2. 外语
  3. It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h

It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h

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问题     It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discov- ered that the production and release 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 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 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 blood-tryp-tophan increases. In later studies we 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. We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion. As we had hypothesized, the blood trypto- phan 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 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 of protein increases blood con- centration 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 re- leased.
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?
选项 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.
答案A
解析 可从文中推出，作者本指望给老鼠喂大量蛋白质后会出现何种情况?第三段一开始surprisingly，现实情况与预想不符。将现实情况取非，即得预想情况。∴A正确，老鼠脑中血清素含量不会下降。B、C、D、E皆不合上述推理。
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