Risky rats gambled on the iffier lever more than half the time. Risk-averse rats were strongly influenced by their last choice; if they picked the risky lever and received a trickle, they picked the consistent lever next time.

“Some are very sensitive to losing, and if they take a risky option and lose, they’re very likely to not go back to it again,” said Paul Phillips, a professor of psychiatry and pharmacology at the University of Washington and a co-author of a commentary about the study. “That’s very common in human behavior. An analogy is a slot machine in Vegas.”

To identify the brain location involved in these decisions, the researchers gave rats a drug used to treat Parkinson’s disease, pramipexole, marketed as Mirapex, which acts on D2 receptors and seems to dampen some patients’ ability to restrain risk-seeking behavior. Risk-averse rats receiving pramipexole turned into risk-taking rats, but the drug had much greater effects when piped directly into the nucleus accumbens than when it was administered to another brain area researchers had thought might be involved.

The scientists used a technique Dr. Deisseroth helped invent fiber photometry, which uses light particles to track activity of neurons tagged with certain proteins. They found that neurons in the nucleus accumbens with D2 receptors transmitted a signal when rats were making their decisions. That signal was much larger if the choice the rat had made had just had been a loser, yielding just a dribble of sucrose. The signal only spiked in non-risky rats, however; it was negligible in rats that always gambled for the sucrose windfall.

So, what to do with those risky rats? Using optogenetics, which Dr. Deisseroth also helped develop, the team stimulated nucleus accumbens neurons with D2 receptors at the very moment of the fateful food-lever decision. That caused the receptors to send strong loss signals to the rats, apparently making them weigh recent losses more heavily, and prompting them to play it safe with their next lever choice.

“It turns out you can explain a large part of whether rats were risky or not by this particular signal at this particular time,” Dr. Deisseroth said. “We saw it happen, and then we were able to provide that signal, and then see that we could drive the behavior causally.”