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  3. It was once believed that the brain was independent of metabolic processes o

It was once believed that the brain was independent of metabolic processes o

游客2025-02-25  21
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. [br] According to the passage, when the authors began their first studies, they were aware that
选项 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.
答案 E
解析 当作者开始其最初研究时，他们已注意到：A．最终需要设计给老鼠喂高蛋白食物的计划。无。B．血中色氨酸难以观测。无。C．老鼠被喂富含色氨酸食物后，血清素含量升高。这是最终实验观察到的结果，不是实验前已知晓的。D．“many neurotransmitters”，文中无。E．正确。老鼠在被注射色氨酸后血清含量增加。见原文L11—16。
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