Experimental evidence suggest the involvement of mGluR-LTD in goal directed learning, and cerebellar circuit adjustment during motor learning. Excessive mGluR-LTD has been linked to Alzheimer disease and Fragile X syndrome, while a loss of mGluR-LTD in the striatum may contribute to Parkinson’s symptoms. Finally, reduced mGluR-LTD in the midbrain has been suggested to confer a vulnerability of drug addiction. The group 1 mGluRs, mGluR1 and mGluR5 are differentially expressed in most brain regions, and co-expressed in some, based on (Ferraguti and Shigemoto, 2006). mGluR1 expression is indicated by the diagonal lines and mGluR5 expression pattern is indicated by the dotted stipple. Gray stipple indicates a lower level of expression.