Passage 1
Caffeine, the stimulant1 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
the human brain of a naturally occurring chemical called
adenosine. Adenosine normally depresses neuron firing
in many areas of the brain. It apparently2 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 bind4 to specific receptors on
neuronal membranes5. 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-
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, caffeines effects have been attri-
buted to its inhibition of the production of phosphodi-
esterase, an enzyme6 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
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 stimulation7. But Snyder et al point out that the
caffeine concentrations needed to inhibit3 the production
of phosphodiesterase in the brain are much higher than
those that produce stimulation. Moreover, other com-
pounds that block phosphodiesterases activity are not
stimulants.
To buttress8 their case that caffeine acts instead by pre-
venting9 adenosine binding10, Snyder et al compared the
stimulatory11 effects of a series of caffeine derivatives12 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 stimulate13 locomotion14 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 structural15 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-isobuty1-1-methylxanthine, which bound very
well but actually depressed16 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
known only for its stimulatory effects, displays this
property, depressing mouse locomotion at very low
concentrations and stimulating17 it at higher ones.
1. The primary purpose of the passage is to
discuss a plan for investigation18 of a phenomenon that is not yet fully19 understood
present two explanations of a phenomenon and reconcile the differences between them
summarize two theories and suggest a third theory that overcomes the problems encountered in the first two
describe an alternative hypothesis and provide evidence and arguments that support it（D）
challenge the validity of a theory by exposing the inconsistencies and contradictions in it
2. According so Snyder et al, caffeine differs from adenosine in that caffeine
stimulates20 behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
has mixed effects in the brain, whereas adenosine has only a stimulatory effect
increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations
permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits21 such release（D）
inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing
3. In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon22 for psychoactive drugs to have
mixed effects in the brain
inhibitory effects on enzymes23 in the brain
close structural relationships with caffeine
depressive effects on mouse locomotion（A）
the ability to dislodge caffeine from receptors in the brain
4. According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT
IBMX
caffeine
adenosine
theophylline（E）
phosphodiesterase
5. Snyder et al suggest that caffeines ability to bind to A1 and A2 receptors can be at least partially24 attributed to
which of the following?
The chemical relationship between caffeine and phosphodiesterase
The structural relationship between caffeine and adenosine
The structural similarity between caffeine and neurotransmitters
The ability of caffeine to stimulate behavior（B）
The natural occurrence of caffeine and adenosine in the brain
6. The author quotes Snyder et al in lines 38-43 most probably in order to
reveal some of the assumptions underlying their theory
summarize a major finding of their experiments
point out that their experiments were limited to the mouse
indicate that their experiments resulted only in general correlations（B）
refute the objections made by supporters of the older theory
Caffeine, the stimulant1 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
the human brain of a naturally occurring chemical called
adenosine. Adenosine normally depresses neuron firing
in many areas of the brain. It apparently2 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 bind4 to specific receptors on
neuronal membranes5. 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-
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, caffeines effects have been attri-
buted to its inhibition of the production of phosphodi-
esterase, an enzyme6 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
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 stimulation7. But Snyder et al point out that the
caffeine concentrations needed to inhibit3 the production
of phosphodiesterase in the brain are much higher than
those that produce stimulation. Moreover, other com-
pounds that block phosphodiesterases activity are not
stimulants.
To buttress8 their case that caffeine acts instead by pre-
venting9 adenosine binding10, Snyder et al compared the
stimulatory11 effects of a series of caffeine derivatives12 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 stimulate13 locomotion14 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 structural15 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-isobuty1-1-methylxanthine, which bound very
well but actually depressed16 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
known only for its stimulatory effects, displays this
property, depressing mouse locomotion at very low
concentrations and stimulating17 it at higher ones.
1. The primary purpose of the passage is to
discuss a plan for investigation18 of a phenomenon that is not yet fully19 understood
present two explanations of a phenomenon and reconcile the differences between them
summarize two theories and suggest a third theory that overcomes the problems encountered in the first two
describe an alternative hypothesis and provide evidence and arguments that support it（D）
challenge the validity of a theory by exposing the inconsistencies and contradictions in it
2. According so Snyder et al, caffeine differs from adenosine in that caffeine
stimulates20 behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
has mixed effects in the brain, whereas adenosine has only a stimulatory effect
increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations
permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits21 such release（D）
inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing
3. In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon22 for psychoactive drugs to have
mixed effects in the brain
inhibitory effects on enzymes23 in the brain
close structural relationships with caffeine
depressive effects on mouse locomotion（A）
the ability to dislodge caffeine from receptors in the brain
4. According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT
IBMX
caffeine
adenosine
theophylline（E）
phosphodiesterase
5. Snyder et al suggest that caffeines ability to bind to A1 and A2 receptors can be at least partially24 attributed to
which of the following?
The chemical relationship between caffeine and phosphodiesterase
The structural relationship between caffeine and adenosine
The structural similarity between caffeine and neurotransmitters
The ability of caffeine to stimulate behavior（B）
The natural occurrence of caffeine and adenosine in the brain
6. The author quotes Snyder et al in lines 38-43 most probably in order to
reveal some of the assumptions underlying their theory
summarize a major finding of their experiments
point out that their experiments were limited to the mouse
indicate that their experiments resulted only in general correlations（B）
refute the objections made by supporters of the older theory