In an effort to develop selective radioligands for in vivo imaging of neuronal nicotinic acetylcholine receptors (nAChRs), we synthesized 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-iodo-A-85380) and labeled it with (125)I and (123)I. Here we present the results of experiments characterizing this radioiodinated ligand in vitro. The affinity of 5-[(125)I]iodo-A-85380 for alpha4beta2 nAChRs in rat and human brain is defined by K(d) values of 10 and 12 pM, respectively, similar to that of epibatidine (8 pM). In contrast to epibatidine, however, 5-iodo-A-85380 is more selective in binding to the alpha4beta2 subtype than to other nAChR subtypes. In rat adrenal glands, 5-iodo-A-85380 binds to nAChRs containing alpha3 and beta4 subunits with 1/1000th the affinity of epibatidine, and exhibits 1/60th and 1/190th the affinity of epibatidine at alpha7 and muscle-type nAChRs, respectively. Moreover, unlike epibatidine and cytisine, 5-[(125)I]iodo-A-85380 shows no binding in any brain regions in mice homozygous for a mutation in the beta2 subunit of nAChRs. Binding of 5-[(125)I]iodo-A-85380 in rat brain is reversible, and is characterized by high specificity and a slow rate of dissociation of the receptor-ligand complex (t(1/2) for dissociation approximately 2 h). These properties, along with other features observed previously in in vivo experiments (low toxicity, rapid penetration of the blood-brain barrier, and a high ratio of specific to nonspecific binding), suggest that this compound, labeled with (125)I or (123)I, is superior to other radioligands available for in vitro and in vivo studies of alpha4beta2 nAChRs, respectively.