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The Science of Sex: Love Potion Number Nine

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Testosterone and estrogen have been the big man (and woman) on campus lately, strutting their stuff as media metaphors, catchphrases for every human drive. But mention oxytocin and even the most thoughtful hedonist may draw a blank. Strange, since this hormone’s effects are even more pervasive than those of the showy sex steroids: oxytocin influences functions as basic as falling in love, having an orgasm, giving birth and breastfeeding. It’s about time the hard-working chemical got a bit more credit.

    

Technically, oxytocin is a peptide: nine amino acids linked in a molecular circle like a bracelet. It is synthesized by nerve cells in the hypothalamus — your brain’s Sex Central — and shipped to the pituitary gland, where it is secreted into the bloodstream. From there, it trips off a wide range of effects, almost all of them involving sex, or the after-effects of sex.

    

Take childbirth. During labor, the distension of the cervix and vagina trigger the release of oxytocin from the pituitary — stimulating muscle contractions and helping push the fetus out (which is why oxytocin is sometimes administered to pregnant women to accelerate labor). Then there’s breast-feeding: when a suckling infant stimulates the nipple, this flips the switch on oxytocin, which causes milk to flow.

    

So far, so fabulous for babies — but what about those of us who’ve outgrown the teat? Oxytocin does plenty for adults. In fact, according to recent research, it plays a major role in orgasmic pleasure. In both sexes, sexual climax triggers a flood of the hormone. This surge may help transport sperm by stimulating muscular contraction in the reproductive tracts of both men and women. But it also makes sex feel so good: when oxytocin is blocked, orgasm occurs, but doesn’t pack nearly the same pleasurable punch.

    

Because oxytocin is such a multi-tasker, interesting cross-reactions can occur. For example, women who are lactating sometimes release milk during sex. (And the milkmaids of old knew a trick for getting the most milk out of a cow: blow into her vagina!) Kinkier yet, breast-feeding can trigger orgasm in some highly sensitive women.

    

The fact that oxytocin influences the subjective feelings associated with orgasm suggests that it has direct effects on the brain, and this is the area of research that has yielded the most interesting findings over the last year or two. A group of researchers at the University of Maryland, led by psychobiologist Sue Carter, has amassed evidence that oxytocin helps establish pair bonds in monogamous rodents. In a study published in February of this year, Carter’s group placed a female prairie vole in a cage with a randomly chosen male: the ultimate blind date. While the two animals were together, the researchers injected the female with oxytocin, mimicking the natural release of the hormone from the pituitary gland. Subsequently, the female lingered near the male with whom she had been housed during the hormone treatment, preferring him over all other males — a reaction suggesting that oxytocin may be the original organic love potion.

    

In July, another group, led by neuroscientist Thomas Insel of Emory University, reported on a related experiment with mice. Using genetic engineering techniques, they created mutant mice that lacked oxytocin. These mice, they discovered, had “social amnesia”: they couldn’t learn to pair up with other mice. Insel speculates that a defect in the oxytocin system might underlie human autism, a condition characterized by difficulty in social bonding.

    

Is Insel jumping the gun by suggesting direct human parallels to these animal studies? Perhaps, but the implications are intriguing. Does the oxytocin released at orgasm help lovers bond emotionally? Does nursing hormonally cement mother and child? Details remain to be explored. Perhaps the presence of oxytocin in the brain strengthens synaptic connections between, on the one hand, the circuits that represent the sights, sounds and smells of a particular person and, on the other, the brain centers that mediate feelings of well-being or sexual arousal.

    

What’s particularly interesting is how plastic and easily conditioned the oxytocin system is. That’s obvious in the case of lactation: a cow will quickly learn to produce milk at the sound of a clanking milk-pail, and a woman at her baby’s cry. This same plasticity could be the basis for a wide range of sexual phenomena — from sexual orientation to fetishism and paraphilias. (Could a rush of oxytocin at the right moment eroticize an innocent high heel?) And as usual with such discoveries, drugs are probably not far behind. Someday soon, I suspect, medicines that mimic or block the powerful effects of this flexible hormone may reshape the landscape of our emotional lives — with potentially enormous repercussions for the way we think of the nature of desire itself.


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