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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 A1and

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 fects 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 fects by first increasing cyclic AMP concentra-

tions in target neurons. Therore, 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 fects 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) fective 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 fects in the brain, a not unusual occurrence with

psychoactive drugs. Even caffeine, which is generally

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

property, depressing mouse locomotion at very low

concentrations and stimulating it at higher ones.

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SAT阅读文章模拟题一例

　　1. The primary purpose of the passage is to

　　(A) discuss a plan for investigation of a phenomenon that is not yet fully understood

　　(B) present two explanations of a phenomenon and reconcile the differences between them

　　(C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two

　　(D) describe an alternative hypothesis and provide evidence and arguments that support it

　　(E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it

　　2. Which of the following, if true, would most weaken the theory proposed by Snyder et al?

　　(A) At very low concentrations in the human brain. both caffeine and theophylline tend to have depressive rather than stimulatory fects on human behavior.

　　(B) The ability of caffeine derivatives at very low concentrations to dislodge adenosine from its receptors in mouse brains correlates well with their ability to stimulate mouse locomotion at these low concentrations

　　(C) The concentration of cyclic AMP in target neurons in the human brain that leads to increased neuron firing can be produced by several different phosphodi esterase inhibitors in addition to caffeine.

　　(D) The concentration of caffeine required to dislodge adenosine from its receptors in the human brain is much greater than the concentration that produces behavioral stimulation in humans.

　　(E) The concentration of IBMX required to dislodge adenosine from its receptors in mouse brains is much smaller than the concentration that stimulates locomotion in the mouse.

　　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 fects in the brain, whereas adenosine has only a stimulatory fect

　　(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

　　(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

　　4. In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon for psychoactive drugs to have

　　(A) mixed fects in the brain

　　(B) inhibitory fects on enzymes in the brain

　　(C) close structural relationships with caffeine

　　(D) depressive fects on mouse locomotion

　　(E) the ability to dislodge caffeine from receptors in the brain

　　5. The passage suggests that Snyder et al believe that if the older theory concerning caffeine’s fects were correct, which of the following would have to be the case?

　　Ⅰ.All neurotransmitters would increase the short-term concentration of cyclic AMP in target neurons.

　　Ⅱ.Substances other than caffeine that inhibit the production of phosphodiesterase would be stimulants.

　　Ⅲ.All concentration levels of caffeine that are high enough to produce stimulation would also inhibit the production of phosphodiesterase.

　　(A) Ⅰ only

　　(B) Ⅰ and Ⅱ only

　　(C) Ⅰand Ⅲ only

　　(D) Ⅱ and Ⅲ only

　　(E) Ⅰ,Ⅱ,and Ⅲ

　　6. According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT

　　(A) IBMX

　　(B) caffeine

　　(C) adenosine

　　(D) theophylline

　　(E) phosphodiesterase

　　7. 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

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

　　8. The author quotes Snyder et al in lines 38-43 most probably in order to

　　(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

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

　　9. The last paragraph of the passage performs which of the following functions?

　　(A) Describes a disconfirming experimental result and reports the explanation given by Snyder et al in an attempt to reconcile this result with their theory.

　　(B) Specifies the basis for the correlation observed by Snyder et al and presents an explanation in an attempt to make the correlation consistent with the operation of psychoactive drugs other than caffeine.

　　(C) Elaborates the description of the correlation observed by Snyder et al and suggests an additional explanation in an attempt to make the correlation consistent with the older theory.

　　(D) Reports inconsistent experimental data and describes the method Snyder et al will use to reanalyze this data.

　　(E) Provides an example of the hypothesis proposed by Snyder et al and relates this example to caffeine’s properties.

　　Correct Answers:DDDADEBBA

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