THE MOOD MOLECULE

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It would be, that is, if not for the neurotransmitters. These chemicals are stored in tiny sacs known as vesicles, located at nerve endings. When an electrical signal reaches the vesicles, they release their load. The neurotransmitter molecules navigate across the synapse and lock into receptors on the neighboring nerve cell--an action roughly equivalent to flipping on a light switch. The second nerve cell wakes up and sends off a jolt of electricity to pass the message along. Their job completed, the neurotransmitter molecules detach and are ferried back to be reabsorbed or destroyed.

That's the simple version of the story. In practice, things are a bit more complicated. To start with, different neurons specialize in releasing different neurotransmitters. Many carry messages that convey facts about the outside world--incoming sounds, patterns of light and so on--and integrate them into useful information. Some neurotransmitters also carry messages of action, telling muscles when to release or contract.

But a small subset of these brain chemicals, especially serotonin, evidently serves an entirely different purpose. As Steven Hyman, director of the National Institute of Mental Health, describes it, "These neurotransmitters modulate raw information and give it its emotional tone." Northwestern University psychiatrist James Stockard puts it more poetically: "A person's mood is like a symphony, and serotonin is like the conductor's baton." Other neurotransmitters help us know our stomachs are full; serotonin tells us whether we feel satisfied. Other chemicals help us perceive the water level in a glass; serotonin helps us decide whether we will think of it as half empty or half full.

With such a broad neurological portfolio, it is no wonder these mood-changing brain chemicals have been implicated in so many mental disorders. And it is not surprising that serotonin appears to be especially important--the first among equals, in a sense. The nerve cells that specialize in serotonin production originate in the raphe nuclei, in a region right atop the spine that nimh's Hyman calls "the deep basement of the brain." From there, these neurons extend vinelike projections called axons up through the brain and down into the spinal column. The axons form a sort of neurological interstate-highway network, over which serotonin supplies are sent to all parts of the nervous system. Other neurotransmitters are restricted to certain regions of the brain or the body; serotonin is just about everywhere.

Its effects vary widely, however, depending on a variety of factors. To begin with, each neurotransmitter can latch onto more than one kind of receptor. As many as 15 distinct receptors have been identified for serotonin alone. And since a given nerve cell may have more or fewer receptors, depending on where in the brain it is located, a jolt of a particular neurotransmitter can generate electrical signals of widely varying strengths. Small wonder, therefore, that serotonin can affect everything from satiety to depression.