HTR4 and Parkinson disease: Furthermore, 5-HT4R agonists restored the facilitation of contextual fear extinction in PD mice by stimulating the cAMP/CREB pathway in the hippocampal dentate gyrus, the area that is innervated by the serotonergic neurons that form the final synaptic ending in a candidate neuronal pathway from the substantia nigra (SN) to the hippocampus [18].