VIP and Parkinson disease: Compared to PD patients with early-stage disease, PACAP and VIP levels in the medium-stage and advanced-stage PD patients were lower, while there were no significant differences in PACAP (χ2 = 4.828; df = 2; and P > 0.05) and VIP (χ2 = 4.158; df = 2; and P > 0.05) levels across the three groups (Figure 3).