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

FUELS FOR LUST

If there's one substance that ultimately makes it possible to get turned on in the first place, testosterone is probably it. "When testosterone is gone," says UCLA's Berman, "for whatever reason—aging, medication—men experience erection and libido problems." Restore the testosterone, and you usually fix those problems.

Women too seem to have problems getting interested in sex when their testosterone levels are too low, which is why Procter & Gamble is experimenting with testosterone patches. Says Altman: "When women are having normal menstrual cycles in their prime reproductive ages, their ovaries make two times more testosterone than estrogen." A few days before ovulation, triggered by surging levels of testosterone—along with other hormones including progesterone and estrogen—sexual desire peaks, according to new research by Martha McClintock of the University of Chicago that dispels a long-held theory that fertility precedes desire.

But for women, at least, estrogen may also be crucial. "Give estrogen to women with decreased desire," says Pfaus, "and you don't restore desire. Give them testosterone alone, and you get a little increase in desire. Give them estrogen and testosterone together, and you get a whopping increase." Why? Some research suggests that testosterone's role in women is diversionary: it attaches to so-called steroid-binding globulins in the blood that would otherwise latch onto estrogen molecules and render them inert. The testosterone is taken away to the liver, while the estrogen is free to make a lust-inducing dash for the brain.

Pfaus argues further that estrogen may be the ultimate love hormone for men as well. "A lot of studies on rats and birds," he says, "show that brains are like giant ovaries, in the sense that testosterone and other androgens are converted into estrogens in the hypothalamus. And this conversion appears to be critical to the expression of male sexual behavior."

THE FEEL-GOOD CHEMICAL

Both testosterone and estrogen trigger desire by stimulating the release of neurotransmitters in the brain. These chemicals are ultimately responsible for our moods, emotions and attitudes. And the most important of these for the feeling we call desire seems to be dopamine. Dopamine is at least partly responsible for making external stimuli arousing (among other things, it's thought to be the pleasure-triggering substance underlying drug addiction). "Being low on dopamine," says the University of Washington Medical School's Heiman, "correlates with being low on desire." And in men dopamine-enhancing drugs (including some antidepressants and anti-Parkinson's medications) can increase desire and erections. So can apomorphine, a Parkinson's drug that latches directly onto the dopamine receptors in brain cells and turns them on.

Another neurotransmitter almost certainly involved in the biochemistry of desire is serotonin, which, like dopamine, plays a role in feelings of satisfaction. Antidepressants like Prozac, which enhance mood by keeping serotonin in circulation longer than usual, can paradoxically depress the ability to achieve orgasm. But "dopamine and serotonin," says Heiman, "appear to interact with each other in a complicated way to impact desire."

So, researchers suspect, do the neurotransmitters epinephrine and norepinephrine, whose usual job is to pump up our energy when we're in danger. Blood-plasma levels of both chemicals increase during masturbation, peak at orgasm and then decline, and by-products of norepinephrine metabolism remain elevated for up to 23 hours after sex. It's not yet clear, though, whether this is a cause or an effect of arousal.

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