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 n m , 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.