Seeing a loved one, even virtually during this period of quarantine and social distancing, can make our faces light up with joy. But how does our brain light up when we see someone we care about? One area that may get excited is the nucleus accumbens, a pleasure-activated brain region critical for forming lifelong romantic relationships. However, finding monogamous mammals is challenging as it's the exception rather than the rule: only ∼10% of mammals commit to a single partner for life. Aside from humans, the prairie vole (Microtus ochrogaster) is another posterchild for committed relationships that can help us to explore how some brains are wired for devotion. As unattached prairie voles form lifelong pair bonds just as quickly in the lab as in the wild, Jennifer Scribner in the lab of Zoe Donaldson at the University of Colorado Boulder, USA, along with collaborators across the USA, recently explored how vole brains change as they're falling in love and have to choose between cuddling with their mate or a lurking Lothario.
First, Scribner's team needed a way to measure brain activity. To do so, they infused a virus into the nucleus accumbens region of the vole's brain that made cells shine brightly whenever calcium was released, which is a reliable proxy for neural activity. The researchers then introduced unattached voles into a chamber where they were free to mingle with two members of the opposite sex – but the date didn't provoke any changes in overall brain activity. Even after the team allowed two voles to bond for a week and then offered one a choice between an interloper and their new spouse, the nucleus accumbens did not light up differently in response to either their spouse or an interloper, even though the newlywed preferred to spend most of its time with its partner. While the nucleus accumbens is essential for forming a bond for life, the animals did not require a change in brain activity in that region to tie the knot.
Scribner then switched focus from the entire nucleus accumbens to single neurons in the structure to find out if any were tuned to their beloved. To do this, Scribner looked at where a vole was headed when choosing between its mate and a stranger and found two unique types of neuron: one that illuminated as a vole approached its partner and another that triggered as it neared a stranger. By counting individual cells, Scribner found increasing numbers of neurons active just before a vole approached its mate, far outnumbering the number of cells triggered en route to a stranger. Remarkably, the more time a vole spent mingling with its partner, the more partner-approach neurons appeared, suggesting that nucleus accumbens cells may become specialized for its mate to promote monogamous interactions.
Now, during the COVID-19 pandemic, more than ever, we appreciate how important social bonds and physical contact are. The findings by Scribner and colleagues in voles suggest that our brains and those of other monogamous mammals may be wired to adapt when falling in love.