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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, The research described in the passage is primarily concerned with answering which of the following questions（）
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In his culture,（）it was, this exchange of manes on pieces of paper was probably a formal politeness, like saying “thank you”
利用中间件Middleware的高层 【12】 可以将底层网络协议与实现技术隔离开来，使得程序员可以将精力集中在应用软件编程上。
Scholars often seem to operate on the assumption that any analysis with a rosy outlook simply does not adequately understand the matter at hand. Ecotourism researchers have not been derelict in this regard, as the literature review earlier showed. All the researchers who have looked at Capirona’s project, however, have been impressed by its grassroots nature and are optimistic about its potential as eco-development (Colvin 1994; Wesche 1993; Silver 1992).All of these researchers, however, visited the community in its early years of operation. As mentioned previously, recent, non-scholarly reports are less positive. Thus there remains some doubt as to the long-term viability of even such a model of indigenous ecotourism development as Capirona. This study originally proposed to study Capimna’s project, but that community was weary of such research visits and refused a request to carry out the study there. Palo Blanco, though completing only its first year of ecotourism developmemt was chosen as an alternate site. Perhaps it should not be surprising that the prospects for ecotourism in Rio Blanco appear, as they did in Capirona quite bright.Ecotourism development efforts differ from mainstream development efforts in that, aside from start-up loans, much or all of the continuing financial support comes from tourists rather than from governments or development agencies. As a result, the two main players many ecotourism endeavor—the hosts and the guests—are driven by differing motivations. The local population hopes to improve its own lot by taking advantage of the curiosity, disposable income, and in some cases, perhaps, good intentions of ecotourists. The tourists want to "explore the natural wonders of the world," whether that be a wildebeest migration across the Serengeti or the march of leaf-cutter ants across the jungle floor (Ryan and Grasse 1991: 166).In contrast to mass tourism, ecotourism permits tourists to seek educational self-fulfillment in the form of travel, and tries to transform that activity into something that benefits the greater good—specifically, to fund environmental preservation, rural development, and even cultural survival. However, in order to satisfy everyone—tourists, environmentalists, tour operators and the local hosts—ecotourism must bring into aliganment a variety of contradictory purposes. Ecotourism promotes feelings among tourists that they are part of the solution when, in fact, the very act of flying a thousand miles or more to their destination consumes resources and pollutes the enviroment (cf. Somerville 1994). The beauty of ecotourism is that it can exploit this egotistic motivation; the flaw is that it is forever limited by it.Even a brief foray into development literature, however, shows that flawed conceptualizations are the rule, not the exception. As development, ecotourism may be no more inchoate than any other approach, and in some ways it is as progressive as any theory. For example, ecotourism twin development goals—conserving the environment and benefiting local peoples—are increasingly seen, both within and outside of tourism circles, as interdependent. Without economic development, many argue that environmental conservation is neither ethical nor sustainable (Boo 1990: 1; West and Brechin 1992: 14; Brandon and Wells 1992). Such conservation can be achieved only by providing local people with alternative income sources which do not threaten to deplete the plants and amamis within the protected zone (Brandon and Wells 1992: 557). Most research on this issue, however, assumes that the protective regulations have been established by the government or another external ageacy. In Rio Blanco, however, the people themselves are already acting to protect their land. A study of the studies available on this topic shows that（）
针对不同的传输介质，Ethernet网卡提供了相应的接口，其中适用于非屏蔽双绞线的网卡应提供______。
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