Cognitive Deficits Associated with Alteration of Synaptic Metaplasticity Precede Plaque Deposition in AβPP23 Transgenic Mice

Synaptic dysfunction is an early event in the development of Alzheimer's disease (AD) and relates closely to the cognitive impairment characterizing this neurodegenerative process. A causative association has been proposed, largely on the basis of in vitro studies, between memory decline, soluble amyloid-beta (A beta) oligomers and alterations of glutamatergic neurotransmission. We aimed here to characterize in vivo N-methyl-D-aspartate receptor (NMDAR)-mediated signaling, at an early stage of AD, before extracellular amyloid plaques are deposited. We assessed the functional link between cognitive abilities and NMDAR-mediated pharmacological responses of six-month-old A beta PP23 transgenic mice (A beta PP23tg), overexpressing the human amyloid-beta protein precursor carrying the Swedish double mutation. We found evidence of cognitive impairments in these mice, indicated by deficits in the delayed-non-matching-to-place task. Alterations of NMDAR-mediated signaling in this mouse model were confirmed by the reduced sensitivity of motor-activation and working memory to pharmacological inhibition of NMDAR activity. At the molecular level, A beta PP23tg mice show hippocampal alterations in the trafficking of synaptic NMDAR subunits NR2A and NR2B and at an ultrastructural analysis show A beta oligomers intracellularly localized in the synaptic compartments. Importantly, the behavioral and biochemical alterations of NMDAR signaling are associated with the inhibition of long-term synaptic potentiation and inversion of metaplasticity at CA1 synapses in hippocampal slices from A beta PP23tg mice. These results indicate a general impairment of synaptic function and learning and memory in young A beta PP23tg mice with A beta oligomers but no amyloid plaques.