CHRNA4 and depressive disorder: Neuronal nAChR genes are able to produce an enormous number of different pentameric combinations of nAChR receptors, located in many sites across the central and peripheral nervous systems, each of them with specific pharmacological roles; for example, the nAChR subunits α4β2 and α4β4 play a role in neurodegeneration and depression, the α3β2 subunit in dopamine release and Parkinson’s disease, the α3β4 subunit in norepinephrine release and in cardiovascular and gastrointestinal action.