L o v e ,  S e x  &  H e a l t h
The Chemistry of Desire
Everyone knows what lust feels like. Scientists are now starting to understand how it happens
By Michael D. Lemonick

January 19, 2004 Health

	photo essay
	Animal Attraction There's more than one way to make hay, as birds, bees and bonobos know
	graphic
	Where Our Sex Drive Comes From Mapping the origins of sex drive on the human body
	remedies
	Love Potions A guide to some of the medical treatments available for what ails our libidos
	self-test
	The Passionate Love Scale Determine just how you feel about that special (or ex-special) someone

Eight years ago, after she had a hysterectomy at age 42, Roslyn Washington was left with an unexpected side effect. Her doctors, who had recommended removing her ovaries as well as her uterus because of fibroid tumors and an ovarian cyst, had warned her about a lengthy recovery period. But, she says, "I was not aware of the fact that there would be a decrease in my sexual life." That's something of an understatement. Washington, an office manager from Silver Spring, Md., who is married and has a teenage daughter, says that after the surgery she felt no sexual desire whatsoever. "I didn't think about it," she says. "I didn't get that urge from a glance or a look or a touch." It was a profound loss. "Without that connection, without the sexual aspects, you feel in some instances like you're really less than a woman," she says.

Then several years later Washington heard a radio ad seeking women like her for a study of testosterone patches. People usually think of testosterone as a male hormone, but women have plenty in their systems too, and researchers have reason to believe that the hormone is involved in the female sex drive. About half of women's testosterone is produced in the ovaries, so the patches were an attempt to replace what had been lost through surgery. Washington signed up and was one of 75 women accepted for the study out of 50,000 who applied; clearly she was not alone in her misery. Twice a week for the next year she affixed a thin, clear patch onto her abdomen, alternating sites over where her right and left ovaries used to be.

Washington didn't know whether she was receiving transdermal testosterone or a placebo. She did know that things were very different. "I hadn't felt like that in years," she says. "I felt stimulated. It was like, 'Oh, yeah, I'd forgotten that's what it feels like.' It was good." Alas, when the trial ended, her desire ebbed.

It's tempting to conclude that Procter & Gamble, manufacturer of the testosterone patch, had found the elusive chemical key to female desire. The study, published in 2000 in the New England Journal of Medicine, reported that many of the women who, like Washington, were on real testosterone had more sexual fantasies and more sex and masturbated more than they had before. But so, albeit to a lesser extent, did women who wore patches with no testosterone at all. For women suffering from lost libido, the placebo effect was almost as strong as that of the hormone. In short, the mere belief that the treatment would rekindle sexual desire was often enough to turn up the heat.

This finding illustrates the promise and the perplexity of research into the biology of human sexuality, where mind, body and experience are endlessly intermingled. People find themselves turned on in obvious situations—slow-dancing together, seeing someone with a sexy body, finding a member of the opposite or same gender to be excitingly sharp-witted or funny. But carnal longings strike at surprising times too—in the wake of a victory by your favorite team (for men, anyway) or at times of fear or even after a tragedy, like the death of a parent.

No matter how lust is triggered, though, sex, like eating or sleeping, is ultimately biochemical, governed by hormones, neurotransmitters and other substances that interact in complicated ways to create the familiar sensations of desire, arousal, orgasm. By understanding how that happens, scientists should in principle be able to help people like Washington for whom sex just isn't working. And indeed, over the past decade or two, scientists have identified many of the pieces of this complex puzzle. It clearly involves testosterone, along with other hormones, including estrogen and oxytocin, and brain chemicals such as dopamine, serotonin and norepinephrine. And there are numerous other bodily chemicals that turn us on, ranging from the commonplace, nitric oxide, to the obscure, vasoactive intestinal polypeptide.

Scientists have also learned that the old notion that 90% of sex is in the mind is literally true: the parts of the brain involved in sexual response include, at the very least, the sensory vagus nerves, the midbrain reticular formation, the basal ganglia, the anterior insula cortex, the amygdala, the cerebellum and the hypothalamus.

If all this sounds complicated, it is. Researchers are still struggling to understand how these pieces fit together and how they might be different for men and women. It's not clear which chemicals of desire are unleashed and under which circumstances, because setting and mood, as women know better than men, can make all the difference between arousal and annoyance.

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