Abstract
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 α4β2, α3β4 and α7 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 α4β2 AChRs was centered at 0.13 µM, a level found in smokers. However, nicotine produced smoldering activation of α3β4 and α7 AChRs at concentrations well above levels found in smokers. The α4β2 expressing cell line contains a mixture of two stoichiometries, namely (α4β2)2β2 and (α4β2)2α4. The (α4β2)2β2 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 α4β2 AChRs, but full agonists on α3β4 and α7 AChRs. It has been reported that cytisine and varenicline are most efficacious on the (α4β2)2α4 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 be sustained.
Highlights
The component of tobacco that drives its compulsive use is nicotine, an alkaloid that acts on nicotinic acetylcholine receptors (AChRs) in the brain
Acute Responses to Nicotinic Agonists Acute responses of a4b2, a3b4 and a7 AChRs were tested after application of a range of concentrations of ACh, nicotine, varenicline, cytisine and sazetidine-A, using the FlexStation with an indicator which is sensitive to changes in membrane potential
We utilize human AChRs cloned in human cells to examine the dual effects of activation and desensitization by nicotinic agonists
Summary
The component of tobacco that drives its compulsive use is nicotine, an alkaloid that acts on nicotinic acetylcholine receptors (AChRs) in the brain. AChRs are a heterogeneous family of ligand-gated cation channels which consist of five homologous subunits arranged around a central pore [1,2]. They are heteropentamers formed of combinations of a and b subunits, or homopentamers formed entirely of a7 subunits [3]. Heteromeric AChRs usually have two ACh binding sites that are formed between adjacent a and b subunits. While the accessory subunit does not usually form part of a binding site for ACh, it has major effects on responses to nicotinic agonists, antagonists and allosteric modulators. There is a third ACh binding site in the (a4b2)2a4 stoichiometry, formed at the interface between the a4 accessory subunit and another a4 subunit [4,5]
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