Previous studies have confirmed the loss of vasoactive intestinal peptide (VIP) neurons in the colon of patients with PD (Wakabayashi et al., 1993) and the loss of excitatory dopaminergic neurons in the colon of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model (Anderson et al., 2007). Here, VIP is linked to Parkinson disease.