The internalization of NMDA receptors (NMDARs) is promoted by monomeric alpha-synuclein (alpha-syn). Because of the pathogenic role of oligomeric alpha-syn, the effect of aggregated alpha-syn on this regulation deserves investigation. Several alpha-syn oligomers were prepared by incubating recombinant human alpha-syn in phosphate-buffered saline (PBS), plasma of normal controls (NC) and patients with Parkinson's disease (PD). The alpha-syn oligomers formed in PBS are not phosphorylated and are different from the alpha-syn oligomers formed in the plasma of NC and PD that are moderately and highly phosphorylated at serine 129, which is a key phosphorylation site of the alpha-syn molecule in PD patients. After being added into the culture medium, the alpha-syn monomers and its oligomers formed in different methods and rapidly entered into MES23.5 dopaminergic cells and induced an increase in the expression of Rab5B, an endocytic protein that has been shown to regulate clathrin-mediated endocytosis of NMDARs. The levels of surface GluN1, a subunit obligatory for the assembly of functional NMDAR, were reduced, but the total GluN1 changes didn't show a parallel reduction of the surface of GluN1, indicating the internalization of GluN1. Compared with the monomers, the oligomers, especially those formed in PD plasma, were more potent in promoting GluN1 internalization, and were abolished by clathrin inhibitor pitstop2. The above results suggest that alpha-syn oligomers, especially those formed in PD plasma, increase the effect of alpha-syn in promoting the internalization of NMDAR GluN1 subunits, possibly through a clathrin-mediated endocytic mechanism.