og-1-5

HHB

Passage 1

Caffeine, the stimulant in coffee, has been called

“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.

5. Snyder et al suggest that caffeine’s ability to bind to A1 and A2 receptors can be at least partially attributed to which of the following?

(A)   The chemical relationship between caffeine and phosphodiesterase

(B)   The structural relationship between caffeine and adenosine

(C)   The structural similarity between caffeine and neurotransmitters

(D)  The ability of caffeine to stimulate behavior（B）

(E) The natural occurrence of caffeine and adenosine in the brain

我选的是D。看了答案分析，觉得B的确正确。可是想不通为什么D不对。原文41-43行

不是说the higher their capacity to bind at the receptors, the higher their ability to stimulate

locomo-tion.么？

请指教，谢谢！！

wangyu73cn

结构关系－－结合A1和A2－－刺激行为，其因果关系是如此的。这里要求找出原因，即B选项，而D选项是结果而非原因，反了。

这说明楼主没有意识到ETS在考什么。而且，阅读的逻辑思路也没有搞清晰。

[此贴子已经被作者于2005-8-30 16:40:49编辑过]

HHB

谢谢！

刚刚开始做阅读，相信会逐渐熟悉ETS阅读的逻辑思路~~

swlfx

以下是引用wangyu73cn在2005-8-30 16:40:00的发言：

结构关系－－结合A1和A2－－刺激行为，其因果关系是如此的。这里要求找出原因，即B选项，而D选项是结果而非原因，反了。

这说明楼主没有意识到ETS在考什么。而且，阅读的逻辑思路也没有搞清晰。

wangyu牛牛