如题, 大全里第10篇咖啡因的那个文章,好像还是OG10第一篇阅读, 第2题weaken题搜遍了CD没人问过, 求牛牛们帮忙给解释一下,D怎么就weaken了S等人的理论了呢?
Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth.” Snyder, Daly and Bruns have recently proposed that caffeine affect 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 A1 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 otherwise 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 caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase’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-1-methylxanthine (IBMX), which bound very well but actually depressed mouse locomotion. Snyder et al suggests 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 psychoactive 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.
2. Which of the following, if true, would most weaken the theory proposed by Snyder et al?
(A) At very low concentrations in the human brain, both caffeine and theophylline tend to have depressive rather than stimulatory effects on human behavior.
(B) The ability of caffeine derivatives at very low concentrations to dislodge adenosine from its receptors in mouse brains correlates well with their ability to stimulate mouse locomotion at these low concentrations.
(C) The concentration of cyclic AMP in target neurons in the human brain that leads to increased neuron firing can be produced by several different phosphodiesterase inhibitors in addition to caffeine.
(D) The concentration of caffeine required to dislodge adenosine from its receptors in the human brain is much greater than the concentration that produces behavioral stimulation in humans.这个是正确答案.
(E) The concentration of IBMX required to dislodge adenosine from its receptors in mouse brains is much smaller than the concentration that stimulates locomotion in the mouse.
不过如果顺便哪个大侠啊或者老师啥的路过,HAPPEN TO KNOW这个题的解题思路,请麻烦留个言,赐教一下我这个好学的孩子吧!作者: veralove3271 时间: 2012-5-29 20:29
1. D 选项不是划线部分的改写。
2. D 选项和划线部分在逻辑上是类似的，但其所要驳斥的对象，以及所依赖的事实是区分的：前者是对 Snyder et al 的质疑，而后者是 Snyder et al 对以前观点的质疑。
2. 划线部分是Snyder et al 这群人对以前的解释的反驳。以前的解释认为，神经递质通过升高目标神经元的 cyclic amp 浓度达到刺激作用，而phosphodiesterase (磷酸二酯酶)破坏 cyclic amp。咖啡因通过阻碍 phosphodiesterase 的产生以保持 cyclic amp 的高浓度，从而达到使人兴奋的效果。
但是，Snyder et al 这群人指出：阻碍 phosphodiesterase 产生的咖啡因的临界浓度高于能使人兴奋的实际浓度，并且其他能阻碍 phosphodiesterase 产生的物质没有刺激效果。换言之，咖啡因并非是通过阻碍 phosphodiesterase 的产生来发挥作用的。这很好的驳斥了旧观点。
3. Snyder et al 认为，adenosine(腺苷酸) 首先附着在神经细胞膜受体上，然后阻止神经递质的释放以放缓大脑活动(神经递质在神经元间传递神经脉冲)。而咖啡因因为与 adenosine 结构类似，所以可以附着在神经细胞膜受体上，以阻止 adenosine 的附着，这样神经递质可以更好的发挥作用，大脑活动得到刺激。
D 选项的意思是说：防止 adenosine 附着在神经细胞膜上的咖啡因的最小浓度高于实际能使人兴奋的浓度。换言之，咖啡因不是通过驱逐 adenosine 来发挥作用的。这实际上削弱了 Snyder 等人的主张。
