Characterization of [³H]ABT-418: A novel cholinergic channel ligand

ABT-418 [(S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole] is a potent and selective agonist at neuronal nicotinic acetylcholine receptors (nAChRs) with cognitive enhancing and anxiolytic activities. [³H]ABT-418 was found to bind with high affinity (K(D) = 2.85 ± 0.14 nM) to membranes prepared from rat brain. Binding of [³H]ABT-418 was characterized by rapid association (T( 1/2 ) = 1.4 ± 0.3 min) and dissociation (T( 1/2 ) = 2.9 ± 0.4 min) half- times. The pharmacology of [³H]ABT 418 binding was consistent with an interaction with the putative α4β2 nAChR subtype. The nAChR agonists, (-)- nicotine, (-) cytisine and (±)-epibatidine, displayed a high affinity (K(i) = 0.8 ± 0.1, 0.2 ± 0.1 and 0.05 ± 0.01 nM, respectively) for [³H]ABT-418 binding sites, whereas among nAChR antagonists examined, only dihydro-β- erythroidine competed with high affinity (K(i) = 32 ± 1.5 nM). Although autoradiography studies indicate that the binding distribution of [³H]ABT- 418 and (-)-[³H]cytisine are largely identical, there are some brain regions including the striatum, olivary pretectal nucleus and the superior colliculus, in which [³H]ABT-418 demonstrates significantly (P < .05) less binding. The data in the present study demonstrate that [³H]ABT-418 binds with high affinity to a population of binding sites in the rat brain that have the pharmacological characteristics of neuronal nAChRs. [³H]ABT-418 may, therefore, serve as a useful radioligand to further probe the observed differences in pharmacological properties between ABT-418 and other nicotinic agonists in vivo.