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.
The author quotes Snyder et al. in highlight text most probably in order to
题目分析：引号in order to题。答案要在引号中的总结句去找，答案就是总结句的改写。大家记得文章中我重点划出的in general吗？因此看完每个选项的第一个单词就可以选出答案B：summarize，只有这个单词是in general的改写。 当然，严谨起见，我们来每个选项看一下： (A) reveal some of the assumptions underlying their theory 第三段说的是实验，没有提假设，错 (B) summarize a major finding of their experiments 总结、主要、发现、实验――多么完美的改写啊。 (C) point out that their experiments were limited to the mouse 没有说过，错 (D) indicate that their experiments resulted only in general correlations 大家要注意这种极端词。文章中如果没有出现过，答案中出现了，一般必然是错的。 (E) refute the objections made by supporters of the older theory 这是第二段的内容。引号出现在第三段，根据文字对应法，答案不可能在第二段。
没有 objections In general--summarize