NPY and major depressive disorder: Interneuron populations are furthersubdivided by co-expression of other calcium-binding proteins (e.g., calbindin (CB)and calretinin (CR)) and neuropeptides (e.g., neuropeptide Y (NPY) andcholecystokinin (CCK)), which together can help characterize vulnerable cellpopulations relevant to the pathological changes associated with MDD and otherstress-related disorders.