CD-1 mice Show Individual Differences in Nicotine Preference in a Modified Two-Bottle Oral Self-Administration Model

Although both animal and human studies reveal significant contributions of genetics to smoking addiction, many human studies were underpowered or biased by potential confounding variables, and animal genetic studies are challenged by limited genetic variations and lack of convincing phenotypes. To address these concerns, we used non-sibling outbred CD-1 mice to evaluate individual differences in nicotine preference with a modified two-bottle oral self-administration model. Animals were first given free access to two bottles, one filled with nicotine dissolved in 2% saccharin and the other with saccharin only. Under this regular two-bottle choice condition, the majority of animals avoided the nicotine solution with limited individual differences. However, when we modified the model by introducing 4 days of exposure to 5% saccharin in the drinking water, the animals significantly increased nicotine consumption in the two-bottle choice test, with about 30% animals showing a nicotine preference. Nicotine preference after 5% saccharin treatment remained elevated throughout the 28 days of the experiment. Further, we found there existed striking individual differences in nicotine consumption after exposure to 5% saccharin, with a range of 0-100% of total liquid consumption. The enhanced individual differences and the ratio of nicotine consumption were observed at different concentrations of nicotine (10-80 μg/ml) and in both adolescents and adults. Further examination on the induction mechanism showed that the long-lasting nicotine preference was not correlated with nicotine consumption before the induction, 5% saccharin consumption, or weight gain during the induction. Although liquid consumption during the 4 days of 5% saccharin exposure was decreased by about 30%, comparable liquid restriction alone for 4 days did not induce nicotine preference. Together, this study showed a strong and stable nicotine preference in CD-1 mice, which was induced by a short-term high concentration of saccharin in the drinking water. Considering the nature and heterogeneity of CD-1 mice, the striking individual differences imply that genetics plays an important role in nicotine preference observed in these animals.