The α5 Subunit Regulates the Expression and Function of α4*-Containing Neuronal Nicotinic Acetylcholine Receptors in the Ventral-Tegmental Area

Human genetic association studies have shown gene variants in the alpha 5 subunit of the neuronal nicotinic receptor (nAChR) influence both ethanol and nicotine dependence. The alpha 5 subunit is an accessory subunit that facilitates alpha 4* nAChRs assembly in vitro. However, it is unknown whether this occurs in the brain, as there are few research tools to adequately address this question. As the alpha 4*-containing nAChRs are highly expressed in the ventral tegmental area (VTA) we assessed the molecular, functional and pharmacological roles of alpha 5 in alpha 4*-containing nAChRs in the VTA. We utilized transgenic mice alpha 5+/+(alpha 4YFP) and alpha 5-/-(alpha alpha 4YFP) that allow the direct visualization and measurement of alpha 4-YFP expression and the effect of the presence (alpha 5+/+) and absence of alpha 5 (-/-) subunit, as the antibodies for detecting the alpha 4* subunits of the nAChR are not specific. We performed voltage clamp electrophysiological experiments to study baseline nicotinic currents in VTA dopaminergic neurons. We show that in the presence of the alpha 5 subunit, the overall expression of alpha 4 subunit is increased significantly by 60% in the VTA. Furthermore, the alpha 5 subunit strengthens baseline nAChR currents, suggesting the increased expression of alpha 4* nAChRs to be likely on the cell surface. While the presence of the alpha 5 subunit blunts the desensitization of nAChRs following nicotine exposure, it does not alter the amount of ethanol potentiation of VTA dopaminergic neurons. Our data demonstrates a major regulatory role for the alpha 5 subunit in both the maintenance of alpha 4*-containing nAChRs expression and in modulating nicotinic currents in VTA dopaminergic neurons. Additionally, the alpha 5 alpha 4* nAChR in VTA dopaminergic neurons regulates the effect of nicotine but not ethanol on currents. Together, the data suggest that the alpha 5 subunit is critical for controlling the expression and functional role of a population of alpha 4*-containing nAChRs in the VTA.