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A new study casts doubt on oxytocin's role as a 'love hormone'

SCOTT SIMON, HOST:

It's known as the love hormone, but a new study suggests that label is misleading. NPR's Jon Hamilton reports on what scientists are learning about oxytocin.

JON HAMILTON, BYLINE: When romance is in the air, a couple's oxytocin levels rise. That's true for both people and prairie voles, mouse-like rodents that mate for life and are often used to study human behavior. Dr. Dev Manoli, a psychiatrist at the University of California, San Francisco, says prairie vole couples share a nest and even co-parent.

DEV MANOLI: One of the behaviors that's really, you know, sort of the most adorable is this huddling behavior, just sort of huddling with each other. They'll sometimes groom. Sometimes they just fall asleep because it's very calming. And that's very specific to the pair-bonded partner.

HAMILTON: Decades of research has suggested that oxytocin is critical to that sort of behavior. So Manoli and a team of scientists did an experiment designed to disrupt pair bonding. They removed fertilized eggs from female prairie voles and edited the genes to neutralize the effects of oxytocin. After that, Manoli says, they let the cells grow.

MANOLI: So we culture them for a few days and then put them into what's called a pseudo-pregnant female.

HAMILTON: An animal that's hormonally ready to carry an embryo. The result was pups that appeared normal. And when these pups grew up, they formed pair bonds just like other prairie voles. Manoli says females were even able to produce milk for their offspring, a process usually mediated by oxytocin.

MANOLI: We were shocked because that was really, really not what we expected. And, you know, my initial response was, OK, we have to do this three more times because we need to be sure that this is 100% real. But also, what's going on?

HAMILTON: Repeated experiments confirmed the finding, which appears in the journal Neuron. Manoli says it's still a mystery how pair bonding occurs in the absence of oxytocin. But he says the study makes one thing clear.

MANOLI: Because of evolution, the parts of the brain and the circuitry that are responsible for pair bond formation don't really rely on oxytocin. They don't need it.

HAMILTON: In other words, Manoli says...

MANOLI: Oxytocin might be "Love Potion No. 9," but one through eight are still in play, right? There's more there than that one entry point.

HAMILTON: Manoli says in retrospect, the result makes sense because pair bonding is essential to a prairie vole's survival, and evolution tends to favor redundant systems for critical behaviors. He says the result also may help explain why giving oxytocin to children with autism spectrum disorder doesn't reliably improve their social functioning.

MANOLI: There's not a single pathway, but rather these complex behaviors have really complicated genetics and complicated neural mechanisms.

HAMILTON: Many scientists who study oxytocin say they're uncomfortable with the term love hormone. Sue Carter of the University of Virginia and Indiana University helped discover the link between oxytocin and monogamy in prairie voles. But she says she never assumed the hormone was acting alone.

SUE CARTER: The process of forming a secure social bond lasting for a very long period of time is too important to restrict to a single molecule.

HAMILTON: Carter says a different molecule called vasopressin also contributes to social bonding, and there may be others, she says, though she believes oxytocin is the major player.

CARTER: We can live without fine clothing. We can even live without too much physical protection. But we cannot live without love.

HAMILTON: Which may be the reason we can love without oxytocin.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.

Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.