Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth. "Snyder, Daly, and Bruns have recently proposed that caffeine affects brain of by countering the activity in 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.
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 a1. propose that caffeine, which is structurally 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 attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMR. A number of neurotransmitters exert their effects by first increasing cyclic AM P concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et aI point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase's activity are not stimulants.
To buttress their case that caffeine acts instead by preventing 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 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 locomotion." Theophylline, a close structural relative of the most 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-isobutyl-1-methylxanthine (IBMX), which bound very 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 known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.
题目分析：主题题，注意文章架构 (A) discuss a plan for investigation of a phenomenon that is not yet fully understood 文章没有说过调查，错 (B) present two explanations of a phenomenon and reconcile（调和） the differences between them 文章没有调和，而是反驳了老理论，错。（reconcile是主题题常见单词，要记住） (C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two 一共只有2个理论，新的和老的，没有第三个，错。 (D) describe an alternative hypothesis and provide evidence and arguments that support it 描述了一个理论（第一段描述理论，第二段说了一个老理论所以第一段的理论是alternative的），提供证据（第三段做实验举例子），提出争议（第四段例外），support it（第三段和第四段都是支持新理论的，尤其是第三段，开头说的就是to buttress） (E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it 新老理论，不是同一个理论本身的矛盾，错
To buttress their case that caffeine acts instead by preventing adenosine binding。。。为了支持上面的argument！ （上面的caffeine's effects have been attributed to its inhibition of the production of phosphodiesterase,也就是个alternative hypothesis，后一段提供了evidence