The β3 subunit increases the density of α6* nAChRs at the PM and is necessary for the nicotine-induced up-regulation of α6β2* nAChRs. (A, C, and E) Representative TIRFM images of Neuro-2a cells transfected with α6-SEP and a combination of β2 with or without β3 nAChR subunits at basic (pH 7.4) and acidic (pH 5.4) conditions. Nicotine was added at listed concentrations 24 h before imaging. (B, D, and F) PMID for α6-SEP* nAChRs. Bars, 10 µm (n = 19–23). Data are mean values ± SEM. n.s., not significant; ***, P < 0.0001.