Effects of Prolonged Nicotinic Ligand Exposure on Function of Heterologously Expressed, Human α4β2- and α4β4-Nicotinic Acetylcholine Receptors

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Abstract
Effects of prolonged nicotinic ligand exposure on the function of human alpha4beta2- and alpha4beta4-nicotinic acetylcholine receptor (nAChR) subtypes were studied using receptors heterologously expressed in SH-EP1 human epithelial cells. Magnitudes of acute, nAChR-mediated, specific Rb-86(+) efflux responses to 1 mM carbamylcholine were reduced after pretreatment with specific nAChR ligands in effects that depended on pretreatment drug dose, duration of drug pretreatment, and duration of drug-free recovery. Fifty percent inhibition of alpha4beta2- nAChR function following 5 min of recovery occurred after 1 min of pretreatment with 1 mM nicotine but also after 1-h pretreatment at 10 nM nicotine. Seventy-five percent loss in function persisted 1 h after drug removal following 15 min or more of exposure to 1 mM nicotine. However, functional recovery was nearly complete after 1 h in drug-free medium following 1 min to 24 h pretreatment with 0.1 to 1 muM nicotine, i.e., in the range of smoker plasma nicotine levels. alpha4beta4-nAChR was similarly sensitive to persistent inactivation by prolonged nicotine exposure. Carbamylcholine exhibited slightly lower persistent inactivation potency than nicotine at both alpha4beta2- and alpha4beta4-nAChR. The nAChR antagonist, mecamylamine, exhibited persistent inactivation potency and efficacy similar to nicotine at alpha4beta2-nAChR but had a reduced effect on alpha4beta4-nAChR. These studies illustrate persistent inactivation of human alpha4beta2- or alpha4beta4-nAChR induced by prolonged exposure to nicotine and show that other ligands induce nAChR persistent inactivation in a subtype-specific manner.