Abstract: Background:  Phosphodiesterase 4 (PDE4), one of the 11 PDE enzyme families that hydrolyze cyclic nucleotides, is critical for controlling intracellular cyclic AMP (cAMP) concentrations and plays an important role in regulating alcohol consumption and mediating memory in dementia such as Alzheimer’s disease (AD). Chronic alcohol consumption can cause alcohol-related dementia and 50-75% of detoxified alcoholics have memory or cognition impairment. However, the role of PDE4 and its mechanism remain to be characterized and elucidated. Methods: Using the water-maze, passive avoidance, or novel object recognition test, we examined the effects of rolipram, a prototypical PDE4 inhibitor, and roflumilast, a potent PDE4 inhibitor which has been approved for treatment of chronic obstructive pulmonary disease (COPD) in humans, on memory loss in APP/PS1 double transgenic mice, a widely used model for AD. In addition, we tested the effects of the PDE4 inhibitors, via ip, intra-gastric, or intra-striatum infusion, on ethanol intake and preference using the mouse two-bottle choice paradigm. Mice deficient in PDE4A, PDE4B, or PDE4D (4AKO, 4BKO, and 4DKO, respectively) and their wild type (WT) controls were tested for ethanol consumption and memory; the latter was measured in the absence or presence of beta-amyloid peptide 1-42 (Abeta42) infused into the dorsal hippocampus. Results:  Similar to rolipram, roflumilast reversed memory deficits in APP/PS1 mice in all the memory tests and reduced ethanol intake and preference in C57BL/6 mice in two-bottle choice. Consistent with the results in the memory tests, roflumilast reduced the loss of neurons and neurocyte apoptosis in AD mice, as shown using HE and Nissl staining. It also reversed the decreased ratio of Bcl-2/BAX in the cerebral cortex and hippocampus of AD mice. In addition, roflumilast reversed the decreased levels of cAMP and expression of phosphorylated cAMP response element-binding protein (CREB) and brain derived neurotrophic factor (BDNF) in AD mice. Compared to the WT controls, 4AKO mice displayed significant decreases in ethanol intake and preference and reversal of Abeta42-induced memory deficits. In contrast, 4BKO mice only mimicked the ability of 4AKO mice to reduce alcohol consumption while 4DKO mice only to reverse Abeta42-induced memory deficits. In addition, levels of cAMP and phospho-CREB (pCREB) were increased in the hippocampus of 4AKO or 4DKO mice, which also showed reversal of Abeta42-induced decreases in pCREB. 
Conclusions:  These data suggest that PDE4 inhibitors such as roflumilast improve learning and memory in AD and reduce ethanol intake and preference likely via cAMP/CREB/BDNF signaling-mediated neuroprotection. PDE4 isoforms have different roles in mediating ethanol-drinking behavior and memory in AD. The results indicate PDE4A as a potential new target for alcohol-related dementia, although studies with animal models of alcohol-related dementia are needed to clarify this.

Key words:  , Phosphodiesterase-4 (PDE4), Alzheimer&rsquo, s disease (AD), alcoholism, cAMP signaling, memory, trangenic mice