VIP and depressive disorder: The PACAP system, in turn, is involved in the molecular pathways of the three main theories of depression (monoamine, neurotrophic, and endocrine), while VIP is shown to be aberrantly expressed in patients with IBS and has been hypothesized to be involved in its development (Albert et al., 2011; Del Valle-Pinero et al., 2015; Bednarska et al., 2017).