When love is in the air, what happens in the brain? For many years, biologists answered, “Oxytocin!” This small protein—only nine amino acids long—is sometimes called the “love hormone” because it is involved in pairing, maternal care and other positive, love-like social behaviors.
But recently, neuroscientists are reconsidering their opinion about oxytocin. Experiments with mice and other laboratory animals suggest that rather than acting as a trigger for prosocial behavior, the molecule may simply sharpen the perception of social cues so that the mice learn to direct their social behavior more precisely. “It turns out it’s not as simple and straightforward as ‘oxytocin equals love,'” he says Gul Dolen, a neuroscientist at Johns Hopkins University. If something similar is true in humans, it could, among other things, add a new wrinkle to attempts to treat social disorders like autism by messing with the oxytocin system.
Neuroscientists have long believed that the release of oxytocin in the brain can be triggered by social interactions with certain individuals, such as offspring or mates, that are important to a person or another animal. And when researchers experimentally blocked the action of oxytocin, the mice lost the ability to recognize socially significant individuals. That suggests the molecule plays a central role in social learning — but researchers don’t know exactly how oxytocin does its job.
That is changing thanks to advances in neuroscience techniques that in recent years have allowed researchers to identify and record the activity of individual oxytocin-producing neurons deep in the brain. And these records tell a story that differs from the old view—in subtle but important ways.
In the primary olfactory center of rat brains, for example, measurements show that oxytocin inhibits the random, noisy firing of nerve cells, thus allowing the neural signals of actual odors to stand out more clearly. (It does this in reverse, by exciting nerve cells known as granule cells, which inhibit other nerve cells.)
“There’s a lot of noise in the brain,” he says Larry Younga behavioral neuroscientist at Emory University who, with an assoc Robert Froemkeexplore ours a new understanding of oxytocin in 2021 Annual Review of Neuroscience. “But when oxytocin is released, it reduces the static, so the signal comes through much more clearly.”
That clarity is familiar to new parents, says Froemke, a neurologist at the Grossman School of Medicine in New York and Young’s co-author. “I have two young children,” he says. “Even two rooms down, the air conditioning is on and I’m fast asleep, the baby starts crying and I’m immediately awake and present, with my pupils fully dilated.”
Oxytocin also enhances the brain’s reward system response, he says Evgenia Kozorovitskyi, a neuroscientist at Northwestern University. This effect can divert the animal’s behavior away from seeking new things in the environment and towards focusing on social rewards.
In prairie voles, for example—which intrigue researchers with their monogamous behavior, rarely seen in rodents—this change makes pairing easier. Among other effects, some of the oxytocin-sensitive cells link the partner’s scent to the reward system. “Pair bonding is kind of like an addiction to a partner,” says Young. “The partner becomes intrinsically rewarding.” Species as diverse as mice and barn voles lack these oxytocin receptors, but, amazingly, they do in human brains, suggesting we may respond more like prairie voles than mice. (oxytocin’s molecular cousin, vasopressin, also plays a role in pair bonding.)
The new understanding that oxytocin sharpens attention to socially relevant cues may help explain the common experience that love makes the world shine. “When you look into your partner’s eyes, oxytocin can make the world brighter,” says Young.
There is an additional complication here, and an important one: If the true role of oxytocin is to clarify social sensory perceptions and not simply promote sociability, then the chemical is likely to have different effects in different contexts. For example, Young notes, oxytocin improves maternal care in mice—an apparent prosocial behavior—but also increases maternal aggression toward unfamiliar individuals.
(Credit: Lev Fried/Shutterstock)
Also, the female prairie voles respond differently to oxytocin depending on whether they had yet formed a pair bond, Young and his colleagues found. In single females, oxytocin reduces noise in the reward system, allowing them to learn to like the smell of a potential mate. In females who have already bonded with a partner, the molecule increases the volume of the reward system to make the partner more rewarding, thereby reducing aggression toward the partner.
Something similar can also apply to humans. In a 2012 study, 30 men who were in monogamous relationships maintained a a little more social distance from an attractive female stranger when they received an intranasal oxytocin spray than when they received a placebo. The effect was not seen in a similar group of 27 single men.
Context dependence of oxytocin may complicate efforts to use oxytocin for treatment autism spectrum disorder. Some therapists already use intranasal oxytocin sprays to treat people with autism, on the theory that it should improve their response to social stimuli. However, a large clinical trial recently found without demonstrable effect.
That’s not surprising, Young says, because the trial didn’t control for the context in which patients received the sprays. If a child is being bullied at school, for example, the treatment offered may not help, but instead may reinforce that negative experience, he notes. Instead, Young believes that any oxytocin-based therapies should be cautious delivered in the safety of a therapy session.
The neuroscientists also note that although oxytocin clearly plays an important role in regulating social behaviors such as pair bonding and parental care, it is not the only factor at work. “Falling in love is a total brain-body experience,” says Kozorovitsky. “There are sensory elements and cognitive elements, and memory is important. Is oxytocin one of the many modulators that mediate all these changes? Absolutely. But can we pin it all on oxytocin? This is definitely an oversimplification.”
10.1146/knowable-021122-1
Bob Holmes is a solidly connected science writer based in Edmonton, Canada. This article originally appeared in Knowable Magazine, an independent journalistic venture from Annual Reviews. You can read the original here.