文章作者 100test 发表时间 2008:01:10 13:30:14
来源 100Test.Com百考试题网
老观点For many years(证明了是新老观点对比型), caffeine’s effects have been attributed to its inhibition阻碍 of the production of phosphodiesterase, an enzyme that breaks down分解 the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore(结论), prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor其实就是指咖啡因, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder(老观点的描述中出现新观点的人名,用来批驳老观点) et al 理由一point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher(比较句) than those that produce stimulation. 理由二Moreover, other compounds that block阻碍 phosphodiesterase’s activity are not stimulants.
新观点的证据To buttress支持(=support) their case that caffeine acts instead by preventing抑制 adenosine binding, Snyder et al compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge排斥 adenosine from its receptors in the brains of mice. (做实验比较刺激能力和排斥能力)“In general*8B,” they reported(引用人物原话,是重要出题点), “the ability of the compounds to compete at the receptors correlates with(正相关性) 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(与结论紧密相连,篇幅小于5行,可以不看), a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both regards.
新观点的例外情况及解释There were some apparent exceptions*9A(不希望不出现的内容,直接跳去看IBMX这种术语) 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抑制 mouse locomotion. Snyder et al suggests that*9A 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(整个问题的解答) in the brain*4A, a not unusual*4 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.
1. The primary purpose of the passage is to