It was once believed that the brain was independent of meta

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
(5) in brain neurons of the neurotransmitter 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
(10) the increase in serotonin observed in rats given
a large injection of the amino acid tryptophan
might also occur after rats ate meals that change
tryptophan levels in the blood. Wefound that,
immediately after the rats began to eat, parallel
(15) elevations occurred in blood tryptophan, brain
tryptophan, and brain serotonin levels. These
findings suggested that the production and release
of serotonin in brain neurons were normally coupled
with biood-tryptophan increases. In later studies we
(20) found that injecting insulin into a rat's bloodstream
also caused parallel elevations in blood and brain
tryptophan levels and in serotonin levels. We then
decided to see whether the secretion of the animal's
own insulin similarly affected serotonin production.
(25) We gave the rats a carbohydrate-containing meal
that we knew would elicit insulin secretion. As we
had hypothesized, the blood tryptophan level and
the concentrations of tryptophan and of serotonin
in the brain increased after the meal.
(30) Surprisingly, however, when we added a large
amount of protein to the meal, brain tryptophan
and serotonin levels fell. Since protein contains
tryptophan, whyshould it depress brain tryptophan
levels? The answer lies in the mechanism that
(35) provides blood tryptophan 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 of protein increases
blood concentration of the other amino acids much
(40) more, proportionately, than it does that of tryptophan.
The more protein is in a meal, the lower is the ratio
of the resulting blood-tryptophanconcentration to
the concentration of competing amino acids, and
the more slowly is tryptophan provided to the brain.
(45) Thus the more protein in a meal, the less serotonin
subsequently produced and released.

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




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
with 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




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 oftryptophan



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




Accordingto 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) ratioof 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 kindthat the
rat will produce and release
(E) number of amino acids the rat's blood will
contain



The authors' discussion of the "mechanism that
provides blood tryptophan to the brain cells" (lines
34-35) is meant to
(A) stimulate further research studies
(B) summarize an area of scientific investigation
(0 help explain why a particular research finding
was obtained
(D) provide supporting evidence for a controversial
scientific theory
(E) refutethe conclusions of a previously mentioned
research study




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




It can be inferred from the passage that which of the
following would be LEAST likelyto 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
(0 Meals that would elicit insulin secretion
(D) ,Meals that had very low concentrations of
tyrosine
(E) Meals that had very low concentrations of
leucine




Itcan 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.