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　　caffeine's effects

　　caffeine，the stimulant in coffee，has been called“the most widely used psychoactive substance on earth.”synder，daly and burns have recently proposed that caffeine affects behavior by countering the activity in the human brain of a naturally occuring chemical called adenosine.adenosine normally depresses neuron firing in many areas of the brain.it apparently does this by inhibiting the release of neurontransmitters，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 readly than they otherwise would.

　　for many years，caffeine's effects have been attributed to its inhibition of the production of phosphodi-esterase，an enzyme that breaks down the chemical called cyclic amp. a number of neurontransmitters 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 synder 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，syder 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 stimulated locomotion.”theophynine，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-isobuty 1-1-methylxanthine （ibmx），which bound very well but actually depressed mouse locomotion.synder 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 known only for its stimulatory effects，displays this property，depressing mouse locomotion at very low concentrations and stimulating it at higher ones.

　　28.the primary purpose of the passage is to____

　　a.challenge the validity of a theory by exposing the inconsistencies and contradictions in it.

　　b.describe an alternative hypothesis and provide evidence and arguments that support it.

　　c.discuss a plan for investigation of a phenomenon that is not yet fully understood.

　　d.summarize two theories and suggest a third theory that overcomes the problems encountered in the first two.

　　29.according to snyder et al，caffeine differs from adenosine in that caffeine

　　a.increases cyclic amp concentrations in target nuerons，whereas adenosine decreases such concentrations.

　　b.has mixed effects in the brain，whereas adenosine has only a stimulatory effect.

　　c.stimulates behavior in the mouse and in humans，whereas adenosine stimulates behavior in humans only.

　　d.permits release of neurontransmitters when it is bound to adenosine receptors，whereas adenosine inhibits such release.

　　30.according to snyder et al，all of the following compounds can bind to specific receptors in the brain except____

　　a.phosphodiesterase. b.caffeine. c.ibmx. d.adenosine.

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