GRM5 and major depressive disorder: Acquisition: Separate PET [(11C)ABP688, dynamic] and MRI (3T). Analysis: The BPND of [11C]ABP688 was quantified using the SRTM for mGluR5 availability. Seed-based functional connectivity analysis was performed using resting-state fMRI data with regions derived from quantitative [11C]ABP688 PET as seeds. Results: Patterns of correlation between [11C]ABP688 BPND and the strength of functional connectivity involving the superior prefrontal cortex were opposite in the depression and control groups.