Abstract

The α4β2 nicotinic acetylcholine receptor (nAChR) is important in normal CNS physiology and in mediating several of the pharmacological effects of nicotine on cognition, attention, and affective states. It is also the likely receptor that mediates nicotine addiction. This receptor assembles in two distinct stoichiometries: (α4)2(β2)3 and (α4)3(β2)2 that are referred to as high (HS) and low (LS) sensitivity nAChRs, respectively, based on a difference in the potency of acetylcholine to activate them. The physiological and pharmacological differences between these two receptor subtypes have been described in heterologous expression systems. However, the presence of each stoichiometry in native tissue remains unknown. In this study, different ratios of rat α4 and β2 subunit cDNA were transfected into HEK293 cells to create a novel model system of HS and LS α4β2 nAChRs expressed in a mammalian cell line. The HS and LS nAChRs were characterized through pharmacological and biochemical methods. Isolation of surface proteins revealed increasing amounts of α4 or β2 subunits in the LS or HS nAChR populations, respectively. Additionally, sazetidine-A displayed different efficacies in activating these two forms of the receptor. Using this model system, a neurophysiological 'two-concentration' acetylcholine paradigm was developed and validated to determine α4/β2 subunit stoichiometry. This paradigm was then utilized in layers I-IV of slices of rat motor cortex to determine the percent contribution of high and low sensitivity α4β2 receptors in this brain region. We report that the majority of α4β2 nAChRs in this brain region possess a stoichiometry of the (α4)3(β2)2 LS subtype.