Consistent with an unsilencing of synapses, deletion of GluN2B (Figure 8C) decreased synaptic failures, whereas deletion of GluN2A had no effect on the rate of failures (Figure 8B). Furthermore, for both GluN2A and GluN2B deletion, the average amplitude from all trials was significantly increased (Figure 8D), consistent with the increases in AMPAR-EPSC (Figure 5B). However, only GluN2A deletion increased the average amplitude of “nonfailures” (Figure 8E), consistent with the increase in mEPSC amplitude (Figure 6A). Taken together, our results suggest that deletion of the GluN2B subunit, given its prominent expression in early postnatal development, increases AMPAR-EPSCs

by a mechanism similar to the deletion of NMDARs

entirely (Adesnik et al., Bortezomib clinical trial 2008). That is, removing NMDARs selleck compound during synaptogenesis results in an increase in the number of functional synapses, possibly by removing a silencing signal, without appreciable change in synaptic strength. Early postnatal deletion of GluN2A, however, clearly increases AMPAR-EPSCs by a distinct mechanism involving an increase in synapse strength without a significant change in the number of functional synapses. We utilized a single-cell genetic approach to address the roles of GluN2A and GluN2B in synapse development. We have recently used this approach to evaluate the composition of AMPAR subunits (Lu et al., 2009) and the role of GluN1 in synapse development, and have shown that this approach reveals cell autonomous effects of the genetic manipulation without competition between neighboring cells (Adesnik et al., 2008). We have shown here, for the first time electrophysiologically, that GluN2A and GluN2B subunits fully account for synaptic NMDAR currents in adult CA1 pyramidal cells. Deletion of GluN2A or GluN2B individually thus allowed for the detailed analysis of pure diheteromeric synaptic populations. The biophysical and pharmacological properties determined for the diheteromeric synaptic NMDARs provided a basis

for a detailed characterization of the developmental time nearly course of the NMDAR subunit switch. We found that CA1 pyramidal cell synapses undergo an incomplete subunit switch and express significant amounts of triheteromeric receptors, while sensory cortical neurons undergo a more complete switch from GluN2B to GluN2A. We then evaluated the functional effects of GluN2 subunit deletion on synapse development and found that, similar to GluN1 deletion (Adesnik et al., 2008), deletion of GluN2B subunits increased AMPAR-EPSCs by increasing the number of functional synapses. Surprisingly, however, GluN2A deletion also increased AMPAR-EPSCs, but this increase was secondary to a postsynaptic strengthening of unitary connections without affecting the number of functional synapses.