Amyloid-␤ (A␤) peptides are produced in high amounts during Alzheimer's disease, causing synaptic and memory dysfunction. However, they are also released in lower amounts in normal brains throughout life during synaptic activity. Here we show that low picomolar concentrations of a preparation containing both A␤ 42 monomers and oligomers cause a marked increase of hippocampal long-term potentiation, whereas high nanomolar concentrations lead to the well established reduction of potentiation. Picomolar levels of A␤ 42 also produce a pronounced enhancement of both reference and contextual fear memory. The mechanism of action of picomolar A␤ 42 on both synaptic plasticity and memory involves ␣7-containing nicotinic acetylcholine receptors. These findings strongly support a model for A␤ effects in which low concentrations play a novel positive, modulatory role on neurotransmission and memory, whereas high concentrations play the well known detrimental effect culminating in dementia.

Memory loss, synaptic dysfunction, and accumulation of amyloid ␤-peptides (A␤) are major hallmarks of Alzheimer's disease (AD). Downregulation of the nitric oxide/cGMP/cGMP-dependent protein kinase/c-AMP responsive element-binding protein (CREB) cascade has been linked to the synaptic deficits after A␤ elevation. Here, we report that the phosphodiesterase 5 inhibitor (PDE5) sildenafil (Viagra), a molecule that enhances phosphorylation of CREB, a molecule involved in memory, through elevation of cGMP levels, is beneficial against the AD phenotype in a mouse model of amyloid deposition. We demonstrate that the inhibitor produces an immediate and long-lasting amelioration of synaptic function, CREB phosphorylation, and memory. This effect is also associated with a long-lasting reduction of A␤ levels. Given that side effects of PDE5 inhibitors are widely known and do not preclude their administration to a senile population, these drugs have potential for the treatment of AD and other diseases associated with elevated A␤ levels.

Objective The goal of this study was to investigate the role of endogenous amyloid-β peptide (Aβ) in healthy brain. Methods Long-term potentiation (LTP), a type of synaptic plasticity that is thought to be associated with learning and memory, was examined through extracellular field recordings from the CA1 region of hippocampal slices, whereas behavioral techniques were used to assess contextual fear memory and reference memory. Amyloid precursor protein (APP) expression was reduced through small interfering RNA (siRNA) technique. Results We found that both antirodent Aβ antibody and siRNA against murine APP reduced LTP as well as contextual fear memory and reference memory. These effects were rescued by the addition of human Aβ42, suggesting that endogenously produced Aβ is needed for normal LTP and memory. Furthermore, the effect of endogenous Aβ on plasticity and memory was likely due to regulation of transmitter release, activation of α7-containing nicotinic acetylcholine receptors, and Aβ42 production. Interpretation Endogenous Aβ42 is a critical player in synaptic plasticity and memory within the normal central nervous system. This needs to be taken into consideration when designing therapies aiming at reducing Aβ levels to treat Alzheimer disease.

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