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  3. Much of the research on hallucinogenic drugs such as LSD has focused on the neurotransmitter serotonin, a chemical that when rel

Much of the research on hallucinogenic drugs such as LSD has focused on the neurotransmitter serotonin, a chemical that when rel

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问题     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 discovered early on 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 arc 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 or, peripheral tissue ( tissue outside the brain). Two different groups of scientists reported than 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 drags 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 number of different laboratories
It can be inferred that researchers abandoned the presynaptic hypothesis because______ .
选项 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
答案C
解析 从第三段的实验结果知道contrary to their expectations，即结果与他们事先的预想相反，故选C。
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