When you get into the car to meet your lover for dinner, your brain’s reward center is likely flooded with dopamine, a hormone also linked to cravings for sugar, nicotine, and cocaine. This rush of dopamine motivates you to navigate traffic to preserve that special bond. However, if you’re just having dinner with a co-worker, that intense rush of dopamine can be reduced to a mere trickle, according to a recent study by neuroscientists at the University of Colorado Boulder.

“We actually discovered a biological signature of desire that helps explain why we want to be with some people more than others,” said senior author Zoe Donaldson, an associate professor of behavioral neuroscience at CU Boulder.

A study recently published in the journal Current BiologyIt is concentrated around meadow voles, which are among the 3-5% of mammals that form monogamous pairs.

Like humans, these furry, wide-eyed rodents tend to mate for long periods of time, share the same home, raise their young together, and experience a pain-like experience when they lose their mates.

By studying these, Donaldson aims to gain a new understanding of what goes on inside the human brain to make intimate relationships possible, and how we experience it neurochemically when those connections are broken. New research answers both questions and shows for the first time that the neurotransmitter dopamine plays a critical role in sustaining love.

“As humans, our entire social world is fundamentally defined by varying degrees of selective desire to interact with different people, whether it’s your romantic partner or your close friends,” Donaldson said. “This study shows that some people leave a unique chemical imprint in our brains that allows us to maintain these connections over time.”

How does love enlighten the brain?

For the study, Donaldson and his colleagues used cutting-edge neuroimaging technology to measure in real time what was happening in the brain as a vole tried to reach its mate. In one scenario, a Polish woman had to press a lever to open the door to her partner’s room. In another, she had to climb over the fence to reunite.

Meanwhile, a small fiber-optic sensor monitored millisecond by millisecond activity in the animal’s brain accumbens (the part of the brain responsible for motivating people to seek rewards, from water and food to medications). (Human neuroimaging studies have shown that the nucleus accumbens lights up when we hold our partner’s hand).

First author Ann Pierce, who worked on the research as a graduate student in Donaldson’s lab, explained that the sensor “lit up like a glow stick” when it detected a dopamine release. When voles press a lever or climb over a wall to see their mates, the fiber “shines like a frenzy,” he said. And the party continued as they hugged and smelled each other.

Conversely, the glow bar goes out when there is a random field mouse on the other side of the door or wall.

“This suggests that dopamine is really important in motivating us to seek out a partner, but also that more dopamine passes through our reward center when we are with our partner than when we are with a stranger,” Pierce said.

Hope for broken hearts

In another experiment, a pair of voles were kept apart for four weeks (about an eternity in a rodent’s lifespan), long enough for the voles to find another mate in the wild. When they reunited, they remembered each other, but their characteristic dopamine rush had almost disappeared. In fact, this trace of desire has disappeared. When it came to their brains, their ex-mates were no different than any other field mouse.

“We see this as a kind of brain reset that allows the animal to continue working and potentially make a new connection,” Donaldson said.

This may be good news for people going through a painful breakup or even losing a spouse; This suggests that the brain has an internal defense mechanism against endless unrequited love.

The authors emphasize that more research is needed to determine how well the findings in voles translate to their larger-brained bipedal counterparts. But they believe their work could have important implications for people who have difficulty forming close relationships or coping with loss, a condition known as prolonged grief disorder.

“We hope that by understanding what healthy connections look like in the brain, we can begin to identify new treatments that will help many people with mental illnesses that impact their social world,” Donaldson said.