Love is Like an Addiction

VOLE LOVE

Some note a study that has linked a single genetic change to a complex behavioral adjustment as particularly elegant. Researchers from Emory University, Atlanta, with colleagues in Boston and Tallahassee, Fla., found that boosting the expression of one gene switched meadow voles, a type of rodent, from a promiscuous to a monogamous lifestyle.¹ "For a new behavior to evolve, you might think a lot of different genes would have to evolve in concert, but I don't think it works that way," says Larry J. Young, associate professor of psychiatry and behavioral sciences at Emory. He says it's possible that this gene, which encodes the vasopressin V1a receptor, lies in a pathway homologous to one underlying human love.

Andreas Bartels, a research scientist at the Max Planck Institute for Biological Cybernetics, Tübingen, Germany, says he's nearly certain that Young's suggestion is correct, because his study "fits like a glove" with Bartels' neuroimaging findings with humans. Bartels and Semir Zeki of University College London ran functional magnetic resonance imaging (fMRI) scans of couples who professed to be deeply in love, as they viewed photographs of their beloveds.² The researchers found significant activations in brain networks rich in receptors for the hormones vasopressin and oxytocin. These are the hormones identified as crucial for pair bonding in a series of animal studies,³ including Young's work with voles, Bartels adds, suggesting the pathways are homologous. "For me this is an absolutely clear-cut story."

One part of the tale to be clarified, though, is the difference in roles between vasopressin and oxytocin. Researchers have found that the hormones have largely similar effects in facilitating pair bonding, although each has different additional functions. Some have suggested vasopressin is more important in male pair bonding, and oxytocin in female pair bonding, but this remains obscure, writes Zuoxin Wang of Florida State University.⁴

An observation that may lead to greater clarity, says Bartels, is that brain regions rich in oxytocin and vasopressin receptors overlap strongly with those rich in dopamine, the neurotransmitter classically associated with the brain's reward system. Young and colleagues propose that long-term partner preference occurs when the vasopressin circuits, which are also known to mediate individual recognition, somehow connect with the dopamine pathway, causing an animal to associate a specific individual with a sense of reward. In key dopamine-rich brain regions, vasopressin V1a is expressed more highly in monogamous than in promiscuous mammals. Its upregulation might be the evolutionary event or events that caused the pathways to connect, they suggest.¹ Again, Bartels concurs: "I think it's emerging that the mechanism of attachment preference uses the dopamine pathway to make attachment a rewarding experience."

No one has pinpointed how, chemically, the vasopressin and dopamine pathways might link in this proposed system. But while a molecular explanation is lacking for that part of the story, it might already be partially worked out in others.

As the hypothesis of a vasopressin-dopamine linkage suggests, and as scientists have found, dopamine is another substance crucial for long-term pair bonding in animals that pair bond. Dopamine seems to achieve this effect by cutting levels of cAMP, a common intracellular signaling molecule, in the brain's nucleus accumbens, Wang argues.⁴ This region is thought to be a key pleasure center. The lower cAMP levels may in turn reduce levels of the enzyme protein kinase A, Wang continues. Although further details haven't been worked out, similar events are believed to underlie the pleasure involved in drug and alcohol use. In love-smitten people, not only these detailed neurochemical events, but also overall brain activation patterns are "similar to what you would see when you take a drug like cocaine," Young says.

LOVE'S MANY FACETS

© 2000 Lippincott Williams & Wilkins

The images show brain regions that are deactivated when subjects viewed pictures of their friends compared to when they viewed pictures of loved partners. Deactivations are right lateralized within the prefrontal cortex, the middle temporal gyrus and the parietal cortex as is apparent in (A), the projections onto cortical surfaces and in (B), the glassbrain projections. In (C), the sagittal section (X = 4 mm) shows deactivations in the posterior cingulate gyrus (pc) and the medial prefrontal cortex (mp). In (D), the coronal section (y = -8 mm) shows deactivation in the left amygdaloid region. (From A. Bartels, S. Zeki, Neuroreport, 11:3829–34, 2004.)

To some researchers, the combination of rodent and human studies presents a tidy picture. "The prairie vole story ... strips away cognitive clutter and allows you to see some of the components [of love] without the human version of cognition lying on top of it," says Sue Carter, professor of psychiatry at the University of Illinois at Chicago.

But others question this neat portrayal. Evan Balaban, an associate professor of psychology at McGill University, Montreal, says the vole findings may be "pretty remote from anything having to do with love." For instance, he suggests, increased vasopressin in voles might facilitate pair bonding not by directly creating a partner preference (a mechanism conceivably related to human love), but by reducing their odor sensitivity, a scenario of little apparent human relevance. Lowering odor sensitivity might facilitate long-term bonds by stifling the normally solitary voles' tendency to avoid each another, Balaban says.⁵

Fisher offers a different objection. She insists that animals experience love, at least somewhat, and that their love is homologous with human love. But in her view, vole pair-bonding is not love: It's attachment. She argues that attachment is what in humans is sometimes called "companionate love."⁶ It's what's left after the fire of romantic attraction dies down; it's the relationship necessary to raise young together and share chores.

Attachment is one of three neural systems Fisher distinguishes in connection with reproduction.⁷ The other two are romantic love, characterized by increased energy and focused attention on a preferred partner, and lust. Each can operate independently, she says, and is associated with a different neurochemistry: attachment with vasopressin and oxytocin, love with dopamine and norepinephrine, and lust with testosterone. "In prairie voles they have been studying attachment," she says, because the research focused on the creatures' lifelong bonds. "We have been studying love."

Like Bartels, Fisher has conducted neuroimaging studies of lovers as they view photographs of their beloveds.⁷ Her results highlighted brain areas associated with dopamine and norepinephrine, in patterns similar, though not identical, to those Bartels found. Both say the differences might have arisen because Fisher's studies required that participants had just fallen in love, whereas Bartel's studies didn't impose this requirement. Thus Bartels could have picked up more signs of what Fisher calls attachment.

If Fisher is correct about attachment being separate from love, it would seem, she says, that scientists haven't gotten very far beyond fMRI studies in examining love's chemistry. But other researchers don't use Fisher's distinctions; some make different ones. "Love is a concept. Concepts are not such solid, real things in the brain. Social attachment, that is a very solid, real thing," says Jaak Panksepp, research professor emeritus of psychobiology at Bowling Green State University, Akron, Ohio. He adds, "Surely that has implications for our understanding of love."

Bartels, for his part, tends to unify his subject of study with a larger theme: love in general, not just the romantic type. "It boils down to the biological function of bonding individuals together," he writes in an E-mail. Bartels and Zeki have conducted separate neuroimaging studies of maternal love, scanning mothers' brains as they viewed pictures of their babies. They found that many of the same regions were activated as in the romantic type.⁸ The zones of overlap, including the oxytocin and vasopressin areas, evidently represent a "core attachment system," Bartels says. He allows that in humans, one might even call them the neural substrates of "pure love."

The challenge remains to clarify how this circuitry works, and whether it really is related to simpler animals' pair-bonding systems, Balaban says. Human love "has all kinds of emotional components, cognitive components, in which if the other partner doesn't happen to be there, we can still talk about [the feelings]," he says. "It's unclear the extent to which mate preference in voles really resembles what we see in humans."