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.
According to the passage, the speed with which tryptophan is provided to the brain cells of a rat varies with the
选项 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.
答案A
解析 色氨酸被供给到鼠的神经细胞的速度和以下哪个相关?A．正确。蛋白质在食物中含量。符合第三段叙述，蛋白质在食物中含量越高，色氨酸的供给速度就越慢。B、C、D、E皆提到“before a meal”、“after a meal”，文中根本没有这种比较。
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