This Is Your Brain on Sex (19 page)

Ultimately the basal ganglia provide a sort of platform for monogamy in prairie voles. With vasopressin receptors present on this brain area, molecular signaling sets these animals up to prefer sex with a pair-bonded partner. When the gene that expresses these receptors is not present, as in the montane and meadow voles, it is more about the booty than any type of bonded relationship. And when it is present, then love, at least as seen in a prairie vole pair-bond, endures—that is, when you use oxytocin, vasopressin, and dopamine-related changes to the brain’s reward circuitry as the working definition of
love
.

Dopamine Reception and Maintenance

The neurochemicals oxytocin, vasopressin, and dopamine work together to help voles make a pair-bond. These compounds, mixed together with a little hot sex (as well as a few other chemicals that have yet to be characterized), transform the attraction from a few sniffs of urine into an attachment. What makes that attachme
nt last? What allows it to persevere, even when another sweet-smelling prairie vole saunters into the picture? It appears that it all comes down to the right type of dopamine receptor.

The nucleus accumbens does not only have a high density of oxytocin receptors in monogamous prairie voles, it also houses two types of dopamine receptors: D1 and D2. Previous work with prairie voles suggests that dopamine’s pleasurable effects are critical to the formation of a pair-bond. Giving female prairie voles a dopamine agonist, a drug that releases ample dopamine into the brain, allowed them to form a selective pair-bond with a male without the inconvenience of actually having sex. When the researchers blocked the D2 receptor using a drug, it stopped the animals from forming a pair-bond, no matter how many bodily fluids they exchanged.

Given that there are multiple dopamine pathways in the brain, Brandon Aragona, now a neuroscientist at the University of Michigan, and his colleagues at Florida State University wondered if a specific pathway might be responsible for pair-bonding. They soon discovered that the type of receptor in the nucleus accumbens that absorbed all the dopamine released from sex or a drug showed quite different effects depending on the type of vole and his current relationship status.

The group took a pair-bonded male and housed him with a female, either his bonded mate or a stranger. Normally in this scenario, the males will cuddle with their mates and get persnickety with the strangers; the bond means that only an animal’s one true love will do, and a different female is viewed as a threat. When Aragona and his colleagues infused the males with a D1 receptor antagonist (a drug that specifically blocked the D1 receptors) directly into the nucleus accumbens, the males were happy to cuddle with whatever female happened to drop in. It was as if their previous pair-bond had never existed. Just a simple infusion and—poof!—“love” disappeared.

Meadow voles naturally have a lot of D1-type receptors residing in their nucleus accumbens. This is true even before they mate. What gives? If D1 is so important to a pair-bond, these guys should be even more faithful than prairie voles. When the researchers blocked the nucleus accumbens D1 receptors in this species of vole, these asocial males all of a sudden became
much more affectionate. Although they did not form a pair-bond, the lack of D1 receptors made them much more amenable to company. These differences make it clear that dopamine works in a variety of ways when it comes to vole relationships.

Aragona’s work suggests that D1 and D2 receptors have distinct roles when it comes to monogamy. D2-type receptors, in the part of the nucleus accumbens projecting to the ventral pallidum (the brain area implicated in attachment) help animals initially form a pair-bond after that inaugural mating session. The dopamine released during mating is taken up by these receptors and makes physical changes to the brain, resulting in the bond. The male now associates his lady, or her unique odor, as it were, with love.

D1-type receptors appear more important to the maintenance of that pair-bond.
³
After the pair-bond is formed, male prairie voles show an increase of D1 receptors in the nucleus accumbens. Aragona and his colleagues hypothesized that, once voles are bonded, it is the D1 receptors that help ensure a male has eyes only for his one true love. All other females would be ignored or attacked. It is these receptors that are facilitating the double whammy of sugar previously discussed.

It’s amazing when you think about it. One chemical can be used in a variety of ways to facilitate these types of social behaviors, and that one chemical makes physical changes to the brain, creating the neural equivalent of a fidelity vow inside the basal ganglia.

Does this differential dopamine receptor action work the same way in other species? Karen Bales, a researcher at the University of California, Davis, and her colleagues looked at D1 receptor binding in titi monkeys (
Callicebus cupreus
), a monogamous primate species. The group used positron emission tomography to measure D1 binding, or how much dopamine was taken up by this specific receptor, in the nucleus accumbens, ventral pallidum, and caudate putamen of adult male titis before and four to eight weeks after forming a pair-bond with a female. The group’s preliminary results showed the opposite effect from what was seen in prairie voles: binding potential did not change in the pair-bonded males. In fact it was the unpaired males that showed an increase in D1 receptors in these three brain areas. Why this difference was found requires further study. It may be that titi monkeys have evolved a dopamine system that works a little differently
from prairie voles’, or there may be a third, unknown variable at work here that has yet to be discovered.
⁴
Bales plans to follow up on this work with titis, adding in scans months after bonds are formed, to better understand what kinds of changes pair-bonds are making to the male titi brain—and how the dopamine system may mediate them.

Dopamine obviously plays a role in making love last, but what regulates its variety of effects may vary. Aragona suggests that unique distributions of these different dopamine receptor subtypes may underlie species-typical mating strategies.
⁵
He was talking about different types of voles in this instance. But given Bales’s work, it’s just as likely that one might observe unique patterns of dopamine receptors across all species, be it prairie voles or titi monkeys or even human beings. Even with similar mating strategies—in this case, a monogamous pair-bond—higher-order mammals may have evolved different dopamine receptor patterns along with their bigger forebrains to facilitate the same sorts of behaviors. It’s entirely possible, but more work is needed before we can know for sure.

Oxytocin and Lasting Pair-Bonds

Humans don’t want just a long-lasting relationship. They also want a relationship of quality: a loving and beneficial connection. It’s assumed that the two go hand in hand, even though we know better. Perhaps understanding the neurochemistry behind a quality pair-bond will offer some answers about how to make love stay.

Cotton-top tamarins (
Saguinus oedipus
), another monogamous breed of monkey, also happen to be cooperative breeders. Moms, dads, and even older siblings all work together to help raise infants. Like humans, they are sexually active not only when they are ovulating; they commonly participate in recreational or “noncontraceptive” sex. Tamarins get down and dirty for the pleasure of the act, not just when the female happens to be fertile. While observing these humanlike tamarin pair-bonds, Charles Snowdon, a zoologist and psychologist at the University of Wisconsin, Madison, noticed that there was quite a bit of variety in relationship quality across different couples. “The animals had very different types of relationships with
each other,” Snowdon told me. “Some were lovey-dovey with lots of physical contact. These couples are always together, always touching. And they are having lots of sex. Other pairs were almost like random molecules moving around without much regard for each other. They seemed to have very little interest in being together. Honestly, you wouldn’t think they were affiliated if you did not know it already.”

You see the same thing in human relationships. Some couples, even after years of marriage, remain very physically and emotionally connected. I know one couple, wishing to be referred to herein as Dirk and Lola (probably so they can act it out as some kind of kinky fantasy later), who have been this way since they met in their teen years. More than ten years of marriage later, Dirk and Lola seem to have trouble being in the same room without touching one another. They have sex “as often as they can,” which, if they had their druthers, would be at least once a day. They both say they go out of their way to do nice things for each other whenever they can. Although they sometimes fight—they say their arguments can get quite heated—they ultimately remain focused on making each other as happy as possible. Even after years and years together they are still in the honeymoon period of their relationship.

Dirk and Lola can make those around them feel a little jealous over the quality of their connection (not to mention ill—there’s only so much happy cuddling folks outside of a pair-bond should have to tolerate). They provide only one side of the relationship quality continuum. On the other side of the spectrum you have couples who act as if they merely endure any time they are required to spend together. You wonder how they’ve managed to stay in a relationship for so long, given their obviously disconnected lives. These are the people you never see touch—in fact you rarely see them together. They may be technically married, but they don’t seem, well, bonded. This is the couple you point at when you say, “I’d rather be alone than be in a relationship like that.”

Previous research has shown that oxytocin is released after cuddling and sexual behavior. This led Snowdon and his team to wonder if pairs of tamarins that were more affectionate, engaging in more sex and cuddling with each other, had higher levels of oxytocin than less affectionate couples.

The research group observed the behavior
and measured urinary hormone levels in fourteen pairs of tamarins over three weeks. Despite oxytocin’s reputation as being a little more important in female pair-bonding, Snowdon did not see an overall difference in oxytocin levels between the sexes. “Within a pair, if one animal had high oxytocin levels, his mate also had high levels,” said Snowdon. “If the female had low levels, so did her guy. It was a very close correlation within couples.”

There was plenty of variation in oxytocin levels across the couples; it was as varied as the types of affectionate behaviors observed. When the group factored that behavior into the analysis, they found that the contact, grooming, and sexual behaviors explained more than half the variance in oxytocin levels. That is, the more cuddly a couple was, the higher its corresponding oxytocin levels. Conversely, the less affectionate a pair was, the lower its corresponding oxytocin levels.
⁶

Snowdon and his team took the analysis a level deeper. Were the same kinds of behaviors driving those oxytocin levels? Or were the males and females getting that boost of the hormone from different kinds of touches? He and his colleagues took a closer look at the data. They found that snuggling and grooming were behind the variance in the females, while for males it came down to how much sex they were having.

In high-oxytocin-level relationships, each member of the pair-bond worked to make sure the other was getting the kind of touching he or she needed. Males initiated the cuddling their mate wanted, and females solicited the sex their mate was after. The findings suggest that there are different behavioral mechanisms behind oxytocin levels and that in the best of relationships, like Dirk and Lola’s perhaps, members of pairs make sure to give their partners what they need.

Snowdon’s results present a bit of a chicken-and-egg problem. Do pair-bonded couples take care of each other in this way because of their high oxytocin levels? Or does putting their partner first lead to elevated oxytocin? It is difficult to sort out, but when I asked Snowdon what he thinks, he didn’t hesitate. “We never got to test this directly,” he said. “But I’m inclined to think it’s the behavior driving the hormones. Good couples are sensitive to what their partners need. And by giving your partner what they need, you up both of your oxytocin levels.”

Snowdon believes this study has direct relevance to humans. “I think we tend to overemphasize the intellectual
and emotional sides of relationships and underestimate the physical. Monkeys reconcile with touch and sexual behavior when something perturbs the relationship. It’s kissing and making up, really. And there’s no reason to think, when human relationships get stressed or there’s some kind of separation, that physical contact wouldn’t be important to maintaining the bond.” He paused for a moment. “It certainly couldn’t hurt.”

Oxytocin and Humans

Corresponding oxytocin levels have also been found in human couples. Ilanit Gordon, a researcher at Yale University, studies oxytocin levels in relationships. She says the findings are consistent across parents and bonded couples. Apparently oxytocin is important in human relationships too.

“So should I get a man tested for his resting oxytocin level before I agree to date him?” I asked her when we met.

“Studies have shown that levels of oxytocin in romantic partners
are correlated. They go up in the first few months of a relationship and then drop to the level we see in parents after that,” she said. “Something is happening there. Do you choose someone like you, who has a similar level of oxytocin? Or does coming together sync you up? It could even be both.” She paused. “In answer to your question, I don’t know that I would test a man before I date him. It’s just one marker out of many. For me, I’d check out how sweet someone is before dating him. That would be good enough for me.”