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  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.
Snyder et al suggest that caffeine’s ability to bind to Al and A2 receptors can be at least partially attributed to which of the following?
选项 A、The chemical relationship between caffeine and phosphodiesterase.
B、The structural relationship between caffeine and adenosine.
C、The structural similarity between caffeine and neurotransmitters.
D、The ability of caffeine to stimulate behavior.
E、The natural occurrence of caffeine and adenosine in the brain.
答案B
解析 咖啡因能附着在A1、A2两个受体之上至少可以部分地归因于：A．提到ph，是老观点内容，必错，和A1、A2根本无关。B．正确。咖啡因和腺苷之间结构上的相似性。见原文L19—22。C．咖啡因和神经元传导物质结构上的相似性。本文无。D．咖啡因的兴奋作用。因果倒置，是咖啡因附着在受体上的能力导致其兴奋作用。E．natural occurrence文中只提到腺苷是这样(L7—8)，咖啡因没有提到。
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