Henry Lester, professor of biology at Caltech, discusses the effects of nicotine addiction on the brain.

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Tobacco originated in the Americas, where humans have known some of its effects for at least 10,000 years. Beginning some 500 years ago, ocean crossings spread tobacco use to all other continents. We are essentially the only species that has learned how to use small amounts of plant toxins — which provide the selective advantage of sickening or poisoning animals who might eat them — for our own purposes. For tobacco, that toxin is nicotine. In addition to their historical medicinal and ritual uses, these substances, over time, have come to serve as guides, models, and touchstones for learning about the brain and opening many fields of neuroscience.

We have learned from tobacco and nicotine that it is possible to isolate single chemicals from plants that cause toxic effects on herbivores and valued effects on people. We’ve learned that it’s possible to define chemical processes in the human brain that are activated, inhibited, or otherwise manipulated by those substances.

According to the text, what is one of the main discoveries made from the study of tobacco and nicotine?