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It was once believed that the brain was independent
of metabolic processes occurring elsewhere in the body.
In recent studies, however, we have discovered that the
production and release in brain neurons of the neuro-
(5) transmitter serotonin (neurotransmitters are compounds
that neurons use to transmit signals to other cells)
depend directly on the food that the body processes.
Our first studies sought to determine whether the
increase in serotonin observed in rats given a large injec-
(10)tion of the amino acid tryptophan might also occur after
rats ate meals that change tryptophan levels in the
blood. We found that, immediately after the rats began
to eat, parallel elevations occurred in blood tryptophan,
brain tryptophan, and brain serotonin levels. These find-
(15) ings suggested that the production and release of sero-
tonin in brain neurons were normally coupled with
blood-tryptophan increases. In later studies we found
that injecting insulin into a rat’s bloodstream also caused
parallel elevations in blood and brain tryptophan levels
(20) and in serotonin levels. We then decided to see whether
the secretion of the animal’s own insulin similarly affected
serotonin production. We gave the rats a carbohydrate-
containing meal that we knew would elicit insulin secre-
tion. As we had hypothesized, the blood tryptophan
(25) level and the concentrations of tryptophan
serotonin in the brain increased after the meal.
Surprisingly, however, when we added a large
amount of protein to the meal, brain tryptophan and
serotonin levels fell. Since protein contains tryptophan,
(30) why should it depress brain tryptophan levels? The
answer lies in the mechanism that provides blood tryp-
tophan to the brain cells. This same mechanism also
provides the brain cells with other amino acids found in
protein, such as tyrosine and leucine. The consumption
(35) of protein increases blood concentration of the other
amino acids much more, proportionately, than it does
that of tryptophan. The more protein in the meal, the
lower is the ratio of the resulting blood-tryptophan
concentration to the concentration of competing amino
(40) acids, and the more slowly is tryptophan provided to
the brain. Thus the more protein in a meal, the less
serotonin subsequently produced and released.
1. Which of the following titles best summarizes the
contents of the passage?
(A) Neurotransmitters: Their Crucial Function in
Cellular Communication
(B) Diet and Survival: An Old Relationship Reexamined
(C) The Blood Supply and the Brain: A Reciprocal
Dependence
(D) Amino Acids and Neurotransmitters: The
Connection Between Serotonin Levels and Tyrosine
(E) The Effects of Food Intake on the Production and
Release of Serotonin: Some Recent Findings
2. According to the passage, the speed with which
tryptophan is provided to the brain cells of a rat varies
with the
(A) amount of protein present in a meal
(B) concentration of serotonin in the brain before a meal
(C) concentration of leucine in the blood rather than on
the concentration of tyrosine in the blood after a
meal
(D) concentration of tryptophan in the brain before a
meal
(E) number of serotonin-containing neurons present in
the brain before a meal
3. According to the passage, when the authors began their
first studies, they were aware that
(A) they would eventually need to design experiments
that involved feeding rats high concentrations of
protein
(B) tryptophan levels in the blood were difficult to
monitor with accuracy
(C) serotonin levels increased after rats were fed meals
rich in tryptophan
(D) there were many neurotransmitters whose
production was dependent on metabolic processes
elsewhere in the body.
(E) serotonin levels increased after rats were injected
with a large amount of tryptophan
4. According to the passage, one reason that the authors
gave rats carbohydrates was to
(A) depress the rats’ tryptophan levels
(B) prevent the rats from contracting diseases
(C) cause the rats to produce insulin
(D) demonstrate that insulin is the most important
substance secreted by the body
(E) compare the effect of carbohydrates with the effect
of proteins
5. According to the passage, the more protein a rat
consumes, the lower will be the
(A) ratio of the rat’s blood-tryptophan concentration to
the amount of serotonin produced and released in the
rat’s brain
(B) ratio of the rat’s blood-tryptophan concentration to
the concentration in its blood of the other amino
acids contained in the protein
(C) ratio of the rat’s blood-tyrosine concentration to its
blood-leucine concentration
(D) number of neurotransmitters of any kind that the rat
will produce and release
(E) number of amino acids the rat’s blood will contain
6. The authors’ discussion of the “mechanism that provides
blood tryptophan to the brain cells” (lines 31-32) is
meant to
(A) stimulate further research studies
(B) summarize an area of scientific investigation
(C) help explain why a particular research finding was
obtained
(D) provide supporting evidence for a controversial
scientific theory
(E) refute the conclusions of a previously mentioned
research study
7. According to the passage, an injection of insulin was
most similar in its effect on rats to an injection of
(A) tyrosine
(B) leucine
(C) blood
(D) tryptophan
(E) protein
8. It can be inferred from the passage that which of the
following would be LEAST likely to be a potential
source of aid to a patient who was not adequately
producing and releasing serotonin?
(A) Meals consisting almost exclusively of protein
(B) Meals consisting almost exclusively of
carbohydrates
(C) Meals that would elicit insulin secretion
(D) Meals that had very low concentrations of tyrosine
(E) Meals that had very low concentrations of leucine
9. It can be inferred from the passage that the authors
initially held which of the following hypotheses about
what would happen when they fed large amounts of
protein to rats?
(A) The rats’ brain serotonin levels would not decrease.
(B) The rats’ brain tryptophan levels would decrease
(C) The rats’ tyrosine levels would increase less quickly
than would their leucine levels
(D) The rats would produce more insulin.
(E) The rats would produce neurotransmitters other than
serotonin.