Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth .” Synder, Daly and Bruns have recently proposed that caffeine affects behavior by countering the activity in (5) 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. (10) 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 struc- (15) turally 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 attri- (20) buted to its inhibition of the production of phosphodi- esterase, an enzyme that breaks down the chemical called cyclic AMP.A number of neurotransmitters exert their effects by first increasing cyclic AMP concentra- tions in target neurons. Therefore, prolonged periods at (25) the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behav- ioral stimulation. But Snyder et al point out that the caffeine concentrations needed to inhibit the production (30) of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other com- pounds that block phosphodiesterase’s activity are not stimulants. To buttress their case that caffeine acts instead by pre- (35) venting 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 (40) 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 locomo- tion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most (45) 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-isobuty1-1-methylxanthine(IBMX), which bound very (50) well but actually depressed mouse locomotion. 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 psychoactive drugs. Even caffeine, which is generally (55) known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.
The chemical adenosine is released by brain cells when those cells are active. Adenosine then binds to more and more sites on cells in certain areas of thebrain, as the total amount released gradually increases during wakefulness. 随着化学物质Adenosine被脑细胞释放出来的数量在清醒时的逐步增加，A物质越来越多地绑定到大脑的某一区域（注意，这里的意思可不是说“绑定的多就清醒”如果错误理解成这样，那就会觉得前后有矛盾，而是“醒着的时候大脑释放A，A逐步越来越多地绑定”）
During sleep, the number of sites to which adenosine is bound decreases. 睡眠时，A物质被绑定的区域减少（我的理解是和前面那句话说的是一回事，A绑定的越来越多，所以可以给更多的A用来绑定的区域当然就少了）
Some researchers have hypothesized that it is the cumulative binding of adenosine to a large number of sites that causes the onset of sleep. 结论：累计的A绑定到大脑某个区域的数量达到一个大的量，这个累积的绑定就引发了睡眠。 注意：THAT从句 that causes the onset of sleep修饰限定的是binding
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发表于 2006-12-21 20:05:00
