Turning a decades-old dogma on its head, scientists at the University of California, San Francisco (UCSF) and Stanford Medicine have found that the receptor for the hormone oxytocin, which has previously been considered essential to forming social bonds, may not play the critical role that scientists have assigned to it for the past 30 years.
The team’s research showed that prairie voles bred without receptors for oxytocin—sometimes referred to as the “love hormone”—showed the same monogamous mating, attachment, and parenting behaviors as regular voles. In addition, females without oxytocin receptors (Oxtr) gave birth and produced milk, though in smaller quantities than ordinary female voles. The results indicate that the biology underlying pair bonding and parenting isn’t purely dictated by the receptors for oxytocin.
The discoveries could suggest new strategies for helping people who struggle with social connection. “If we can find the key pathway that mediates attachment and bonding behavior, we’ll have an eminently druggable target for alleviating symptoms in autism, schizophrenia, and many other psychiatric disorders,” said Stanford University neurobiologist and research co-lead Nirao Shah, MD, PhD.
“While oxytocin has been considered ‘Love Potion #9,’ it seems that potions 1 through 8 might be sufficient,” added psychiatrist Devanand Manoli, MD, PhD, a member of the UCSF Weill Institute for Neurosciences. “This study tells us that oxytocin is likely just one part of a much more complex genetic program.” Manoli is a co-senior author of the team’s paper, which is published in Neuron, and titled “Oxytocin receptor is not required for social attachment in prairie voles.”
Prairie voles are among a small group of mammals that display long-term social attachment between mating partners, the authors explained. After mating, they form lifelong partnerships known as pair bonds. “Pair-bonded voles share parental responsibilities, prefer the company of their partner over unknown members of the opposite sex, and actively reject potential new partners,” the team explained. “Pair-bonded animals prefer to huddle with their partners compared with exploring unfamiliar conspecifics of the opposite sex.”
Because prairie voles are one of the few mammalian species known to form lifelong monogamous relationships, researchers study them to better understand the biology of social bonding. Research carried out in the 1990s using drugs that prevent oxytocin from binding to its receptor found that voles were unable to pair bond when oxytocin signaling was blocked, giving rise to the idea that the hormone is essential to forming such attachments. In fact, the team noted, “Many pharmacological studies show that signaling via the oxytocin receptor (Oxtr) is critical for the display of social monogamy in these animals … decades of research implicates both Oxt and its cognate receptor Oxtr in a large repertoire of parenting behaviors.”
The current project emerged from shared interests between Manoli and co-senior author Shah. Shah had been interested in the biology of oxytocin and social attachment in prairie voles since teaching about oxytocin studies decades earlier. Manoli, who wanted to investigate the neurobiology of social bonding, joined Shah’s lab in 2007 as a postdoctoral scholar.
The team wanted to know whether pair bonding really was controlled by oxytocin receptor signaling. For this study, 15 years in the making, the investigators applied genetic technology to confirm if oxytocin binding to its receptor was indeed the factor behind pair bonding. They used CRISPR gene editing to generate prairie voles that lack functional oxytocin receptors. Then, they tested the oxytocin-receptorless voles to see whether the animals could form enduring partnerships with other voles. To the researchers’ surprise, the mutant voles formed pair bonds just as readily as did the normal (wild-type; WT) voles. “Surprisingly, male and female prairie voles homozygous for each of the three distinct loss-of-function Oxtr alleles displayed pair-bonding,” the investigators stated.
“The patterns were indistinguishable,” said 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 … We were all shocked that no matter how many different ways we tried to test this, the voles demonstrated a very robust social attachment with their sexual partner, as strong as their normal counterparts.”
The researchers wondered whether oxytocin receptor signaling might be similarly dispensable for its other functions—parturition, parenting, which, in prairie voles, is a shared responsibility between the two parents, and milk release during lactation.
Again, they found that voles lacking the Oxtr receptor made great parents. “We observed that Oxtr-null parents interacted equivalently with their pups compared with their WT counterparts,” they wrote. “Both WT and mutant parents spent the majority of their time in the nest, in direct contact with their litters, and, in the case of mothers, nursing pups … Our findings, consistent across multiple paradigms, three labs, and three null alleles of Oxtr, are in contrast to prior studies that highlight the importance of Oxtr signaling in pair-bonding and parental behaviors in prairie voles.”
More surprising for Manoli and Shah than the pair bonding findings was the observation that a significant percentage of the female voles were able to give birth and provide milk for their pups. “We found that mutant voles are not only able to give birth, but actually nurse,” said Shah. Both male and female mutants engaged in the usual parental behaviors of huddling, licking, and grooming, and were able to rear pups to weaning age.
So, while oxytocin is likely to have a role in both birth and lactation, it may be more nuanced than previously thought, Manoli said. Female voles without the oxytocin receptors proved perfectly capable of giving birth, in the same timeframe and in the same way as the regular animals, even though labor has been thought to rely on oxytocin.
The results help to clear up some of the mystery surrounding the hormone’s role in childbirth. Oxytocin is commonly used to induce labor, but blocking its activity in mothers who experience premature labor isn’t better than other approaches for halting contractions.
The greatest surprise was that the female voles lacking oxytocin receptors were still able to produce milk and feed pups. For decades, it has been considered that oxytocin binding to its receptor is essential for milk ejection and parental care. But the newly reported study found that half of the mutant prairie vole females were able to nurse and wean their pups successfully, indicating that oxytocin signaling plays a role, but it is less vital than previously thought.
The mutant prairie voles did have more limited milk release compared with normal voles, however. As a result, fewer of their pups survived to weaning age, and those that did survive were smaller compared to the pups of normal prairie voles. Nevertheless, the fact that the voles could nurse at all is in contrast to equivalent studies in oxytocin receptor-deficient mice, who completely failed to lactate or nurse, and whose pups consequently died within a day or so of being born. “Female mice null for Oxtr show a complete failure of milk letdown and nursing behavior such that none of their pups survive,” the authors noted.
They further hypothesized that this species difference could be due to the inbred nature of laboratory mouse strains in contrast to the genetically heterogenous voles. “It could be that inbreeding in mice has selected for a large dependence on oxytocin signaling, or this may represent a species-specific role of oxytocin receptor signaling,” said Shah.
“The result, Shah added, “overturns conventional wisdom about lactation and oxytocin that’s existed for a much longer time than the pair bonding association. 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.’”
When asked why their results differ from previously published studies that used drugs to block oxytocin receptor signaling, the authors pointed to the key difference between genetic and pharmacological studies: precision. “A key difference between our work and preceding studies is that the latter was conducted in adult animals, using pharmacological or viral misexpression strategies to determine a role for Oxtr function in behavior,” they wrote. Manoli added, “Drugs can be dirty, in the sense that they can bind to multiple receptors, and you don’t know which binding action is causing the effect. From a genetics perspective, we now know that the precision of deleting this one receptor, and subsequently eliminating its signaling pathways, does not interfere with these behaviors.”
Also, while most pharmacological studies suppress oxytocin receptor signaling in adult animals, for their work, Manoli and colleagues switched it off when the voles were embryos. “We’ve made a mutation that starts from before birth,” pointed out Shah. “It could be that there are compensatory or redundant pathways that kick-in in these mutant animals and mask the deficits in attachment, parental behaviors, and milk let-down.”
Manoli and Shah focused on understanding the neurobiology and molecular mechanisms of pair bonding because it is thought to hold the key to unlocking better treatments for psychiatric conditions, such as autism and schizophrenia, that interfere with a person’s ability to form or maintain social bonds. Over the past decade, much hope was pinned on clinical trials using oxytocin to address those conditions. But those results were mixed, and none has illuminated a clear path to improvement.
“For at least the last ten years people have been hoping for the possibility of oxytocin as a powerful therapeutic for helping people with social cognitive impairments due to conditions ranging from autism to schizophrenia,” Manoli said. As the investigators further explained, “Based on observations in prairie voles and other mammals, including humans, clinical trials have used exogenous Oxt or small molecule ligands to Oxtr to ameliorate the deficits in social attachment and cognition seen in multiple psychiatric conditions; however, these studies have yielded mixed results.” But as Manoli noted, “This research shows that there likely isn’t a magic bullet for something as complex and nuanced as social behavior.” The authors added, “Together with these clinical studies, our observation that Oxtr signaling is not required genetically for pair bond formation or parenting in prairie voles suggests that we require a more refined understanding of the molecular pathways underlying social attachment behaviors.”
The researchers said their study strongly suggests that the current model—a single pathway or molecule being responsible for social attachment—is oversimplified. This conclusion makes sense from an evolutionary perspective, they said, given the importance of attachment to the perpetuation of many social species.
“These behaviors are too important to survival to hinge on this single point of potential failure,” said 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.”
The discovery points the researchers down new paths to improving the lives of people struggling to find social connection. “New genetic models such as the Oxtr prairie vole mutants we have generated may better allow the rigorous dissection of the molecular and circuit mechanisms mediating attachment behavior and its disruption in disease,” the authors stated. “Whole-animal mutants better represent what may occur in patients with mutations associated with neuropsychiatric disorders, and molecular genetic approaches in prairie voles now permit us to test directly the impact of such genetic disruptions in the context of complex social and attachment behaviors.”
Working with prairie voles presented an obstacle, but one worth overcoming. Because prairie voles are not commonly used in genetic studies like laboratory mice, the team needed to develop all of their molecular tools and protocols from scratch. Now that they have these vole-specific pipelines and tools, the authors are excited about the doorways this opens, both for them and for other researchers.
“We’re very happy to be part of a community and to have this technology that we can share,” says Manoli. “Now we have this trove that we can start to mine. There are so many other questions that prairie voles could be interesting and useful for answering, both in terms of potential clinical implications for models of anxiety or attachment and also for basic comparative biology.”