In addition, several studies have revealed neuroprotective effects of genetic deletion of mGlu5 or mGlu5 antagonists in 6-OHDA- or MPTP-treated animals [60,61,62,63,64,65,66,67] suggesting that antagonism of mGlu5 may retard further neurodegeneration and improve motor symptoms in early PD. This evidence concerns the gene GRM5 and Parkinson disease.