新东方在线 www.koolearn.com 网络课堂电子教材系列 GMAT阅读讲义GMAT 阅读讲义主讲：王鹏欢迎使用新东方在线电子教材GMAT 核心阅读Passage 1Caffeine, the stimulant in coffee, has been called“the most widely used psychoactive substance on Earth .”Synder, Daly and Bruns have recently proposed thatcaffeine affects behavior by countering the activity in(5) the human brain of a naturally occurring chemical calledadenosine. Adenosine normally depresses neuron firingin many areas of the brain. It apparently does this byinhibiting the release of neurotransmitters, chemicalsthat 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 onneuronal membranes. There are at least two classesof these receptors, which have been designated A1 andA2. Snyder et al propose that caffeine, which is struc-(15) turally similar to adenosine, is able to bind to both typesof receptors, which prevents adenosine from attachingthere and allows the neurons to fire more readily thanthey otherwise would.For many years, caffeines effects have been attri-(20) buted to its inhibition of the production of phosphodi-esterase, an enzyme that breaks down the chemicalcalled cyclic AMP.A number of neurotransmitters exerttheir effects by first increasing cyclic AMP concentra-tions in target neurons. Therefore, prolonged periods at(25) the elevated concentrations, as might be brought aboutby a phosphodiesterase inhibitor, could lead to a greateramount of neuron firing and, consequently, to behav-ioral stimulation. But Snyder et al point out that thecaffeine concentrations needed to inhibit the production(30) of phosphodiesterase in the brain are much higher thanthose that produce stimulation. Moreover, other com-pounds that block phosphodiesterases activity are notstimulants.To buttress their case that caffeine acts instead by pre-(35) venting adenosine binding, Snyder et al compared thestimulatory effects of a series of caffeine derivatives withtheir ability to dislodge adenosine from its receptors inthe brains of mice. “In general,” they reported, “theability of the compounds to compete at the receptors(40) correlates with their ability to stimulate locomotion inthe mouse; i.e., the higher their capacity to bind at thereceptors, the higher their ability to stimulate locomo-tion.” Theophylline, a close structural relative of caffeineand the major stimulant in tea, was one of the most(45) effective compounds in both regards.There were some apparent exceptions to the generalcorrelation observed between adenosine-receptor bindingand stimulation. One of these was a compound called3-isobuty1-1-methylxanthine(IBMX), which bound very(50) well but actually depressed mouse locomotion. Snyderet al suggest that this is not a major stumbling block totheir hypothesis. The problem is that the compound hasmixed effects in the brain, a not unusual occurrence withpsychoactive drugs. Even caffeine, which is generally(55) known only for its stimulatory effects, displays thisproperty, depressing mouse locomotion at very lowconcentrations and stimulating it at higher ones.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 it2. 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 suchrelease(E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration inthe brain, whereas adenosine inhibits only neuron firing3. In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon forpsychoactive drugs to have(A) mixed effects in the brain(B) inhibitory effects on enzymes in the brain(C) close structural relationships with caffeine(D) depressive effects on mouse locomotion(E) the ability to dislodge caffeine from receptors in the brain4. According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT(A) IBMX(B) caffeine(C) adenosin