Ampakine CX516 ameliorates functional deficits in AMPA receptors in a hippocampal slice model of protein accumulation.

AMPAkines are positive modulators of AMPA receptors, and previous work has shown that these compounds can facilitate synaptic plasticity and improve learning and memory in both animals and humans; thus, their role in the treatment of cognitive impairment is worthy of investigation. In this study, we have utilized an organotypic slice model in which chloroquine-induced lysosomal dysfunction produces many of the pathogenic attributes of Alzheimer's disease. Our previous work demonstrated that synaptic AMPA receptor function is impaired in hippocampal slice cultures exhibiting lysosomal dysfunction leading to protein accumulation. The present study investigated the effect of the AMPAkine CX516 on AMPAR-mediated synaptic transmission as well as the CX516 induced modification of single channel AMPA receptor properties in this organotypic slice-culture model. In whole cell recordings from CA1 pyramidal neurons in chloroquine-treated slices we observed a significant decrease in AMPAR-mediated mEPSC frequency and amplitude indicating synaptic dysfunction. Following application of CX516, these parameters returned to nearly normal levels. Similarly, we report chloroquine-induced impairment of AMPAR single channel properties (decreased probability of opening and mean open time), and significant recovery of these properties following CX516 administration. These results suggest that AMPA receptors may be potential pharmaceutical targets for the treatment of neurodegenerative diseases, and highlights AMPAkines, in particular, as possible therapeutic agents.

Lysosomal dysfunction produces distinct alterations in synaptic alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid and N-methyl-D-aspartate receptor currents in hippocampus.

The early processes that lead to synaptic dysfunction during aging are not clearly understood. Dysregulation of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors may cause age-related cognitive decline. Using hippocampal slice cultures exhibiting lysosomal dysfunction, an early marker of brain aging that is linked to protein accumulation, we identified alterations to AMPA and NMDA receptor-mediated synaptic currents. The miniature and spontaneous excitatory postsynaptic currents that were examined after 3, 6, and 9 days of lysosomal disruption showed progressive changes in amplitude, frequency, and rise and decay kinetics. To investigate whether modifications in specific channel properties of single synaptic receptors contributed to changes in the amplitude and time course of synaptic currents, we examined the single channel properties of synaptic AMPA and NMDA receptors. The channel open probability and the mean open times showed decreases in both receptor populations, whereas the closed times were increased without any change in the channel conductance. The Western blot analysis revealed a progressive decline in synaptic markers including glutamate receptor subunits. These results indicate that lysosomal dysfunction leads to progressive functional perturbation of AMPA and NMDA receptors in this slice model of protein accumulation, suggesting that age-related cognitive decline could result from altered glutamate receptor function before reductions in synaptic density.

Amyloid beta-peptide Abeta(1-42) but not Abeta(1-40) attenuates synaptic AMPA receptor function.

The brains of Alzheimer's disease (AD) patients have large numbers of plaques that contain amyloid beta (Abeta) peptides which are believed to play a pivotal role in AD pathology. Several lines of evidence have established the inhibitory role of Abeta peptides on hippocampal memory encoding, a process that relies heavily on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function. In this study the modulatory effects of the two major Abeta peptides, Abeta(1-40) and Abeta(1-42), on synaptic AMPA receptor function was investigated utilizing the whole cell patch clamp technique and analyses of single channel properties of synaptic AMPA receptors. Bath application of Abeta(1-42) but not Abeta(1-40) reduced both the amplitude and frequency of AMPA receptor mediated excitatory postsynaptic currents in hippocampal CA1 pyramidal neurons by approximately 60% and approximately 45%, respectively, in hippocampal CA1 pyramidal neurons. Furthermore, experiments with single synaptic AMPA receptors reconstituted in artificial lipid bilayers showed that Abeta(1-42) reduced the channel open probability by approximately 42% and channel open time by approximately 65% and increased the close times by several fold. Abeta(1-40), however, did not show such inhibitory effects on single channel properties. Application of the reverse sequence peptide Abeta(42-1) also did not alter the mEPSC or single channel properties. These results suggest that Abeta(1-42) but not Abeta(1-40) closely interacts with and exhibits inhibitory effects on synaptic AMPA receptors and may contribute to the memory impairment observed in AD.

Neural cell adhesion molecule-associated polysialic acid inhibits NR2B-containing N-methyl-D-aspartate receptors and prevents glutamate-induced cell death.

The neural cell adhesion molecule (NCAM) and its associated glycan polysialic acid play important roles in the development of the nervous system and N-methyl-D-aspartate(NMDA)receptor-dependent synaptic plasticity in the adult. Here, we investigated the influence of polysialic acid on NMDA receptor activity. We found that glutamate-elicited NMDA receptor currents in cultured hippocampal neurons were reduced by approximately 30% with the application of polysialic acid or polysialylated NCAM but not by the sialic acid monomer, chondroitin sulfate, or non-polysialylated NCAM. Polysialic acid inhibited NMDA receptor currents elicited by 3 microm glutamate but not by 30 microm glutamate, suggesting that polysialic acid acts as a competitive antagonist, possibly at the glutamate binding site. The polysialic acid induced effects were mimicked and fully occluded by the NR2B subunit specific antagonist, ifenprodil. Recordings from single synaptosomal NMDA receptors reconstituted in lipid bilayers revealed that polysialic acid reduced open probability but not the conductance of NR2B-containing NMDA receptors in a polysialic acid and glutamate concentration-dependent manner. The activity of single NR2B-lacking synaptosomal NMDA receptors was not affected by polysialic acid. Application of polysialic acid to hippocampal cultures reduced excitotoxic cell death induced by low micromolar concentration of glutamate via activation of NR2B-containing NMDA receptors, whereas enzymatic removal of polysialic acid resulted in increased cell death that occluded glutamate-induced excitotoxicity. These observations indicate that the cell adhesion molecule-associated glycan polysialic acid is able to prevent excitotoxicity via inhibition of NR2B subunit-containing NMDA receptors.