In addition, it was found in a 6-OHDA-induced PD rat model that intraperitoneal injection of VIP can increase the number of 5-hydroxytryptamine (5-HT)-positive cells and the relative expression of mRNA in the dorsal raphe nucleus (DRN) and reduce the expression of depression-related neuroendocrine hormones (Wang et al., 2015). Here, VIP is linked to depressive disorder.