Resumo

Soluble oligomers of the amyloid-beta peptide (AβOs) accumulate in the brains of Alzheimer’s disease (AD) patients and are increasingly recognized to be responsible for synapse dysfunction and early memory loss in AD. AβOs have been shown to inhibit long-term potentiation (LTP), facilitate long-term depression (LTD), and induce endocytosis of AMPA and NMDA receptors. Recent studies have shown that activation of dopamine D1/D5 receptors reinforces excitatory transmission via phosphorylation of AMPA and NMDA receptor subunits (GluR1 and NR1, respectively) by protein kinase A (PKA). Phosphorylation of GluR1 and NR1 by PKA favors its insertion into extra-synaptic and synaptic sites, respectively, rendering the receptors more resistant to endocytosis. We show, by immunocytochemistry, that treatment of rat embryonic (E18) hippocampal neurons in culture (20 DIV) with AβOs (400 nM for 4h) significantly decreased GluR1 (54,7 ± 6,5 %) and NR1 (56,0 ± 4,7 %) levels at the surface of dendrites. In parallel, western blotting analysis revealed that total cellular levels of both proteins remained unchanged, confirming the disruption of receptor trafficking by AβOs. A selective D1/D5 receptor agonist, (±)-6-Chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine hydrobromide (SKF81297), blocked the endocytosis of both GluR1 and NR1 induced by AβOs (94,5 ± 13,0 % and 95,4 ± 1,5 %, respectively). On the other hand, the protection exerted by SKF81297 was abrogated by (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-te trahydro-1H-3-benzazepine hydrochloride (SCH23390), a selective D1/D5 receptor antagonist, further substantiating the involvement of D1/D5 receptors. Phosphorylation of AMPA receptor subunit GluR1 at Ser845 (pS845-GluR1) is required for synaptic insertion of the receptor during LTP; conversely, dephosphorylation at this specific site by calcineurin increases receptor endocytosis and is required for the induction of LTD. We found that AβOs induce a decrease in pS845-GluR1 dendritic levels (63,8 ± 2,5 %), which was sensitive to a calcineurin inhibitor, FK-506 (20 nM). SKF81297 prevented the reduction in pS845-GluR1 levels (95,0 ± 6,6 %), providing a mechanism by which SKF81297 blocks AβOs-induced loss of surface AMPARs. This result was also confirmed by western blotting analysis of total pS845-GluR1 levels. SKF81297 further prevented the impairment of LTP by AβOs in acute rat hippocampal slices, indicating the functional relevance of these findings. Results suggest that specific stimulation of D1 receptors may provide a novel pharmacological approach to prevent synapse failure and memory decline in AD.