If serotonin is the Zen-master among neurotransmitter, is Pollyanna, responsible for the highs of infatuation, new love, joy, , and . But like all roller-coaster rides, dopamine highs have their dangers.

We use the term "falling in love" even though the first giddy days of a new romance feel buoyant, free from gravity, as if some perfectly placed wind were boosting us skyward. That sense of becoming airborne is so unforgettable that years later we may recall it longingly and wonder why relationships now seem so bound-to-Earth. We may even gaze on our partners with some dissatisfaction, measuring them against that lost intensity.

But the gleam of infatuation—if current theory is right—may be largely the product of unexpectedly potent brain chemistry. And the primary ingredient in that chemical brew is dopamine, a neurotransmitter best known for its ability to initiate muscle movement (and thus a key factor in diseases in which that ability is lost, such as Parkinson's). These days dopamine's profile is on the rise--the neurotransmitter became a celebrity in its own right when it graced the cover of Time magazine last May. But while that story focused on dopamine's role in , new research suggests that the neurochemical may be similarly important in triggering the joyously obsessive nature of first love. Rutgers University anthropologist Helen Fisher, Ph.D., suspects that love's initial all-consuming sizzle is part pure lust, part pure dopamine. "My prediction is that dopamine is an essential part of infatuation," says Fisher, who is now scanning the brains of wildly infatuated people, probing for that dopamine drive. "Dopamine," she notes, "is already associated with euphoria, , loss of , and a rush of motivation." In other words, the dazzling beginnings of love.

But as Fisher's comments suggest, such behaviors have far broader ramifications. Dopamine now seems everywhere in the brain: running through four main brain pathways, picked up by five different types of receptors—each with several subtypes, many still just being defined. Suddenly, the neurotransmitter is the target of research into , attention, , self-confidence, and goal-direction.

"Dopamine, how do I love thee? Let me count the ways," jokes George Koob, Ph.D., a professor of neuropharmacology at the Scripps Research Institute in La Jolla, California. Excitement about dopamine is now so high, says Koob, that the danger is not underestimating its reach but exaggerating it: "Today's gig is that dopamine is a kind of everyman's neurotransmitter because it does everything. And the fact is, it doesn't."

What does it do, then? The long-observed link to Parkinson's remains dead-on. Starting in the mid-brain and reaching to structures such as the basal ganglia, dopamine clearly functions as a kind of spark plug, initiating motor behavior. When the spark fails, the brain's ability to order appropriate muscle movement fails, too. "You are trapped in your body," is the way Koob describes it.

In addition, dopamine appears to influence attention and the ability to concentrate. This is why researchers think that some dopamine-enhancing drugs, such as Ritalin, help control the jittery, unfocused behaviors of .

Still the most intense excitement among researchers is over the role of dopamine within the limbic system, a brain region that helps regulate emotions. Basically, dopamine brightens and highlights our connections with the world around us, says David Goldman, Ph.D., a neuro-scientist with the National Institute of and Alcoholism. "It's essential for associating something that happens with the feeling of pleasure." In other words, it reinforces behaviors that make us feel good.

Koob tells an anecdote about , Ph.D., the Yale University psychologist who made famous the whole notion of reinforcement. According to the story—cheerfully shared among dopamine researchers—the famed psychologist's class once decided to experiment on him. So for an entire lecture half the class smiled at him, half glared. By the end of the session, he was only talking to the beaming side of the group. "And that, we assume, would have been a dopamine-driven response," Koob says.

In this sense, dopamine seems a sort of Pollyanna among neurotransmitters: It only really responds to the good stuff. In experiments with monkeys, scientists rewarded some tasks with juice (apparently beloved by the primates) and punished other tasks with puffs of air blown in their faces (equally hated.) The monkeys learned to anticipate either reward or . But the dopamine neurons in their mid-brain only got active when juice/pleasure was foreseen.

It seems logical to ask, then: If a person is born with a highly responsive dopamine system, wouldn't he or she be more tuned to receiving pleasure, to feeling rewarded? Would, in turn, the person with a sluggish response tend to be unmotivated and less exuberant overall? And what creates that outlook--the dopamine itself, the receptors that process dopamine's chemical messages, or some combination of the two? These are questions researchers are still sorting through.

But there is a growing consensus that the dopamine network does influence everyday behaviors. There is even speculation that by pursuing certain behaviors—single-mindedly focusing on a project, for instance—a person might increase that dopamine drive. Richard Depue, Ph.D., professor of human development at Cornell University, points out that goal-directed behavior (or the lack of it) tends to stand out as a major trait. Put simply, some of us are motivated to pursue goals and others are not. Depue believes it's the dopamine system that explains that difference. "When our dopamine system is active, we are more positive, excited, and eager to go after goals and rewards, whether it's food, , money, , or professional achievement," he says. In fact, he suggests that people who are goal-directed are not only more motivated but are generally happier. "We have strong evidence that feelings of elation [that occur] because you are moving toward achieving an important goal are biochemically based, though they can be modified by experience."

If we compare dopamine to serotonin—another "happiness" neurotransmitter—it becomes clear that dopamine requires a cautious approach. Serotonin is a Zen master among neurotransmitters, linked to tranquillity, reason, calm. The best-selling on the market--the Prozac family—keep serotonin levels high by preventing neurons from reabsorbing the neurotransmitter after it has been released, thus increasing its availability. Prozac users talk of an emotional leveling, a gradual disappearance of the depressive free-fall. "Prozac is not an abusable drug and never will be," Koob says. "There's no tangible effect the first time you take it. It just slowly helps regulate the system."

There's nothing Zen about dopamine; it doesn't call up thoughts of monasteries and , but of more breathless images--a teenager standing up in a plummeting roller-coaster car, perhaps. There's a body of work, still debated by researchers, that links dopamine directly to novelty-seeking and behaviors.

It's not surprising, then, that dopamine is suspected of facilitating the effects of not just Ritalin but more troubling drugs as well. As alcohol concentrations build in the body, the brain responds by releasing more dopamine. also seems to stimulate a dopamine rush. And with that comes a high-flying sense of pleasure, power, , a jazzed-up sense of energy. It's well known that rats and other lab animals will perform tasks devotedly and sometimes desperately in return for a cocaine hit once they become addicted. That behavior continues unless scientists block their dopamine transport system. Then the drug suddenly falls into the ho-hum category. Without dopamine, cocaine would apparently be nothing more than white, powdery stuff with a funny taste.

Richard Mailman, Ph.D., a professor of and pharmacology at the University of North Carolina at Chapel Hill, points out that a pleasure-reward system in the brain is, in an evolutionary sense, a terrific idea. It pushes us toward accomplishment, achievement, improving ourselves. "But it's supposed to be a subtle system," he says. "With cocaine, you get a jackhammer effect." There are hints, in fact, that the body tries to keep dopamine in check. And there's growing evidence that too much dopamine—notably in the limbic system—plays a role in the development of . A number of antipsychotic medications seem to suppress goal-oriented behaviors and interest in the world, a strong suggestion that the drugs are suppressing dopamine's effects.

Mailman emphasizes that dopamine does not work in isolation; indeed, many scientists suspect that dopamine and serotonin may interact—in some complicated and poorly understood ways—to produce illnesses such as schizophrenia. But researchers are currently fascinated by the dopamine angle, with its hints of how thin the line may be between intensity and insanity. One of the newest theories holds that dopamine initiates not only thoughts of pleasure, but perhaps initiates thought itself. "The idea is that schizophrenia is a result of dopamine hyper-activity," Koob explains. "The person is bombarded with so many thoughts their ability to pay attention to what's real and what's not is overwhelmed."

Given all this, perhaps it's reassuring that Fisher suspects that the dopamine high of first love is meant to be a merely transient pleasure. "When I first started looking at the properties of infatuation, they had some of the same elements of a cocaine high: sleeplessness, loss of a sense of time, absolute focus on love to the detriment of all around you. People walk out of marriages, abandon children. Infatuation can overtake the rational parts of your brain."

Fisher theorizes that different neurochemicals influence different stages of male-female relationships, which she categorizes as lust (driven mainly by ), /infatuation (dopamine), and . As we move into attachment—long-standing relationships—it may be that serotonin or hormones, such as , that are associated with nurturing behaviors become more important. For partners and parents, such stability is essential. It is those who never grow out of infatuation, who remain obsessed with love, that may eventually become dangerous lovers—stalkers, for instance.

On the other hand, Fisher's theory also holds hope for those who look back wistfully, wishing they could recreate those heady days of early romance. Fisher suspects that , in particular, may suppress the dopamine spark in relationships. But not necessarily forever. Later in the relationship the chemistry may return; long-time partners may somehow "fall" in love all over again, and be carried away on a dopamine high.

And people who have lost one partner thus may tumble again for another. Among those couples that Fisher is studying are newly met partners in nursing homes, people in their 70s and 80s, whose infatuation is just as intense as that shared by 20-year-old lovers. "I think there's a wonderful message in that," she says.

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BY DEBORAH BLUM

Deborah Blum is a Pulitzer Prize-winning science writer, the author of Sex and the Brain (Viking), and a professor in the School of Journalism at the University of Wisconsin.