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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.
--THE CUDDLE HORMONE
Endocrinologists have known for years that oxytocin, released by the pituitary gland, ovaries and testes, helps trigger childbirth contractions, milk production during nursing and the pelvic shudders women experience during orgasm (and possibly the contractions during male orgasm as well). The hormone is believed to play a vital role in mother-child bonding and may do the same for new fathers: oxytocin surges when a new dad holds his bundle of joy. Some researchers also think of oxytocin as a cuddle chemical. Preliminary studies by psychiatrist Kathleen Light at the University of North Carolina have found that oxytocin levels rise after couples hold hands, hug or watch romantic movies. It also may be what makes you want to stay with your partner until the morning after sex. Those who can relate to Billy Crystal's "How long do I have to lie here?" scene from When Harry Met Sally might question whether oxytocin affects both genders equally.
But there's increasing evidence that oxytocin is also involved in deeper bonding. It certainly plays that role in a much studied little rodent called the prairie vole, which is famous for its fidelity to its mate. The critter's brain releases a rush of oxytocin as it bonds with its beloved. Block the chemical, and voles fail to make a connection. Inject more of the hormone, and they fall for each other even faster.