mGluR5 in particular has attracted considerable interest because of its potential involvement in Alzheimer's Disease (AD) [32] and PD [33], [34], its role in learning and memory [35], [36], and its abundant expression in the frontal cortex, limbic system, and caudoputamen [37]—brain regions selectively affected in AD and PD. Here, GRM5 is linked to Parkinson disease.