward system, which uses hormones like dopamine to motivate us to seek out or avoid objects or experiences, from an angry boss to a cheeseburger. Inside your reward system, and the reward system of other animals, is a structure called the nucleus accumbens (NA). Your NA contains two categories of dopamine receptor cells called DR1 and DR2.
For this experiment, the researchers focused on DR2 cells. They implanted teeny-tiny optical fibers in the brains of lab rats, then taught the rats to gamble. (Fun fact: this is not the first time rats have learned to play the odds.)
Each rat was set up with a little game center equipped with a hole. When they felt like playing, the rats could poke their noses into the hole, which would trigger the appearance of two levers. Pulling one lever produced sugar water—the same amount every time, no matter what, like a steady paycheck. The other lever was more like a freelance career. Most of the time, pulling lever 2 yielded a little bit of sugar water, but every so often it would pay off with a much bigger helping. The rats could (and did) play the game 200 times a day.
As expected, about two-thirds of the rats repeatedly went for the dependable sugar water salary. The other third were bred-in-the-bone freelancers. Even after the researchers switched the levers, the rats kept to their preferences. But just like in the real world, some of the conservative rats occasionally went for the risky lever instead. If their risk paid off that first time, they’d keep taking the risk. If it didn’t, they’d go back to their steady sugar paycheck.
While the rats were gambling the day away, the researchers were watching their DR2 cells. They found that just before the conservative rats chose a level, DR2 activity spiked. When the scientists used the optical fibers to light up the risky rats’ DR2 cells, they became more risk-averse, but only as long as the fibers were lit. As soon as the light went off, they went back to their risky behavior.
Then the researchers gave the rats small doses of pramipexole, a Parkinson’s disease drug that is notorious for causing impulsive gambling in patients. Sure enough, once the drug was in their system, the salaried rats turned to the high-risk freelance life.
In other words, high DR2 activity in the nucleus accumbens kept conservative rats conservative. “It looks as though we have found a brain signal that, in most individuals, corresponds to a memory of a failed risky choice,” Deisseroth said. “It seems to represent the memory of that recent unfavorable outcome, manifested later at just the right time when it can, and does, modify an upcoming decision.”
“Humans and rats have similar brain structures involved,” said Karl Deisseroth, MD, PhD, professor of bioengineering and of psychiatry and behavioral sciences. “And we found that a drug known to increase risk preference in people had the same effect on the rats. So every indication is that these findings are relevant to humans.”