Hallucinogen-induced changes in the effects of serotonin on behavior

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

　　 a new and more attractive hypothesis was suggested

　　 no research was reported that supported the hypothesis

　　 research results provided evidence to counter the hypothesis

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

　　 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?

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

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

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

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

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

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