OG16 RC 84

sylvia2014

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 bloodtryptophan

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.

84. According to the passage, an injection of insulin was most similar in its effect on

rats to an injection of

(A) tyrosine

(B) leucine

(C) blood

(D) tryptophan

(E) protein
答案是D
但是我觉得有点不太严谨
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.

从这个句子我感觉是injecting insulin造成了blood and brain tryptophan levels 和serotonin

levels的同时上升,所以tryptophan和insulin有相似effect是逻辑上很奇怪，因为insulin本身就会造成tryptophan increase

求大神解答

woodfishyang

LZ我觉得这道题的重点是 also caused parallel elevations, 肯定你会猜测到是不是前面有什么导致parallel elevations。结果就在前一句文章明确提到：
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......
答案就是D