Displacement of [¹²⁵I]epibatidine binding to immunoimmobilized nAChRs. αCtxMII (A) and MLA (B) were used to displace ¹²⁵I-Epi binding to immunoimmobilized striatal nAChRs. Three types of nAChR preparation have been tested in these experiments: β2 immunoimmobilized nAChRs from α6-/- animals [β2(α6-/-)] and α6-immunoimmobilized nAChRs from α4-/- [α6 (α4-/-)] or α6+/+ [α6 (α6+/+)] animals. According to our IPP experiments, these three preparations are likely to contain primarily α4β2, α6β2, and a mix of α4α6β2 and α6β2 nAChRs, respectively. Data points were fitted to a one- or two-site model using the LIGAND program (see Table 1 for fitting parameter values). For both ligands, this analysis revealed the existence of a heterogeneous population of binding sites only in α6-immunoimmobilized nAChRs from Wt mice. This result is in agreement with the existence of α4α6β2 nAChRs. Each point is the mean ± SEM of at least four separate determinations.