CCK and type 2 diabetes mellitus: The discrepancies between OLETF and GK rats may be due to the different pathological properties of type 2 diabetes: chronic progression of type 2 diabetes with obesity and temporal alterations of plasma insulin levels in OLETF rats; continuous hypoinsulinemia and non-obesity in GK rats [53]; however, the causal mechanisms of alterations of CCK- and PV-positive neurons and ATF4 co-expression in OLETF rats are still unclear.