ADCYAP1 and Parkinson disease: Serum PACAP levels were significantly lower in PD patients than in healthy controls [(76.02 ± 43.78) pg/ml vs. (154.96 ± 76.54) pg/ml, P < 0.001] and serum VIP levels were significantly lower in PD patients than in the healthy controls [(109.56 ± 15.39) pg/ml vs. (136.46 ± 24.16) pg/ml, P < 0.001].