The brain receptor targets of the incretins, GLP-1 and GIP; specifically, GLP-1R and GIPR, were probed by Western blot and their protein expression levels were found to be preserved across age in mouse cerebral cortex (Fig. 1 A1, A2), were maintained in rat striatum following 6-OHDA lesioning (Fig. 1 B1), and also were confirmed present in human SNc in PD (Fig. 1 C1, C2)). The gene discussed is GIP; the disease is Parkinson disease.