Aim: a2 nAChR subunit mRNA expression in mice is most intense in the olfactory bulbs and interpeduncular nucleus. We aimed to investigate the properties of a2* nAChRs in these mouse brain regions.Methods: a2 nAChR subunit-null mutant mice were engineered. Pharmacological and immunoprecipitation studies were used to determine the composition of a2 subunit-containing (a2*) nAChRs in these two regions.Results: [125I]Epibatidine (200 pmol/L) autoradiography and saturation binding demonstrated that al deletion reduces nAChR expression in both olfactory bulbs and interpeduncular nucleus (by 4.8±1.7 and 92卤26 fmol-mg"1 protein, respectively). Pharmacological characterization using the 02-selective drug A85380 to inhibit [125I]epibatidine binding proved inconclusive, so immunoprecipitation methods were used to further characterize a2* nAChRs. Protocols were established to immunoprecipitate 02 and 04 nAChRs. Immunoprecipitation specificity was ascertained using tissue from 02- and 04-null mutant mice, and efficacy was good (>90% of (32* and >80% of 04* nAChRs were routinely recovered). Conclusion: Immunoprecipitation experiments indicated that interpeduncular nucleus a2* nAChRs predominantly contain (32 subunits, while those in olfactory bulbs contain mainly 04 subunits. In addition, the immunoprecipitation evidence indicated that both nuclei, but especially the interpeduncular nucleus, express nAChR complexes containing both 02 and 04 subunits.