HTR4 and Parkinson disease: Moreover, we also demonstrated that administration of the phosphodiesterase IV (PDE IV) inhibitor rolipram [20] and/or the serotonin 5-hydroxytryptamine receptor 4 receptor (5-HT4R) agonists prucalopride and velusetrag [18] restored facilitation of contextual fear extinction in PD mice via increasing hippocampal cAMP levels.