In male mice, the overexpression of MMP-9 led to increased conflict-based anxiety, which is consistent with the increased anxiety levels observed in the novelty-suppressed feeding test in animal models of depression, such as the chronic corticosterone [25] and obesity [53] models, both of which are characterized by elevated MMP-9 brain levels. This evidence concerns the gene MMP9 and obesity due to melanocortin 4 receptor deficiency.