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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 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-tryptophan 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 tryptophan level and the concentrations of tryptophan and of 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 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 is in a 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.

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?

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当前版本由 ya**** 更新于2015-11-05 09:37:33 感谢由 ya**** 对此题目的解答所做出的贡献。
文章第三段谈及了蛋白质,因此定位于第三段,其首句surprisingly,however说明了作者之前的假设与第三段的发现相违背
A正确,对fell取非成为would not decrease
B错误,与文章相反
C错误,与T物质无关
D错误,与胰岛素无关
E错误,与第一段词汇neurotransmitters无关

题目讨论 (6条评论)

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  • 用户头像
    847716opn

    看到innitially就一直在找作者以前的观点,其实however后面就可以判断作者以前的观点
    0 0 回复 2019-09-25 20:16:03
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    Joyce L

    对降低感到惊讶证明预期是不会降低而不是会增高
    0 0 回复 2019-07-28 01:36:55
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    zoraaa

    initially
    太粗心了
    0 0 回复 2019-03-03 20:57:22
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    Richard1996

    题干 initially......................
    0 0 回复 2018-11-30 13:12:53
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    小冷烤鸡中

    定位错误,应由protein定位在第三段。
    0 0 回复 2018-06-16 17:29:24
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    ***5593

    第三段首句when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell. fell=decrease, 为什么B错????
    0 0 回复 2016-05-10 13:30:11
    • 评论用户头像

      ***5593回复***5593

      因为题干

      0 0 回复 2016-05-10 13:31:30

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