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Scientists have known for nearly a century that when you give mice access to their home cage as a reward, it boosts their ability to navigate through a maze.

“Going home is pretty much like biting into a nice piece of chocolate,” said Felix Mayer, Ph.D., a postdoctoral fellow at ��������’s Brain Institute. But how exactly that happened was unclear — until now.

In a new study published in the journal Neurochemistry International, Mayer and Randy Blakely, Ph.D., executive director of ��������’s Brain Institute and a professor of biomedical science in ��������’s Schmidt College of Medicine, have shown that when mice return home, it causes a surge of dopamine — a signal for pleasure. Surprisingly, the magnitude of this surge was comparable the response to a single dose of cocaine, a drug that is widely consumed for its rewarding effects.

To test mice brain chemistry, the two neuroscientists used a relatively new technique called fiber-photometry. It’s an imaging technique that collects and measures light in deep brain regions in real-time and it can be used on freely moving, living mice.

The researchers introduced a gene that makes a special protein into a brain region that is involved in the brain’s reward circuitry. The thing about this protein is that it is a sensor that emits light when it binds to dopamine. By inserting a hair-thin fiber into the mouse’s brain to catch the light signal and send it to an external receiver, they could measure second-to-second changes in dopamine across the course of the experiment.

"Going home is pretty much like biting into a nice piece of chocolate.”

— Felix Mayer, Ph.D., a postdoctoral fellow at ��������’s Brain Institute

Yet, it was serendipitous that the two neuroscientists made this discovery. They were testing the fiber-photometry technique for a completely different study, and it was Mayer who noticed the change in dopamine when moving the mice between experimental cage and home-cage. “This is an occasion where the unexpected turns out to be more important than perhaps what we were pursuing in the first place,” Blakely said. “In this particular case, it was nice because it really opened up a new area of research for us and for many others, potentially, which has to do with the role that dopamine plays signaling comfort.”

After realizing this discovery, the resulting study was fairly simple, he said. Mayer and Blakely documented the dopamine surge of mice in an unfamiliar, experimental cage compared to their own home-cage, with familiar bedding, water and smells. They compared this to the dopamine response to cocaine. In both cases, they found an increase in dopamine levels of about tenfold. “When the mouse goes home, the response is comparable to instant reward that it gets with cocaine,” Mayer said.

Ultimately, the scientists said they hope their work leads to better research and treatments for mood disorders, such as depression, autism and attention-deficit hyperactivity disorder, which have links to altered dopamine signaling. “We need better ways of testing our animals so that we can identify better medications,” Blakely said. “So many people don’t respond well enough to current medications for behavioral disorders. Some of this may simply be the context in which the tests are done? Maybe we’ve got a leg up on that. ������������