Acute Activation, Desensitization and Smoldering Activation of Human Acetylcholine Receptors

The behavioral effects of nicotine and other nicotinic agonists are mediated by AChRs in the brain. The relative contribution of acute activation versus chronic desensitization of AChRs is unknown. Sustained ‘‘smoldering activation’ ’ occurs over a range of agonist concentrations at which activated and desensitized AChRs are present in equilibrium. We used a fluorescent dye sensitive to changes in membrane potential to examine the effects of acute activation and chronic desensitization by nicotinic AChR agonists on cell lines expressing human a4b2, a3b4 and a7 AChRs. We examined the effects of acute and prolonged application of nicotine and the partial agonists varenicline, cytisine and sazetidine-A on these AChRs. The range of concentrations over which nicotine causes smoldering activation of a4b2 AChRs was centered at 0.13 mM, a level found in smokers. However, nicotine produced smoldering activation of a3b4 and a7 AChRs at concentrations well above levels found in smokers. The a4b2 expressing cell line contains a mixture of two stoichiometries, namely (a4b2)2b2 and (a4b2)2a4. The (a4b2)2b2 stoichiometry is more sensitive to activation by nicotine. Sazetidine-A activates and desensitizes only this stoichiometry. Varenicline, cytisine and sazetidine-A were partial agonists on this mixture of a4b2 AChRs, but full agonists on a3b4 and a7 AChRs. It has been reported that cytisine and varenicline are most efficacious on the (a4b2)2a4 stoichiometry. In this study, we distinguish the dual effects of activation and desensitization of AChRs by these nicotinic agonists and define the range of concentrations over which smoldering activation can b