OG10-1-6：看了几遍了，还是不明白。请教。感谢感谢。

“the most widely used psychoactive substance on Earth .”

Synder, Daly and Bruns have recently proposed that

caffeine affects behavior by countering the activity in

(5) 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.

(10) 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 propose that caffeine, which is struc-

(15) turally 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 attri-

(20) buted to its inhibition of the production of phosphodi-

esterase, an enzyme that breaks down the chemical

called cyclic AMP.A number of neurotransmitters exert

their effects by first increasing cyclic AMP concentra-

tions in target neurons. Therefore, prolonged periods at

(25) the elevated concentrations, as might be brought about

by a phosphodiesterase inhibitor, could lead to a greater

amount of neuron firing and, consequently, to behav-

ioral stimulation. But Snyder et al point out that the

caffeine concentrations needed to inhibit the production

(30) of phosphodiesterase in the brain are much higher than

those that produce stimulation. Moreover, other com-

pounds that block phosphodiesterase’s activity are not

stimulants.

To buttress their case that caffeine acts instead by pre-

(35) venting 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

(40) 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 locomo-

tion.” Theophylline, a close structural relative of caffeine

and the major stimulant in tea, was one of the most

(45) 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-isobuty1-1-methylxanthine(IBMX), which bound very

(50) well but actually depressed mouse locomotion. Snyder

et al suggest that this is not a major stumbling block 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

(55) known only for its stimulatory effects, displays this

property, depressing mouse locomotion at very low

concentrations and stimulating it at higher ones.

(A)   reveal some of the assumptions underlying their theory

(B)   summarize a major finding of their experiments

(C)   point out that their experiments were limited to the mouse

(D)  indicate that their experiments resulted only in general correlations（B）

(E)   refute the objections made by supporters of the older theory

答案B，我选了E。

答案B，我选了E。

文章结构：第一段：snyder的观点--咖啡因通过抑制adenosine的附着，来达到刺激的效果；第二段：旧观点--咖啡因通过抑制phosphodiesterase的产生来达到刺激（然而snyder认为抑制phosphodiesterase需要的咖啡因浓度比咖啡因本身产生刺激的浓度要高很多）；第三段：snyder做了个试验来证明他的观点（第一段中提到），其中有些特例是不影响snyder的观点。

我觉得我对文章结构的理解应该没错吧，可是我就是不明白：1）为什么第三段的那个试验就证明了snyder的观点？什么逻辑，想不通。2）虽然我没想通我上面提到的这个问题，可是我觉得从全文逻辑来看，第6题答案应该选个类似“support snyder‘s view“等等，我觉得很多题都是这样答案啊。都不用看全句，通过第一个动词就可以判断。所以我选了E，觉得还沾点边，不过看了OG的解释。E确实也是瞎说了些文章没提到的咚咚。那么B为什么对？还请高人指点。