Fractalkine (CX(3)CL1) enhances hippocampal N-methyl-D-aspartate receptor (NMDAR) function via D-serine and adenosine receptor type A2 (A(2A)R) activity

Background: N-Methyl-D-aspartate receptors (NMDARs) play fundamental roles in basic brain functions such as excitatory neurotransmission and learning and memory processes. Their function is largely regulated by factors released by glial cells, including the coagonist D-serine. We investigated whether the activation of microglial CX(3)CR1 induces the release of factors that modulate NMDAR functions. Methods: We recorded the NMDAR component of the field excitatory postsynaptic potentials (NMDA fEPSPs) elicited in the CA1 stratum radiatum of mouse hippocampal slices by Shaffer collateral stimulation and evaluated D-serine content in the extracellular medium of glial primary cultures by mass spectrometry analysis. Results: We demonstrated that CX(3)CL1 increases NMDA-fEPSPs by a mechanism involving the activity of the adenosine receptor type A2 (A(2A)R) and the release of the NMDAR coagonist D-serine. Specifically (1) the selective A(2A)R blocker 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine (SCH58261) and the genetic ablation of A(2A)R prevent CX(3)CL1 action while the A(2A)R agonist 5-(6-amino-2-(phenethylthio)-9H-purin-9-yl)-N-ethyl-3, 4-dihydroxytetrahydrofuran-2-carboxamide (VT7) mimics CX(3)CL1 effect, and (2) the selective blocking of the NMDAR glycine (and D-serine) site by 5,7-dicholorokynurenic acid (DCKA), the enzymatic degradation of D-serine by D-amino acid oxidase (DAAO) and the saturation of the coagonist site by D-serine, all block the CX(3)CL1 effect. In addition, mass spectrometry analysis demonstrates that stimulation of microglia and astrocytes with CX(3)CL1 or VT7 increases D-serine release in the extracellular medium. Conclusions: CX(3)CL1 transiently potentiates NMDAR function though mechanisms involving A(2A)R activity and the release of D-serine.