Deletion of the NR2A subunit prevents developmental changes of NMDA-mEPSCs in cultured mouse cerebellar granule neurones

We investigated the role N-methyl-d-aspartate (NMDA) receptor subunits play in shaping excitatory synaptic currents in cultures of cerebellar granule cells (CGCs) from NR2A knockout (NR2A-/-) and wild-type (+/+) mice. Cultures were maintained in a condition that facilitates the occurrence of functional synapses, allowing us to record NMDA-miniature excitatory postsynaptic currents (mEPSCs) in addition to NMDA receptor-mediated whole-cell currents at three ages in vitro. Whole-cell NMDA current density decreased with development in both strains though currents from NR2A-/- neurones demonstrated greater sensitivity to CP101 606, an NR2B subunit specific blocker. Sensitivity to Mg(2+) blockade decreased with age in vitro in +/+ but not in NR2A-/- CGCs. Immunocytochemistry revealed that dendrites and somas displayed distinct NR1 and NR2A subunit clusters which became increasingly colocalized in +/+ neurones. Qualitatively the overall NR2B subunit staining pattern was similar in +/+ and NR2A-/- neurones throughout development, suggesting that the NR2B subunit distribution is not mediated by the NR2A subunit. In addition, staining with markers for excitatory synapses showed that expression of NR2A subunit (but not NR2B) increases at both synaptic and extrasynaptic sites in +/+ neurones during development. In parallel, NMDA-mEPSCs were faster in +/+ compared with NR2A-/- neurones at all time points studied, suggesting that the NR2A subunit begins to replace NR2B-rich NMDA receptors even at early stages of development. Many NR2A-/- neurones were devoid of NMDA-mEPSCs at the later time point, and transfection of the NR2A subunit in these neurones restored fast decay and the occurrence of NMDA-mEPSCs. Taken together, our results indicate that the NR2A subunit is mainly responsible for the developmental changes observed in the maturation of excitatory synapses.