One study found that mGluR5 (–/–) [also known as Grm5(–/–)] mice displayed more depression-like behaviors (e.g., learned helplessness, social withdrawal, and anhedonia) than control mice following exposure to various stressful stimuli, while several other studies found that mGlu5 receptor antagonism was associated with antidepressant-like effects and that mGlu5 receptor knockout mice displayed an antidepressant-like behavioral phenotype (50, 62). Here, GRM5 is linked to major depressive disorder.