The researchers took two sets of mice - one was exposed to nicotine and the other wasn't. Both were then exposed to cocaine. They found that the nicotine-exposed mice were more likely to seek out cocaine again, by a great deal (78%!).
They found that exposure to nicotine caused epigenetic changes (specifically histone acetylation) in genes related to the reward system in the brain, allowing changes in the expression of certain genes during exposure to cocaine, inducing a more addictive experience.