DPP4 and metabolic syndrome: Based on these reports, several studies have evaluated the therapeutic potential of agonists or antagonists of the PACAP/VIP receptors (including PACAP and/or VIP themselves), and an inhibitor of dipeptidyl peptidase-4 (DPP-4), a common degradation enzyme for PACAP, VIP, GIP, and GLP-1, for the treatment of metabolic syndrome, including diabetes mellitus (Ahrén, 2008, 2009; Verspohl, 2009; Chapter et al., 2010).