| First Author | Zhang HJ | Year | 2023 |
| Journal | J Neurosci | Volume | 43 |
| Issue | 1 | Pages | 2-13 |
| PubMed ID | 36028313 | Mgi Jnum | J:352614 |
| Mgi Id | MGI:7707601 | Doi | 10.1523/JNEUROSCI.2484-21.2022 |
| Citation | Zhang HJ, et al. (2023) Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by In Vitro Cellular Studies and In Vivo PET Imaging. J Neurosci 43(1):2-13 |
| abstractText | 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 pK(a) and high affinity for alpha4beta2-type nicotinic receptors (alpha4beta2Rs) are trapped in intracellular acidic vesicles containing alpha4beta2Rs in vitro Nicotine, with lower pK(a) and alpha4beta2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligands in vivo in male and female mouse brain and identifying the trapping brain organelle in vitro as Golgi satellites (GSats). Two PET (18)F-labeled imaging ligands were chosen: [(18)F]2-FA85380 (2-FA) with varenicline-like pK(a) and affinity and [(18)F]Nifene with nicotine-like pK(a) and affinity. [(18)F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in alpha4beta2R-containing GSats. In contrast, [(18)F]Nifene kinetics were rapid, consistent with its binding to alpha4beta2Rs but no trapping. Specific [(18)F]2-FA and [(18)F]Nifene signals were eliminated in beta2 subunit knock-out (KO) mice or by acute nicotine (AN) injections demonstrating binding to sites on beta2-containing receptors. Chloroquine (CQ), which dissipates GSat pH gradients, reduced [(18)F]2-FA distributions while having little effect on [(18)F]Nifene distributions in vivo consistent with only [(18)F]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 alpha4beta2R-containing GSats, which results in very slow release long after nicotine is gone after smoking.SIGNIFICANCE STATEMENT Mechanisms 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 alpha4beta2R-containing acidic vesicles. Using a fluorescent-labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were alpha4beta2R-containing Golgi satellites (GSats). 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. |
