HTR4 and Parkinson disease: In the present study, we demonstrated that the 5-HT4R agonists prucalopride and velusetrag improved the facilitation of contextual fear extinction in the PD mice without affecting the mRNA expression levels of hippocampal 5-HT4R. These results suggest that 5-HT4R agonists could be potentially useful as a therapeutic drug for treating cognitive deficits in PD.