Nicotine Preference and Gene Expression: The Role of Cd81

Abstract

Smoking is a leading preventable cause of death worldwide. Variation in smoking behavior arises in large part from individual perception of nicotine’s rewarding effects, and the propensity for nicotine dependence. Attempts to elucidate molecular mechanisms underlying these factors have so far been insufficient for developing effective treatments for, or reliable predictors of dependence. Thus, identifying genetic determinants of nicotine addiction is of vital importance. This can most readily be accomplished using laboratory mice. We used two-bottle choice nicotine self-selection to assess differences in nicotine consumption between the C57BL/6J and A/J inbred strains, and in a separate experiment, C57BL/6J Cd81 knockout mouse strains. We quantified the relative motivation to seek nicotine. Our unique set of experimental design parameters allowed unprecedented success in distinguishing between the strains and sexes through voluntary nicotine consumption. The cohorts exhibited distinct nicotine consumption levels. Most showed increasing consumption with time, indicating tolerance effects. We measured signs of nicotine withdrawal in the C57BL/6J mice, and found without exception, each cohort became nicotine dependent. Our nicotine self-selection paradigm satisfies all accepted criteria for animal models of alcoholism. On this basis, we regard our design as being a valid model for nicotine dependence. During self-selection, we determined that Cd81 loss-of-function significantly increased nicotine preference. However, previous studies had indicated Cd81 loss-offunction produced a reduction in cocaine preference. Thus, we investigated further by comparing gene expression in wild-type and Cd81 knockout mice at baseline and after nicotine treatments. We measured the expression of genes for dopamine receptors (Drd1, Drd2S, Drd2L, Drd3), the dopamine transporter (DAT), phosphodiesterases (Pde4b, Pde4d), and the tetraspanin Cd81, in the mesocorticolimbic pathway. Our results indicated the following: (i) CD81 function was essential for normal transcriptional response to nicotine, (ii) baseline expression of Pde4b and DAT were each influenced by Cd81 genotype in key brain areas and (iii) the baseline expression of Pde4b and DAT correlated with nicotine consumption behaviors. Finally, we speculate that Cd81, Pde4b and DAT work in concert to modulate nicotine preference and that this Cd81-associated pathway may function in a drug-specific manner. In conclusion, our results support the use of laboratory mice in nicotine selfselection for assessing nicotine preference. We found that CD81 influences nicotine consumption and transcriptional activity of dopamine signaling-associated components. Our hypothesis that CD81, PDE4B and DAT participate in a single pathway, working in concert to modulate drug consumption will be tested in future experiments.