1 NMDA receptor activation by spontaneous glutamatergic neurotransmission

Under physiological conditions N-methyl-D-aspartate (NMDA) receptor activation requires coincidence of presynaptic glutamate release and postsynaptic depolarization due to the voltage-dependent block of these receptors by extracellular Mg 2+. Therefore, spontaneous neurotransmission in the absence of action potential firing is not expected to lead to significant NMDA receptor activation. Here, we tested this assumption in layer IV neurons in neocortex at their resting membrane potential (~-67 mV). In long duration stable recordings, we averaged a large number of miniature excitatory postsynaptic currents (mEPSCs) (>100) before or after application of AP-5 a specific blocker of NMDA receptors. The difference between the two mEPSC waveforms showed that the NMDA current component comprises about 20% of the charge transfer during an average mEPSC detected at rest. Importantly, the contribution of the NMDA component was markedly enhanced at membrane potentials expected for the depolarized up-states (~-50 mV) that cortical neurons show during slow oscillations in vivo. In addition, partial block of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor component of the mEPSCs did not cause a significant reduction in the NMDA component indicating that potential AMPA receptor-driven local depolarizations did not drive NMDA receptor activity at rest. Collectively these results indicate that NMDA receptors significantly contribute to signaling at rest in the absence of dendritic depolarizations or concomitant AMPA receptor activity.