UB-165: A Novel Nicotinic Agonist with Subtype Selectivity Implicates the α4β2* Subtype in the Modulation of Dopamine Release from Rat Striatal Synaptosomes

Presynaptic nicotinic acetylcholine receptors (nAChRs) on striatal synaptosomes stimulate dopamine release. Partial inhibition by the α3β2-selective α-conotoxin-MII indicates heterogeneity of presynaptic nAChRs on dopamine terminals. We have used this α-conotoxin and UB-165, a novel hybrid of epibatidine and anatoxin-a, to address the hypothesis that the α-conotoxin-MII-insensitive subtype is composed of α4 and β2 subunits. UB-165 shows intermediate potency, compared with the parent molecules, at α4β2* and α3-containing binding sites, and resembles epibatidine in its high discrimination of these sites over α7-type and muscle binding sites. (±)-Epibatidine, (±)-anatoxin-a, and (±)-UB-165 stimulated [ 3 H]-dopamine release from striatal synaptosomes with EC 50 values of 2.4, 134, and 88 nm, and relative efficacies of 1:0.4:0.2, respectively. α-Conotoxin-MII inhibited release evoked by these agonists by 48, 56, and 88%, respectively, suggesting that (±)-UB-165 is a very poor agonist at the α-conotoxin-MII-insensitive nAChR subtype. In assays of 86 Rb + efflux from thalamic synaptosomes, a model of an α4β2* nAChR response, (±)-UB-165 was a very weak partial agonist; the low efficacy of (±)-UB-165 at α4β2 nAChR was confirmed in Xenopus oocytes expressing various combinations of human nAChR subunits. In contrast, (±)-UB-165 and (±)-anatoxin-a were similarly efficacious and similarly sensitive to α-conotoxin-MII in increasing intracellular Ca 2+ in SH-SY5Y cells, a functional assay for native α3-containing nAChR. These data support the involvement of α4β2* nAChR in the presynaptic modulation of striatal dopamine release and illustrate the utility of exploiting a novel partial agonist, together with a selective antagonist, to dissect the functional roles of nAChR subtypes in the brain.