PTGS2 and major depressive disorder: To address the potential role of hippocampal COX-2 in the mechanisms of the susceptibility to MDD-like behavior, we used a variant of the original CMS model [84,85] that is based on the induction of decreased sensitivity to reward (anhedonia) as the core depressive symptom [86,87], and on the previously observed individual susceptibility of 50–70% of C57BL6 mice to this condition [88,89,90,91].