Passage10(10/63)-T3

bletyvette

Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth.” Snyder, Daly and Bruns have recently proposed that caffeine affect behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next. Like many other agents that affect neuron firing, adenosine must first bind to specific receptors on neuronal membranes. There are at least two classes of these receptors, which have been designated A₁ and A₂. Snyder et al (et al: abbr. (Lat) 以及其他人，等人) propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would.

For many years, caffeine’s effects have been attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase’s activity are not stimulants.

To buttress their case that caffeine acts instead by preventing adenosine binding, Snyder et al compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge adenosine from its receptors in the brains of mice. “In general,” they reported, “the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse; i.e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both regards.

There were some apparent exceptions to the general correlation observed between adenosine-receptor binding and stimulation. One of these was a compound called 3-isobutyl-1-methylxanthine (IBMX), which bound very well but actually depressed mouse locomotion. Snyder et al suggests that this is not a major stumbling block (stumbling block: n.障碍物, 绊脚石) to their hypothesis. The problem is that the compound has mixed effects in the brain, a not unusual occurrence with psychoactive drugs. Even caffeine, which is generally known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.

3.According so Snyder et al, caffeine differs from adenosine in that caffeine

(A) stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
(B) has mixed effects in the brain, whereas adenosine has only a stimulatory effect
(C) increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations
(D) permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release（D）

(E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing
想问一下这道题答案D对应原文的地方在哪里？找了很久都没有找到。。。谢谢啦~

jj0615

建议看看小安的分析，这里可以用文字对应法，adenosine出现在第一段，在GMAT中的考点一般是在第一次出现的地方。
这题目也不难做，要记得第一段的框架，就是caffeine和adenosine的关系------C阻碍A，注意是关系。回文定位， Snyder, Daly and Bruns have recently proposed that caffeine affect behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain.It apparently does this by inhibiting the release of neurotransmitters....
A中是实验内容不对
Bmixed effects说的是IBMX
C说AMP没有提到adenosine，因为说的是老观点
E同C

DeliciaAn

在第一段靠后的位置，但是只是知道部分答案。
Snyder et al propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would.(第3题D选项，但是不知道为什么是permits release of neutotransmitters)