Neurotransmitter of the '90s


When a stray thought niggles your brain, reminding you to stop biting your nails, you'll stop because you want to and because you can-your brain's cells have no trouble communicating the message to your body. But many people aren't so lucky. Their brain produces too much or too little serotonin, one of several chemicals that transmit impulses between nerve cells, or neurons.

Accumulating evidence reveals that serotonin is unlike other neurotransmitters. It does not have receptors localized in a few specific areas of the brain. Instead, nerve cells tipped with serotonin-sensitive binding sites cluster deep within the brain stem and send neuronal tentacles snorkeling out through the gray matter. Hence, their uptake and release of serotonin ultimately affects much of our mental life.

o Take the case of suicidality. Brain tissue from the frontal cortex of suicide victims has fewer serotonin-releasing neurons than normal brain&-but more that suck up the transmitter, suggesting an attempt to compensate for serotonin lack by upping receptor sensitivity. Still, low serotonin producers suffer from suicidal thoughts during acute stress. Some may be born with "suicidal" brains. Measurements of serotonin debris in spinal fluid may identify those at risk.

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o Serotonin imbalance seems to underlie obsessive-compulsive disorder (0CD), an anxiety-related condition marked by intrusive thoughts and repetitive behaviors such as hand-washing. Neurotransmitter activity may be abnormal in the frontal cortex, caudate, and cingulate gyrus, all rich with serotonin neurons.

o Surging serotonin elsewhere may bring on bliss. People who take the illegal drug MDMA, or Ecstasy, get a ,'serotonin high" as MDMA-responsive cells release large amounts of the neurotransmitter, stimulating receptors in middle layers of the cerebral cortex. In high doses, Ecstasy destroys serotonin nerve fibers. A less toxic form of MDMA might help those with illnesses like AIDS face death more calmly.

o Serotonin may interact with sex hormones to orchestrate sexual behavior, animal studies show. In female rats, serotonin usually inhibits sexual behavior. But as estrogen and progesterone make their cyclic rise, they dampen serotonin neurons in the hypothalamus, and the females go into heat. In males, serotonin does the opposite, promoting sexual activity in the presence of testosterone. Serotonin may yield better ways to control sexual dysfunction in men and ovulation in women.

o Serotonin aids and abets learning and memory formation. As serotonin is released in response to a stimulus, it attaches to the receptors of the next cell down the line, raising its excitability level and increasing the chance it will become part of a circuit that encodes memory. Serotonin also enhances the neuron's electrical impulse, creating enduring memory. These responses turn on at different stages of development and underlie two distinct types of learning-sensitization and dishabituation.

As scientists finally get a handle on the complexities of serotonin, they also implicate it in schizophrenia, depression, alcohol abuse, chronic pain, premenstrual syndrome, eating disorders, and panic attacks. "Serotonin is only one of the molecules in the orchestra," says Yale neuropsychologist Thomas J. Carew. "But rather than being the trumpet or the cello player, it's the band leader who choreographs the output of the brain."


The thinking brain. In six layers, makes up 80% of brain. Stimulation of serotonin nerve fibers in the mid-layers induces a blissful feeling of oneness with life.


Regulates limbic system, or emotional brain, notably emotional states pertaining to interpersonal relations, especially those of a sexual nature. Serotonin imbalance here linked to 0CD.


Controls complex motor function, expressive speech; also faculties of planning and mental representation of outer world. Imbalance of serotonin here linked to suicidal thoughts during stress.


Regulates both voluntary and automatic movements. Serotonin imbalance here implicated in 0CD. Hormonal command center interregulates nervous and endocrine systems; translates emotion into physical response. Serotonin here dampens sexual activity in females, stirs it in males.


Serotonin-sensitive cells originate here in primitive brain stem and branch widely, influencing broad range of functions.


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