The neuropeptide Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) and its receptors PAC1 and VPAC2 are widely expressed in the nervous system. The investigators found that PACAP treatment of neuronal cell cultures increases expression of Brain Derived Neurotrophic Factor (BDNF) that plays an important role in the etiology of psychiatric disorders and action of antidepressants. For the first time, the investigators demonstrated that treatment by Paroxetine and Citalopram significantly decreases PAC1 and VPAC2 and upregulates PACAP mRNA expression, whereas Imipramine shows an opposite effect. Moreover, PACAP, PAC1 and VPAC2 expression is highly correlated with BDNF expression. Their in vivo studies show that Imipramine reduces BDNF and increases PAC1 mRNA expression in murine hippocampus, suggesting that antidepressants may affect neuronal plasticity through PACAP-BDNF interactions. Based on their observations in experimental systems, the investigators hypothesize that PACAP signaling system may be involved in the etiology of depression and mechanism of antidepressant action. The investigators will evaluate this hypothesis by examining serum PACAP levels, effect of antidepressants on PACAP levels, and gene polymorphisms of PACAP and its receptors in major depressive disorder patients. This study will enhance the investigators' understanding of PACAP's role in the etiology of depression and antidepressant treatment and will provide a basis to evaluate PACAP pathway as a potential target for diagnostics and novel antidepressants drug discovery.