Reading comprehension Home Exercise- 05, PASSAGE A

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 unnaturally 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 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 Aᵪ and Aᵦ. 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 barks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged perios 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 behaviousral 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 compare 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- I- 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 his generally known only for – its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.

1.       The primary purpose of the passage is to

               A.      Discuss a plan for investigation of a phenomenon that is not yet fully understood. 

               B.      Present two explanations of a phenomenon and reconcile the differences between them.

              C.      Summarize two theories and suggest a third theory that overcomes the problems encountered in the first two.

               D.      Describe an alternative hypothesis and provide evidence and arguments that support it.

2.       According Snyder et al, caffeine differs from adenosine in that caffeine

               A.      Stimulates behavior in the mice and in humans, whereas adenosine stimulates behavior in humans only

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

                C.      Increases cyclic AMP concentrations in target neurons , whereas adenosine decreases such concentrations’

                 D.      Permits release of  neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release

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

               A.      Mixed effects in the brain

              B.      Inhibitory effects n the brain

              C.      Close structural relationships with caffeine

               D.      Depressive effects  on mouse locomotion

Answer and Explanations

1.         D    The first paragraph introduces a recent hypothesis about how caffeine affects behavior. The second paragraph looks at an earlier, widely accepted hypothesis and then presents the objections to it made by the scientists proposing the more recent hypothesis. The third and fourth paragraphs provide evidence to support that newer hypothesis. Since most of the passage is devoted to the recent hypothesis, clearly the primary purpose must be to present that hypothesis to readers.

2.        D    The first Paragraph leads the reader thorough how adenosine and caffeine work in the brain, according to Snyder et al. Adenosine depresses neuron firing by inhibiting the release of neurotransmitter; it is able to achieve this by binding to specific receptors on neuronal membranes. Caffeine interrupts this process by binding to the receptors, which prevents adenosine from attaching to them, and the neurons then fire more readily than they otherwise would.

3.       A     IBMX binds to the adenosine receptors, but instead of acting as a stimulant as other caffeine derivatives do, it was found to depress locomotion in mice. Snyder et al explain that IBMX has mixed effects in the brain, a not unusual occurrence with psychoactive drugs.