CREB1 and Alzheimer disease: Another class of inhibitors to exert anti-AD effects are phosphodiesterase (PDE) inhibitors, which have been shown to effectively restore memory function through enhancing levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP)—critical second messengers for neuronal signalling in the nitric oxide/cGMP/cAMP-responsive-element-binding (CREB) pathway [107].