VIP and Parkinson disease: The present study demonstrated that central nigrostriatal dopaminergic denervation (one of the main pathologic hallmarks of PD), was associated with decreased cholinergic neurons in the SPN, decreased ACh, VIP and nNOS, but upregulated M2R and M3R in the colorectum, resulting in colorectal dysmotility, which contributed to outlet obstruction constipation (Figure 6).