2. 外语
  3. Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth. " Syn-der, Daly and Br

Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth. " Syn-der, Daly and Br

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问题     Caffeine, the stimulant in coffee, has been called "the most widely used psychoactive substance on Earth. " Syn-der, Daly and Bruns have recently pro- posed that caffeine affects behavior 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 Al and A2. Snyder et al 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 oth- erwise 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 AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP 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 al point out that the caf- feine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodi- esterase’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 caffeine and the major stimulant in tea, was one 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-l-methylxan-thine(IBMX), which bound very well but actually depressed mouse locomo- tion. 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 psychoac- tive 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 primary purpose of the passage is to
选项 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.
答案D
解析 主题题型：A．讨论一个考察现在还未被完全理解的现象的计划。无。B．给出对一现象的两种解释，并调和两者差异。文中没有调和内容。Snyder等人的看法和以前的看法自始至终是对立的。C．给出两种理论的总结，然后提出能解决前两个理论所遇到的问题的第三种理论。文中无第三种理论。D．正确。描述一个替代的设想并给出证据和论证过程。Snyder等人的理论就是他们代替不正确的长久以来看法的替代理论，作者提到了证据，并给出了Snyder等人论证老理论为非，以及一个例外并不能影响其理论有效性的过程。E．挑战一个理论有效性，指出其内部矛盾。Snyder等人在论证老理论为非时，并没有指出其内部的矛盾。
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