Greider and her team found that in mice bred to feel indifferent to nicotine, smoking changes the number of aversion receptors in the brain and alters their signaling. They then infected the mice with a virus that introduced nicotine receptors in an area of the VTA dominated by two types of neurons — dopamine and GABA neurons.The researchers then dose the mice with nicotine and scanned their brains while simultaneously observing their behavior. The mice’s dopamine neurons triggered nicotine aversion — while the GABGA neurons triggered the feeling of reward.People generally think of dopamine as a reward chemical, but it is more of a motivation signal, says Greider. When it’s active, it increases the motivation to do anything — rewarding or not.“Here we showed that it leads to the motivation to avoid nicotine,” she says.