Passage 38

It was once believed that the brain was independent of metabolic<sup>1 processes occurring elsewhere in the body.In recent studies, however, we have discovered that the production and release in brain neurons of the neuro transmitter serotonin 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 injection 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 elevations2 occurred in blood trypTOPhan, brain trypTOPhan, and brain serotonin levels. These find- ings suggested that the production and release of serotonin in brain neurons were normally coupled with blood-trypTOPhan increases. In later studies we found that injecting insulin into a rats bloodstream also caused parallel elevations in blood and brain trypTOPhan levels and in serotonin levels. We then decided3 to see whether the secretion4 of the animals own insulin similarly affected5 serotonin production. We gave the rats a carbohydrate- containing meal that we knew would elicit6 insulin secretion. As we had hypothesized, the blood trypTOPhan 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,why should it depress brain trypTOPhan levels? The answer lies in the mechanism7 that 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 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 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?
Neurotransmitters: Their Crucial Function in Cellular8 Communication
Diet and Survival: An Old Relationship Reexamined
The Blood Supply and the Brain: A Reciprocal Dependence9
Amino Acids and Neurotransmitters: The Connection Between Serotonin Levels and Tyrosine
The Effects of Food Intake10 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
amount of protein present in a meal
concentration of serotonin in the brain before a meal
concentration of leucine in the blood rather than on the concentration of tyrosine in the blood after a meal
concentration of trypTOPhan in the brain before a meal
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
they would eventually need to design experiments that involved feeding rats high concentrations of protein
trypTOPhan levels in the blood were difficult to monitor with accuracy
serotonin levels increased after rats were fed meals rich in trypTOPhan
there were many neurotransmitters whose production was dependent on metabolic processes elsewhere in the body.
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 carbohydrates11 was to
depress the rats trypTOPhan levels
prevent the rats from contracting diseases
cause the rats to produce insulin
demonstrate that insulin is the most important substance secreted12 by the body
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
ratio of the rats blood-trypTOPhan concentration to the amount of serotonin produced and released in the rats brain
ratio of the rats blood-trypTOPhan concentration to the concentration in its blood of the other amino acids contained in the protein
ratio of the rats blood-tyrosine concentration to its blood-leucine concentration
number of neurotransmitters of any kind that the rat will produce and release
number of amino acids the rats blood will contain
6. The authors discussion of the mechanism that provides blood trypTOPhan to the brain cells is meant to
stimulate13 further research studies
summarize an area of scientific investigation14
help explain why a particular research finding was obtained
provide supporting evidence for a controversial scientific theory
refute the conclusions of a previously15 mentioned research study
7. According to the passage, an injection of insulin was most similar in its effect on rats to an injection of
tyrosine
leucine
blood
trypTOPhan
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?
Meals consisting almost exclusively of protein
Meals consisting almost exclusively of carbohydrates
Meals that would elicit insulin secretion
Meals that had very low concentrations of tyrosine
Meals that had very low concentrations of leucine
9. It can be inferred from the passage that the authors initially16 held which of the following hypotheses about what would happen when they fed large amounts of protein to rats?
The rats brain serotonin levels would not decrease.
The rats brain trypTOPhan levels would decrease
The rats tyrosine levels would increase less quickly than would their leucine levels
The rats would produce more insulin.
The rats would produce neurotransmitters other than serotonin</sup>