Metabotropic glutamate receptors, particularly mGluR5, have been implicated in antidepressant action and mood regulation.7 mGluR5 antagonists have been shown to have antidepressant effects;8, 12, 37, 38 therefore, an understanding of how mGluR5 availability varies in mood disorders may help devise appropriate treatment strategies. This evidence concerns the gene GRM5 and mood disorder.