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. This evidence concerns the gene GRM5 and post-traumatic stress disorder.