# Involvement of Vasoactive Intestinal Peptide Family Members in Diabetic Keratopathy
## 3 Expression of Neuropeptides in the Cornea
### Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP)  
PACAP, encoded by the gene Adcyap1, exists in 27 and 38 amino acid forms. Both forms are bioactive; however, PACAP38 is more than 100-fold more abundant than PACAP27 in neuronal tissues [98; 99]. PACAP cytoprotective effects are mediated by the activation of specific receptors, which share with VIP. However, the type I PACAP receptor (PAC1R) has a high affinity for PACAP as compared to VIP [100]. Neurons and endocrine cells express different PAC1R splice variants, which differ at the N terminus or in the third intracellular loop (IC3) of this GPCR [101]. Therefore, the multimodal effects played by PACAP depend on its concentration as well as on the receptor splice variants expressed in the specific tissue and cell, leading to adenylate cyclase (AC) or phospholipase C (PLC) pathways activation [102; 103]. The effects of PACAP are also mediated by the stimulation of an intracellular astrocyte-derived neurotrophic factor known as activity-dependent protein (ADNP) [104; 105; 106; 107]. Furthermore, through the PKA signaling pathway, the activation of PAC1R also transactivates EGFR, resulting in the activation of the ERK signaling pathway as detected in different pathological conditions, such as cancers, amyotrophic lateral sclerosis, diabetic retinopathy, and corneal alteration [108; 109; 110; 111; 112; 113]. PACAP is largely distributed in the body (Figure 3) where it plays different roles at behavioral and cognitive levels [114; 115; 116; 117]. The peptide is involved in the regulation of car diovascular, gastrointestinal, urinary, and endocrine functions [118; 119; 120; 121; 122; 123; 124]. Moreover, it exerts an important role in reproduction, pregnancy, early development, and aging [125; 126; 127; 128; 129; 130; 131]. In the eye, PACAP and its high-affinity receptor PAC1 are expressed in the retinal layers [132; 133; 134; 135; 136; 137], in the ciliary body, in the optic nerve, in the conjunctiva, and in the iris [50; 133; 138]. Moreover, the expression of PACAP and its receptors has been largely demonstrated in the cornea. In fact, PACAP was detected in nerve terminals running in the stroma and sending off some branches into the epithelium [50]. PACAP and PAC1R immunoreactivity was detected in corneal epithelium, endothelium, as well as in stroma [50; 139]. The literature largely described the protective role played by PACAP in the cornea. The treatment with the peptide enhanced cell proliferation and ensured the integrity of the corneal endothelium barrier subjected to growth factor deprivation [139]. Moreover, PACAP shown to protect the human corneal endothelial cells (HCECs) derived from donors' cornea against the ultraviolet B radiation insult, leading to cell death and the alteration of barrier integrity [140]. The exogenous peptide accelerated corneal epithelial wound healing after laser-assisted in situ keratinocytes (LASIK) surgery, increasing up to 75% of the corneal sensitivity eight weeks after the operation. In fact, the peptide significantly increased the neurite outgrowth of trigeminal ganglion cells [141], confirming the positive role played by PACAP in corneal nerve regeneration and corneal sensitivity. PACAP alone or connected to the N-terminal agrin domain (Nta) with a genetic engineering method promoted corneal epithelial cell proliferation and differentiation, preventing the apoptosis of injured nerves and promoting the growth of nervous processes [142]. Moreover, a new recombinant PACAP-derived peptide (named MPAPO) was shown to promote corneal epithelial cell proliferation and trigeminal ganglion cell axon regeneration. PACAP exerts also a key role in tear secretion. In fact, PACAP knockout mice developed dry eye-like symptoms, which comprised corneal keratinization and reduced tear production [143]. Interestingly, PACAP eyedrops promoted tear secretion through the activation of the AC/cAMP/PKA pathway, leading to the translocation of aquaporin 5 from the cytosol to the membrane of lacimal acinar cells [144].