Chapter 4 Neurons, Hormones, and the Brain 127that something about a disorder leads to abnor-
mal neurotransmitter levels instead of the other
way around. (If you have read Chapter 1, you’ll
recall that correlation does not imply causation.)
Although drugs that boost or decrease levels of
particular neurotransmitters are sometimes effec-
tive in treating severe cases of clinical depression,
this fact does not necessarily mean that abnormal
neurotransmitter levels cause the depression. After
all, aspirin can relieve a headache, but headaches
are not caused by a lack of aspirin!
Many of us regularly ingest things that affect
our own neurotransmitters. For example, sero-
tonin levels decrease after a protein-rich meal and
increase after a high-carbohydrate meal. And most
recreational drugs produce their effects by block-
ing or enhancing the actions of neurotransmitters,
as do some herbal remedies. St. John’s wort, which
is often taken for depression, prevents the cells
that release serotonin from reabsorbing excess mol-
ecules that have remained in the synaptic cleft; as
a result, serotonin levels rise. Many people do not
realize that such remedies, because they affect the
nervous system’s biochemistry, can interact with
other medications and can be harmful in high doses.
Watch the Video In the Real World:
Neurotransmitters at mypsychlabHormones: Long-Distance Messengers.
Hormones, which make up the second class of
chemical messengers, are produced primar-
ily in endocrine glands, such as the pancreas,
ovaries, testes, and adrenal glands. They are
released directly into the bloodstream, which
carries them to organs and cells that may be
far from their point of origin. Hormones have
dozens of jobs, from promoting bodily growth
to aiding digestion to regulating metabolism.
Neurotransmitters and hormones are not alwayshormones Chemical
substances, secreted by
organs called glands, that
affect the functioning of
other organs.endocrine glands
Internal organs that
produce hormones and
release them into the
bloodstream.• Acetylcholine affects neurons involved in
muscle action, arousal, vigilance, memory, and
emotion.
• Norepinephrine affects neurons involved in
increased heart rate, the slowing of intesti-
nal activity during stress, learning, memory,
dreaming, waking from sleep, and emotion.
Two other important neurotransmitters are
distributed throughout the brain, like taxis that
travel in all directions as needed:
• GABA (gamma-aminobutyric acid) is the
major inhibitory neurotransmitter in the brain.
• Glutamate is the major excitatory neurotrans-
mitter in the brain; it is released by about
90 percent of the brain’s neurons.
Harmful effects can occur when neurotrans-
mitter levels are too high or too low. Abnormal
GABA levels have been implicated in sleep and
eating disorders and in convulsive disorders, in-
cluding epilepsy. People with Alzheimer’s dis-
ease lose brain cells responsible for producing
acetylcholine and other neurotransmitters, and
these deficits help account for their devastating
memory problems. A loss of cells that produce
dopamine accounts for the tremors and rigidity of
Parkinson’s disease. In multiple sclerosis, immune
cells overproduce glutamate, which damages or
kills glial cells that normally make myelin.
We want to warn you, however, that pinning
down the relationship between neurotransmitter
abnormalities and behavioral or physical abnor-
malities is tricky. Each neurotransmitter plays
multiple roles, and the functions of different sub-
stances often overlap. Further, it is always possible
Actor Michael J. Fox has Parkinson’s disease, which in-
volves a loss of dopamine-producing cells. He has used his
fame to draw public attention to the disorder, and despite
noticeable tremors, he continues to act—even launching a
television series about a newsman with Parkinson’s.
Robert Leighton/The New Yorker Collection/ http://www.cartoonbank.com