Drop that bow, Cupid — scientists may have discovered a biological signature of desire that underlies strong interpersonal bonds and may explain why getting over an ex or mourning a loved one hurts so much.
The caveat: The research was in lovesick rodents, not in people.
In a new study, scientists looked at the brains of prairie voles (Microtus ochrogaster), a type of rodent that forms monogamous relationships. When the voles are separated from their partners and then reunited, their brains release dopamine in a key part of the brain's reward center, called the nucleus accumbens. More of the chemical messenger is released in this scenario than when they meet unfamiliar voles.
In other words, a vole's partner leaves a mark on its brain, according to the study, which was published Jan. 12 in the journal Current Biology. However, the researchers also found that, after a long separation, the partner no longer triggers that chemical spark.
"Why we want to be with some people more than we want to be with other people is quite literally a readout of dopamine release in your nucleus accumbens," co-senior study author Zoe Donaldson, an associate professor of molecular, cellular and developmental biology at the University of Colorado, Boulder, said in a video.
This long-lasting chemical imprint likely motivates animals to maintain bonds with their partners over time, Donaldson said, suggesting the findings in voles may also apply to humans.
Related: Is love at first sight real?
It's well established that dopamine release in the nucleus accumbens is involved in reward-seeking behavior. However, until now, it wasn't fully understood how dopamine signaling differs when animals interact with long-term partners, as opposed to strangers of the same species. This could provide hints as to how dopamine motivates animals — including humans — to spend more time with some peers over others.
To investigate this, the study authors separated vole couples and made them complete physical tasks, such as climbing over a fence or pressing a lever to open a see-through door, to get back to their partner. However, in some rounds of the experiment, they were presented with the same tasks to get to a vole they'd never seen before.
As the voles performed these tasks, the researchers measured how much dopamine was released in their nucleus accumbens, in real time using neuroimaging.
They found that more dopamine was released as the voles worked to reach their partners than when they were about to interact with a stranger. There was another dopamine surge when the voles eventually succeeded and interacted with their partner, but not when they met new voles.
In a separate experiment, the team separated the voles for four weeks — long enough that, in the wild, they'd likely find a new partner — to simulate the rodent equivalent of a break-up. When reunited with their former partners, the voles' dopamine signature had disappeared. The voles hadn't forgotten each other, as evidenced by the fact that they still spent longer huddling with each other than with strangers, but their brains effectively didn't distinguish their "ex" from any old vole.
This suggests that the brain has an inherent protective mechanism that enables it to move on and avoid long-term emotional suffering, the study authors claim.
"We think of this as sort of a reset within the brain that allows the animal to now go on and potentially form a new bond," Donaldson said in a statement.
More research is needed to see whether these findings in voles translate to humans. If they do, they could have implications for people who struggle to form close relationships or for those who struggle to recover after losing a loved one, Donaldson told The Guardian.
In the latter case, this can manifest as a condition called prolonged grief disorder. "It is possible that, for these people, their partner dopamine signal isn't adapting after loss, essentially stalling their processing of the loss," Donaldson told The Guardian. "A larger goal of my research is to identify ways to help those with prolonged grief disorder by identifying the biological changes that help them integrate a loss and re-engage with life."
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