Characterization of Human α4β2-Nicotinic Acetylcholine Receptors Stably and Heterologously Expressed in Native Nicotinic Receptor-Null SH-EP1 Human Epithelial Cells

Naturally expressed nicotinic acetylcholine receptors composed of α4 and β2 subunits (α4β2-nAChR) are the predominant form of high affinity nicotine binding site in the brain implicated in nicotine reward, mediation of nicotinic cholinergic transmission, modulation of signaling through other chemical messages, and a number of neuropsychiatric disorders. To develop a model system for studies of human α4β2-nAChR allowing protein chemical, functional, pharmacological, and regulation of expression studies, human α4 and β2 subunits were stably introduced into the native nAChR-null human epithelial cell line SH-EP1. Heterologously expressed α4β2-nAChR engage in high-affinity, specific binding of ³H-labeled epibatidine (H-EBDN; macroscopic K_D = 10 pM; k_on = 0.74/min/nM, k_off = 0.013/min). Immunofluorescence studies show α4 and β2 subunit protein expression in virtually every transfected cell, and microautoradiographic studies show expression of ¹²⁵I-labeled jodo-deschloroepibatidine binding sites in most cells. H-EBDN binding competition studies reveal high affinity for nicotinic agonists and lower affinity for nicotinic antagonists. Heterologously expressed α4β2-nAChR functional studies using ⁸⁶Rb⁺ efflux assays indicate full efficacy of epibatidine, nicotine, and acetylcholine; partial efficacy for 1,1-dimethyl-4-phenyl-piperazinium, cytisine, and suberyldicholine; competitive antagonism by dihydro-β -erythroidine, decamethonium, and methyllycaconitine; noncompetitive antagonism by mecamylamine and eserine; and mixed antagonism by pancuronium, hexamethonium, and d-tubocurarine. These results demonstrate utility of transfected SH-EP1 cells as models for studies of human α4β2-nAChR, and they also reveal complex relationships between apparent affinities of drugs for radioligand binding and functional sites on human α4β2-nAChR.