NPY and major depressive disorder: Similarly, the expression of neuropeptide Y (NPY), a ligand for the DPP4 receptor, has been proposed as a biomarker for diagnosing PTSD, depression, and other neuropsychiatric illnesses (Canneva et al., 2015; Schmeltzer et al., 2016; Gołyszny and Obuchowicz, 2019).