PDE5A and Alzheimer disease: Although classically described as brain tissue atrophy accompanied by the presence of β-amyloid plaques and neurofibrillary tangles, AD is now also known to be associated with the increased CNS expression of numerous PDE mRNA species, including PDE2, PDE4A, PDE4B, PDE5, PDE7 and PDE8B as demonstrated in AD human brain tissue and rodent experimental models [28,209,210,211,212,213].