Stimulation of dopamine release by nicotinic acetylcholine receptor ligands in rat brain slices correlates with the profile of high, but not low, sensitivity α4β2 subunit combination

α4β2 neuronal nicotinic receptors (nAChRs) can exist in high and low sensitivity states possibly due to distinct stoichiometries during subunit assembly: (α4)2(β2)3 pentamer (high sensitivity, HS) and (α4)3(β2)2 pentamer (low sensitivity, LS). To determine if there is a linkage between HS or LS states and receptor-mediated responses in brain, we profiled several clinically studied α4β2* nAChR agonists for the displacement of radioligand binding to α4β2 [3H]-cytisine sites in rat brain membranes, effects on stimulation of [3H]-dopamine release from slices of rat prefrontal cortex and striatum, and activation of HS and LS human α4β2 nAChRs expressed in Xenopus laevis oocytes. Binding affinities (pKi) and potency (pEC50) values for [3H]-dopamine release closely correlated with a rank order: varenicline > (−)-nicotine > AZD3480 > dianicline ≅ ABT-089. Further, a good correlation was observed between [3H]-dopamine release and HS α4β2 pEC50 values, but not between [3H]-dopamine release and LS α4β2. The relative efficacies of the agonists ranged from full to partial agonists. Varenicline behaved as a partial agonist in stimulating [3H]-dopamine release and activating both HS and LS α4β2 nAChRs expressed in oocytes. Conversely, ABT-089, AZD3480 and dianicline exhibited little efficacy at LS α4β2 (<5%), were more effective at HS α4β2 nAChRs, and in stimulating cortical and striatal [3H]-dopamine release ≥30%. In the presence of α-conotoxin MII to block α6β2* nAChRs, the α4β2* α-conotoxin-insensitive [3H]-dopamine release stimulated by these ligands correlates well with their interactions at HS, but not LS. In summary, this study provides support for HS α4β2* nAChR involvement in neurotransmitter release.

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