Kinetic Modeling of Weak Base nAChR Ligand Selective Trapping within Intracellular Acidic Vesicles: Insights into Mechanisms Underlying Nicotine Addiction and Smoking Cessation

Abstract

Recent work from our group has characterized selective trapping of nicotinic acetylcholine receptor (nAChR) ligands within neurons in the brain (Govind et al. eLife 20171). The ligands are weak bases that bind with high-affinity to α4β2-type AChRs (α4β2Rs), such as the anti-smoking drug varenicline (Chantix) and epibatidine. Selective trapping occurs within α4β2Rs-containing acidic vesicles of cells and neurons. Slow release of trapped varenicline reduces the effects of long-term nicotine exposure. Selective trapping is further regulated by nicotine exposure, which increases the number of acidic vesicles. Nicotine, also a weak base, is not trapped because its lower pKa and lower affinity to α4β2Rs1. Here, we present a kinetic model that accounts for the biphasic behavior of these weak bases leaving the cell after accumulating in vesicles and binding to the nAChRs on the cell-surface and within intracellular vesicles. Accumulation and release of molecules in the cell by diffusion into and out of cytosol and vesicles is calculated with the Fick's Law of diffusion (neutral molecule) and the Nernst-Planck equation (ion, Trapp and Horobin 2005). The ligand binding to α4β2s is modeled using empirical receptor-ligand kinetics. Solving our coupled differential equations numerically, we found that the fast release phase is mainly caused by unbinding of protonated ligands from the α4β2Rs not in the membrane of the acidic vesicles. The slow phase is caused by combination of trapping of the protonated ligands inside the acidic vesicles, and the α4β2R high-affinity binding sites in the vesicle lumen keeping the ligands out of the vesicle solution. This interpretation is supported by the experimental data and can help us further understand the mechanism of nicotine addiction and smoking cessation drugs.