Public Health, Spring 1998

Social attachment may be more biologic than soulful, according to neuroscientist Tom Insel, who is uncovering surprising clues to monogamy, parenting, and other behaviors with the help of voles.

Love & Sex: The Vole Story

by Kate Egan

In Shakespeare's enchanted midsummer forest, a quick sprinkle of magic dust allowed for some very unlikely pairs to fall suddenly, hopelessly, in love. How did Puck so masterfully manage these affairs of the heart? Emory scientists have an answer. They say it's really the brain that orchestrates the forming of such heartfelt, lasting emotional bonds. These bonds make up the pulse of life--life with lovers, family, friends, business associates. Virtually every form of psychopathology is characterized by abnormal social attachments. Yet, very little is known about social bond formation: its anatomy, chemistry, and physiology.

It may sound a bit clinical to the romantics among us, who envision the complex processes of falling in love or being a loyal friend as more soulful than biologic. However, according to Thomas R. Insel, Emory neuroscientist and director of the Yerkes Regional Primate Research Center, the various forms of attachment, including parent-infant, male-female, and filial, are not unique to humans.

Insel suggests that "at least the neural basis of attachment can be investigated in animal models." These neural pathways, he proposes, also may prove to be important in treating clinical disorders such as autism and schizophrenia, both of which result in social isolation and detachment.

Neuroscientist Tom Insel, who has studied social attachment for the past 15 years, is known for developing the best laboratory model, a rodent from the Midwest call a vole.

Having studied social attachment for 15 years, Insel is known for developing the best laboratory model, using a mouse-sized rodent found in the Midwest called a vole. Voles come in various species, including prairie and montane voles. These two species are 99% genetically identical, "but that last 1% results in some very different social behaviors," Insel explains wryly.

Different, indeed. Field biologists in the wild discovered that highly social prairie voles are the model of family values. They naturally form lasting, monogamous pair bonds (male-female) after mating and prefer the company of their mate to others. The male reacts aggressively toward other males once he's bonded with a female. Vole breeding pairs nest together, and both parents provide extensive and prolonged parental care, with offspring remaining in the nest for several weeks beyond weaning.

By contrast, the montane vole is a loner, nesting in isolation. Love doesn't last beyond a brief interlude. Montanes, who do not form a pair bond after mating, breed promiscuously. The males make terrible dads. Even the females abandon offspring soon after birth.

In the laboratory, Insel and his research team have been able to duplicate the natural behavioral differences of voles in the field. Lining the walls of Insel's laboratory are stacks of variously shaped cages and mazes which help him quantify affiliate behaviors. For instance, he knows that when placed in large cages, prairie voles will spend more than 50% of their time huddled together with their mates. Montane voles will spend less than 5% of the time close to another individual. He has seen that when a pair-bonded prairie vole dies, the surviving partner prefers living alone to taking a new mate. As in the wild, prairie vole dads in the lab are extremely involved with their pups, the very model of Mr. Mom. When separated from their parents, prairie vole pups get very upset, as demonstrated in ultrasonic distress calls. Laboratory montanes, true to form, take no interest in their pups whatsoever, and pups take it all in stride.

As it turns out, one of the major contributing factors to these enormous differences are the peptide hormones oxytocin (OT), active in females, and vasopressin (AVP), active in males. In mammals, cells in the hypothalamus produce these hormones. In rodents, scientists have associated OT and AVP with various social behaviors such as parenting behavior (including parturition and nursing), social memory, territorial behavior, and aggression. Other mammals, like sheep, also exhibit maternal behaviors when the oxytocin switch is turned on. "It's remarkable, really, that the two different peptide systems have been adapted for different roles in male and female mammals," says Insel. "Apparently, at least in voles, they are activated by pair bonding."

As for humans, the role of central oxytocin or vasopressin in forming enduring, selective bonds has remained largely unexplored. "Across human cultures, sexual behavior is consistently associated with pair bonding," Insel says, "although sex is neither necessary nor sufficient for human pair bond formation." In men, AVP peaks during arousal, and oxytocin peaks with ejaculation. While no data from humans regarding OT or AVP in pair bonding exist, studies in monkeys show that increased transmission of the peptides does increase social interaction.

Insel and his colleagues, Zuoxin Wang and Larry Young, are further investigating OT and AVP and their wide-ranging effects in a series of behavioral, cellular, and molecular studies.

Peptide hormones play a part

In Insel's laboratory, colleague Zuoxin Wang records a female prairie vole's preference in choosing a mate. Typically, prairie voles form lasting, monogamous pair bonds and prefer the company of their mates to others. They are the model of family values, with both parents providing prolonged care. By contrast, montane voles fail to form pair bonds after mating and breed promiscuously. The males make terrible dads, and the females abandon offspring soon after birth.

Young works with genetically altered mice that come in two varieties: a knock-out mouse, in which the animal is bred without a particular gene, or a knock-in mouse, bred with a certain gene grafted on from another species.

For the prairie vole, the business of choosing a partner for life begins during the very first mating experience. A female prairie vole enters puberty not at a specific age, but after she's exposed to a chemical in the urine of an unrelated male. Within a day, she becomes sexually receptive and mates repeatedly with an unrelated male. In the process, she forms a selective and enduring preference for that male.

To see if oxytocin was really the key ingredient to this bonding behavior, Insel and his colleagues took a closer look at the biochemical dimensions of the vole brain. They found that even in the absence of mating, when OT is injected into the brain of a female prairie vole, she forms a bond with the male to whom she is exposed. Similarly, an OT antagonist blocked the preferences induced by mating. To confirm the link between OT and bonding, Insel's team plans to actually measure cellular levels of OT in the brain during mating.

But OT is definitely not the end of the story. "Oxytocin and vasopressin appear to be very important," says Insel, "but they're only one link in a neurochemical chain needed for these complex social behaviors." To identify the next link in the chain, Wang moves to the cellular level. In his current study, preliminary results show that dopamine, known for its association with reward and addiction, also is involved in regulating partner preferences in female prairie voles. In fact, during mating female voles pump a vastly increased amount of cellular dopamine into the nucleus accumbens, an area of the brain involved in memory, learning, and reward. A dopamine antagonist directly injected into the accumbens also induces the partner preference, even without mating--telling us that in addition to OT, mating preferences may also depend on dopamine release.

So, how do increased OT and dopamine affect the vole dating game? One hypothesis is that mating imprints an emotional memory on the new lovebirds. "It appears that as the female mates she learns new information--perhaps the specific olfactory pheromones of her partner--that become a memory associated with mating," says Insel. So if she's then placed with a new male, she simply does not react to his olfactory pheromones; she prefers the memory of her first love. And this memory processing, apparently, is dependent on the presence of OT or AVP and dopamine.

Is there a structural change in the brain that accompanies this learning change? When the female prairie vole is exposed to a male, she begins estrus, thus increasing estrogen levels in the blood. In a remarkable finding, Insel's team recently discovered that estrogen actually induces new neuronal cell growth in the brain. This marks the first evidence of a steroid hormone regulating neurogenesis in mammals. The researchers also noted a clear migration of the new cells in the brain from the subventricular zone, in the midbrain, up to the olfactory bulb, in the forebrain. "It's a very distinct pathway," says Insel. "The estrogen may be what prepares the animal for learning the new memory."

According to Insel, he hopes "next to see if the new neurons have estrogen receptors and then to identify the neurons' functional significance." The team wants to eventually test this newfound knowledge in primates. They are already working on mapping OT, AVP, and dopamine receptors in rhesus monkeys.

Naturally, these studies beg the question: do human brains undergo change when they mate or fall in love? Can we, too, generate neurons with a little extra estrogen? "Perhaps we'll be able to answer that question someday, with PET imaging, and other new technologies as they develop," says Insel. "But we're definitely not there yet."

The estrogen-neurogenesis link in voles does, however, make an irresistible parallel with recent clinical reports that women who take hormone replacement therapy (estrogen) at menopause appear to be protected against Alzheimer's disease, and that women may fare better than men in neuronal repair after serious head injury. Insel cautions that much more research is needed before making assumptions about these associations.

How do oxytocin, vasopressin, dopamine, and estrogen all interact to seal a relationship? Estrogen may prepare the vole by sending her into estrus, thus making her receptive to mating. The subsequent act of mating stimulates OT release, which encourages dopamine release. Dopamine, and its system for reward and pleasure, is positively linked to the mate--thus making sex the gateway to the formation of a long, pleasurable relationship. So far, it's still just a theory. "No one really knows yet if OT is driving dopamine, or if the reverse is true," says Insel, "but the associations indicate that they may all be vital in turning a neutral experience into a strongly positive and reinforced one that lasts for life."

Understanding the dating game

In a remarkable finding, Insel's team recently discovered that estrogen actually induces new neuronal cell growth in the brain.

Insel's earlier work showed that a difference in the distribution of the hormones' receptors--not the amount of hormone itself--could make the difference between monogamy and promiscuity. Insel has already mapped the receptors for OT and AVP in prairie and montane vole brains. He now hopes to match specific social behaviors to the various receptor-laden areas in the brain. For instance, is one location associated with pair bonding, and another with paternal care? Can we transform a deadbeat dad into a model of responsibility with the flip of a genetic switch?

To identify the specific molecular mechanisms shaping these biological differences, Insel's group zeroes in on a tiny but powerful target: DNA. For this, Insel turns to Larry Young, the acknowledged "gene jock" of the group.

Young's genetic studies are trying to identify the "hot spot" on the receptor gene that is responsible for the variation in social behaviors among species. "Throughout evolution, AVP and OT receptor distribution has changed," he explains. "When it changes behavior favorably, it is selected for. Now we're trying to see if we can alter social behavior in the lab the same way evolution has over millions of years."

Young works with genetically altered mice, rather than voles. The technology to breed mice with manipulated genes has been around for more than 10 years, but the technology for doing so in voles is still in development. Transgenic mice come in two basic varieties: a "knock-out" mouse, in which a mouse is bred without a particular gene; or a "knock-in," bred with a certain gene grafted on from another species.

Young is currently working with knock-ins to identify the genetic "hot spot" that captains the journey of the AVP receptor, directing its expression toward a particular area of the brain. Young believes the responsible party is the promoter sequence, found in the front of the receptor gene.

Young's knock-in mice have the AVP receptor gene from a prairie vole incorporated into their DNA. The result is a curious combination. As expected, they show patterns of receptors different from a regular mouse, but they don't exactly look like a prairie vole either. They even look a little different from one another. "In each mouse egg I injected, the AVP gene migrated onto a different chromosome," says Young. "So the gene is living in a slightly different environment in each mouse, making the receptors look a little different in each one. This tells us the promoter is only partially responsible for where the gene is expressed. The chromosomal environment is also a determinant." The subsequent step, with the functional gene successfully integrated, will be to see how it affects behavior.

Young is now trying to create the world's first transgenic vole. He's attempting to create a kinder, gentler, more social montane vole--montanes normally being the antisocial variety--by giving the montane a receptor gene from a prairie vole.

The implications of all his genetic fiddling are intriguing. Clearly, Insel thinks so. But he emphasizes, too, what this research is not. This research is not a hunt for the recipe for fairy love dust or an elixir for fidelity. Instead, it may lead to answers to the more pressing problems of millions of people who are unable to form normal social bonds at all: those with autism and schizophrenia. Currently, antipsychotic medications are the only treatment for these devastating illnesses. While the drugs help manage hallucinations, they have no effect on social deficits.

In parallel clinical studies, Insel and Gail McGee, director of Emory's Autism Resource Center, have begun screening autistic kids for genetic mutations in the oxytocin gene. Although results are years away, with luck, a tiny field rodent may help speed the process. Uncovering the neurobiology of an extremely complicated disease which robs people of the very fundamentals of human interaction and communication may start with the humble vole.

New technology to explain age-old behaviors

Gene jock Larry Young is searching for a genetic hot spot on the receptor gene that is responsible for the variation in social behavior among species.


When I'm 94 | Love and Sex: The Vole Story | Salty Doc | The Quest for Excellence
Letters | In Brief | Development | Alumni News

Copyright © Emory University. All Rights Reserved..
Web version by Jaime Henriquez.