Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed byIn VitroCellular Studies andIn VivoPET Imaging

Abstract

A question relevant to nicotine addiction is how nicotine and other nicotinic receptor membrane-permeant ligands, such as the anti-smoking drug varenicline (Chantix), distribute in brain. Ligands, like varenicline, with high pKaand high affinity for α4β2-type nicotinic receptors (α4β2Rs) are trapped in intracellular acidic vesicles containing α4β2Rsin vitro. Nicotine, with lower pKaand α4β2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligandsin vivoin male and female mouse brain and identifying the trapping brain organellein vitroas Golgi satellites (GSats). Two PET18F-labeled imaging ligands were chosen: [18F]2-FA85380 (2-FA) with varenicline-like pKaand affinity and [18F]Nifene with nicotine-like pKaand affinity. [18F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in α4β2R-containing GSats. In contrast, [18F]Nifene kinetics were rapid, consistent with its binding to α4β2Rs but no trapping. Specific [18F]2-FA and [18F]Nifene signals were eliminated in β2 subunit knock-out (KO) mice or by acute nicotine (AN) injections demonstrating binding to sites on β2-containing receptors. Chloroquine (CQ), which dissipates GSat pH gradients, reduced [18F]2-FA distributions while having little effect on [18F]Nifene distributionsin vivoconsistent with only [18F]2-FA trapping in GSats. These results are further supported byin vitrofindings where dissipation of GSat pH gradients blocks 2-FA trapping in GSats without affecting Nifene. By combiningin vitroandin vivoimaging, we mapped both the brain-wide and subcellular distributions of weak-base nicotinic receptor ligands. We conclude that ligands, such as varenicline, are trapped in neurons in α4β2R-containing GSats, which results in very slow release long after nicotine is gone after smoking.SIGNIFICANCE STATEMENTMechanisms of nicotine addiction remain poorly understood. An earlier study usingin vitromethods found that the anti-smoking nicotinic ligand, varenicline (Chantix) was trapped in α4β2R-containing acidic vesicles. Using a fluorescent-labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were α4β2R-containing Golgi satellites (GSats).In vivoPET imaging with F-18-labeled nicotinic ligands provided additional evidence that differences in PET ligand trapping in acidic vesicles were the cause of differences in PET ligand kinetics and subcellular distributions. These findings combiningin vitroandin vivoimaging revealed new mechanistic insights into the kinetics of weak base PET imaging ligands and the subcellular mechanisms underlying nicotine addiction.