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发布时间：2006-06-23 17:05     点击：

　　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 neuro- (5) transmitter 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 injec- (10)tion 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 find- (15) ings suggested that the production and release of sero- tonin 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 (20) 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 secre- tion. As we had hypothesized, the blood tryptophan (25) level and the concentrations of tryptophan serotonin in the brain increased after the meal.

　　1. Which of the following titles best summarizes the contents of the passage?

　　(A) Neurotransmitters: Their Crucial Function in Cellular Communication

　　(B) Diet and Survival: An Old Relationship Reexamined

　　(C) The Blood Supply and the Brain: A Reciprocal Dependence

　　(D) Amino Acids and Neurotransmitters: The Connection Between Serotonin Levels and Tyrosine

　　(E) The Effects of Food Intake on the Production and Release of Serotonin: Some Recent Findings

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