GRM5 and major depressive disorder: Notably, in one study (Baldassarri et al., 2023), individuals with co-occurring major depressive disorder and tobacco use did not significantly differ in mGlu5 availability from those with major depressive disorder alone, though individuals with co-occurring PTSD and tobacco use exhibited higher mGlu5 availability in the orbitofrontal, dorsolateral prefrontal, and anterior cingulate cortex than individuals with PTSD alone.