CHRNA4 and Parkinson disease: Oligomeric α-synuclein, particularly the larger oligomeric α-synuclein aggregates studied here, selectively inhibits hα4β2-nAChR function in a dose-dependent and non-competitive manner, providing the basis for a new hypothesis that α-synuclein can directly modulate hα4β2-nAChR function, which in turn may contribute to cholinergic signaling deficits in PD.