PDE5A and Alzheimer disease: Recently, an upregulated PDE5 expression has been found in the brains of mild AD patients, and studies have confirmed that PDE5 inhibitors exhibit therapeutic effects on AD patients by improving learning and memory, and reversing cognitive impairment (Boccia et al., 2011; Hosseini-Sharifabad et al., 2012; Al-Amin et al., 2014).