接下来的几天给大家整理了历届gre考试阅读真题及解析，下面这篇是1998年4月gre考试阅读真题SECTION A部分。复习gre考试阅读的时候，gre考生应该以往年的真题为复习材料，从真题中总结阅读规律和特点才是制胜法宝。

　　Much of the research on hallucinogenic drugs such as LSD has focused on the neurotransmitter serotonin, a chemical that when released from a presynaptic serotonin-secreting neuron causes the transmission of a nerve impulse across a synapse to an adjacent postsynaptic, or target, neuron. There are two major reasons for this emphasis. First, it was discoveredearly on (early on: adv 在早期)that many of the major hallucinogens have a molecular structure similar to that of serotonin. In addition, animal studies of brain neurochemistry following administration of hallucinogens invariably reported changes in serotonin levels.

　　Early investigators correctly reasoned that the structural similarity to the serotonin molecule might imply that LSD’s effects are brought about by an action on the neurotransmission of serotonin in the brain. Unfortunately, the level of technical expertise in the field of brain research was such that this hypothesis had to be tested on peripheral tissue (tissue outside the brain). Two different groups of scientists reported that LSD powerfully blockaded serotonin’s action. Their conclusions were quickly challenged, however. We now know that the action of a drug at one site in the body does not necessarily correspond to the drug’s action at another site, especially when one site is in the brain and the other is not.

　　By the 1960’s, technical advances permitted the direct testing of the hypothesis that LSD and related hallucinogens act by directly suppressing the activity of serotonin-secreting neurons themselves—the so-called presynaptic hypothesis. Researchers reasoned that if the hallucinogenic drugs act by suppressing the activity of serotonin-secreting neurons, then drugs administered after these neurons had been destroyed should have no effect on behavior, because the system would already be maximally suppressed. Contrary to their expectations, neuron destruction enhanced the effect of LSD and related hallucinogens on behavior. Thus, hallucinogenic drugs apparently do not act directly on serotonin-secreting neurons.

　　However, these and other available data do support an alternative hypothesis that LSD and related drugs act directly at receptor sites on serotonin target neurons (the postsynaptic hypothesis). The fact that LSD elicits “serotonin syndrome”—that is, causes the same kinds of behaviors as does the administration of serotonin—in animals whose brains are depleted of serotonin indicates that LSD acts directly on serotonin receptors, rather than indirectly through the release of stores of serotonin. The enhanced effect of LSD reported after serotonin depletion could be due to a proliferation of serotonin receptor sites on serotonin target neurons. This phenomenon often follows neuron destruction or neurotransmitter depletion; the increase in the number of receptor sites appears to be a compensatory response to decreased input. Significantly, this hypothesis is supported by data from a number of different laboratories.

　　17. According to the passage, which of the following is one of the primary factors that led researchers studying hallucinogenic drugs to focus on serotonin?

　　(A) The suppression of the activity of serotonin-secreting neurons by the administration of hallucinogens

　　(B) The observed similarities in the chemical structures of serotonin and hallucinogens

　　(C) The effects the administration of hallucinogens has on serotonin production in the human brain

　　(D) Serotonin-induced changes in the effects of hallucinogens on behavior

　　(E) Hallucinogen-induced changes in the effects of serotonin on behavior

　　18. It can be inferred that researchers abandoned the presynaptic hypothesis because

　　(A) a new and more attractive hypothesis was suggested

　　(B) no research was reported that supported the hypothesis

　　(C) research results provided evidence to counter the hypothesis

　　(D) the hypothesis was supported only by studies of animals and not by studies of human beings

　　(E) the level of technical expertise in the field of brain research did not permit adequate testing of the hypothesis

　　19. Which of the following best expresses the main idea of the passage?

　　(A) Research has suggested that the neurotransmitter serotonin is responsible for the effects of hallucinogenic drugs on the brain and on behavior.

　　(B) Researchers have spent an inadequate amount of time developing theories concerning the way in which the effects of hallucinogenic drugs occur.

　　(C) Research results strongly suggest that hallucinogenic drugs create their effects by acting on the serotonin receptor sites located on target neurons in the brain.

　　(D) Researchers have recently made valuable discoveries concerning the effects of depleting the amount of serotonin in the brain.

　　(E) Researchers have concluded that hallucinogenic drugs suppress the activity of serotonin-secreting neurons.

　　20. The research described in the passage is primarily concerned with answering which of the following questions?

　　(A) How can researchers control the effects that LSD has on behavior?

　　(B) How are animals’ reactions to LSD different from those of human beings?

　　(C) What triggers the effects that LSD has on human behavior?

　　(D) What technical advances would permit researchers to predict more accurately the effects of LSD on behavior?

　　(E) What relationship does the suppression of neuron activity have to the occurrence of “serotonin syndrome”?

　　21. Which of the following best defines “serotonin syndrome” (line 46) as the term is used in the passage?

　　(A) The series of behaviors, usually associated with the administration of serotonin, that also occurs when LSD is administered to animals whose brains are depleted of serotonin

　　(B) The series of behaviors, usually associated with the administration of LSD, that also occurs when the amount of serotonin in the brain is reduced

　　(C) The maximal suppression of neuron activity that results from the destruction of serotonin-secreting neurons

　　(D) The release of stores of serotonin from serotonin-secreting neurons in the brain

　　(E) The proliferation of serotonin receptor sites that follows depletion of serotonin supplies in the brain

　　22. Which of the following best describes the organization of the argument that the author of the passage presents in the last two paragraphs?

　　(A) Two approaches to testing a hypothesis are described, and the greater merits of one approach are indicated.

　　(B) The assumptions underlying two hypotheses are outlined, and evidence for and against each hypothesis is discussed.

　　(C) A phenomenon is described, and hypotheses concerning its occurrence are considered and rejected.

　　(D) The reasoning behind a hypothesis is summarized, evidence supporting the hypothesis is presented, and research that counters the supporting evidence is described.

　　(E) A hypothesis is discussed, evidence undermining the hypothesis is revealed, and a further hypothesis based on the undermining evidence is explained.

　　23. The author’s attitude toward early researchers’ reasoning concerning the implications of similarities in the structures of serotonin and LSD molecules can best be described as one of

　　(A) complete agreement

　　(B) reluctant support

　　(C) subtle condescension

　　(D) irreverent dismissal

　　(E) strong opposition