The ( α 4) 3 ( β 2) 2 Stoichiometry of the Nicotinic Acetylcholine Receptor Predominates in the Rat Motor Cortex.

The α 4 β 2 nicotinic acetylcholine receptor (nAChR) is important in central nervous system 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: ( α 4)2 ( β 2)3 and ( α 4)3 ( β 2)2 , which are referred to as high-sensitivity (HS) and low-sensitivity (LS) nAChRs, respectively, based on a difference in the potency of acetylcholine to activate them. The physiologic and pharmacological differences between these two receptor subtypes have been described in heterologous expression systems. However, the presence of each stoichiometry in native tissue currently remains unknown. In this study, different ratios of rat α 4 and β 2 subunit cDNA were transfected into human embryonic kidney 293 cells to create a novel model system of HS and LS α 4 β 2 nAChRs expressed in a mammalian cell line. The HS and LS nAChRs were characterized through pharmacological and biochemical methods. Isolation of surface proteins revealed higher amounts of α 4 or β 2 subunits in the LS or HS nAChR populations, respectively. In addition, sazetidine-A displayed different efficacies in activating these two receptor stoichiometries. Using this model system, a neurophysiological "two-concentration" acetylcholine or carbachol paradigm was developed and validated to determine α 4/ β 2 subunit stoichiometry. This paradigm was then used in layers I-IV of slices of the rat motor cortex to determine the percent contribution of HS and LS α 4 β 2 receptors in this brain region. We report that the majority of α 4 β 2 nAChRs in this brain region possess a stoichiometry of the ( α 4)3 ( β 2)2 LS subtype.