• Posted January 31, 2023

Is Oxytocin Really the 'Love Hormone'? Rodent Research Raises Doubt

The "love hormone" oxytocin might not play the critical role in forming social bonds that scientists have long believed, a new animal study suggests.

Prairie voles bred without receptors for oxytocin display the same monogamous mating, attachment and parenting behaviors as regular voles, according to researchers.

"While oxytocin has been considered 'Love Potion No. 9,' it seems that potions 1 through 8 might be sufficient," said co-senior researcher Dr. Devanand Manoli, a psychiatrist with the University of California, San Francisco's Weill Institute for Neurosciences. "This study tells us that oxytocin is likely just one part of a much more complex genetic program."

Prairie voles are one of the few mammals known to form lifelong monogamous relationships, so they are a natural fit for researchers looking into the biology of social bonding.

Studies performed in the 1990s showed that oxytocin-blocking drugs left voles unable to pair-bond, researchers said in background notes. As a result, oxytocin gained its reputation as a love hormone.

To dig further into that relationship, researchers used gene-editing tools to create prairie voles that lacked functional oxytocin receptors. They then tested the altered voles' ability to form enduring relationships with other voles.

The researchers were surprised to find that the mutant voles paired up as readily as normal voles.

"The patterns were indistinguishable," Manoli said in a university news release. "The major behavioral traits that were thought to be dependent on oxytocin -- sexual partners huddling together and rejecting other potential partners as well as parenting by mothers and fathers -- appear to be completely intact in the absence of its receptor."

Female prairie voles also were capable of giving birth and providing milk for their pups, another surprise given that oxytocin likely plays a role in both birth and lactation, the researchers said.

Oxytocin has been considered essential for milk ejection for many decades, researchers said. Nevertheless, half of the mutant females were able to nurse and wean their pups successfully -- a sign that the hormone plays a less vital role than previously thought.

The findings were published Jan. 27 in the journal Neuron.

"This overturns conventional wisdom about lactation and oxytocin that's existed for a much longer time than the pair-bonding association," co-senior researcher Dr. Nirao Shah, a neurobiologist with Stanford Medicine, said in the release. "It's a standard in medical textbooks that the milk letdown reflex is mediated by the hormone, and here we are saying, 'Wait a second, there's more to it than that.'"

The mechanisms by which humans pair-bond with each other have long been thought the key to unlocking better treatments for mental health conditions like autism and schizophrenia, both of which interfere with a person's ability to form social attachments, researchers said.

Much hope has been pinned on clinical trials using oxytocin to treat such conditions, but results have been mixed.

While results in animal studies may not always be seen with humans, this study strongly suggests it's too simple to think of a single factor like oxytocin as being wholly responsible for social attachment.

"These behaviors are too important to survival to hinge on this single point of potential failure," Manoli said. "There are likely other pathways or other genetic wiring to allow for that behavior. Oxytocin receptor signaling could be one part of that program, but it's not the be-all end-all."

More information

The Cleveland Clinic has more about oxytocin.

SOURCE: University of California, San Francisco, news release, Jan. 27, 2023