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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.
According to Snyder et al, caffeine differs from adenosine in that caffeine
选项 A、stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only.
B、has mixed effects in the brain, whereas adenosine has only a stimulatory effect.
C、increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations.
D、permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release.
E、inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing.
答案D
解析 Snyder等人认为，咖啡因和腺苷的区别在于：A．咖啡因使人鼠共同兴奋，腺苷只能使人兴奋。显然无。B．咖啡因在神经中有多种作用，腺苷只有兴奋作用。前半句对，后半句说反了，腺苷抑制神经兴奋。C．咖啡因增加神经元中AMP含量，腺苷减少这种含量。腺苷根本和AMP无关。D．正确。当咖啡因附着在腺苷的受体上时，会促成神经元传导物质的释放，而腺苷抑制此种作用。见原文L19—25，咖啡因附着腺苷受体上阻止腺苷附着。因此它肯定会促进神经元传导物质的释放。
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