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.
In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon for psychoactive drugs to have
选项 A、mixed effects in the brain.
B、inhibitory effects on enzymes in the brain.
C、close structural relationships with caffeine.
D、depressive effects on mouse locomotion.
E、the ability to dislodge caffeine from receptors in the brain.
答案A
解析 对于IBMX说出的(异常的)实验结果，Synder等人指出精神兴奋药具有以下哪种特点并不稀奇?A．正确。在脑神经中的混合作用。见原文L72—75。B．对脑神经中酶的抑制作用。未提。C．和咖啡因结构相似。未提。D．对老鼠抑制运动。仅说IBMX有此特点，不能推而广之至一般兴奋药。E．将咖啡因从脑神经受体上移去。无。
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