Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by in vitro Cellular Studies and in vivo PET Imaging
A question relevant to nicotine addiction is how nicotine and other nicotinic receptor membranepermeant ligands, such as the anti-smoking drug varenicline (Chantix), distribute in brain. Ligands, like varenicline, with high pKa and high-affinity for 4{beta}2-type nicotinic receptors (4{beta}2Rs) are trapped in intracellular acidic vesicles containing 4{beta}2Rs in vitro. Nicotine, with lower pKa and 4{beta}2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligands in vivo in mouse brain and identifying the trapping brain organelle in vitro as Golgi satellites (GSats). Two PET 18F-labelled imaging ligands were chosen: [18F]2-FA85380 (2-FA) with varenicline-like pKa and affinity and [18F]Nifene with nicotine-like pKa and affinity. [18F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in 4{beta}2R-containing GSats. In contrast, [18F]Nifene kinetics were rapid, consistent with its binding to 4{beta}2Rs but no trapping. Specific [18F]2-FA and [18F]Nifene signals were eliminated in {beta}2 subunit knockout mice or by acute nicotine injections demonstrating binding to sites on {beta}2-containing receptors. Chloroquine, which dissipates GSat pH gradients, reduced [18F]2-FA distributions while having little effect on [18F]Nifene distributions in vivo consistent with only [18F]2-FA trapping in GSats. These results are further supported by in vitro findings where dissipation of GSat pH gradients blocks 2-FA trapping in GSats without affecting Nifene. By combining in vitro and in vivo imaging, 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 4{beta}2R-containing GSats, which results in very slow release long after nicotine is gone after smoking. SignificanceMechanisms of nicotine addiction remain poorly understood. An earlier study using in vitro methods found that the anti-smoking nicotinic ligand, varenicline (Chantix) was trapped in 4{beta}2R-containing acidic vesicles. Using a fluorescent labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were 4{beta}2R-containing Golgi satellites. In vivo PET 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 combining in vitro and in vivo imaging revealed new mechanistic insights into the kinetics of weak base PET imaging ligands and the subcellular mechanisms underlying nicotine addiction.