The reasons why people smoke are varied, but research has demonstrated that genetic influences on various aspects of nicotine addiction are a major factor. There also is a strong genetic influence on measures of nicotine sensitivity in mice. Despite the established contribution of genetics to nicotine sensitivity in mice and humans, no naturally occurring genetic variation has been identified that demonstrably alters sensitivity to nicotine in either species. However, one genetic variant has been implicated in altering nicotine sensitivity in mice is a T529A polymorphism in Chrna4, the gene that encodes the nicotinic receptor (nAChR) α4 subunit. The Chrna4 T529A polymorphism leads to a threonine to alanine substitution at position 529 of the α4 subunit. To more definitively address whether the Chrna4 T529A polymorphism does, in fact, influence sensitivity to nicotine, knockin mice were generated in which the threonine codon at position 529 was mutated to an alanine codon. Compared to Chrna4 T529 littermate controls, the Chrna4 A529 knockin mice exhibited greater sensitivity to the hypothermic effects of nicotine, reduced oral nicotine consumption and did not develop conditioned place preference to nicotine. The Chrna4 A529 knockin mice also differed from T529 littermates for two parameters of acetylcholine-stimulated 86Rb+ efflux in midbrain: maximal efflux and the percentage of α4β2* receptors with high sensitivity to activation by agonists. Results indicate that the polymorphism affects the function of midbrain α4β2* nAChRs and contributes to individual differences in several behavioral and physiological responses to nicotine thought to be modulated by midbrain α4β2* nAChRs.

Aim: α2 nAChR subunit mRNA expression in mice is most intense in the olfactory bulbs and interpeduncular nucleus. We aimed to investigate the properties of α2* nAChRs in these mouse brain regions. Methods: α2 nAChR subunit-null mutant mice were engineered. Pharmacological and immunoprecipitation studies were used to determine the composition of α2 subunit-containing (α2*) nAChRs in these two regions. Results: [ 125 I]Epibatidine (200 pmol/L) autoradiography and saturation binding demonstrated that α2 deletion reduces nAChR expression in both olfactory bulbs and interpeduncular nucleus (by 4.8±1.7 and 92±26 fmol·mg -1 protein, respectively). Pharmacological characterization using the β2-selective drug A85380 to inhibit [ 125 I]epibatidine binding proved inconclusive, so immunoprecipitation methods were used to further characterize α2* nAChRs. Protocols were established to immunoprecipitate β2 and β4 nAChRs. Immunoprecipitation specificity was ascertained using tissue from β2-and β4-null mutant mice, and efficacy was good (>90% of β2* and >80% of β4* nAChRs were routinely recovered). Conclusion:Immunoprecipitation experiments indicated that interpeduncular nucleus α2* nAChRs predominantly contain β2 subunits, while those in olfactory bulbs contain mainly β4 subunits. In addition, the immunoprecipitation evidence indicated that both nuclei, but especially the interpeduncular nucleus, express nAChR complexes containing both β2 and β4 subunits.

Approximately 80% of smokers initiate tobacco use during adolescence, suggesting that nicotine initiation and nicotine dependence have a substantial age component. There also is a substantial genetic influence on smoking behaviors such as age of initiation and the development of nicotine dependence. The goal of this study was to examine both genetic background and age dependent effects on oral nicotine self-administration and anxiety-like behaviors in mice. Two inbred mouse strains (C3H/Ibg and C57BL/6J) were assessed for oral nicotine preference during early adolescence (postnatal day 24–35), middle adolescence (postnatal day 36–47), late adolescence (postnatal day 48–59), adulthood (postnatal day 60+) and 2 months following their initial exposure to nicotine. Mice also were assessed for innate anxiety using an elevated zero maze to determine if age and/or genetic background influenced anxiety-like behaviors. Results indicated that initial nicotine preference and nicotine preference two months after an initial exposure are both strain and age dependent. Age also had an effect on some baseline anxiety measures but strain differences for most zero maze measures were present throughout all age groups. In general, early adolescent C3H mice exhibited greater nicotine preference while C57 mice displayed greater preference during middle adolescence and upon a second exposure to nicotine. In contrast, C57 mice exhibited reduced anxiety across all ages tested. These studies indicate that genetic background should be considered when evaluating age-dependent effects of drugs of abuse and baseline anxiety-like behaviors.

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