Neurotransmitter of the '90s

Serotonin

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.

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."

CEREBRAL CORTEX

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.

CINGULATE GYRUS

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.

FRONTAL CORTEX

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.

CAUDATE NUCLEUS

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.

NUCLEI

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

Diagram: (SCOTT MACNEILL)