Effects of chronic ligand exposure on nicotinic acetylcholine receptors

Abstract

To illuminate effects of chronic nicotinic ligand treatment on nicotinic acetylcholine receptor (nAChR) function, responses to acute agonist challenge after nicotinic ligand pre-exposure were studied using ion efflux assays. Losses in function of heterologously-expressed, human α4β2- and α4β4-nAChR were observed following exposure to concentrations of nicotine at or below levels of nicotine present in the brain and serum of human smokers and showed both time- and concentration-dependence. α4β2- and α4β4-nAChR functional responsiveness to acute agonist challenge following prolonged exposure to various nicotinic receptor agonists or antagonists also was evaluated to determine whether these agents block or mimic nicotine effects and to gain insight as to mechanisms underlying nAChR functional changes. For all agonists tested, prolonged exposure induced functional losses at both α4β2- and α4β4-nAChR that persisted beyond removal of the drug. A strong positive correlation between acute functional potency of agonists and the amount of functional loss induced by pre-exposure to the agonists was found. Some but not all antagonists tested were able to induce persistent functional inactivation of α4β2- and α4β4-nAChR, but more weakly than agonists. However, sensitivity to antagonist pre-exposure allowed discrimination between closely related nAChR subtypes. Pre-exposure to pancuronium, a competitive antagonist of nAChR, had no lasting effect on nAChR function after its removal, but instead protected nAChR from nicotine-induced persistent inactivation. In studies to explore the idea that open channel block is involved in nAChR persistent inactivation, local anesthetics (LAs) were found to act mostly as non-competitive antagonists at α1-, α3β4*-, α4β2- and α4β4-nAChR subtypes. Prolonged exposure to some LAs also induced persistent functional losses at α4β2- and α4β4-nAChR, and some evidence was obtained suggesting that other LAs may protect against nicotine-induced persistent inactivation of α4β2- or α4β4-nAChR. Collectively, these studies provide insight into changes in nAChR functional state induced by prolonged nicotinic ligand exposure for diverse nAChR subtypes including those found in the brain. These findings have implications for nicotine dependence and for anticipated nicotinic therapies of potential clinical importance in treatment of a variety of neurological and psychiatric conditions.