More recently, we have reported that PACAP gene-deficient mice develop dry eye-like symptoms such as corneal keratinization and tear reduction, and have shown that PACAP eye drops stimulate tear secretion via an adenylyl cyclase/ cyclic adenylyl cyclase monophosphate/protein kinase A (AC/cAMP/PKA) cascade, which in turn stimulates the translocation of aquaporin 5 from the cytosol to the membrane of lacrimal acinar cells to bring about an increase in water permeability [3]. Here, ADCYAP1 is linked to dry eye syndrome.