Phosphodiesterase inhibition facilitates cognitive restoration in rodent models of age-related memory decline

BACKGROUND: Previous studies have shown that cyclic nucleotide phosphodiesterase type 5 (PDE5) inhibition with the drugs sildenafil and vardenafil can enhance spatial performance and object recognition in rodent models of learning and memory. OBJECTIVE: We review recent studies on PDE5 inhibition and report novel data that specifically tests the systemic effects of both pharmacological agents in aged rats using two different spatial learning/memory paradigms. METHODS: The 14-unit T-maze was used as a test of egocentric spatial processing that requires rats to learn a series of left/right turns to avoid mild footshock. The Morris water maze is a test of allocentric spatial learning that requires the acquisition of place information to localize a hidden platform relative to distal room cues. RESULTS: In both cases, acquisition (i.e., learning performance) was not improved, however after a one week drug washout period, aged animals demonstrated improved spatial memory retention compared to aged controls, ruling out simple performance effects. CONCLUSIONS: These findings are discussed in relation to recent reports on the use of PDE inhibitors to treat Alzheimer's disease (AD) dementia and age-related memory impairments. While some report promising pre-clinical results, others note that PDE5 may not be an appropriate target in AD due to a lack of localization within critical brain structures where therapeutic activity is needed. Despite these limitations, PDE5 inhibition may produce beneficial effects via several mechanisms that target predisposing risk factors leading to increased incidence of memory impairment in aged individuals and influence memory consolidation mechanisms that preserve long-term retention of cognitive information.