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What You Think You Know About Oxytocin – The Love Hormone – Is Probably Wrong

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Oxytocin, popularly known as the “love hormone” or “cuddle hormone,” is a chemical produced by the brain that plays a crucial role in feelings of affection, attachment, and pleasure.

Researchers have extensively examined the societal effects of the hormone, and are currently gaining a deeper understanding of its functions.

A new study from UC San Francisco and Stanford Medicine suggests that the receptor for oxytocin, a hormone thought to be important for making social bonds, may not be as important as scientists have thought for the past 30 years.

The research team discovered that prairie voles raised without receptors for oxytocin displayed the same monogamous mating, bonding, and parental behaviors as ordinary voles, and their findings were published today in Neuron. Also, females who didn’t have oxytocin receptors gave birth and made milk, but not as much as regular female voles.

The findings suggest that oxytocin receptors, popularly referred to as the “love hormone,” are not the only biological factors governing pair bonding and parenting.

While oxytocin has been referred to as “Love Potion #9,” senior author of the study and member of the UCSF Weill Institute for Neurosciences psychiatrist Devanand Manoli, MD, PhD, indicated “that potions 1 through 8” may be enough. This work shows that oxytocin is probably just a small component of a much more intricate genetic pathway.

CRISPR Voles Have a Big Secret

Since prairie voles are one of the few mammalian species known to develop lifelong monogamous pairings, scientists study them to get a deeper understanding of the biology of social bonding.

Voles were shown to be unable to pair bond in studies conducted in the 1990s using medicines that prevented oxytocin from binding to its receptor. This led researchers to conclude that the hormone is necessary for the development of such relationships.

The project originated from common interests between Manoli and co-senior author Nirao Shah, a neurobiologist who was previously at UCSF and now at Stanford Medicine. 

The two used new genetic technology to establish whether oxytocin binding to its receptor was actually the cause causing pair bonding in their 15-year-long research. They employed CRISPR to create prairie voles deficient in oxytocin receptors. Then, they put the mutant voles in groups with other voles to see if they could stay together.

To the surprise of the researchers, the mutant voles bonded with each other just as easily as normal voles.

“The patterns were indistinguishable,” points out Manoli. “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.” 

Lactation and Labor

Even more remarkable than the pair bonding was the fact that a considerable proportion of the female voles were able to give birth and produce milk for their pups.

According to Manoli, oxytocin probably plays a more complex function in nursing and delivery than was previously believed. Despite the belief that labor depends on oxytocin, female mice lacking receptors were found to be fully capable of giving birth in the same manner and at the same time as other animals.

The findings shed light on some of the mysteries regarding the hormone’s involvement in childbirth: Oxytocin is often used to start labor, but stopping it from working in mothers who go into labor too soon isn’t better than other ways to stop the contractions.

But the researchers were surprised at how the animals produced milk and fed their pups. It has been believed for decades that oxytocin binding to its receptor is essential for milk ejection and parental care. However, half of the mutant females were able to successfully nurse and wean their pups, indicating that oxytocin signaling plays a role, but is less essential than previously believed.

“We found that mutant voles are not only able to give birth, but actually nurse,” adds Shah.

“This overturns conventional wisdom about lactation and oxytocin that’s existed for a much longer time than the pair bonding association,” comments Shah. “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.’” 

Hope for a Social Link

It is believed that knowing the neurobiology and molecular underpinnings of pair bonding holds the key to developing more effective therapies for mental disorders, such as autism and schizophrenia, that hinder an individual’s capacity to build or sustain social ties.

Over the last ten years, a lot of optimism has been placed on clinical studies that use oxytocin to treat such disorders. However, the outcomes were inconsistent, and none have shown a definite course for development.

The data, according to the researchers, clearly shows that the present hypothesis, which attributes social attachment to a single route or molecule, is oversimplified. From an evolutionary standpoint, they said that this result makes sense considering the significance of attachment to the survival of many social animals.

“These behaviors are too important to survival to hinge on this single point of potential failure,” adds Manoli. “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.” 

Researchers can now find new ways to help people who have trouble making friends.

“If we can find the key pathway that mediates attachment and bonding behavior,” Shah adds, “We’ll have an eminently druggable target for alleviating symptoms in autism, schizophrenia, many other psychiatric disorders.”

Image Credit: Getty

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