Question

Read the following and answer the questions.
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 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-isobuty1-1-methylxanthine(IBMX), which bound very 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 known only for its stimulatory effects, displays
this property, depressing mouse locomotion at very low concentrations and stimulating it at higher
ones.

Snyder et al suggest that caffeine’s ability to bind to A1 and A2 receptors can be at least partially attributed to which of the following? *

1 point

The chemical relationship between caffeine and phosphodiesterase

The structural relationship between caffeine and adenosine

The structural similarity between caffeine and neurotransmitters

The primary purpose of the passage is to *

1 point

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

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

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

According so Snyder et al, caffeine differs from adenosine in that caffeine *

1 point

stimulates 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

permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release

According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT *

1 point

caffeine

phosphodiesterase

theophylline

Other:

In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon for psychoactive drugs to have *

1 point

mixed effects in the brain

inhibitory effects on enzymes in the brain

close structural relationships with caffeine

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Answer 1

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Explanation:

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