Outside-in and inside-out mechanisms of nicotine signaling. In the outside-in mechanism, nicotine binds directly to nAChR channels in the plasma membrane, causing membrane depolarization, calcium influx, and eventual channel densitization. In the inside-out mechanism, nicotine travels inside the cell and stabilizes particular nAChR subunit combinations in the ER. This leads to increased channel trafficking to the plasma membrane and a modified distribution of subunits, which modifies endogenous acetylcholine signaling.