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 last paragraph of the passage performs which of the following functions?
选项 A、Describes a disconfirming experimental result and reports the explanation given by Snyder et al in an attempt to reconcile this result with their theory.
B、Specifies the basis for the correlation observed by Snyder et al and presents an explanation in an attempt to make the correlation consistent with the operation of psychoactive drugs other than caffeine.
C、Elaborates the description of the correlation observed by Snyder et al and suggests an additional explanation in an attempt to make the correlation consistent with the older theory.
D、Reports inconsistent experimental data and describes the method Snyder et al will use to reanalyze this data.
E、Provides an example of the hypothesis proposed by Snyder et al and relates this example to caffeine’s properties.
答案A
解析 最后一段起什么作用：A．正确。描述了一个不支持(其观点的)实验结果，并且给出了Snyder等人试图使这个结果和其观点相一致所做的解释。IBMX的结果是一个例外，Synder等人将这种现象解释为在精神兴奋药中，混合现象是常出现的。B．“咖啡因之外的精神兴奋药”。未提。C．“使这种观点和老理论一致”。未提。D．没有“reanalyze this data”。E．给出Snyder等人假设的一例子。此段IBMX例子和Snyder等人设想的主要内容是不相符的。
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